Instrument Transformer Construction & Working

For measurement of large currents and high voltages in AC circuits, specially constructed accurate ratio transformers are used in conjunction with low range AC instruments. These specially constructed transformers are known as instrument transformers and are of two types:

  • Potential Transformers (PTs)
  • Current Transformers (CTs)

These instrument transformers are also used in power system in conjunction with protective relays. For safety purposes, the secondaries of these transformers are grounded.
 

Current Transformer Construction & Working

 
Current transformers are used in AC power circuits to feed the current coils of indicting and metering instruments (ammeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low current instruments suitable for measurement of high current and isolate them from high voltage.
 
The current transformer basically consists of an iron core on which a primary and one or two secondary windings are wound. The primary winding has one or two turns of thick wire and is connected in series with the load. It carries the actual power system current. Primary current ratings vary from 10 A to 3000 A or more.
 
The secondary winding has a large number of turns of fine wire. It is connected across current coils of indicting and metering instruments and protective relays. The secondary current ratings are of the order of 5 A, 1 A, and 0.1 A. The latter is used for static relays. If for any reason the instrument connected to the secondary of CT is to be removed then the secondary of CT must be short-circuited by a fairly thick wire.
 
The ratio of primary current to the secondary current is known as transformation ratio of the CT. The transformation ratio of a CT is usually high.
 
The product of voltage and current on the secondary side when it is supplying its maximum rated value of current is known as the rated burden and is measured in volt-amperes (VA). The volt-ampere rating of CTs is low (5 – 150 VA) as compared to that of power transformers.
 
Also current in the secondary of CTs is governed by the current in the primary winding i.e. power circuit current. But in the case of power transformers, it is governed by load impedance.
 
instrument transformer working
 

Potential Transformers Construction & Working

 
Potential transformers are used in AC power circuits to feed the potential coils of indicting and metering instruments (voltmeters, watt-meters, energy-meters) and protective relays. These transformers make the ordinary low voltage instruments suitable for measurement of high voltage and isolate them from high voltage.
 
The PTs are highly accurate ratio step down transformers. Its primary winding has a large number of turns and is always connected across the supply system. Its secondary winding has few number of turns and is connected to the potential coil of indicting and metering instruments and protective relays. The primaries of PT are rated from 400 V to several thousand volts and secondaries always for 110 V.
 
The ratio of the rated primary voltage to the rated secondary voltage is known as turn or transformation ratio of PT.
 
The burden is the total external volt-ampere load on the secondary at rated secondary voltage.
 
The rated burden of a PT is the VA burden which must not exceed if the transformer is to operate with its rated accuracy.
 
The maximum burden is the greatest VA load at which the PT will operate continuously without overheating its winding beyond the permissible limits.
 
Let the voltage to be measured of a power system is 11 kV. It is impossible to measure such a high voltage directly by a voltmeter. Therefore, a PT having secondary to primary turn ratio 1:100 is used in conjunction with a voltmeter which steps down the voltage from 11 kV to 110 V as shown in the figure.
 
potential transformer
 
For measurement of power in a high voltage power system, both CT and PT are used. The CT is used to step down the system current and the PT is used to step down the system voltage up to the required value. The potential coil (PC) of the wattmeter is connected across the secondary of PT and the current coil (CC) of the wattmeter is connected across the secondary of CT as shown in the figure.
 
power measurement in ht circuit
 

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