Automatic Generation Control Of A Multi Area Thermal System With Tcps Smes

CHAPTER 1

INTRODUCTION

There are volumes of research articles which have been appeared in the literature regarding Automatic Generation Control (AGC)/LFC of single area/multi area power system considering various control strategies. There are two variables of interest, namely, frequency and tie-line power exchanges.

Their variations are weighted together by a linear combination to a single variable called the Area Control Error (ACE).The AGC problem has been augmented with the valuable research contributions from time to time, like AGC regulator designs incorporating parameter variations/uncertainties, load characteristics, excitation control, and parallel ac/dc transmission links.

There are different types of power plants which supply reliable and good quality of electricity to their consumers. Power plants are dependent on type of energy source. So the researchers are investigating different types of power plant. Power plants can be thermal, hydro, wind, solar, nuclear and some other type source of electricity generation.

1.1 Approaches to realistic systems

However, the implementation of AGC strategy based on a linearized model on an essentially nonlinear system does not necessarily ensure the stability of the system. Considerable attention has been paid by researchers to consider the system nonlinearities to develop a robust controller. It is shown in the literature that governor dead-band nonlinearity tends to produce continuous oscillations in the area frequency and tie-line power transient response which produces destabilizing effect on the system. The first attempt in the area of AGC has been to control the frequency of a power system via the fly wheel governor of the synchronous machine. This technique was subsequently found to be insufficient, and a supplementary control was included to the governor with the help of a signal directly proportional to the frequency deviation plus it’s integral.

Control techniques

A lot of control techniques are proposed by the researches in there pioneer work to design LFC controllers. The controllers are based on:

Classical control techniques.

LQR based controlling techniques.

Proportional, Derivative, Integral controlling techniques.

Soft computing techniques/Artificial intelligence (AI) techniques.

Fuzzy logic based techniques.

Neural network based techniques.

Genetic Algorithm based techniques

Particle Swarm based techniques.

Other hybrid techniques

Description of load frequency control techniques are described by different researchers.

1.3. FACTS CONTROLLERS

With the rapid development of power electronics, Flexible AC Transmission Systems (FACTS) devices have been proposed and implemented in power systems. FACTS devices can be utilized to control power flow and enhance system stability. Particularly with the deregulation of the electricity market, there is an increasing interest in using FACTS devices in the operation and control of power systems with new loading and power flow conditions. A better utilization of the existing power systems to increase their capacities and controllability by installing FACTS devices becomes imperative. Due to the present situation, there are two main aspects that should be considered in using FACTS devices the first aspect is the flexible power system operation according to the power flow control capability of FACTS devices. The other aspect is the improvement of transient and steady-state stability of power systems. FACTS devices are the right equipment to meet these challenges.

1.3.1 Definition of FACTS:

According to IEEE, FACTS, which is the abbreviation of Flexible AC Transmission Systems, is defined as follows:

Alternating current transmission systems incorporating power electronics based and other static controllers to enhance controllability and power transfer capability.

The basic applications of facts-devices are:

Power Flow Control.

Increase of Transmission Capability.

Voltage Control.

Reactive Power Compensation.

Stability Improvement.

Power Quality Improvement.

Power Conditioning.

Flicker Mitigation.

Interconnection of Renewable and Distributed Generation and Storages.

Fig 1.1 shows the basic idea of facts for transmission systems. The usage of lines for active power transmission should be ideally up to the thermal limits. Voltage and stability limits shall be shifted with the means of the several different facts devices. It can be seen that with growing line length, the opportunity for facts devices gets more and more important.

The influence of facts-devices is achieved through switched or controlled shunt compensation, series compensation or phase shift control. The devices work electrically as fast current, voltage or impedance controllers. The power electronic allows very short reaction times down to far below one second.

Fig 1.1 Operational limits of transmissions lines for different voltage levels

The development of facts-devices has started with the growing capabilities of power electronic components. Devices for high power levels have been made available in converters for high and even highest voltage levels. The overall starting points are network elements influencing the reactive power or the impedance of a part of the power system. Fig 1.2 shows a number of basic devices separated into the conventional ones and the facts-devices.

For the facts side the taxonomy in terms of 'dynamic' and 'static' needs some explanation. The term 'dynamic' is used to express the fast controllability of facts-devices provided by the power electronics. This is one of the main differentiation factors from the conventional devices. The term 'static' means that the devices have no moving parts like mechanical switches to perform the dynamic controllability. Therefore most of the facts-devices can equally be static and dynamic.

Fig 1.2 Overview of major FACTS-Devices

The left column in fig 1.2 contains the conventional devices build out of fixed or mechanically switch able components like resistance, inductance or capacitance together with transformers. The facts-devices contain these elements as well but use additional power electronic valves or converters to switch the elements in smaller steps or with switching patterns within a cycle of the alternating current. The left column of facts-devices uses thyristor valves or converters. These valves or converters are well known since several years. They have low losses because of their low switching frequency of once a cycle in the converters or the usage of the thyristors to simply bridge impedances in the valves.

The right column of facts-devices contains more advanced technology of voltage source converters based today mainly on insulated gate bipolar transistors (IGBT) or insulated gate commutated thyristors (IGCT). Voltage source converters provide a free controllable voltage in magnitude and phase due to a pulse width modulation of the IGBT’s or IGCTS. High modulation frequencies allow to get low harmonics in the output signal and even to compensate disturbances coming from the network. The disadvantage is that with an increasing switching frequency, the losses are increasing as well. Therefore special designs of the converters are required to compensate this.

1.4 TYPES OF FACTS CONTROLLERS:

In general, FACTS Controllers can be divided into four categories:

1. Series controllers

2. Shunt controllers

3. Combined series-series controllers

4. Combined series-shunt controllers

1.4.1 SERIES CONTROLLERS:

A series controller could be variable impedance such as capacitor reactor or power electronics based variable source of main frequency, sub synchronous and harmonic frequencies (or a combination) to serve the desired need. In principle, all series controllers inject voltage in series with the line. Even variable impedance multiplied by the current flow through it, represents an injected series voltage in the line.

Different types available are as follows:

Static Synchronous Series Compensator (SSSC).

Interline Power Flow Controller (IPFC).

Thyristor- Controlled Series Reactor (TCSR).

Thyristor-Switched Series Reactor (TSSR).

Thyristor-Controlled Series Capacitor (TCSC).

Thyristor-Switched Series Capacitor (TSSC)

SHUNT CONTROLLERS:

In principle all shunt Controllers inject current into the system at the point of connection. Even variable shunt impedance connected to the line voltage causes a variable current flow and hence represents injection of current into the line. Different types available are listed below.

Static Synchronous Compensator (STATCOM).

Static Synchronous Generator (SSG).

Static Var Compensator (SVC).

Thyristor Controlled Reactor (TCR).

Thyristor Switched Reactor (TSR).

Thyristor Switched Capacitor (TSC).

Static Var Generator (SVG).

COMBINED SERIES-SERIES CONTROLLERS:

This could be a combination of separate series controllers, which are controlled in a coordinated manner in a multiline transmission system. The real power transfer ability of the unified series-series controller , referred to as interline power flow controller, makes it possible to balance both the real and reactive power flow in the lines thereby maximize the utilization of the transmission system.

Interline Power Flow Controller(IPFC)

COMBINED SHUNT-SERIES CONTROLLER:

This could be a combination of separate series and shunt controllers, which are controlled in a coordinated manner or UPFC with the series and shunt elements. In principle, combined shunt and series controllers inject current into the system with the shunt part of the controller and voltage in series in the line with the series part of the controller. When series and shunt controllers are unified, there can be a real power exchange between the series and shunt controllers via the power link. Different types are listed below.

Unified Power Flow Controller (UPFC).

Thyristor-Controlled Phase Shifting Transformer (TCPST).

1.5 TYPICAL FACTS DEVICES AND THEIR FUNCTIONS:

In these four typical FACTS devices are considered in detail: TCSC (Thyristor Controlled Series Capacitor), TCPST (Thyristor Controlled Phase Shifting Transformer), UPFC (Unified Power Flow Controller) and SVC (Static Var Compensator).

Fig.1.3: Functional diagrams of facts devices

TCSC is a typical series FACTS device that is used to vary the reactance of the transmission line. Since TCSC works through the transmission system directly, it is much more effective than the shunt FACTS devices in the application of power flow control and power system oscillation damping control. The UPFC is the most powerful and versatile FACTS device due to the facts that the line impedance, terminal voltages, and the voltage angles can be controlled by it as well. Similar to the UPFC, TCPST is also a typical combined series-shunt FACTS device, which can be used to regulate the phase angle difference between the two terminal voltages. SVC is a shunt FACTS device that can be used to control the reactive compensation.

1.6 OBJECTIVE OF THESIS

The main objective of the thesis is Automatic Generation Control (AGC) is to maintain the balance between power output of the electrical generator and load demand. It is response to the changes in the system and tie-line loading. This function is normally termed as load frequency control (LFC).

LITERATURE SURVEY

Tripathy S C and Balasubramanian [12] are explains to the improvement in automatic generation control (AGC) with the addition of a small-capacity superconducting magnetic energy storage (SMES) unit is studied. Time-domain simulations were used to study the performance of the power system and control logic. Optimization of gain parameters and the stability studies were carried out by the second method of Lyapunov. Suitable methods for the control of SMES units are described.

Rajesh Joseph Abraham, D Das, Amit Patra[10] is explains to the . Comparison of the dynamic responses demonstrates the effectiveness and efficiency of the suggested SMES–TCPS combination in suppressing and stabilizing the oscillations of frequency and tie-power deviations as well as in reducing the settling time. 

R J Abraham, D Das, A Patra[9] is explains to gain settings of the integral controllers with and without considering TCPS are optimized using integral squared error technique following a step load disturbance in each of the areas by minimizing a quadratic performance index. Analysis reveals that a TCPS is quite capable of suppressing the frequency and tie-power oscillations effectively under the occurrence of sudden load changes in any of the areas when compared with that obtained without TCPS.

Miniesy, S.M., and Bohn, E.V A two-level controller for interconnected power plants is discussed. Each plant has a first level local optimal or suboptimal controller. The second level of control is an intervention control performed by a central coordinator. If a sudden system disturbance causes the system angular acceleration to exceed preset tolerances a priority interrupt to the central coordinator

1.8 ORGANIZATION OF THESIS

The complete project thesis is divided into six chapters as follows:

In Chapter 1, it provides the introduction to the thesis, defines its objective and the FACTS Controllers and the types of FACTS Controllers of the thesis.

In Chapter 2, it gives brief description about the literature survey and Automatic Generation Control (AGC) and Super Conducting Magnetic Energy Storage (SMES) is discussed details.

In Chapter 3,It gives brief description of the proposed concept and the TCPS and SMES ,system investigated is discussed details.

In Chapter 4, it describes overall simulation model with and without TCPS, SMES Controllers by using MATLAB/SIMULINK model.

In Chapter 5, the conclusion of the project and scope for the future work are presented.

CHAPTER2

AUTOMATIC GENERATION CONTROL

2.1 INTRODUCTION

The system will be in equilibrium, when there is a balance between the power demand and the power generated. As the power in AC form has real and reactive components: the real power balance; as well as the reactive power balance is to be achieved. There are two basic control mechanisms used to achieve reactive power balance (acceptable voltage profile) and real power balance (acceptable frequency values). The former is called the automatic voltage regulator (AVR) and the latter is called the automatic load frequency control (ALFC) or automatic generation control (AGC).

2.1.1 Generator Voltage Control System

The voltage of the generator is proportional to the speed and excitation (flux) of the generator. The speed being constant, the excitation is used to control the voltage. Therefore, the voltage control system is also called as excitation control system or automatic voltage regulator (AVR).For the alternators, the excitation is provided by a device (another machine or a static device) called exciter. For a large alternator the exciter may be required to supply a field current of as large as 6500A at 500V and hence the exciter is a fairly large machine. Depending on the way the dc supply is given to the field winding of the alternator (which is on the rotor), the exciters are classified as:

DC Exciters;

ii) AC Exciters; and

iii) Static Exciters.

Accordingly, several standard block diagrams are developed by the IEEE working group to represent the excitation system. A schematic of an excitation control system is shown in Fig2.1.

Fig2.1: A schematic of Excitation (Voltage) Control System.

A simplified block diagram of the generator voltage control system is shown in Fig2.2. The generator terminal voltage is compared with a voltage reference to obtain a voltage error signal ∆V. This signal is applied to the voltage regulator shown as a block with transfer function. The output of the regulator is then applied to exciter shown with a block of transfer function. The output of the exciter is then applied to the field winding which adjusts the generator terminal voltage. The generator field can be represented by a block with a transfer function. The total transfer function is

Where,

The stabilizing compensator shown in the diagram is used to improve the dynamic response of the exciter. The input to this block is the exciter voltage and the output is a stabilizing feedback signal to reduce the excessive overshoot.

Fig2.2: A simplified block diagram of Voltage (Excitation) Control System.

2.1.2Performance of AVR Loop

The purpose of the AVR loop is to maintain the generator terminal voltage within acceptable values. A static accuracy limit in percentage is specified for the AVR, so that the terminal voltage is maintained within that value. For example, if the accuracy limit is 4%, then the terminal voltage must be maintained within 4% of the base voltage. The performance of the AVR loop is measured by its ability to regulate the terminal voltage of the generator within prescribed static accuracy limit with an acceptable speed of response. Suppose the static accuracy limit is denoted by Ac in percentage with reference to the nominal value. The error voltage is to be less than (Ac/100) ∆|V|ref. From the block diagram, for a steady state error voltage;

For constant input condition, (s→0)

Where, K= G (0) is the open loop gain of the AVR. Hence,

-1}

Automatic Load Frequency Control

The ALFC is to control the frequency deviation by maintaining the real power balance in the system. The main functions of the ALFC are:

To maintain the steady frequency;

Control the tie-line flows; and

Distribute the load among the participating generating units. The control (input) signals are the tie-line deviation (measured from the tie line flows), and the frequency deviation ∆f (obtained by measuring the angle deviation ∆δ). These error signals ∆f and ∆P tie are amplified, mixed and transformed to a real power signal, which then controls the valve position. Depending on the valve position, the turbine (prime mover) changes its output power to establish the real power balance. The complete control schematic is shown in Fig2.3

Fig2.3: The Schematic representation of ALFC system

For the analysis, the models for each of the blocks in Fig2.3 are required. The generator and the electrical load constitute the power system. The valve and the hydraulic amplifier represent the speed governing system. Using the swing equation, the generator can be modeled by .

Expressing the speed deviation input, .This relation can be represented as shown in Fig2.4.

Fig2.4.The block diagram representation of the Generator

The load on the system is composite consisting of a frequency independent component and a frequency dependent component. The load can be written as where, is the change in the load; is the frequency independent load component; is the frequency dependent load component. = D∆ω where, D is called frequency characteristic of the load (also called as damping constant) expressed in percent change in load for 1% change in frequency. If D=1.5%, then a 1% change in frequency causes 1.5% change in load. The combined generator and the load (constituting the power system) can then be represented as shown in Fig2.5

Fig2.5.The block diagram representation of the Generator and load

The turbine can be modeled as a first order lag as shown in the Fig2.6

Fig2.6.The turbine model.

is the TF of the turbine; is the change in valve output (due to action). is the change in the turbine output the governor can similarly modeled as shown in Fig2.7. The output of the governor is by where ∆Pref is the reference set power, and ∆ω/R is the power given by governor speed characteristic. The hydraulic amplifier transforms this signal ∆Pg into valve/gate position corresponding to a power. Thus

∆PV(s) = (Kg/(1+sTg))∆Pg(s).

Fig2.7: The block diagram representation of the Governor

All the individual blocks can now be connected to represent the complete ALFC loop as shown in Fig2.8

Fig2.8: The block diagram representation of the ALFC

2.3 Steady State Performance of the ALFC Loop

In the steady state, the ALFC is in ‘open’ state, and the output is obtained by substituting s→0 in the TF. With s→0, and, become unity, then, (note that = = = = ; that is turbine output = generator/electrical output = load demand)

When the generator is connected to infinite bus (∆f = 0, and ∆V = 0), then = . If the network is finite, for a fixed speed changer setting ( = 0), then

If the frequency dependent load is present, then

If there are more than one generator present in the system, then where,

The quantity β= (D + 1/) is called the area frequency (bias) characteristic (response) or simply the stiffness of the system.

2.4 Concept of AGC (Supplementary ALFC Loop)

The ALFC loop shown in Fig 2.8, is called the primary ALFC loop. It achieves the primary goal of real power balance by adjusting the turbine output ∆Pm to match the change in load demand ∆PD. All the participating generating units contribute to the change in generation. But a change in load results in a steady state frequency deviation ∆f. The restoration of the frequency to the nominal value requires an additional control loop called the supplementary loop. This objective is met by using integral controller which makes the frequency deviation zero. The ALFC with the supplementary loop is generally called the AGC. The block diagram of an AGC is shown in Fig2.9. The main objectives of AGC are i) to regulate the frequency (using both primary and supplementary controls); ii) and to maintain the scheduled tie-line flows. A secondary objective of the AGC is to distribute the required change in generation among the connected generating units economically (to obtain least operating costs).

Fig2.9: The block diagram representation of the AGC

2.5 AGC in a Single Area System

In a single area system, there is no tie-line schedule to be maintained. Thus the function of the AGC is only to bring the frequency to the nominal value. This will be achieved using the supplementary loop (as shown in Fig.2.9) which uses the integral controller to change the reference power setting so as to change the speed set point. The integral controller gain KI needs to be adjusted for satisfactory response (in terms of overshoot, settling time) of the system. Although each generator will be having a separate speed governor, all the generators in the control area are replaced by a single equivalent generator, and the ALFC for the area corresponds to this equivalent generator.

2.6 AGC in a Multi Area System

In an interconnected (multi area) system, there will be one ALFC loop for each control area (located at the ECC of that area). They are combined as shown in Fig 2.10 for the interconnected system operation. For a total change in load of ∆PD, the steady state

Deviation in frequency in the two areas is given by where,

Fig.2.10. AGC for a multi-area operation.

2.7. Superconducting magnetic energy storage

Energy stored in a normal inductor will fade out rather quickly due to the ohmic resistance in the coil when the power supply is disconnected. Obviously this wills not be acceptable energy storage for use in a power system. The ohmic resistance has to be removed before an inductor can work for this purpose. This is possible by lowering the temperature of the conductors, and by this making the conductors superconducting. A superconducting wire is in a state where the resistance in the material is zero. In this state the current in a coil can flow for infinite time. This can also be seen from the time constant of a coil τ = LR where R goes to zero and τ then goes to infinity.

There are constraints for a superconducting wire to stay superconducting. The conductor has to be operated below a critical temperature, below a critical current and below a critical magnetic field. There should also be some safety margin between the critical values and the operating conditions. There are several types of superconducting material. They can be divided into two groups. High Temperature Superconductors (HTS) and Low Temperature superconductors (LTS). The HTS types are cooled to 77 K using liquid nitrogen. A LTS is generally cooled using liquid helium to 4.2 K. There are advantages and drawbacks attended with both the technologies.

SMES has a rather poor energy storage capacity compared to pumped hydro or Compressed Air Energy Storage (CAES). There are conceptual design studies for large scale SMES systems which can operate in diurnal power compensation having the ability to store large amounts of energy. This makes use of several thousand coils to store the energy. However this is only a study case and a real system is not likely to be constructed in the near future. While the energy density of a SMES system is low, one of the main properties of SMES is the ability to deliver large amounts of energy in a very short time, or said in another way, deliver high power. Combined with very short response time, this makes SMES one of the most suitable energy storage solutions to compensate for fast power fluctuations. Summed up the features of SMES are the following:

• Capability of absorbing and delivering large amounts of power.

• High efficiency.

• Long lifetime.

• Short response time.

• Completely static construction, low maintenance.

• All electric energy storage

2.8 The System Topology

The system which is studied in this thesis is shown in Fig. 2.11. The components in the system are a wind turbine, a gear b ox, an induction generator, two transformers and a connection to a main grid and the SMES and converter system. The power in the system is generated by the wind turbine. This is connected through an ideal gearbox to an induction generator. The gearbox has a transmission ratio of 1/100. The wind turbine is on the slow rotating side. The wind turbine is based on the one used in . This has a rated wind speed of 12 m/s and rated power of 2 MW. The wind turbine has no pitch control which makes the power output completely dependent on the wind speed. This may seem unrealistic, but the scope of this thesis is studying the compensation of power fluctuations due to wind speed variations using SMES. A slow pitch control would not contribute to this smoothing. The induction generator is of the normal squirrel cage type. It has a rated power of 2 MW. The generator is connected to the grid through a transformer (T1) having a ratio of 690/1100 V. The main grid is connected to magnetize the induction generator, and supply the constant frequency and stiff voltage. The main grid is also the part of the system which it is desirable to control the power flow to. The converter is connected in shunt with the grid through a transformer (T2). The ratio of this is 1100/1100 V. The purpose of it is mainly to act as a filter. The converter is of current source converter type as distinct from the converters in the thesis thesis which were voltage source converter and a DC-DC chopper. All the lines in the system are modeled as ideal, having no impedance. The capacitances at the input of the converter acts as a filter together with T2. The system frequency is 50 Hz.

Fig 2.11: Single line diagram of the system being studied.

2.8.1 per unit representation of the system

Per unit (Pu) system analysis generally makes the calculations a lot easier. Especially when calculations are made in a system where there are several voltage levels. If the reference values are selected properly, transformation to a pu-system removes the transformers from the calculations. All of the different values of voltage, current, impedance and power will normally be in the interval 0 – 1.0. This makes the comparison between different voltage levels easier than if the physical values were used. It is easy to distinct between normal state and fault state. There are different strategies for deciding the base values for voltage. In power system analysis the RMS values of the phase to ground voltage or line to line voltage is normally set as voltage.
However in this thesis the base for the voltage is chosen as the peak value of the phase to ground voltage.

2.8.2 The converter

The ideal converter topology used to connect the SMES system to the rest of the network will have no harmonic distortion, no usage of reactive power and no losses. These requirements are of course not possible to fulfill, and the choice of topology will be a compromise between the different aspects. A line commutated converter using thyristors has low on-state losses and the thyristor devices can cope with large amounts of power. The disadvantages are lagging power factor and high low order harmonics pollution. Neither does a thyristor converter provide the same degree of control as a self-commutated converter. The requirement of a present strong grid is also a drawback considering line-commutated converters. Because of these disadvantages a self-commutated converter is chosen. When choosing the converter type among self-commutating types there are mainly two different types to choose from. They are the voltage source converter type (VSC) and the Current Source Converter type (CSC. In a VSC the DC-voltage will always have the same polarity. Bidirectional power flow of the converter is achieved by reversing the DC-current polarity. In a CSC the DC-current will always flow in the same direction and the bidirectional power flow is achieved by reversing the DC voltage polarity. The VSC has a big capacitor on the DC-side. This is sized to be large enough to sustain the voltage in the DC-link on a constant level for the expected operating conditions. Because the current flow can be bidirectional, the so called converter valves also have this feature. As Fig. 2.12a shows this is normally solved having a diode connected in anti-parallel with the switching device which very often is an IGBT. Because the DC-voltage never switches polarity, there is no need for reverse blocking capability. A CSC will need a blocking diode in series connection with the switching device in absence of reverse blocking capabilities in a normal IGBT. The CSC and VSC topology have quite different properties, and because of that they have different advantages and disadvantages. The VSC has the advantage of having lower harmonic pollution. The CSC needs capacitors on

(a) Voltage source converter.

(b) Current source converter.

Fig 2.12

The AC-side to filter out harmonics. The reason for this is the rapid changing current pulses which are created from the switching of the continuous DC current. The capacitors provide a stiff bus interfacing the converter. These filter capacitors are expensive and bulky and are clearly a disadvantage compared to the VSC. A drawback with the VSC is that the switches are more vulnerable to high short circuit currents. If two switches on the same leg are on at the same time the DC-voltage will be short circuited and a large current will flow. This can cause failure of the converter. In a CSC the short circuit current is limited by the coil. In fact a short circuit of one leg normally happens several times during one cycle. Another drawback regarding the VSC is the capacitor on the DC-side. This has limited lifetime compared to the inductor in a CSC. Despite these drawbacks the VSC configuration is by far the preferred over the CSC. The reasons have normally been economics and performance.

In this master thesis the purpose is to study a CSC and its feasibility in SMES applications. The converter which has been used is the CSC shown in Fig. 2.12a. This converter is different from the converter used in the thesis. The fact that the superconducting coil is a current source requires another converter in addition to the VSC, a DC-chopper. This is placed on the DC-side to directly manage the current in the coil. The process of feed in and extraction of energy is quite different in the two topologies. In a CSC the energy management is directly to and from the coil as there is only one converter interfacing the power system. In the VSC/DC-chopper topology the energy management in the SMES is a two-step procedure. If extracting energy from the SMES, the energy is as a first step withdrawn from the DC-link capacitor. The capacitor is then charged again by the superconducting coil. If feeding energy into the SMES, the capacitor is first charged and then it charges the superconducting coil. Compared to a CSC solution the VSC will as a consequence of this have faster response because of the DC-link capacitor which can deliver large current.

The CSC linking the system and the superconducting coil.

The VSC and DC-DC chopper linking the system and the superconducting coil in the thesis.

Fig 2.13: The two different converter topologies to connect the superconducting coil to the grid.

Rents quicker than an inductor as this does not allow instantaneous change of current. The larger inductance, the slower response time is present. A disadvantage regarding the capacitor is as mentioned its limited lifetime. This ad to the running expenses of the converter. In a CSC topology the DC-DC chopper is superfluous. Because of this the control system can be made simpler than for the VSC solution. A drawback of the conventional SC is the requirement of blocking diodes. This increases the amount of semiconductors the current has to flow through, and thereby increase the conduction losses. A reverse blocking IGBT would help decreasing these losses as this does not require a blocking diode. On the other hand the VSC/DC-chopper topology has fewer semiconductor devices conducting in the VSC part, but it also has semiconductors in the DC-chopper which contribute to losses. The number of semiconductors the current flows through is actually higher for the VSC/DC-chopper solution than for the CSC. The reason for this is that three half-legs are on compared to two in the CSC. Together with the devices in the chopper this equals five devices to pass compared to four in the CSC. However two of the half-legs in the VSC will share the current making the load on each unit smaller.

2.8.3 Sizing of the converter

The two constraining factors on the switching units are applied voltage and the current in the SMES coil. The voltage applied gives the rate of change of the current according to the following equation:

From this follows that the SMES will be able to charge and discharge faster the higher the applied voltage is. This also implies that the power in the SMES will be higher. The rated turbine power in the system is as mentioned 2 MW. To cope with power oscillations from the turbine the SMES system should be able to cover fluctuations in the megawatt range. The purpose of the system in this thesis is to cover for fast power fluctuations, not for diurnal variations. On basis of this the rated power of the converter is selected to be 1.5 MW. As is well known this is equal to 1.5 MJ/s. To store 1.5 MJ the current flowing, can be calculated using:

The inductance of the superconducting coil is 1.8 H, a value found to be suitable in the thesis. The rated current is then 1290 A. This will be the maximum operating current and the switches must be rated to cope with this current. Reverse blocking diodes will as mentioned decrease the losses. However these IGBTs are not rated for high power usage.

ABB has in a document given guidelines to choose the voltage ratings of high power semiconductors. These are taken into account when devices have been chosen here. It is important that there are safety factors compared to the operating voltage. On the other hand the semiconducting devices should not be selected having to high ratings as this would increase the losses. The CSC is subjected to AC-voltage, and it is the peak of this which is interesting. The peak of the AC-voltage is calculated and a certain safety margin, x is added:

2.8.4 Losses

These loss considerations are based on the theory. In an ideal semiconductor the current conduction capabilities are infinite at the same time as the voltage drop is zero. The transition from OFF-state to ON-state happens instantaneously. However these characteristics are not present in the real world, which means there are power losses involved when operating semiconductor devices and the losses in a semiconductor can be divided into two categories. Conduction losses and switching losses.

Fig 2.14: The power dissipation in a semiconductor.

Because the device has a small on-state voltage. The latter arise because the current does not rise in an instant when the device is turned on, nor does the voltage fall immediately. When switching off, the current does not fall instantaneously and the voltage does not rise in an instant either. Fig. 2.14 shows the principal of losses in a semiconductor which is first switched on, then conducts for a time and then is switched off. is the voltage which the device blocks, Io is the current conducted when fully on, Von is the on state voltage. There are some exaggerations in the fig to make the points clearer. The magnitude of,which in reality is in the order of a few volts is exaggerated compared with. The length of ton is on the other hand
shortened compared to the length of (on) and (off). The coloured areas correspond to the energies lost during one cycle.

fr

2.8.5 Switching frequency selection

The converter will operate on a fixed switching frequency. The value of this will influence on the switching losses in the converter and the harmonic pollution. The higher switching frequency, the higher switching losses, but lower harmonic content. The selection is therefore a compromise between these two factors. The maximum switching frequency is dependent on the minimum pulse width and the temperature on the junction. It is used when considering this. The maximum theoretical switching frequency is constrained by the turn on and turn-off switching times and the rise and fall times of the current. This time added up is an estimate of the total switching time, and should not exceed 5 % of the switching period. The intention of this is just that there has to be time for current conduction during a switching period in addition to “lost” time in the switching moments.

2.9 Control system

The objective of the control system is to smooth out the power flow from the generator to the power system. Ideally the combined power output from generator and SMES will be constant. The control unit to maintain this is rather simple and contains only one PI-regulator see Fig. 2.14. The input to this regulator is the difference between the reference power and the measured power flow to the grid. The reference power is a constant value, set in advance. The PI-regulator is tuned using trial and error, and the output is forwarded into a block which uses so called ABC theory to calculate current references. This theory and its employment in the system will be further explained in the next section. The parameters of the PI-regulator are found
to be as following:

Fig 2.15: The PI power controller.

2.10 ABC theory

The ABC theory in this system, also called instantaneous ABC theory is used to construct the reference currents for the modulation. The ABC theory is generally used to compensate for reactive power. However in this system it is used for compensating active power. The concept of the theory will first be explained. Then its implementation in this system will be explained. The idea of the ABC theory is to determine the active part of a total load current which has both active and reactive components, or active and non-active current as the terminology. The goal is to deliver the same amount of energy from the source to the load, without having to transport reactive power on the lines. The method of doing this is to establish minimized, instantaneous active current components which fulfill the energy
constrain mentioned. The difference between this new minimized active current and the uncompensated load current is the non-active current which now comes from a type of compensator, as seen in Fig. 2.16. The determination of the instantaneous active current is carried out using e.g. the Lagrange Multiplier Method. The objective of this method is to find the extreme values of a function whose domain is constrained to lie within some particular boundaries. The active,, and non-active, , currents of any given load current, are given by the following equation:

(a) Uncompensated currents.

Compensated currents.

Fig 2.16: The concept of active and non-active currents.

The goal is to minimize the load current, and the constraint is that the non-active current components, do not generate any three phase instantaneous active power.

(a) Positive power fluctuation.

(b) Negative power fluctuation.

Fig 2.17: The different flow directions of current into the SMES.

So how can this theory are to be applied to the SMES power conditioner the idea is that the current from the induction generator also consists of an active and non-active part. These do not correspond to active and reactive power, but reference power and power fluctuations. The reference power is the power generation from the generator set according to predictions of wind speed, and the power fluctuations arise according to wind speed fluctuations around the reference wind speed. The active part of the current gives the reference power, and the non-active gives the power fluctuations.

The meaning of this is that the SMES will supply the non-active current. If the power fluctuation is positive the sum of the active and non-active current components will be larger than the reference current, and the opposite if the power fluctuation is negative. Vital detail is that the non-active current can flow in both directions. What distinguish this from the conventional ABC theory is of course that it is used for compensating active power. However reactive power fluctuations will also be included in this compensation as it is part of both the reference current and the fluctuating currents. This is a drawback of this control system as it is not possible to compensate for active and reactive power separately. Another control strategy could be to implement the p q-theory, which enables this feature.

2.11 SUMMARY

In Chapter 2, it gives brief description about the literature survey and Automatic Generation Control (AGC) and Super Conducting Magnetic Energy Storage (SMES) is discussed details.

CHAPTER3

PROPOSED CONCEPT

3.1 INTRODUCTION

The ultimate objective of automatic generation control (AGC) is to maintain the balance between power output of the electrical generator and load demand so as to keep the frequency within the acceptable limits, in response to the changes in the system and tie-line loading. This function is normally termed as load frequency control (LFC) [1]. The power systems are widely interconnected for its applicability all over the globe. Interconnection not only enhances system reliability but also improves the system efficiency. Since the system is wide and complex, for the faithful operation, the analysis of the system is of greater importance. Currently system became too complex with addition of more utilities, which may leads to a condition where supply and demand has got a wide gap [2]. Due to heavy load condition in tie-lines by electric power exchange results in poor damping which may leads to inter-area oscillation.

Since the loading conditions are unpredictable, this makes the operation more complex. It has been a topic of concern, right from the beginning of interconnected power system operation. In this context, Automatic Generation Control plays a vital role in the power system operation. Several works have been carried out for the AGC of interconnected power systems for last few decades [3]-[6]. Earlier works in this field proposed many ideas to enhance to system stability when there is sudden drift in the demand. However thermal power plants has got its own associated operational constraints, most of the proposed solutions so far for AGC have not been implemented [7]. But a few efforts were made to attenuate the oscillations in system frequency and tie-line power interchange.

The use of power electronic devices for power system control has been widely accepted in the form of flexible AC transmission system (FACTS) devices which provide more flexibility in power system operation and control [1]. This extra flexibility permits the independent adjustment of certain system variables such as power flows, which are not normally controllable [7]. Thyristor-controlled phase shifter (TCPS) is a device that allows dispatchers to change the relative phase angle between two system voltages, thereby helping them to control real power transfers between the two interconnected power systems. It attenuates the frequency of oscillations of power flow following a load disturbance in either of the areas, as well. Phase shifters also provide series compensation to augment stability. The high-speed responses of phase shifters make them attractive for use in improving stability. A TCPS is expected to be an effective apparatus for the tie-line power flow control of an interconnected power system.

Usually sudden changes in power requirement are met by kinetic energy of generator rotor, which effectively damp electromechanical oscillations in power system [2]. Use of fast acting storage devices in the system also improves the transient performance by supplying stored energy after the sudden load perturbation. [8] Has proposed a control strategy for TCPS to providing active control of system frequency and thereby to damp the system frequency and tie-power oscillations by controlling the phase angle of TCPS. Authors of [10] have made an attempt to improve the transient performance analysis in hydro-hydro system with SMES, TCPS controllers. This work gives an insight into application of FACT devices especially series connected, to damp out inter area oscillations. A maiden attempt to use energy storage to enhance the system performance appeared in [10], also considered an interconnected hydro-thermal system with capacitive storage devices and TCPS to stabilize low frequency oscillation so as to improve the transient performance of the system.

With advent of FACTs devices and energy storage devices many research work are made to damp out the tie line oscillation with some of them like TCPS, SSSC etc. Literature survey reveals that the most of the work in relation with application of FACTS devices and storage systems in AGC problems were considered separately in frequency control and tie-line power control [10-12]. But there are less work devoted to coordinated action of FACTS devices and storage systems. Thus this thesis deals with an attempt to connect thermal-thermal interconnected system with Thyristor Controlled Phase Shifter (TCPS) in tie-line and in coordination with Superconducting Magnetic Energy Storage (SMES) system. The effect of TCPS and SMES in coordinated with two area thermal system were investigated and controllers were designed.

3.2 THYRISTOR CONTROLLED PHASE SHIFTER (TCPS)

Fig.3.1.Schematic diagram of two area system with TCPS & SMES

TCPS is a device that changes the relative phase angle between the system voltages. Therefore the real power flow can be regulated to mitigate the frequency oscillations and enhance power system stability [7]. In this study, a two-area multi-unit thermal power system interconnected by a tie-line is considered. Fig. 3.1 shows the schematic representation of the two-area interconnected power system considering a TCPS in series with the tie-line. TCPS is placed near Area 1. Practically, in an interconnected power system, the reactance to-resistance ratio of a tie-line is quite high and the effect of resistance on the dynamic performance is not that significant. Because of this, the resistance of the tie-line is neglected. 3.3Superconducting Magnetic Energy Storage (SMES)

The SMES unit contains a DC superconducting coil and a 12-pulse converter, which are connected to grid through a Y-Δ/Y-Y transformer. The superconducting coil can be charged to a set value from the utility grid during steady state operation of the power system. The DC magnetic coil is connected to grid via inverter/ rectifier arrangement. The charged superconducting coil conducts current which is immersed in a tank containing helium. The energy exchange between the superconducting coil and the electric power system is controlled by a line commutated converter. When there is a sudden rise in the load demand, the stored energy is almost released through the converter to the power system as alternating current. As the governor and other control mechanisms start working to set the power system to the new equilibrium condition, the coil current changes back to its initial value and are similar for sudden release of load [12].

3.4 SYSTEM INVESTIGATED

Fig.3.2 Linearized model of an interconnected thermal-thermal system

Fig.3.2 shows linearized model of an interconnected power system with AGC comprises two control areas. The two areas are connected through a tie-line which allows the power exchange between the control areas. Area 1 consists of two reheat thermal power generation units and Area 2 comprises two non-reheat thermal generation units. The frequency in the power system is being maintained by controlling the driving torques of the thermal turbine. The reheat turbine gives a fast response component due to the High Pressure (HP) stage and a much slower Low Pressure (LP) due to reheat delay. A Generation Rate Constraint (GRC) of 10 % p.u. MW/min and 3% p.u.MW/min for non-reheat and reheat thermal systems respectively[9, 13]. GRCs are taken into account since the rapid power increase would draw out excessive steam from boiler system to cause steam condensation due to adiabatic expansion. KI1 and KI2 are the integral gain settings in area 1 and area 2 respectively.

3.4.1. State space representation

The power system model considered being a linear continuous-time dynamic system can be represented by the standard state space model. The standard state space form of the system can be expressed as

(3.1)

Where x, u and p are the states, control input, and load disturbance input vectors and A, B and ߬are the respective matrices of appropriate dimensions associated with them. The dynamic state variables are chosen from the linearized power system model as shown in Fig. 3.2.

3.4.2 Mathematical problem formulation

The automatic generation control is incorporated in the interconnected power system is to meet the frequency regulation, i.e. to restore the frequency to its scheduled value as quickly as possible and minimize the oscillations in the tie line power flow between the control areas. To the above requirements, integral controller gains of the control areas (KI1and KI2) are optimized to have better dynamic response. Integral Squared Error (ISE) technique is used for formulating the objective function to obtain the optimum integral gain settings.

The quadratic performance index defined by

3.2

Where, is the incremental change in frequency, is the incremental change in tie-line power. The objective function is minimized for 1% step load disturbance in either of the areas in the presence of GRCs. The ISE criterion is used because it weighs large errors heavily and small errors lightly.

3.4 SUMMARY:

In this chapter 3 is briefly explains to the proposed concept to the introduction, TCPS and SMES, system investigated is discussed details.

CHAPTER 4

MATLAB/SIMULATION RESULTS

There are three cases:

Without TCPS and SMES Controllers

Without TCPS and SMES Controller in area2

4.2 With TCPS and SMES Controllers

4.1 Without TCPS and SMES Controllers:

Fig.4.1 without TCPS and SMES Controllers

Fig.4.1.1 Deviation of Frequency in area1

Fig.4.1.2 Deviation of frequency in area2

Fig.4.1.3 Change in power output of generator in area-1

Fig.4.1.4 Change in power output of generator in area-2

Fig4.1.5 Tie line power oscillations

Without TCPS and SMES Controller in area 2 :

Fig.4.2.1: Without TCPS and SMES Controllers

Fig4.2.2: Frequency Deviation in area 1

Fig4.2.3: Frequency deviation in area 2

Fig4.2.4: Waveform of Deviation in tie-line Power Flow

Fig4.2.5: Power output in frequency deviation in area 1

Fig4.2.6: Power output in frequency deviation in area 2

Fig4.2.7: Power output in frequency deviation in load change in area1

Fig4.2.8: Power output in frequency deviation in load change in area2

4.3 With TCPS and SMES Controllers:

Fig.4.3 with TCPS and SMES Controllers

Fig4.3.1 Block Diagram of TCPS

Fig4.3.2 Block diagram of SMES

Fig4.3.3 Waveform of Deviation in Frequency of Area-1

Fig4.3.4 Waveform of Deviation in Frequency of Area-2

Fig4.3.5 Waveform Of Deviation In tie-line Power Flow

Fig4.3.6 Power output in frequency deviation in area 1

Fig4.3.7 Power output in frequency deviation in area2

Fig4.3.8 Power output in area1

Fig4.3.9 Power output in area2

4.3 SUMMARY:

In this chapter 4 is to explain Simulation results with and without TCPS and SMES controllers is explained.

CHAPTER5

CONCLUSION

In this thesis, a coordinated control of SMES and TCPS has been proposed for a two-area multi-unit interconnected thermal power system. Gain settings of the integral controllers without and with SMES-TCPS combinations are optimized using ISE technique in the presence of GRCs by minimizing a quadratic performance index. A control strategy has been proposed to control the SMES-TCPS combination which in turn controls the frequency deviation in the control areas and inter-area tie-line power flow. From the simulation studies it is revealed that the SMES-TCPS combination can effectively control phase angle (TCPS action) and supply or absorb power when there is change in system power levels (SMES action). Thus SMES-TCPS combination will in turn suppress the oscillations in the area frequencies and tie-power following sudden load disturbance.

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Elgerd, O.I., and Fosha, C.: Optimum megawatt frequency control of multi-area electric energy systems, IEEE Trans. Power Appar. Syst., , pp. 556-563,1970.

Cohn, N.: Techniques for improving the control of bulk power transfers on interconnected systems, IEEE Trans. Power Appar. Syst., pp. 2409- 2419,1971.

Miniesy, S.M., and Bohn, E.V.: Two level control of interconnected power plants, IEEE Trans. Power Appar. Syst., , pp. 2742 -2748, 1971, 90, (6)

Hiyama, T.: Design of decentralized load frequency regulators for interconnected power systems, IEE Proc., Gener. Transm. Distrib. 1982.

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