Previously, the UL safety standard required that an SPD withstand a continuous current conduction of five amperes for at least seven hours. The specification related to situations when the SPD is required to clamp voltage to a safe level, allowing the unit to withstand a continuous short circuit originating from the power system. During this test the device is not allowed to explode, start a fire or generate smoke.
Use of SPDs containing standard metal oxide varistors (MOVs) that are normally designed to conduct short impulse currents to clamp high-energy impulses is a challenge. No standard MOV can withstand 5A for seven hours without overheating. The practice in the past was to disconnect the MOV during the UL test to prevent it failing. If a disconnection mechanism was not included in the SPD, fuses were installed in front of the SPD to achieve the same result. With either solution the wind turbine is left unprotected during a power surge, just as it needs most protection.
Since introduction of the revised standard, an SPD must be able to withstand a current of 1000 amps for seven hours. The new rule means that SPDs designed with mechanisms that disconnect the unit in self protection are not enabled to disconnect up to 1000A. As a result, a large number of SPD types based on standard MOVs will no longer meet the UL requirements.
Alternatives have to be found. Spark gaps could theoretically be used, but spark gaps are generally not allowed inside cabinets due to the open blasting. Spark gaps also have higher clamping voltages (the voltage which is present when the SPD suppresses a surge) than MOVs. This clamping voltage has to be much lower than the immunity level of the protected equipment.
Varistors are still the best components for protection against fast transients, such as the huge differences in current resulting from lightning strikes. New technology based on a single high capacity MOV is available and seems to be the best approach for achieving optimum protection featuring high energy absorbing capacity, low clamping voltage and an ability to handle several hundreds of surges.