The setting of over-current protection of low-voltage power supply system is basically calculated according to the setting rules of short-circuit protection device of low-voltage power grid in coal mine, which must meet the requirements.

1. 1 Calculation of current setting value of overcurrent protection device of low-voltage feeder switch

IZ=IQe+Kx∑Ie[ 1]

Where: iz-current setting value of overcurrent protection device;

IQe- rated starting current of maximum capacity motor;

Kx- demand coefficient, 0.5 ~1;

∑Ie- sum of rated currents of other motors.

1. 1.2 Calculation of setting current of the selected short-circuit protection device;

Ue Id=

2(∑R)2+(∑X)2[2]

Where: ID-two-phase short-circuit current at the farthest point of the protection range of the protection device;

UE-rated voltage of secondary side of transformer;

σr—— the sum of resistance values of one phase during short circuit;

σx- the sum of one-phase reactance values during short circuit.

1. 1.3 operation reliability verification of protection device;

Id IZ≥ 1.5[3]

Problems in setting calculation of 1.2

With the development of coal mining technology and the increase of high-yield and efficient working faces, the power of coal mine machinery and equipment is increasing. For example, the power of shearer, scraper conveyor and belt conveyor has reached more than 1000 kW, and the direction of working face has also reached 1~2 km, which has brought certain difficulties to the power supply design of mining working face. Due to the increase of equipment power, the starting current IQe increases, and the setting current value IZ of overcurrent protection increases accordingly, which directly leads to the decrease of action sensitivity coefficient, and the short-circuit protection device cannot operate reliably. At the same time, with the increase of power supply distance, the line impedance increases, the minimum two-phase short-circuit current decreases, and the action sensitivity coefficient also decreases, so the short-circuit protection device cannot operate reliably. This is a common problem in the design of low voltage power supply system at present.

1.3 traditional solution

Generally speaking, the sensitivity coefficient of protection device is improved by increasing the minimum two-phase short-circuit current. The main methods are:

1) Increase the section of trunk or branch cables. With the increase of cable section, the line impedance will decrease and the short-circuit current will increase;

2) Try to reduce the cable length, and the line impedance will also decrease with the decrease of the line length, that is, the short-circuit current will increase;

3) replace the method of large-capacity transformer or transformer parallel connection to increase short-circuit current;

4) increase the sectional protection switch to shorten the protection range and improve the minimum short-circuit current within the corresponding protection range;

5) The mobile substation is used for power supply, mainly to reduce the length of low-voltage power supply cable, extend the high voltage to the mining area as far as possible, and improve the minimum short-circuit current within the protection range.

For fully mechanized mining face with more high-power equipment, the cross section of low-voltage power supply cable is 95mm2, and there is not much room to increase the cross section of cable except parallel cable. Moreover, some trunk cables in fully mechanized mining face have been used in parallel, which is very uneconomical, more difficult to use and maintain, and there are many accidents. The geological conditions of some fully mechanized mining faces are poor, and the roadway is severely deformed by extrusion. In order to prevent the mobile substation from moving too close to the working face, the roadway section becomes smaller and it is difficult to move, the mobile substation and the pumping station are placed far away from the working face. Although the mobile substation is used for power supply, the problem of long power supply distance cannot be avoided.

Second, the new idea of power supply design

With the application of new technology and equipment, the action coefficient can be improved by reducing the setting value.

2. 1 Reduce the motor starting current IQe

From the formula [1], it can be seen that the setting current IZ can be reduced as long as the starting current IQe of the motor is reduced.

2. 1. 1 Start one by one with each motor.

In a working machine driven by multiple motors, each motor is started one by one. This method is suitable for shearer, scraper conveyor and belt conveyor. Imagine that a scraper conveyor is driven by two motors, the rated current of a single motor is Ie, and the direct starting current is 6 times, then the simultaneous starting current of the two motors of the scraper conveyor reaches 12 times Ie. When starting one by one, the maximum starting current is 6Ie+Ie=7Ie, which is nearly doubled. If it is a working machine driven by multiple motors, the starting current will be reduced even more.

2. 1.2 adopts two-speed motor.

Using two-speed motor on scraper conveyor can greatly reduce the starting current. The two-speed motor starts winding at low speed and runs at high speed. The rated power and current of the low-speed winding are about 0.5 times that of the high-speed winding, and the starting current is small. After starting, switch to high-speed winding. Since the working machine has been started, the starting current after switching to the high-speed winding is much lower than the direct starting current.

2. 1.3 adopts various soft start technologies.

Belt conveyor adopts various soft start technologies. For example, hydraulic and viscous soft start technology, auxiliary hydraulic motor runner system and electric soft start system, such as variable frequency speed regulation system and silicon controlled switched reluctance system. Especially in the variable frequency speed regulation system, under the control of the computer, the soft start time can be adjusted at will, and the starting current can be reduced to 1~2 times of the rated current, which is very effective in solving the problem of excessive starting current of high-power equipment.

2.2 choose the most suitable setting value IZ

Because the short-circuit setting of the old-fashioned switch electronic protection is based on the rated current multiple of the switch, the interval between two adjacent gears is very large, so the maximum starting current cannot be avoided when choosing a lower gear, and the reliability action coefficient is too small when choosing a higher gear, so the method of adding a cable has to be adopted, which is neither economical nor convenient for maintenance. At present, the new generation of switches widely use single chip microcomputer technology, and its setting value can be adjusted almost continuously, which provides more space for the selection of setting value IZ and avoids these problems existing in the old switches.

2.3 Using phase-sensitive protection technology

The traditional short-circuit protection of feed switch depends on the sampling of current signal. For long-distance power supply lines, the terminal short-circuit current is small, which can not be distinguished from the starting current of large motors, so it is difficult to set. Sensitive protection can distinguish short-circuit current from motor starting current. The principle is that the sampling method is current and power factor, that is, the signal is sampled not only by current, but also by power factor:

u=Icosθ[4]

Where: u- sampling signal of protection device;

Current signal;

Cosθ- power factor.

When the motor starts, the current is large, but the power factor is small, generally 0.2~0.5, while the power factor of short-circuit current is large, which is about equal to 1. If the starting current is as large as the short-circuit current, the total starting current signal will be half smaller than the short-circuit current signal through the phase-sensitive conversion unit, so it can be separated, so it will not trip by mistake. This protection technology is especially suitable for long-distance tunneling faces.