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Chapter 9—Wind Power Plants 9–7 allows for isolation of any section of underground cable that has a fault in it. The two ends of the loop are connected to circuit breakers labeled CB2. These are connected together on the low voltage side of another stepup transformer which increases the voltage from 12.47 kV to that voltage required for transmitting power to the utility. This may be 34.5, 69, 115, 169, 230, or 345 kV, depending on the utility and the existing transmission lines near the windfarm site. There will be another circuit breaker CB3 on the high voltage side of this transformer, so the entire windfarm can be isolated from the utility if desired. Figure 2 shows three turbines connected to each unit transformer. This number may be either smaller or larger depending on detailed economic studies. The transformers are a relatively expensive part of the distribution network and are proportionally less expensive in larger sizes, so it will probably be desirable to have T 1 be rated at perhaps 1000 kVA or even larger if available. This means that 20 to 30 turbines rated at 50 kVA could be connected to one transformer. The limitation to this would be the cost, losses, and voltage drops in very long sections of 480 V cable. A good windfarm design would include a check of at least two transformer sizes and the total cost of transformers plus underground cable for each size. 4 SELECTION OF SIZES, LOW VOLTAGE EQUIPMENT Once the windfarm network is selected, sizes of transformers, circuit breakers, and conductors can be selected. These sizes will vary with the rating of the wind turbine and with the total number of turbines in the windfarm. Any rearrangement of the network of Fig. 2 will also cause sizes to be changed. The following discussion presents some of the key features of selecting component sizes, but should not be considered complete or adequate to actually prepare construction plans for a windfarm. Circuit Breakers The circuit breaker CB1 of Fig. 2 might be designed and sold as a part of the wind turbine package, in which case it would not need to be considered here. Some manufacturers do not include it as a part of the turbine, so we will discuss it briefly. There are two separate functions required here. One is the interruption of high currents during a fault and the other is the more frequent connecting and disconnecting of the generator from the utility grid in daily operation. These functions are typically performed by two separate pieces of equipment, the first being called a circuit breaker, and the second being called a contactor or starter. They are normally mounted in a steel enclosure and sold as a package. Table 9.1 shows some selected prices from a Westinghouse catalog for the Class A206 full voltage starter, sold as an enclosed combination with a circuit breaker. The prices are list for the year 1989. Actual costs may vary substantially from those listed, but the relative costs Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
Chapter 9—Wind Power Plants 9–8 should be valid. Table 9.1. Price list for motor starters NEMA Rated List Size Current Price 0 18 906 1 27 936 2 45 1326 3 90 1896 4 135 4314 5 270 9627 6 540 19659 7 810 26403 8 1215 38745 9 1800 51273 The left hand column shows the NEMA (National Electrical Manufacturers Association) size. The larger sizes indicate larger enclosures with greater rated currents, shown in the next column. Each NEMA size can be purchased for nominal motor voltages of 200, 230, 460, and 575 V. The corresponding nominal line voltages are 208, 240, 480, and 600 V. A windfarm in the United States would most likely use 480 V. There are actually several different types of enclosures. For example, the general purpose enclosure would be used in indoor, dry locations. The watertight enclosure would be used in very wet locations, like a dairy barn where a high pressure hose may be used to clean equipment. The hazardous location starter would be used in grain elevators or refineries where dust or fumes could be a problem. The values given in this table are for a dust tight and rain proof enclosure which would be appropriate for a windfarm. The turbine generator should have a rated operating current listed on its nameplate. If not, this can be computed from the maximum generator power and the voltage. For a voltage of 480 V and a power of P tur ,thisis I trat = P tur √ 3(480) cos θ (6) If the power factor cos θ is not specified, it can be assumed as being about 0.85 without serious error. The circuit breaker CB1 would be located at the base of the turbine tower if supplied by the turbine manufacturer or at T 1 if supplied by the windfarm developer. Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001
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