A wind farm in Scotland was granted planning permission including a connection in principal was offered, however SSE were concerned of the number of wind turbine transformers that could potentially be energised simultaneously. Energisation of unloaded transformers results in a magnetising inrush current with a high amplitude.
The inrush currents can have many unfavorable effects, including operation failure of transformer differential protection, deterioration of the insulation and mechanical support structure of windings and reduced power quality of the system. Without controlled switching the energisation may occur at any time on the voltage wave producing high current peak, when the transformer core is driven into saturation.
The wind turbines power transformers, a vital components of the 11kV electric power network, required the protective relays with very high dependability, security, and speed of operation. But the magnetising peak current, which is often generated when the transformer is energised, may have caused false tripping of the differential relay protection, therefore a reduction of peak current would be necessary. Some methods have been to reduce high peak currents. Pre-insertion of series resistors and synchronous closing of circuit breakers are examples of the available mitigation techniques.
However, Engineering Power Solutions concluded that the most convenient and cost effective solution in this case was to control transformer energisation through operational procedures by means of controlled step sequencing methods, switching on transformers individually through a PLC based system.