1. Introduction

Potential transformers (PTs) play a crucial role in power systems by stepping down high-voltage signals to measurable levels for protection, metering, and control applications. In medium-mounted draw-out carts, which are widely used in switchgear systems, the proper installation and electrical connection of PTs are essential for ensuring accurate voltage measurement, reliable protection operation, and overall system safety. This paper comprehensively details the key aspects of PT installation and electrical connection within such carts.

2. Installation of Potential Transformers in Medium-mounted Draw-out Carts

2.1 Mechanical Installation Preparation

Cart Structure Compatibility: Before installing the PT, ensure that the medium-mounted draw-out cart is designed to accommodate the physical dimensions and weight of the selected PT. The cart should have a dedicated compartment with sufficient space to house the PT, considering its height, diameter, and any additional accessories such as terminal boxes or protective covers.
Inspection of PT: Thoroughly inspect the PT for any physical damage, loose components, or signs of deterioration before installation. Check the integrity of the insulation, the condition of the windings, and the tightness of terminal connections. Verify that the PT meets the relevant electrical and mechanical specifications for the intended application.

2.2 Mounting Procedures

Securing the PT: Use appropriate mounting hardware, such as bolts, nuts, and washers, to firmly secure the PT to the designated mounting plate or structure within the cart. Ensure that the PT is level and properly aligned to prevent any mechanical stress or vibration during operation. Torque the mounting bolts to the manufacturer's recommended values to guarantee a stable and secure installation.
Clearance and Spacing: Maintain adequate electrical and mechanical clearances around the PT. This includes ensuring sufficient distance from other live components, conductive parts, and the cart's enclosure to prevent electrical arcing, short circuits, and mechanical interference. Refer to relevant electrical standards (e.g., IEC, IEEE) for the minimum clearance requirements based on the rated voltage of the PT and the system.

2.3 Integration with the Cart's Mechanical Systems

Interlocking Mechanisms: Medium-mounted draw-out carts are often equipped with interlocking mechanisms to prevent incorrect operation and ensure safety. Integrate the PT installation with these interlocking systems to ensure that the PT can only be inserted or removed when the cart is in the proper position and the electrical circuit is safely isolated. For example, the PT should be mechanically interlocked with the cart's draw-out mechanism to prevent it from being withdrawn while under load.
Movement and Positioning: Ensure that the PT can move smoothly within the cart during the insertion and withdrawal processes. Lubricate any moving parts, such as guide rails or rollers, as per the manufacturer's instructions to reduce friction and prevent wear. Check that the PT's position indicators or limit switches are properly installed and calibrated to accurately show the PT's operating position (e.g., connected, disconnected, or test position).

3. Electrical Connection of Potential Transformers

3.1 Primary Electrical Connection

Terminal Connection: Connect the primary winding of the PT to the high-voltage circuit. This is typically done through busbars or cable connections. Ensure that the primary terminals of the PT are properly insulated and securely connected to the high-voltage conductors. Use appropriate connectors, such as compression lugs or bolted connections, and apply anti-oxidation compounds to prevent corrosion and ensure good electrical contact.
Phase Sequence and Polarity: Pay strict attention to the phase sequence and polarity during the primary connection. Incorrect phase sequence can lead to inaccurate voltage measurement and improper operation of protection and control devices, while incorrect polarity can cause protection relays to malfunction. Use phase indicators and polarity testers to verify the correct connection before energizing the system.

3.2 Secondary Electrical Connection

Wiring to Instrument Transformers and Relays: Connect the secondary windings of the PT to the associated instrument transformers, meters, relays, and control devices. Use insulated cables with appropriate cross-sectional areas to handle the secondary current and voltage ratings of the PT. Label the secondary wires clearly to identify the phase, winding, and function to facilitate troubleshooting and maintenance.
Grounding: Properly ground the secondary circuit of the PT. A reliable ground connection is essential for protecting personnel and equipment from electrical hazards and for ensuring accurate voltage measurement. Connect the secondary grounding terminal of the PT to the grounding busbar of the switchgear using a low-impedance conductor. Follow the grounding requirements specified in relevant electrical codes and standards.

3.3 Isolation and Protection Devices

Fuse Installation: Install fuses on the primary and secondary sides of the PT to provide overcurrent protection. Select fuses with appropriate ratings based on the PT's rated current and short-circuit withstand capacity. The fuses should be able to quickly interrupt the current in case of a fault to prevent damage to the PT and other connected equipment.
Surge Protection: Consider installing surge protection devices, such as metal-oxide varistors (MOVs), on the primary and secondary sides of the PT to protect against voltage surges caused by lightning strikes, switching operations, or other transient overvoltage events. These devices help to limit the voltage levels and divert the surge currents, safeguarding the PT and the connected electrical systems.

4. Testing and Commissioning after Installation

Insulation Resistance Test: After completing the installation and electrical connection, perform an insulation resistance test on the PT to check the integrity of its insulation. Use a high-voltage insulation tester to measure the resistance between the primary and secondary windings, the windings and the grounded enclosure, and between the different phases. Compare the test results with the manufacturer's specifications and the acceptable limits specified in relevant standards.
Turns Ratio and Polarity Test: Conduct a turns ratio test to verify the accuracy of the PT's voltage transformation ratio. Use a turns ratio tester to measure the ratio of the primary voltage to the secondary voltage. Additionally, perform a polarity test to confirm that the PT's polarity is correct. Incorrect turns ratio or polarity can lead to inaccurate voltage measurement and malfunction of protection and control systems.
Functionality Test: Test the functionality of the PT by energizing the system and verifying the output voltage at the secondary terminals. Check that the measured voltage values are within the acceptable tolerance range and that the voltage signals are correctly transmitted to the connected instruments, meters, and relays. Ensure that the protection and control devices operate as expected based on the PT's voltage output.

5. Conclusion

The installation and electrical connection of potential transformers in medium-mounted draw-out carts are critical processes that require careful attention to detail. By following proper installation procedures, ensuring correct electrical connections, and conducting thorough testing and commissioning, the reliability, accuracy, and safety of the power system can be significantly enhanced. Regular maintenance and inspection of the PT and its associated connections are also essential to ensure long-term trouble-free operation.

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Installation and Electrical Connection of Potential Transformers (PTs) in Medium-mounted Draw-out Carts