Top 05 Viva Question & Answer about Current Transformer (CT)

1. Why Current Transformer (CT) secondary should not be open? Explain it.

A Current Transformer (CT) secondary should not be open-circuited because it can lead to several dangerous and damaging consequences:

1. High Voltage Generation: When the secondary of a CT is open, the current that would normally flow through it is interrupted. This can cause a significant increase in voltage across the secondary terminals due to the transformer’s nature to maintain current balance. This high voltage can exceed insulation ratings and potentially cause insulation breakdown.
2. Damage to the CT: The excessive voltage can damage the CT itself, leading to overheating or even destruction of the transformer. This damage might occur due to arcing or flashover within the CT or to the connected devices.
3. Safety Hazards: The high voltages generated can pose a serious safety risk to personnel working near the equipment. It can lead to electrical shock or arc flash incidents.
4. Measurement Errors: If the secondary circuit is open, accurate current measurement becomes impossible. This can lead to incorrect readings and poor system performance analysis.
5. Equipment Malfunction: Many systems rely on accurate current readings for protection and control. An open CT secondary can cause relays or other protective devices to operate incorrectly, potentially leading to system failures or outages.

For these reasons, it’s critical to always ensure that the secondary side of a CT is connected to a load or shorted appropriately when not in use.

2. A Current Transformer (CT) is “step-up transformer” or “step-down transformer”?

A Current Transformer (CT) is not typically referred to as a “step-up transformer.” Instead, it is more accurately described as a “step-down transformer” because it reduces high primary current to a lower secondary current for measurement and protection purposes.

Reasons for the “Step-Down” Designation:

1. Current Reduction: The primary function of a CT is to transform a high primary current (the current flowing through the conductor) into a lower, manageable secondary current. For example, a CT might take 100 A from the primary and output 5 A in the secondary.
2. Safety and Measurement: The reduced current makes it safe to measure and process without risking damage to measuring instruments and ensuring operator safety.
3. Secondary Impedance: The CT’s design allows for a secondary circuit that can handle lower currents, which is more suitable for standard measurement equipment and relays.

Clarification:

The term “step-up transformer” typically refers to transformers that increase voltage from primary to secondary (e.g., stepping up voltage for transmission). In contrast, CTs focus on stepping down current while maintaining the same power level (ignoring losses), adhering to the principle of conservation of energy.

In summary, CTs are designed to safely reduce current levels, and thus they are better classified as step-down transformers.

N.B:

Potential Transformer (PT):
– Function: Measures high voltage levels and produces a lower, proportional voltage for measurement or protection.
Type: PTs are also considered step-down transformers since they reduce high voltages to a lower level suitable for measurement.

In summary, both CTs and PTs are step-down transformers, as they reduce high current and high voltage levels to lower, manageable levels for measurement and protection systems.

 

3. What is “Burden on a Load of a current transformer (CT)”?

The “burden” on a current transformer (CT) refers to the load connected to its secondary winding. It is an important concept because it affects the accuracy and performance of the CT. Here’s a more detailed explanation:

Definition of Burden

• Burden is usually expressed in ohms (Ω) or as a power measurement (volt-amperes, VA). It represents the impedance of the devices connected to the CT’s secondary side, such as measuring instruments, relays, or protective devices.

Importance of Burden

1. Accuracy: Each CT is designed to operate within a specific burden range. If the burden is too low, it can cause saturation, leading to inaccurate readings. Conversely, if the burden is too high, it may not allow enough current to flow, also affecting accuracy.
2. Safety: A proper burden ensures that the secondary circuit does not exceed safe current levels. Open-circuiting the secondary of a CT can generate high voltages, which can be dangerous.
3. Performance: The burden affects how well the CT can respond to changes in the primary current. It should be matched to the application requirements to ensure reliable operation.

Calculating Burden

To determine the appropriate burden for a CT, you can use the following formula:

Burden (VA) =Secondary Voltage (V)×Secondary Current (A)

04. What are the rated parameters are different Current Transformer (CT) classes and what’s significance?

Answer: Current Transformer (CT) classes are defined based on their performance characteristics, which ensure that they accurately measure and transform high currents into a lower, measurable value for protection, metering, or monitoring applications. Each CT class corresponds to specific parameters that are crucial for the intended application, and these classes are specified according to standards like IEC (International Electrotechnical Commission) and ANSI (American National Standards Institute).

Here are the key rated parameters that define the different CT classes, along with their significance:

1. Accuracy Class (or Ratio Class)

• Definition: The accuracy class indicates how close the CT’s output is to the actual current. It is expressed as a percentage of error at a specific load or current.
• Significance:
  • For protection CTs, the accuracy class must be sufficiently high to ensure correct operation of protection relays, even during fault conditions. A CT with a higher accuracy class (e.g., 5P, 10P) is needed for reliable fault detection.
  • For metering CTs, a high accuracy is essential for precise energy measurement, often with classes like 0.1, 0.2, 0.5.

Example:

• 0.2 class means the CT will have no more than a 0.2% error at rated conditions.
• 5P means the CT will have a maximum error of 5% under normal operating conditions and may go up to a certain error limit during fault conditions.

2. Burden

• Definition: The burden is the load (in ohms) connected to the secondary of the CT, often represented as VA (volt-amperes). It refers to the power that the CT can supply to the connected equipment (meter, relay, etc.) without exceeding its accuracy limits.
• Significance:
  • If the burden is too high, the CT’s secondary voltage can increase, leading to incorrect measurement or relay operation.
  • A low burden can cause a large secondary current, which may also affect accuracy.

Example:

• A CT with a rated burden of 5 VA can safely drive meters, relays, and other equipment that draw up to 5 VA.

3. Rated Primary Current (Ip)

• Definition: The rated primary current is the maximum current that the CT is designed to measure without saturation.
• Significance: The rated primary current must be selected according to the maximum expected current in the system to avoid saturation of the CT, which would lead to incorrect secondary readings.

Example:

• A CT rated for 100 A is designed to accurately transform currents up to 100 A in the primary circuit.

4. Rated Secondary Current (Is)

• Definition: The rated secondary current is the output current from the CT when the rated primary current is flowing. Typically, this is either 1 A or 5 A for most CTs.
• Significance: This is the standard output used by meters and protection equipment, and it is important for ensuring compatibility with these devices.

Example:

• A CT with a secondary rating of 5 A will output 5 A when 100 A flows through the primary (assuming the CT ratio is 100:5).

5. Saturation Characteristics (or Knee Point Voltage)

• Definition: The knee point is the point at which the CT begins to saturate, meaning it no longer provides a proportional output to the input current. This is especially important in protection CTs.
• Significance:
  • Protection CTs need to be designed to avoid saturation during fault conditions, ensuring that the relay system continues to operate correctly even during high currents.

Example:

• A 10P10 class CT has a knee point voltage that ensures it remains accurate up to 10 times its rated current (i.e., 10 times its primary rated current).

6. Short Circuit Current (Isc)

• Definition: The short circuit current refers to the maximum current the CT can handle safely under short-circuit conditions without damage.
• Significance: The CT must be able to withstand high currents during faults without causing damage to its winding or insulation.

Example:

• A CT with a short-circuit current rating of 20 times its rated primary current will not be damaged even if 20 times the rated current flows through it.

7. Rated Primary-to-Secondary Current Ratio

• Definition: This is the ratio of primary current (Ip) to secondary current (Is) and indicates the scaling factor used to reduce high currents into measurable, lower currents.
• Significance: The correct CT ratio ensures that the current is measured and relayed accurately to protective and metering equipment.

Example:

• A CT with a ratio of 100:5 will reduce 100 A of primary current to 5 A of secondary current.

8. Overload Capacity (or Multiplying Factor)

• Definition: This refers to the maximum multiple of the rated current that the CT can handle without degradation of accuracy or reliability, typically during short-duration fault conditions.
• Significance: It ensures the CT remains accurate during fault conditions where the current exceeds its rated capacity for a short time.

Example:

• A CT rated 5P10 can measure currents up to 10 times its nominal rating without significant error, which is useful in protection systems for handling faults.

9. Temperature Rating

• Definition: The temperature rating defines the range of operating temperatures for the CT without affecting its performance or safety.
• Significance: CTs in harsh environments (high or low temperatures) need to be selected based on their temperature ratings to ensure proper function and longevity.

Example:

• A CT might have a temperature range of -20°C to +70°C, meaning it can operate reliably within that temperature range.

05. How to demagnetize a completely magnetize CT and why?

To demagnetize a completely magnetized Current Transformer (CT), you can follow these methods:

1. AC Excitation Method: Apply an alternating current (AC) through the primary winding. Gradually increase the frequency and slowly reduce the voltage to zero to neutralize the residual magnetism in the core.
2. Demagnetizing Coil: Use a demagnetizing coil wrapped around the CT core. Apply an AC current, which creates a reversing magnetic field, gradually reducing the magnetization.
3. DC Pulse Reversal: Apply a short reverse DC pulse to the primary winding to flip the magnetization and neutralize the core.
4. Signal Generator Method: Use a signal generator to apply a low-frequency AC signal and reduce its amplitude gradually until the core is demagnetized.

Always proceed with caution and ensure that the proper equipment is used.

Why Demagnetization is Important:

A magnetized CT, especially if saturated, will provide incorrect current readings, leading to faulty protection relay behavior or inaccurate energy metering. Demagnetization ensures that the CT can function accurately once again, and it’s particularly crucial in the case of protection CTs where correct relay operation is vital.

Conclusion

In summary, the burden on a current transformer is crucial for ensuring accurate measurements and safe operation. It should be carefully considered during the design and installation of CTs to achieve optimal performance.