Which of the following devices is used to reduce currents and voltages to lower values and to isolate protective relays from high voltages of the power system?

Explanation:

Current Transformers (CTs) and Voltage Transformers (VTs)

Definition: Current Transformers (CTs) and Voltage Transformers (VTs) are types of instrument transformers used in electrical power systems to step down high currents and voltages to lower, safer values for measurement, protection, and control purposes. They provide isolation between high-voltage power circuits and low-voltage metering and relay circuits.

Working Principle:

• Current Transformers (CTs): CTs are designed to produce a reduced current accurately proportional to the current in the circuit being measured. They consist of a primary winding, which carries the high current, and a secondary winding, which produces a lower current. The ratio of primary to secondary current is determined by the turns ratio of the transformer. This allows for the safe and accurate measurement of high currents using standard low-current instruments.
• Voltage Transformers (VTs): VTs, also known as potential transformers (PTs), are used to step down high voltages to lower, standardized values suitable for measurement and protection. Similar to CTs, VTs have a primary winding connected to the high-voltage circuit and a secondary winding that provides a reduced voltage. The voltage ratio is defined by the turns ratio of the transformer, ensuring that the secondary voltage is a precise fraction of the primary voltage.

Advantages:

• Provide electrical isolation between high-voltage power circuits and low-voltage measurement and protection circuits, enhancing safety.
• Enable accurate measurement and monitoring of electrical parameters such as current and voltage in high-voltage systems.
• Facilitate the protection of power systems by providing input to protective relays, which can detect faults and initiate corrective actions.

Disadvantages:

• Accuracy can be affected by factors such as burden (load on the secondary winding), temperature, and aging of the transformer.
• CTs can saturate under high fault currents, potentially leading to measurement errors and affecting the performance of protective relays.
• VTs require proper insulation and construction to withstand high voltages and prevent breakdowns.

Applications: CTs and VTs are widely used in electrical power systems for various purposes, including:

• Measurement and metering: Accurate measurement of current and voltage for billing and monitoring purposes.
• Protection: Providing input to protective relays to detect and isolate faults in the power system.
• Control: Supplying input to control systems for regulating and managing power system operations.
• Monitoring: Continuous monitoring of electrical parameters to ensure the stability and reliability of the power system.

Correct Option Analysis:

The correct option is:

Option 3: Transducers (CTs and VTs)

This option correctly identifies the devices used to reduce currents and voltages to lower values and to isolate protective relays from high voltages of the power system. CTs and VTs play a crucial role in ensuring the safe and accurate measurement, protection, and control of electrical power systems. By stepping down high currents and voltages, they enable the use of standard low-current and low-voltage instruments, enhancing safety and reliability.

Additional Information

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

Option 1: Protective relay

Protective relays are devices used to detect abnormal conditions in electrical power systems, such as faults, and initiate actions to isolate the affected part of the system. While protective relays are crucial for system protection, they do not directly reduce currents and voltages. Instead, they rely on inputs from CTs and VTs to perform their protective functions.

Option 2: Circuit breaker

Circuit breakers are devices used to interrupt the flow of current in electrical circuits under fault or overload conditions. They provide protection by disconnecting the faulty section of the power system. However, circuit breakers do not reduce currents and voltages; they are designed to handle and interrupt high currents and voltages directly.

Option 4: Buchholz relay

The Buchholz relay is a type of gas-actuated relay used in oil-filled transformers to detect internal faults, such as gas accumulation or oil flow due to a fault. While the Buchholz relay plays a vital role in transformer protection, it does not reduce currents and voltages to lower values. It is specifically designed for transformer protection and not for general current and voltage reduction.

Conclusion:

Understanding the roles and functions of different devices in electrical power systems is essential for selecting the appropriate equipment for specific applications. CTs and VTs are critical components for reducing high currents and voltages to lower, safer values, enabling accurate measurement, protection, and control of power systems. By providing isolation between high-voltage circuits and low-voltage instruments, CTs and VTs enhance the safety and reliability of electrical power systems.