Overview

In China's 3~35kV power supply systems, most are neutral point ungrounded systems. When single-phase grounding occurs, the grid can still operate with faults, greatly reducing operating costs and improving system reliability. However, this method tends to cause arc flash overvoltage and interphase short circuits when single-phase grounding current is large, causing hazards to power supply equipment. The previous solution was to add arc suppression coils at the neutral point to compensate for capacitor current to reduce the probability of arc flash at fault points. Obviously, the purpose of this method is to eliminate arc flash, but due to the many characteristics of arc suppression coils themselves, it is difficult to effectively compensate capacitive current, especially the high-frequency components that cause insurmountable harm to power supply equipment. An intelligent arc extinguishing device uses a set of phase-controlled vacuum contactors to ground the fault phase, achieving complete arc flash elimination.

Usage environment

1. During normal operation, the microcomputer controller continuously detects the voltage signal provided by the PT. If the system experiences PT disconnection, single-phase metal grounding, or single-phase arc grounding, the auxiliary secondary open delta voltage immediately switches from low to high, the microcomputer controller starts, and determines the fault type and phase based on changes in PT secondary voltage.

2. If the PT single-phase disconnection fault occurs, the device outputs the switch contact signal and can simultaneously transmit data remotely to the microcomputer monitoring system via RS485 (or RS232) interface.

3. If it is a single-phase metallic grounding fault, the device outputs a switch contact signal. According to user requirements, the microcomputer controller can also send action commands to the vacuum contactor, and data can be transmitted remotely to the microcomputer monitoring system via RS485 (or RS232) interface.

4. If it is a single-phase arc flash grounding fault, the microcomputer controller issues an action command to the vacuum contactor, instructing it to quickly convert unstable arc flash grounding into stable metallic grounding. The device outputs the switch contact signal, and can also transmit data remotely to the microcomputer monitoring system via RS485 (or RS232) interface.

Basic functions and features of the device

1. It can limit the system's atmospheric and operating overvoltages to low voltage levels, ensuring the insulation safety of the power grid and electrical equipment.

2. The device operates quickly, capable of operating within 30ms~40ms, quickly eliminating intermittent arc flash and stable arc flash grounding faults, suppressing arc flash grounding overvoltage, preventing further accident escalation, and reducing the accident trip rate of the line.

3. It can quickly and effectively eliminate system resonant overvoltage, preventing long-term resonant overvoltage from damaging system insulation and damaging surge arresters and small inductive loads installed in the power grid.

4. After the device activates, it allows a 200A capacitor current to pass continuously for at least 2 hours. Users can only handle the faulty line after completing the switching operation of the transfer load.

5. It can accurately locate single-phase ground fault lines, which is important for preventing further expansion of accidents and reducing the workload of operation and maintenance personnel.

6. According to the working principle of the device, its overvoltage limiting mechanism is independent of the magnitude of the grid's capacitive current to ground, so its protection performance does not change with changes in grid operation. It can be used by both large and small grids, and grid expansion is unaffected.

7. The voltage transformer in this device can provide system voltage signals to metering instruments and relay protection devices, replacing conventional PT cabinets.

8. Capable of measuring the single-phase grounding capacitor current of the system.

9. The device is simple, compact, and easy to install and commission. It is suitable for substations and also for high-voltage plant power systems in power plants; It is suitable for both new stations and retrofitting old power stations.

10. High cost performance, offering a high performance-to-price ratio compared to arc suppression coil systems.

The main components and functions of the device

1. Combined overvoltage protector TBP composed of high-capacity ZNO nonlinear components

TBP is a high-capacitance zinc oxide overvoltage protector. Compared with conventional zinc oxide arresters (MOA), it has the following advantages:

(1) The TBP combined overvoltage protector adopts a structure combining high-capacitance ZNO nonlinear resistors and discharge gaps. Due to the coordination between the gap element and the ZNO valve plate, the protector's charge rate and power frequency aging issues are resolved.

(2) The impact coefficient of the TBP combined overvoltage protector is 1, and the discharge voltage values are equal under various voltage waveforms, unaffected by the overvoltage waveform. The overvoltage protection value is accurate and offers excellent protection performance.

(3) The TBP combined overvoltage protector uses a four-star connection, which can greatly reduce phase-to-phase overvoltage. Compared with conventional arresters, it reduces phase-to-phase overvoltage by 60-70%, greatly improving protection reliability.

The TBP combined overvoltage protector is a device in this device that limits various types of overvoltage, mainly used to limit atmospheric and operating overvoltage.

2. Phase-controllable high-pressure vacuum contactor (KA-KC).

This is a high-voltage vacuum AC contactor with three phases in a separate unit, with one end of each phase connected to the busbar and the other end grounded. During normal operation, the vacuum switch is in an open state and operated under the control of a microcomputer controller. The phases are locked, and when one phase closes and the busbar is grounded, neither phase of the other two phases closes.

3. Multifunctional microcomputer controller WZK (core technology).

The multifunctional microcomputer controller is the technical core component of this device. It uses a PIC microcontroller with high anti-interference capability as its core processor, which consists of two microcontrollers and offers fast fault handling speed.

It mainly has the following functions and features:

(1) High degree of automation. All functions of the microcomputer controller are executed automatically, requiring no manual operation, making maintenance and operation simple.

(2) Good anti-interference performance and high reliability. The microcomputer controller uses secondary power technology to resist various electromagnetic interferences. All interfaces use optical/electrical isolation to eliminate interference from the power supply and primary system. The controller is also equipped with a Watchdog self-reset circuit to enable automatic device reset, avoiding various external electromagnetic interference.

(3) Comprehensive protection functions. Since the microcomputer controller uses dual processors as processing units, it can simultaneously perform comprehensive control of arc suppression, resonance suppression, and wire selection, with very fast speed. Moreover, the device can perform high-voltage fuse alarms for stack voltage transformers and fault alarms for secondary voltage transformer signal circuits.

(4) Voltage measurement function. The microcomputer controller can measure the system's voltage and display it in digital form.

(5) Remote data transmission function. The microcomputer controller is equipped with RS485/232 communication interfaces, enabling real-time system fault information and device operation status to be reported to the control center.

(6) Fault log query function. The microcomputer controller has memory and can store system fault information (including fault type, fault time, etc.) long-term, allowing users to query system fault records as needed.

(7) Accurate wire selection function. The WZK microcomputer controller has designed a wire selection module capable of single-phase ground wire selection for multiple outgoing lines (32 lines per busbar segment). It uses the "incremental mutation method" wire selection principle, ensuring accurate selection of grounding lines regardless of the type of grounding occurring in the system.

4. High-voltage current-limiting fuse FU2

The high-voltage current-limiting fuse is the backup protection component of the entire device, with the following functions:

◇ Large breaking capacity, up to 63KA;

◇ Rapid breaking, with switching time less than 0.3ms;

◇ Good current limiting effect, limiting fault current to less than one-fifth of the short-circuit current and inrush current.

◇ Low breaking arc voltage: During fuse breaking, the arc voltage is very low, and when used in systems below rated voltage, the arc voltage will decrease further. Therefore, 12KV fuses can be used in 7.2Kv systems without the risk of damaging system insulation.