Instrument Transformer Calculations and Dimensioning For Substation Protection

Substation protection calculations

The purpose of this document was to get to know the theory behind instrument transformers in substations and to create a calculation template in Microsoft Excel which would simplify the current transformer calculation process.

The way of implementing different calculations to be print ready to customers was to make several sheets for different protection functions. The basic instrument transformer theory is covered in the first segment followed by basic relay protection functions.

In the dimensioning segment the used standards for instrument transformer from IEC are presented along with different types of current transformers that are specified in the IEC 61869 instrument transformer standard.

Relay protection

Instrument transformers are a key part of relay protection. The measured signals from current and voltage transformer secondary coils are used in the relay to detect fault situations and quickly isolate the fault from the network.

1. Protection Functions

Different protection functions are used to protect different parts of the electrical grid. Different protection functions set different requirements for the protective current transformer.

1.1 Line Distance Protection

Line distance protection is one of the most used and important functions of high voltage transmission line protection. The impedance of a transmission line is proportional to its length. If the measured impedance of the transmission line is smaller than the calculated impedance of the transmission line then the distance protection function will trip.

Distance protection is also called impedance protection.

Transmission lines vary in length and distance protection is not ideal for protecting short transmission lines.

1.2 Line Differential Protection

Differential protection compares the current amplitude and/or –phase angle of both ends of the transmission line. If the measured value exceeds a set value, circuit breakers will trip at both ends of the transmission line and isolating the faulty section of the network.

Line differential is usually a better option than distance protection for short transmission lines. Differential protection is also commonly used in transformer and generator protection.