From Transmission and Distribution Electrical Engineering, Third Edition


Current and voltage transformers are required to transform high currents and voltages into more manageable quantities for measurement, protection and control. This chapter describes the properties of current transformers (CTs) and voltage transformers (VTs), and how to specify them for particular applications.


5.2.1 Introduction

A current transformer is used to transform a primary current quantity in terms of its magnitude and phase to a secondary value such that in normal conditions the secondary value is substantially proportional to the primary value. IEC 60044 (instrument transformers) covers CTs and VTs, superseding IEC 60185 (current transformers for measuring and protective applications) and its predecessor IEC 185.

5.2.2 Protection CT classifications

Protection CTs, unlike measuring CTs, may be required to operate at many times full load current. Linearity under these conditions is not of great importance. The essential point is that saturation must be high enough to drive the magnetizing current and the secondary current under fault conditions. 5P or 10P classification

Several terms are used in connection with CTs and these are described below:

Rated primary (or secondary) current

This value, marked on the rating plate of the CT, is the primary or secondary current upon which the performance of the transformer is based.

Rated transformation ratio

The rated transformation ratio is the ratio of rated primary current to rated secondary current and is not necessarily exactly equal to the turns ratio.

The magnetizing current depends upon the magnitude of the primary...

Products & Services
Instrument Transformers
nstrument transformers are used to step-down current or voltage to measurable values. They provide standardized, useable levels of current or voltage in a variety of power monitoring and measurement applications.
Current Transformers
Current transformers are used to step down current in a very predictable fashion with respect to current and phase. They are generally used as inputs to current instruments.
Single Phase Transformers
Single-phase transformers output single-phase AC power, typically while stepping-up or stepping-down the voltage between two circuits.
Voltage and Potential Transformers
Voltage and potential transformers are used to measure voltage (potential). The secondary voltage is substantially proportional to the primary voltage and differs from it in phase by an angle that is approximately zero.
Harmonic Mitigating Transformers
Harmonic mitigating transformers use phase-shifting, electromagnetic flux cancellation, and source impedance to reduce harmonic currents in electrical distribution systems.

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