STATIC SYNCHRONOUS SERIES COMPENSATOR

STATIC SYNCHRONOUS SERIES COMPENSATOR

ABSTRACT

The synchronous series compensator controls the compensating voltage over an inductive and capacitive range and is independent of the magnitude of the line current. It is resistant to resonance i.e. oscillations at higher amplitudes and certain frequencies. The static synchronous series compensator in addition to the series compensation it can also compensate the drop in voltage across the resistive component of the line impedance. The compensation of the real part impedance can maintain high reactance to resistance ratio even when the line has a very high degree of series compensation. The power oscillation damping can be increased to a large extent by the coordination modulation of the real and reactive compensation. The static synchronous series compensator is base on the voltage source converter which converts the direct current input voltage to the alternating current output voltage and it results in the compensation of the reactive power of the system. By adjusting the amplitude of the voltage source converter the variations in the generation and absorption of the reactive power can be controlled to a very large extent and thus improving the voltage stability and reducing the power quality problem issues.

INTRODUCTION

The performance of transmission lines, those of short medium and long can be improved by the series compensation. Series compensation consists of a capacitor bank placed in series with each phase conductor of the line. Series compensation decreases the impedance of the line which is the main cause in drop of voltage and principal factor for determining the maximum power that the line can transmit. The reactance of the capacitor bank can be known by compensating a specific amount of inductive reactance of the line. This defines the term compensation factor which is given by the ratio of the capacitive reactance of the capacitor bank to the inductive reactance of line per phase.

Usage of the switching power converters for the generation of reactive power to provide compensation for transmission and as well as distribution lines has been realised and a prototype static synchronous series compensator has been installed using a thyristor based voltage course inverter. (Commissioned at the Sullivan substation of Tennessee valley Authority (TVA) power system). (A book on power system analysis by John J. Grainger and William D. Stevenson). This further lead to the concept of unified power flow Controller (UPFC) for the dynamic power flow control. Usually static synchronous series compensators are used as regulating devices in the transmission networks. There are several advantages and applications for these static synchronous compensators. These are introduced in 1990s to deal with the power quality problems. These have also been used in the renewable energy distribution and transmission. In the power distribution system regulation of the load voltage is very important to compensate the time varying loads. For this purpose reactive power sources are commonly used for regulation in case of disturbances. The control bandwidth is very large. So in order to handle that the static compensators based on voltage source converters are being proposed. For the effective regulation of the load voltage the static compensators are modelled by the axis theory for three phase systems. The effect of the change in the parameters of distribution system is studied with regard to dynamics of the static synchronous compensator. Firstly the model is used to address the problem of regulation of load voltage with the static synchronous compensator to control the current source. Subsequently nonlinear and linear controllers are designed and the performance is compared with the simulations obtained. Then the static synchronous compensator that is controlled is combined with the distribution system model and load voltage controller. In this way the static synchronous series compensator helps in the regulation of the voltage in distribution and transmission systems. Basic structure of the static synchronous series compensator is as shown below:

The structure of the static synchronous series compensator mainly consists of the voltage source converter. This uses the control strategies of pulse width modulation to control the magnitude of the voltage. With the help of this voltage source converter the magnitude of voltage can be controlled and subsequently the generative reactive power and absorption of the reactive power can be controlled to deal with the power quality problem issues and voltage stability problems.

3. FUNDAMENTAL PRINCIPLES AND CHARACTERISTICS

The characteristics of the static synchronous series compensator are as given below:

  • Power-angle characteristics
  • Capability of power exchanging
  • Immune to resonance
  • Volt-ampere rating of the compensator

3.1 Power-angle characteristics:

The impedance in the line is due to the capacitor bank. The voltage is proportional to the current which is the function of the angle of transmission. Voltage changes as the angle changes. The graph determining the power angle characteristics is as shown below: 6

By observing the graph we come to know that maximum power is obtained at the angle of 90 degrees. By reversing the polarity of the voltage injected we can decrease the power to be transmitted. Apart from the negative and positive power flows of the system the static synchronous series compensator has an excellent response time and the transition from the positive to negative power flow through zero voltage injection is very continuous. For applications requiring the power flow control the determination of the power-angle characteristics is very essential.

3.2 Capability of power exchanging:

The static synchronous series compensator can resolve the changes between the reactive and active power with the alternating current system by simply controlling the angular position of the voltage with respect to the line current. The static synchronous series compensator has significant application potential due to capability of active power exchange. Simultaneous compensation of both the resistive and reactive components of the series line impedance so that the ratio between the reactance and resistance is maintained high. The static synchronous series compensator with an suitable direct current supply which can be powered simply will be able to inject the voltage in the phase in addition to the compensating voltage with the effect of the resistive drop on the power distribution and transmission. During the periods of acceleration the static synchronous series compensator can apply the maximum line compensation for increasing the active power and absorb the power to include the damping resistance in series with the line. Similarly when the time of deceleration then the static synchronous series compensator can operate the compensation i.e. the inductive compensation to reduce the active power transmitted and provide the effect of negative resistance to supply extra power to the line for damping. Even the effective energy storage is required for the ideal damping; a more economical way would be the usage of the energy sink that is when the static synchronous series compensator will absorb power during the angular accelerations which is transferred to the energy sink. Therefore the static synchronous series compensator will solve the problems of transfer of active and reactive power control in the alternating and as well as the direct current system. The static synchronous series compensator (SSSC) is suitable for both the systems. The capability of exchange of the power is very essential for the solid state approach to the static synchronous series compensation of the transmission lines.

3.3 Resistant to Resonance:

Resonance is the direction of the system to vary repetitively at greater amplitudes and a few frequencies than at other frequencies. These frequencies are known as systems resonance frequencies. Resonance occurs when the system is able to transfer energy between the different storage equipments. For example in case of pendulum the kinetic energy and potential energy. (Wikipedia) Resonance occurs in many types of waves. Resonance of different types like mechanical, electromagnetic etc. Series capacitor provides opposite voltage that develops across the line impedance at the fundamental frequency of 60hertz to improve the power that is transmitted.

Capacitor is a function of frequency and it leads to resonance that is the disturbance at different frequencies with other reactive impedances present in the network. This frequency is less than that of the fundamental frequency. At this frequency disturbances occur in the systems as it is less than the fundamental frequency and therefore it leads to the disturbances in the mechanical systems and as well as the electromagnetic disturbances and this is very well known as the sub synchronous resonance which results in great damage to the generators.

A considerable effort has been made to improve the inherent frequencies characteristics of the capacitor in the dominant sub synchronous band of the frequency by the connection of parallel connected reactor called the thyristor-controlled reactor (TCR) to make it resistant to the disturbances in the system. The series capacitor can be combined with the thyristor-controlled reactor with required inputs then it would operate only at specific fundamental frequency and the impedance at other frequencies would be zero theoretically and as well as practically. (Colin D. Schauder, IEEE Power delivery) The static synchronous series compensator has very small inductive output impedance that which is provided by the leakage inductance of the injection transformer. So if we need to increase the transmitted power we need to provide impedance on the line at the fundamental frequency. By using the static synchronous series compensator the line can be compensated in case of drop in voltage at fundamental frequency that is 60 hertz. Static synchronous series compensator has a very instantaneous response and thus is very efficient in reducing the oscillations that is the disturbances in the system. Apart from the resistance to disturbances the static synchronous series compensator behaviour is different from the behaviour of the series capacitor.

In practical cases the voltage across the line would be very high and is limited by the static synchronous series compensator. It is very essential to know that the static synchronous series compensator would require an external power supply to retain all the internal losses in the system so that the improvement in the transmission angle takes place.

In this way the static synchronous series compensator helps in improving the system performance by reducing the losses occurring in the system due to the disturbances. Therefore we conclude that the static synchronous series compensator is very helpful as it can be resistant to the oscillations in the system and thus improves the performance of the system in all ways.

3.4 voltage-ampere rating of the compensator:

The static synchronous series compensator will provide capacitive compensating voltage that is independent of the current at its specified rating at which the voltage of the compensator would be initiated to zero which is called as the by-pass operation. The voltage-ampere rating of the static synchronous series compensator is defined as the product of the voltage rating of the static synchronous series compensator and the line current of the static synchronous series compensator.

In practice the maximum current represents the maximum steady state current and as well as the over current. When we compare different static synchronous series compensator schemes we come to know that the maximum current represents the steady state current and the over current line current. Capacitor provides a compensating voltage that is directly proportional to the current then the voltage range of the static synchronous series compensator becomes the function of the line current.

The voltage ampere ratings of the system are very essential for the improvement of the system performance. Therefore the consideration of the static synchronous series compensator voltage-ampere rating is very important for growth of the performance and well as to reduce the internal losses of the system. The compensation varies in a step like manner by opening and closing the switches of the static synchronous series compensator. During the period of minimum compensation all the switches will be closed and during the period of the maximum compensation all the switches will be open.

The voltage produced by the static synchronous series compensator is proportional directly to the current produced by the compensator. There will be a double voltage occurring across the static synchronous series compensator is due to the conventional thyristor and this is because that the thyristor can be turned off at the current zero which forces the voltage across the capacitor bank to be zero.

This results in the increase in the voltage across the capacitor bank. Therefore we can see that the voltage-ampere rating of the static synchronous series compensator is very important characteristic to be considered for the improvement of the system performance.

4. MODELLING OF STATIC SYNCHRONOUS SERIES COMPENSATOR:

In very heavily stressed power systems voltage lapse that is reduction of the voltage takes place. This even sometimes leads to complete collapse of the voltage as well and as a result of this many disturbances occur in the system which will degrade the performance of the system. This problem can be avoided to an extent by taking into consideration few important things such as the transmission of the power in the system and as well as the consumption or absorption of power in the system. (A. M. Gole and others. "Guidelines for Modelling Power Electronics" IEEE Transaction on power delivery).

The static synchronous compensator has been studied for many years and found that it is the most efficient and appropriate device to control this voltage collapse and improve the regulation of the voltage. Therefore these static synchronous compensators are very widely used for the improvement of voltage regulation. This even increases the stability of the system so that the system remains in the stable region even in case of disturbances and large oscillations. The static synchronous series compensators are usually modelled by the power-voltage bus or the PQ bus depending on the application of the compensator. We come across few problems during modelling of the static synchronous series compensator such as the losses during the steady state are neglected and the limit of the current of the capacitor is not able to represent accurately. Later this static synchronous series compensator is studied and has been proposed a model for it by injecting the power into it and is also called as the power injection model of the static synchronous series compensator.

4.1 Structure and working of the Static Synchronous series Compensator:

The model of the static synchronous series compensator mainly consists of the transformer which is connected in shunt, a voltage source converter, a direct current capacitor, a controller and a magnetic circuit. The capacitor is connected in the circuit because if there is no energy storage device in the model then the converter will not be able to generate or absorb the power and the operation will be limited only to certain domains therefore the inclusion of the capacitor in the model is very important.

The exchange of the reactive power of the system is controlled by the voltage amplitude of the voltage source converter. If the amplitude of the voltage source converter is increased then the power flows from the static synchronous series compensator to the alternating current system and the system generates the reactive power and if the voltage amplitude of the voltage source converted is decreased then the current starts flowing from the alternating current system to the static synchronous series compensator and the absorption of the power takes place. (Electric power systems research, volume 76, august 2006).

Therefore by controlling the output of the voltage source converter the current can be controlled from maximum to minimum so that the generation and absorption of the reactive power is under control and leads to appropriate working of the system effectively and efficiently thus improving the performance of the system. So in order to make this possible the capacitor plays a very major role therefore inclusion of it is very essential in the static synchronous series compensator model.

4.2 Controlling of the static synchronous series compensator:

Nowadays the power width modulation control is becoming the more important and practical technology for the applications related to power systems. The effect of the losses due to fast switching of the electronic devices is more on the calculation of power flow in the power width modulation control and has a very large impact on the charging and discharging of the capacitor. Therefore this must be kept in mind when modelling the static synchronous series compensator.

So as seen above the power injection model can be used to implement the power width modulation on it so that the voltage amplitude can be adjusted appropriately and hence controls the generation and absorption of the reactive power. In this way the modelling of the static synchronous series compensator is done taking into consideration the important aspects such as the generation and as well as the consumption of power, the voltage amplitude control using the pulse width modulation strategy and even the charging and discharging of the capacitor.

5. APPLICATIONS OF THE STATIC SYNCHRONOUS SERIES COMPENSATOR:

5.1 For power quality problems:

The power quality problems issues have become very common in the recent times and are more important for the improvement in the performance of the system. The main issues of power quality problems are the reliability of the supply of power to the consumers and the maintenance of the supply voltage in the considerable range for uninterrupted power supply. The power quality issues results from both the increase in the demand of electricity from the consumers and also due to the regulations of the government that have to be followed legally. Due to these power quality problem issues the consumers may face many problems technically and the consequences of it are also very bad. Disturbances such as fluctuations in the voltage, corona, flickers etc can damage the house hold electrical appliances and as well as the industrial appliances leading to shut down of the industries.

Therefore these power quality problems must be minimised to a very large extent to deliver consumers uninterrupted power supply and as well as to prevent the damage. In order to make this possible we need a device that provides the compensation to the internal and external disturbances or oscillations or harmonics occurring in the system. For these reasons at first static var capacitors have been employed to reduce these power quality problems and to keep the voltages stable and improve the power factor of the system and even provide balance of the generation and consumption of the reactive power. But in the recent times the solid state devices are being employed in many applications for effective function of the devices such as the solid state compensators which are otherwise called as the static synchronous series compensators. And later it is observed that the role of the static synchronous series compensator is extremely high in reducing the power quality issues by controlling the voltages and maintaining them at the required level and as well as providing balance between the generation of the reactive power and consumption of the reactive power. The main component of the static synchronous series compensator is the voltage source converter which uses the pulse width modulation for the regulation of the voltage amplitude as it is essential for controlling the increase and decrease in the current level which resemble the generation level of reactive power and as well as the absorption level of the reactive power. The multi-pulse scheme used helps in reducing the harmonics to a very large extent. The converter in the static synchronous series compensator is modelled using the switching device components and the storage devices like capacitors which help in the charging and discharging. In this way the static synchronous series compensator plays a major role in dealing with the power quality problems.

5.2 Voltage and Angle Stability issues:

In the recent years the increase in demand for electricity is growing higher and as a result the focus on the quality of the power to be delivered is also increased. Especially in the local areas the increase for reactive power is becoming very essential as the generation there is being reduced at a large extent.

In this competitive world the demand for the consumers expect for a uninterrupted power supply and this is possible only with loss less generation, transmission and distribution of the power. Many latest technologies have evolved to reduce the problems of power quality and power supply especially the power controlled devices which help in dealing with these power quality issues very efficiently. One of such devices that are being used very rapidly is the static synchronous series compensator which helps in improving the quality of the power by responding instantaneously to the changes in the system and as well as control the transfer limits. The static synchronous series compensator modelled will help in the voltage regulation thus by improving the voltage stability of the system. Voltage stability is a very essential aspect to be considered when dealing with the power quality problem issues.

The load of the static synchronous series compensator is modelled depending on the time domain characteristics and impedance characteristics so that it enables for the fast and slow transitions in the system as and when required. Using the pulse width modulation in the static synchronous series compensator is becoming very common to control the voltage amplitude and thus by improving the voltage stability. And this is done by the voltage source converters. This application of pulse width modulation helps in reducing the losses due to switching and thus by improving the stability as explained in the before section. And as voltage stability improves which results in the improvement of the angle. By implementing these and by programming we can define the results from which we can determine how the steady state operating conditions change certain parameters of the power systems. Depending on the results we can check the parameters that are causing these power quality problem issues and appropriately provide the solution to correct them and improving the voltage and as well as the angle stability.

5.3 Static synchronous series compensator in wind farms:

In wind farms the power quality problems and power control issues are very common and identifying these problems is very essential for obtaining efficient performance of the power system. The static synchronous series compensator is used in the wind farms for purpose of stabilization of the voltage of the grid, to reduce the disturbances in the system and sudden load changes. In wind farms usually the grids are connected very strongly and closer so that the voltage and frequency can be restored back in the case of disturbances. Delay in the restoring of the voltage and frequency after the occurrence of disturbance is not good for the system and it leads to the case of weak grid interconnection and instability of voltage. So in this case the static synchronous series compensator is very useful to retaining the voltage stability to the system and the power system to work very efficiently as before in case of disturbances or oscillations.

Effective power quality problems are very essential for the voltage stability in the period of faults like the three phase faults especially the short circuit faults. When the short circuit fault occurs then the terminal voltage drops which depends on the fault. In this situation the wind turbine will not be able to transfer the generated power produced by the system which leads to the imbalance between the input mechanical power generated and output mechanical power generated. Due to this imbalance between the input power and output power it will be difficult for the system to retain back after the fault has been cleared as the system needs more reactive power.

For all the wind turbines the capability to sustain the changes in the power system and voltage drops is very essential to improve the performance of the power system. The static synchronous series compensator is a series connected compensator device that is capable of generator and absorption of the reactive power of the system which leads to the performance improvement. In the static synchronous series compensator capacitors are connected to improve the voltage of the system. These capacitors connected are capable of improving the voltage stability limit but it is not very sensitive to the changes in the voltage of the system. Therefore we can conclude that the use of static synchronous series compensator is very essential in wind farms to improve the performance of the system by reducing the variations in the voltage fluctuations and thus by increasing the voltage stability.

5.4 Static synchronous series compensator for hydrogen generation:

The static synchronous series compensator combined with the battery energy storage system is used for the design of the hydrogen generator. As we know that the static synchronous series compensator can work only in the lagging and leading mode of operation and thus it can only control the reactive power of generation and as well as absorption.

Static synchronous series compensator is such a system that can generate hydrogen gas with the help of electrical energy from the wind farms and at the same time it can also provide smooth and loss less line power and constant voltage magnitude. This static synchronous series compensator converts the wing energy to the hydrogen gas and this gas can be used for the generation of electricity whenever the wind power is not sufficient and is not possible.

This conversion of the wind power to the hydrogen gas is possible by the component called the electrolyser which is employed with the static synchronous series compensator. Therefore we can say that by using the static synchronous series compensator the hydrogen gas can be generated from the wind energy.

6. ADVANTAGES AND DISADVANTAGES OF STATIC SYNCHRONOUS SERIES COMPENSATOR:

6.1 Advantages of the static synchronous series compensator:

The static synchronous series compensator has many functional characteristics and compensational features. The solid state devices used in the modelling of static synchronous series compensator make it more efficient and accurate. The advantages of this static synchronous series compensator can be explained as follows:

The capacitor and inductor in the static synchronous series compensator model can internally generate a controllable compensating voltage which is independent of the magnitude of the line current. It can keep the ratio of reactance to the resistance high and is independent of the compensation. This is because of the ability of the static synchronous series compensator to interface with the direct current power supply and can provide compensation for the resistance connected in the system which is otherwise called as the line resistance compensation. As we know that the model of the static synchronous series compensator consists of the energy storage device which helps in reducing the disturbances or oscillations occurring in the system during the situation of over voltages. The static synchronous series compensator can control the reactive power generated and as well as the reactive power absorbed in the system and eventually provides reactive compensation. The characteristics of the static synchronous series compensator make it very resistant to the resonance occurring in the system. Static synchronous series compensators have relatively many applications such as in wind farms etc where it is used to deal with the power quality problems and as well as the voltage stability problems.

6.2 Disadvantages of static synchronous series compensator:

The cost of this compensator is very high because of its voltage ampere rating of the equipment. As the voltage ampere rating alone does not determine the cost of the equipment but still it plays a major role in determining the effective cost of the system. In this way we can define the advantages and disadvantages of the static synchronous series compensator. As we can see that from above the advantages of the compensator are more when compared to the disadvantages of the compensator this static synchronous series compensator is being widely used for solving the power quality issues and voltage-angle stability problems.

7. CONCLUSION:

The static synchronous series compensator is another method used for the series capacitive line compensation. The normal series compensator provides the compensation of the impedance of the line where as the static synchronous series compensator is a solid state device that generates the compensating voltage that is independent of the line current.

The main component is the design of the static synchronous series compensator is the voltage source converter that provides great functional and operating characteristics to the system which is not achievable by the normal series compensator. The capability of the static synchronous series compensator to exchange both the reactive power generated and absorbed as well as the active power generated and absorbed along with the resistive drops in the system and therefore increases the ratio of the reactance and resistance of the system. As the static synchronous series compensation provides the simultaneous compensation of the reactive and active power which results in minimising the disturbances or the oscillations that occur in the power system.

Therefore we can conclude that the static synchronous series compensator offers flexibility to many applications there these compensators are widely used. The solid state devices present in the system makes it more efficient and accurate in the results. Many power quality issues and voltage stability issues can be solved using these static synchronous series compensator which is very essential for the accurate performance of the power system which helps in delivering the uninterrupted power supply to the consumers depending on the demand.

8 REFERENCES:

1. IEEE Transactions on power delivery by Laszlo Gyugyi , Colin D. Schauder and Kalyan K. Sen.

2. Electric power systems research, volume 76, august 2006.

3. F. Dong, B.H. Chowdhury, M.L. Crow and L. Acar, Improving voltage stability by reactive power reserve management, IEEE Trans. Power Systems

4. Application of STATCOM Controllers for Power Quality Improvement -Romyslaw Mienski. Ryszard. Pawelek and H A Wasiak

5. J.G. Slootweg, W.L. Kling, "Modeling of Large Wind Farms in Power System Simulations," IEEE Power Engineering.

6. Arthit Sode-Yome and N. Mithulananthan Electric Power System Management, Asian Institute of Technology.

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8. A. M. Gole and others. "Guidelines for Modelling Power Electronics. IEEE Transaction on power delivery.

9. A book on Power system analysis by John J. G Grainger and William D. Stevenson JR.

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