Potential Transformer Operation Guide

Potential Transformers (PT/VT) are essential electrical equipment in power systems, primarily used to proportionally step down high voltages to standard low voltages, providing accurate voltage signals for metering instruments, relay protection, and automatic control devices. Selecting a reliable potential transformer supplier is the first critical step toward ensuring the equipment’s long-term safe and stable operation, which is crucial for power system reliability. This guide outlines the fundamental safety requirements, standard operating procedures, and emergency principles for potential transformer operation and maintenance.

Part One: PT Safety Operation and Routine Maintenance

Adhering to strict safety standards and technical requirements is critical during the routine operation of potential transformer. To ensure stable PT performance, the following key regulations and precautions must be mastered.

I. Basic Safety and Grounding Principles

1. Single-Point Grounding Principle for Secondary Side: All secondary windings of the potential transformer (including spare windings) must have reliable protective grounding, and only one grounding point is permitted. This regulation prevents high voltage from entering the low-voltage circuit, thereby protecting personnel, and avoids circulating currents caused by multiple grounding points that could affect measurement accuracy.
2. Prohibited Operations: The secondary side of the potential transformer is strictly prohibited from short-circuiting. A short circuit on the secondary side will generate excessive current, which could lead to transformer burnout or fuse operation.

II. Operating Conditions and Accuracy Requirements

1. Load Control Requirements: The load connected to the PT’s secondary windings must be within the specified load range for its accuracy class. This ensures the transformer operates within its rated accuracy level, guaranteeing the precision of metering and protection.
2. Permissible Overvoltage Operating Limits:
   • Continuous operation is permitted at 1.2 times the rated voltage.
   • For transformers in effectively grounded neutral systems, operation is permitted at 1.5 times the rated voltage for 30 seconds.
   • For transformers in non-effectively grounded neutral systems, operation is permitted at 1.9 times the rated voltage for 8 hours if the system lacks automatic protection for earth faults.

III. Maintenance Guide for Specific Potential Transformer Types

The maintenance approach varies widely depending on the specific types of potential transformer in service, particularly regarding insulation and structure.

1. Capacitive Voltage Transformer (CVT) Requirements:
   • The externally connected damping resistor must be connected before the transformer is put into service, as it is used to suppress ferroresonance.
   • If a capacitor voltage divider unit is damaged during operation, care must be taken to readjust the transformer’s error after replacement.
2. Insulation Medium Status Check: Oil-immersed PTs with severe oil leakage or capacitive PTs with oil seepage from the capacitor unit must be immediately taken out of service.
3. Abnormal Sound Warning: If the Voltage Transformer exhibits abnormal noises, it should be immediately taken out of service.

Part Two: Standard Procedures for Energizing and De-energizing

Strict standard procedures and precautions must be followed when putting a Potential Transformer into service or taking it out of service. These procedures, which must align with the specific quality assurance standards and documentation provided by the instrument transformer supplier, are detailed below:

I. De-energizing (Taking out of Service) Provisions

1. Pre-withdrawal of Protection: Before de-energizing, protection and automatic devices that might maloperate due to voltage loss (e.g., distance protection, high-frequency protection, low-voltage protection, bus differential protection) must be withdrawn. This step prevents the sudden loss-of-voltage signal from causing false tripping of relay protection.
2. Switching Sequence (De-energizing): When taking out of service, first withdraw the secondary fuses or quick-break switches (isolating the secondary load), and then open the primary side disconnect switch (interrupting the primary power).
3. Safety Measures for Maintenance: When the Voltage Transformer is taken out of service for maintenance, the secondary fuses must be removed or the secondary quick-break switch must be opened to prevent back-charging and ensure the safety of maintenance personnel.

II. Energizing (Putting into Service) Provisions

1. Switching Sequence (Energizing): The sequence is reversed when energizing: first close the primary side disconnect switch, and then install the secondary fuses or close the quick-break switch.
2. Post-Operation Verification:
   • After closing the primary side disconnect switch, verify that the check relay is energized or de-energized, confirming the correct position indication of the disconnect switch.
   • After installing the secondary fuses or closing the quick-break switch, verify that the voltmeter readings are correct, confirming normal PT power supply.

Part Three: Abnormal Phenomena and Emergency Shutdown Regulations

When an instrument transformer like a potential transformer exhibits abnormal phenomena, accurate judgment criteria and decisive emergency handling principles are key to ensuring equipment and personnel safety.

I. Primary Principle for Fault Handling

Before the cause of a fault is identified, the secondary load must not be switched to a circuit supplied by a normally operating Voltage Transformer, and parallel operation of secondary circuits is prohibited. This measure prevents the spread of the fault and avoids affecting normally operating equipment and protection circuits.

II. Abnormal Phenomena Requiring Immediate Shutdown

A Voltage Transformer must be immediately taken out of service under any of the following conditions:

1. Insulation and Discharge Faults: Internal abnormal noise or discharge sound; severe discharge due to cracked porcelain insulators; severe discharge from the PT that threatens safety during operation; severe surface cracking or discharge on resin-cast transformers.
2. Internal Thermal and Structural Faults: Overheating phenomena observed through infrared temperature measurement; abnormal noise, odor, smoke, or fire internally; high-voltage fuses blow continuously twice; abnormal expansion and deformation of the metal expansion device; pressure relief device (explosion-proof diaphragm) has ruptured.
3. Insulation Medium Faults: Severe oil leakage, causing the oil level indicator to show no oil; SF₆ gas pressure gauge needle is in the red zone (for gas-insulated transformers).

Conclusion and Summary

The potential transformer is a critical component for achieving accurate metering and reliable protection in power systems, making its stable operation decisive for grid security. Ensuring the long-term reliability of this vital equipment necessitates strict routine maintenance, standardized operating procedures, and timely emergency response—all of which are indispensable key aspects. Mastering and rigorously executing these operational guidelines is not merely a technical requirement but a professional commitment to power system safety and stable supply. All operation and maintenance personnel must maintain high vigilance and internalize these regulatory requirements as standard practice to secure the reliable operation of PT equipment.