Mitigation Voltage Sag in Power System by Static Synchronous Compensator

Problem statement: The disturbance in power system is unavoidable sit uation. It cause in voltage sag in power system. Approach: This study applied the STATCOM to mitigate voltage of the system. The mathematical model of power system equi pped with a STATCOM is systematically derived. The presented mathematical model is shown how it effects on voltage sag improvement. The simulation results are tested on a Single Machine I nfinite bus. The proposed method is equipped in sample system with severe disturbance. The variatio n voltage of the system without and with a STATCOM is plotted and compared. Results: It was found that the system without a STATCOM has high voltage variation especially voltage sags wher eas with a STATCOM the system voltage has small voltage sages. Conclusion: From the simulation results, the STATCOM can mitig a e voltage of power system.


INTRODUCTION
Voltage sag is one of the most important power qualities in customer aspects. It is unavoidable in power system with fault or disturbance (Hochgraph and Lasseter, 1998;Uzunovic et al., 1997;Al-Rawi at al., 2007;Hingorani and Gyugyi, 2000;Song and Johns, 1999). The application of advanced power electronic such as Flexible AC Transmission (FACTS) and High Voltage Direct Current (HVDC) system is the one possible way to mitigate voltage sag. There are various forms of FACTS devices such as Static Synchronous Series Compensator (SSSC), Static Synchronous Compensator (STATCOM), Unified Power Flow Controller (UPFC) (Ahmad and Al-Husban, 2009;Barbuy et al., 2009;Kumkratug, 2010;Magaji and Msutafa, 2009;Hannan et al., 2009;Zarate-Minano et al., 2010).
A Static Synchronous Compensator (STATCOM) is a member of the FACTS family that is connected in shunt with system. The STATCOM consists of a solid state voltage source converter with GTO thyristor switches or other high performance of semi-conductor and transformer. The STATCOM can electrically mimic reactor and capacitor by injecting a shunt current in quadrature with the line voltage. The reactive power (or current) of the STATCOM can be adjusted by controlling the magnitude and phase angle of the output voltage of the shunt converter (Ahmad and Al-Husban, 2009) This study applied the STATCOM to mitigate voltage of the system. The mathematical model of power system equipped with a STATCOM is systematically derived. The presented mathematical model is shown how it effects on voltage sag improvement. The simulation results are tested on a Single Machine Infinite bus system.

MATERIALS AND METHODS
Mathematical model: Figure 1a shows the single line diagram of Single Machine Infinite Bus (SMIB) system without a STATCOM and the corresponding equivalent circuit is shown in Fig. 1b. Here X 1 is the equivalent reactance between the machine internal bus and the bus m and X 2 is the equivalent reactance between bus m and the infinite bus. The generator is represented by a constant voltage source (E') behind transient reactance (X d ' ). The active power balance at bus m is given by: Here: After some mathematical manipulations of Eq. 1-Eq. 3, the voltage angle at bus m of the system without STATCOM is given by: The reactive power balance at bus m is given by: Here: and: After some mathematical manipulations of Eq. 5-7, the voltage magnitude at bus m of the system without STATCOM is: It can observe from the Fig. 1b and 2a-c that the STACOM doesn't affect on the active power balance and then the voltage angle equation at bus doesn't change written by: However, the STATCOM affects on reactive power balance given by: After some mathematical manipulations, the voltage magnitude at bus m of the system with a STACOM is given by: From Eq. 8 and 12, the voltage magnitude at bus m of the system with a STATCOM is given by: Here: C = X 1 X 2 /(X 1 +X 2 ) Voltage sag and improvement: It can be seen from Eq. 9, 12 and 13 that the voltage magnitude at bus m (V m ) depends on machine angle (δ) and current injection of STATCOM (I q ). Figure 3 shows the V m -δ curve. At steady state, the operating points of the system are V m0 and δ 0 , respectively. When the system is subject to disturbance, the δ significantly increases and thus the voltage dramatically decreases. The V m can be improved by injecting the current I q in positive sign as can be seen in Fig. 3. Our objective is to maintain V m is at V m0 . The control strategy of I q is given by:

RESULTS
The presented mathematical model and control strategy is used to study the effect of STATCOM on voltage sag improvement. The diagram of the system is shown in the Fig. 4  It is considered that a three phase self clearing fault appears at middle of the line 2 and the fault is cleared by opening the circuit breaker at the both ends. Figure 5 shows the comparison of the voltage sag of the system without and with a STATCOM.

DISCUSSION
It can be obviously seen from the Fig. 5 that the voltage of system without a STATCOM is continuously osculation. The voltage magnitude of the system at steady state is round 1 p.u. where at the post fault the minimum voltage magnitude is decreased to 0.5 p.u. This may cause in the damage of the electrical equipment. However, with a STATCOM a minimum voltage is around 0.8 pu. Moreover it can return to the stable equilibrium by 5 sec.

CONCLUSION
This study applies the Static Synchronous Compensator (STATCOM) to mitigate the voltage sag in power system. The mathematical model of the STACOM is systematically derived. The presented mathematical model has shown that the system voltage can be regulated by the STATCOM. The simulation results are tested on Single Machine Infinite Bus