Page 483 - Industrial Power Engineering and Applications Handbook

Instrument and control transformers: application and selection 151457
15.1 introduction associated with the same power system and interrupting
devices as a switchgear assembly, they should generally
meet the requirements for a switchgear assembly. except
Transformers are used in an auxiliary circuit, linked to a for small variations in the test requirements. For more
power circuit, to indicate, measure and control its voltages details refer to the following publications:
and currents. They find application in a switchgear or a
controlgear assembly and a switchyard. It would be
impracticable to produce indicating and measuring 1 For voltage transformers
instruments or protective devices to operate at high to IEC 60044-2 and IEC 60186 (for two-winding
very high voltages or currents. The universal practice, transformers such as CVTs)
therefore, is to transform the high voltages, say, 415 V 2 For current transformers
and above. and currents above 50 A to reasonably low IEC 60044- 1 and IEC 60044-6
values, as discussed later, for these applications. Indicating
and measuring instruments and protective devices are SECTION I: VOLTAGE
designed for these reduced values. The transformers used
to transform voltages are known as voltage transformers* TRANSFORMERS
and those to transform currents as current transformers.
Below we discuss their classifications, basic requirements 15.4 General specifications and
and design parameters.
design considerations for
voltage transformers (VTs)
15.2 Types of transformer

These transformers develop a voltage on the secondary.
15.2.1 Voltage transformers (VTs) substantially proportional to the voltage on the primary
(there being no knee point saturation. as is sometimes
The\e may be classified as follows: required in CTs (Section 15.6.l(viii)).

I Instrument voltage tramformers 15.4.1 Instrument voltage transformers
(i) Conventional two-winding, electromagnetic
voltage transformers
(ii) Residual voltage transformers (RVTs) and 1 Rated primun voltage
(iii) Capacitor voltage transformers (CVTs). These This will generally be the nominal sy\tem voltage, except
may be used for metering or protection, with for transformers connected between a phase and the ground
very little difference between the two as noted or between the neutral and the ground, when the primary
later. voltage will be considered as I/? 3 times the nominal
2 Control transformers systems voltage (V,)

15.2.2 Current transformers (CTs) 2 Rated secondary voltage
InEurope and Asian nations this is generally 110 or 1 101
These may be classified as: y;3 V, (63.5 V) for phase-to-phase or phase-to-ground
auxiliary circuits respectively. In @e USA and Canada
1 Instrument current transformers these voltages are 120 oy_ 120/ % 3 V for distribution
(i) Measuring current transformers systems and 1 IS or 1 IS/ %'3 V for transmission systems.
(ii) Protection current transformers and
(iii) Special-purpose current transformers, class 'PS'.
2 Interposing current transformers 3 Rated frequeizcx
3 Summation current transformers This may be 50 or 60 Hz as the system may require. The
4 Core balance current transformers (CBCTs) permissible variation may be considered as +2% for
measuring as well as protection VTs. These limits are
based on the recommended variations applicable for a
15.3 Common features of a voltage switchgear assembly (IEC 60439- 1 ) or an electric motor
and a current transformer (Section 1.6.2).

4 Insulation systems
15.3.1 Design parameters (service conditions These transformers may be PVC taped. thermoplastic
and likely deratings) (polypropylene) moulded, fibreglass taped. polyester resin
The\e are similar to parameters for a switchgear assembly cast or epoxy resin cast depending upon the system voltage
as discussed in Section 13.4. Since they are directly and the surroundings. HT indoor transformers, for instance,
are generally polyester or epoxy resin cast, and are
economical with good dielectric properties. They are
* Potential transformer (PT) is not the appropriate word to identify resistant to humid, chemically contaminated and hazardous
an instrument voltage transformer. areas. Outdoor HT transformers, however. may be epoxy