One of the most critical devices on any electricity network is the circuit breaker which can be described as the silent sentinel, standing guard over the network and only being called into action when a fault occurs.
The circuit breaker needs to operate within its composite tripping time (relay trip time + circuit breaker main contact opening time) to ensure correct discrimination with upstream circuit breakers and therefore minimise the number of customers disconnected during a fault operation. A slow tripping circuit breaker will not only result in unnecessary disruptions to electricity supplies but maintaining high fault currents for extended durations can stress the network and cause damage to plant and equipment.
Often the problem that caused a slow trip is temporarily cleared during the first trip operation because the stickion in the mechanism is freed up. Traditional diagnostic testing procedures usually require the circuit breaker to be isolated and removed from service, therefore not only has the opportunity to detect the cause of the defect been missed but also the tests do not focus on the condition of the operating mechanism apart from overall speed of operation.
Conventional testing has normally required removing the circuit breaker from service. However, isolating the circuit breaker is time consuming as it requires a planned outage involving the issue of switching instructions, safety documents and will often involve several engineers or technicians onsite.
This process resulted in vital information relating to the first trip not being captured and often the problem that caused a slow trip in a circuit breaker was temporarily cleared during this first trip operation.
When a slow trip operation occurred the focus tended to be on timing tests to determine if the problem lay within the protection relay or the circuit breaker operating mechanism. Rarely were the problems due to slow operation of the protection relay. Timing of the circuit breaker main contacts merely indicated the circuit breaker was operating within its design specification. Therefore the initial tests were inconclusive, so the focus then turned to inspecting the main operating mechanism and lubricating the numerous mechanical components. In some cases this further compounded the problem as the wrong type of lubricant was used.
Diagnostic Testing for Slow Tripping Circuit Breakers
The failure of conventional timing tests to diagnose the causes of slow tripping circuit breakers lead to the search for a technique that would pinpoint the cause during the critical first trip. Although timing tests can confirm that circuit breakers overall operation is within set limits, it was discovered that monitoring the current flowing through the trip coil provided a very powerful methodology for analysing the readiness of a circuit breaker to trip. Trip coil profiling was developed as a condition monitoring technique where deviation from a standard profile could help to pinpoint a potential problem within either the trip coil or circuit breaker main operating mechanism.
Analysis of a Trip Coil Profile
This diagram shows a typical trip coil profile with its distinct shape where each stage of the trip coil mechanism can be identified through its operation and problems identified if the profile deviates from a signature trace
Development of Portable Technology for measuring Trip Coil Profiles
Initially the monitoring of trip coil profiles required the hard wiring of equipment into the circuit breaker. However this was cost prohibitive and could only be justified for circuit breakers located at extremely critical nodes on the network. The development of portable, light weight, hand held devices with non invasive connections to monitor the key current and voltage parameters has revolutionised the testing of circuit breakers. The process of testing a circuit breaker only takes a few minutes, enabling a condition assessment of the operating mechanism to be obtained during the first trip operation. As well as indicating the health of the circuit breaker trip & close coil mechanisms, other useful parameters are obtained which include -
Furthermore this information is immediately available for onsite analysis and corrective maintenance can be targeted at a defect. The advantage of a portable hand held device means that it can be considered as an invaluable addition to the maintenance technicians toolkit that can be used at any circuit breaker location.