A Review on Mid-point Compensation of a Two-machine System Using STATCOM

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1 Volume-4, Issue-2, April-2014, ISSN No.: International Journal of Engineering and Management Research Available at: Page Number: A Review on Mid-point Compensation of a Two-machine System Using STATCOM Gaurav Tembhurnikar 1, Ajit Chaudhari 2, Nilesh Wani 3, Atul Gajare 4, Pankaj Gajare 5 1 PG Student, Department of Electrical Engineering, INDIA 2 Associate Professor, Department of Electrical Engineering, INDIA 3,4,5 Assistant Professor, Department of Electrical Engineering, INDIA ABSTRACT Regarding the power system large wind farms are greatly affected stability and control issues. So it requires a deep study to overcome this potential problems and it requires advanced control and compensating devices to avoid & recover large disturbances this paper involve the use of Static Synchronous Compensator (STATCOM) for stabilizing the grid voltage after grid-side disturbances such as a three phase short circuit fault, temporary trip of a wind turbine and sudden load changes. These will help to maintain and regulate the proper voltage. The DC voltage at individual wind turbine (WT) inverters is also stabilized to facilitate continuous operation of wind turbines during disturbances. Keywords - STATCOM, Dynamic Compensator, SVC, Transient Stability, V-Q Response. I. INTRODUCTION STATCOM is basically a voltage source converter, VSC that converts a dc voltage at its input terminals into three-phase ac voltages at fundamental frequency of controlled magnitude and phase angle. VSCs use pulse width modulation, PWM, technology, which makes it capable of providing high quality ac output voltage to the grid or even to a passive load (Uzunovic, 2001). STATCOM provides shunt compensation in a similar way as SVC but utilizes a voltage source converter rather shunt capacitors and reactors (Machowski, 1997). The basic principle of operation of a STATCOM is the generation of a controllable AC voltage source behind a transformer leakage reactance by a voltage source converter connected to a DC capacitor. The voltage difference across the reactance produces active and reactive power exchanges between the STATCOM and the power system (Wang and Li, 2000). The effect of stabilizing controls on STATCOM controllers have been investigated also in several recent reporting (Wang and Li, 2000), (Wang, 1999). Figure 1: Line diagram of STATCOM 1.1 Features of STATCOM A STATCOM has both turn-on and turn-off control capability (IGBTs). It generates an output ac voltage from a dc voltage. The ac voltage is controllable both in magnitude and phase angle. Flexible Alternating Current Transmission Systems (FACTS) devices, namely STATIC synchronous Compensator (STATCOM), Static Synchronous Series Compensator (SSSC) and Unified Power Flow Controller (UPFC), are used to control the power flow through an electrical transmission line connecting various generators and loads at its sending and receiving ends. FACTS devices consist of a solid-state voltage source inverter with several Gate Turn off (GTO) thyristor switch-based valves and a DC link capacitor, a magnetic circuit, and a controller. The quality of AC waveforms generated by the FACTS devices depends on the valves and the various configuration magnetic circuits. The inverter configuration used in this paper can be utilized to build a voltage source inverter. 109

2 II. SYSTEM MODELLING 2.1 TWO MACHINE SYSTEM WITH MIDPOINT DYNAMIC COMPENSATOR For the symmetrical system shown in the figure 2, the doubling is shown by the increase in the p/δ curve from a to b in figure 3. Without the compensator, curve a is given by the equation; P = Where E is now the emf behind transient reactance (E ) of the two the generators and X1 is the total series reactance, equal to the sum of the line and transformer reactance (which are assumes identical). The effect of shunt capacitance is ignored. With an ideal compensator that holds the midpoint voltage constant at the value E the power angle curve b is obtained according to the equation; We will compare it with the equal area criterion to match the relative transient stability with and without a dynamic shunt compensator. First consider a case without compensation. Assume that a fault occurs between circuit breaker a and b in figure 2 and is cleared by a and b circuit breakers. Curve-1 in figure 3 shows the pre fault transient power angle curve which has a maximum of Curves shown are during the fault. A curve 3 result after the faulted line section is removed and differs from curve 1 is that X l is replaced by 3. E is assumed constant through the first swing period. If there is no compensator, the system can, in principle, be pre loaded to the transient stability power limit P 1 for the prescribed fault, such that the available decelerating energy A 2 just balances the accelerating energya 1. In practice, the power level would be somewhat less than this, to provide a stability margin. This curve is applicable only if the transient compensator voltage/current characteristics is flat, if the compensator responds instantaneously, and if it has sufficient capacity of current. But actually it is not possible to get this condition. The small positive slope in the power-angle curve from b to c, while the limited capacitive current capability breaks the curve at point D, the compensator behaving as a fixed capacitor at higher load angle, curve d. Figure 2: Two-machine system with midpoint dynamic compensator Figure 3: Figure 5: Equal area method illustrating increased transient stability with dynamic shunt compensation The effect of shunt compensator at midpoint with rapid response is shown in figure 5 Curve 1,2 and 3 of figure 3 are replaced by the higher curves 1, 2 and 3, respectively. During the pre-fault power P 1 and the same fault duration, the decelerating area available is now larger and is only partly used up, leaving a margin as indicated in figure 5. In other language, the transient stability limit is improved, that is, the power transfer can be improved up to the point where all of the final margin is used up. In stability margin due to capacitive current of compensator it decreases in figure 5 to the right of point D. III. BASIC CONFIGURATION OF STATCOM Transient power/angle curves with & without dynamic shunt compensation. 2.2 THEORY OF TRANSIENT STABILITY IMPROVEMENT By using thyristor-controlled reactors (TCR) and thyristor-switched capacitors (TSC), SVC provides voltage regulation and dynamic reactive power for VAR absorption and production respectively. A STATCOM accomplishes the same effect by using a VSC to synthesize a voltage waveform of variable magnitude with respect to the system voltage. The STATCOM branches provides both production and reactive power absorption capability and 110

3 incase of an SVC requires separate branches for each. The STATCOM, with the use of PWM, perform faster response and thereby improves power quality. This is very useful to flicker from disturbances caused by electric arc furnaces at steel mills. To increase the power transfer capability by installing an SVC or STATCOM in transmission networks and it is limited by post-contingency voltage criteria or under voltage loss of load probability. Determining the optimum mix of dynamic and switched compensation is a challenge. Control systems are designed to keep the normal operating point within the middle of the SVC or STATCOM dynamic range. The voltage-sourced converter (VSC) is the basic electronic part of a STATCOM, which converts the dc voltage into a frequency, and phase. There are different methods to realize a voltage sourced converter for power utility application and it is based on harmonics and loss considerations, pulse width modulation (PWM) or multiple converters are used. Inherently, STATCOMs have a symmetrical rating with respect to inductive and capacitive reactive power. For example, the rating can be 100 MVAR inductive and 100 MVAR capacitive. For asymmetric rating, STATCOMs need a complementary reactive power source. Figure 6 shows Static Synchronous Compensator (STATCOM) used for midpoint voltage regulation on a 500-kV transmission line. Figure 6: Schematic Diagram of STATCOM Figure7: Phasor diagram for inductive load operation In the last one decade commercial using of Gate Turn-Off thyristor (GTO) devices with high power handling capability, and the advancement of other types of power-semiconductor devices such as IGBT s have to led the development of controllable reactive power sources utilizing electronic switching converter technology. These technologies additionally offer considerable advantages over the existing ones in terms of space reductions and performance. The GTO thyristor enable the design of solid-state shunt reactive compensation equipment based upon switching converter technology. This concept was used to create a flexible shunt reactive compensation device named Static Synchronous Compensator (STATCOM) due to similar operating characteristics to that of a synchronous compensator but without the mechanical inertia. By using of Flexible AC Transmission Systems (FACTS), it is gives a new family of power electronic equipment emerging for controlling and optimizing the performance of power system, e.g. STATCOM, SSSC and UPFC. The use of voltage-source inverter (VSI) has been widely accepted as the next generation of reactive power controllers of power system to replace the conventional VAR compensation, such as the thyristor-switched capacitor (TSC) and thyristor controlled reactors (TCR). A FACT is the acronym for Flexible AC Transmission Systems and refers to a group of resources used to overcome certain limitations in the static and dynamic transmission capacity of electrical networks. The IEEE defines FACTS as alternating current transmission systems incorporating power-electronics-based and other static controllers to enhance controllability and power transfer capability. Purpose of these systems is to supply the network as quickly as possible with inductive or capacitive reactive power that is adapted to its particular requirements and improving transmission quality and the efficiency of the power transmission system. The inevitable globalization and liberalization of energy markets associated with growing deregulation and privatization are increasingly resulting in bottlenecks, uncontrolled load flows, instabilities, and even power transmission failures. Power supplies are increasingly dependent on distributed power plants with higher voltage levels, a greater exchange within meshed systems, and transport to large load centers over what are often long distances. In future this type of power transmission must be implemented safely and cost effectively. Implementing new transmission systems and components is a long-term strategy for meeting these challenges. For the short and medium term, modern transmission technologies can be employed at comparatively little expense to rectify or minimize bottlenecks and substantially improve the quality of supply. It is possible to postpone investing in new plants and, as a result, to achieve critical advantages over the competition especially important in de-regulated energy markets in which power supply companies are subject to extreme pricing pressure. As a world leader in the power transmission and distribution industry, Siemens has developed a number of modern, flexible, high-capacity FACTS for efficiently and reliably regulating voltage, impedance, and phase angle when transmitting power over high-voltage lines. From the other side of view, the FACTS principle is mainly depend on the advanced technologies of power 111

4 electronic techniques and algorithms into the power system, to make it electronically controllable. Much of the research upon which FACTS rests evolved over a period of many years. Nevertheless, FACTS, an integrated technology, is a novel concept that was brought to fruition during the 1980s at the Electric Power Research Institute (EPRI) for applications of North American army objectives. FACTS can capitalize on the many ideas taking place in the area of high-voltage and high-current power electronics, to improve the control of power flows in networks during both steady-state and transient conditions. In the present time of the power network electronically controllable has initiated a change in the way that power plant equipment is designed and built as well as the technology that goes into the planning and operation of transmission and distribution networks. These achievements may also enhance the method energy exchanges are done, as high-speed control of the path of the energy flow is now feasible. FACTS own a lot of promising benefits, technical and economical, which get the benefits of electrical equipment devices, operators, and research groups around the world. FACTS controllers have been installed in various regions of the world. The well different types are: load tap-changers transformer, static VAR compensators, phase-angle regulators, thyristorcontrolled series compensators, static compensators, interphase power controllers and unified power flow controllers. This thesis covers in breadth and depth the modeling and simulation methods required for a thorough study of the steady-state and dynamic operation of electrical power systems with FACTS controllers. The characteristics of a given power system evolve with time, as load grows and generation is added. If the transmission grid capacity is not updated sufficiently the power network becomes vulnerable to steady state and transient stability problems, as stability margins will be narrower. The powerful of the transmission grid to transmit power has constraint by one or more of the following steady-state and dynamic limitations: Angular stability, Voltage stability, Thermal limits, Transient stability, and Dynamic stability. Mainly restrictions on power exchange can be controlled by installing new transmission and generation circuits. Also, FACTS controllers can achieve the same tasks to be met with no huge changes to system layout. These limits affect the packages of the power to be transferred without block out to transmission lines and electric apparatuses. From the operational point of view, FACTS technology is concerned with the ability to control, in an adaptive trend, the directions of the power flows throughout the network, where before the advent of FACTS, high-speed control was very limited. FACTS controllers save a lot of benefits such as reduction of operation and transmission investment cost, increased system security and system reliability, maximize power transfer capabilities, and an overall enhancement of the quality of the electric energy delivered to customers. In many practical situations, it is desirable to include economical and operational considerations into the power flow formulation, so that optimal solutions, within constrained solution spaces, can be obtained. The ability to control the line impedance and the buses voltage magnitudes and phase angles at both the sending and the receiving ends of transmission lines, with almost no delay, has significantly increased the transmission capabilities of the network while considerably enhancing the security of the system. 3.1 FACTS PROVIDE Easy & rapid voltage regulation, increased power transfer over long AC lines, damping of active power oscillations, and load flow control in meshed systems, Thereby significantly improving the stability and performance of existing and future transmission systems. So by using the FACTS devices, many of large industries and power sector companies will be able to better utilize their existing transmission networks, substantially increase the availability and reliability of their line networks, and improve both transient and dynamic network stability while ensuring a better quality of supply. i) ASKS OF FACTS DEVICES Control voltage under various load conditions. Balance reactive power (voltage, transmission losses). Increase the stability of power transmission over long distances. Increase active power stability. ii) STATCOM FEATURES It provides compact and reduced size. System voltage support and stabilization by smooth control over a wide range of operating conditions. Dynamic response following system contingencies. High reliability with modular construction parallel and converter design. Flexibility of reconfiguration to Back To Back power transmission or UPFC (Unified Power Flow Controller) and other configurations. 3.2 OPERATION MODE OF THE STATCOM Active power exchange between the STATCOM and the EPS can be at minimum extent. This means that the inverter cannot provide active power to the AC system form the DC accumulated energy if the output voltage of the inverter goes before the voltage of the AC system. The exchange between the inverter and the AC system can be controlled adjusting the output voltage angle from the inverter to the voltage angle of the AC system. On the other hand, the inverter can absorb the active power of the AC system if its voltage is delayed in respect to the AC system voltage. 112

5 Following figure shows the operation mode of STATCOM for on load operation, capacitive operation and inductive operation. Figure 8: Operation mode of STATCOM 3.3 V-I CHARACTERISTIC OF A STATCOM The STATCOM smoothly and continuously controls voltage from V1 to V2. However, if the system voltage exceeds a low-voltage (V1) or high-voltage limit (V2), the STATCOM acts as a constant current source by controlling the converter voltage (Vi) appropriately. Thus, when operating at its voltage limits, the amount of reactive power compensation provided by the STATCOM is more than the most-common competing FACTS controller, namely the Static Var Compensator (SVC). This is because at a low voltage limit, the reactive power drops off as the square of the voltage for the SVC, where Mvar=f(BV2), but drops off linearly with the STATCOM, where Mvar=f(VI). This makes the reactive power controllability of the STATCOM superior to that of the SVC, particularly during times of system distress. Quicker response time (A STATCOM has a step response of 8 ms to 30 ms). This helps with compensation of negative phase current and with the reduction of voltage flicker. Active power control is possible with a STATCOM (with optional energy storage on dc circuit). This could further help with system stability control. No potential for creating a resonance point. This is because no capacitor banks or reactors are required to generate the reactive power for a STATCOM. The STATCOM has a smaller installation space due to no capacitors or reactors required to generate MVAR, minimal or no filtering, and the availability of high capacity power semiconductor devices. Designs of systems of equal dynamic ranges have shown the STATCOM to be as much as 1/3 the area and 1/5 the volume of an SVC. A modular design of the STATCOM allows for high availability (i.e., one or more modules of the STATCOM can be out-of-service without the loss of the entire compensation system). IV. MODELING OF THE STATCOM AND ANALYSIS 4.1 Operating Principles The fundamental phasor diagram of the STATCOM terminal voltage with the voltage at PCC for an inductive load in operation, neglecting the harmonic content in the STATCOM terminal voltage, is shown in figure 7. Ideally, increasing the amplitude of the STATCOM terminal voltage Voa above the amplitude of the utility voltage Vsa causes leading (capacitive) current Ic to be injected into the system at PCC. Iac, the real component of Ic, accounts for the losses in the resistance of the inductor coil and the power electronic converter. Ideally, if the system losses can be minimized to zero, Ic_a, would become zero, and Ic would be leading at perfect quadrature. Then, Voa, which is lagging and greater than Vsa, would also be in phase with Vsa. The STATCOM in such a case operates in capacitive mode (when the load is inductive). 4.2 Modeling The modeling is carried out with the following assumptions: 1. All switches are ideal. 2. The source voltages are balanced. 3. Rs represents the converter losses and the losses of the coupling inductor. 4. The harmonic contents caused by switching action are negligible. Figure 9: V-I characteristic of a STATCOM In addition the STATCOM has other advantages compared to an SVC, such as: 113

6 4.3 Analysis of model i) STATCOM Dynamic Response Figure 10: V ref signal (dotted lines) along with the measured positive-sequence voltage v m at the STATCOM We will now verify the dynamic response of our model. Open the STATCOM dialog box and select "Display Control parameters". Verify that the "Mode of operation" is set to "Voltage regulation" and that "External control of reference voltage Vref" is selected. Also, the "droop" parameter should be set to 0.03 and the "Vac Regulator Gains" to 5 (proportional gain Kp) and 1000 (integral gain Ki). Close the STATCOM dialog block and open the "Step Vref" block (the red timer block connected to the "Vref" input of the STATCOM). This block should be programmed to modify the reference voltage Vref as follows: Initially Vref is set to 1 pu; at t=0.2 s, Vref is decreased to 0.97 pu; then at t=0.4 s, Vref is increased to 1.03; and finally at 0.6 s, Vref is set back to 1 pu. Also, make sure that the fault breaker at bus B1will not operate during the simulation (the parameters "Switching of phase A, B and C" should not be selected). Figure 11: The reactive power Q m absorbed (positive value) or generated (negative value) by the STATCOM Run the simulation and look at the "VQ_STATCOM" scope. The first graph displays the Vref signal (magenta trace) along with the measured positivesequence voltage Vm at the STATCOM bus (yellow trace). The second graph displays the reactive power Qm (yellow trace) absorbed (positive value) or generated (negative value) by the STATCOM. The signal Qref (magenta trace) is not relevant to our simulation because the STATCOM is in "Voltage regulation" and not in "Var Control". Looking at the Qm signal we can determine that the closed-loop time constant of the system is about 20 ms. This time constant depends primarily on the power system strength at bus B2 and on the programmed Vac Regulator gains of the STATCOM. To see the impact of the regulator gains, multiply the two gains of the Vac Regulator Gains by two and rerun the simulation. You should observe a much faster response with a small overshoot. Looking at the Vm and Vref signals, you can see that the STATCOM does not operate as a perfect voltage regulator ( Vm does not follow exactly the reference voltage Vref). This is due to the regulator droop (regulating slope) of 0.03 pu. For a given maximum capacitive/inductive range, this droop is used to extend the linear operating range of the STATCOM and also to ensure automatic load sharing with other voltage compensators (if any). Set the droop parameter to 0 and the voltage regulator gains back to 5 (Kp) and 1000 (Ki). If you then run a simulation, you will see that the measured voltage Vm now follows perfectly the reference voltage Vref. ii) Comparison between STATCOM & SVC under fault condition Figure 12: Measured voltage Vm on both systems We will now compare our STATCOM model with a SVC model having the same rating (+/- 100 MVA). If you double-click on the "SVC Power System" (the magenta block), you will see a SVC connected to a power grid similar to the power grid on which our STATCOM is connected. A remote fault will be simulated on both systems using a fault breaker in series with fault impedance. The value of the fault impedance has been programmed to produce 30% voltage sag at bus B2.Before running the simulation; you will first disable the "Step Vref" block by multiplying the time vector by 100. You will then program the fault breaker by selecting the parameters "Switching of phase A, B and C" and verify that the breaker is programmed (look at the "Transition times" parameter) to operate at t=0.2 s for a duration of 10 cycles. Check also that the fault breaker inside the "SVC Power System" has the same parameters. Finally, set the STATCOM droop back to its original value (0.03 pu). 114

7 Figure 13: The measured reactive power Qm generated by the SVC and the STATCOM Run the simulation and look at the "SVC vs STATCOM" scope. The first graph displays the measured voltage Vm on both systems (magenta trace for the SVC). The second graph displays the measured reactive power Qm generated by the SVC (magenta trace) and the STATCOM (yellow trace). During the 10-cycle fault, a key difference between the SVC and the STATCOM can be observed. The reactive power generated by the SVC is pu and the reactive power generated by the STATCOM is pu. We can then see that the maximum capacitive power generated by a SVC is proportional to the square of the system voltage (constant susceptance) while the maximum capacitive power generated by a STATCOM decreases linearly with voltage decrease (constant current). This ability to provide more capacitive power during a fault is one important advantage of the STATCOM over the SVC. In addition, the STATCOM will normally exhibit a faster response than the SVC because with the voltagesourced converter, the STATCOM has no delay associated with the thyristor firing (in the order of 4 ms for a SVC). [3] Reactive power control in electric systems by Timothy J.E. Miller. [4] Hingorani, N.G. and L. Gyugyi, Understanding FACTS, IEEE Press, New York. [5] Macho ski, J., Power System Dynamics and Stability, John Wiley & Sons. [6] Hamad, A.E., Analysis of Power System Stability Enhamsement by Static VAr Compensators, IEEE Transactions on power Systems, 1(4): [7] M. Sajedihir, Y. Hoseinpoor, P.Mosadegh Ardabili, T. Pirazadeh., Analysis and Simulation of a STATCOM for Midpoint Voltage Regulation of Transmission Lines, Australian Journal of Basic and Applied Sciences, 5(10): V. CONCLUSION In this Paper, We observed the voltage can be 500 KV transmission Line at Mid-point which can be used in the industrial lines as well as interconnected power grid network of transmission Lines. We have compared the output and performance of SVC and STATCOM with respect to the analytical and Simulation studies. So, from the output characteristic of SVC and STATCOM. We can conclude that both these devices improve the behavior of power system under the transient voltage condition by using MATLAB based modeling and simulation. REFERENCES [1] Zhou, E.Z., Application of static VAR compensators to Increase Power System Damping, IEEE Transactions on power system, 8(2): [2] Wang, H.F. and F. Li, Design of STATCOM Multivariable Sampled Regulator, Int. conf. on Electric Utility Deregulation and power Tech, City University, London. Copyright Vandana Publications. All Rights Reserved. 115