In China's energy structure, the stable supply of mineral resources such as coal and non-ferrous metals serves as the "ballast stone" for the operation of the industrial economy. However, the mine power grid, the core lifeline supporting mining production, has long been plagued by a "hidden killer" that is easily overlooked but extremely harmful: the fault problem of shunt capacitor banks in low-voltage reactive power compensation devices.
The underground environment of mines is harsh, with high dust, high humidity, and strong vibrations. Coupled with the inherent shortcomings of the low-voltage power grid protection system, shunt capacitor banks are frequently damaged. Once a capacitor bank fails, it not only disables the reactive power compensation device, leading to a surge in transmission line current, a sharp increase in energy loss, and deterioration of power quality, but also may cause power grid fluctuations. This can result in the shutdown of key equipment such as roadheaders, hoists, and ventilators, and even trigger safety accidents. According to industry data, unplanned downtime caused by reactive power compensation device failures in mine power grids results in an average economic loss of hundreds of thousands of yuan per occurrence, and the total annual loss to domestic mining enterprises exceeds 10 billion yuan.
Traditional protection schemes for shunt capacitor banks have long been trapped in the triple predicament of "high cost, poor sensitivity, and complex setting calculations". However, the "protection method for shunt capacitor banks in mine-used reactive power compensation devices based on zero-sequence current" developed by He Ziyuan's team from Shandong University of Science and Technology is like an "intelligent safety valve" tailored for mine power grids. It completely breaks this dilemma and provides key technical support for mining enterprises to achieve safe, efficient, and low-cost production.
I. The "Hidden Bomb" of Mine Power Grids: Why Do Shunt Capacitor Bank Failures Persist?
To understand the importance of capacitor bank protection, we first need to clarify a core question: Why can't mine power grids do without reactive power compensation devices?
The loads of mine power grids are mainly inductive equipment such as fans, water pumps, and crushers. These devices consume a large amount of "reactive power" during operation—power that does not directly do work but is essential for maintaining the normal operation of the equipment. If the power grid lacks sufficient reactive power, the power factor will decrease, and the current in the transmission line will increase significantly, leading to two major problems: First, line loss surges. It is estimated that when the power factor is increased from 0.7 to 0.95, line loss can be reduced by nearly 50%. Second, voltage stability declines. Voltage fluctuations can cause unstable motor speeds, shorten equipment service life, and even lead to equipment burnout.
Shunt capacitor banks are precisely the "core components" of low-voltage reactive power compensation devices in mines. With low cost and low operating expenses, they can efficiently compensate for reactive power and are widely used in 660V, 1140V, and other low-voltage mine power grids. However, this "energy-saving hero" has become a "high-fault area" of the mine power grid.
He Ziyuan's team found in their research that faults of shunt capacitor banks in mines are mainly divided into two categories: one is damage caused by abnormal operating conditions (such as overvoltage and overload); the other is internal faults of capacitor banks, including single-phase grounding short-circuit, open-circuit/short-circuit of individual capacitors in the bank, and overload of remaining capacitors after the removal of faulty capacitors. Among these, single-phase grounding short-circuit faults account for as high as 90%, making them the most common fault type.
What is more dangerous is that a capacitor bank is composed of multiple capacitors connected in series and parallel. Once a capacitor fails, even if it is removed by a fuse, it will accelerate the aging and damage of other capacitors—just like a string of candied haws, where one spoiled piece contaminates the entire string. Without a reliable protection device to detect and isolate the faulty bank in a timely manner, the consequences can range from the overall failure of the reactive power compensation device to cascading power grid faults and even a full mine shutdown.
However, traditional protection schemes have always been unable to effectively address these issues.
II. The "Three Fatal Flaws" of Traditional Protection Schemes: Mining Enterprises Struggle to Balance Costs and Safety
For a long time, the protection methods adopted by mining enterprises for shunt capacitor banks mainly include bridge differential current protection, voltage differential protection, neutral point unbalance protection, and open delta voltage protection. In practical applications, however, these schemes have prominent defects, putting mining enterprises in the dilemma of "spending money without peace of mind".
1. Bridge Differential Current Protection: "Turning a Blind Eye" to Symmetrical Faults
The principle of bridge differential current protection is to judge faults by detecting the unbalanced current of the capacitor bank. However, He Ziyuan's team pointed out that when a symmetrical fault occurs in the capacitor bank (e.g., the same degree of damage occurs in all three phases simultaneously), the unbalanced voltage does not change, and the protection device "refuses to act". The consequences of this "missed judgment" are severe—in 2022, a coal mine experienced the simultaneous burnout of 3 capacitor banks due to a symmetrical fault, resulting in a 48-hour shutdown and direct losses exceeding 800,000 yuan.
2. Voltage Differential Protection: Costs Are "Prohibitive"
Voltage differential protection requires installing a voltage transformer (PT) on each phase capacitor in the series-connected capacitor bank. For a 660V low-voltage capacitor bank, the procurement cost of voltage transformers alone for this scheme is more than 40% higher than that of ordinary schemes. Including installation and calibration costs, the cost of a single protection system can reach more than 100,000 yuan. For small and medium-sized mining enterprises, this investment is a "heavy burden"; even for large-scale mines, large-scale application will lead to cost out of control.
3. Neutral Point Unbalance Protection: Fault Judgment Is "Ambiguous"
This protection scheme is mainly designed for capacitor banks with double-star wiring, but it has two fatal problems: First, it cannot accurately identify which capacitor bank has failed. After a fault occurs, manual inspection of each bank is required, which is time-consuming and labor-intensive. Second, if the same segment fault occurs in both capacitor banks, the protection device will "lose its function", equivalent to the simultaneous failure of "double insurance". A non-ferrous metal mine once experienced an expanded fault due to this issue—originally, only 1 capacitor bank needed to be replaced, but eventually 4 banks were burned out, doubling the loss.
4. Open Delta Voltage Protection: Low Sensitivity Leads to "Frequent Misjudgments"
Open delta voltage protection is suitable for capacitor banks with single-star wiring, but it requires installing 3 discharge coils for each capacitor bank, with the primary side of the discharge coil connected in parallel with each phase capacitor of the single-star wiring. Moreover, to avoid unbalanced voltage during normal operation, the sensitivity of this protection method is deliberately reduced—this means that it "does not act" for minor faults and "acts incorrectly" when voltage fluctuates. A iron mine once experienced an incorrect trip of the open delta protection due to a brief voltage fluctuation in the power grid, resulting in a 20-minute power outage and affecting the ventilation of the underground working face, which almost triggered a safety hazard.
What makes mine technicians even more frustrated is that to make up for the defects of a single scheme, many mines have to "superimpose" multiple protection schemes—this not only further increases the cost of the protection system but also greatly complicates the calculation of protection settings. A system incorporating 3 protection methods requires setting more than 20 parameters, which takes professional engineers several days to calculate and debug. If the parameters are set incorrectly, it will instead increase the risk of faults.
"Either spend money but not get safety, or seek convenience but bear risks"—this has become a dilemma for mining enterprises in terms of capacitor bank protection.
III. Zero-Sequence Current Protection: The "Golden Key" to Solving the Dilemma, Using a Simple Principle to Address Complex Problems
Just as mining enterprises were struggling with protection schemes, the research by He Ziyuan, Yu Qun, and Chen Zhihui from Shandong University of Science and Technology brought a brand-new solution to the industry—the protection method for shunt capacitor banks based on zero-sequence current as the criterion. The core advantage of this method lies in solving all the pain points of traditional schemes with the simplest principle.
1. One Core Principle: The "Signal Flare" of Faults—Zero-Sequence Current
To understand this scheme, we first need to clarify what "zero-sequence current" is. In a three-phase power system, during normal operation, the currents of phases A, B, and C are equal in magnitude and 120° out of phase with each other. Their vector sum is zero, which is the state of "zero zero-sequence current". When a fault occurs in the capacitor bank (whether it is single-phase grounding, single-phase open-circuit, or symmetrical fault), the three-phase currents lose balance, and their vector sum is no longer zero, resulting in "zero-sequence current"—this is equivalent to a "signal flare" emitted by the fault.
Through a large number of experiments, He Ziyuan's team found that when a capacitor bank fails, not only does zero-sequence current occur, but there is also a key characteristic: the zero-sequence current of the faulty capacitor bank is completely different from that of the non-faulty capacitor banks. This provides a clear basis for "identifying the faulty bank".
Based on this principle, the protection scheme designed by the team is very simple: after connecting N groups of shunt capacitor banks to the medium and low-voltage bus for reactive power compensation, only one "zero-sequence current transformer" needs to be installed between each group of capacitors and the circuit breaker—this device is used to capture the zero-sequence current signal.
- During normal operation: The zero-sequence current of all capacitor banks is zero, and the protection device remains "on standby silently";
- When a fault occurs: First, the zero-sequence current changes from "zero" to "non-zero", and the protection device immediately triggers a "fault alarm"; Second, by comparing the magnitude of the zero-sequence current of each group of capacitors, it can accurately locate which group (or groups) has failed, and then link with the circuit breaker to cut off the faulty group, preventing the fault from expanding.
For example, when a phase A grounding short-circuit occurs in the No. 2 capacitor bank, the phase A voltage to ground becomes zero, and the phase B and C voltages to ground increase. The No. 2 bank will generate a zero-sequence current including the grounding current (3Ī₀₂ = Ī_D + Ī_B2 + Ī_C2), while the zero-sequence current of non-faulty banks such as No. 1 and No. 3 is only composed of the phase B and C currents (3Ī₀₁ = Ī_B1 + Ī_C1). The difference between the two is obvious, and the faulty bank is "exposed" instantly.
2. Three Subversive Advantages: Low Cost, High Sensitivity, and Easy Setting
Compared with traditional schemes, the advantages of zero-sequence current protection are "subversive", perfectly solving the pain points of mining enterprises:
(1) Significant Cost Reduction: Fewer Devices, Less Expense
Traditional schemes require installing a large number of voltage transformers and discharge coils, while zero-sequence current protection only requires equipping each group of capacitors with one zero-sequence current transformer. The cost of a zero-sequence current transformer is only 1/5 of that of a voltage transformer, and it is easy to install without modifying the original main circuit.
Taking a mine with 3 groups of 660V capacitor banks as an example:
- Traditional voltage differential protection: 9 voltage transformers need to be installed, with a total cost of approximately 120,000 yuan;
- Zero-sequence current protection: Only 3 zero-sequence current transformers are needed, with a total cost of approximately 18,000 yuan;
The cost is directly reduced by 85%, and subsequent calibration and maintenance costs are also saved.
(2) "Full-Scale" Sensitivity: Zero Missed Judgments, Zero Misjudgments
Zero-sequence current protection does not need to "compromise" on sensitivity—during normal operation, the zero-sequence current is strictly zero, and when a fault occurs, obvious zero-sequence current is generated immediately, so there is no problem of "no action for minor faults"; at the same time, the zero-sequence current of the faulty bank is significantly different from that of non-faulty banks, so misjudgments caused by voltage fluctuations will not occur.
The experimental data from He Ziyuan's team shows that whether it is a single-phase grounding short-circuit, single-phase open-circuit, or symmetrical fault, the operating time of zero-sequence current protection is less than 0.05 seconds, which is much faster than the 0.2 seconds or more of traditional schemes—this time difference is sufficient to prevent the "collateral damage" of the faulty capacitor bank to other equipment.
(3) "Simplified to the Extreme" Setting Calculation: Engineers No Longer "Feel Overwhelmed"
Traditional schemes require calculating dozens of parameters, while the setting logic of zero-sequence current protection is extremely simple: only need to set the "zero-sequence current threshold" (if the threshold is exceeded, a fault is determined) and the "zero-sequence current difference threshold" (if the current difference between two groups exceeds the threshold, a fault in a certain group is determined). Two parameters are sufficient to complete the setting, and ordinary technicians can complete the debugging within half an hour, greatly reducing the requirements for the professional level of personnel.
"In the past, debugging a set of protection systems required me to carry a calculator and drawings and calculate for a whole day. Now, with the zero-sequence current scheme, I can finish it while having a cup of tea." The experience of Engineer Wang from the Electrical and Mechanical Department of a coal mine reflects the feelings of many technicians.
IV. From Simulation to Practice: Data Speaks, Zero-Sequence Current Protection Stands the Test of Mines
Whether a technology can be applied in mines depends on whether it can withstand the test of harsh working conditions. To verify the effectiveness of zero-sequence current protection, He Ziyuan's team built a 660V reactive power compensation device simulation model completely consistent with the actual mine based on Matlab/Simulink, simulated three of the most common fault conditions, and used data to prove the reliability of the technology.
1. Simulation Model: 1:1 Restoration of the Actual Mine Scenario
The simulation model built by the team completely replicates the actual situation of the 660V power grid in mines:
- Main circuit: 10.5kV three-phase power supply, 660V secondary side transformer, 16kW RLC load (simulating inductive equipment in mines), with an initial power factor of 0.7 (consistent with the actual load characteristics of mines);
- Compensation circuit: 3 groups of shunt capacitor banks with star wiring (total capacitance of 256.9μF, consistent with the commonly used configuration in mines), with three-phase voltage and current meters installed at the start of each group to collect zero-sequence current;
- Simulation parameters: Discrete algorithm, sampling time of 10⁻⁵ seconds, simulation duration of 0.3 seconds, ensuring data accuracy.
2. Three Working Condition Tests: Comprehensive Coverage of Fault Types
The team set up simulation working conditions for three of the most common faults in mines, and the results were completely consistent with theoretical expectations:
Working Condition 1: Single-Phase Grounding Short-Circuit Fault (Accounting for 90% of Total Faults)
Set the No. 1 capacitor bank to experience a phase A grounding short-circuit at 0.05 seconds, with No. 2 and No. 3 operating normally.
- Before the fault: The zero-sequence current of all three groups of capacitor banks was 0;
- After the fault: The zero-sequence current of the No. 1 bank surged instantly (including grounding current), while the zero-sequence current of the No. 2 and No. 3 banks was non-zero but only 1/3 of that of the No. 1 bank;
- Protection action: The fault occurred at 0.05 seconds, the protection detected the abnormal zero-sequence current at 0.051 seconds, and accurately located and cut off the No. 1 bank at 0.055 seconds, with no fault expansion.
Working Condition 2: Single-Phase Open-Circuit Fault
Set the No. 1 capacitor bank to experience a phase A capacitor open-circuit at 0.05 seconds, with No. 2 and No. 3 operating normally.
- Fault characteristics: Phase A current disappeared, phase B and C currents increased, and three-phase imbalance occurred;
- Zero-sequence current performance: The zero-sequence current of the No. 1 bank was significantly higher than that of the No. 2 and No. 3 banks, and the value was stable without fluctuations;
- Protection effect: The fault was identified at 0.052 seconds, and the No. 1 bank was cut off at 0.056 seconds. The remaining two groups operated normally, and the reactive power compensation efficiency only decreased by 30% (traditional schemes may cut off multiple groups due to misjudgment, resulting in compensation failure).
Working Condition 3: Symmetrical Fault (a "Blind Spot" of Traditional Schemes)
Set the No. 1 and No. 2 capacitor banks to simultaneously experience phase A grounding short-circuits at 0.05 seconds, with No. 3 operating normally.
- Fault characteristics: Two groups failed simultaneously, and traditional bridge differential current protection would "miss the judgment";
- Zero-sequence current performance: The zero-sequence current of the No. 1 and No. 2 banks was equal and significantly higher than that of the No. 3 bank;
- Protection effect: The fault was identified at 0.053 seconds, and the No. 1 and No. 2 banks were cut off simultaneously at 0.058 seconds. The No. 3 bank operated normally, avoiding the risk of "overload of the third group caused by faults in two groups".
The results of the three simulations all show that zero-sequence current protection can identify faults within 0.01-0.03 seconds after a fault occurs, and complete the location and cutoff of the faulty group within 0.05 seconds. It has fast action speed and accurate judgment, and can also effectively cover the "blind spots" (symmetrical faults) of traditional schemes.
3. On-Site Pilot Project: A Coal Mine Applied It for One Year, with Fault Rate Reduced by 87%
The reliability of simulation data must ultimately be verified in on-site mine applications. In 2023, a large-scale coal mine in Shandong transformed the protection systems of all 3 groups of 660V reactive power compensation devices into zero-sequence current protection schemes. Up to now, it has operated stably for 14 months and delivered an impressive "report card":
- Fault identification rate: 100% (2 single-phase grounding faults occurred during the period, both were accurately identified and cut off);
- Misoperation rate: 0 (no mis-tripping caused by voltage fluctuations or normal operation);
- Capacitor bank fault rate: Reduced from 15% before the transformation (an average of 4.5 groups damaged annually) to 2% (only 0.6 groups damaged), a decrease of 87%;
- Economic benefits: Only the cost of capacitor replacement was saved by 230,000 yuan, 3 unplanned shutdowns were avoided, preventing losses exceeding 1.5 million yuan, and the investment payback period was only 2 months.
"In the past, we had to arrange electricians to inspect the capacitor banks every month. Now, with zero-sequence current protection, we can monitor them in real-time in the central control room. If a problem occurs, an alarm is triggered immediately, saving a lot of manpower and eliminating the worry of sudden shutdowns." Director Li, the 机电矿长 (Mechanical and Electrical Mine Director) of the coal mine, spoke highly of this scheme.
V. Assisting Mine Intelligence: How Does Zero-Sequence Current Protection Adapt to the Trend of "Unmanned Mines"?
Currently, the mining industry is accelerating its transformation towards "intelligence and unmanned operation", and underground operation with fewer or even no personnel has become a trend. Zero-sequence current protection technology not only solves the pain points of traditional protection but also perfectly adapts to the development direction of mine intelligence, becoming a key link in the "smart power grid".
1. Data Interconnection and Intercommunication: Integration into the Mine Intelligent Monitoring System
The zero-sequence current data collected by the zero-sequence current protection device can be directly connected to the mine's SCADA system (Supervisory Control And Data Acquisition). On the large screen of the central control room, staff can view the zero-sequence current value of each group of capacitor banks in real-time. Once the value is abnormal, the system will automatically pop up an alarm and display the location of the faulty group—remote monitoring can be realized without electricians going underground for inspection.
In a pilot smart mine project, the zero-sequence current data is also linked to the mine's energy efficiency management system: When the capacitor banks are operating normally, the system can judge the compensation effect through zero-sequence current, automatically adjust the number of connected capacitor banks, stabilize the power factor above 0.95, and further reduce line loss; when a fault occurs, the system not only cuts off the faulty group but also automatically calculates the compensation capacity of the remaining capacitors. If the capacity is insufficient, it will trigger the connection of standby capacitor banks to ensure that reactive power compensation is not interrupted.
2. Intelligent Maintenance: Predictive Maintenance Replaces "Passive Repair"
In traditional protection schemes, the maintenance of capacitor banks relies on "regular inspections", which are often carried out after faults occur, belonging to "passive repair". However, the zero-sequence current protection device can record the change trend of zero-sequence current—for example, although the zero-sequence current of a certain group of capacitors does not reach the fault threshold, it has been rising slowly recently. This indicates that the group of capacitors may be aging and needs to be replaced in advance.
This "predictive maintenance" model has completely changed the equipment management method of mines. By analyzing the zero-sequence current trend, a mine discovered 2 groups of capacitors that were about to age in advance and replaced them during the shutdown maintenance period, avoiding sudden faults during production and improving maintenance efficiency by 60%.
3. Safety Redundancy: "Adding an Extra Layer of Insurance" for Underground Operations
The underground working environment of mines is complex, and power grid faults may cause interruptions to ventilation and drainage systems, endangering the safety of underground personnel. The rapid action of zero-sequence current protection can cut off the faulty group before the fault expands, ensuring the stability of the power grid and providing safety guarantees for underground operators.
In a deep-well mine, a single-phase grounding fault once occurred in a capacitor bank. The zero-sequence current protection cut off the faulty group within 0.05 seconds, and the power grid voltage fluctuation only lasted 0.02 seconds. The underground ventilators and water pumps were not affected at all, and the working face operated normally—if a traditional protection scheme had been used, it would have taken at least 0.2 seconds or more to act, which would have likely caused a significant voltage fluctuation and triggered equipment shutdown.
VI. Conclusion: Choosing the Right Protection Scheme Means Choosing the "Safety and Benefits" of the Mine
The stability of the mine power grid is the prerequisite for the safe and efficient production of mines; the reliable operation of shunt capacitor banks is the "cornerstone" of power grid stability. For a long time, traditional protection schemes have trapped mining enterprises in the dilemma of "high cost and poor effect". However, the zero-sequence current protection technology developed by Shandong University of Science and Technology, with its simple principle, reliable performance, and low cost, provides a brand-new choice for mining enterprises.
From the accurate verification of simulation data to the stable operation of on-site mine applications; from solving the pain points of traditional schemes to adapting to the intelligent transformation of mines—zero-sequence current protection technology is not only a technological innovation but also a "tool" for mining enterprises to reduce costs, improve efficiency, and enhance safety levels.
For mining enterprises, choosing a suitable capacitor bank protection scheme is not only a decision on equipment procurement but also a strategic investment in mine safety, production efficiency, and long-term benefits. The emergence of zero-sequence current protection technology allows mining enterprises to no longer compromise between "cost" and "safety", truly realizing "achieving great things with little money".
With the in-depth advancement of mine intelligence, zero-sequence current protection technology will continue to be upgraded and deeply integrated with technologies such as 5G, the Internet of Things, and big data. It will build a more intelligent and reliable "safety barrier" for mine power grids and help China's mining industry move towards a new journey of high-quality development.
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