Non-uniform field breakdown in SF 6

Highly divergent fields can exist in GIS under certain conditions as, for example, when a needle-like free metallic particle is attracted to the inner conductor or is deposited on the surface of an insulator. Such defects can result in very low breakdown levels and, with large defects (e.g. particles several mm long), failure can occur even at the working stress of the equipment. For this reason, there have been many laboratory studies of the breakdown characteristics of highly non-uniform field gaps in SF₆.

These studies have shown that there are two distinct types of breakdown, depend-ing on the rate at which the voltage is applied to the gap. When the stress is applied relatively slowly, as with alternating voltage or long rise time switching surges, corona space charge plays an important part in controlling the field distribution by

200

V(kV) 100

1 2

pressure (bar)

3 4

breakdown

onset

r₀= 5 mm

3 mm

2 mm 1.5 mm

Figure 2.4 AC corona onset and breakdown characteristics for a 40 mm rod–plane gap in SF₆[4]

the so-called corona stabilisation process [3]. With shorter rise time surges (light-ning impulse or fast transients), breakdown occurs directly by a stepped leader mechanism [5]. For both cases, the breakdown voltage is lower when the high field electrode is positive and most attention has therefore been given to breakdown under positive point conditions.

2.4.1 Corona stabilised breakdown

Figure 2.4 shows AC voltage–pressure characteristics for point-plane gaps in SF₆[4].

The shape of these curves is typical of all non-uniform field gaps with slowly varying voltage applied, in that there is (a) a broad pressure region over which the breakdown voltage is much higher than the (streamer corona) onset voltage and (b) a critical pressure at which breakdown occurs directly at onset (i.e. the first streamer leads directly to breakdown).

The peak in the mid-pressure range is due to the effects of space charge injected by streamer activity around the point.

For a positive point, for example, the electrons generated by the corona are quickly removed at the point while the positive ions diffuse relatively slowly into the low field region. This space charge tends to shield the point and stabilises the field there to a level close to the onset value.

As the voltage is raised, the space charge density (and the shielding effect) inten-sifies, and a voltage considerably above onset is required to cause breakdown. The breakdown usually occurs as a result of filamentary (leader) discharges developing around the shielding space charge, so that spark channels in the stabilisation region typically take a very irregular, curved path.

As the pressure is increased, the individual streamers become more intense, the corona region becomes confined to a smaller region at the tip of the point and the stabil-isation becomes less effective, so that the breakdown voltage is reduced. Eventually, the shielding effect is lost, and the streamer which forms at onset is able to initi-ate a discharge which develops completely across the gap at the onset voltage. This discharge has been shown to be identical to the stepped leader discharge which is found to occur in non-uniform field gaps under fast pulse conditions.

2.4.2 Leader breakdown

With fast-fronted surges, where the voltage passes rapidly through the theoretical streamer onset level, the initial streamers can be very intense and may lead to the formation of a highly ionised leader channel before there is time for space charge stabilisation of the field at the tip of the protrusion [5].

In addition to the rate-of-rise of voltage, the statistics of appearance of initiatory electrons may play an important role. For negative-point conditions, electrons are produced by field emission; with the positive point, however, the trigger electrons result from detachment from negative ions in the vicinity of the point [6].

Before the stress is applied, the gas contains a negative-ion population of a few thousand ions per cm³. (These are produced by the action of cosmic rays, which typically generate∼10 ion pairs per cm³per second in gases at atmospheric pressure.) For discharge initiation to occur, it is necessary to find one of these ions in the very small critical volume where (α > η). This critical volume is vanishingly small at the theoretical onset level and increases with voltage.

For a fast-fronted wave, the field may therefore be well above the minimum onset level when inception occurs so that the streamer corona is more vigorous than would be the case for AC or DC stress.

If the streamer corona is large enough, a stepped leader discharge may be initiated. The mechanism of the stepped leader may be summarised, with reference to Figure 2.5, as follows.

During the dark period a–b which follows the initial corona, charge separation in the streamer filaments generates a succession of ionising waves which build-up their conductivity. Eventually, one of the streamer filaments is transformed into a highly conducting leader channel step; this behaves essentially as an extension to the point electrode and a new corona burst immediately occurs at its tip b. The range of this second corona determines the length of the second channel step c. During each streamer’s dark period, there are regular reilluminations of the leader channel, probably associated with the relaxation processes which are occurring in the streamer filaments.

The leader propagates into the gap in steps typically of a few mm until the streamer activity is too weak for further channel steps to form. If the voltage is high enough, the leader can cross the gap, resulting in breakdown. As the interstep interval is

∼100 ns for p ≈ 3 bar, the breakdown formative time lag can be greater than 1 μs.

As the field along the leader channel is much lower than that in the streamer filament, breakdown can occur by the stepped leader process at much lower voltages

time

a b c d

Figure 2.5 Schematic of leader development

than would be required for streamer breakdown. For point-plane gaps, the minimum leader breakdown voltage is found to be almost independent of pressure and average breakdown fields of∼25 kV/cm are typical of short (20–50 mm) gaps [7].

In configurations where the background field in the low field region is falling less steeply (as, e.g., for the case of a particle fixed to the inner conductor of a GIS), leader breakdown can occur at average fields of only∼15 kV/cm.

Figure 2.6 shows V –p characteristics in SF6for corona-stabilised breakdown and for the minimum breakdown voltage under impulse conditions. As p₁is typically only about 0.5 bar, the minimum breakdown voltage in non-uniform fields at pressures typical of GIS is determined by the conditions for leader propagation in the absence of preexisting corona space charge. Models have been developed [8, 9] which allow the conditions for direct leader inception and propagation to be predicted for a wide range of geometries.

It must be emphasised that, under surge conditions, the leader propagation field is the minimum level at which breakdown can occur. Depending on the statistics of initiation of the primary streamer, there is a probability of corona stabilisation occurring so that, even for lightning impulse, the 50 per cent probability voltage–

pressure characteristic will exhibit a stabilisation peak. It is important therefore to determine the low probability breakdown level when carrying out surge breakdown tests in inhomogeneous fields.