ABB SPAU 121 C, overvoltage and udervoltage

The combined overvoltage and undervoltage relay SPAU 121 C is a secondary relay that is connected to the voltage transformers of the protected object. The relay generally measures the one phase-to-phase voltage of the system. On operation the relay trips the circuit breaker and/or provides alarm, in accordance with the selected functions and given configuration of the relay. When the phase-to-phase voltage exceeds the set start value U gt; of the overvoltage stage, the overvoltage stage starts generating a start signal SS1. When, at definite time mode of operation, the set operate time t gt; or, at inverse time mode of operation, the calculated operate time t gt;, has expired, the overvoltage stage operates generat- ing a trip signal TS1. In the same way the undervoltage stage starts generating a start sig- nal SS2 when the measured value goes below the start value U lt; and when the set operate time t lt; has expired, the undervoltage stage operates, generating a trip signal TS2. The overvoltage stage can be given definite time characteristic or inverse time characteristic. When inverse time characteristic is selected two sets of curves named A and B are available. The undervoltage stage can also be given defi- nite time characteristic or inverse time charac- teristic. When inverse time characteristic is se- lected one curve named C is available. Operation of the undervoltage stage can be blocked by means of a control signal BS applied to the opto-isolated external control input of the relay. The overvoltage and undervoltage relay is pro- vided with two output relays for tripping and four output relays for signalling purposes. One of the signalling relays is dedicated for the self- supervision system of the relay.