Introduction
Voltage changes continuously according to the varying electrical demand,transmission lines utilization, system control by the control centers, and emergencysituations occurred in the system. Since customers require voltage quality, at deliverypoints, to meet the agreed criteria, it is the control centers’ responsibility to control thevoltage so that it can satisfy the agreement.Controlling the voltage is regional problems. In other words, the voltage controllingproblems are needed to be solved separately by each control area. This can be achievedby providing sufficient reactive power sources for controlling voltage level as specified bythe agreement. The voltage controlling problems can be divided into two situations, whichare normal situation, and emergency situation. The reactive power sources are classifiedinto two types, static and dynamic. The static type consists of shunt capacitor, and shuntreactor. The dynamic type is the source of reactive power produced by generators, SVC,and OLTC that operates automatically. The control centers need to organize and compilethe constantly updated information concerning the availability of relevant equipment. Alsothe control centers have to provide procedures, in writing, required for properly controllingof the equipment in order to support normal situation, and emergency situation that maylead to voltage collapse and cascade tripping both of which can cause a blackout..


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