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1.5MW dynamic resistor
1.5MW dynamic resistor
Figure 1: Comparison between current control method and new resistor control method (click to enlarge)
Figure 1: Comparison between current control method and new resistor control method (click to enlarge)

Dynamic Resistive Frequency Control

Hydro Tasmania completed construction and commissioning of a substantial upgrade to the power station control system in 2010. The new control system employs a large resistive load which can be varied rapidly in order to absorb excess wind generation rather than spill it through shutting down or throttling back wind turbines. Instead of reducing output, the wind turbines are allowed to produce as much power as possible, with the excess generation to be absorbed by the resistor. As the resistor can be adjusted rapidly, this effectively converts spilled wind into "spinning" reserve that can be used to supplement diesel generation. Maintaining the power balance between generation and demand in this way allows the resistor to maintain system frequency.

Shifting system reserve requirements away from the diesels enables the control system to reduce diesel output to minimum levels, reducing fuel use. Whilst this can occur only during periods of wind spill, a further increase in renewable energy generation will increase the amount of time that the system experiences spill, and thus increase the time that diesels can be run at minimum loading. This will in turn reduce the amount of overall energy contribution from the diesel generators.

Figure 1 shows the effect of the new control method on diesel output and wind utilisation over a few hours. Note that under the new control method the diesel output remains fairly steady around the minimum output of 480kW, while under the old methodology the diesel generation would have increased due to fluctuations in wind farm output. Over a year these savings equate to a substantial amount of diesel fuel saved. The wind spill component of the chart is treated as system reserve.

The resistor frequency control enables the running diesel generators to remain on minimum loading whenever the island’s load is less than the combined solar and wind output. Modelling of the King Island energy demand compared with historic wind data and expected solar output indicates that this will occur for about half of the year – resulting in significant diesel fuel and thus GHG emission savings.