Power system blackout

CHAPTER1: INTRODUCTION

In power system, blackouts have been the most problem occur in the interconnected grid which results in large loss. Power system blackout is one of the major challenges of electric utilities in the world such as blackout of Greece, Italy, North America, Sweden and Denmark. However, in recent years this phenomenon has considerably increased base on the frequency and the severity of this problem. This may be due to the consequence of new regulations and restriction restricted by power system deregulation. In fact, it is not possible to completely prevent these power deficiency occur. However, proper monitoring, control and protection schemes, frequency and the severity of this phenomenon occur may be reduced. Some kind of operation contributes to the recently occurred blackouts such as Incorrect of protective systems, voltage instability, and lack of under frequency load shedding. Under frequency load shedding (UFLS) scheme is proposed to enhance the reliability of power systems to against system failure and fault occur. This scheme is classified as protection scheme or system protection scheme. System protection scheme is defined as protection strategies designed to detect a particular condition which is known to cause failures to the power system and prepare some kind of predetermined action counteract the observed condition in a controlled manner. System protection scheme is specially designed to detect abnormal conditions at the same time take predetermined corrective action, other than the isolation of faulted element, to preserve system integrity and provide acceptable system performance. The objective of using the protection scheme is to increase power system reliability especially in term of security during extreme contingencies and to improve power system operation as well. Under frequency load shedding (UFLS) scheme is one of the most commonly used as a protection schemes. This kind of protection scheme is employed due to the effectiveness to counter power system failure regardless what kind of disturbance it is applied. Under frequency load shedding (UFLS) scheme is conventionally designed to preserve the balance power in the island during fault occur. Under frequency load shedding (UFLS) is a very important approach to prevent frequency decline. It should have capability not only to shed load under different operating modes when local systems are connected to the main systems, but also capable to maintain the frequency stability when local systems are islands. The frequency decline, caused by the power imbalance between generation and demand is considered as serious problems which lead to the excess load. The frequency decline may cause a permanent damage to the turbine blades and plant themselves lead to failures because at low frequency their auxiliary are not be able to maintain normal output when the frequency is about 10 to 15% below normal. The primary method to bring back to the nominal frequency level is to shed amount of load. In power systems protection scheme, the frequencies are widely used as a setting in UFLS design. Under frequency load shedding (UFLS) must be performed quickly to arrest power system frequency decline by decreasing power system load to match available generating capacity. Extreme frequency decline can occur within seconds. An automatic under frequency load shedding (UFLS) scheme is applied to restore the system frequency to an acceptable level following a major system emergency which can cause a generation deficiency. In conventional under frequency protection design the only measured parameter of the system involved in decision making is frequency. Conventional UFLS schemes are designed to prevent extreme frequency decline because of disturbance which lead to imbalance between generation and demand. Excessive frequency decline may cause damage to the equipments of power system particularly turbine blades in power plant at frequency below 47.5Hz. During normal operation of power system the amount of generation is equal to the amount of demand. Whenever the faults occur, either the amount of generation is decreases or the amount of load is suddenly increases, the balance of power is violated and the frequency falls at the predetermined threshold the portion of the load will be shed in a few steps to equalize the amount of demand and generation to prevent the system collapse. The loads to be shed in this system are shed constant load feeders and are not selected adaptively. In the other hand, these systems always drop the same load regardless of the location of disturbance. In this system loads are classified in three groups of non vital, semi vital and vital loads. The system usually shed non vital load. Sometime in severe condition semi vital loads may also be shed. Although this kind of protection scheme is easy to implement, but it is suffer in term of adaptability when various kind of fault is experience. In the other hand, regardless of the severity of the disturbance, setting of the under frequency load shedding scheme is the same or constant. This kind of scheme may be introduced over shedding or under shedding for small disturbance or large disturbance respectively. In this project rate of change of frequency is proposed to enhance the adaptability of under frequency relays. By using this load shedding method, fast reactions could be obtained for major system failures. Adaptive under frequency load shedding (AUFLS) scheme can prevent complete system blackouts in the case of large or small disturbance. In other word, adaptive under frequency load shedding (AUFLS) scheme is specially designed to counter any kind of in coming disturbance applied. Rate of change of frequency, df/dt is the indicator to detect the magnitude of disturbance or faults. The rate of change of frequency df/dt is an instantaneous indicator of power imbalance and is presently used with the frequency function to provide a more selective or faster operation. To make the rate of change of frequency df/dt as power deficiency indicator additional information about the system is required such as voltage, spinning reserve, load and etc. Such information may be communicated to the relay. There are three types of the under frequency relays available for load shedding scheme purpose. They are electromechanical relays, solid-state relays and digital relays. The purpose of the under frequency relay is for monitoring the frequency of an alternating current power line and protect the system by giving signal when ever the frequency drop below predetermine value for a specific length interval. The frequency of grid system must be maintain at 50Hz, if the frequency drop below nominal value the protection scheme must be initiated in order to maintain its generator on line even at low frequency. Frequency drop in power line may take several steps to measure. One of the methods is by using oscillator, the frequency which is substantially higher than the nominal frequency being measured, and counting the number of pulses from the oscillator during the period. When the line frequency decrease, the period will correspondingly increase, and therefore a high number of oscillator output pulse will be obtained. It is no desired to detect under frequency condition which result in load shedding when an under frequency condition does not exist in fact, primarily because of the inconvenience may cause customer unsatisfied. It is also because of the difficulty and duration taken to restore power to those customers. The objective of the invention of under frequency relay is to provide method of detecting an under frequency condition on an alternating current power line including the step of sensing by means of electrical circuit. After the power deficient has been cleared, the amount of the power generation and the power demand is approximately the same. Therefore the frequency of the system will bring back to the acceptable level because the power imbalance is proportional to the frequency deviation. Thus, when the frequency is synchronous with the frequency of the grid system, the islanded locations will connected back to the grid in case of the fault has been cleared.

1.1 OBJECTIVE:

AIM 1: To design adaptive under frequency load shedding scheme for small Distributed Generation.

AIM 2: To develop model to predict the rate of frequency (df/dt) decay during system disturbance.

1.2 SCOPE OF THESIS

The scopes of work are:

  1. Gathering all suitable method for load shedding scheme from reliable resources such as journal or internet.
  2. Chose one suitable method which is can adapt any disturbance the network applied. Study and understand well the method and design the algorithm.
  3. Write program into C++ programming language based on the written algorithm.
  4. Modeling simulation network by using Power System CAD (PSCAD) software. Interfacing C code to the PSCAD software.
  5. Testing adaptive under frequency load shedding scheme in the simulation modeling.
  6. The result is studied fro different power imbalance.

CHAPTER2: LITERATURE REVIEW

2.1 OVERVIEW

In our interconnected system all the power demand and power supply is in the balance mode or it is called in synchronism. They are all synchronous with the grid system in term of generator speed or frequency to maintain its stability. All synchronous generators are in the same speed or frequency, and not one of them run at slower or faster to ensure the generator run at rated speed. Before disturbances occur, the frequency is maintained at 50Hz so that the prime movers will drive generator at constant speed. Since the frequency is directly proportional to the speed of the generator, hence when the frequency decreases the generator speed will decreases as well. When there is a disturbance because of fault or failures occur there is instability within the system. The relay grading will set the time to discriminate the fault location to protect the equipment and to ensure that there is no total blackout occurs. If we were not discriminating the safe location it may cause large loss and damage the equipment. Disturbance is classified as large, medium and small so that we can know what type of disturbance occur to take predetermines action. The fault location is being isolated that location is called islanding. Islanding is condition where the distributed generation (DG) and local bus is disconnected from the grid due to fault or failure. When ever islanding occurs, the interconnected grid will not feed the energy to the islanded location until the entire fault has been cleared. During islanding, the distributed generation (DG) needs to supply sufficient power to the local bus because there are loads connected at the bus as well to ensure the security of the load after islanding is guaranteed. Whenever islanding occur, the distributed generation (DG) has to run at reference speed to safe the local load. After islanding, the frequency may drop and the speed of the generator is reduced as well. To maintain at nominal frequency the prime mover have to drive the generator much faster. When the distributed generation (DG) is not capable to supply enough power to the load the power imbalance is establish which is the power of demand is greater than the power of supply. To ensure the power is always available, the spinning reserve is activated. In the case of the spinning reserve take to longer time to supply power or it can not capable to maintaining supply, thus under frequency load shedding scheme is initiated to prevent frequency decline or power imbalance. In the extreme contingency the loads have to shed to prevent from total cascaded blackout occur. Frequency decline is detected by under frequency relay. Under frequency relay is very sensitive to the change of the frequency due to the power imbalance between generation and consumption. In this project, the rate of frequency change is used to detect the power deficiency. The rate of frequency decay is small if there is large imbalance between the supply and demand or it is called large disturbance. In the other word, if there is small disturbance occur then the rate of frequency decay is large. The rate of change of frequency is inversely proportional to the power imbalance. The larger the power imbalance the smaller the rate of frequency change. If the disturbance is large the rate of the frequency decline is small which indicate that the frequency will drop faster and proper action must be taken quickly. Under frequency relay must be performs quickly to arrest power system of frequency change (decline) by decreasing power system load to match with the available generating capacity. Adaptive under frequency load shedding is used for purpose to drop the load in a proper manner when imbalance power established regardless the amount of the power deficiency it is experience. The value of the rate of frequency change is a powerful indicator to achieved proper load shedding scheme. This method will shedding load based on the magnitude of disturbance without introducing any under shedding or over shedding which lead to the improper load shedding and may cause large loss. This kind of protection scheme is called Adaptive Under frequency Load Shedding Scheme (AUFLS). This scheme is usually designed to protect power system from total blackout which is the load to be shed is programmed to specified which load will be safe the most. The load must be safe the most is usually related to the government office. This method is compared with the conventional load shedding which is shed the same location which can cause the importance of the load. Usually, the loads to be shed are chosen from the loads which have low degree of importance and the loads are not concentrated at any specific area. For the purpose of preserving the island, the amount of the load and supply must be equal. The power demand and supply is calculated to determine the amount of load to shed and the value will be used to select which load will be shed based on the importance of the load. The less importance of the load, the first load will be shed and vice versa. The value of the rate of change of frequency is influence the value of the load to be shed. Because the large disturbance the large load is to shed. After load is reduced the total load is also reduced then the demand is approximately equal to the power supply. Therefore the frequency is bringing back to the nominal frequency, but slightly below the nominal level. The power of the total load might be loss in a small quantity. After the fault has been cleared the islanding location and the grid will reconnect in the case the islanding location and the grid is synchronous in term of frequency.

2.2 ELECTRICAL GRID SYSTEM

When we say about the power industry, "grid" is a term used for an electricity network which may support all or some of the following three obvious operations:

  1. Electricity generation
  2. Electric power transmission
  3. Electricity distribution

1. Electricity generation.

The main component in power generation of power system is synchronous generator or also known as alternator. Synchronous generator comprises of two rotating field, which is from rotor driven at synchronous speed and excited by dc current and the other field is produced by three phase armature currents in the stator windings. The dc current is provided by the excitation systems for the rotor windings. In the older units, the dc current exciters are providing by dc generators with rotating rectifiers, known as brushless excitation systems. The generator voltage and reactive power flow is controlled by the generator excitation system. Ac generators can generate high power at high voltage, typically 30KV due to lack of the commutator. Typically in power plant the size of generators can be varied from 50MW to 1500MW. The prime movers will produce mechanical power to move the turbine blade. The prime movers usually come from hydraulic turbines at waterfalls, steam turbines whose energy comes from the burning of coal, gas and nuclear fuel, gas turbines, or occasionally internal combustion engines burning oil. Steam turbines usually operate at 3600 rpm and 1800 rpm for 2 and 4 poles respectively, the generators to which they are coupled are cylindrical rotor. Hydraulic turbines, particularly those operating with a low pressure, operate at low speed. Their generators are typically a salient type of rotor with many poles. In the grid system, several synchronous generators are operating in parallel switch other generator to provide the total power needed. They are connected at the common point known as BUS.

2. Electric power transmission

The purpose of an overhead transmission network is to transfer electric energy from generating unit at various locations to the distribution system which ultimately supplies the load. Transmission lines also interconnect neighboring utilities which permits not only economic dispatch of power between regions during normal conditions, but also the transfer of power regions during emergencies. Standard transmission voltages are established in the United States by the American National Standards Institute (ANSI). Transmission voltage lines operating at more than 60KV are standardizing at 69KV, 115KV, 138KV, 161KV, 230KV, 345KV, 500KV and 765KV line to line. Transmission voltages above 230KV is considered to as extra high voltage (EHV). High voltage transmission lines are terminated in substations, which are called high voltage substations, receiving substations or primary substations. Switching stations are function for switching circuit in and out of service. The voltage is stepped down to a value more suitable for the next part of the journey toward the load at the primary substations. Very large industrial customers may be served from the transmission system. Sub transmission network is referred as the portion of the transmission system that connects the high voltage substations through step-down transformers to the distribution substations. There is no clear delineation between transmission and sub transmission voltage levels. The sub transmission voltage level usually ranges from 69KV to 138KV. Some large industrial customers may be served from the sub transmissions for maintaining the transmission line voltage.

3. Electricity distribution

The distribution system is those parts which connect the distribution substations to consumers service entrance equipment or in the other word part of supply electricity which is dealing with the customer. The primary distribution lines are usually in the range of 4 to 4.5KV and supply the load in a well defined geographical area. Some small industrial customers usually served directly from the primary feeders. The secondary distribution network stepped down the voltage for utilization commercial and residential consumers. Lines and cable are usually not exceeding a few hundred feet in length then deliver power to the individual consumers. The secondary distribution serves most of the consumers at a level 240/120V, single phase, three wire, 208Y/120V, three phases, four wire, 480Y/227V. The typical power for home is derived from a transformer which stepped down the primary feeder voltage to240/120V using a three wire line. Distribution systems are both overhead and underground. The growth of underground distribution systems has been extremely rapid and as much as 70% of new residential construction is served underground.

In an electricity grid system, electricity demand and supply must be balance at all times, any significant imbalance power may cause grid instability or severe voltage fluctuations, and cause failures within the grid system. Total generation capacity is therefore sized to correspond to total peak demand with some margin of error and allowance for contingencies (such as plants being off-line during peak demand periods or outages). Operators will generally plan to use the least expensive generating capacity (in terms of marginal cost) at any given period, and use additional capacity from more expensive plants as demand increases. Demand response in most cases is targeted at reducing peak demand to reduce the risk of potential disturbances or failure, avoid additional capital cost requirements for additional plant, and avoid use of more expensive and/or less efficient operating plant. Consumers of electricity will also pay lower prices if generation capacity that would have been used is from a higher-cost source of power generation.

Demand response may also be used to increase demand during periods of high supply and/or low demand. Some types of generating plant must be run at close to full load (such as nuclear), while other types may yield at negligible marginal cost (such as wind and solar). Since there is usually limited load to store energy, demand response may attempt to increase load during these periods to maintain grid stability. For example, in the province of Ontario in September 2006, there was a short period of time when electricity prices were negative for certain users. Energy storage such as Pumped-storage hydroelectricity is a way to increase load during periods of low demand for use during later periods. Use of demand response to increase load is less common, but may be essential or efficient in systems where there are large amounts of generating capacity that cannot be easily cycled down.

Some grids may use pricing system that are not real-time, but easier to implement (users pay higher prices at the day and lower prices at night, for example) to provide some of the benefits of the demand response mechanism with less demanding technological requirements. For example, in 2006 Ontario began implementing a "Smart Meter" program that implements "Time-of-Use" (TOU) pricing, which tiers pricing according to on-peak, mid-peak and off-peak schedules. During the winter, on-peak is defined as morning and early evening, mid-peak as mid-day to late afternoon and off-peak as night-time; during the summer, the on-peak and mid-peak periods are reversed, reflecting air conditioning as the driver of summer demand. In 2007, prices during the off-peak were C$0.034 per KWh and C$0.097 during the on-peak demand period, or just less than three times as expensive. As of 2007, few utilities had the meters and systems capability to implement TOU pricing, however, and most customers are not expected to get smart meters until 2008-2010. Eventually, the TOU pricing (or real-time pricing) is expected to be mandatory for most customers in the province.

2.3 DISTRIBUTED GENERATION

Distributed generation (DG) is anticipated to become more important in the future generation system. The current literature, however, does not use a consistent definition of DG. The relevant issues and aims at providing a general definition for distributed power generation in competitive electricity markets. In general, DG can be defined as electric power generation within distribution networks or on the customer side of network. In addition, the terms distributed resources, distribution capacity and distributed utility are discussed. Network and connection issues of the generation are presented.

Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many small energy resources.

Distributed Generation are considered to be important in improving the security of energy supplies by decreasing dependency on imported fossils fuels and in reducing the emissions of greenhouse gases. Distributed Generation usually referred to the local generation of electricity in the case of cogeneration system, heat for industrial processes or space heating. Basically, distributed generation takes place close to the point where the energy is actually demanded or it is called local generation. Distributed Generation is not centrally planned and mostly conducted or produced by independent power producers or consumers and it is not centrally dispatched. Usually Distributed Generation is produce smaller than 50MW. Distributed Generation is connected to the electricity distribution network or grid. All renewable energy system mostly is also distributed generation systems. Distributed Energy Resources usually referred to the distributed electricity generation and electricity storage. Generally, distributed generation will used a portion of the electricity for local use and the rest will be fed into the grid. The heat, on the other hand, is usually used locally due to the costly transport and may cause large loss. Distributed Generation is usually used for domestic, commercial and industrial purpose. The purpose of the distributed generation is central rather than distributed is due to the economy of scale, efficiency fuel capability and lifetime. Therefore, by increasing size of production unit increases the efficiency and decreases the cost per MW. However, in term of economy advantage, the small units are benefiting from continuing technological developments, awhile large units are already fully developed. The other reason to keep building large power plants in fuel capability. Coal is not economically suitable for DG in fact it is most abundant fossil fuel with steady suppliers all over the world and a stable price. Additionally, large power plants will remain the prime source of electricity within 25 50 years lifetime. Currently, industrial countries generate most of their electricity in large centralized facilities, such as fossil fuel (coal, gas powered) nuclear or hydropower plants. These plants have excellent economies of scale, but usually transmit electricity long distances and can affect to the environment. Most plants are built this way due to a number of economic, health & safety, logistical, environmental, geographical and geological factors. For example, coal power plants are built away from cities to prevent their heavy air pollution from affecting the populace, in addition such plants are often built near collieries to minimize the cost of transporting coal. Hydroelectric plants are by their nature limited to operating at sites with sufficient water flow. Most power plants are often considered to be too far away for their waste heat to be used for heating buildings. Low pollution is a main advantage of combined cycle plants that burn natural gas. The low pollution permits the plants to be near enough to a city to be used, perhaps even in the same building. This also reduces the size and number of power lines that must be constructed. Typical distributed power sources in a Feed in Tariff(FIT) scheme have low maintenance, low pollution and high efficiencies. In the past, these traits required dedicated operating engineers, and large, complex plants to pay their salary and reduce pollution. However, modern embedded systems can provide these traits with automated operation and a renewable, such as sunlight, wind and geothermal. This reduces the sizes of the power plant that can show a profit. With everything interconnected, and open competition occurring in a free market economy, it starts to make sense to allow and even encourage distributed generation (DG). Smaller generators, usually not owned by the utility, can be brought on-line to help supply the need for power. The smaller generation facility might be a home-owner with excess power from his solar panel or wind turbine. It might be a small office with a diesel generator. These resources can be brought on-line either at the utility's behest or by owner of the generation in an effort to sell electricity. Many small generators are allowed to sell electricity back to the grid for the same price they would pay to buy it.

2.3.1 SYNCHRONOUS GENERATORS

Large scale of power is produced by three phase synchronous generators which are also known as alternators. Synchronous generator is either driven by steam turbines, hydro turbines, or gas turbines. The stationary part or so called stator is placed armature windings. The armature winding is specifically design for generation of balanced three-phase voltages and are arranged to develop the same number of magnetic poles as the field winding which is in the rotor. The field which relatively draw small power (0.2 3 percent of the machine rating) for its excitation is placed on rotor. The rotor is also occupied by one or more damper winding as a short circuit winding. The prime mover drive the rotor at constant speed and its field circuit is excited by direct current. Slip rings and brushes will be providing the excitation by means of dc generator also known as exciters mounted on the same shaft as the rotor of the synchronous machine. However, for modern excitation systems, they usually use ac generator with rotating rectifiers which is also known as brushless excitation. The generator voltages and reactive power flow is controlled by the generator excitation to keep maintain. The rotor of the synchronous generator may either kind of cylindrical or salient construction. The cylindrical type is also known as round rotor, has one distributed winding and a uniform air gap. Synchronous generator is driven by steam turbines are design for high speed at 3600 rpm or at 1800 rpm for two and for pole machines respectively. The rotor of these generators has a relatively large axial length and small diameter to limit the centrifugal forces. About 70% of the large synchronous generators are cylindrical rotor type raging from 150 to 1500MVA. The salient type of rotor has concentrated windings on the poles and non uniform air gaps. The synchronous generator usually driven by hydraulic turbines in hydroelectric power plant.

2.3.2 POWER SYSTEM DYNAMIC:

The tendency of power system to develop restoring forces equal to or greater than the disturbing forces to maintain the state of equilibrium is known as stability. If the forces tending to hold the machines in synchronism with one another are sufficient to overcome the disturbing forces, the system is said to remain stable (to stay in synchronism). The stability problem is considered with the behavior of the synchronous generator after disturbances occur. In the easy way, stability problems generally divided into two major categories which is steady state stability and transient stability. Steady state stability can be defined as the ability of the power system to regain synchronism after small and low disturbances, such as gradual power changes. It is convenient to assume that the disturbances causing the changes disappear. The motion is free, and stability is assured if the system is returned to its original state. Such a behavior can be determined in a linear system. It is assumed that the linear automatics control, such as voltage regulator and governor is not active. An extension of steady state study is dynamic study. The dynamic study is concerned with small disturbance lasting for along time with the inclusion of automatic control devices. The transient stability studies deal with the effect of large, sudden disturbance such as sudden occurrence of fault, the sudden outage of a line or the sudden application or removal of loads. Transient stability studies are needed to ensure that system can withstand the transient condition following a major disturbance. Frequently, such studies are conducted when new generating and transmitting facilities are planned. The studies are helpful in determining such things as the nature of the relaying system needed, critical clearing time of circuit breakers, voltage level of, and transfer capability between systems.

SWING EQUATION:

Under normal operating conditions, the relative position of the rotor axis and the resultant magnetic field axis is fixed. The angle between the two is known as power angle or torque angle. Whenever disturbances occur rotor will accelerate or decelerate with respect to the synchronously rotating air gap (mmf), and a relative motion begins. The equation describing this relative motion begins. The equation describing this relative motion is known as the swing equation. If, after this oscillatory period, the rotor locks back into synchronous speed, the generator will maintaining its stability. In the case of the disturbance does not involve any net change in power, the rotor return to its original position. if the disturbance is created by a change in generation, load, or in network conditions, the rotor comes to a new operating power angle relative to the synchronously revolving field.

Consider a synchronous generator developing an electromagnetic Te and running at the synchronous speed wsm. If Tm is the driving mechanical torque, then under steady state operation with losses neglected we have

Tm=Te (1)

A departure from steady state due to the disturbance result in an accelerating (Tm>Te) or decelerating (Tm

Ta=Tm-Te (2)

If J is the combined moment of inertia of the prime mover and generator, neglecting frictional and damping torques, from laws of rotation we have

J(dw/dt) = Tm Te = Ta (3)

Where

J is the moment of inertia(kg-m2)

w Is the angular velocity(rad/s)

dw/dt is the angular acceleration(rad/s2)

Ta is the resulting torque(N-m)

2.3.3 AUTOMATIC GENERATION CONTROL (AGC)

If the load on the system is increased, the turbine speed drops before the governor can adjust the input of the steam to a new load. Whenever the change in the value of the speed diminishes, the error signal becomes smaller and the position of the governor fly balls gets closer to the point required to maintain a constant speed. However, the constant speed will not be the set point, and there will be an offset. One way to restore the speed or frequency to its nominal value is to add an integrator. The integral unit monitors the average error over a period of time and will overcome the offset. Because of its ability to return a system to its set point, integral action is known as rest action. As the system load changes continuously, the generation is adjusted automatically to restore the frequency to the nominal value. This scheme is also known as automatic generation control (AGC). In an interconnected system consisting of several pools, the role of the AGC is to divide the loads among systems, station, and generators so as to achieve maximum economy and correctly controlled interchanges of tie-line power while maintaining a reasonably uniform frequency. During large transient disturbances and emergencies, AGC is bypassed and load shedding is applied. The generator excitation system maintains generator and controls the reactive power flow. The generator excitation of older systems may be provided through the slip rings and brushes by means of dc generator mounted on the same shaft as the rotor of the synchronous machine. However, modern excitation system usually use ac generators with rotating rectifiers, and are known as brushless excitation. A change in the real power demand affects essentially the frequency, whereas a change in the reactive power affects mainly the voltage magnitude. The interaction between voltage and frequency controls is generally weak enough to justify their analysis separately. The source of the reactive power is come from generators, capacitors and reactors. The generator reactive powers are controlled by excitation. Other supplementary methods of improving the voltage profile on electric transmission systems are transformer load-tap changers, switch capacitors, step voltage regulators, and static var control equipment. The primary means of generator reactive power control is the generator excitation control using automatic voltage regulator (AVR). The function of the AVR is to hold the terminal voltage magnitude of a synchronous generator at a specified level. An increase in the reactive power load of the generator is accompanied by a drop in the terminal voltage magnitude. The voltage magnitude is sensed through a potential transformer on one phase. This voltage is rectified and compared to a dc set point signal. The amplified error signal controls the exciter field and increases the exciter terminal voltage. Thus the generator field current increased, which result in increase in the generated emf. The reactive power generation is increased to a new equilibrium, raising the terminal voltage to the desired value. The factors which contribute power generation at minimum cost are operating efficiencies, fuel cost and transmission losses. A program called dispatch was developed to find the optimal dispatch of generation for an interconnected power system. The optimal dispatch may be considered within the framework of Load Frequency Control (LFC). Digital computer is included in the control loop which scans the unit generation and tie- line flows in direct digital control systems. These setting are compared with the optimal settings which are derived from solution of the optimal dispatch program. If the actual settings are deviate from optimal values, the computer will generates the raise/lower pulses which are not sent to the individual units. The other program will also be considered in the tie-line power contracts between the areas. Parallel with the development of modern control theory, several concept are included in the automatic generator control (AGC) which is go beyond the simple tie line bias control. The basic approach is the use of more extended mathematical models. The automatic generator control (AGC) can also be used to include the representations of the dynamic area and the complete system as well. Other concepts of modern control theory are also can be employed, such as state estimation and optimal control with linear regulator utilizing constant feedback gains. In addition to the structures which aim at the control of deterministic signal and disturbances, there are scheme which use stochastic control concepts. The generator excitation system should be maintaining voltage and controlling flow of reactive power. For older system generator excitation may be provided through slip rings and brushes by means of dc generators mounted on the same shaft as the rotor of the synchronous machine, but for modern generator excitation systems usually use ac generators with rotating rectifiers and are known as brushless excitation. Basically, when ever power demands change, it may result in frequency change, whereas a change in reactive power will result a change in the voltage magnitude essentially. The relationship between voltage and frequency controls is generally weak enough to justify their analysis separately. Reactive power is draws from generator, capacitor and reactors. Field excitation will control the generator reactive power. In addition methods of improving the voltage profile on electric transmission systems are transformer load tap changers, switched capacitors, step regulators and static var control equipment. The generator excitation control primarily using automatic voltage regulator (AVR) for generator reactive power control. The purpose of the automatic voltage regulator (AVR) is to hold the terminal voltage magnitude of a synchronous generator at a specified level. If the reactive power loads of the generator increase the terminal voltage magnitude will decrease. The potential transformer will detect the voltage magnitude. Terminal voltage will rectify and compared to the dc set point signal. The exciter field will controlled by the amplified error signals to increase the exciter terminal voltage. Therefore, the generator field current is increased and directly increase the generated emf. The reactive power generation is established in a new equilibrium, and increase the terminal voltage to the desired value.

2.3.4 RESPONDS OF THE GENERATOR DURING ISLANDING

Island operation occur whenever one or more distributed generation (DG) continues to energize a part of the grid after the connection to the rest of the system has been lost. Islanding operation can be in either intentional or unintentional. Intentional islanding is the purposeful sectionalized of the utility system during widespread disturbances to create power island. These islands can be designed to maintain a continuous supply of power during disturbances of the main distribution system. When the disturbances are comes on a distributed utility system, the grid sectionalized by itself. This protection is really a system protection of last resort. This scheme are supposes that the integrity of the system cannot be maintained in spite of the automatic load shedding, for every possible emergency. Instead of allowing the system to disintegrate by the tripping of generators and transmission lines as the disturbance develops, the islanding scheme itself sectionalizes the whole system into sustainable small systems each consisting of a group of generating stations and a group of load that can be supplied by these generating stations. In effect each group becomes a sustainable island and hence the name islanding scheme. The distributed generation can then supply the load power demand of the islands created until reconnection with the main utility systems occurs. As the demand for more reliable and secure power systems with greater power quality increases, the concepts of distributed generation (DG) have become more popular. This popularity of DG concepts has developed simultaneously with the decrease in manufacturing costs associated with clean and alternative technologies, like fuel cells, biomass, micro- turbine, and solar cell systems. Although the costs associated with these technologies have continued to decrease more work is essential to make these technologies readily available. To make these distributed energy resource (DER) technologies more economically viable and energy efficient, power electronics based conversion systems need to be developed for proper conditioning of the energy to be delivered to the current three phase power system. These power conversion systems (PCS) allow for increased reliability, security and fewer downtimes by incorporating intentional islands into the utility grid without having to add or replace the existing transmission system.

When distributed generation (DG) and their local load (the island) are isolated from a larger grid. If the distributed generation or so called synchronous generator and its prime mover ( a turbine or a reciprocating engine) and the load of this small "island" that can be isolated from the grid and powered by this generator and it's prime mover is 25 MW. So, when this island and it's generator are isolated from the larger grid some load has to be shed (automatically disconnected) in order not to exceed the rating of the generator's prime mover. In this example, a total of 5 MW of load would have to be disconnected from the isolated island in order not to exceed the rating of the generators' prime mover if it were to be operated at rated frequency when isolated from a largergrid.

In order for a prime mover and it's generator to energize a load at a constant frequency the prime mover needs to be operated in that way that it is controlling frequency in response to changes in load. This is usually called Isochronous Governor mode, or Isochronous Speed Control mode. And a prime mover and its generator can only produce power at rated frequency up to the rating of the synchronous generator's prime mover.

When the generator connected to a larger grid in parallel with other generators and their prime movers the normal mode of operation for the prime mover governor (control system) is Droop Speed Control mode, this is because some other "entity" is controlling the frequency of the grid. when connected to a larger grid in parallel with other generators and their prime movers a generator can only produce power up to the rating of its prime mover.

So, when a prime mover and its generator are suddenly disconnected from a larger grid and are to be provide power to a local load (the island) independent of the grid, the prime mover's governor is usually switched to Isochronous Speed Control mode. If the load of this small island exceeds the rating of the generator's prime mover then some of the load must automatically be disconnected, and this is referred to as load shedding.

Usually, there is a contact on the breaker that connects the island to the larger grid that serves to tell the prime mover governor to switch from Droop Speed Control mode to Isochronous Speed Control mode *and* to initiate load shedding to reduce the load below the generator prime mover's rating.

Depending on how fast the load is shed when the grid tie breaker opens and also depending on how fast the prime mover's governor (control system) can react to the change in load, what usually happens if the island load is initially greater than the prime mover's rating is that the frequency will decrease. Once the load of the island has been reduced to at least the rating of the generator prime mover and the prime mover's governor has responded to the change in load, then the frequency should return to rated and remain at rated as long as the load of the island does not exceed the rating of the generator's prime mover.

As long as the island load is not allowed to exceed the rating of the generator's prime mover it should be able to respond to any change in load and still maintain rated frequency. That is, the prime mover's control system (the governor) should be able to respond to any change in load up the rating of the prime mover and still be able to maintain frequency.

The generator produces power at a frequency that is proportional to the speed at which it's rotor is being turned by the prime mover (the turbine or reciprocating engine). When operating a small island of isolated load, the amount of load must be less than the rated power of the prime mover driving the synchronous generator, or else the frequency will not remain at rated value.

It's not the generator which controlling frequency or the amount of load, but it's the control system (the governor) of the generator's prime mover which is controlling the frequency (when operating in Isochronous Speed Control mode) or the load (when operating in Droop Speed Control mode in parallel with other generators and their prime movers). A generator is just a device for converting torque (from a prime mover) into amperes. Those amperes can then be transmitted over wires to motors which convert the amperes into torque. (Lighting is a way to convert amperes into heat, heat so hot that it produces light.) So, the load of a generator is proportional to the amount of torque being produced by it's prime mover. And that torque is a function of the energy which flowing into the prime mover (usually fuel or steam or water or wind).

2.3.5 DROOP SPEED CONTROL

The speed of the synchronous generator is a function of frequency or generator speed. Because of its name synchronous generator indicate that no generator can go faster or slower than the speed that is dictated by the frequency. All are in synchronism to maintaining grid stability. In other word, they are all connected together and their rotors are locked into synchronism with each other (magnetically), the prime movers which are mechanically tied to the generators can not change their speed either. Synchronous generator is connected to a grid with other generators which driven by their prime mover respectively. In the large interconnected grid, the frequency of the generator is controlled by the frequency of the grid, the frequency must be constant and in synchronism with the other generator. Therefore, the speed of the prime mover is in a function of the frequency indicating that the prime mover is inject constantly, hence the speed of the prime mover is fixed as well. In the grid, all synchronous generators must be run at the same speed. Not one of them runs at faster or slower than other generator. If the synchronous generators run at 50.0001Hz, all generators must be run at same speed. To make the prime mover (which is providing the torque input that the generator is converting to amps that is being converted to torque by motors which are also connected to the grid) stably control its power output while connected in parallel with the other generators and prime movers on the grid, the control system employ straight proportional control also known as droop control or droop speed control. The generator is sharing load during controlling stably power output when connected to a grid in parallel with the other generators. Droop speed control will consider both the prime movers speed reference and actual speed which is in the function of the grid frequency. The power output is directly proportional to the speed reference, hence to increase the power output, the speed references is increased. The speed actually can not change the increased error between reference and actual speed is converted to increase fuel flow which lead to increased fuel flow. Therefore, the speed will increase due to extra torque as well. The generators convert that extra torque to the extra current. This operation is done very smoothly and all the prime movers and their generators behave nicely and work together to provide the load. If the load on the generator is to be increased, then the turbine speed reference is increase again, the error between the actual speed and the speed reference increases again, which increases the fuel flow directly increases the torque and the amps. Droop speed control is directly proportional control in the strictest, purest sense of the word. There is no integral action or reset to increase the fuel to make the actual speed be equal to the reference speed. The actual speed can not achieve reference speed because it is physically not possible. Therefore, droop speed control take this disadvantages to stably control fuel in proportion error. The error can occur either one of two reasons which is a change in the speed reference or a change in actual speed. When the grid frequency or the generator speed change, the control system automatically responds to the change because the error changes and adjusts the fuel to try to compensate for the change in actual speed relative to the speed reference. The errors between speed reference and actual speed for all generators within large infinite grid are fairly constant. When the frequency change, the error also change because the frequency is proportional to the generator speed hence it will change error of speed as well. If the frequency in the grid decreases, then the error increases this will lead increases in fuel of all the machines because they are all connected together to the grid. Each prime movers governor will respond to a change of frequency as a function of the amount of droop that the control system has been programmed. A 1% change in the frequency of the machine with 5% droop will result a 20% change in load, nominally, supposing the machine was running at 80% of load or less to begin with. A unit with 4% droop will respond with a 25% change in load, nominally, again presuming the machine was running at 75% or less than rated load to begin with. the prime mover can not increase its output power further if the machine is operate at rated power output on droop speed control whenever the frequency is decrease. The prime movers always operating at rated power output when interconnected in the grid system. Even though they are in droop speed control mode they can not add additional load by increasing their power output when the grid frequency decreases. Whenever a machine which is to have 5% droop speed control will normally reach rated output when the speed reference reaches approximately 105% of rated speed. A machine with 4% droop will reach its rated power when the speed reference is 104%. The power produced by the generator heavy duty gas turbine operating on most fuels is generally directly proportional to the fuel flow rate. The rate of fuel injected is directly proportional to the fuel stroke reference (FSR), which is a reference for the opening of the fuel control valve and/or the fuel flow rate through the control valve. The purpose of droop speed control primarily is to allow a prime mover and its generator to smoothly and stably share in supplying the load of a grid while operating in parallel with other prime movers and their generators. The secondary purpose of droop speed control is to help to maintain grid frequency when it varies from desired. When the grid frequency decreases because of the power imbalance (generally the amount of load is greater than generation), hence droop speed control increases the output to try to help support the local load at particular time. The amount of generation must be balance with the amount of the load.

Droop speed control is referred to as proportional control. The amount of power produced is directly proportional to the error between the turbine speed reference and actual speed. This is explaining that how the fuel is controlled during parallel operation with other prime movers and generator connected to a grid supplying a load. Prime movers mostly uses something similar, it is not only for stable control of fuel flow from no load to rated load based on simple parameters, but it is also useful when trying to maintain load on a grid when the grid frequency is not at the rated output. The speed error will decrease when ever the grid frequency increases above rated. Droop speed control does not try to make the actual speed of generator equal to the speed reference. It is based on the fact that there will be an error because under normal circumstances the grid frequency is stable at 100% and therefore, the actual turbine speed is stable at 100%, and the amount of fuel flow is proportional to the error of speed which is different of actual speed and reference speed. A machine which have 4% droop setting in generally new and clean conditions being operated at ambient temperatures less than nameplate rated will usually reach exhaust temperature control or Base Load at TNR greater than 104% and the load will be greater than nameplate rated value. Generally for every 1% change in TNR the unit power output will increase by approximately 25% of rated power output and this value is corresponding to the 4% droop speed control. In the case of a machine is not new or in other word in dirty compressor, high inlet filter differential pressure and increased tolerances in the axial compressor and/or turbine sections, will not be as efficient and the change in load for the change in TNR will be slightly depending on the severity of the condition less than specified values. A machine with 4% droop speed control might have only 23% increases power output and 1% change in speed error. Therefore, with unclean condition the machine will not be able to achieve optimal result. Droop speed control is only part of the change in the power output relative to the speed error, and that portion is related to rated power output not actual power output under less than rated conditions. The desired rated power output of the machine is only achieve if the machine in new and clean condition. The main purpose of the droop speed control is to control prime mover governor to allow a prime mover and its generator to smoothly and stably share in supplying the load of a grid wile operating in parallel with other prime movers and their generators.

2.4 ESTIMATION MAGNITUDE OF DISTURBANCES (EMD)

In this project the auxiliary indicator to determine the magnitude of disturbance is rate of frequency change, df/dt. The power imbalance is proportional to the rate of frequency change, df/dt. Therefore we can recognize what kind of the disturbance the system experience to predict the action to be taken. If the disturbance experience is large , the rate of change of frequency is smaller and if the disturbance applied is small, the rate of frequency change is large. In the other word, the power imbalance or disturbance is inversely proportional to the rate of frequency change, df/dt. Under frequency relay is very sensitive to the frequency deviation. If the power in the system is not equal which is imbalance between power demand and power supply, the frequency will decline proportionally with the power imbalance. This frequency decline will establish slope, these slope will be using as indicator either the disturbance is large, medium or small. From this point of view, we can use the value of rate of frequency change as threshold to shed the load. To shed loads the value of rate of change of frequency is included in several range. Therefore, we can classified what suitable range of rate of frequency change for small, medium and large disturbance. In this project the instantaneous value of df/dt is taken as auxiliary indicator. After the disturbance has been recognize, the appropriate load will be shed simultaneously. Therefore, the frequency is bring back to the acceptable level rapidly without introducing under shedding or over shedding.

Large Disturbance: this diagram shows that the sharp drop of the frequency indicate that the islanding experience large disturbance.

Medium Disturbance: this diagram shows that the frequency drop is slightly faster and it is classified as medium disturbance because the frequency does not fall as fast as in large disturbance.

Small Disturbance: this diagram shows that the frequency drop is slower than the medium disturbance and this kind of disturbance is classified as small disturbance.

The rate of the frequency change is calculated by determine the value of the slope of frequency drop, the equation is shown below.

The short time interval, will give more accurate value. In the other word t2 and t1 must be in the short duration to get accurate value.

s1 is representing the small disturbance which is the slope is bigger because the frequency is fall slowly.

s2 is representing the slightly greater from small disturbance, which is the slope is slightly smaller than the small disturbance.

s3 is representing the medium disturbance which is the slope is quite small because the frequency is drop quite faster.

s4 is representing the slightly greater from medium disturbance, which is the slope is slightly smaller than the medium disturbance.

s5 is representing the large disturbance which is the slop is very small because the frequency is drop rapidly.

CHAPTER3: METHODOLOGY

3.1 INTRODUCTION

ADAPTIVE UNDER FREQUENCY LOAD SHEDDING SCHEME

The method that employed in this project is adaptive LD df/dt characteristic scheme. In this method the amount of load to be shed is a function of df/dt variable. The load to be shed in based on the value of the df/dt. Because rate of change of frequency of the system will determine the magnitude of the disturbance, hence load to be shed is proportional to the value of the df/dt. Adaptive LD df/dt is very powerful characteristic to achieve proper load shedding. Whenever large disturbance occur, this scheme will shed large load and in the case of small disturbance occur, small load will be shed and the same cases for medium disturbance. In other word, this load shedding scheme will introduce proper load shedding and enhancing the power system reliability without introducing large loss. This type of scheme is designed several times for different levels of df/dt. The adaptive Under Frequency Load Shedding Scheme is specially designed to encounter for any level of disturbance, with its related df/dt, minimum frequency of the system does not fall to the below a certain value. This method is designed for each value of df/dt a suitable value of LD is calculated where LD is the amount of the load to be shed.

3.2 ADAPTIVE LD -df/dt CHARACTERISTIC SCHEME.

  1. DETERMINATION OF THE EXPECTED OVERLOAD:
  2. The expected overload will determine the amount of the protection is to be provided. The value is get from the following formula.

  3. DETERMINATION OF THE NUBMBER OF LOAD SHEDDING STEPS:
  4. In the adaptive under frequency load shedding scheme, the load will be shedding simultaneously based on the magnitude of the disturbance and it will result faster recovery of frequency decline. Compared to the conventional load shedding, the take a few step to shed load and it will result under shedding or over shedding because they shed constantly regardless what kind of the disturbance it is applied. Hence it will result slower frequency recovery. In adaptive under frequency load shedding scheme, the main powerful tool is the rate of frequency change df/dt variable, so the load to be shed is programmed based on the several range of the df/dt value.

  5. DETERMINATION OF THE AMOUNT OF LOAD TO BE SHED:
  6. The first step is to calculate the amount of load to be shed to maintain frequency above minimum permissible frequency for maximum expected overload. Total amount of load to be shed is calculated by the following equation:

    Where

    LD = total load that must be shed

    L = expected overload

    f = minimum permissible frequency

    d = load reduction factor

    fn = nominal frequency (50Hz)

    The value of the load must be shed is included in the several range of the rate of the frequency change. Because LD value is proportional to the value of the rate of frequency change. The disturbance is large if the value of the rate of frequency change is large and directly the load to be shed is also large. Therefore, the value of large load to be shed is included in the large value of df/dt interval. Adaptive under frequency load shedding scheme will be shed the load simultaneously or lump sum based of the magnitude of the disturbance. Compare to the conventional under frequency load shedding scheme, the load to be shed is divide into several percentage and shed following several steps. This method will lead to the under shedding or over shedding.

    LD df/dt CHARACTERISTIC SCHEME:

    This characteristic scheme is very important part in adaptive under frequency load shedding scheme. This part will distinguish between the conventional and adaptive under frequency load shedding scheme. In this method the amount of the to be shed is in the function of the df/dt. The characteristic of LD df/dt is shown below.

    From the diagram, this method indicates that the load to be shed is in a function of df/dt. If the smallest the value of the df/dt the larger the load is to be shed. For example, Df2 is large disturbance and its value is smaller, the load to be shed is LD3 which indicate that the largest load is to be shed. This correlation is explaining that when the large disturbance occur, the frequency decline is very fast which result the smaller value of df/dt and this indicate that the larger value of load must be shed to prevent excess frequency decline. In the case of Df1, the load to be shed is LD2, this indicate when medium magnitude of disturbance occur, the medium value of load is to be shed. The same case for the small disturbance.

3.2.1 ALGORITHM

  1. START
  2. Insert values of
    • Generation power, pg
    • Frequency, f
    • Power load1, pload1
    • Power load2, pload2
    • Power load3, pload3
    • Power load4, pload4
    • Power load5, pload5
    • Power load6, pload6
    • Power load7, pload7
    • Power load8, pload8
    • Power load9, pload9
  3. Calculate value of total power load, pt
  4. Total power load, pt = power load4, pload4 + power load5, pload5 + power load6, pload6 + power load7, pload7

    + power load8, pload8 + power load9, pload9

    Or

    Total power load, pt =pload4+pload5+pload6+pload7+pload8+pload9

    Pload1 = pload4+pload5

    Pload2=pload6 +pload7

    Pload3 =pload8+pload9

  5. Calculate expected overload, L
  6. or

  7. Calculate d and LD
  8. Calculate rate of frequency change, df/dt
  9. Set several range of rate of change of frequency change, df/dt
  10. df/dt=s1=-0.0001 for small disturbance which is large value of rate frequency drop

    df/dt=s2 =-0.002 for slightly bigger than smaller disturbance which is slightly less than s1

    f/dt=s3=-0.004 for medium disturbance which is medium value of rate of frequency drop

    df/dt=s4=-0.005 for slightly large than medium disturbance which is slightly smaller than s3

    df/dt=s5=-0.007 for large disturbance which is give smaller value of rate frequency change

    The rate of frequency change is divided into several range

    if s2< df/dt

    if s3< df/dt

    if s4< df/dt

    if s5< df/dt

    if df/dt>s5

  11. The value of the load to be shed, LD is compared to the load which is selected to shed. If the load to be shed is less or equal to the value of the selected load, the corresponding breaker of the load will be tripped.
  12. If LD<= selected load then the breaker will trip(sl)

  13. Breaker trip.
  14. If logic 1 is send to the breaker, the breaker will be trip if logic 0 is send the breaker will close.

    In the PsCAD software when the breaker close(load energize) the breaker is colored red while when the breaker trip(load not energize) the breaker is colored green.

    BRK=1(OPEN) GREEN

    BRK=0(CLOSE) RED

  15. EXIT
  16. 3.2.2 FLOW CHART

    3.3 PROPOSED SIMULATION MODELLING

    In this project, model of network is developed by using software, simulation is run by using Power System CAD (PSCAD). The calculation is done by interfacing C++ code to the PSCAD. The accurate result is obtained by using C++ and faster respond of breaker can be using C++ or FORTRAN.

    3.3.1 INTRODUCTION TO PSCAD SOFTWARE

    HISTORY OF PSCAD

    PSCAD was first conceptualized in 1988 and began its long evolution as a tool to generate data files for the EMTDC simulation program. In its early form, Version 1 was largely experimental. Nevertheless, it represented a great leap forward in speed and productivity, since users of EMTDC could now draw their systems, rather than creating text listings. PSCAD was first introduced as a commercial product as Version 2 targeted for UNIX platforms in 1994. It arrived as a suite of associated software tools that performed circuit drafting, runtime plotting/control and off-line plotting.

    When Version 3 for Windows arrived in 1999, it sought to push the envelope by introducing a simulation system that could be built in a modular form. Systems could now be built up using interconnecting drawing blocks, compiled individually and having their own private data space. This modular system improved accuracy and correctness of the simulation. In addition, Version 3 brought some new usability by fully integrating the drafting and runtime systems of its predecessor. This integration produced an intuitive environment for both design and simulation.

    PSCAD Version 4 represents the latest developments in power system simulation software. With much of the simulation engine being fully mature for many years, the new challenges lie in the advancement of the design tools for the user. The goal is to produce software that is both powerful and easy to use. Version 4 retains the strong simulation models of its predecessors while bringing to the table an updated and fresh new look and feel to its windowing and plotting. New single-line representations and new compiler enhancements improve both the accuracy and reliability of the simulation. New editors and easier navigation mean that finding your way and maintaining larger systems is far easier to do. Portability to Version 3 means updating to Version 4 is as simple as point, click, and run. These and many other enhancements are why PSCAD has been, and is, the professional's choice for transients simulation.

    WHAT IS PSCAD

    PSCAD (Power Systems CAD) is a powerful and flexible graphical user interface to the world-renowned, EMTDC solution engine. PSCAD enables the user to schematically construct a circuit, run a simulation, analyze the results, and manage the data in a completely integrated, graphical environment. Online plotting functions, controls and meters are also included, so that the user can alter system parameters during a simulation run, and view the results directly.

    PSCAD comes complete with a library of pre-programmed and tested models, ranging from simple passive elements and control functions, to more complex models, such as electric machines, FACTS devices, transmission lines and cables. If a particular model does not exist, PSCAD provides the flexibility of building custom models, either by assembling them graphically using existing models, or by utilising an intuitively designed Design Editor.

    The following are some common models found in systems studied using PSCAD:

    • Resistors, inductors, capacitors
    • Mutually coupled windings, such as transformers
    • Frequency dependent transmission lines and cables (including the most accurate time domain line model in the world!)
    • Current and voltage sources
    • Switches and breakers
    • Protection and relaying
    • Diodes, thyristors and GTOs
    • Analog and digital control functions
    • AC and DC machines, exciters, governors, stabilizers and inertial models
    • Meters and measuring functions
    • Generic DC and AC controls
    • HVDC, SVC, and other FACTS controllers
    • Wind source, turbines and governors

    PSCAD, and its simulation engine EMTDC, have enjoyed close to 30 years of development, inspired by ideas and suggestions by its ever strengthening, worldwide user base. This development philosophy has helped to establish PSCAD as one of the most powerful and intuitive CAD software packages available.

    3.3.2 INTERFACING C CODE TO THE PSCAD SOFTWARE

    In this project C language programming is used to give better calculation result. PSCAD itself is using FORTRAN as a compiler, so that C code have to interface with PSCAD with several method.

    1. PASTE C CODE FILE TO PROJECT

    - First, we have to copy C code file from CINTERFACE example, and paste it inside our project. C code is written inside this C code file. Any programmed we want to write is written inside the C code file for C language only.

    The ways we write C code inside the C code file is slightly different for make it readable by the PSCAD software. The header must be include * whether it is type of double or float or integer. Anything is not come from PSCAD must be attach with the *.

    NOTE: what ever variable we want to used inside the program, must be declare in the header. If not it may cause error.

    The variable is attach with * to make the readable by the PSCAD software.

    2. CREATE NEW COMPONENT

    -New component is created to determine how many input and output we desire to do. Usually the left hand side is input and right hand side is the output.

    3. CALL C SUBROUTINE

    - The way we call c subroutine is by call c code from the c code file through the new component script.

    Write header in the c code file like following header

    And call c code through the new component script

    Therefore, anything program written in the c code file will be read by calling through new component script. The flow of operation is shown below.

    3.3.3 TAKE VALUE FROM CONTINUOUS DATA.

    In the PSCAD software the value input we want to use is must be stored or taken to ensure the right value is use for programming C. the way to store data is explain below.

    1) Create new component

    The new component for store the data is differentiating with the new component for interfacing with C Code. The input is set accordingly with the input of the C interface component, this is because the stored data is using for the input of c code interface. And obviously the output of this new component is the same with the input of the c interface component.

    2) Write at what time we want to read the input at new component script.

    This figure shows that the value for L4 will be read from 9 second onward because the time is set greater or equal to 9 second (GE is in Fortran mean greater or equal). In the case we want the value at instantaneous time, we can set the time is equal to ==. And this data will be send to the input of the c code interface. The data we want to stored is address from the generated FORTRAN compiler like shown below.

    The address of the data is taken from this diagram. Cautious must be taken in the writing program to store data FORTRAN is used.

    The whole process of interfacing and storing data is shown above; this flow process is easily to understand and save time to understand. For the summary of this part, the data is catch or stored from the meter by call the address of the variable and set the time in the FORTAN script when we want to start storing the data. This data is used for the input of the calculation in the c code programming to run the process and after that the output is send to the channel and can be displayed in the graph or control panel.

    3.4 SIMULATION MODELLING

    In this project, the network modeling is in single phase diagram. The Grid system comprises of 3BUS, BUS1 BUS2 and BUS3 is feed 33KV, 11KV and 11KV respectively. The load is connected to the BUS3 which is 11KV feeder. Six load is connected to the same BUS to get power supply. The distributed generation (DG) is rated at 4.51MVA or 1.0 p.u. When ever fault occur at the grid system, BREAKER1 is trip and the load is islanded to prevent from the total blackout.

    3.4.1 MEASURING DATA

    Figure above shows that the power generation, voltage, frequecy, torque, power load and breaker 1 is monitored and measured. Those variable are measure their data through the channel and can be displayed by using graph frame or control panel. Next step is take the data or storing the data to be use.

    3.4.2 STORING DATA

    Figure above is block diagram of new component created to take value from the continuous data. the data is taken from the meter which read the variable data. From figure, pload1 to pload9, w2(frequency) and Pdg1(power generation) is input to the new component for storing the input data. the output of the new component is the data which is has been stored and ready to be use in programming.

    3.4.3 BREAKER CONTROL

    Figure 3.4.30 above shows the block of control created from new component. This diagram is illustrate how the breaker is control from the C Code. The input from the continuous data is process inside the block and give the output in the right hand side. The program will determine which breaker will be trip depending on the what kind of disturbance it is applied. The OR gate is function to hold the state of breaker after load shedding, therefore the load has been shed is maintain shed.

    3.4.4 GENERATOR SPEED CONTROLLER

    Figure 3.4.40 above is torque-speed controller. The current frequency is compared with the speed reference which is 1p.u or 50Hz. This comparison will produce error if the speed or frequency deviate from the nominal speed, and this error will send to the droop speed control to produce more power to bring back the frequency to the nominal. In this project the droop control is 5% which is generate excess 20% of the rated generation. And the power generated is compared with the rated speed reference to ensure that the speed or frequency is between in the acceptable level. And finally this signal is send to adjust the torque directly change the speed. This controller will run in its droop mode when ever the BREAKER1 is trip which is indicate the network is islanded and speed will decrease. In the normal condition the speed error is does not exist and the generation is constant.

    3.4.5 DERIVATIVE OF FREQUENCY (df/dt)

    Figure 4.4.50 shows how the derivative of frequency is obtain from the continuous data. Frequency is input and the FORTRAN comment line is set to derivative and the output is derivative of frequency or the other word is rate of frequency change, df/dt. The rate of change is set to 0.001 second which mean the time interval is set to 0.001s. And this time interval will give the accurate answer of the frequency derivative.

    CHAPTER4:

    4.1 CASE 1(For 0.9 p.u of power generation)

    DISCUSSION

    In this case power generation is 4.09MVA and the power load is 5.25MW. The power generated is 0.9 from the rated power. There is imbalance of power exist and some load has to be shed to match with the power generated. The expected overload, L is calculated and the value is 0.2934 which is indicate that the loss of 23 percent of total generation. The load to be shed is 1.1623MW which indicate that this amount of power must be shed to bring back the frequency to nominal. This amount is the minimum number of load must be shed. From the frequency graph, there is slope exist and it is indicate that there is power imbalance. For this case the rate of frequency change measured is -0.0003 Hz/s which is mean the frequency is drop -0.0003Hz in a second. This value is not critical yet and hence it is consider as a small disturbance. Therefore, this value of frequency change is fall at the first interval which is between -0.001 and -0.0001. if the load shedding is not initiate the frequency drop at 45Hz which cause turbine blade damage. And by initiate load shedding scheme the frequency drop at 49.6Hz which is in the safe mode. The appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this modeling network, the priority load is load 6 which is the larger load in this network. To choose load to be shed, the power imbalance is calculated and we got 1.19MW. This number of load must be shed either equal or slightly equal. For this case the closer load is power load1 which have 1.2MW. Therefore, power load1, load4 and load5 are shed by tripping breaker 2, 5, and 6 respectively. All the loads to be shed are shed simultaneously by tripping the breaker at the same time setting. Therefore, the frequency will recover faster and bring back to the acceptable level. So, the remain power load is 4.05MW which is approximately equal to the power generation which is 4.09MW. Hence, the frequency is bringing back to the nominal value. Network diagram shows that the corresponding load is shed. The breaker is trip by give signal logic 1 to the breaker. Figure of 4.10, 4.11 and 4.12 shows the behavior of frequency when without load shedding, with load shedding and comparison between them respectively. The frequency without load shedding finish at slightly below load shedding frequency because the frequency can not bring back to nominal due to generator trip. Load shedding succeed to bring the frequency rise at 49.6Hz to prevent from total blackout. Figure 4.13 illustrate simulation model after load shedding.

    4.2 CASE 2(. For 0.8 Power Generation)

    DISCUSSION

    In this case power generation is 3.608MVA and the power load is 5.25MW. The power generated is 0.8 from the rated power. There is imbalance of power exist and some load has to be shed to match with the power generated. The expected overload, L is calculated and the value is 0.4550 which is indicate that the loss of 31 percent of total generation. The load to be shed is 1.6MW which indicate that this amount of power must be shed to bring back the frequency to nominal. This amount is the minimum number of load must be shed. From the frequency graph, there is slope exist and it is indicate that there is power imbalance. For this case the rate of frequency change measured is -0.00217 Hz/s which is mean the frequency is drop -0.00217Hz in a second. This value is quite critical and hence it is consider as a slightly greater than small disturbance. Therefore, this value of frequency change is fall at the second interval which is between -0.004 and -0.002. if the load shedding is not initiated the frequency is drop at 42Hz which cause the turbine blade damage. And by initiate load shedding scheme the frequency will drop at 48.9Hz which is in the safe mode. The appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this modeling network, the priority load is load 6 which is the larger load in this network. To choose load to be shed, the power imbalance is calculated and we got 1.642MW. This number of load must be shed either equal or slightly equal. For this case the closer load is total of power load1 and power load8 which have 1.2MW and 0.45MW respectively. Therefore, power load1, load4, load5 and load8 are shed by tripping breaker 2, 5, 6 and 8 respectively. All the loads to be shed are shed simultaneously by tripping the breaker at the same time setting. Therefore, the frequency will recover faster and bring back to the acceptable level. So, the remain power load is 3.6MW which is approximately equal to the power generation which is 3.608MW. Hence, the frequency is bringing back to the nominal value. Network diagram shows that the corresponding load is shed. The breaker is trip by give signal logic 1 to the breaker. Figure 4.20, 4.21 and 4.22 shows the behavior of frequency when without load shedding, load shedding and comparison between them respectively. Load shedding brings the frequency rise at 48.9Hz and go back to the nominal level. The frequency is not exactly 50Hz but slightly below that. Figure 4.23 illustrate simulation model after load shedding.

    4.3 CASE 3(For 0.7 Power Generation)

    DISCUSSION

    In this case power generation is 3.157MVA and the power load is 5.25MW. The power generated is 0.7p.u from the rated power. There is imbalance of power exist and some load has to be shed to match with the power generated. The expected overload, L is calculated and the value is 0.6629 which is indicate that the loss of 40 percent of total generation. The load to be shed is 2.02MW which indicate that this amount of power must be shed to bring back the frequency to nominal. This amount is the minimum number of load must be shed. From the frequency graph, there is slope exist and it is indicate that there is power imbalance. For this case the rate of frequency change measured is -0.00409 Hz/s which is mean the frequency is drop -0.00409Hz in a second. This value is critical and hence it is consider as a medium disturbance. Therefore, this value of frequency change is fall at the third interval which is between -0.005 and -0.004. if the load shedding is not initiated the frequency is drop at 38Hz which cause the turbine blade damage. And by initiate load shedding scheme the frequency will drop at 48.2Hz which is in the safe mode. The appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this modeling network, the priority load is load 6 which is the larger load in this network. To choose load to be shed, the power imbalance is calculated and we got 2.093MW. This number of load must be shed either equal or slightly equal. For this case the closer load is total of power load1 and power load3 which have 1.2MW and 1.05MW respectively. Therefore, power load1, load4, load5, load3, load8 and load9 are shed by tripping breaker 2, 4, 5, 6, 9 and 10 respectively. All the loads to be shed are shed simultaneously by tripping the breaker at the same time setting. Therefore, the frequency will recover faster and bring back to the acceptable level. So, the remain power load is 3.0MW which is approximately equal to the power generation which is 3.157MW. Hence, the frequency is bringing back to the nominal value. Network diagram shows that the corresponding load is shed. The breaker is trip by give signal logic 1 to the breaker. Figure 4.30, 4.31, 4.32 shows the behavior of frequency when without load shedding, load shedding and comparison between them respectively. For this case, frequency with load shedding is slightly odd, because the gap between load shed and power generation is large which 0.157MW is. so the behavior of graph is influence by the response of the generator. Fortunately the frequency is still can be bring back to acceptable level. Figure 4.33 illustrate simulation model after load shedding.

    4.4 CASE 4(For 0.6 Power Generation)

    DISCUSSION

    In this case power generation is 2.706MVA and the power load is 5.25MW. The power generated is 0.6p.u from the rated power. There is imbalance of power exist and some load has to be shed to match with the power generated. The expected overload, L is calculated and the value is 0.940 which is indicate that the loss of 48 percent of total generation. The load to be shed is 2.41MW which indicate that this amount of power must be shed to bring back the frequency to nominal. This amount is the minimum number of load must be shed. From the frequency graph, there is slope exist and it is indicate that there is power imbalance. For this case the rate of frequency change measured is -0.00508 Hz/s which is mean the frequency is drop -0.00508Hz in a second. This value is very critical and hence it is consider as a large disturbance. Therefore, this value of frequency change is fall at the fourth interval which is between -0.007 and -0.005. If the load shedding is not initiated the frequency is drop at 35Hz which cause the turbine blade damage. And by initiate load shedding scheme the frequency will drop at 47.5Hz which is in the border of the safe mode. The appropriate load to be shed is properly chosen. Because the priority load can not be shed, in this modeling network, the priority load is load 6 which is the larger load in this network. To choose load to be shed, the power imbalance is calculated and we got 2.544MW. This number of load must be shed either equal or slightly equal. For this case the closer load is total of power load1, power load7 and power load9 which have 1.2MW, 0.75MW and 0.6MW respectively. Therefore, power load1, load4, load5, load7 and load9 are shed by tripping breaker 2, 4, 5, 8 and 10 respectively. All the loads to be shed are shed simultaneously by tripping the breaker at the same time setting. Therefore, the frequency will recover faster and bring back to the acceptable level. So, the remain power load is 2.7MW which is approximately equal to the power generation which is 2.706MW. Hence, the frequency is bringing back to the nominal value. Network diagram shows that the corresponding load is shed. The breaker is trip by give signal logic 1 to the breaker. Figure 4.40, 4.41 and 4.42 shows the behavior of frequency when without load shedding, load shedding and comparison between them respectively. For this case the frequency is drop at the border of the safe mode which is 47.5Hz. after load shedding the frequency is bring back to the acceptable level. Figure 4.43 illustrate the simulation model after load shedding.

    CHAPTER 5: CONCLUSION

    Adaptive under frequency load shedding scheme is designed in this project to prevent from extreme frequency drop which might cause turbine blade damage and cause total blackout in power system. The Adaptive LD-df/dt Characteristic scheme is applied for this project. This characteristic is very powerful to prevent from under shed or over shed which may cause large loss. Figure 4.44 shows the summary of Adaptive LD-df/dt Characteristic scheme. This method is proven by changing the power imbalance between power generation and power load. The power imbalance is represented as disturbance, the greater the imbalance between power generation and power load the greater disturbance exist. The rate of frequency change is used as an indicator to determine the severity of the disturbance and therefore predetermine action can be taken. The instantaneous of frequency change is taken to predict the power imbalance inside the system. Simulation model is developing to predict the outcome of the scheme. The algorithm is written in the sequence to properly shed the appropriate load. The rate of frequency change can predict the disturbance. Simulation modeling shows the network after and before the load shedding is initiated. When the load shedding is not applied the frequency is drop at 35Hz for the worst case which cause damage turbine blade and cause total blackout. Whenever, Adaptive UFLS is applied , The result is load to be shed is corresponding to the severity of the disturbance and load to be shed is shed simultaneously which result faster frequency rise.

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