The mystery of nuisance tripping incidents in transformer protection that worry engineers

Transformer Failure Incidents

This technical article deals with transformer failure incidents due to nuisance tripping caused by various design flaws, operational conditions, or improper protection relay settings. The focus is on differential protection and ground protection, as they account for a considerable number of false tripping.

I will try to explain two different case studies of primary substation 132/66 kV ∆/∆ and investigate the incident of feeders breakers tripped open although most of the loads were disconnected, and what went wrong and caused the transformers to go out of service.

Table of Contents:

1. Introduction to Power Transformers
2. Transformer Types
   1. Applications that Impact Transformer Protection
   2. Transformer Failure Statistics
3. Transformer Protection
   1. Differential Protection
   2. Ground Fault Protection
4. Case Studies

1. Introduction to Power Transformers

Transformers are electrical devices that transfer power between two circuits at different voltage and current values using the principle of induction discovered by Faraday in 1831. Devices that are based on the Faraday law like inductors used to be in laboratories till the AC was launched near the end of the 19th century.

The development of the AC power system and transformers almost occurred at the same time because they belong to they are interrelated.

Transformers vary in types, rating, and design to suit the different connected load types, but they follow the same principle of operations and they do have the same construction (more or less).

In the following, a list that contains most transformer components could be identified:

• Magnetic circuit: It consists of a core and yoke that provides a path for magnetic flux.
• Electric circuit: It is the primary and secondary winding that form the transformation voltage ratio. Transformers are categorized as shell type or core type according to the primary/secondary winding arrangement.
• Dielectric circuit: It is composed of insulation used in different places within transformers.
• Tank and accessories: Accessories differ from transformer to transformer based on the specifications and applications. They include conservator, breather, explosion vent, radiator, bushings, and many others.

Transformers are prone to nuisance tripping as a result of design flaws, operational conditions, improper relay settings, or otherwise. Transformers are a major component of substations and many times it is not affordable to have accidental outages, especially for bulk substations that serve large areas.