Repetitive VT Failure – Root Cause Analysis

Uninterrupted electricity supply is key to continuous industrial operations to avoid unplanned trip or shutdown and consequent revenue loss. Substations or switchyards set up by industries comprise of elements such as lightning arrestors (LA), isolators (IS), circuit breakers (CB), power transformers (PT), current transformers (CT), voltage transformers (VT), etc., that must operate efficiently to meet electrical demand.

Research establishes that reliability of VT is dependent on load-shedding ratio. Load shedding of fully loaded transformer may create high voltage transients on the main bus causing insulation breach in the network resulting in earth fault. Primary winding of VT is also susceptible to such insulation breach. Short circuit in VT causes blowing of fuse, cracking of VT, melting of VT cast resin, loss of metering, loss of information to protection system resulting in shutdown and blackouts irrespective of coordinated protection systems.

Below is a case study to analyze the failure of voltage transformers during transient conditions.

Problem Statement 

During operation a plant was experiencing chronic failure of Voltage Transformers (VT) installed at its 33 kV bus. The VT was repeatedly changed, and many countermeasures were implemented, but the failure reoccurred.

APPROACH

Manav Energy analyzed the transient performance of the network to determine the switching overvoltage at 33 kV bus. It designed a snubber circuit to subdue the effect of switching transient for saving the VT from further failures.

Benefits

This study enables the O & M team to understand operational parameter that results in VT failure. It enables to determine implementable corrective measures to prevent chronic disruptions and keep the O & M cost minimal.