Instrument Transformer (CT & PT)

Instrument Transformer (CT and PT)

(What is instrument Transformer? Explain it. Or explain about current transformer and potential transformer. Or explain construction details and working of CTs & PTs )

Current, voltage, power and impedance are the fundamental parameters of a power system. Condition of power system whether it is healthy or not will be determined by variation of these parameters from the values of those at normal operating condition. So the detection of fault and hence clearance of fault can be achieved if we can monitor these quantity throughout the operation. Normally power system variables are of very large magnitude in the range of Kilo Volts and Kilo Amps. It is very much costly and even impossible to use that quantity directly for control and protection but what we can do is that to reduce in proportionate amount and use that reduced quantity in required purpose. Transformer of special design used for that purpose are called instrument transformer. They are of two types:

i. Current transformer 

ii. Voltage transformer

Current transformer

Heavy amount of current flowing through the power line needs to be continuously monitors so that any fault conditions can be detected and prevention for the damages can be done. For this separate circuit is needed where proportionate amount of current of the power line can be achieved. This is achieved by the help of current transformer. The current ratings of the current transformer are generally 5 or 1 amps. In case of CTs primary current is the line current, which may be of heavy amount,and is unaffected by the condition of the secondary circuit. So, primary current is dominant factor.

CTs are further classified as

i. Protective CTs – used in association with relays, trip coils and pilot wires etc.

ii. Measuring CTs – used in conjunction with ammeter, wattmeter etc.

CT construction

There are two types of construction of current transformers.

a. Bar type current transformer

b. Wound type current transformer

                    Fig: Current Transformer 

Operation of  CTs ( Working):

Operation of current transformer is also based on the principle of faradays laws of electromagnetic induction. Primary carries the line current it produces alternating flux that will be linked with the secondary winding and current will also flow in the closed path of the secondary circuit. Current transformers are used to reduce the current flowing so that it will have larger no of turns in the secondary. Ratio of no of rated current in the secondary and primary is the rated ratio of the CT and the ratio of the actual values of the secondary and the primary current is the actual ratio of the CT. Vector diagram for a CT is shown in figure below which might be helpful on understanding the operation of the CT.

*Why secondary side of CT should not be opened while in operation?

CT is always operated with closed secondary circuit. If secondary terminals are left open the counter mmf produced by secondary winding will be zero and hence the large flux will flow through the core so that larger heat loss in the core and core may be burn out. Similarly, large emf appears in the secondary terminal.

Hence it may be dangerous to the working personnel in the secondary side. Hence secondary of the CT should never be opened while primary is carrying current.

Potential Transformer:

They are used to reduce the power system voltage to standards values and rto physically isolate the relays and other instrument (meter) from the high voltage of the power system.The voltage rating of the secondary winding of the voltage transformer (VTs) used in practice as 110V for the line or 100√3 V for line to neutral.

For the safety the secondary should be completely isolated from high voltage primary and should be grounded for affording protection to the operator.In case of measuring devices according scaling can be sine to direct read the HV reading.

Operation:

Operation of the VTs is same as that of the power transformer. Just the difference is that it handles small amount of power and low voltage on the secondary side. It is connected across the line (i.e. Is connected in parallel), so that voltage whose respective quantity is required can be obtained at the secondary of the transformer and that can be applied for measurement of protective scheme. Vector diagram of the PT is shown in the figure below which might be helpful in understanding the operation of the PT.

For the proper operation of the PT following specification of the PTs are required,

i. Rated primary voltage.

ii. Rated secondary voltage.

iii. Rated burden.

iv. Supply frequency

v. number of phases

vi. Class of accuracy

vii. Insulation level, power frequency and impulse voltage withstand.

viii. Limits of dimension, types of construction etc.

Difference between Current Transformer & Potential Transformer:

Current Transformer (CT) and Potential Transformer (PT) are instrument transformers basically used for measurement and protection purpose in power systems. 

A Current Transformer (CT) is used to obtain reduced current signals for purpose of measurement, control and protection. They reduce the higher current to lower values which are suitable for operation of Relays and other instruments connected to their secondary winding.

While a Potential Transformer (PT) used to reduce the voltage to lower values and to provide isolation between high voltage power network and the relays and the other instruments that are connected to their secondary.

Major differences between the Current Transformer (CT) and Potential Transformer (PT) are listed below.

Current Transformer:

Top Core Type CT is shown in figure below.

 The secondary of a C.T cannot be open circuited on any circumstance when it is under service.

 A CT may be considered as a series transformer.

 The primary current in a C.T is independent of the secondary circuit conditions (burden).

 The primary winding of the CT is connected in series with the line carrying the current to be measured. Hence it carries of the full line current.

 With the help of CT, a 5A ammeter can be used measure a high current like 2000A by using 2000/1 CT.

Potential Transformer:

 The secondary of a P.T can be open circuited without any damage being caused either to the operator or the transformer.

 P.T may be considered as a parallel transformer.

 The primary current of a P.T depends upon the secondary circuit conditions (burden).

 The primary winding P.T is connected across the line of voltage to be measured. Hence the full line voltage is impressed across its terminal.

 With the help of P.T, a 120V voltmeter can be used to measure very high voltages like 11 kV.

A Potential / Voltage Transformer (PT / VT) is represented as shown below.