Production total December = 2093.05 kWHrs Operational hours December = 72.75 Hrs Total production for 2014 = 20887.5 kWHrs
At the beginning of December the machine a number of circuit breakers where trip during the turbine speeding up phase of the start-up sequence. An inspection of the spinning rectifier inside the generator revealed that a number of electrical bridges had been formed across the diodes. In addition some of the isolation plastic material had melted andburned due to electrical arcs forming across the busbar at the end of the generator. The damage is shown in figure 1.
Figure 1 (top left) Inspecting carbon bridges across pinning rectifier diodes. (top right) Moulton plastic from pole isolation. (bottom left) location of arcs which formed between busbars. (bottom right) burned pole isolation material.
The significant damage that was found left no other option than to completely remove the generator in order to gain sufficient access to make the repairs. This is a difficult ad length process but after the generator is uncoupled from the turbine shaft it can be rotated through 90 degrees using the internal crane which gives adequate access to the spinning rectifier components for removal and repair. The damaged diodes where removed and replaced from stock. The burnet pole isolation sheathes where extracted but because it was not possible to buy replacements parts they had to be rebuilt by hand. This was achieved by cutting out all burnet material and then layering up epoxy soaked fiberglass until the voids where filled. The tubular shape of the sheathes were then restored using a lathe. This was not the first instance of generator short circuits (due to carbon bridges) but previous failures had been less severe. To avoid further repetition of the issue all elements of the spinning rectifier were painted with a special low conductivity paint. In this way the carbon dust could build up but would not be able to make contact between the generator poles. Also, an automated dust extraction system is being investigated. The generator removal and repair of the rectifier are shown in figure 2.
Figure 2 (top) lifting the uncoupled generator to gain access to the service hatch. (bottom) repair and isolation (painting) of the spinning rectifier components.
After a week hard labour the generator was closed up and re-installed successfully and operation was resumed.