Diagnostic measurements on instrument transformers – Part I
Abstract The article presents the most common diagnostic measurements on instrument transformers (current and voltage transformers) used to assess their condition and reliability. The measurements...
byFlorian PREDL, Dr. Michael FREIBURG, Dr. Martin ANGLHUBER
Abstract
The article presents the most common diagnostic measurements on instrument transformers (current and voltage transformers) used to assess their condition and reliability. The measurements reveal possible failures which can occur due to aging processes during the lifetime of an instrument transformer. The failures are either related to the transformers’ electro-magnetic circuit (for example, short, or open circuits) or to their insulation (for example, partial discharge, moisture ingress). Case studies are presented to underline the importance of conducting regular diagnostic tests.
Keywords: instrument transformer, electro-magnetic circuit, insulation, diagnostic tests
1. Introduction
Instrument Transformers (ITs) are the eyes and ears of the electrical supply system. Although they are less expensive and can be replaced much faster than power transformers or generators, a severe failure can cause serious danger to people and significant damage and costs to the surrounding equipment, putting the overall system at risk. Without these ITs, key components such as transmission lines, power transformers, and generators cannot operate, causing loss of revenue or supply interruptions. Therefore, ITs should be tested on a regular basis to reveal the possible failures which can occur due to aging processes during their operation.
2. A classification and overview
In general, diagnostic measurements on ITs can be subdivided into two main groups:
• measurements on the electro-magnetic circuit
• measurements on the insulation system
2.1 Measurements on the electro-magnetic circuit
As part of the first group, measurements on the electro-magnetic circuit involve the magnetic iron core, the primary and secondary winding and, in case of a Capacitive Voltage Transformer (CVT), the compensation reactor and the capacitor stack. A measurement of the accuracy (ratio error and phase displacement) involves the electrical circuit of an IT. ITs which are used in metering applications require a high accuracy. Inductive current and voltage transformers and capacitive voltage transformers can develop ratio and phase deviations after some time in service. Shorted turns in current transformers and broken capacitive layers in the capacitor stack of a CVT are often undetected. This can lead to: readings errors, loss in revenue, and in some cases, a complete breakdown.
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