The differential relays normally response to those faults which occur in side the differential protection zone of. Thus, if a numerical 87t differential relay is directly applied for the overall differential protection of a par, and set to compensate for vector group yy0, the false differential current will appear as soon as par phase angle shift is different from zero degrees. In this simple scheme, we can assume that under normal operating conditions, the current entering into the piece of equipment under protection is equal. The biased differential relay is a lowimpedance relay and compares with the highimpedance relay as follows. Mitsubishi electric fa site introduces information in latest information, product information, technological material, and the manual, etc. This automatic increase in the trip point allows for errors in the relay system that is proportional to the current level. In this type of relay, there are restraining coils in addition to the operating coil of the relay. Biaseddifferential protection theory and application eep. Differential protection of transformer differential relays. The biased beam relay also called percentage differential relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected section. Pdf easy method for testing transformer differential relays. Use of 87t relay principles for overall differential.
The above disadvantages are overcome to a great extent in biased beam relay. A currentbased solution for transformer differential protection. Easy method for testing transformer differential relays. The operating characteristics of the relay is shown in fig. The threephase differential protection incorporated in ref542plus is primarily designed for the protection of twowinding power transformers and highvoltage motors. Ca generator differential relay is a single phase, induction disc type with inverse timing characteristics. Can operate with cts with a lower knee point and less accuracy, since the relay is of a lowimpedance type. Protection relays differential relays 2 using biased relay. Principles of differential relaying the restraint characteristic what needs to be realised is that the first one is properly termed the restraint characteristic rc whilst the latter is an operating characteristic. Current differential line protection setting consideration.
In most differential relays, the bias trip point is not a fixed value but calculated from a starting point with a curve that increases as current increases. Moreover buchholz relay is provided in transformer for detecting any internal fault in the transformer but differential protection scheme detects the same in more faster way. The principles of differential protection you must. The bias feature compensates for any ct inaccuracy or saturated cts. A currentbased solution for transformer differential. Protection relays manual download mitsubishi electric fa. Siprotec protection relays figure 1 shows a simple differential protection scheme, also known as a merzprice scheme. Differential relay current differential relay biased.
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