The Role of Switchgear
What is switchgear and how does it fit into the power system?
Switchgear is critical to the protection and control of medium voltage systems in many utility and industrial applications. Often these devices, consisting of a control house and multiple circuit breakers, serve as the primary means for rerouting power through the various branches of distribution circuits. They also provide overcurrent protection against through faults, which provides a means for circuit interruption when these over current conditions are present.
Two Failure Modes of Switchgear
Breaker Related Failures
Loss of lubrication, lack of preventative maintenance, and deterioration due to age are the greatest causes of failure for the breaker. Symptoms of breaker failure include slowed breaker interrupting times, failure to operate, thermal deterioration of rear disconnects, and damage to protected equipment due to excessive through fault exposure.
Bus Related Failures
Exposure to moisture, dust or other contaminants is the highest cause of switchgear bus related failures. Partial discharge (PD) is a leading indication of insulation breakdown occurring on the operating bus. Typically, partial discharge most commonly occurs in systems greater than 2000 volts and if left unaddressed can result in a complete breakdown of insulation systems. Due to this breakdown of insulation, there is often a conductive path created from either phase-to-phase or phase-to-ground allowing an arcing event to occur which often results in system failure. Symptoms of PD could include corona buildup, tracking on the bus/insulation or flashover events.
Online Switchgear Monitoring
Difficulties of Offline Maintenance
Offline maintenance of switchgear can be a difficult task and is a time-consuming process. It requires racking out of breakers which exposes rear contacts to excessive wear and possible misalignment. More importantly, the crew who performs the testing could be susceptible to potential arc flash hazards. With offline testing, slow interrupting times can often go undetected due to operation of the breaker before testing. The incipient stages of insulation failure can also often go undetected.
Convenience of Online Monitoring
Online switchgear monitoring is an easy, comprehensive approach to managing switchgear. With online monitoring, asset managers have a viable test of breaker operations every time the breaker operates. Monitoring allows for direct maintenance on the breakers that need it. Monitoring can offer a comprehensive look at breaker operations and bus PD activities.
What areas should be monitored on a switchgear?
- Breaker – track the breaker to make sure it is working properly
- Motor – keep an eye on motor starts, current and runtimes
- Heater – watch for heater insulation deterioration due to moisture ingress
- Bus – observe PD as a leading indicator of insulation breakdown
- GIS Breakers – consider real-time pressure readings for GIS breakers and operational bus, forecast leakage rates, and provide a mass gas loss for reporting.
- Cable – look at the cable in the gear itself and also the feeder cable
What should I look for in a switchgear monitor?
A monitor should
- Have an easy-to-use webpage to define breaker parameters, assignments and features to be monitored
- Monitor multiple bays with a single monitor
- Capture waveforms of every operation for each individual breaker (trip coil signature, close coil signature, load current and aux contacts)
- Offer PD monitoring for operational bus and circuit breakers
- Monitor SF6 pressure for GIS breakers
- Have smart waveform overlay which allows for detection of minor changes in breaker operation
- Record coil current
- Provide motor and heater monitoring