The Wavelet Relay

The basic function of power system protective relays is the detection of anomalous and/or potentially harmful or undesirable operating conditions. The conditions may occur in the steady state or as transients. Most relays detect steady state conditions, they issue trip signals, and they cause protective steps to be taken -- usually the operation of a circuit breaker. Power system protective relays are complex engineering designs because they must expect the unexpected. They must detect failures and harmful modes of operation; and if they themselves fail, they must detect this as well. Their design is often made more robust by the addition of backup systems that detect failures or other undesired operation of the main system. Many power engineers agree that power system protective relaying is as much an art as it is an engineering science. The basic technology of power system relaying includes such factors as: voltage and current measurement; digital and analog signal processing; impedance and phasor techniques; threshold detection; frequency domain techniques; time domain techniques; microprocessor applications; solid state designs; induction disk technology; time signal methods and timing; and overvoltage and overcurrent detection.In many cases, the basic function depicted is insufficient for the accurate operation of the protective relay: in some cases, some normally expected operating conditions may appear to the relay to be abnormal and indicative of fault conditions; but processing and analysis shows that the measured system states are in fact indicative of normally expected conditions. A false trip on such normally expected conditions would cause outage or disruption of the system load, and this may be intolerable and it is certainly undesirable. Therefore, logic and processing must be inserted and designed into the relay to prevent such false trips.Researchers at Arizona State University have devised a new technique involving the use of wavelets as signal processing tool for the detection and distinction of normally expected transients from undesired and intolerable operating conditions. Examples include inrush current for transformers; inrush currents for shunt capacitors; overvoltages from capacitor switching; transients resulting from line switching in which reactors are present; and other switching transient voltages and currents. The wavelet is itself a transient and it is an efficient method to detect transients.