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

Matching the Error Types and Causes in Current Transformers (CTs):

In a Current Transformer (CT), various sources of errors arise due to the physical and electrical characteristics of the transformer. These errors are classified into different types, and each error type has specific causes associated with it. Understanding the relationship between these errors and their causes is crucial for ensuring the accurate performance of CTs in electrical systems.

The correct matching of error types with their causes is:

Option 3: 1 → B, 2 → C, 3 → D, 4 → A

Let us analyze each error type and its corresponding cause:

1. Ratio Error (1 → B):

Explanation: Ratio error occurs when there is a discrepancy between the actual transformation ratio of the CT and its nominal or designed transformation ratio. This discrepancy primarily arises due to the magnetizing current requirement of the core. Magnetizing current is the current needed to establish the magnetic flux in the CT core, and it does not contribute to the secondary current. As a result, it causes a deviation in the actual ratio of primary to secondary currents, leading to ratio error.

Key Cause: Magnetizing current requirement (B).

2. Phase Angle Error (2 → C):

Explanation: Phase angle error is the angular displacement between the primary current and the secondary current of a CT. Ideally, the primary and secondary currents should be in phase (for a purely resistive burden). However, due to non-ideal properties of the core material (such as hysteresis and eddy current losses), there is a phase shift between the two currents. These non-ideal core properties affect the accuracy of the CT and result in phase angle error.

Key Cause: Non-ideal core material properties (C).

3. Hysteresis Error (3 → D):

Explanation: Hysteresis error is caused by the residual magnetism or remanence in the CT core. When the core is magnetized by the primary current, some residual magnetism remains in the core even after the current is removed. This residual flux affects the accuracy of the CT and leads to hysteresis error. It is particularly significant in applications where the CT is subjected to varying primary currents.

Key Cause: Residual magnetism in the core (D).

4. Leakage Reactance Error (4 → A):

Explanation: Leakage reactance error arises due to the leakage flux in the CT windings. Leakage flux is the portion of the magnetic flux that does not link both the primary and secondary windings. This flux creates reactance in the windings, which impacts the accuracy of the CT. The leakage reactance causes a voltage drop in the windings, thereby introducing errors in the secondary current.

Key Cause: Leakage flux in windings (A).

Important Information

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

Option 1: 1 → A, 2 → B, 3 → D, 4 → C

This option incorrectly associates the causes with the error types. For example, it matches ratio error with "leakage flux in windings" (A), which is incorrect because ratio error is primarily caused by the magnetizing current requirement (B). Similarly, other mismatches make this option incorrect.

Option 2: 1 → B, 2 → A, 3 → D, 4 → C

This option also contains incorrect associations. For instance, it matches phase angle error with "leakage flux in windings" (A), which is incorrect because phase angle error is caused by non-ideal core material properties (C). The other pairings are also mismatched.

Option 4: 1 → B, 2 → A, 3 → C, 4 → D

In this option, hysteresis error is incorrectly matched with "non-ideal core material properties" (C) instead of "residual magnetism in the core" (D). Leakage reactance error is also mismatched with "residual magnetism in the core" (D) instead of "leakage flux in windings" (A).

Conclusion:

Understanding the sources of errors in CTs and their respective causes is essential for ensuring the accuracy and reliability of these devices. The correct matching of error types with their causes is as follows:

• Ratio Error: Magnetizing current requirement (B).
• Phase Angle Error: Non-ideal core material properties (C).
• Hysteresis Error: Residual magnetism in the core (D).
• Leakage Reactance Error: Leakage flux in windings (A).

Option 3 correctly identifies these relationships, making it the correct choice.

If a power transformer has a star connected primary and a delta connected secondary then the CT connections on its primary and secondary sides should be: Delta and Star respectively.

Correct Option: Option 4

Detailed Explanation:

In power transformers, the configuration of the primary and secondary windings impacts the Current Transformer (CT) connections. When the primary side of a transformer is star-connected and the secondary side is delta-connected, it is crucial to ensure that the CT connections are made appropriately to maintain accuracy in current measurement and protection schemes.

Reasoning for Delta and Star CT Connections:

1. Star-Connected Primary Winding:

• The star connection on the primary winding results in line currents being equal to the phase currents. This simplifies the CT connections on the primary side.
• To accurately measure the line currents in the star connection, the CTs should be connected in a delta configuration. This is done to account for phase displacement and to ensure correct phase relationships for protection and metering purposes.
• In a star connection, the voltage across each phase winding is lower (line voltage/√3), which reduces insulation requirements and makes the system efficient for high-voltage transmission.

2. Delta-Connected Secondary Winding:

• The delta connection on the secondary winding causes the line currents to differ from the phase currents. The relationship between line and phase currents in a delta connection is that the line current is √3 times the phase current.
• To accurately measure the line currents in the delta connection, the CTs should be connected in a star configuration. This is necessary to ensure proper current transformation and phase angle relationships for protection devices like differential relays.
• Delta connections on the secondary side also help in providing a path for the third harmonic currents, which eliminates distortion and ensures stable operation.

3. Phase Displacement Between Star and Delta Connections:

• In a star-delta transformer, there is a 30° phase displacement between the primary and secondary windings. This displacement must be accounted for in CT connections to ensure proper operation of protection systems.
• By connecting CTs in delta on the primary side and star on the secondary side, the phase displacement is effectively compensated, ensuring that the protection systems, such as differential relays, function correctly.

4. Protection Scheme Accuracy:

• One of the primary reasons for choosing specific CT connections is to ensure the accuracy of protection schemes like differential protection.
• If the CTs are not connected in the appropriate configuration, the phase displacement and current magnitude differences between the primary and secondary windings may lead to incorrect operation of protection devices.
• By using delta CT connections on the star side and star CT connections on the delta side, the current magnitudes and phase relationships are matched, ensuring the correct operation of the protection system.

Important Information:

To further analyze the question, let’s evaluate the other options:

Option 1: Delta and Delta respectively

This option is incorrect because using delta CT connections on both the primary (star-connected winding) and the secondary (delta-connected winding) would not correctly account for the phase displacement between the windings. This could lead to inaccuracies in current measurement and protection schemes.

Option 2: Star and Delta respectively

This option is incorrect because using star CT connections on the primary (star-connected winding) would not account for the phase displacement and would result in incorrect current measurements. Additionally, delta CT connections on the secondary (delta-connected winding) would not align with the required phase relationships for protection schemes.

Option 3: Star and Star respectively

This option is incorrect because using star CT connections on both the primary and secondary sides would fail to account for the phase displacement between the star and delta windings. This misalignment would cause protection systems, such as differential relays, to malfunction.

Option 4: Delta and Star respectively

This is the correct option because:

• Delta CT connections on the primary (star-connected winding) ensure proper phase alignment and current transformation.
• Star CT connections on the secondary (delta-connected winding) account for the phase displacement and ensure accurate current measurement and protection system operation.

Option 5: Not provided

This option is invalid as it does not offer a relevant solution to the given problem.

Conclusion:

The correct CT connections for a power transformer with a star-connected primary and a delta-connected secondary are delta on the primary side and star on the secondary side. This configuration ensures proper current transformation, accurate phase alignment, and reliable operation of protection systems. Other options fail to meet these requirements and would result in inaccuracies or protection system malfunctions.

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. 

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