Book at a Glance

PART II - Switchgear Assemblies and captive (Emergency) Power Generation

Chapter 15. Instrument and control transformers:applications and selection

• Introduction
Types of Transformers
Common features of a voltage and a current transformer
General specifications and design considerations for voltage transformers
Precautions to be observed while installing a voltage transformer
Current transformers
Short-time rating and effect of momentary peak or dynamic currents
Summary of specifications of a CT
Precautions to be observed when connecting a CT
Test requirements
Nonconventional methods of current measurement (sensors and transducers)
Rogowski coils

Types of transformers
Voltage transformers (VTs)
These may be classified as follows:
1. Instrument voltage transformers
(i) Conventional two-winding, electromagnetic voltage transformers
(ii)  Residual voltage transformers (RVTs) and
(iii) Capacitor voltage transformers (CVTs). These may be used for metering or protection, with very little difference between the two as noted later.
2. Control transformers

Current sensors
A conventional CT is the most used device to measure current of a circuit. But its magnetizing current causes a small phase displacement (d) in its secondary current  with reference to primary current I1 as shown in Figure 15.18. This displacement introduces a phase error (Table 15.8) in the accurate measurement of primary current. Lower the p.f. of the circuit higher the error. It poses a limitation for instruments and devices that are current operated and demand for accurate measurement of circuit current in turn to provide reliable measurements, such as testing instruments, measuring current and energy. This limitation of a conventional CT is overcome through electrically isolated (having no magnetic effect) current sensors or transducers. Some prominent transducers developed so far and available in the market are noted below,
– Resistive shunts
– Hall effect current sensors
– Faraday effect optical sensors
– Zero flux current sensors
– Rogowski current transducers or Rogowski coils
– Digital optical instrument transformers

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