Transformer Energization Studies

1.??????Study Methodology

Transformer energization is a regular operation in transmission and/or distribution networks. There are mainly two issues which may be consequential for the transformer itself and the associated network up to two nearby buses and are: -

1- RMS voltage dips/sags due to inrush current

2- Transient over voltages (TOVs) due to parallel network resonance

Transformer energization is, in majority of cases, a planned operational event for which the associated voltage dip/sag must be limited to withstand level of voltage sensitive industrial loads. On the second category, inrush current interacts with the power system and can result in long duration resonant TOVs. This second category issue is a major concern at weak systems nodes (Higher inductances) having long lines and cables (Higher capacitances). This inductance capacitance combination creates lower resonant frequencies and the chances of TOVs are increased [4].

This study comprises of multiple phases like: -

a)??????Defining study cases for the project

b)?????Model Validation

>?Validation of source data (POC with minimum short circuit level)

>?Validation of shunt reactor losses

>?Validation of transformer parameters

>>Inrush current

>>No load losses

>>Full load losses & short circuit impedance

1.1????????Transformer Energization Scenarios

In the light of ER P28 guidelines, three types of events must be evaluated for each project. The events defined for this project, having four power transformers,will be as follows:

1.1.1????????Frequent Events

The frequent event, for this project, will the energization/de-energization of a single transformer when other transformers are in operation. In this event, both inrush currents caused by the energization of said transformer and sympathetic interaction caused by the already in-operation transformers and shunt reactor will be assessed.

1.1.2????????Infrequent Events

The frequent event, for this project, will be the simultaneous energization/de-energization of two transformers when other transformers are in operation. In this event, both inrush currents caused by the energization of said transformer and sympathetic interaction caused by the already in-operation transformers and shunt reactor will be assessed.

1.1.3????Very Infrequent Events

The very infrequent event, for this project, be the simultaneous energization/de-energization of all the transformers. In this event the cumulated effect of the inrush currents and voltage dips at the POC will be assessed.

2.????ER P28 Recommended Events Results

The frequent, infrequent and very infrequent event are simulated to check the response of the network. A sample result with ER P28 (UK Grid Code) envelope is here for reference.

Transformer energization study depends upon the residual flux and its orientation, time of energization of transformer with respect to voltage/current waveform, source strength and loading conditions of transformer during energization.

Careful studies before commissioning, good engineering practices during energization events and continuous monitoring can prevent the untoward situations.