Requirement
When a transformer is energised, two main phenomena occur; an inrush current and a voltage drop at the primary side. The amplitudes of these two phenomena depend greatly on the remanent (or residual) magnetic flux, the transformer’s saturation curve and the instant of energisation. Before being commissioned, it should be checked whether the transformer is compliant or not with the local electrical grid standards.
Before the commissioning of a 40MVA power transformer on the UK electrical grid (33kV), the customer wanted to check if the transformer complies with the national standards in terms of voltage drops (« P28: Planning limits for voltage fluctuations caused by Industrial, Commercial and Domestic equipment in the United Kingdom ») during its energisation process.
If the voltage drop exceeds the limits, the transformer will not be compliant and therefore unfit to be commissioned. Mitigation devices should be installed in order to reduce the voltage.
Realization
For the case of this transformer commissioning, Capsim built a numerical model of the electrical network on EMTP-RV. The properties of the power transformer such as the saturation curve and the residual magnetic flux were included. Based on the worst cases scenarios for the energisation process, it has been observed that the voltage drop exceeds the P28 standards limit values.
Results
Mitigation devices could be sized with the model to limit inrush current effect at the expected level and fulfill standard requirements.
Evolution of the currents at the primary when the transformer is switched on
