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Effects of Low Resistance Measurement Instruments on Protection and Control Devices

Effects of Low Resistance Measurement Instruments on Protection and Control Devices

Powertest Conference

This paper will provide an insight into the factors that can go wrong during micro ohm measurements and how to prevent those using a proactive approach.
Author(s): Dinesh Chhajer and Daniel Carreño
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Low resistance micro ohm measurement is a routine diagnostic test performed on Circuit Breakers (CB) and bus bars in medium and high voltage substations. This test is important to check the integrity of CB contacts and detection of high resistance joints and terminations for installed bus work. Although the test is very simple and easy to perform, it can prove problematic for other devices connected to the same circuit. 

The DC current used for measuring the low resistance can affect the protection and control circuitry in adverse ways. Typical problems encountered are Current Transformer (CT) magnetization, accidental tripping of differential relays and inconsistent and unreliable low resistance measurements. The paper will address the root cause of these encountered problems. It will recommend precautions to take prior to performing the test and discuss in detail the recommended practices to avoid any misoperation of connected protection and control devices. 

The recommended practices will be supported by both a real life case study and lab simulations to thoroughly explain the impact of quality of DC current used for the low resistance measurements, effect of in service tap CT ratio, and associated settings of connected relays. This paper will provide an insight into the factors that can go wrong during micro ohm measurements and how to prevent those using a proactive approach.