Instrument Transformer- What is CT and PT?

Instrument Transformer- What is CT and PT?

INSTRUMENT TRANSFORMERS

Instrument transformers are the specially constructed accurate ratio transformers which are used for AC measurements  in conjunction with relevant measuring instruments. The instrument transformers are used to measure high voltage, current and power in AC system.

The ammeter shunt and voltmeter multiplier are also used to measure high current and high voltage respectively but the instrument transformers have following advantages over them:

  1. With the help of instrument transformers, the high current and high voltage can be measured using small range instruments. Usually the range of ammeter used with instrument transformer is 0-5A and the range of voltmeter is 0-110V.
  2. Instrument transformers provides safety to observer  because the measuring instruments can be located far away from high voltage circuit.
  3. The high current can be measured in power system without breaking the circuit by using clamp on current transformer.

 

TYPES

Instrument transformers are divided into two types:

  1. Current Transformer
  2. Potential Transformer

CURRENT TRANSFORMER

Current transformer, as the name suggests, are used to measure the high value of current in AC system. The current transformer is used in conjunction with ammeter to measure the current.  Current transformer is used for measuring current when the value of current is more than the safe current limit of ammeter. The current transformer is whether connected with ammeter or with the current coil of wattmeter or energymeter.

current-transformer-diagram
Current Transformer

The current transformer is basically a step up transformer which step up the voltage level of circuit and hence step down the current level of system. The primary winding of current transformer has very few turns of thick wire and it is connected in series with line whose current is to be measured. The secondary winding has large number of turns and directly connected with ammeter or current coil of wattmeter or energymeter.

The current transformer is considered as short circuited because the secondary winding of CT has very small load impedance. Current transformer working is different from power transformer because it works under short circuit conditions. The current in secondary winding of CT is depends upon the current flowing through primary winding rather than load impedance. Based on the construction, The current transformer are divided into two types:

  • Clamp-On Current Transformer
  • Bar Type Current Transformer

Clamp-On Current Transformer

Clamp-On Current Transformer is a type of current transformer in which the core can be opened with the help of clamp. The conductor whose current is to be measured, can be inserted in the core. The single conductor act as primary winding of CT and the secondary winding is wound on the core.

clamp-on-current-transformer
Clamp-On Current Transformer

Usually the current measuring devices are inserted in series with the circuit to measure the current for which, the circuit has to be opened but in case of clamp on current transformer, there is no need to break the circuit for measuring current. An ammeter is connected with the secondary winding of CT measures the current flowing through conductor directly.

Bar Type Current Transformer

The bar type current transformer consists of a circular ring type laminated magnetic core. The secondary winding of current transformer is wound on core and the ammeter is connected with secondary winding. The conductor whose current is to be measured act as primary winding. The conductor is inserted inside the magnetic core and ammeter directly measures the current flowing through conductor.  These type of current transformers are used with protective relays  or with the instruments placed on panels.

current-transformer-types
Types of Current Transformer

POTENTIAL TRANSFORMER

Potential transformers as the name suggests, are used in measurement of high voltage with the help of voltmeter. The potential transformers are used to measures the high AC voltage in conjunction with voltmeter and the voltmeter is connected on the secondary side of potential transformer. Potential transformer (PT) are also known as voltage transformer (VT). The potential transformers are basically shell type transformers because this type of construction provide higher accuracy. For voltage up to 6.6kV the dry type transformer are used and for voltage above 11KV the oil immersed transformers are used.

The potential transformers are basically step down transformers. The primary winding of PT has very large numbers of turns whereas the secondary winding has very small numbers of turns. Both windings are magnetically coupled and the voltage to be measured applied across primary side and measuring instrument are connected across secondary side. The secondary winding of potential transformer is either connected with voltmeter or with the potential coil of wattmeter or energymeter.

potential-transformer-diagram
Potential Transformer

Potential transformer working principle is similar to power transformer but the power loading of PT is very small as compared to power transformer. Sometimes the loading of potential transformers are as low as few volt amperes. In case of power transformers, the exciting current is very small fraction of secondary load current but in case of potential transformers, the exciting current and secondary load current are almost of same order.

 

DIFFERENCE BETWEEN CT AND PT

  1. The current transformer is considered as series transformer because the primary winding of CT is connected in series with the line whose current is to be measured. On the other hand, Potential transformer is considered as parallel transformer.
  2. The secondary winding of current transformer is under short circuited conditions because the ammeter connected on secondary side has very low resistance. On the other hand the secondary of potential transformer is considered as open circuited because the voltmeter connected across secondary side has very high resistance.
  3. The primary winding current of current transformer is independent of secondary winding conditions rather it depends upon the line current. In case of potential transformer, the primary winding current is depends upon secondary burden.
  4. In case of current transformer low voltage is applied across primary winding  because it is connected in series with the line whereas in case of potential transformer, full line voltage is applied across primary winding.
  5. The exciting current in case of current transformer varies over a large range because the primary current of CT varies over large range. On the other hand, the line voltage applied across potential transformer is constant and thus the exciting current in PT varies over a restricted range only.

 

POWER MEASUREMENT USING INSTRUMENT TRANSFORMER

Instrument transformer (CT and PT) are used to measure power or energy in high voltage AC power system. Both CT and PT are used at same time for power measurement purpose.  Potential transformer or PT is used to step down the voltage and current transformer or CT is used to step down the current.

current-transformer-and-potential-transformer
Power Measurement using CT and PT

Current transformer is connected in series with the line while the potential transformer is connected in parallel across the line. The secondary winding of current transformer is connected with the current coil of energymeter or wattmeter. The potential coil of energymeter or wattmeter is connected with the secondary winding of potential transformer. Thus the energymeter or wattmeter measures the energy or power of the system directly.

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