Mine leakage current protection devices

Despite the widespread use in the vast majority of low-voltage (up to 1000 V) industrial and domestic networks of power supply systems with a grounded neutral , in underground workings according to PB ^{[1], the} use of such systems is strictly prohibited due to the fact that earth faults in such networks ( at their proper design and execution), although on and off very quickly, but is accompanied by an electric arc and sparks that can cause fires and explosions, as in underground mining is often observed allocation mine g for (mostly methane , sometimes - propane , butane ), and the present dust prone to explosions (e.g. coal dust).

In this case, the safety regulations (safety rules) are required to fulfill the power supply networks of underground workings only with an isolated neutral, short circuits to the case in which do not cause sparks and electric arcs (due to the small currents of the earth fault in the network with isolated neutral). At the same time, networks with isolated neutral are also not without drawbacks:

• since the short circuit of one phase to the housing (to the ground) is not accompanied by high currents, such damage will not cause a change in mode and the operation of protective equipment, while the damaged area may be in operation for a long time
• if the insulation of one of the phases is damaged, the voltage of the undamaged phases will increase to linear (approximately 1.73 times) and when a person touches this phase, his body will be under linear voltage; at the same time, high humidity is usually observed in the mine, in addition, a person located in an underground mine experiences increased sweating, all this leads to a decrease in the electrical resistance of his body. Touch voltage increased to a linear value and a strong decrease in the electrical resistance of the body under the influence of external factors can lead to a large current flowing through the body and death
• the magnitude of the current flowing through the human body in networks with an insulated neutral of high voltage (above 1000 V) and in extended cable low-voltage networks is characterized not so much by active leakage currents as by the capacitance of the phases relative to the ground and even with good insulation in such a network, a person touches the conductive surface can be fatal

Based on the foregoing, in underground workings, it is necessary to use a power supply system only with isolated neutral and mandatory use of special protective devices that automatically monitor the insulation of the network and measure leakage currents to the ground and if the insulation resistance decreases below the required value, as well as the leakage current increases above the set value, disconnecting damaged section, in addition, such a device should not allow the damaged section of the network to turn on.

This eliminates the dangerous modes for humans that may occur on the network. The influence of the capacitive component in the low-voltage short circuit is not significant, but if the low-voltage protected area has an extensive network and the capacitive component can affect the severity of the personnel’s defeat when touching live parts of the network; Usually, special separate devices are used to compensate for the capacitive component, which are not included in the design of the RUZTU (although there are designs with combined leakage relays and a compensation device, for example, AZAK).

The principle of this device is to apply a low-current operational voltage to a controlled power supply network with a frequency other than 50 Hz (direct current is most convenient for these purposes) and create an artificial neutral from three ballast complex resistances (inductors or active resistors); in this case, between the artificial neutral and the ground (for which a special external earthing switch is used), a sensitive organ is switched on (in the simplest case, an electromechanical relay), when a leak occurs, a zero sequence voltage appears between the ground and the artificial neutral (neutral bias). Upon detection of increased leakage currents to the ground, a signal is supplied to the release of the switching device (for example , a circuit breaker ), which disconnects the network. This principle of fixing leakage currents is applied because the RUZTU must control the leakage currents when the line is turned on and when it is turned off and not allow the device to turn on (preventively). Thus, the main purpose of the RUZTU is similar to an RCD : protecting people from electric shock, preventing fires, but in this case the differential current is measured using a special differential toroidal current transformer in the RCD.

At the same time, the RUZTU does not perform the selective type (they do not disconnect only the damaged section, departing from the low-voltage switchgear (switchgear) of the given transformer): The RUZTU is installed at the beginning of the line near the transformer substation, which controls the operation of the input switch and when any network appears on any feeder In case of leakage, all loads are disconnected, this is due to the fact that modern mining equipment has strong technological connections and when at least one unit is disconnected along the mutual x lock stops the entire complex. When the RUZTU is triggered and the introductory machine is turned off, the maintenance personnel turn it on again and then turn on the starters installed on the mechanisms in turn; when you try to turn on the starter, which feeds the damaged area, all lines will be disconnected again according to the signal of the RUZTU. If we take into account that the distance from the district TP to the distribution center (distribution point) can be from 200-500 m or more, then to determine the damaged area, the maintenance personnel must go up to 1-2 km or more, which requires a lot of time and more equipment downtime . To speed up the search for the damaged area in the starters, a blocking leakage relay (BRU) is built in, while when the input circuit breaker is disconnected by the signal of the RUZTU, the maintenance personnel re-supply power, switching on the damaged section will prevent the BRU of this starter, which significantly reduces the time to search for the damaged line, the use of remote inclusion of the input switch (with RP lava) will further accelerate the search.

According to the current GOST 31612-2012, RUZTU in addition to continuous monitoring of the active earth leakage current must:

• be able to measure the resistance of the network relative to the ground using the built-in ohmmeter
• have a measuring current of not more than 10 mA
• operate at a single-phase leakage current of not more than 25 mA, taking into account the measuring current and the largest capacitance and mains voltage
• have a response resistance (the minimum value of the network resistance relative to the ground at which a trip signal should be supplied) of the network protection device:
  • 127 V - 3.3 kOhm per phase,
  • 220 V - 10 kOhm per phase,
  • 380 V - 10 kOhm per phase,
  • 660 V - 30 kOhm per phase,
  • 1140 V - 60 kOhm per phase
• have a voltage of the measuring current source not more than the voltage of the controlled network, the preferred value is 100V
• have their own response time of the protection device no more (with a single-phase leakage resistance of 1 kOhm) for voltage:
  • 380, 660 V - not more than 0.1 s,
  • 1140 V - not more than 0.07 s
• operate when the mains voltage drops to 0.6 nominal, due to phase-to-phase overlapping of the phases by an electric arc or by closing the arc from phase to case
• equipped with a special terminal for connecting an additional ground electrode, and must also have a separator
• have a manual control device
• equipped with a device for prohibiting the inclusion of the network when the protection against leakage currents is disabled
• have such response characteristics that, when touching live parts, a charge of more than 50 mCl should not pass through the human body, which can be achieved by capacitive compensation or (and) a phase short circuit
• control the network resistance even when the switching device is turned off and prevent the network from turning on when the resistance is low
• have a measuring current when the device is off, not more than 5 mA; recommended value 1 mA
• have a return to its original state after tripping no more than 1.5 times lower than the tripping resistance
• have an open circuit voltage of the measuring current source with blocking of at least 50 V, but not more than the mains voltage; recommended value 100 V

RUZTU according to GOST should be subjected to the following types of tests:

• acceptance , according to which an experimental batch of products is tested in an amount of at least 5 pieces:
  • single phase leakage current detection
  • determination of dielectric strength
  • determination of measuring current
  • determination of response time at 0.8 of the nominal response resistance
  • determination of conditions of operation and return in blocking mode
  • verification of device immunity to false alarms
  • checking the function of self-control of device elements
• acceptance tests , to which each device should be subjected after manufacturing, the volume and norms of acceptance tests are determined by the technical conditions for a particular device
• typical ones are produced only when the design of the device, its circuit diagram, etc. are changed. The type testing program provides for the study of those quantities that may change as a result of changes
• periodic tests are carried out during operation, the volume and standards of tests are regulated by the relevant technical specifications for a particular type of device, and periodic tests should be carried out at least once every 2 years.

The most common in domestic practice are the RUZTU of the AZUR brands (unified mine protection apparatus): AZUR 1, AZUR 2, AZUR 3, AZUR4, AZUR 4PP, AZUR 4MK (microcontroller). Previously, RUZTU of the RUV brands (developer Prof. Leibov R.M., DPI) was widely used, UAKI (automatic isolation control device, AZAK (protection and automatic compensation device) - a combination of UAKI device and capacitive compensation device (manufacturer - Prokopyevsky mine automation plant )

• GOST 31612-2012 Mining leakage current protection devices for networks up to 1200 V. General technical requirements

• PB 05-618-03. Safety regulations in coal mines.
• Ozernoy M. I. "Electrical equipment and power supply of underground mining and coal mines" M., "Nedra", 1975