WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–55
equate to meet load, with generators and transmission lines turning off in a domino fashion.
Emergency control action is necessary in this state to keep as much of the system as possible
from collapse.
In the restorative state, control action is taken to pick up lost load and reconnect the
system. This can easily take several hours to accomplish.
System disintegration may result in wind generators operating in an island. A simple
island, consisting of two wind turbines, two loads, and a capacitor bank used for voltage
control in the normal state, is shown in Fig. 29. If the wind turbines are using induction
generators, there is a good possibility that these generators will draw reactive power from the
capacitor bank and will continue to supply real power to the loads. This can be a planned
method of operating a turbine independently of the utility system, as we shall see in the next
chapter. Without the proper control system, however, the voltage and frequency of the island
may be far away from acceptable values. Overvoltage operation may damage much of the load
equipment as well as the induction generators themselves. Frequencies well above rated can
destroy motors by overspeed operation. Under frequency operation may also damage motors
and loads with speed sensitive oiling systems, including many air conditioning systems.
Figure 29: Electrical island.
In addition to the potential equipment problems, there is also a safety hazard to the utility
linemen. They may think this particular section of line is dead when in fact it is quite alive. In
fact, because of the self-excitation capability of the induction generator, the line may change
from dead to live while the linemen are working on it, if the wind turbines are not placed in
astopmode.
All of these problems can be handled by proper system design and proper operating procedures,
but certain changes in past operating procedures will be necessary. In the past, the
control, monitoring, and protection functions at a distribution substation have generally been
performed by separate and independent devices. Information transfer back to the dispatch
center was very minimal. Trouble would be discovered by customer complaints or by utility
service personnel on a regular maintenance and inspection visit to the substation. An
increasing trend is to install SCADA systems at the substations and even to extend these
systems to the individual customer. A SCADA system will provide status information to the
dispatcher and allow him to make necessary adjustments to the distribution system. At the
customer level, the SCADA system can read the meter and control interruptible loads such
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001