Instrument Transformer-Current Transformer and Voltage Transformer

Instrument transformer are special type of transformer. Because we use them to increase range of ordinary ammeters and voltmeters. We use them in AC system measurement for measurement of electrical quantities. It also find it application in protective relays for the protection of power system.

The basic function of instrument transformer is to step down the AC system voltage and current. Because the level of voltage and current is very high. So, it is very costly and difficult to design the measuring instruments for measurement of high level quantities. That is why these instruments are popular in modern power system.

Limitations of Shunts and Multipliers

We have studied “shunts” and “multipliers” for extension of range of ammeters and voltmeters respectively. The use of shunt and multipliers have the following limitations

Limitations of Shunts 

  1. They can measure dc only. On AC measurement we can not achieve accuracy.
  2. They have a limit of extension and we can not be use it for very high currents. They have a capacity limited to few hundred amperes.
  3. For proper measurements, time constant of the shunt and meter should be same. Hence we need a separate shunt for each instrument.
  4. The problem of insulation of instruments and the shunts is quite difficult large currents.
  5. The measuring circuit is not isolated from the power circuit. This may create problem at high current measurements.

Limitations of Multipliers:

  • They can work satisfactorily up to 1000 V
  • In this case also, measuring circuit is not electrically isolated from the power circuit and that creates problems.
  • The other limitations of shunts also apply with multipliers.

TYPES OF INSTRUMENT TRANSFORMERS

The instrument transformers are specially designed transformers with accurate ratio of transformation. We can use it only for AC measurement.

Instruments transformers are of two types:

  • (1) Current transformers
  • (2) Potential or Voltage transformers

CURRENT TRANSFORMER (C.T)

We use them to measure alternating current, which exceeds the safe value of ammeter. The Figure shows a Current Transformer. The primary winding of Current Transformer carries current to be measured. The current is step down i.e., a C.T., is a “step down” transformer. The secondary winding is connected to an ordinary ammeter. Thus the C.T. steps down the current equal to the full scale deflection value or safe value of the ammeter.

The primary winding of C.T. is in series with the main line, whose current we want to measure. The primary current therefore is independent of the load on the secondary.

Construction of Current Transformer

The primary winding of Current Transformer type instrument transformer consists of few turns of thick wire. Hence there is no appreciable voltage across it. The secondary of the C.T. has large no. of turns of fine wire determined by the turn ratio.

Current Transformer
Current Transformer

The ammeter is connected directly across the secondary terminals. Thus the secondary operates nearly under short circuit conditions. The current in the secondary is not governed by its load impedance rather it depends upon its primary current.

The C.T. value are in volt amperes (VA) and they arc available between 15 to 30 VA.

Important Note

Potential Transformer

As mentioned already the ratio of transformation (K) is equal to the voltage ratio (V2/V1) and turn ratio (N2/N1). But in case of CT the ratio (K) is not equal to the turn ratio. Moreover its value depends upon magnetizing and loss components of the excilator current.Potential Transformer

This causes various errors. Therefore, it needs proper construction of Current transformer. For this purpose the core of C.T. should have low reactance. Arid low core loss to keep the magnetizing and loss components at low value. For this the material used for the core should have

  • High permeability
  • Low value of flux density
  • Low hysteresis
  • And low eddy losses.

TYPES OF Сurrent Transformer Based on Construction

(1) Clamp on/ Clip on / Tong tester

This C.T. can be used with a single conductor. The core of the C.T. can be split opened with help of a “trigger switch” provided with the device. Thus the core can be clamped around live conductor to measure the current.

Clamp of Meter
Clamp of Meter

This arrangement avoids breaking the circuit for connecting C.T. The single live conductor acts as a primary and the secondary is wound on the core of the C.T. The ammeter can be connected in the secondary. It is a portable instrument and generally used by scientist in laboratories.

(2) Bar Type Current Transformer

It has a circular ring type core, on which secondary is wound. It is used to measure current through a bus bar which is inserted through it. The ammeter is connected in the secondary. The instruments placed on panel boards can be used.

Precautions in the use of Current Transformer

The C.T. should always be used with secondary short circuited through ammeter, wattmeter or relay etc. It should not be used with open secondary while primary carrying current. Here is the explanations

(1) In normal operation short circuited secondary of a C.T has demagnetizing ampere turns and the flux density in the core is very low as compared in the primary. 

But if the secondary is open circuit. Then the demagnetizing effect of secondary disappears and flux density in the core increases many times. This large flux induces a very high voltage in the secondary. It is so great that it may be dangerous to the operator.

(2) In open circuit conditions, the C.T. may also be damaged. As Insulation break down may occur. Iron losses are increases to a great value and it may cause over heating of the core.

The C.T.’s are usually provided with a “short circuiting switch”. This should be closed before the secondary is opened for removal of the ammeter etc.

Note:The C.T. should never be operated with open circuited secondary.

POTENTIAL TRANSFORMER (P.T.)

We use it for measuring alternating voltages which exceed the safe value of voltmeter. They are also called as “voltage transformers“.

High voltage is first stepped down by using a potential transformer. This process of stepping down the voltages for the purpose of measurement we call as “Extension of instrument range“. This method of extension of range instruments is superior to shunts and multipliers.

The primary winding of P.T. is connected across the voltage V to be measured. This voltage is stepped down by the P.T.. An ordinary voltmeter is across the secondary of the P.T. Thus the P.T. steps down the voltage to the full scale deflection value of the voltmeter.

Construction of Potential Transformer

The P.T. in general are shell type power transformers. Except that the load on very small. We use P.T. to measure high voltage. They are also used to operate relays in power system. For safety, their secondary is completely insulated from high voltage primary. Also grounded for providing protection to the operator. The normal secondary voltage rating is 110 V.

The P.Ts. are basically step down transformers. The primary has large number of turns and secondary has small number of turns. A shell type construction is suited to P.T. as they give more accuracy. The P.T.’s up to 6.6 KV are “dry” type and their high rating are usually oil immersed type.

 Important

  • (1) The P.Ts. are like shell type power transformers. They should be designed to give a constant transformation ratio and a smaller phase angle. The primary and secondary windings are coaxial. It is to decrease the leakage reactance. The oil filled bushings are used to reduce the size.
  • (2) A potential transformer has larger core.
  • (3) The output of P.T. is always small and its size is quiteere is no problem due to rise in temperature.
  • (4) Usually, cotton tape and varnished cambric are used as insulation. Hard fiber “separators” are also used in between the coils.
  • (5) There is no danger, if the secondary of a P.T. is left open circuited.

BURDEN OF AN INSTRUMENT TRANSFORMER

It is convenient to express load across the secondary winding. As the output in VA at the rated secondary voltage. This is known as Burden of Instrument Transformer. The rated burden is the VA loading of the Instrument transformer. It is permissible without the errors exceeding the limits for the particular class of accuracy.

It is also specified in ohms and it is the ratio of secondary voltage to the secondary current.

Burden = Secondary voltage (V2)/Secondary current (I2)

ADVANTAGES OF INSTRUMENT TRANSFORMERS OVER SHUNTS & MULTIPLIERS

  1. Very large currents or voltages can be measured through Instrument Transformer.
  2. The readings do not depend upon resistance or  inductance or capacitance of the transformer. Note that the primary and secondary windings of a Instrument transformer are not pure inductive coils. But they consist of all the three parameters i.e. R, L and C.
  3. The ammeter/voltmeter is connected in the secondary winding. In this way, the measuring current is isolated from the power circuit which is very important point in high voltage system.
  4. Several measuring instruments can be operated from a single instrument transformer
  5. The instrument transformers can be used to measure power or energy consumed by a circuit. In this case, a wattmeter or energy meter is connected with secondary of the instrument transformer. Instrument Transformer can also be used in power system to operate various relays.
  6. They can also indicate power factor, frequency etc. of a power system.
  7. They enable the small range instruments to measure large range. The 5A ammeter can read up to 1000 A and a 110V voltmeter can read up to 66 KV

The only disadvantage of instrument transformers is that they cannot be used for dc measurements.

ERRORS IN INSTRUMENT TRANSFORMERS

(a) Ratio Error:

For satisfactory operation of an instrument transformer (I.T) the transformation ratio should be constant and within limits. It has been seen that they are not constant but vary with power factor of the load. The error so produced is called Ratio error.

The ratio of working component (Iw) of the exciting current to the secondary current of an instrument transformer is called its ratio error.

Ratio error = Working current of exciting current/Secondary current

The ratio between actual ratio of current transformation and the normal ratio is known as ratio correction factor (RCF)

RCF = Actual ratio/Normal ratio

Phase Error (Fig. 8.25)

The primary and secondary currents should by a 180° out of phase in a C.T. But in actual practice, the phase is not exactly 180° but slightly less. The angular displacement a between primary and secondary currents 1, and l2 (re versed) is called phase angle error or simply “phase error” of a C.T.

Similarly the angular displacement between primary voltage V, and secondary voltage V2(reverse) is called “phase error” or simply “phase error” of a P.T.

In case of a C.T, current ratio is more significant while phase angle error has no significance, so long the C.T. is feding an ammeter. But if a C.T. feeds a wattmeter’s current coil, then the phase error will produce error in measurement of power.

Similarly in case of P.T. voltage ratio is more significant while phase angle has no significance, so long the P.T. is connected to a voltmeter. But if a P.T. is connected to a wattmeter’s pressure coil. Then the phase error will produce an error in the measurement of power.

Minimizing ratio and phase errors :

The cause of ratio and phase error is the presence of magnetizing component of the primary current. This component can be minimized by the following methods:

  • By using high permeability materials for the core of the CT. Now a days, materials like hyperlink (50% Fe, 50%M) and permalloy (N1 = 5%. Co = 50%) are used.
  • By providing a suitable turn ratio i.e., no. of turns of secondary can be reduced by one or two turns.
  • By using a shunt across primary or sesecondary. If the secondary current is too large.

Read more: Measuring instruments