Grounding Causes of Cable Lines and the Principles of Incident Handling
Our 220 kV substation is located far from the urban center in a remote area, surrounded primarily by industrial zones such as Lanshan, Hebin, and Tasha Industrial Parks. Major high-load consumers in these zones—including silicon carbide, ferroalloy, and calcium carbide plants—account for approximately 83.87% of our bureau’s total load. The substation operates at voltage levels of 220 kV, 110 kV, and 35 kV.
The 35 kV low-voltage side mainly supplies feeders to ferroalloy and silicon carbide plants. These energy-intensive factories are built close to the substation, resulting in heavy loads, short feeder lines, and severe pollution. These feeders are predominantly connected via cables, sharing a common cable trench. Therefore, any line fault poses significant risks to the substation. This paper analyzes the causes of 35 kV line faults and discusses corresponding countermeasures. In February 2010, a 220 kV substation under our bureau frequently experienced grounding faults on the 35 kV II bus and 35 kV III bus, as detailed in Table 1.
1 Analysis of Grounding Causes in Cable Lines
According to our bureau’s 2010 statistics on cable incidents, the main causes of cable line failures were as follows:
Temperature effects: At facilities such as Sanyou Chemical, high temperatures in furnace transformers and cable terminations led to insulation breakdown. This occurred in approximately 18 incidents, requiring the fabrication of 15 cable terminations.
High cable density in cable trenches: At Rongsheng Yinbei Ferroalloy Plant, manhole covers fell and damaged cables in the trench, causing short circuits and fires that affected other plants’ cables. A total of 51 cable splices were made.
Severe customer overloading: Plants such as Huanghe Ferroalloy, Pengsheng Metallurgy, Lingyun Chemical, and Rongsheng Yinbei Ferroalloy operated cables under long-term overload conditions, accelerating cable aging and increasing temperatures. Especially during hot summers, thermal stress caused insulation breakdown in cables and terminations, requiring approximately 50 cable terminations.
Mechanical damage: Excavators during construction and earthworks severed cables, causing fractures and insulation damage. A total of 25 cable terminations and splices were made.
Cable quality issues: Defects such as air bubbles in insulation or broken shielding during manufacturing led to 9 accidents, requiring 9 cable terminations and splices.
Damage during cable laying: Excessive pulling tension due to long cable runs caused scraping by sharp objects, resulting in 13 cable damage incidents.
Poor cable termination workmanship: Insufficient technical expertise and improper procedures during installation led to moisture ingress in cable insulation. A total of 16 cable splices and terminations were fabricated.
Surface discharge on cable terminations: Heavy pollution from high-energy-consuming plants led to contaminants settling on cable equipment. Dirty cable termination surfaces, combined with rain or humid weather, caused surface flashover, damaging insulation and leading to breakdowns. In such cases, 13 cable terminations were replaced.
2 Principles for Handling Cable Grounding Faults
Standard procedures exist for handling 35 kV cable grounding faults. However, in our bureau, these voltage-level lines primarily serve high-energy consumers with large individual capacities (minimum 12,500 kVA), direct supply loads, heavy loading, and high currents.
Sudden load shedding causes significant grid disturbances. Moreover, cable grounding faults are difficult to locate, and prolonged fault duration increases risks. If not addressed promptly, such faults can endanger grid safety, placing higher demands on dispatchers. Some 35 kV customers are coal mines or chemical plants—classified as critical users. Power outages for these users may lead to casualties, fires, or explosions. Therefore, customers are categorized as general or critical, with the following handling principles:
For general customers (mainly silicon carbide and ferroalloy plants), once a faulted line is identified, contact the customer immediately to disconnect the load and de-energize the faulty line promptly. For non-cooperative customers, enforce load shedding with warning measures.
For critical customers such as coal mines and chemical plants, instruct them to transfer loads to backup power sources. If no backup exists, prepare for outage before taking the faulty line out of service.
Given the strong overload capability of smelting furnaces, for substations and lines operating under long-term heavy load, if current exceeds 90% of the current transformer rating, enhance monitoring, notify customers to reduce load, and implement a three-step process: notification → warning → forced load shedding, to ensure equipment safety.
For customers with frequent cable failures, require enhanced line inspections and regular maintenance during scheduled outages, performed by qualified professional contractors to ensure reliable operation.
Strict quality control from the source: For dedicated-line customers, require submission of all relevant documentation to the dispatch center and signing of a "Dispatch Agreement" before commissioning. Customers without a signed agreement or with incomplete/inadequate documentation shall not be connected to the grid.
For cable trenches with excessive and dense cabling, recommend limiting cable numbers to prevent fault propagation and minimize incident escalation.
3 Conclusion
Safe grid operation requires not only careful dispatching and dedication but also proficient use of legal tools to protect both personnel and equipment. Especially when dealing with power customers, the "Dispatch Agreement" must be fully utilized to regulate customer behavior, ensure proper operation, and prevent disputes. It is essential to understand customer line characteristics, load profiles, capacities, and usage patterns in daily operations, enabling swift, accurate, and decisive fault response, and ensuring the safe and stable operation of the power grid.
