When a transformer is first energized or reenergized after a short interruption, the following phenomena can be observed:
- A transient inrush current up to 10 to 15 times larger than rated transformer current can flow for several cycles.
- A voltage drop at the primary side.
Worst case inrush happens when the primary winding is connected at an instant around the zero-crossing of the primary voltage with residual flux left on the transformer core. inrush currents may cause fuses, reclosers, or relays to falsely operate.
Before being commissioned, it should be checked whether the transformer is compliant or not with the local electrical grid standards.
If the voltage drop or inrush current exceed the limits, the transformer will not be compliant and therefore unfit to be commissioned. Mitigation devices should therefore be installed in order to reduce the voltage drop and inrush current.
The way we proceed:
We perform transformer energization analysis using EMTP-RV software.
We accurately model the transformer and all the upstream equipement untill the power supply.
While modeling the transformer, we pay a particular attention at the folowing:
- Transformer Core Saturation,
- Mutual inductances betwen windings.
Also, different switching scenarios are analyzed: Remanent (or residual) magnetic flux, and the instant of energization.
We can carry out on site measurements to obtain the missing equipment parameters and validate the simulation model.