High-voltage resistor features corona discharge shield

Historically, HVDC systems have been based on line commutated converter technology. Voltage source converter technology offers a more compact solution and is used where space is restricted, such as on offshore wind farms, however resistors are needed to limit the very high currents developed upon start up.

Very high voltages can ionise air. Known as corona discharge, this ionisation can cause radio interference, reduce system efficiency and potentially damage equipment. Even when not absorbing energy, the resistor is still connected to the high voltage circuit and so corona discharge needs to be accounted for.

Working with industry specialists, Cressall designed and fitted corona shields, smooth aluminium bars on the top and bottom of the structure that reduce the electrical stresses and evenly distribute the electrical field to minimise ionisation potential.

To prove that corona discharge would not be generated, the electrical stresses on the resistor were measured by conducting a radio interference voltage test at 275kV.

“Because the voltages and energies are so high, we've had to systematically design and test the PIRs” explained Martin Nicholls, Sales Director, Cressall. “As well as the RIV tests, we have conducted high current density testing to confirm the design will safely absorb the high energies applied in such a short time. By impulse testing at 475kV, we've made sure that the insulation and the air gaps will not break down with the application of high voltages. In addition, using Ansys design software, we've ensured that the structures can withstand wind speeds of up to 55mph. All in all, we are extremely pleased with how the PIRs responded to all testing.”