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IJECT Vol. 5, Issue 3, July - Sept 2014 ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) Reactive Power Compensation in Transmission Lines Using Static Var Compensator by Simulation in ETAP Tamojit Chakraborty Dept. of Electrical Engineering, Netaji Subhash Engineering College, Kolkata, West Bengal, India Abstract The study of shunt connected Flexible AC Transmission System (FACTS) device is a connected field with the Reactive Power Compensation and the better mitigation of transmission in Power Systems. The FACTS technology based on Power Electronics devices is used to enhance existing transmission capabilities in order to make the Power Systems network more flexible with independent operation thus increasing the power transfer capability. [1] Static VAR Compensator (SVC) is one of the shunt connected FACTS device, which can be utilized for the purpose of reactive power compensation. This paper attempts to simulate the distribution network substation by introducing SVC at the load ends using the Electrical Transient Analyzer Program (ETAP) environment. A comparative study is made before and after using the FACTS device and subsequent results have been shown. Keywords FACTS, AC Transmission, Power Electronics, SVC, ETAP. I. Introduction During the past two decades, the increase in electrical energy demand has presented higher requirements from the power industry. More power plants, substations, and transmission lines need to be constructed. However, the most commonly used devices in present power grid are the mechanically-controlled circuit breakers. The long switching periods and discrete operation make them difficult to handle the frequently changed loads smoothly and damp out the transient oscillations quickly. In order to compensate these drawbacks, large operational margins and redundancies are maintained to protect the system from dynamic variation and recover from faults. This not only increases the cost and lowers the efficiency, but also increases the complexity of the system and augments the difficulty of operation and control. Severe black-outs happened recently in power grids worldwide and these have revealed that conventional transmission systems are unable to manage the control requirements of the complicated interconnections and variable power flow. Therefore, investment is necessary for the studies into the security and stability of the power grid, as well as the improved control schemes of the transmission system. Different approaches such as reactive power compensation and phase shifting have been applied to increase the stability and the security of the power systems. The demands of lower power losses, faster response to system parameter change, and higher stability of system have stimulated the development of the Flexible AC Transmission systems (FACTS). FACTS has become the technology of choice in voltage control, reactive/active power flow control, transient and steady-state stabilization that improves the operation and functionality of existing power transmission and distribution system. The achievement of these studies enlarge the efficiency of the existing generator units, reduce the overall generation capacity and fuel consumption, and minimize the operation cost. II. Static VAR Compensators (SVC): Static Var Compensator is “a shunt-connected static Var generator w w w. i j e c t. o r g or absorber whose output is adjusted to exchange capacitive or inductive current so as to maintain or control specific parameters of the electrical power system (typically bus voltage)”. [2] SVC is based on thyristor without gate turn-off capability. The operating principal and characteristics of thyristor realize SVC variable reactive impedance. SVC includes two main components and their combination: 1. Thyristor-controlled and Thyristor-switched Reactor (TCR and TSR) 2. Thyristor-switched capacitor (TSC). TCR and TSR are both composed of a shunt-connected reactor controlled by two parallel, reverse-connected thyristor. TCR is controlled with proper firing angle input to operate in a continuous manner, while TSR is controlled without firing angle control which results in a step change in reactance. TSC shares similar composition and same operational mode as TSR, but the reactor is replaced by a capacitor. The reactance can only be either fully connected or fully disconnected zero due to the characteristic of capacitor. With different combinations of TCR/TSR, TSC and fixed capacitors, a SVC can meet various requirements to absorb/ supply reactive power from/to the transmission line. Fig. 1: Static VAR Compensators (SVC): TCR/TSR, TSC, FC and Mechanically Switched Resistor III. Other FACTS Devices Converter-based Compensator: Static Synchronous Compensator (STATCOM) is one of the key Converter-based Compensators which are usually based on the voltage source inverter (VSI) or current source inverter (CSI). Unlike SVC, STATCOM controls the output current independently of the AC system voltage, while the DC side voltage is automatically maintained to serve as a voltage source. Mostly, STATCOM is designed based on the VSI. Compared with SVC, the topology of a STATCOM is more complicated. The switching device of a VSI is usually a gate turn-off device paralleled by a reverse diode; this function endows the VSI advanced controllability. Various combinations of the switching devices and appropriate topology make it possible for a STATCOM to vary the AC output voltage in both magnitude and phase. Also, the combination of STATCOM with a different storage device or power source endows the STATCOM the ability to control the real power output. STATCOM has much better dynamic International Journal of Electronics & Communication Technology 269 IJECT Vol. 5, Issue 3, July - Sept 2014 performance than conventional reactive power compensators like SVC. The gate turn-off ability shortens the dynamic response time from several utility period cycles to a portion of a period cycle. STATCOM is also much faster in improving the transient response than a SVC. This advantage also brings higher reliability and larger operating range [3]. IV. Series-Connected Controllers As shunt-connected controllers, series-connected FACTS controllers can also be divided into either impedance type or converter type. The former includes Thyristor-Switched Series Capacitor (TSSC), Thyristor-Controlled Series Capacitor (TCSC), Thyristor-Switched Series Reactor, and Thyristor-Controlled Series Reactor. The latter, based on VSI, is usually in the form of a Static Synchronous Series Compensator (SSSC). The composition and operation of different types are similar to the operation of the shunt-connected peers [4]. V. Load Flow Analysis Load-flow studies are very common in power system analysis. It allows us to know the present state of a system, given previous known parameters and values. The power that is flowing through the transmission line, the power that is being generated by the generators, the power that is being consumed by the loads, the losses occurring during the transfer of power from source to load, and so on, are iteratively decided by the load flow solution, or also known as power flow solution. In any system, the most important quantity which is known or which is to be determined is the voltage at different points throughout the system. Knowing these, we can easily find out the currents flowing through each point or branch. This in turn gives us the quantities through which we can find out the power that is being handled at all these points. In earlier days, small working models were used to find out the power flow solution for any network. Because computing these quantities was a hard task, the working models were not very useful in simulating the actual one. It’s difficult to analyze a system where we need to find out the quantities at a point very far away from the point at which these quantities are known. Thus we need to make use of iterative mathematical solutions to do this task, due to the fact that there are no finite solutions to load flow. The values more often converge to a particular value, yet do not have a definite one. Mathematical algorithms are used to compute the unknown quantities from the known ones through a process of successive trial and error methods and consequently produce a result. The initial values of the system are assumed and with this as input, the program computes the successive quantities. Thus, we study the load flow to determine the overloading of particular elements in the system. It is also used to make sure that the generators run at the ideal operating point, which ensures that the demand will be met without overloading the facilities and maintain them without compromising the security of neither the system nor the demand. The objective of any load-flow analysis is to produce the following information: • Voltage magnitude and phase angle at each bus. • Real and reactive power flowing in each element. • Reactive power loading on each generator. VI. Load Flow Equation Solution Methods To start with by solving the load flow equations, we first assume values for the unknown variables in the bus system. For instance, let us suppose that the unknown variables are the magnitude of 270 International Journal of Electronics & Communication Technology ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) the voltages and their angles at every bus except the Slack bus, which makes them the load bus or the PQ bus. In this case, we assume the initial values of all voltage angels as zero and the magnitude as 1p.u. Meaning, we choose a flat voltage profile. We then put these assumed values in our power flow equations, knowing that these values don’t represent the actual system, even though it should have been describing its state. So, now we iterate this process of putting in the values of voltage magnitudes and angles and replacing them with a better set. So, as the flat voltage profile keeps converging to the actual values of the magnitudes and angles, the mismatch between the P and Q will reduce. Depending on the number of iterations we use and our requirements we can end the process with values close to the actual value. This process is called as the iterative solution method. The final equations derived in the previous section are the load flow equations where bus voltages are the variables. It can be seen that these equations are nonlinear and they can be solved using iterative methods like: 1. Gauss-Seidel method 2. Newton-Raphson method VII. Power Flow Analysis with SVC Fig. 2 below shows a power plant which is connected to a larger system via a double line transmission system (upper part). The lower part of the figure shows simplified the voltages at the end of the transmission system for various load cases: 1. Heavy load 2. Light load 3. Outage of one line during heavy load condition 4. Load rejection at the end of the line According to the loading conditions voltage decreases and increases will occur with larger deviations at contingency conditions. An SVC will be typically designed in size to limit voltage deviations during normal load conditions and a good voltage profile is kept for this operation. At other contingency conditions larger voltage deviation will occur due to the sizing for normal conditions. Fig. 2: Voltages at the End of a Transmission System Under Various Operating Conditions Fig. 3 below shows the case of load rejection. The voltage rises rapidly in will be reduced by the voltage control means of the system i.e. voltage controllers in power plants. If a SVC is connected close the voltage will also rise rapidly but will be reduced in only a few cycles by the fast reaction of a SVC. The first peak cannot be influenced because the SVC control must first observe the increase and can only react afterwards. w w w. i j e c t. o r g ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) Fig. 3 Action of a SVC on Load Rejection Fig. 4 below shows a transmission system with strong active power oscillations after a severe line fault followed by fault clearing and switch off the faulted line. IJECT Vol. 5, Issue 3, July - Sept 2014 Without a SVC the oscillations continue at low damping for a long time. Using a SVC with voltage control already helps in damping of the oscillations and reducing the oscillation time. Using an SVC with a specific POD (power oscillation damping) control function will even damp out the oscillations quite faster and increase thus the margin in system stability [5]. VIII. Case Study A Single line diagram of 33/11 KV Distribution Substation is taken with fifteen buses (from Bus 1 to Bus15) as shown in Fig. 1. It consists of two power transformers (T1 and T2), each having capacity of 3 MVA and four distribution transformers (T3, T4, T5 and T6). There are four static loads (from Load 1 to Load 4). There are two outgoing feeders connected to each of power transformers. Incoming voltage level is 33KV and the distribution voltage level is 11KV. Load receives a voltage of 0.435 KV. Bus 1is swing Bus. Buses from 2 to 7 are PV Buses and Buses from 8 to 15 are PQ Buses. Power source to this system is provided by Utility, U1. In order to see effect of two SVCs on voltage profile, losses and power flows at each bus in given single line diagram. Fig. 4: Damping of Power Oscillations by SVC Fig. 5: ETAP Load Flow analysis calculates Bus voltages, Branch Power Factors, Currents and Power Flows throughout the Electrical system in single line diagram. ETAP allows for swings, voltage regulated, unregulated power sources with multiple power grids and generator connections. It is capable of performing on both radial and loop systems. ETAP allows feeding of all these above values in w w w. i j e c t. o r g International Journal of Electronics & Communication Technology 271 IJECT Vol. 5, Issue 3, July - Sept 2014 ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) single line diagram for load flow analysis. ETAP provides three load flow calculation methods: Newton-Raphson, Fast-Decoupled, and Accelerated Gauss- Seidel. They possess different convergent characteristics, and sometimes one is more favourable in terms of achieving the best performance. Any one of them is selected depending on system configuration, generation, loading condition, and the initial bus voltage. The following two figures shows load flow analysis of the above shown single line diagram with and without using SVC. Fig. 6: Load Flow Without Using SVC 272 International Journal of Electronics & Communication Technology w w w. i j e c t. o r g ISSN : 2230-7109 (Online) | ISSN : 2230-9543 (Print) IJECT Vol. 5, Issue 3, July - Sept 2014 Fig. 7: Load Flow Using SVC By using two SVCs, average value of voltage is changed from 77.366 to 78.071 i.e. increased by 0.705 units. Thus using two SVCs in the single line diagram at location bus 9-10 where static load is present, it has been found that, we have increased voltage profile and increased active power. Reduction of losses, increase of power transfer capability and voltage profile can also be optimized by number of other optimization methods such as simulated annealing, fuzzy logic. References [1] Guneet Kour, Dr. G.S.Brar, Dr. Jaswanti, International Journal of Engineering Science and Technology (IJEST), Optimal Placement of Static VAR Compensator in Power System. [2] H. K. Tyll, Senior Member, IEEE,"Application of SVCs to Satisfy Reactive Power Needs of Power Systems". [3] Y.H. Song, A.T. Johns,“Flexible ac transmission systems (FACTS)”, IEEE 1999. [4] Juan Dixon (SM), Luis Morán (F), José Rodríguez (SM), Ricardo Domke, Reactive Power Compensation Technologies, State of-the-Art Review. [5] A. K. Chakraborty, A. E. Emanuel,“A Current regulated Switched Capacitor Static Volt Ampere Reactive Compensator”, IEEE Transactions [6] Power Electronics by Dr. P.S Bhimbra [7] Elements of Power System Analysis by Stevenson w w w. i j e c t. o r g International Journal of Electronics & Communication Technology 273