KYN28A-12 Metal-Clad Withdrawable Switchgear (PT Cabinet)

1. Product Overview

The KYN28A-12 PT cabinet is a special 12kV indoor medium voltage armored withdrawable central switchgear, specially configured for bus voltage sampling, system voltage monitoring and lightning surge suppression in 10kV power distribution systems. It is installed on the main busbar between the incoming cabinet and feeder cabinets, operating in coordination with incoming cabinets, metering cabinets and transformer outgoing cabinets to form a complete medium-voltage power distribution system.

Different from incoming cabinets equipped with vacuum circuit breakers, the PT cabinet has no main circuit breaker. Its core components are voltage transformers, high-voltage fuses and lightning arresters. It adopts the same four-compartment fully isolated metal arc-proof partition structure as standard KYN28A-12 series, with IP4X cabinet protection grade and complete five-prevention mechanical & electrical interlock system. Widely applicable to industrial plants, commercial power stations, photovoltaic substations, residential distribution rooms and various overseas medium-voltage power supply projects.

2. Main Internal Equipment Configuration

Primary Circuit Core Devices

1. Three-Phase Voltage Transformer (PT) The core component of the PT cabinet, connected in parallel to the three-phase main busbar. It steps down 10kV high primary voltage into standard 100V secondary voltage signals, which are transmitted to energy meters, comprehensive protection relays and power monitoring devices for voltage measurement, overvoltage/undervoltage judgment and power factor calculation.
2. High-Voltage Current-Limiting Fuse for PT Protection Installed on the primary side of each voltage transformer. When short-circuit or overload faults occur inside PT, the fuse will blow rapidly to isolate the faulty transformer, preventing phase-to-phase short-circuit of the main busbar and large-scale power failure accidents.
3. Zinc Oxide Lightning Arrester Connected in parallel with the three-phase busbar. It restrains switching overvoltage and lightning induced surge voltage, absorbing instantaneous overvoltage energy to protect all subsequent high-voltage equipment from insulation breakdown damage.
4. Main Busbar & Branch Insulated Copper Busbars TMY insulated copper bars connect the PT cabinet with adjacent switchgears to transmit 10kV three-phase high voltage to the primary side of PT and lightning arresters.
5. Earthing Switch Arranged inside the cable compartment for safe discharge of residual high voltage during equipment maintenance, mechanically interlocked with the PT trolley and cabinet door.
6. High-Voltage Live Display Sensors Inductive sensors mounted on primary busbars to detect electrified state and trigger warning indicators on the cabinet front panel.

Secondary Control & Monitoring Components

1. Voltage Measuring Meters & Indicator Lights Installed in the upper secondary instrument compartment, displaying real-time three-phase bus voltage, PT fuse breakage alarm and live bus status.
2. Secondary Terminal Block Centralized wiring unit for PT secondary voltage signal transmission, transmitting voltage signals to all protection devices and monitoring systems of the whole high-voltage room.
3. Signal Transmission Module Supports RS485 communication, uploading real-time bus voltage data, abnormal voltage alarms and PT fault signals to the intelligent power monitoring background for unattended operation management.
4. Auxiliary Control Buttons & Fault Alarm Buzzer Realize local status inspection and abnormal sound-light alarm for voltage loss, PT fuse failure and unbalanced three-phase voltage.

Cabinet Safety Structural Parts

• Four independent sealed compartments: bus compartment, PT trolley compartment, cable compartment and secondary instrument compartment, separated by arc-resistant metal partitions to block fault arc diffusion.
• Top pressure relief channel: automatically releases high-temperature high-pressure gas generated by short-circuit faults to avoid cabinet explosion.
• Full set of five-prevention interlock mechanism to eliminate all dangerous misoperation behaviors.

3. Working Principle

3.1 Bus Voltage Sampling Principle

Three-phase 10kV high voltage flows into the PT cabinet through the main copper busbar and acts on the primary winding of parallel-connected voltage transformers. Based on electromagnetic induction principle, PT converts high primary voltage into low standard secondary voltage. The secondary voltage signals are transmitted to multifunctional meters and microcomputer protection devices through secondary cables, providing basic voltage data for the whole power distribution system.

The primary high-voltage fuses serve as the unique short-circuit protection for PT. Once internal winding short-circuit or insulation breakdown happens inside the transformer, the fuse will melt instantly to cut off the primary circuit and isolate the fault, avoiding serious busbar short-circuit risks.

3.2 Overvoltage Suppression & Lightning Protection Principle

Lightning arresters are parallelly connected to each phase busbar and reliably connected to the station grounding grid. When lightning strikes or switching operation generates instantaneous overvoltage, the resistance of zinc oxide lightning arrester drops sharply, forming a transient conductive channel to release surge current into the underground grounding network. After overvoltage disappears, the arrester restores insulation state automatically, effectively limiting overvoltage amplitude and protecting CT, PT, circuit breakers and transformers from insulation damage.

3.3 System Voltage Monitoring & Fault Alarm Principle

The secondary voltage signal output by PT is sent to the comprehensive protection system, which realizes multiple voltage monitoring functions:

1. Overvoltage protection: trigger alarm or interlock tripping when bus voltage exceeds the upper limit;
2. Undervoltage protection: alarm when voltage drops below the rated threshold, judging power loss or line breakage;
3. Three-phase voltage unbalance monitoring: send fault signals when voltage deviation of three phases exceeds the set range;
4. PT disconnection alarm: detect abnormal voltage loss caused by blown fuses and output sound-light alarm.

All voltage operation data and fault records can be uploaded remotely via communication modules for centralized monitoring.

3.4 Five-Prevention Interlock & Maintenance Safety Principle

The PT cabinet adopts integrated mechanical and electrical interlock to prevent misoperation:

1. The PT trolley cannot be pushed into the working position if the earthing switch is closed;
2. The earthing switch can only be closed after the PT trolley is withdrawn to the test position and the primary busbar is fully de-energized;
3. The cabinet door cannot be opened when the PT trolley is in working position with live busbar;
4. PT fuses are inaccessible for replacement when the primary loop is electrified.

During maintenance, pull out the PT trolley, close the earthing switch to discharge residual high voltage on busbars and equipment, forming a zero-voltage safe maintenance environment for inspection, replacement of PT and lightning arresters.

3.5 Cabinet Grounding Protection Principle

The cabinet metal shell, arc-proof partitions, PT support frames, lightning arrester bases and earthing switch are all connected to the overall station grounding grid via flat steel. If insulation aging causes cabinet leakage, leakage current will be quickly guided into underground earth to prevent electric shock accidents for field operators.

4. Core Functions of KYN28A-12 PT Cabinet

1. Real-Time Three-Phase Bus Voltage Sampling Provide stable standard voltage signals for metering, protection and monitoring equipment of the whole high-voltage distribution system, serving as the core voltage data source of the power station.
2. PT Short-Circuit Fault Isolation Protection Matched high-voltage fuses rapidly cut off faulty voltage transformers to prevent busbar short-circuit and large-area power outages.
3. Lightning & Switching Overvoltage Suppression Built-in zinc oxide lightning arresters absorb surge overvoltage, protect all medium-voltage equipment from insulation breakdown damage.
4. Multiple Voltage Abnormality Monitoring & Alarm Detect overvoltage, undervoltage, three-phase unbalance and PT fuse breakage, output sound-light local alarms and remote fault signals.
5. Remote Voltage Data Upload & Centralized Monitoring Support RS485 communication interface, realize remote real-time voltage reading and fault record storage, matching intelligent unattended power distribution station construction.
6. Full Arc Isolation & Safe Maintenance Guarantee Independent arc-proof compartments, top pressure relief device and complete five-prevention interlock greatly reduce safety hazards during operation and maintenance.

5. Main Technical Parameters

• Rated voltage: 12kV
• Rated frequency: 50Hz / 60Hz (customizable for global overseas markets)
• Rated current of main busbar: 630A, 1250A
• Rated short-time withstand current: 20kA, 25kA, 31.5kA
• Cabinet protection grade: IP4X
• Accuracy class of voltage transformer: 0.2 / 0.5
• Operating ambient temperature: -10℃ ~ +40℃
• Application environment: Indoor dry medium-voltage power distribution room

6. Foreign Trade Promotion Advantages (Suitable for Website Display)

1. Standard KYN28A-12 modular design, compatible with mainstream international medium voltage power distribution standards, convenient for matching with other switchgears;
2. Withdrawable PT trolley structure, quick replacement and maintenance without cutting off the whole busbar power supply, low maintenance cost;
3. Integrated lightning arrester design, combining voltage sampling and overvoltage protection into one cabinet, saving overall room layout space;
4. High precision voltage transformer meets metering and monitoring requirements of power grids in Asia, Africa, Southeast Asia and the Middle East;
5. Dual frequency design of 50Hz/60Hz, applicable to most overseas national power supply standards;
6. Complete arc suppression and pressure relief safety structure, high reliability for heavy industrial and outdoor substation supporting projects;
7. Intelligent communication function supports remote monitoring, conforming to the construction trend of modern smart power grids.