Partial discharge monitors are used to monitor partial discharge activities in high voltage equipment such as transformers, on-load tap changers (OTLC), bushings, switchgear and cables. High-voltage transformers convert power-level voltages from one level or phase to another. Transformers that use paper and oil decompose over time, causing the oil to become increasingly saturated with water. Left undetected, partial discharge or corona may occur and cause thermal faults or arcing. OTLCs are often used to control secondary terminal voltage regulation, but can fail due to contact wear, misalignment, overloading, or component fatigue. High-voltage bushings are designed to transfer load currents in and out of grounded enclosures, but are prone to sudden failures.
Partial discharge monitors can detect increased capacitance before bushing currents reach unsafe levels. Switchgear interrupts the very high currents caused by fault conditions, but fails at the breaker level. To prevent system failure, partial discharge monitors are used to detect parameters such as mechanism timing, arc duration time, and contact interruption duty. High-voltage cables degrade over time and experience partial discharge when small voids in insulation or terminations flash over internally as the voltage rises and falls during each half cycle.
Partial discharge monitors are used to detect tiny pulses of radio frequency (RF) current and correlate these values with the monitored phase. Selecting partial discharge monitors requires an analysis of specifications, features, and approvals.
Product specifications include power input, input correction, range, memory, outputs, dimensions and weight. Maximum pulse, absolute voltage peak, and frequency range are also important input-signal parameters to consider. Environmental conditions include operating temperature, storage temperature, and humidity range. In terms of features, partial discharge monitors may include integral software or use network protocols such as TCP/IP. Remote access can be achieved via a local area network (LAN), supervisory control and data acquisition (SCADA) system, or Internet connection.
Product features also include three-phase measurements, noise rejection by pulse shaping and rise-time detection, individual alert thresholds per phase, and local or remote alerts and alarms. Approvals for a partial discharge monitor include the CE Mark and compliance with specifications such as EN6101-1 for safety and EN61326 for electromagnetic compatibility (EMC).