ENANTIOSELECTIVE ACYL TRANSFER REACTIONS FOR THE KINETIC RESOLUTION OF ALCOHOLS, AMINES AND AMIDES

2.5.1 Streamer-controlled breakdown

The design stresses used in GIS are low enough (<50 per cent E_crit)that streamer inception will not occur even at the full rated impulse level. However, scratches or other small defects on the inner electrode surface may result in streamer formation.

streamer breakdown

leader breakdown

corona-stabilised (AC or DC) breakdown

minimum impulse breakdown level

streamer onset

gas pressure, p

p₁ p_c

voltage,V

Figure 2.6 Idealised V–p characteristics for minimum impulse (direct leader) break-down and corona-stabilised (AC or DC) breakbreak-down in a point-plane gap in SF₆[7]

For a small scratch or protrusion (h ≈ 100 μm), the field perturbation is very localised and the geometric field has to be quite high in order to achieve streamer onset.

Such defects would probably be detected only under impulse-voltage test conditions at levels close to the BIL.

At such high voltages, the conditions for breakdown (whether by the leader process or by a direct streamer channel mechanism) are automatically satisfied at onset. The breakdown voltage of the GIS may therefore be predicted on the basis of the streamer criterion:

exp  _x

0

(AE(x)− Bp) dx 

= Nc

provided that E(x), the spatial distribution of the perturbed field, is known.

As discussed earlier, the statistics of discharge initiation can play an important role in determining the probability of breakdown in perturbed quasi-homogeneous fields and this has significant implications for insulation coordination on SF6-insulated equipment. Various models have been developed for calculating surge breakdown probability on the basis of the negative-ion density distribution and the evolution of the critically-stressed volume with time during the surge [10, 11].

2.5.2 Leader breakdown

For large defects, such as needle-like particles of several mm length attached to the high voltage conductor, the onset voltage for streamer corona will be low.

This means that the onset voltage is lower than the leader propagation voltage and breakdown is preceded by corona. This is the situation discussed in the section on non-uniform fields, where either corona-stabilised or direct leader breakdown may occur, depending on the voltage waveform.

2.5.3 Particle-initiated breakdown

Free conducting particles (FCPs) are the most common cause of failure in GIS, and long, thin particles are most dangerous because of the strong field enhancement associated with such defects.

If FCPs are present in a coaxial system they become charged by the applied field and, at a relatively low voltage, lift-off will occur. If the particle is rod shaped, it will stand up on the outer conductor and corona onset will occur. For DC stress, parti-cles will cross the gap at the onset voltage. For AC conditions the partiparti-cles initially make small hopping excursions at the outer electrode. As the stress is increased, the excursions become longer and, because of inertial effects and the bouncing action at the electrode, the bounce interval becomes longer. At each contact with the elec-trode, a PD occurs and the interval between contacts is an important parameter in the assessment of the severity of particle-induced PD activity in GIS.

As the voltage is increased, particles may cross the gap to the upper electrode where they will receive a new charge, which depends on the polarity and magnitude of the applied voltage at the instant of contact, and will then move back towards the lower electrode. The crossing does not necessarily lead to breakdown, and a voltage increase will usually be necessary for the conditions for leader breakdown to be achieved.

In coaxial electrode systems, breakdown is most likely to occur when a particle strikes the inner conductor just as the voltage reaches a positive maximum [12].

The particle then behaves like a needle fixed to the HV conductor.

Early studies of the mechanism of AC particle-initiated breakdown in GIS led to some confusion, as the particle-triggered breakdown voltage–pressure characteristics did not show the strong peak observed with AC stress applied to a coaxial electrode system having a particle fixed to the inner conductor [13]. This led to the consider-ation of mechanisms such as density reduction in the wake of the moving particle, and the possible triggering action of the microdischarge which occurs at the instant of contact.

The actual reason for the disparity between the fixed and free particle data may be inferred from the earlier discussion on corona-stabilised and direct leader breakdown.

For the fixed particle, there is enough time under AC conditions to guarantee effective stabilisation at each voltage maximum, resulting in the typical peaked V–p character-istic. With the free particle, however, the sudden arrival of the particle, together with the step function increase in the field at the tip as a result of the small spark occur-ring on contact, means that the behaviour is more similar to that for a fixed particle subjected to a fast fronted impulse voltage. Under these conditions, the minimum breakdown voltage is that associated with direct leader breakdown, which is almost independent of pressure (see Figure 2.6).

In practice, the probability of breakdown in GIS at each contact will be a func-tion of the particle charge, velocity and orientafunc-tion, the phase and magnitude of the applied voltage, the statistics of breakdown of the particle–electrode microgap and the probability of formation of a stabilisation corona. It may therefore be necessary to wait for a relatively long period (20–30 s) to ensure that a test particle will not trigger breakdown at a given voltage.

Measurements made in a 125/250 mm coaxial system with fixed and free particles showed good agreement between the (t = 20 s) AC breakdown voltage with free particles and the minimum breakdown voltage with 1μs rise time impulses applied to a particle fixed to the inner conductor [14]. As the minimum fixed point impulse breakdown level corresponds to breakdown by a leader mechanism, it is clear that models of leader inception and propagation can be used to predict the conditions of particulate contamination which will result in breakdown in GIS.

Present indications are that, for normal working stresses, particles of length less than∼4 mm should not be able to cause breakdown [9]. However, smaller particles may be scattered onto the surfaces of insulating barriers or spacers, where they may cause breakdown under subsequent impulse stresses. It is important, therefore, to ensure that free conducting particles of significant size (≥1 mm) are not present in GIS under working conditions.