05-03-2013, 03:26 PM
Dynamic Performance of 48-pulse STATCOM, SSSC and UPFC controller
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Abstract:
The paper investigates the dynamic operation of both static synchronous compensator (STATCOM) and static synchronous series compensator (SSSC) based on a new full model comprising a 48- pulse gate turn off thyristor voltage source converter for combined reactive power compensation and voltage stabilization of the electrical grid net work. The complete digital simulation of the STATCOM and SSSC within the power system is performed in the MATLAB/ simulation environment using the power system block set (PSB).The STATCOM scheme and the electrical grid network are modelled by specific electrical blocks from the power system block set, while the control system is modelled using simulink.Two controllers for the STATCOM and SSSC are presented in this paper based on a decoupled with voltage and current control strategy. The performance of both STATCOM and SSSC schemes connected to the 500-kv grid are evaluated. The proposed to ensure the stable operation of the STATCOM under various load conditions. it is shown that phase-locked loop(PLL) inherent delay has a great effect on dynamic operation of SSSC and new auxiliary regulator is proposed to enhance the dynamic performance of the SSSC.the proposed control schemes are validated by digital simulation.
INTRODUCTION :
In the last decade, commercial availability of Gate Turn –Off (GTO) thyristor switching devices with high-power handling capability and the advancement of the other types of power –semiconductor devices such as IGBTs have led to the development of fast controllable reactive power sources utilizing new electronic switching and converter technology. These switching technologies additionally offer considerable advantages over existing methods in terms of space reductions and fast effective damping. The GTO thyristors enable the design of the solid-state shunt reactive compensation and active filtering equipment based upon switching convertor technology. These power quality devices (PQ Devices) are power electronic converters connected in parallel or in series with transmission lines, and the operation is controlled by digital controllers. The interaction between these compensating devices and the grid network is preferably studied by digital simulation. Flexible alternating current transmission systems (FACTS) devices are usually used for fast dynamic control of voltage, impedance, and phase angle of high-voltage ac lines. FACTS devices provided strategic benefits for improved transmission system power flow management through better utilization of existing transmission assets, increased transmission system security and reliability as well as availability, increased dynamic and transient grid stability, and increased power quality for sensitive industries (e.g., computer chip manufacture). The advent of FACTS systems is giving rise to a new family of power electronic equipment for controlling and optimizing the dynamics equipment for controlling and optimizing the dynamic 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 flexible reactive power compensation to replace other conventional VAR compensation, such as the thyristor-switched capacitor (TSC) and thyristor controlled reactor (TCR). This paper deals witty a novel cascade multilevel converter model, which is a 48- pulse (three levels) source converter. The voltage source converter described in this paper is a harmonic neutralized, 48- pulse GTO converter.
DYNAMIC PERFORMANCE OF THE STATCOM :
The basic STATCOM model consists of a step-down transformer with leakage reactance Xt, a three-phase GTO VSI, and a dc side capacitor. The ac voltage difference across this transformer leakage reactance produces reactive power exchange between the STATCOM and the power system at the point of interface. The voltage can be regulated to improve the voltage profile of the interconnected power system, which is the primary duty of the STATCOM. A secondary damping function can be added to the STATCOM for enhancing power system dynamic stability. The STATCOM‘s main function is to regulate key bus voltage magnitude by dynamically absorbing or generating reactive power to the ac grid network, like a thyristor static compensator. This reactive power transfer is done through the leakage reactance of the coupling transformer by using a secondary transformer voltage in phase with the primary voltage (network side). This voltage is provided by a voltage-source PWM inverter and is always in quadrature to the STATCOM current. The STATCOM device operation can be illustrated by the phasor diagrams shown in Fig. 1. When the secondary voltage (VS) is lower than the grid system bus voltage (VB), the STATCOM acts like an inductance absorbing reactive power from the grid bus. When the secondary voltage (VS) is higher than the bus voltage (VB), the STATCOM acts like a capacitor generating reactive power to the grid bus. In steady-state operation and due to inverter losses, the bus voltage (VB) always leads the inverter ac voltage by a very small angle to supply the required small active power losses
DIGITAL SIMULATION STATCOM MODEL:
A novel complete model using the 48-pulse digital simulation of the STATCOM within a power system is presented in this paper. The digital simulation is performed using the MATLAB/Simulink software environment and the Power System Block set (PSB). The basic building block of the STATCOM is the full 48-pulse converter-cascade implemented using the MATLAB/Simulink software it was shown in the Fig.2. The control process is based on a novel decoupled current control strategy using both the direct and quadrature current components of the STATCOM. The operation of the full STATCOM model is fully studied in both capacitive and inductive modes in a power transmission system and load excursion. The use of full 48–pulse STATCOM model is more accurate than existing low-order or functional models.
POWER SYSTEM DESRIPTION:
The test system is a simple power system 500-kV network grid equipped with the SSSC and its novel controller, which connected in series with the transmission system. Modeling the SSSC compensator, including the power network and its controller in MATLAB/Simulink environment, requires using ―electric blocks‖ from the power system blockset and control blocks from thr Simulink library. A Mvar SSSC device is connected to the 500-kV grid network. Fig. 5 shows the single line diagram that represents the SSSC and the 500/33-kV grid network. The feeding network is represented by an its equivalent Thevenin (bus B1) where the voltage source is a 500 kV with 10 000 MVA short circuit level with a resistance of 0.1 p.u. and an equivalent reactance of 0.3 p.u. followed by the 500-kV radial transmission system connected to bus B2. The full system parameters are given in appendix. The SSSC FACTS device consists mainly of the 48–pulse GTO-voltage source converter model that is connected in series with the transmission line at Bus B1 by the coupling transformer T1. The dc link voltage Vdc is provided by capacitor C, which is charged with an active power taken directly from the ac network. The novel full 48-pulse GTO-VSC model results in less harmonic distortion than other 6-, 12-, and 24-pulse converters or functional models usually used to represent SSSC device operation.