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PT cabinet function and composition

Release time:

2025-05-16

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Abstract

Function and Composition of PT Cabinet

The PT cabinet (voltage transformer cabinet) is an indispensable core device in the power system, mainly used for key functions such as voltage measurement, relay protection, and energy metering. The following is a detailed description of its function and composition:

I. Functions of the PT Cabinet

Voltage Measurement and Conversion

Through the voltage transformer (PT), high voltage (such as 10kV, 35kV) is proportionally converted to low voltage (such as 100V) for use by instruments, protection devices, and electricity meters.
Provides measurement voltage signals for power meters, voltmeters, etc., ensuring real-time monitoring of the power system operating status.

Relay Protection

Provides abnormal signals such as overvoltage, undervoltage, and loss of voltage, triggering the operation of relay protection devices (such as tripping) to protect power equipment and the power grid.
Supports bus insulation monitoring and automatic switching of backup equipment.

Operation and Control Power Supply

Provides power to the small busbar at the top of the high-voltage cabinet panel, supporting the operation and control circuit power supply of other high-voltage cabinets.

Energy Metering

Completes energy metering in conjunction with current transformers (CTs), providing data support for electricity billing (in some scenarios, PT is shared with the metering cabinet).

Safety Isolation and Protection

Electrical isolation between the high-voltage side and the low-voltage side is achieved through insulation design to ensure the safety of maintenance personnel and equipment.
Fuses or circuit breakers prevent short circuits on the secondary side, preventing PT damage.

Lightning Protection and Overvoltage Protection

Surge arresters can suppress lightning strikes or operating overvoltages, protecting the PT and subsequent equipment from damage.

II. Composition of the PT Cabinet

The core components and their functions of the PT cabinet are as follows:

Voltage Transformer (PT)

Core Function : Converts high voltage to low voltage (e.g., 10kV→100V).
Structural Features : Usually uses electromagnetic or capacitive design, with high accuracy (error <1%) and good insulation performance.

Fuse or Circuit Breaker

Function : Protects the PT secondary circuit from short circuits or overloads.
Common Configurations : Fuses (such as RN type) are used in small-capacity systems, and circuit breakers are used in large-capacity or automated systems.

Surge Arrester

Function : Absorbs lightning strikes or operating overvoltages, preventing PT insulation breakdown.
Installation Location : Usually connected to the high-voltage side or neutral point of the PT.

Isolation Switch

Purpose : Disconnects the PT from the high-voltage busbar during maintenance to ensure operational safety.
Type : GN30, GN19 (10kV), GN27 (35kV), etc.

Ferroresonance Suppression Device

Function : Suppresses ferroresonance (PT may cause resonance overvoltage due to ground faults in ungrounded neutral systems).
Type : Ferroresonance resistors, microcomputer ferroresonance suppressors, etc.

Wiring Terminals and Secondary Circuit

Function : Connects the primary side (high voltage) and secondary side (low voltage) of the PT, and distributes signals to instruments, protection devices, etc.
Design Requirements : Ensure reliable wiring and avoid poor contact.

Enclosure and Structure

Material : Removable metal armored structure, providing mechanical protection and electromagnetic shielding.
Observation Window : Facilitates real-time monitoring of the PT operating status (such as fuse blowing, surge arrester operation).

Small Busbar

Function : Transmits the low-voltage signals output by the PT to other high-voltage cabinets to supply power to the measurement, protection, and control circuits.

III. Key Technical Parameters of the PT Cabinet

Parameter	Typical Value
Rated Voltage	10kV/35kV (primary side), 100V (secondary side)
Accuracy Class	0.2S/0.5S (metering grade), 1.0 (protection grade)
Ratio	10kV/√3 : 100V/√3 (common)
Insulation Withstanding Voltage	≥32kV (1 minute, power frequency)
Short-Time Thermal Stability	105℃ (1 hour, 1.2 times rated current)

IV. Application Scenarios of the PT Cabinet

Substation : Monitors bus voltage and provides protection signals.
Industrial Power Distribution System : Provides voltage measurement and protection for factory equipment.
Power Automation System Provides voltage signals to the SCADA system for remote monitoring.
User-side Power Distribution Cooperates with the metering cabinet to complete energy metering (e.g., commercial buildings, data centers).

V. Selection and Maintenance of PT Cabinets

Selection Points

Select the PT ratio and insulation level according to the system voltage level (10kV/35kV).
Distinguish between measuring PTs (0.2S grade) and protection PTs (1.0 grade) according to accuracy requirements.
Consider whether a resonance suppression device is needed (mandatory for ungrounded neutral systems).

Maintenance Notes

Regularly calibrate PT accuracy (replace if error > 1%).
Check the status of fuses and surge arresters, and replace aging components in a timely manner.
Avoid short circuits on the secondary side of the PT (strictly prohibit direct short-circuiting of the output terminals).

The PT cabinet converts high voltage to low voltage through voltage transformers, providing the power system with voltage signals required for measurement, protection, and metering, while also having overvoltage protection and safety isolation functions. Its core components include PTs, fuses, surge arresters, isolating switches, etc., and are widely used in substations, industrial power distribution, and automation systems. Correct selection and maintenance of PT cabinets are key to ensuring the stable operation of the power system.

Key words:

What is the difference between a PT cabinet and a metering cabinet?

Bending limits of different metals in sheet metal bending