Current Transformer MCQ Quiz in বাংলা - Objective Question with Answer for Current Transformer - বিনামূল্যে ডাউনলোড করুন [PDF]

Concept:

The actual ratio of a current transformer is given by:

\(R = n+{I_W\over I_S}\)

where, n = Turns ratio

I_w = Wattage loss component 

I_s = Secondary current

Calculation:

\(n = {I_P \over I_S}\)

\(n = {1000 \over 5} =200\)

I_w = 6A

I_s = 5A

\(R = 200+{6\over 5}\)

R = 201.2

Current Transformer​:

• A Current Transformer (CT) is used to measure the current of another circuit.
• CTs are used worldwide to monitor high-voltage lines across national power grids.
• Load on Current Transformer is called as a burden because Current Transformer secondary winding can’t be kept open, high voltage can do the insulation failure.
• So it is necessary to load the Current Transformer secondary winding continuously and ultimately it acts as a burden on the Current Transformer.
• The burden of Current Transformer can be specified as Volt-Ampere absorbed at a certain Power Factor.

Clip-on type transformer:

• Clip-on type current transformer is also known as clamp-on type current transformer.
• Clamp meters rely on the principle of magnetic induction to make non-contact AC current measurements.
• Electric current flowing through a wire produces a magnetic field. Since alternating current frequently reverses polarity, it causes dynamic fluctuations in the magnetic field which are proportional to the current flow.
• A current transformer inside the clamp meter senses the magnetic fluctuations and converts the value to an AC current reading. This type of measurement is convenient for measuring very high AC currents.
• While measuring, the primary is connected to feeder and the secondary is connected to ammeter. The current transformer steps down the high feeder current to low current which can be measured by using ammeter.

Nominal ratio, \({K_n} = \frac{{200}}{1} = 200\)

Actual transformation ratio,

\(R = \frac{{{I_p}}}{{{I_s}}} = \frac{{100}}{{0.495}} = 202.02\)

\(Ratio\;error = \frac{{Nominal\;ratio - Actual\;ratio}}{{Actual\;ratio}} \times 100\)

\(= \frac{{200 - 202.02}}{{202.02}} \times 100\)

The primary winding of the current transformer is connected in series with the transmission line whose current is to be measured.

If you try to connect a CT in parallel with another resistance or load, the coil of the CT will seem to offer negligible resistance or simply it will work as a short circuit.

Note:

Concept:

• A current transformer is a type of instrument transformer that is used to measure and monitor the current in a circuit.
• It transforms the high current from the primary side into a lower current on the secondary side, while maintaining the phase relationship between the two.
• The primary and secondary currents are ideally in phase (i.e., the phase angle between them is zero). However, in practice, there can be a small phase shift due to the inherent characteristics of the transformer, such as winding resistance, leakage reactance, etc. This phase shift is typically measured as the angle between the primary current and the secondary current, when reflected back to the primary current.

Important PointsCurrent Transformer:

• A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to the primary current and differs in phase from it by ideally zero degrees.
• Instrument current transformer is used in electrical power system for stepping down currents for metering and protection purposes. 

Equivalent circuit of CT:


Vector Diagram of CT:

Is – Secondary current.
Es – Secondary induced emf.
Ip – Primary current.
Ep – Primary induced emf.
KT – Turns ratio = Numbers of secondary turns/number of primary turns.
I0 – Excitation current.
Im – Magnetizing component of I0.
Iw – Core loss component of I0.
Φm – Main flux.

The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.

• Current transformer works on the principle of shorted secondary
• It means that the burden on the system Zb is equal to 0
• Thus, the current transformer produces a current in its secondary which is proportional to the current in its primary

Important Points:

• Most important precaution in use of a CT is that in no case should it be open-circuited (even accidentally)
• As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened
• This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable
• Again, due to large flux in the core, the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT
• This large voltage across the secondary terminals will be very dangerous and will lead to the insulation failure and there is a good chance that the person who is opening the CT secondary while the primary is energized will get a fatal shock

Concept:

The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.

• The current transformer works on the principle of short secondary
• It means that the burden on the system Zb is equal to 0
• Thus, the current transformer produces a current in its secondary which is proportional to the current in its primary

Important Points:

• The most important precaution in the use of a CT is that in no case should it be open-circuited (even accidentally)
• As the primary current is independent of the secondary current, all of it acts as a magnetizing current when the secondary is opened
• This results in deep saturation of the core which cannot be returned to the normal state and so the CT is no longer usable
• Again, due to the large flux in the core, the flux linkage of secondary winding will be large which in turn will produce a large voltage across the secondary terminals of the CT
• This large voltage across the secondary terminals will be very dangerous and will lead to insulation failure and there is a good chance that the person who is opening the CT secondary while the primary is energized will get a fatal shock

A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value.

The ratio of the primary current and the secondary current is known as a current transformer ratio of the circuit. The current ratio of the transformer is usually high. The secondary current ratings are of the order of 5A, 1A and 0.1A. The current primary ratings vary from 10A to 3000A or more.

The symbolic representation of the current transformer is shown in the figure below.

• Answer-

For any transformer

Turn Ratio is given by

\(\frac{{{N_1}}}{{{N_2}}} = \frac{{{V_1}}}{{{V_2}}} = \frac{{{I_2}}}{{{I_1}}}\)

Since for current transformer secondary current (F₂) are high as compared to primary current so the number of secondary turns is very high as compared to the number of primary turns.

Protective Relay:

• A relay is a device that detects the fault and supplies information to the circuit breaker for circuit interruption.
• The energy to supply power to the relay is provided by the current transformer and the potential transformer.
• The circuit to be protected carries a large current (normal or fault current) and is reduced to a suitable value for relay operation with the help of a current transformer.
• The figure given below shows a typical relay circuit.

• The relay circuit can be divided into three parts:
  • The primary winding of a current transformer (C.T.) is connected in series with the circuit to be protected. The primary winding often consists of the main conductor itself.
  • The second circuit is the secondary winding of C.T. connected to the relay operating coil.
  • The third circuit is the tripping circuit which consists of a source of supply, the trip coil of the circuit breaker, and the relay stationary contacts.
• Under normal load conditions, the e.m.f. of the secondary winding of C.T. is small and the current flowing in the relay operating coil is insufficient to close the relay contacts.
• This keeps the trip coil of the circuit breaker unenergised. Consequently, the contacts of the circuit breaker remain closed and it carries the normal load current.
• When a fault occurs, a large current flows through the primary of C.T. This increases the secondary e.m.f. and hence the current through the relay operating coil.
• The relay contacts are closed and the trip coil of the circuit breaker is energized to open the contacts of the circuit breaker.