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The problem of voltage or current unbalance is gaining more attention recently with the increasing awareness on power quality. Excessive unbalance among the phase voltages or currents of a three phase power system has always been a concern to expert power engineers. An unbalanced voltage supply can cause power electronic converters to generate more harmonic distortions. It may result in the malfunction of sensitive three-phase electronic equipment. In fact, the voltage and current unbalance has been regarded as one of the basic power quality attributes (Birt et al., 1976). The asymmetry in transmission lines and loads produces a certain degree of unbalance in real power systems. Under these conditions, low quantities of negative and zero sequence voltages can be observed in power networks. These magnitudes are considered to be a disturbance whose level must be controlled by power quality standards to maintain the electromagnetic compatibility of the system (Mamdouh Abdel-Akher et.al, 2005). In order to cope with these kind of problems and increase usable power transmission capacity, Flexible AC transmission systems (FACTS), where developed and introduced to the market. FACTS philosophy was first introduced by Hingorani (Hingorani, N.G, 1988) from the Electric power research institute (EPRI). The objective of FACTS devices is to bring a system under control and to transmit power as ordered by the control centers, it also allows increasing the usable transmission capacity to its thermal limits. With FACTS devices we can control the phase angle, the voltage magnitude at chosen buses and/or line impedances (Mahdad.b et al., 2006) In practical installation of FACTS in power system, there are six common requirements as follows (Mahdad.b et al., 2007) : 1. What Kinds of FACTS devices should be installed? 2. Where in the system should be placed? 3. How much capacity should it have? 4. How to coordinate dynamically the interaction between multiple FACTS and the network to better exploit FACTS devices? 5. How to estimate economically the optimal size and number of FACTS to be installed in a practical network? 6. How to adjust dynamically the three phase recative power in unbalanced network?

Optimal Location and Control of Flexible Three Phase Shunt FACTS to Enhance Power Quality in Unbalanced Electrical Network