I. Underground Power Supply Network:
The underground power supply system consists of a surface substation, an underground central substation, a mining area substation, an explosion-proof mobile substation, a transformer station, and the cables connecting them.
The surface substation (6KV/10KV) is the central hub for power supply in the entire mine, responsible for power receiving, transformation, and distribution. The underground pump house central substation is the core underground substation, responsible for supplying power to the mining area substation, rectifier substation, main drainage pump, and loads near the bottom of the mine. The mining area substation is the center for transformation and distribution in the mining area, responsible for supplying power to the mining area loads and roadway excavation loads. The industrial face distribution point is the power distribution center for the mining face and nearby roadways, supplying power to the working face and nearby loads.
Figure 1 Mine power supply process
II. Classification of Underground Power Supply Systems
1. Neutral point directly grounded system
This system is also known as a high grounding current system. In such a system, when one phase is grounded, another grounding point appears besides the neutral point, forming a short-circuit loop. The current in the grounded phase is very large, and to prevent equipment damage, the power supply must be cut off quickly, resulting in low power supply reliability and a high risk of power outages. However, when a single-phase grounding fault occurs in this system, the clamping effect of the neutral point prevents a significant increase in the voltage to ground of the non-faulty phases, which is beneficial to the system insulation.
Figure 2 Neutral point directly grounded system
2. Neutral point not directly grounded system
This is a unique power system in my country, with three main operating methods: ungrounded, grounded via an arc suppression coil, and directly grounded. The neutral point of the power system can operate in various ways, including ungrounded, grounded via a resistor, grounded via an arc suppression coil, or directly grounded. In a three-phase system with an ungrounded neutral point, when one phase experiences a ground fault: firstly, the voltage to ground of the two ungrounded phases increases to √3 times, which is equal to the line voltage. Therefore, in this type of system, the insulation level to ground should be designed based on the line voltage.
This is mainly because it has the following advantages: First, under normal power supply conditions, the phase line voltage to ground can be kept constant, so that two different voltages, 220V and 380V, can be provided to the outside (to the load) to meet the different power needs of single-phase 220V (such as light bulbs and electric heaters) and three-phase 380V (such as motors).
Figure 3 shows that the neutral point is not directly grounded.
In mine operation systems, my country mainly adopts neutral point non-grounded systems. This system has a large zero-sequence impedance value, which is generally higher than other impedance values. Therefore, when leakage or short circuit faults occur underground, the generated current is small and is unlikely to cause significant damage.
III. Hazards of electrical leakage in underground mines
gas explosion
Methane is a flammable gas. Its explosive potential is limited by a concentration range, approximately 5% to 16%.
When the gas concentration is below 5%, it will not explode when exposed to fire, but it can form a combustion layer around the flame. When the gas concentration is 9.5 %, its explosive power is the greatest. When the gas concentration is above 16%, it loses its explosiveness, but it will still burn when exposed to fire in the air.
However, its explosion limit is not constant; it is also affected by factors such as temperature, pressure, and the mixing of coal dust, other combustible gases, and inert gases.
When the insulation of mine cables is damaged, leakage current will be generated. Since the energy required to ignite methane is very low, if the electric arc generated by the leakage current breaks down the air, it will ignite it.
Figure 4 Gas explosion accident
Detonator detonation
Detonators are a primary initiating material in blasting engineering. Their function is to generate initiation energy to detonate various explosives, detonating cords, and detonation tubes. They are divided into two types: fire detonators and electric detonators. Electric detonators are used in underground coal mines. Electric detonators are further divided into instantaneous electric detonators and delayed electric detonators. Delayed electric detonators are further divided into second-delay electric detonators and millisecond-delay electric detonators.
Electric detonators are widely used in coal mining. A short circuit can detonate an electric detonator, leading to a safety accident. If an electric detonator detonates, it means there is an extremely high current, exceeding the safe current for the human body. If this current flows through the human body, it can have a significant impact on life safety.
Figure 5 Electric detonator control box
IV. Selectivity of Residual Current Protection
When a leakage fault occurs, the selective protection of the residual current device (RCD) is mainly reflected in two aspects:
Longitudinal selectivity
This refers to the leakage current protection system detecting and disconnecting a faulty circuit in the underground mine, while simultaneously allowing normal circuits to continue operating. Its working principle is shown in Figure 2. Leakage protection devices are installed in A, B1, B2, C1, C2, C3, and C4. A, B1, and B2 are feeder switches, C1, C2, C3, and C4 are magnetic starters, and K1 and K2 are leakage fault points. If the leakage fault occurs at point K1, the selective leakage current protector in component C4 will react, disconnecting the leakage point from the overall circuit. The selective leakage current protector in feeder switch B2 will not activate; thus, the faulty section is disconnected, but the normal section continues to operate normally.
Figure 6. Principle of Selective Protection
Horizontal selection
This refers to a situation where only the branch where a leakage current fault occurs is disconnected by the leakage current protection system, while other normal branches continue to operate normally, as shown in Figure 2, where leakage current protection devices are located at points A, B, and C. When a leakage current fault occurs at a point such as K1, the selective leakage current protection device in the magnetic starter C4 or the branch feeder switch B2 of the leakage current protection system reacts to the leakage current fault, disconnecting the branch where the leakage current fault occurred at point K1, while the other devices do not react. Under current technology, the aforementioned longitudinal selectivity relies on a time delay, that is, a step-by-step delay from the load end to the power supply end for each leakage current protection device. When a fault occurs at one location, nearby leakage current protection devices can react quickly. When the load end is closer to the leakage current fault, the leakage current protection device closer to the load end reacts promptly and disconnects the faulty section. Due to the unique time delay of this device, after the faulty line is disconnected, it returns to its initial position, thus achieving the purpose of protection. Leakage current faults can be solved using selective leakage current protection theory, which will greatly improve downhole production efficiency.
V. Leakage Protection Scheme
Leakage current protector
GB3836-3, "Electrical Apparatus for Explosive Gas Atmospheres (Increased Safety Type)," specifies that in TT or TN systems in mines...
It is recommended to use a residual current device (RCD) with a rated residual operating current not exceeding 300mA. It is particularly recommended to prioritize an RCD with a rated residual operating current of 30mA. This RCD should have a maximum disconnect time of no more than 5 seconds at residual operating current, and no more than 0.15 seconds at 5 times the rated residual operating current.
Additional DC power protection system
The additional DC power supply Uz flows into the three-phase power grid through the three-phase reactor SK, forming a DC path through the insulation resistance to ground. The three-phase reactors are connected in a star configuration, forming an artificial neutral point. The DC current Iz first flows into the ground through the positive terminal of the additional DC power supply, passing through the insulation resistances ra, rb, and rc of the ground. It then enters the three-phase power grid, flowing through the three-phase reactor SK, the zero-sequence reactor LK, and a kiloohm/milliammeter, finally returning to the negative terminal of the additional power supply through a DC relay, forming a closed circuit.
Figure 7. Structure of the additional DC power supply method
Intelligent leakage protection system
Selective residual current protection (RCD) systems are a type of low-voltage power grid protection system. This system incorporates the advantages of other RCD systems, organically combining the safety of bypass grounding, the characteristics of online insulation resistance monitoring, the comprehensiveness of DC monitoring, and the selectivity of zero-sequence current direction to form an effective RCD protection system. The system consists of a feeder switch with reclosing capability, a magnetic starter, DC-detection RCD protection, interlocking, and time-delay modules. Regardless of the location or nature of the RCD fault or electric shock accident, the DC-detection RCD at the main switch de-energizes the main switch. All switches and magnetic starters are de-energized due to voltage loss. After approximately 0.5 seconds, the RCD interlocking is activated, selectively locating the leakage point and interlocking the switches or magnetic starters of the faulty branch. Afterward, the main switch closes and supplies power to the uninterlocked circuits.
Figure 8 System Structure Diagram
VI. Conclusion
Coal remains the primary energy source, and coal mining is inherently dangerous. Therefore, selecting the appropriate leakage protection device is crucial for ensuring worker safety and normal production. As coal mining extends deeper, higher standards are being set for electrical safety. Various causes of leakage must be tested and studied, while also incorporating optimization principles of the power supply system. Currently, addressing leakage issues in mines is an urgent task.
