ABB SPAD 346 C3 Differential Protection

Motor and generator protection

The calculation process for setting up differential protection applications for motors or generators is very similar to that of power transformers, but the vector group should be set to Yy0. The starting ratio (S) is calculated as the sum of the CT accuracy on both sides, the relay action accuracy (4%), and the required margin (usually 2.5-5%). The second turning point (I ₂ tp/In) may cause CT saturation due to current during motor start-up, with a typical setting value of 1.0. The generator is usually between 1.0-1.5, and the motor and generator usually do not require second or fifth harmonic blocking.

Combination protection of motor and autotransformer

The differential relay measures the phase current on both sides of the protected object, and the protection area includes the autotransformer and motor. The current between all autotransformer/motor combinations and the network must be measured, and its setting is the same as that of ordinary motor protection applications. The autotransformer will have a typical starting sequence during the motor starting process, from power supply to acting as a parallel reactor and then to bypass.

Variable frequency motor and its power transformer protection

SPAD 346 C can only be used to protect power transformers that supply power to frequency converters, and is not suitable for protecting power transformers or motors powered by frequency converters, as the fundamental frequency component in the relay is numerically filtered through a Fourier filter and is not suitable for measuring the output of frequency converters.

Short overhead or cable protection

SPAD 346 C can be used for differential protection of overhead or cable lines. When the distance between measurement points is long, an intermediate CT may be required to reduce CT load. Using 1A secondary current can reduce CT load, and the actual accuracy limit factor (Fa) of CT can be calculated to ensure compliance with requirements.

Summary

This guide describes how to select and calculate differential protection settings for SPAD 346 C protection relays, introduces the working principle and setting effect of relays, and presents the calculation process through examples. It also describes protection application examples for three winding power transformers, motors, and generators, discusses the applicability of SPAD 346 C relays in frequency converter applications, and finally describes differential protection for short overhead or cable lines through examples of calculating the actual accuracy limit factor of intermediate CT.