Particle movement pattern in a 1-φ GIB under abnormal conditions

Power system comprises of various over voltages which are transient in nature namely lightning and switching phenomenon under abnormal conditions which cause flashover in Gas Insulated Bus-duct (GIB) if not protected properly. So, there is a necessity for GIS to have withstanding capability of lightning and switching. Otherwise they may cause shut down of the system. In this work, 1050 kV Lightning Impulse and 750 kV Switching Impulse is superimposed on normal frequency voltage class of 100 kV, 132 kV and 145kV and are given to 1-ΦGIB for finding peak radial movement for Al and Cu particles. The movement patterns for both lightning and switching over voltages are compared. All the simulation results are shown in this paper.


Introduction
Today, Gas Insulated Substations (GIS) are widely used because of their excellent aesthetic and technical merits to Air Insulated Substation (AIS). GIS is more reliable and safer than AIS in many aspects. One of the important parameters affecting the system of GIS is the contamination of the metallic particle [1]. They may approximate any shape. Over voltages which are transient in nature are two types namely lightning and switching phenomenon. Switching phenomenon is internal to the system. Lightning phenomenon is external to the system and depends on climatic conditions. The gas that is widely used in GIS is the compressed SF6gas as it possesses wonderful dielectric strength and properties of arc quenching.In this paper, particleradial peak movement was found in1-Φ GIB for both lightning and switching phenomenon superimposed on normal frequency voltages and are compared.Lightning and switching over voltages are shown as a double exponential wave shape [12][13][14][15][16][17][18][19].

Mathematical modelling
Here, particle is assumed to be wire like in nature and its position is assumed to be on theouter enclosure surface, and the particle may move upwards from rest provided it possess lift-off field and at the same time, acquires charge in the field presence The motion of the particle equation finally is shown as [9]:

Result analysis
All the results are tabulated in Table 1 and Peak radial particle movement was found for particle length of 12mm, radius of 0.3mm are obtained with for voltage class 100KV, 132KV and 145KV.With an advanced C language program, simulation is done [7]. It is observed from the table and graphs that as the voltage level increases both for lightning and switching impulse, particles of Al show greater movement in the radial direction compared to Cu particles due to lighter mass [6]. Also, with the comparison of lightning and switching over voltages as depicted in table 1, the particle movement is higher for lightning impulse as against switching impulse over voltages. This is due to the magnitude high for lightning impulse of 1050 kV , 1.2/50µsec. and this can cause severe flashover and the system get shutdown.
This problem can be eliminated with coating of dielectric of suitable thickness of the inner surface of the outer enclosure of the GIB[4-5]. Also, with this, the system integrity will improve , the performance of the dielectric will be good and the breakdown risk will be reduced. Figures 2 to 9 show the Peak radial movements of lightning and switching impulse over voltages superimposed for both particles of Al &Cu.

Conclusion
Particle trajectory was found using a Mathematical model along with the peak movement in the direction of radial in nature. An advanced C program language is used for simulation. It can be inferred that especially particles of Al are more affected and influenced with increase of voltage levels have higher movement than Cu particles with lightning impulse than switching impulse. Due to lighter in weight, it is also observed that particles of Al are highly effected by the voltage than copper particles. The results of simulation is done for 100KV,132KV, 145KV, and are analysed and presented.