Mine flameproof vacuum feed switch (hereinafter referred to as feed switch) is suitable for underground mines and other surrounding media containing explosive gases. In the three-phase power grid without neutral point grounding, it is used as the main distribution switch or branch switch. This article introduces that the switch uses an intelligent protector, reliable performance, accurate action, with undervoltage, overload, short circuit, leakage blocking, selective leakage protection, and can be connected to a remote shunt trip button.

At present, domestic feeder switch products are generally control cores composed of discrete components or single-chip microcomputers, with complex structures and difficult installation and debugging. The anti-interference ability is poor, the failure rate is high, and the dispersion of the components has a great influence on the reliability and stability of the protection. The display uses ordinary digital tube or indicator light, which is not intuitive. The foreign similar products are also generally composed of PLC system, and the technological level is higher than that of the domestic. However, once a failure occurs, the repair cannot keep up, affecting production. In addition, the price of foreign products is extremely high, more than ten times that of similar domestic products.

In response to this situation, the NA200 small integrated PLC is used in mine explosion-proof vacuum feeder switches. According to the technological requirements of the feed switch, it provides a more cost-effective, smarter and more optimized control scheme. The core of the device adopts technologically advanced industrial programmable controller NA200 PLC and human-machine interface (text display and keyboard setting) to form an intelligent comprehensive protection controller, which makes the power distribution device reliable and safe. The feed switch can monitor the underground power grid parameters in real time, and timely display and alarm over and under voltage, phase failure, leakage, overload, short circuit and other faults.

Feed switch structure and protection principle

The flameproof enclosure of the feed switch is rectangular, and is connected to the pry-shaped base with four M12 bolts. The upper and lower cavities of the flameproof enclosure are the wiring cavity and the main cavity. The wiring cavity is above the main cavity, and it gathers all the input and output terminals of the main circuit and the control circuit. The main circuit power inlet terminal is covered with a protective plate. There are two main loop inlet and outlet horns on both sides of the wiring cavity, and cables with outer diameters of φ68～φ78 can be introduced. The main cavity is composed of the main cavity shell and the front door containment, which is mainly equipped with a main body core frame and a kilovolt power control switch. The front door adopts a quick opening door structure.

The AC vacuum circuit breaker is installed above the left side of the core frame of the main body. The three incoming wires are directly connected to the incoming terminals of the circuit breaker, and the three outgoing wires are connected to the outgoing terminals on the reverse side of the core frame. The kilovolt-level power control switch is installed above the right side wall of the main cavity, and is connected to the operating handle outside the main cavity by a connecting sleeve. The overvoltage absorption device is installed under the circuit breaker, and the control transformer, current transformer and zero-sequence transformer are installed on the reverse side of the main body core frame. The front of the core frame is also equipped with intermediate relays, fuses, reactors, transformers, and two plug connectors for connecting the plug and socket from the front door and the seven-core terminal in the wiring chamber.

The front door is equipped with observation windows, buttons, test buttons, protector setting buttons, protectors, protector power supply, PLC and display screen, etc. There is a reliable mechanical interlock between the front door and the housing. When you need to open the front door of the feed switch, you must first turn the handle of the power control switch on the right wall to the off position (middle position) to disconnect the control power. The circuit breaker automatically opens due to the power loss of the control circuit, and then screwed into the locking lever, so that the inner end enters the locking hole of the power switch handle, and the power is turned on, off, locked and disconnected. The outer end can only be released from the limit block on the front door to unlock the front door, and then the front door can be opened. The method of closing the front door is opposite to the opening procedure, so that the closing operation can only be performed after the front door of the feed switch is locked.

The line protection function of the distribution switch needs to collect three-phase current, three-phase voltage, zero-sequence current, zero-sequence voltage, a line voltage, insulation and phase sensitive signals on the high-voltage transformer. These signals pass through the low-voltage transformer, then become the standard signal of DC through rectification and filtering, and finally enter the PLC to control the opening output of the circuit breaker.

The PLC calculates the incoming voltage signal. The voltage calculation result is greater than 1.2 times the rated voltage, indicating overvoltage, and the overvoltage protection will operate within 5s. The voltage calculation result is less than 0.8 times the rated voltage, indicating undervoltage, and the undervoltage protection will operate within 5s. The calculation result of the three-phase unbalance is greater than 0.7, it shows the phase failure, and the protection action within 10～20s. Any one phase in the current calculation result is within the range of 6 to 10 times the rated current, indicating a short circuit, and the short circuit protection will act within 0.08s. When any phase in the current calculation result is within the range of 1.2 to 6 times the rated current, an overload is displayed, and the overload protection operates according to the inverse time limit characteristic. If a leakage fault occurs on the line, the line will be cut off according to the phase sensitive and voltage signal leakage protection action.

Control System

The PLC uses the NA200 series programmable controller of Nanda Automation Technology Co., Ltd. NA200 series PLC is a small integrated PLC produced by Nanda Automation Technology Co., Ltd. In this application, the CPU module selects the CPU201-2401 module integrated with 14-point input/10-point relay output on the body. The analog input selects the unique leakage protection OPD201-1201 module of Nanda Automation Technology Co., Ltd. It has 12-bit precision, collects zero-sequence voltage, zero-sequence current, system voltage and system current, and completes power calculation at the same time. This module can meet the harsh environment in the underground, and can meet the power supply protection requirements of the power supply and distribution system whose neutral point is not grounded. The display component of the system uses the four-line blue text screen of Nanda Automation Technology Co., Ltd., and communicates with the PLC through the serial port with Modbus RTU protocol, and displays the collected values in real time. The control system measurement point configuration is as follows:

1)Input measuring point

2)Output measuring point

3)Analog Input

Control function

The main functions of the feed switch control system through software programming are:

1) Parameter setting rated current:

The setting range of rated current is 100A----630A.

Power supply voltage: The setting range of the power supply voltage is 660V and 1140V second gear.

Short circuit multiple: the setting range is 3----10 times continuously adjustable.

2) Leakage protection

Leakage blocking: 1140V system 42K blocking, 50K recovery, 660V system 23K blocking, 28K recovery.

Leakage failure: 23K protection for 1140V system and 12.2K protection for 660V system.

3) Overcurrent protection

According to the requirements of the coal industry standard, different action times are determined according to the ratio of different actual currents/rated currents. In order to improve the protection accuracy and sensitivity, phase B, A, and C use different precision transformers.

4) Short circuit protection

When a short-circuit fault occurs, the PLC gets a slowly rising voltage signal, because there is a certain "mechanical action delay" from when it sends a cut-off command to when the switch actually moves. In order to make the action value accurate and at the same time meet the time requirement of not more than 100 ms, the judgment value of PLC must be ahead of the required action value, thereby making up for the adverse consequences caused by "mechanical action delay".

5) Module self-test protection

This program has the module self-checking function:

When the PLC fails, the device has the following protection. PLC self-diagnosis: when the expansion module fails, there is a "module failure" alarm screen.

6) Performance test function

Leakage test: It is realized by using a relay with an external 1K resistor.

Short circuit test: The simulated current is the product of “rated current” and “short circuit multiple”.

Overload test: 1.9Ie, the action time is 60 seconds, because the action time is longer, so the countdown display is set in the program.

7) Fault memory function:

The device can memorize the last 50 faults, and can report 14 faults respectively, including action time and action value. When entering the fault memory screen and turning pages up and down, it has a humanized style, which changed the way of turning around in the past.

8) Start-up time memory function:

This device can memorize "cumulative charging time" and "cumulative pull-in time".

In addition to the basic functions listed above, this feeder switch also has functions such as leakage resistance, voltage, current and power display, gas overrun, high voltage action, and low voltage action. The control system of this power distribution switch is based on the characteristics of my country's coal mine electrical equipment and incorporates international advanced intelligent technology and structural forms. The use of a PLC control system instead of discrete electronic components or single chip microcomputers realizes the control, protection and measurement of the underground power supply system. It has the characteristics of good performance, high reliability, strong stability and fast development cycle.