CAUSES OF POWER SYSTEM TRANSIENTS

There are different causes for power system transients. For example, lightning strokes to the wires in the power system or to ground and component switching either of network components or end user equipment can produce transients. Nature of power system transients are very much event dependent.

Short duration events can be classified into three classes:

1. Events that can be identified by their fundamental frequency magnitude. Voltage magnitude in such events goes through significant changes for long periods. The changes are well apart and observable with respect to time. This enables magnitude estimators to identify and resolving events having significant changes. These are observed mainly in fault induced events, transformer saturation, induction motor starting, etc. Voltage dips with duration typically between 50ms and several seconds and interruptions with duration from several seconds up to many hours are associated with such transient events.

2. Events having significant changes in the fundamental frequency magnitude but of short duration. In such events, it is very difficult to extract voltage magnitude of transients. These are normally observed in fuse-cleared faults and self-extinguishing faults.

3. Events of very short duration (transients) for which the fundamental frequency magnitude does not offer important information. For this class, the higher frequency components of the signal must be considered for a thorough characterization and classification.

Based on waveform shape, power system transients, can be classified into

1. Oscillatory transients
2. Impulsive transients
3. Multiple transients

1) IMPULSIVE TRANSIENTS

An impulsive transient is defined as a sudden change in the steady state condition of voltage, current or both, that is unidirectional in polarity either positive or negative. Analysis of impulsive transients is done by their rise and decay times. Impulsive transients are damped quickly by the resistive circuit elements and do not propagate far from their source. Thus their effects are localized.

Impulsive transients are common during lightning. Lightning stroke may appear directly or by indirect induction. When a lightning stroke hits a transmission line (direct stroke) an impulsive over voltage is induced. They have high magnitude. Lightning over voltage can also be induced by nearby strokes to the ground or between clouds. These over-voltages are of lower magnitude than those produced by direct strokes. Normally impulsive transient shows a sudden rise followed by an exponential decay. But in some cases, lightning transient shows a sudden rise followed by a sudden drop and an oscillation with relatively small amplitude.

2) OSCILLATORY TRANSIENTS

Oscillatory transient is alternating in nature. It shows a damped oscillation with a frequency ranging from a few hundred hertzs up to several Mega hertzs. Oscillatory transients can be mathematically derived by the homogenous solution to linear differential equations. As the electric power system can approximately be described by a set of linear differential equations, oscillatory transients are the “natural transients” in electric power system. For this reason, oscillatory transients dominate over impulsive transients. For example, oscillatory transient can be caused by the energizing of a capacitor bank where, frequency of oscillation is mainly determined by the capacitance of the capacitor bank and the short circuit inductance of the circuit feeding the capacitor bank (capacitor energizing). Another common cause of oscillatory transient is event of energizing of transmission line.

3) MULTIPLE TRANSIENTS WITH A SINGLE CAUSE

Multiple transients are combination of many overlapped transients occurred due to more than one switching action. For example, in three phase system the switching action in the individual phases rarely take place at the same instant. Such events produce multiple transients. Current chopping and re-strike are other two major causes of multiple transients. Current chopping is done when the current during opening of a circuit breaker becomes zero before the natural zero crossing. This results in transients of high over-voltages. Re-strike may occur when a capacitor is de-energized by a slowly moving switch where the voltage over the capacitor increases faster than the voltage-withstand of the gap between the contacts of the switch.

EFFECTS OF TRANSIENTS ON POWER SYSTEM

Transients are very much related to the operation and performance of different parts of power system as well as loads and measuring and protective devices also. Nature and duration of _power system transients are related to correct operation of circuit breakers, and over voltage due to switching of high voltage lines. High magnitudes of voltage transients break insulations of the system. High magnitude of current transients can burn out devices and instruments. Transients can cause mal-operation of relays and mal-tripping of circuit breakers. Frequent number of direct or indirectly induced oscillatory transients may change the magnetic properties of core materials used in electric machines.