What is the function of a PT cabinet in a high-voltage power distribution system?

In high-voltage power distribution systems, the PT panel (voltage transformer panel) plays a crucial role and can be considered the "voltage sensing center" or "voltage monitor" of the entire system.

The following is a detailed breakdown of the main functions of the PT panel:

Core Function: Voltage Conversion and Electrical Isolation

This is the most fundamental function of the PT panel.

Voltage Conversion: High-voltage systems typically have voltage levels of 6kV, 10kV, 35kV, or even higher. Directly measuring such high voltages with instruments is extremely dangerous and uneconomical. The core component within the PT panel is the voltage transformer, a specialized transformer that precisely converts high voltages (e.g., 10kV) into a standard lower voltage (usually 100V).

Electrical Isolation: While performing voltage conversion, it also establishes electrical isolation between the primary (high-voltage) and secondary (low-voltage) sides. This means that measuring instruments, protective devices, and operators connected to the secondary side are all in a safe, low-voltage circuit, completely isolated from dangerous high voltages.

Specific Applications Derived from Core Functions

With a safe, standard 100V voltage signal, the PT panel provides the foundation for the following key functions:

1. Measurement and Metering

Powering Instruments: Providing voltage signals to the voltmeter, frequency meter, power factor meter, and other devices on the distribution panel. This allows operators to monitor key system parameters such as voltage level and frequency in real time.

Energy Metering: Providing voltage signals to the energy meter. The energy meter, combined with the current signal from the current transformer, accurately calculates the active and reactive power consumed by the line, which serves as the basis for electricity bill settlement with customers.

2. Relay Protection
This is the most critical protection function of a PT switch. It provides voltage judgment for many protective devices.

Overvoltage Protection: When the system experiences abnormally high voltage due to lightning strikes, operational issues, or other factors, the protection device activates, disconnecting the circuit to protect equipment insulation.

Undervoltage Protection: When the system voltage drops significantly due to a short circuit, fault, or other factors, the protection device activates. For example, this prevents critical motors from self-starting when voltage is restored and potentially impacting the power grid.

Undervoltage Protection: When power is completely lost, this device prevents equipment from self-starting after a sudden power recovery, ensuring personal and equipment safety.

Ground Fault Protection (Zero-Sequence Voltage): In systems where the neutral point is ungrounded or grounded via an arc suppression coil, measuring the voltage (zero-sequence voltage) across the PT open-delta winding can determine whether a single-phase ground fault has occurred.

Distance Protection: In more complex transmission line protection, simultaneous voltage and current measurements are required to calculate impedance and determine the distance to the fault.

Power direction protection: Determines the direction of fault power and is used for lines with dual-side power supplies to ensure the selectivity of protection action.

3. System Operation and Monitoring

Synchronous Closing: When two power systems need to operate in parallel (for example, when a backup power source is activated or a generator is connected to the grid), the voltage amplitude, frequency, and phase on both sides must be checked for consistency. The PT provides the synchronization device with voltage signals from both sides, ensuring that the circuit breaker can only be closed when the necessary conditions are met, thus avoiding significant current surges.

Insulation Monitoring: By monitoring three-phase voltage imbalance, it can assist in determining the insulation condition of the system.

Providing Operating Power: The PT's secondary winding sometimes has a separate "residual voltage winding" specifically used to provide 220V AC operating power for lighting, control circuits, signal circuits, etc. within the distribution room.

Typical Configuration of a PT Panel

A complete PT panel typically includes:

Voltage transformer: Core equipment, typically consisting of two sets (one for measurement and protection, the other for insulation monitoring/open delta).

High-voltage fuse: Protects the PT itself from damage caused by primary-side short-circuit faults.

Isolating switch/trolley: Facilitates commissioning, decommissioning, and maintenance of the PT.

Secondary circuit miniature circuit breaker: Protects the secondary low-voltage circuit from overloads and short circuits.

Lightning arrester: Prevents damage to the PT from overvoltage surges.

Related measurement and protection devices.

Summary

Simply put, the function of a PT switchgear is to safely convert dangerous high voltages to standard low voltages, providing essential voltage signals for system measurement (meters), protection (relays), and control (synchronizers). Without PT switches, the safe, accurate, and reliable monitoring and protection of high-voltage distribution systems would be impossible. They are essential equipment for ensuring the safe, stable, and economical operation of power systems.

As a professional power equipment or engineering service provider, Shaanxi Huadian can provide users with solutions for PT cabinets and the entire power distribution system.

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