1.1 the concept of GIS
Sulfur hexafluoride enclosed combined electrical appliances, internationally known as "gas insulated switchgear" (Gas lnsulated Switchgear) referred to as GIS, it will be a substation in addition to the transformer other than the primary equipment, including circuit breaker, isolating switch, grounding switch, voltage transformer, current transformer, lightning arrester, bus, cable terminal, in and out of the line bushing, etc., optimized design organically combined into a whole.
Characteristics of 1.2 GIS
1 miniaturization: due to the use of insulation performance of sulfur hexafluoride gas insulation and arc extinguishing medium, so can greatly reduce the volume of substation, to achieve miniaturization.
2 high reliability: because all the charged parts are sealed in inert SF6 gas, the reliability is greatly improved. In addition, it has excellent seismic resistance.
3 Good safety: the live part is sealed in the grounded metal shell, so there is no danger of electric shock. SF6 gas is a non-combustible gas, so there is no fire hazard.
4 put an end to the adverse effects on the outside: because the live part is closed by the metal shell, the electromagnetic and electrostatic shielding is realized, the noise is small, and the anti-radio interference ability is strong.
5 short installation cycle: due to miniaturization, the whole machine can be assembled and tested in the factory and delivered to the site in the form of units or intervals, thus shortening the installation period on site and improving reliability.
6 Convenient maintenance and long maintenance cycle: Because of its reasonable structure and layout, 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 due to miniaturization, low ground, daily maintenance is convenient.
1.3 Significance of GIS Partial Discharge Detection
-With the development of urban power grid construction, the number of GIS substations is increasing;
-The internal space of GIS is extremely limited, the working field strength is very high, and the insulation margin is relatively small;
-Once insulation defects occur in GIS, it is easy to cause equipment failure, resulting in long power outages and high maintenance costs;
-There have been several serious GIS accidents 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 failures can be detected in advance;
-Monitoring partial discharge signals in the handover test of GIS and regular monitoring of GIS in operation are effective means to ensure safe operation.
Definition of 2.1 Partial Discharge
Partial discharge: Under the action of an electric field, only part of the insulation between conductors is broken down (there is no electrical discharge between the conductors across the applied voltage).
Features: Partial discharge is a kind of pulse discharge, which will produce a series of physical phenomena and chemical changes such as light, sound, electrical and mechanical vibration in and around the power equipment. This provides a detection signal for monitoring the internal insulation state of the power equipment.
Although these discharges are generally small in magnitude, they can lead to progressive degradation and eventual breakdown, and therefore the presence of these discharges needs to be detected by non-destructive testing.
Causes of 2.2 Partial Discharge
Reason: it is due to local electric field distortion, local field intensity concentration, resulting in the insulation medium in the local range of gas discharge or breakdown caused. It may occur on the edge of the conductor, or on the surface or inside of the insulator.
Several Typical Defects in GIS
Due to the existence of needle-like protrusions in the insulator or on the high-voltage conductor, the electric field is uneven, the dielectric is uneven, there are bubbles and magazines and other reasons, so that the electric field strength of the local area of the insulator reaches the breakdown field strength.
