Superconducting cables, as a new generation of power transmission technology, have become a powerful tool for solving power supply bottlenecks in urban core areas and promoting green transformation of the power grid due to their zero resistance, low loss, and large capacity characteristics. However, its operation relies on extreme low temperature environments (about -196 ℃) and precision control systems, involving multiple technical challenges such as low temperature maintenance, quench protection, and mechanical adaptation. Below, we will elaborate on the key points and practical experience of superconducting cable operation from three dimensions: how to stabilize the core issues and response practices, how to standardize the operation process, and how to repair typical problems and solutions, combined with actual cases.
1, Core issues and practical solutions for the operation of superconducting cables
(1) Low temperature environment maintenance: the stability of liquid nitrogen system is the "lifeline" of operation
Superconducting materials require a liquid nitrogen environment (-196 ℃) to exhibit zero resistance characteristics, therefore maintaining a low temperature environment is the primary task. The core challenges lie in the control of heat leakage in the liquid nitrogen circulation system (environmental heat intrusion can cause liquid nitrogen vaporization, disrupting low-temperature conditions), efficient operation of the refrigeration unit (requiring continuous replenishment of cooling capacity), and dynamic balance of system pressure and flow rate.
Dealing with practice:
1. Multi layer insulation design: The cable body is wrapped in a double-layer flexible vacuum insulation tube to reduce external heat intrusion (such as the insulation tube design of the Shanghai 35kV demonstration project, which has only 1/10 of the heat loss of traditional cables);
2. Multi machine parallel refrigeration system: Multiple refrigeration units are configured to run in parallel, and the number of units to be turned on is dynamically adjusted according to the cooling capacity requirements (the Shenzhen 10 kV project uses domestically produced large cooling capacity GM refrigeration units to solve the problem of efficient heat exchange in small spaces);
3. Real time monitoring and redundant backup: Temperature, pressure, and flow sensors are deployed at key nodes of cable entrances, exits, and refrigeration units (9 working wells are set up in Shanghai, each equipped with liquid nitrogen monitoring equipment). Once abnormalities are detected (such as temperature exceeding ± 2 ℃), the backup refrigeration unit is immediately started to ensure stable low-temperature environment.
(2) Overvoltage protection: a technological leap from "passive power-off" to "active self recovery"
Overheating (the phenomenon of superconducting materials suddenly restoring resistance due to temperature, current, or magnetic field exceeding critical values) is the most serious operational failure of superconducting cables, which may lead to local overheating, insulation damage, and even equipment burnout. Traditional protection methods rely on rapid power outages, but can lead to power outages and affect user experience.
Dealing with practice:
1. Multi parameter fusion monitoring: Real time collection of cable temperature, current, and voltage data through fiber optic temperature measurement, current sensors, and voltage transformers (the Shenzhen project deployed fiber optic vibration measurement devices along the 400 meter cable line to achieve millimeter level temperature sensing);
2. Intelligent quench protection device: Developed an integrated device of "quench trip self recovery". When a sudden increase in resistance (such as exceeding 0.1m Ω) is detected, the device cuts off the fault current within 10 milliseconds and rapidly cools down through the refrigeration system, allowing the superconducting material to enter the superconducting state again (Shanghai Engineering's protection device has achieved self recovery after 3 quench cycles without affecting user power supply);
3. Electromagnetic ring network design: Construct redundant power supply paths on the grid side, and maintain power supply through ring network switching during power outages (the Shenzhen project is connected to the dual power ring network in Futian Central District, and the load transfer rate during power outages reaches 100%).
(3) Mechanical Performance Adaptation: The 'Flexibility Challenge' in Installation and Operation
Superconducting cables consist of multiple layers such as superconducting tapes (only 0.4 millimeters thick), buffer layers, and protective layers, and their mechanical strength is much lower than traditional copper cables. Excessive traction force, small bending radius, or vibration during installation may cause strip breakage or interlayer delamination.
Dealing with practice:
1. Customized laying process: Determine key parameters through 1:1 simulation experiments (such as Shanghai Engineering reproducing the complex environment of the central urban area in Wujing Town, Minhang District, measuring the maximum allowable traction force of the superconducting cable to be 8kN and the minimum bending radius to be 1.5 meters);
2. Specialized laying equipment: research and development of small angle and large drop laying equipment (such as the Shenzhen project using the "mud water balance top pipe" and "large angle bypass" processes to solve the problem of narrow underground pipe galleries in old urban areas);
3. Dynamic stress monitoring: Real time monitoring of cable tension during the laying process (fiber Bragg grating sensors are used in the Shenzhen project, and automatic alarms are triggered when the tension deviation exceeds ± 5%), and vibration monitoring through intelligent ground studs during operation (vibration sensors are installed in all 9 working wells of the Shanghai project, and shock absorption measures are activated when the vibration frequency exceeds 10Hz).
(4) Insulation and Thermal Management: A Dual Test of "Low Temperature+High Voltage"
Superconducting cables operate in a liquid nitrogen environment (-196 ℃) and must withstand voltages of 35kV or even higher. The insulation material must have both low-temperature toughness and high-voltage resistance. In addition, cable terminals (interfaces connected to the conventional power grid) may experience local high temperatures due to heat leakage, which can affect insulation performance.
Dealing with practice:
1. Composite insulation design: using a composite insulation structure of solid insulation materials (such as epoxy resin) and liquid nitrogen (the insulation layer thickness of Shanghai 35kV cables is only 20mm, and the corona resistance is twice that of traditional cables);
2. Terminal insulation optimization: The terminal adopts a vacuum multi-layer insulation structure (the terminal heat leakage rate of the Shenzhen project is less than 0.5W/m, which is 30% lower than the international standard), and low-temperature glue is filled at the interface to prevent insulation gaps caused by liquid nitrogen vaporization;
3. Regular insulation testing: Use a megohmmeter to measure the main insulation resistance every quarter (with a requirement of ≥ 1000M Ω), and conduct annual dielectric loss testing (the three-phase dielectric loss factor of Shanghai Engineering is all<0.5%, far below the warning value of 1%).
2, Standardized operation process of superconducting cables
The operation of superconducting cables must strictly follow the four stage process of "pre cooling test grid connection operation and maintenance", and key parameters must be recorded at each step to ensure traceability.
(1) Pre cooling stage: gradual cooling from room temperature to -196 ℃
Pre cooling is a critical step in starting operation, and it is necessary to avoid thermal stress damage caused by rapid cooling (such as superconducting tape breakage or joint detachment). The specific process is as follows:
1. System evacuation: Use a vacuum pump to evacuate the internal pipeline of the cable to a vacuum degree of 1 × 10 ⁻ ³ Pa, remove impurities (such as moisture and air), and prevent pipeline blockage at low temperatures;
2. Nitrogen blowing: Slowly blow the pipeline with room temperature nitrogen (flow rate ≤ 5m ³/h) to further remove residual impurities;
3. Liquid nitrogen pre cooling: Inject liquid nitrogen at a rate of 0.5 ℃/min and gradually reduce the cable temperature (the pre cooling time for Shanghai project is 48 hours, and the final temperature stabilizes at -196 ℃± 2 ℃).
(2) Flow test: a practical exercise to verify the rated current carrying capacity
After pre cooling is completed, the cable's current carrying capacity needs to be verified through a current carrying test. The experiment adopts the "current superposition method":
1. Three phase short circuit at the end of the cable, connect a voltage regulator at the beginning, and gradually increase the current (starting from 10% of the rated current, increasing by 10% every 30 minutes);
2. Monitor the voltage and current phases of each phase (with a required phase difference of ≤ 5 °), as well as the temperature (with a liquid nitrogen outlet temperature of ≤ -190 ° C);
When the current reaches the rated value (such as the rated current of 2160A for a 35kV cable in Shanghai) and stabilizes for 24 hours, the test is qualified.
(3) Grid connected operation: 24/7 guarantee of "online monitoring+intelligent operation and maintenance"
After grid connection, the following parameters need to be monitored in real-time through an online monitoring platform:
1. Liquid nitrogen system: inlet pressure (0.3-0.5MPa), outlet temperature (-196 ℃± 2 ℃), flow rate (10-15L/min);
2. Electrical parameters: current (≤ rated value), voltage (± 5% rated voltage), dielectric loss (≤ 1%);
3. Environmental parameters: working well temperature and humidity (temperature ≤ 30 ℃, humidity ≤ 70%), vibration (≤ 5Hz).
The operation and maintenance team adopts a "three-dimensional inspection+centralized monitoring" mode: daily manual inspection of the work well (checking whether the insulation pipe is frosted and whether the refrigeration machine is running abnormally), weekly analysis of online monitoring data (if the liquid nitrogen flow fluctuates by more than ± 10%, pipeline blockage needs to be checked), and monthly infrared temperature measurement (terminal temperature ≤ -180 ℃ is normal).
(4) Regular maintenance: preventive maintenance of "status assessment+component replacement"
Comprehensive maintenance is required every year of operation:
1. Insulation performance evaluation: Measure the main insulation resistance (≥ 1000M Ω) and dielectric loss factor (≤ 0.5%);
2. Mechanical performance inspection: Check whether there are cracks in the superconducting tape through X-ray inspection (no damage to the tape was found during the 3-year operation of the Shanghai project);
3. Maintenance of refrigeration system: replace refrigeration oil, clean heat exchanger (the maintenance cycle of refrigeration machine in Shenzhen project is 2000 hours).
3, Possible problems and countermeasures during operation
Despite continuous technological optimization, superconducting cable operation may still experience malfunctions due to environmental changes, equipment aging, or operational errors, and targeted response strategies need to be developed.
(1) Problem 1: Abnormal increase in liquid nitrogen temperature (such as outlet temperature>-190 ℃)
Reasons: Heat leakage from the insulation tube (such as damage to the vacuum layer), refrigeration machine failure (such as compressor wear), and blockage of the liquid nitrogen pump (accumulation of impurities).
answer:
1. Immediately inspect the appearance of the insulation pipe (frost areas may be leakage points), use a vacuum gauge to measure the vacuum degree of the insulation layer (<1 × 10 ⁻ ² Pa is normal), and if the leakage point is small, seal it with low-temperature glue; If the leakage point is large, replace the insulation pipe;
2. Switch to the backup refrigeration unit (Shanghai project is equipped with 2 main refrigeration units and 1 backup unit, with a switching time of less than 5 minutes);
3. Turn off the liquid nitrogen pump and blow back the pipeline with nitrogen gas (pressure 0.2MPa) to remove impurities (the Shenzhen project was once blocked by copper shavings left during construction, but the pipeline was restored to normal after blowing back).
(2) Problem 2: Overload triggering (sudden increase in resistance>0.1m Ω)
Reasons: Overcurrent (such as sudden increase in user load), local overheating (poor contact of strip welding points), magnetic field interference (nearby large motors).
answer:
1. The protective device automatically trips (Shenzhen project trip time<10ms), cutting off the fault current;
2. Check the current record (if there is a sudden increase in load, contact the user to adjust the electricity plan; if there is a problem with the welding point, re weld and test the resistance);
3. Start the refrigeration unit to accelerate the cooling process (target temperature -196 ℃), and reconnect to the grid after the resistance returns to 0 (Shanghai engineering once triggered a power outage due to a sudden increase in load, which automatically restored power supply after 30 minutes).
(3) Problem 3: Cable strip breakage after laying (such as insulation resistance<100M Ω)
Reason: Excessive traction force (over 8kN), small bending radius (<1.5 meters), and high lateral pressure (>5kN/m).
answer:
1. Immediately stop laying and use optical fiber to detect the location of the fracture (accuracy ± 1 meter);
2. Cut off the broken section, replace the spare strip (with the same model as the original strip), re weld and perform insulation treatment (the Shenzhen project once caused the strip to break due to a small bending radius, and the replacement passed the test);
3. Adjust the laying parameters (such as reducing the traction speed to 0.5m/min and increasing the diameter of the bending guide wheel).
