1.1 the concept of GIS

Sulfur hexafluoride enclosed combined electrical appliances, internationally known as "Gas lnsulated Switchgear" (GIS) for short, will be a transformer in a substation in addition to the primary equipment, including circuit breakers, disconnectors, grounding switches, voltage transformers, current transformers, arresters, buses, cable terminals. Through optimum design, the intake and outlet bushing can be organically combined into a whole.

1.2 the characteristics of GIS

Miniaturization: Because of the use of sulfur hexafluoride gas with excellent insulation performance as insulation and arc extinguishing medium, the volume of substation can be greatly reduced to achieve miniaturization.

2. High reliability: Because the live part is all sealed in inert SF6 gas, the reliability is greatly improved. In addition, it has excellent seismic performance.

3. Good safety: The live part is sealed in the grounded metal shell, so there is no electric shock danger. SF6 gas is non-combustible gas, so there is no fire risk.

4. Absolute external adverse effects: Because the charged part is enclosed by metal shell, shielding electromagnetic and static electricity, low noise, strong anti-radio interference ability.

5. Short installation period: Because of miniaturization, it can be assembled and tested in the factory, and then delivered to the site in the form of units or intervals. Therefore, it can shorten the construction period of site installation and improve the reliability.

6. Easy maintenance and long maintenance cycle: Because of its reasonable structure layout and advanced arc extinguishing system, the service life of the product is greatly improved, so the maintenance cycle is long, the maintenance workload is small, and because of the miniaturization, the ground is low, so the daily maintenance is convenient.

1.3 Significance of Partial Discharge Detection in GIS

With the development of urban power grid construction, the number of GIS substations is increasing.

- The internal space of GIS is very limited, the workfield strength is very high, and the insulation margin is relatively small.

Once insulation defects occur in GIS, it is very easy to cause equipment failure, resulting in a long blackout time and high maintenance costs.

- Several serious GIS accidents have occurred in China, and the view that GIS equipment is maintenance-free has not been accepted in the past.

GIGRE survey shows that more than 50% of GIS faults can be found in advance.

Monitoring partial discharge signal in GIS handover test and regular monitoring of GIS in operation are effective means to ensure safe operation.

2.1 Definition of Partial Discharge

Partial Discharge: Under the action of electric field, only part of the insulation between conductors is broken down (not between conductors with applied voltage).

Characteristic: Partial discharge is a kind of pulse discharge, which can produce a series of physical phenomena and chemical changes such as light, sound, electrical and mechanical vibration in the internal and surrounding space of power equipment. This provides a detection signal for monitoring the internal insulation status of power equipment.

Although the magnitude of these discharges is usually small, they can lead to gradual deterioration and eventual breakdown. Therefore, the existence of these discharges needs to be detected through non-destructive testing.

2.2 Causes of Partial Discharge

Cause: It is caused by partial electric field distortion and local electric field concentration, which leads to gas discharge or breakdown in partial range of insulating medium. It may occur at the edge of the conductor, or at the surface or inside of the insulator.

Several Typical Defects in GIS

Because of the pinpoint protrusion in the insulator or on the high voltage conductor, the electric field is not uniform, the dielectric is not uniform, there are bubbles and magazines and other reasons, the electric field strength in the local area of the insulator reaches the breakdown field strength.