Study Of Power System Under Transient Condition

Transient over voltages have often been blamed for the malfunctioning of electrical power systems. This seminar paper is an overview which seeks to describe the state of the power system under transient condition. It also provides information on the rate of occurrence, on the wave shape, and on the energy associated with the transients, as a function of the location within the power system. Also, the basic principles of coordinated protection with specific experimental examples are equally examined. The use of several categories of installations is recommended with a matrix of power system over voltages specified for controlled situations.

 

The term “transient” denotes the voltage and current components that occur during the transition from one steady state (typically sinusoidal) to another steady state in a power system (Mihalic,  2003).  They show very high rise of voltage and current in short period of time. Power system transients may generate in the system itself or may propagate to the system from other external system. They may be unidirectional or oscillatory in shape. Though such events are of short duration, they create very high magnitude of voltage and current. Transients affects the performance of power system devices as well as measuring, controlling and protective devices. Generally, power system transients are short duration phenomena of less than one cycle (usually with frequency of 50Hz) and its nature is very much event dependent. Accurate modeling of the power system transients and the characterization of measured transients along with their sources and effects are very important. Since substantial over voltages (more than two or three per unit) are involved, it may be difficult to suppress because of its considerable energy. The most frequent type of transient over voltage in ac power systems is a decaying oscillation, with frequencies between 5 and 500 kHz. Also, local load switching operations especially if it involves re strikes in the switchgear devices, will also produce higher voltages at lower energy levels.

Capacitor switching is one of the most common switching events on power systems. Capacitors are used to provide reactive power (in units of vars) to correct the power factor, which reduces losses and supports the voltage on the power system. They are a very economical and generally trouble free means of accomplishing these goals (Hasler, 1979). Thus, the use of capacitors on power systems is quite common and will continue to be. One draw-back to the use of capacitors is that they yield oscillatory transients when switched. Some capacitors are energized all the time (a fixed bank), while  others  are  switched  according  to  load  levels. Various control means, including time, temperature, voltage, current, and reactive power are used to determine  when  the  capacitors  are switched . It is common for controls to combine two or more of these functions, such as temperature with voltage over-ride. On distribution feeders with industrial loads, capacitors are frequently switched by time clock in anticipation of an increase in load with the beginning of the working day. Figure 3 shows the  one-line  diagram  of  a  typical  utility feeder capacitor switching situation. When the switch is closed, a transient is observed up line from the capacitor at the monitor location. In this particular case, the capacitor switch contacts close at a point near the system voltage peak.