Is it safe?

Is it safe?

Special Edition - Dry-type Transformers

There is always pressure to re-energize a power transformer after a fault trips it out – but to do so comes with risks that need to be understood and addressed.

  1. Introduction

When primary station equipment trips out, it is often the substation maintenance engineers who must review the situation, get the data needed to make an informed decision, and then ‘make the call’. There is often ‘pressure’ to re-energize a large power transformer after a fault, especially when the cause of the fault is ‘clearly’ evident. In the story here, taken from a technical paper presented at the Doble Client Conference in 2012 by Mike Wolf of National Grid USA (now at First Energy) [1], two faults occurred in quick succession, tripping a large power transformer on a site where construction work was being undertaken to expand the station with the transformer adjacent to the site access road.


  1. Initial faults

The transformer in question, a 1969 Westinghouse unit, rated at 30/40/50 MVA at 115–34.5 kV providing power to residential and commercial customers. A support insulator broke on an incoming overhead, causing a grounded conductor to fall on two 34.5 kV circuits, which produced two successive faults, one on each line.

The initial 115 and 34.5 kV CTs which make up the differential protection, showed B-phase currents 180° out of phase, and thus a fault external to the transformer as the current is seen going through the transformer, with the fault cleared after 5 cycles. This overhead linelocked out on overcurrent.

The subsequent fault on a different line was also on the B-phase, which initially showed a 180° out of phase pattern, which quickly moved to 132°, which represents a fault inside the transformer’s differential zone taking some current to the ground.

To read the article, subscribe and choose the option which suits you best. We offer both free and paid options, and the registration takes only a minute.