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Analysis of Internal Insulation Failures in GIS Equipment and Insulation Testing Methods

Oliver Watts
Field: Inspection and testing
China
High-Voltage Conductor Spikes
During the installation of high-voltage conductors, accidental bumps or scratches can cause metallic spikes on the conductor surface, as shown in Figure 1. Under power-frequency voltage, the ionization effect of high electric fields at the spike tips generates charged particles, which may suppress partial discharge (PD) or breakdown. However, under impulse voltage, the ionization process induced by the strong electric field does not have sufficient time to develop, making PD and breakdown more likely to occur.
Contaminants on Insulator Surface
During GIS assembly, on-site cleaning is often insufficient, allowing dust to enter the GIS and deposit on insulator surfaces. In some cases, poor manufacturing processes leave gummy residues on insulators. These defects frequently cause breakdowns during on-site withstand voltage tests. The energy released during breakdown typically removes the contaminants, making it difficult to find any traces on the insulator surface or other components during post-breakdown disassembly analysis. Figure 2 shows an insulator that experienced on-site breakdown, with no visible anomalies on its surface.
Loose Metal Components
During transportation or operation, mechanical vibrations can cause shielding covers, other metal components, and fastening screws to loosen. Poor electrical contact in such cases leads to partial discharges (PD), which over time can escalate into breakdown accidents. Figure 3 illustrates the installation structure of a shielding cover prone to such issues.
Metal Powders Inside Enclosure
During transportation or operation, mechanical vibrations can cause friction between metal components, generating metal powders. Inadequate on-site hygiene during installation may leave dust or metal particles on the inner surface of the enclosure. Additionally, partial discharges due to poor electrical contact can produce metal or metal compound particles. Figure 3 shows powders generated by discharge from poor contact in a shielding cover. During operation, the jumping of metal powders can lead to breakdown accidents.
GIS Insulation Defect Testing Methods
Withstand Voltage Test
Withstand voltage tests are required during handover and after major overhauls. DL/T 555-2004 Guidelines for On-Site Withstand Voltage and Insulation Tests of Gas-Insulated Metal-Enclosed Switchgear specifies the requirements and methods for on-site tests [4]. Alternating voltage is sensitive to free conductive particles and other impurities, making it suitable for detecting defects such as contaminants on insulator surfaces, loose metal components, and metal powders inside the enclosure. Impulse voltage, effective for identifying contaminants and abnormal electric field structures, is ideal for detecting metal spikes and internal metal powders.
Partial Discharge (PD) Test
During on-site withstand voltage tests, PD measurement should be conducted simultaneously. The pulse current method is currently the primary approach for measuring PD signals under power-frequency test voltage. However, this method often fails to detect defects like metal spikes and internal metal powders. Thus, PD measurement during impulse withstand voltage tests is necessary. To avoid interference in the test circuit under impulse voltage, high-frequency, ultra-high-frequency (UHF), or ultrasonic detection methods can be used.
Live PD Detection and On-line Monitoring
For defects such as loose metal components and metal powders generated during operation, live PD detection and on-line monitoring should be actively implemented. Depending on sensor principles, live detection methods include UHF and ultrasonic techniques. Live detection is suitable for periodic inspections, while on-line monitoring is ideal for tracking known defects.
Conclusions and Outlook
GIS internal insulation defects primarily include four types: high-voltage conductor spikes, insulator surface contaminants, loose metal components, and internal metal powders. To prevent these defects from escalating into failures, insulation tests and PD detection should be conducted during handover and operation. For common defects like metal spikes and powders during handover tests, PD detection under impulse voltage should be prioritized.
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About experts
Oliver Watts
China
Expertise area
Inspection and testing
Professional article
66
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