Unsang Mga Kasagaran Ang MVDC Grounding Makapaduli Sa Sistema?

Pagsulay ug Pag-handle sa DC System Grounding Faults sa Substations

Kon mag-occur ang DC system grounding fault, mahimong ikategoryahan kini isip single-point grounding, multi-point grounding, loop grounding, o reduced insulation. Ang single-point grounding gikahibaloan usab isip positive-pole ug negative-pole grounding. Ang positive-pole grounding mahimong mag-resulta sa misoperation sa protection ug automatic devices, samantalang ang negative-pole grounding mahimong mag-lead sa failure to operate (e.g., relay protection o tripping devices). Kon may ground fault na, makabuo kini og bag-ong ground path; kinahanglan nga maputli niini agad. Kon dili, kon mag-occur ang second o daghan pa ka ground, mahimong mag-lead kini sa serious faults o accidents.

Sa normal operation, ang insulation resistance sa positive ug negative poles sa DC system sa ground mao ang 999 kΩ. Apan, kon madampi ang outdoor equipment, makababa ang DC system’s insulation resistance. Ang alarm threshold sa 220V DC system tipikal nga 25 kΩ, ug 15 kΩ sa 110V system. Ang State Grid Hubei Maintenance Company naghahatag og importansya sa grounding hidden dangers ug gibataas ang alarm standard: ang warning ma-trigger kon mobaba ang insulation ngadto sa 40 kΩ sa 220V systems ug 25 kΩ sa 110V systems. Kini naghatag og oportunidad aron maputli ang mga potensyal nga problema bago matapos ang insulation degradation ngadto sa full ground fault.

Bag-o, tungod sa prolonged severe weather ug extended plum rain season uban sa mataas nga humidity, ang anim ka 500 kV substations sa probinsya adunay varying degrees of reduced DC insulation o direct grounding:

• Enshi ug Anfu: mobaba ang insulation ngadto sa 40 kΩ

• Shuanghe: positive-pole grounding

• Jiangxia: positive-pole grounding

• Junshan: overall insulation reduction

• Xian Nv Shan: mobaba ang insulation, negative-to-ground sa 18 kΩ

• Xinglong: positive-pole grounding

Case Analysis of Recent DC System Insulation Issues:

(1) 500 kV Enshi & Anfu Substations:
Ang DC insulation monitoring devices nagpakita og mobaba ang insulation ngadto sa 40 kΩ. Pasinati, mobalik ang insulation ngadto sa acceptable range. Batasan sa past experience, ang probable cause mao ang moisture ingress sa thermal relay sa outdoor disconnect switch mechanism enclosures.

(2) 500 kV Jiangxia Substation:
Pasunod sa DC ground fault, ang secondary maintenance personnel nagsusi sa insulation monitor ug wala nabati og abnormal signals. Ang field voltage measurements nagpakita og 0 V sa positive pole to ground. Gamiton ang DC grounding detector, natrace ang fault sa moisture-affected contact sa density relay sa #2 bus tie control cabinet. Pasinati ang pag-remove sa faulty contact, mobalik ang DC system insulation ngadto sa normal.

Challenges in DC Grounding Troubleshooting:
Ang locating ug handling sa DC grounding defects mao ang challenging. Ang mga fault kasagaran mobalik pinaagi sa changes sa weather, ug difficult ang pag-identify sa fault points. Makapugos usab ang multi-point grounding. Ang pinakarecente nga grounding issues gikan sa reduced insulation sa outdoor equipment contacts o cables. Ang contributing factors mao ang aging components uban sa degraded insulation ug prolonged rain nga nag-cause og moisture ingress o equipment failure.

Enhancing DC Grounding Response Capability:
Effective handling requires coordinated efforts, standardized procedures, and integration of operation and maintenance (O&M) systems:

• Safety Procedures:
  Bago han-ayo sa DC ground fault, clear all personnel from relevant areas, especially those working on secondary circuits. At least two personnel must be present during fault location and repair. Prevent accidental DC short circuits or additional grounding. Implement safety measures to avoid protection misoperations.

• Fault Location Strategy:
  Follow the principles: microprocessor-based detection first, then manual; external before internal; secondary before primary; signals before control. First, use the DC insulation monitoring device to locate the fault. If data is inaccurate, proceed with manual inspection.

• Rapid Response Protocol:
  O&M staff must immediately collect alarm messages and abnormal signals from the insulation monitor. Secondary teams should quickly organize emergency repairs. If the monitor accurately identifies the faulty circuit, disconnect its power and observe if insulation recovers. If not, use a DC grounding detector to scan all DC circuits, identify suspect circuits, and test by power disconnection.

• Precise Fault Isolation:
  Once the faulty circuit is identified, use schematics to pinpoint potential grounding points. Test by disconnecting suspected terminals. After confirmation, apply reliable insulation isolation. Coordinate closely with primary equipment teams to eliminate the fault promptly.

Preventive Measures to Reduce DC Grounding Incidents:

• Improve operating environments. Install air conditioning in areas with inadequate temperature control. Seal terminal boxes, switch mechanism enclosures, and disconnect switch enclosures properly. Ensure cabinet doors are rainproof.

• During regular inspections or transformer installations, thoroughly check whether gas relays, oil flow relays, oil level gauges, thermometers, and pressure relief devices have proper rain shields. Verify secure wiring box installation, presence of sealing gaskets, and that secondary cables are properly routed and undamaged.

• Use scheduled outages to replace vulnerable outdoor secondary components that are frequently operated or continuously energized.

• Eliminate design flaws or poor workmanship. Ensure secondary circuits are complete during commissioning—avoid parasitic circuits, loops, or crossovers. Pay attention to cleaning and dust removal during protection and automatic device inspections.

• For technical upgrades or new construction, strictly follow design drawings. Conduct thorough pre-construction drawing reviews. Prevent DC I/II segment mixing, AC/DC mixing, and parasitic circuits that could cause DC system anomalies.

• Strengthen operation, maintenance, and inspection of DC systems, DC distribution panels, and insulation monitoring devices across all substations. Ensure monitoring devices accurately reflect grounding locations, enabling rapid isolation by maintenance personnel.