Induction Generator in Distributed Generation

Investigation of performance of Induction Generator in Distributed Generation

1. Introduction:

Power Engineering has been developed to a greater extent over the past few years. The generated power is transmitted over a considerable distance and to avoid losses during the transmission, the power is fed to the distribution network and delivered to the customers. In Europe, the generation of power depends on the natural resources like oil, gas, coal, etc and from renewable sources like wind, solar, tidal, etc. The International Energy Agency predicted that, the demand in Europe would increase by 10% by the year 2020 (Roberts, Liss & Saunders, 1900). Due to the scarcity of natural resources, an alternative approach is made with renewable energy sources and efficient electric power systems. This encourages the distributed generation, whose primary reason is to reduce the emission of CO2 as a part of Kyoto protocol. Since the energy density of renewable energy sources is less than fossil fuels, the demand can be met by spreading them globally. The main focus of this project is the type of generator that can be used in distributed generation.

As the demand for electric energy has been increasing there is a need for installing new thermal, hydro, nuclear or coal fired power stations which would increase the capital cost when compared to the potential rewards. So the alternative path to meet this growing demand is small scale generation, this undercuts the central station utility cost. The range of generation would be from few kilowatts to 100MW which includes micro turbines, fuel cells, photovoltaic systems, wind energy systems, diesel engines, gas turbines and battery storage. With small generation producing power nearer to the user they are subjected to less power failures and the reliability is high. A general definition of DG is an electric energy source connected directed to the distribution network or load centre. Several reasons which has made DG an attractive proposition are, technically attractive, environmentally attractive, commercially attractive and, to an extent politically attractive [Steven, 1983].

Customers benefit from the success of DG because:

1. The use of distributed generation will allow improvement in the dispatch ability of resources and improve the integrity of the transmission and distribution systems.

2. Identification and use of alternatives to power generation, transmission and systems control would improve the load leveling, load management and overall power quality.

3. The system will become robust in its ability to tolerate natural disasters, suffers less damage and minimize the dependence upon the need for immediate restoration of the grid system.

4. Overall system reliability will improve.

2. Literature review:

At present, both synchronous and induction generators are widely used in distributed generation networks. A comparison is made to study the performance of each generator to find what would be the best choice to install in distributed generation. The literature review discusses on the following:

1. Characteristics of Synchronous and Induction generators.

2. Impact of distributed generation.

3. Protection scheme for Distribution network.

4. Project description.

5. Network analysis with synchronous and induction generator.

6. Summary.

2.1 Characteristics of Synchronous and Induction Generators:

Synchronous Generators:

Generators which run at constant speed are known as Synchronous generators (Guile & Patterson, 1969). This type of generator maintains the same speed throughout the operation and operates at a wide range of power factor for both leading and lagging phase angles. The excitation system and prime mover of the synchronous generator needs more attention when synchronous generators are used in power generation. Prime mover is an external device which applies torque to the shaft of the generator which converts the mechanical energy into electrical energy (Guile & Patterson, 1969). As synchronous generators are not inherently self-starting, an excitation system is required and the power required to excite the generator will be 1% of the rating of the generator.

Induction Generators:

Induction generators are induction motors with simple modifications in construction in order to operate them as generators. Squirrel cage induction generators are the most common type of generators and in special type of generation wounded type is employed. Induction generator operates in the circular locus, which is a major difference in operation as that of a synchronous generator (Traister, 1983). During the operation, the interaction between the magnetic flux of the stator and rotor produces a counter torque in opposition to the torque produced by the prime mover. In induction generator, the voltage and frequency control cannot be achieved because it would be the same as of the network to which it is connected whereas; the load on the generator can be controlled by adjusting the speed of the prime mover (Miller, 1982).

2.2 Impact of Distributed Generation:

Distribution network are designed to supply from the bulk supply point to the customer through the network. When a distributed generator is added to the network it would have certain impacts on the performance of the network. The obvious impact would be the increase in fault level of the network. The other impacts are change in the power flow that would increase the power losses, and it would also affect the power quality. These impacts incur variety of cost for the Distribution Network Operators (DNO). When dispersed generators are connected to the network, then it has to accept certain conditions designed to reduce the network reinforcement, in order to accommodate the generator (Guile & Patterson, 1969). Where the real and reactive power output is controlled, such control would impose a cost on the generator in terms of energy not supplied where the regulatory frame work allows it, so the DNOs could offer generation connection options that reflect this trade-off between reinforcement costs and cost of constraints.

Other major issue of distributed generation which has impact on the network is the uncontrollable output from some generators, which would affect the network, so required steps has to be done in order to control the output (Roberts, 1990). Sometimes alternative storage can be done in order to regulate the output of these types of generators. Next would be the monitoring and control of the generators, as said earlier the generation operation would be within a certain conditions, so the DNOs would need a certain monitoring and control system to control the distribution generation to operate within the agreed condition (Roberts, 1990). All these factors have to be taken in to consideration when a distributed generator is connected to the network, in this project some of the aspects like voltage profile, stability, fault level of the network with synchronous and induction generator is done to know which would be the best selection of generator for embedded generation.

2.3 Protection scheme for Distribution network:

The protection scheme is necessary for any network in order to avoid the damage caused due to any faulty operation of the components in the network. The network is protected by a technique called Islanding. Islanding refers to the isolation of an electric grid from other components in the network during fault condition (Jenkins, 2000). This technique shuts down the generation network during abnormal conditions like over load, over voltage or frequency and in under voltage or frequency. When a network with induction generator is islanded, it develops a very large distorted voltage. In this condition, it becomes an undesirable operating condition in distributed generation. But, synchronous generators are compatible with this condition i.e. in islanding mode and using synchronous generator would be the best option.

2.4 Project Description:

A simple distribution network is built using the simulation software ERACS power system package and analyzed with and without embedded generators for various load conditions. The ratings of the network components like grid, transformers, bus bars, etc are assumed. When the network is analyzed with and without generators for maximum load conditions the following issues are taken into account. They are

1. Change in network voltage

2. Increase in fault level

3. Quality of power delivered by the network

4. Network protection

5. Stability of the network.

It is ensured that the network is properly designed in order to avoid the power quality of the network. Depending on the situation, the distributed generation plant can either increase or decrease the voltage received by the other users and can cause transient voltage variations when a generator is connected or disconnected from the network. Similarly, the network system designed using power electronic devices will introduce harmonic current which leads to voltage distortion (Jenkins, 2000). Once the network is carefully designed by taking the mentioned above issues into account, the network's performance with synchronous and induction generator is evaluated by varying the load from minimum and maximum for the following conditions.

1. Without generators but with maximum load.

2. Without generators but with minimum load.

3. With Synchronous generator and load at maximum.

4. With Synchronous generator at maximum but load minimum.

5. With Induction generator and load at maximum.

6. With induction generator and load at minimum.

The network is said to be stable, when there is a balance between the power consumed and power generated at each busbars. In practice, the power system will not be in steady state during the operation as the loads and generation are constantly changing. So the stability of the network is studied with synchronous and induction generator.

2.5 Network analysis with Synchronous and Induction Generator:

A synchronous generator connected to the network is operated as constant active power source so they do not take part in the system frequency control. So the mechanical power supplied is considered as constant. i.e., the regulator and the prime mover dynamics are neglected and this is same for the induction generator, this option leads to results that are more generic. A synchronous generator can be operated in two different modes of control. One aim is to maintain a constant terminal voltage and the other is to maintain constant power factor (Steven, 1983). Here we have selected the constant terminal voltage mode in order to obtain a constant terminal voltage. This helps to study the voltage profile of the network for maximum and minimum load conditions which determines the systems stability. Certain factors have to be considered which would affect the voltage profile of the network when connected to synchronous generators:

1. Steady-State voltage regulation.

2. Maximum number of generators installed without affecting the voltage profile.

3. Voltage variation due to generator disconnection.

Once the networks stability is determined under normal conditions, it is also studied under faulty condition. An unpredictable event in which a short circuit occurs between the conductors or earth, this abnormal conducting path is called a fault (Steven, 1983). A fault analysis is done in a network in order to find damage that the fault current would cause to the network. And to select the appropriate circuit breaker to prevent the damage caused to the network. In fault analysis a distinction between balanced and unbalanced has to be made. A balanced fault is one in which all three phase of the network is affected and the symmetry between the voltage and current in the three phases is not altered. And unbalanced fault, it creates asymmetry in the network and requires a complex analysis based on symmetrical components.

In a similar way, a steady state analysis and fault analysis is done to study the network's stability embedded with induction generator for minimum and maximum load conditions.

2.6 Summary:

` By following the above tests and experiments, it is obvious to study the performance of the distribution network with and without synchronous and induction generator for minimum and maximum load conditions

3. Research question:

How the performance of the induction generator is more efficient than synchronous generator in distributed generation?

4. Aims and Objectives:

The aim of the project is to investigate the performance of induction generator in distributed generation. Using MATLAB/SIMULINK, a typical generation and distribution network will be developed in order to examine its performance under various conditions i.e. with and without the generators for maximum and minimum loading conditions. Initially, the network will be studied without any generators and the per unit voltage at each bus bar is gained. The performance of the network is then analyzed with induction and synchronous generators. The main objective of the project is as follows:

1. To study the distribution network parameters like per unit voltage, busbar voltage and fault currents with and without generators for maximum and minimum load conditions.

2. To carry out the tests and experiments on the distribution network for various conditions of load.

3. To make a comparative study on both generators with the network parameters.

4. To assess the future developments to improve the performance of the network

5. Project Approach, Methodology and Data analysis:

The approach is the breakdown of the project on how it will be carried out. It involves the following steps:

A study on the issues in power generation: This discusses the increase in demand, scarcity of supply of fossil fuels, avoiding losses during transmission, power plant construction, capital, etc. This also deals with the environmental impacts, gaseous emission from fossil fuel, alternative methods of power generation from renewable energy sources like wind, solar, photovoltaic's, tidal, etc. Few issues on making the distributed generation global wide is discussed.

Network design for dynamic analysis: This section explains the design parameters and ratings of the network components designed using the simulation software MATLAB/SIMULINK. It includes the schematic of the network with all the design parameters for different modes of generation with and without synchronous and induction generator.

Steady state analysis of the Distribution network: In this section, we study the impact of distributed generation in the network with and without induction generator during load flow and faulty conditions. The data's i.e. parameters like per unit voltage, busbar voltage and fault current are recorded to examine the network's performance.

Dynamic analysis of the distribution network: The stability and voltage profile of the network without generators from minimum to maximum load condition is analyzed and the network's performance is studied. Similarly, the network is analyzed with synchronous and induction generator from minimum to maximum load conditions and the performance of the network is evaluated by recording the per unit volte, busbar voltage and the fault current.

Future developments: This division focuses on the future developments in power generation to improve the network's performance and reliability. This also deals with complex system configuration, operational problems and cost associated with this scheme in power generation. In future, power electronic devices can be employed in order to improve the performance of the network.

The data analysis can be done by recording the parameters of the network which in turn helps to analyze the performance of the network.

6. Ethical consideration:

According to the ethical considerations, this project is carried out by following the school law and regulations in a laboratory as it is a simulation project based on some tests and experiments.

7. Potential Limitations:

The project can be done practically rather than by software simulation but, it has got certain limitations. They are as follows;

1. Time constraint.

2. High investment.

3. Large area requirement.

4. Market regulation and

5. Technical issues.

8. Conclusion:

The continuous development in distributed generation would benefit the society as a whole as it offers significant benefits to the environment. The development of network needs more care as it involves synchronous and induction generator. Therefore, the network needs a protective device which responds quickly under faulty conditions. The network is analyzed with both synchronous and induction generator at minimum and maximum load to study the voltage profile, stability and fault currents which determines the performance of the network. The performance of the network can be improved by introducing power electronic devices. In order to avoid the self excitation of the power electronic devices, a control system is required to control the flow of reactive power in the network. However, complex system configuration, operational and cost associated problems with this scheme recommends the use of induction generator in distributed generation. Due to the time constraint, the network performance can be studied in detail by employing voltage regulation scheme and power electronic devices in future.

9. References:

1. Bird, J. O. (2007) Electrical circuit theory and technology. 3rd edn. Amsterdam ; London: Elsevier/Newnes.

2. Guile, A. E. & Paterson, W. (1969) Electrical power systems. Edinburgh: Oliver & Boyd, Electronic and electrical engineering texts ; 2.

3. Jenkins, N. & Institution of Electrical Engineers. (2000) Embedded generation. London: Institution of Electrical Engineers, IEE power and energy series ; 31.

4. Miller, T. J. E. (1982) Reactive power control in electric systems. New York: Wiley.

5. Pansini, A. J. (1992) Guide to electrical power distribution systems. Prentice Hall.

6. Roberts, L. E. J., Liss, P. S. & Saunders, P. A. H. (1990) Power generation and the environment. Oxford: Oxford University Press, Science, technology and society series: 6.

7. Steven, R. E. (1983) Electrical machines and power electronics. Wokingham: Van Nostrand Reinhold.

8. Traister, J. E. (1983) Handbook of power generation : transformers and generators. Prentice-Hall.

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