PT or VT Working principle, Errors (concept only), Accuracy class, Burdens, Specifications, Precautions.

 Voltage transformers are used for measurement and protection.

Potential transformer or voltage transformer gets used in electrical power system for

stepping down the system voltage to a safe value which can be fed to low ratings meters

and relays. Commercially available relays and meters used for protection and metering, are

designed for low voltage.

These transformers develop a voltage on the secondary, substantially proportional to the

voltage on the primary (there being no knee point saturation, as is sometimes required in

CTs

Voltage transformers are connected across the points at which the voltage to be measured

for metering and /or protection.

They may be either single phase or three phase. Rating of VT is lesser as compared with that

of power transformer.

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) – 5-limb and

(iii) Capacitor voltage transformers (CVTs). These may be used for metering or protection,

with very little difference between the two.

2 Control transformers

Electromagnetic voltage transformers, in which primary and secondary are wound on

magnetic core like in usual transformer.

a)Capacitive Voltage Transformers (CVT), in which the primary voltage is applied to a series

capacitor group. The voltage one or few of the capacitor is taken to auxiliary

electromagnetic voltage transformer. The secondaries are taken for measurement as well as

protection.

b)Coupling capacitor voltage transformer- which serves as a combined VT and coupling

capacitor ( for Power Line Carrier Communication, PLCC)

c)Capacitor – bushing voltage transformer

The construction of a voltage transformer differs from that of a power transformer in that

different emphasis is placed on cooling, insulation and mechanical design. The rated

output seldom exceeds a few hundred VA and therefore the heat generated normally

presents no problem.

PT or VT may be PVC taped, thermoplastic (polypropylene) moulded, fibreglass taped,

polyester resin cast or epoxy resin cast, oil immersed depending upon the system voltage

and the surroundings.

HV indoor transformers, for instance, are generally polyester or epoxy resin cast or oil

cooled and are economical with good dielectric properties. They are resistant to humid,

chemically contaminated and hazardous areas.

Outdoor HV transformers, however, may be epoxy resin cast, oil or SF6 insulated and oil or

SF6 cooled. Epoxy insulation provides better mechanical and constructional qualities. They

are resistant to humid, contaminated and corrosive atmospheres and are suitable for all HV

systems. They are mechanically strong and can bear shocks and impacts.

The primary winding is made of large number of turns of fine wire while the secondary

winding consists of only few turns. The voltmeter and pressure coil of wattmeter are

connected with the secondary terminals of PT.