Instrument Transformers MCQ Quiz - Objective Question with Answer for Instrument Transformers - Download Free PDF

The correct answer is option 2.

Different types of transformers

• Current transformer: A current transformer (CT) is used to step down current for measurement purposes. It is a specific type of instrument transformer designed to reduce high currents to a lower, more manageable level that can be safely and accurately measured by standard electrical instruments. 
• Power transformer: A power transformer is a fixed device that converts power from one circuit to another without changing the frequency.
• Auto transformer: Autotransformers are ideal for voltage adjustment for commercial and industrial machines. They provide an efficient, low cost way of serving the proper voltage to motors and compressors, lathes, CNC machines, and other industrial equipment requiring a step up or down from a building's service voltage.
• Voltage transformer: Voltage transformers (VT), also called potential transformers (PT), are a parallel-connected type of instrument transformer. They are designed to present a negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.

The correct answer is option 1.

Different types of transformers

• Current transformer: A current transformer (CT) is used to step down current for measurement purposes. It is a specific type of instrument transformer designed to reduce high currents to a lower, more manageable level that can be safely and accurately measured by standard electrical instruments. 
• Power transformer: A power transformer is a fixed device that converts power from one circuit to another without changing the frequency.
• Auto transformer: Autotransformers are ideal for voltage adjustment for commercial and industrial machines. They provide an efficient, low cost way of serving the proper voltage to motors and compressors, lathes, CNC machines, and other industrial equipment requiring a step up or down from a building's service voltage.
• Voltage transformer: Voltage transformers (VT), also called potential transformers (PT), are a parallel-connected type of instrument transformer. They are designed to present a negligible load to the supply being measured and have an accurate voltage ratio and phase relationship to enable accurate secondary connected metering.

Burden of an instrument transformer

The burden is defined as the load connected across its secondary potential transformer.

The burden is specified as VA.

The burden in PT is given by:

\(R_{Burden}={V_{out}\over I_{sec}}\)

The rated burden of a PT is a VA burden that must not be exceeded if the transformer is to operate with its rated accuracy.

From the above explanation, we found Both A and R are true and R is the not the correct explanation of A. 

Explanation:

Instrument Transformer Error Reduction

Definition: An instrument transformer is a device used to isolate and transform high current or voltage levels into lower, manageable levels suitable for measurement and protection systems. However, instrument transformers are prone to errors, including ratio error and phase angle error, which can affect the accuracy of measurements.

Correct Option Analysis:

The correct option is:

Option 4: Using low reluctance core

Using a low reluctance core is one of the most effective ways to reduce errors in an instrument transformer. The core material plays a crucial role in the accurate transformation of electrical signals. Reluctance is the opposition to magnetic flux in the core, and a low reluctance core ensures efficient magnetic flux linkage between the primary and secondary windings. This reduces core losses and enhances the accuracy of the transformer.

How low reluctance core reduces errors:

• Improved Magnetic Flux Linkage: A low reluctance core facilitates better magnetic flux linkage between the primary and secondary windings, minimizing flux leakage. This improves the accuracy of the voltage or current transformation.
• Reduction in Hysteresis Loss: Low reluctance materials typically exhibit lower hysteresis loss, which contributes to reducing the ratio error and phase angle error.
• Minimized Core Saturation: Core saturation is a condition where the core cannot handle additional magnetic flux, leading to inaccuracies. A low reluctance core is less prone to saturation, ensuring consistent performance.
• Enhanced Signal Fidelity: Accurate transformation of electrical signals is critical for measurement and protection systems. A low reluctance core ensures minimal distortion of the signal.

Applications: Instrument transformers with low reluctance cores are commonly used in power systems for accurate measurement and protection purposes, including current transformers (CTs) and potential transformers (PTs).

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Using high reluctance core

This option is incorrect as using a high reluctance core would increase the opposition to magnetic flux in the transformer. High reluctance leads to greater flux leakage, higher core losses, and increased errors. It is, therefore, counterproductive for improving the accuracy of instrument transformers.

Option 2: Using small cross-sectional area

Using a small cross-sectional area for the core can lead to higher flux density, which may result in core saturation and increased hysteresis losses. This adversely affects the performance and accuracy of the transformer. Hence, this option is not suitable for reducing errors.

Option 3: Using low permeability materials

Low permeability materials are not ideal for transformer cores because they reduce the ability of the core to support magnetic flux. This leads to flux leakage and higher core losses, causing errors in the instrument transformer. High permeability materials are preferred for efficient flux linkage.

Conclusion:

In instrument transformers, the choice of core material significantly impacts the accuracy and efficiency of the device. Using a low reluctance core is the most effective way to reduce errors, as it enhances magnetic flux linkage, minimizes core losses, and ensures accurate signal transformation. Other options, such as using high reluctance cores, small cross-sectional areas, or low permeability materials, are counterproductive and can result in increased errors and compromised performance. Understanding the principles of core design is essential for optimizing the performance of instrument transformers in power systems.

Explanation:

Resistance-Voltage Divider

Definition: A resistance-voltage divider is an electrical circuit that uses resistors to produce a fraction of its input voltage as output. It is commonly used in electronic circuits to scale down voltages to a desired level.

Working Principle: The voltage divider works on the principle of Ohm's Law, which states that the voltage across a resistor is proportional to the current flowing through it and its resistance. By arranging two or more resistors in series, the input voltage can be divided among them. The voltage drop across each resistor is directly proportional to its resistance.

The correct option is: Option 4: Kilo Volts

The symbols of different equipment used in single line diagram are shown below.

Hence, the given symbol is for the potential transformer.

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 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:

• 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 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 primary is energized will get fatal shock.

Concept:

Current transformer:

• A current transformer is a device that is used to measure high alternating current in a conductor.
• In the figure shown below the conductor act as the primary winding of a single turn that passed through the circular laminated iron core.
• The secondary winding consists of a large number of turns of fine wire wrapped around the core.
• Due to transformer action, the secondary current transforms into a lower value.

​

Let, Np is a number of turn in the primary winding

Ns is the number of turns in the secondary winding.

Ip and Is are primary and secondary turns respectively.

Therefore, the secondary current is given by,

\({I_s} = {I_p} × \frac{{{N_p}}}{{{N_s}}}\)

• Therefore, the current transformer changes the current into a lower value that can easily be measured by the measuring instrument.
• Along with a Potential transformer (PT), a current transformer (CT) can measure Power and energy also.
• Hence, CT used with Ammeter, Wattmeter, and Watt-hour meter.
   

​Calculation:

Given, N_p = 1, N_s = 8, I_p = 50 A

\({I_s} = {I_p} × \frac{{{N_p}}}{{{N_s}}}\)

I_s = 50 × (1 / 8)

I_s = 6.25 A

Current Transformer (C.T.): 

• It is a type of instrument transformer that produces an alternating current in primary winding which is proportional to the current to be measured.
• The primary current of the transformer is dictated by the load current.
• Current transformers reduce high currents to a much lower value and provide a convenient way of safely monitoring the actual electrical current flowing in an AC transmission line using a standard ammeter.
• The principle of operation of a basic current transformer is slightly different from that of an ordinary voltage transformer.
• The current transformer consists of only one or very few turns as its primary winding.
• This primary winding can be of either a single flat turn, a coil of heavy-duty wire wrapped around the core or just a conductor or bus bar placed through a central hole.

Calculation:

Given that CT ratio = 100 : 5 = 20 : 1

The ammeter reading = 2.5 A

ie secondary current of current transformer = 2.5 A

Line current = primary current of CT × Secondary current = 20 × 2.5 = 50 A

Potential transformer (PT):

• A potential transformer (PT) is an instrument transformer used to transfer the voltage from a higher value to a lower value.
• It is a step-down transformer.
• Its secondary voltage is very low compared to primary 
• It is used in high voltage lines for operating the potential coils of wattmeter, voltmeter, relays, etc.
• Generally secondary of PT is rated for 110 V.
• PT always connected in parallel to the transmission line.
• The nominal ratio is defined as the ratio of rated primary voltage to the rated secondary voltage.

• In PT, if the load increases then both the ratio error and phase angle error increase.

In high voltage A.C. circuits, the measurement cannot be done by using the method of extension of ranges of low range meters by providing suitable shunts.

In such conditions, specially constructed accurate ratio transformers are used. These transformers are used to isolate the instruments from high current and high voltage A.C. circuits.

These are generally classified as

(i) Current transformer - large alternating currents can be measured

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

• Current transformer works on the principle of shorted secondary.
• It means that burden on the system Zb is equal to 0.
• Thus, 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 accidently).
• 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 primary is energized will get fatal shock.

Transformation Ratio:

Transformation Ratio of current transformer (CT), \({K_C} = \frac{{{I_1}}}{{{I_2}}}\)

Transformation Ratio of a potential transformer (PT), \({K_V} = \frac{{{V_2}\;}}{{{V_1}}}\)

Where,

I1 = Primary side current of CT

I2 = Secondary side current of CT

V1 = Primary side voltage of PT

V2 = Secondary side voltage of PT

Secondary burden:

The nominal ratio of an instrument transformer does not remain constant as the load on the secondary charges. It changes because of effect of secondary current and this causes errors in the measurement. The specific loading at rated secondary winding voltage is specified such that the errors do not exceed the limits. Such a permissible load is called the burden of an instrument transformer.

The burden across the secondary of an instrument transformer is specified as V2/I2

The ratio of transformation in the case of potential transformers decreases with increases in power factor of Secondary burden.

The nominal ratio of an instrument transformer, does not remain constant as the load on the secondary charges. It changes because of effect of secondary current and this causes errors in the measurement. The specific loading at rated secondary winding voltage is specified such that the errors do not exceed the limits. Such a permissible load is called burden of an instrument transformer.

Industrial analyser is used for single-phase or three-phase measurements of the:

• Active power
• Reactive power
• Apparent power
• Power factor
• Phase angle
• Energy
• Voltage
• Current