POWER SYSTEM DAMPING USING STATCOM

Abstract

Owing to various reasons like ever-increasing demand of electric power, deregulation, restrictions on construction of new transmission corridors etc., the modem power system now a days are forced to carry quite a large amount of power, thereby operating in a much stressed condition. Under this stressed condition, power transfer between two areas over a tie-line often exhibits low frequency inter-area oscillations. Low frequency oscillations were first observed in Northern America Power Network in early 1960's during a trial interconnection of the Northwest Power pool and Southwest Power pool. Later they have been also reported in many other countries. As the low frequency oscillations constrain the power carrying capacity of the line, it threatens power system security as well as limits the most efficient utilization of the transmission system. Therefore, this problem has caused wide concern and attracted the attention of the researchers since 1960's. In the early days, the excitation control of the synchronous machines was remedy available for controlling these oscillations. Later, power system stabilizers (PSS) were developed and added to the excitation system loop to enhance the damping of these low frequency oscillations. However, a PSS is generally more effective in damping local modes, but in certain cases it may not be sufficient to provide the necessary damping for inter-area oscillations. Therefore, for enhancing the power system security, development of most effective method of damping the inter-area mode of low-frequency oscillation is still a major area of research today. Now, in the last one and half decades power electronic device based equipments, commonly known as Flexible AC Transmission (FACTS) controllers, have been implemented in various parts of the world to achieve better utilization of the existing power grid. Because of their capability of fast power control, these FACTS controllers can be used effectively to damp out the inter-area oscillations. Among the various FACTS controllers, Static Synchronous Compensator (STATCOM) is one of the most prominent equipments. It is a shunt connected equipment whose main objective is to control the bus voltage. However, by using the voltage controller only, the STATCOM may not be able to damp out the inter-area oscillations effectively and in some cases may even aggravate the problem. Therefore, an auxiliary damping controller is often needed to be designed, which would provide a superimposed damping signal to its voltage loop.