Background
A wind farm in Scotland was granted planning permission and a connection in principal was offered. However, SSE was concerned about 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.
These inrush currents can have many unfavourable outcomes, such as:
- Operational failure of transformer differential protection
- Deterioration of the insulation and mechanical support structure of windings
- Reduced power quality of the system
Without controlled switching, the energisation may occur at any time on the voltage wave producing a high current peak. When the transformer core is driven into saturation.
The wind turbines’s power transformers, a vital component of the 11kV electric power network, required protective relays with very high dependability, security, and speed of operation. However, 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.
In this case, Engineering Power Solutions concluded that the most convenient and cost-effective solution was to control transformer energisation through operational procedures employing controlled step sequencing methods, switching on transformers individually through a PLC-based system.
