Figure 10.20 shows a typical protection arrangement for a wind farm of fixed-speed wind turbines with generator voltages of 690 V and with a collection circuit voltage of 11 kV. The 11 kV circuit is fed from a 33/11 kV Delta/Star wound transformer with the 11 kV neutral grounded either directly or through a resistor. The 11/0.69 kV transformers are also wound Delta/Star and so the 690 V neutral points of each circuit may be directly grounded. The neutral point of the generators is not connected to ground.
There are a number of zones of protection. At the base of the wind turbine tower a 690 V circuit breaker (usually a moulded case type as shown in Figure 10.1) will be fitted to protect the pendant cables and the generator. This is shown as Zone D.
Zone C in Figure 10.20 is the 690 V cables running from the turbine transformer to the tower-base cabinet. Fuses or another moulded case circuit breaker may be
Figure 10.20 Protection of a Wind Farm with an 11 kV Connection Circuit (RMU - Ring Main Unit)
fitted to the 690 V side of the turbine transformer to provide protection of the cables and also a point of isolation so that all the electrical circuits of the turbine may be isolated without switching at 11 kV. In some designs of wind turbine, the main incoming busbar at the bottom of the electrical cabinet at the tower base consists of exposed conductor. The electrical protection of this area needs careful consideration as there is the possibility of dropping tools or other equipment on to these busbars. Zone B is the 11kV/690V transformer including the region around its 690 V terminals. This is a particularly difficult zone to protect as the 11 kV fuses must be robust enough to withstand the magnetizing inrush current of the transformer while sensitive enough to detect faults on the 690 V terminals when the fault current will be limited by the impedance of the transformer. This problem is common to all 11 kV/400 V transformers used on the public distribution network and a typical UK solution is to use an 11 kV combination fuse-disconnect (sometimes known as a switch-fuse). Faults on the 11 kV winding of the transformer will lead to high fault currents which are cleared by the 11 kV fuse. However, for faults on the low-voltage terminals, the fuse is unable to clear these low fault currents effectively and so, once the fuse operates, a striker pin operates a mechanism to open the disconnect switch to clear all the phases. A more expensive solution adopted on some industrial installations is to use so-called restricted earth fault protection to detect currents leaking to ground from the low-voltage winding and terminal area but this requires an 11 kV circuit breaker. There is obvious commercial pressure to reduce the costs of transformer protection on a wind farm as there is a transformer for each wind turbine. However, safety considerations require that all credible faults can be detected and cleared.
Zone A is the 11 kV cable circuit and this is protected in the conventional fashion by overcurrent and earth fault relays operating an 11 kV circuit breaker. The 33/11 kV transformer is protected in a similar manner to a transformer of the same rating used on the public distribution system.
As wind-turbine ratings increase and larger wind farms are constructed, 11 kV circuits cease to be cost-effective and so a number of large wind farms have been constructed with a collection voltage of 33 kV. Figure 10.21 shows a typical arrangement where a 33 kV wind farm is connected directly to a 33 kV public utility network. This arrangement poses a number of additional difficulties for the electrical protection. 33 kV switch fuses are not readily available and so it can be difficult to provide comprehensive protection of the 33 kV/690 V transformer for the full range of prospective fault currents. Effective protection for a single phase to ground fault on the low-voltage terminals is particularly difficult. One attempt to overcome these difficulties is to use a source protection relay to detect all faults on the 33 kV circuit, transformer and low-voltage terminals (Haslam, Crossley and Jenkins, 1999). Although a suitable technique has been shown to be technically successful this type of relay is not in production as the commercial demand for it is too low.
In the arrangement of Figure 10.20 the 33/11 kV transformer provides two useful features when considering electrical protection. Its impedance allows easier grading of overcurrent relays and, as it blocks the passage of zero-sequence current, fast acting earth fault relays can be applied to the wind-farm circuit. With direct connection to the utility circuit these desirable features are not available and, in
Wind farm fuse
X circuit breaker
Figure 10.21 Protection of a Wind Farm with a 33 kV Connection Circuit some countries, all embedded generation schemes are required to install an interface transformer even if its voltage ratio is 1:1. This is not an economic solution but does illustrate the difficulty of connecting a wind farm without a main transformer.
Electrical protection of wind farms follows conventional distribution engineering practice with the network as the main source of fault current. In the protection of a public supply network it is important only to isolate the faulty section or circuit and so maintain supply to as many customers as possible. This discrimination of the protection is less important in a wind farm as only some loss of generation will occur if correct discrimination is not achieved and so simpler and lower cost protection may be appropriate. The difficulty remains, however, of ensuring that effective protection is installed to detect all credible fault conditions even with limited prospective fault currents.
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