Laboratory - Large Corona Cage: Set-up and method
4.6 TEST PROCEDURES
For each conductor bundle the following procedures were followed:
4.6.1. Dry Conditions
The tests required that the air about the conductors and within the corona cage cylinder remained dryand relatively free of moisture. No flames were required for this stage.
The voltage on the conductor bundle was raised to a point above the normal operating voltage and gradually decreased in steps of gradient (kV/cm). The gradient was then held at specific gradients to record both quasi-peak and time domain data. The voltage was therefore raised to a point where at least some corona activity was visible and from there decreased until no corona was visible.
A reduction in potential rather than an increase in potential ensures that the cut-off of the corona current is accurately measured since the cut-off (which is the threshold for the onset of corona current) will always occur at the same potential [s].
For each step, the induced corona noise propagated along the line was measured after a short period allowing the surface gradient on the conductor surface to "settle". Inmost cases the quasi- peak levels of that noise was recorded.
4.6.2. Rain Conditions
The acceptable rain rate required for tests of this nature was 2 mm/min which was classified as
"heavy rain". No flames were required for this stage.
Again the voltage on the conductor bundle was first increased to a point where corona was clearly visible and then decreased from the higher gradient. This was done with the water sprayers above the corona cage providing the heavy rain conditions. The surface gradient was again reduced in steps with rain falling and paused at the required gradients. After a "settling"
period in which the droplets on the conductor were in a steady state, measurements were taken.
[s] Seep75 [18]
The test was stopped once no further corona was visible or the level of activity had dropped significantly.
4.6.3. Fire Conditions
Three sources of fire were considered:
(1) Gas fire (a fire source with no particles), (2) grass fire and (3) sugar cane fire.
The gradient (in kV/cm) on the conductor bundle under test, was increased from a low gradient up to gradients where either:
(1) flashover occurred, (2) very loud audible corona was generated or (3) until the voltage started deteriorating due to the limited power transformer supply capacity.
The increase in gradient was done in controlled steps in accordance with the observed activity. In contrast to dry air and rain conditions, flames do not present a uniform presence around the conductors within the corona cage. As such, the accuracy of the extinguishing gradient cannot be guaranteed and hence the benefits associated with reducing the gradient do not apply for the fire tests. I n addition, t he maximum gradient before flashover0 ccurs cannot beg uaranteed a s the flames do not provide a singular plasma which remains stable. The transformers used to supply the power tot he corona cage were both sensitive and expensive tor eplace - therefore it was preferable to avoid as many flashovers as possible. The decision was therefore to increase gradient rather than decrease gradient.
The gas fire was produced with a balloon burner requmng low pressure gas. The burner temperature was controlled to a lower temperature to prevent the conductors under test from melting. In the Eskom transmission network very little annealing is seen and as such no excessive temperatures were used in the simulation. In this manner a reasonably repeatable procedure was possible.
The challenge in the procedure was to ensure that the flame and resultant heat generated, remained near the conductors. Winds at very low velocities were capable of causing a deviation of the flame path from the conductors. The gas flames were nearly transparent in bright sunlight conditions where the sunshine matched the light frequencies emanating from the flames. The flames had to be described in both width and depth in0 rder to guarantee that0 bserved high
The starting gradient was always considerably lower than would normally be experienced on the operational transmission lines. For each step the existing induced corona noise propagating along the line was measured and in most cases the quasi-peak 1evels0 f that noise w as recorded as accurately as possible. A "settling" time with the fire tests were inappropriate as the flames were rarely steady.
The balloon burner had an activation switch which allowed the operator to remotely initiate a fire. The burner was then operated several times at each surface gradient selected. That is, the chamber was energised to the required gradient and only then was a flame without particles applied to the chamber. A large increase in the audible corona was considered as an early warning of imminent electrical breakdown in this test.
Sugar cane was also provided. The sugar cane fire tests in the cage were managed rather than controlled. The sugar cane was assembled vertically within metal scaffolding below the wire mesh cylinder in order not to interfere with the electrical characteristics of the applied gradients about the conductor bundles under test. The fuel was set on fire and personnel moved to safety before the line was then energised. At this point the flames start to build up and visual observations were matched to high frequency electrical activity observed on the monitoring equipment.
This process enabled t he author to a Iso monitor the influences0 f smoke a nd particles on the generated corona levels. No "settling" time was possible with these tests and the quasi-peak levels were not recorded as no steady state conditions were possible.