500kV AC Frequency Resonant Hipot for High Voltage Withstand Testing
High Voltage withstand testing is the best way to ensure that new electrical assets are fit for purpose before connection to the grid. The advantage of a hipot test over a soak test is clear and in summary the following key benefits are achieved -
- The AC Hipot is a controlled energy device that limits the amount of fault current that flows when a dielectric breakdown occurs. This provides immensely improved safety when compared with a soak test, whereby the new asset is connected to the grid and everyone hopes it doesn't flashover.
- The AC Hipot test will stress the insulation at a voltage significantly higher than the network voltage (in accordance with the relevant standards and NEM requirements) to give a far greater level of certainty that the plant is not on the verge of imminent failure.
- The cost to perform an AC Hipot is very favourable when compared with the cost of an unscheduled outage and the direct and consequential damages that flow from such an event.
Megavar will be providing high voltage AC withstand testing with our new state of the art Frequency Resonant High Voltage Test System. The system is designed to optimise portability, so that we can respond rapidly and cost effectively to our clients test requirements nationally.
The system uses novel design techniques to achieve power to weight ratios that outstrip the competitors aged test systems and allow us to manoeuvre in tight yards with limited clearances. Our unit is capable of testing a range of apparatus including -
- Air insulated switchgear (AIS)
- Gas insulated switchgear (GIS)
- Instrument transformers
- Reactors and Capacitors
The general concept of resonant test sets, allows for a much greater output power to input power relationship, resulting in smaller components and reduced weight.
- Variable Frequency Resonant Test Systems vary the output frequency to tune the system to the load capacitance creating a resonance circuit.
- Output frequency is typically in the range of 30 to 300 Hz.
- Systems utilize a 3-phase input and have a larger tuning range than variable inductance test sets. Because they are smaller in size than other systems, the variable frequency resonant design is ideal for commissioning tests of GIS and cable.
- Variable frequency test sets allow performance of tests within a frequency range close to power frequency.
- The relatively low testing power required, compared to conventional non-compensated test sets, and the ability to make a direct comparison of the factory and field tests, favors the Variable Frequency Resonant Test System as the most practical on-site test system.
Resonance is defined as the condition at which the net inductive reactance cancels the net capacitive reactance. The resonant circuit must have both capacitance and inductance. (In addition, resistance will always be present due either to the lack of ideal elements, or the control offered on the shape of the resonance curve.)
When resonance occurs, the energy absorbed at any instant by one reactive element is exactly equal to that released by another reactive element within the system. In other words, energy pulsates from one reactive element to the other. Therefore once the system has reached a state of resonance, it requires no further reactive power since it is self-sustaining. The total apparent power is then simply equal to the average power dissipated by the resistive elements.
The average power absorbed by the system will also be at a maximum at resonance. The commonly used measure of the quality in a resonant circuit is the quality factor, or Q. The power source of resonant circuits operating in the resonant mode (exciter and regulator) is used to supply the dissipated energy. Q is approximately equivalent to the ratio of the output kVA to the input kVA. Given kVA requirements of the load and the Q of the test sytem, the input power can be obtained by dividing the kVA by the Q.