Protection Relay Testing Using Damped Oscillatory Waves
Liberalization of the energy markets worldwide has necessarily led to changes in the power distribution networks and billing systems. A technological revolution is taking place which requires modernization and expansion of the existing networks. Power companies must constantly monitor voltage, current and frequency at all points in the network, particularly in remote unmanned substations where a central control office will be responsible for switching transformers on or off line as the need arises. Communication between the remote substations and control office is usually via telephone lines using high speed communication protocols. The data is derived from a number of monitor points by devices called measuring and protection relays. These can simultaneously monitor multiple points and transmit the data via a communication interface. Traditionally, substations have employed air insulated switchgear in “fields” spread over a large area. Switching events within these can generate disturbance which is transmitted directly or by an electromagnetic field to the control elements. Monitor devices co-located in sub stations have to be tested for immunity to these switching disturbances. Traditionally, damped oscillatory waves have been employed with frequencies of 100 kHz and 1 MHz up to 2.5 kV. The IEC 61000-4-12 was written to provide the basis for damped oscillatory wave tests at these frequencies. As part of the modernization in sub stations, the air insulated switchgear is being replaced with more compact gas insulated switchgear using SF6 gas as the insulator. This requires a jacket to contain the gas and ensure insulation of the high voltage contacts. Switching transients still occur but are now present at frequencies and voltages much higher than experienced in the past. In reality, transients have been measured up to 50 MHz oscillation frequency.
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