WIND ENERGY SYSTEMS - Cd3wd

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