WIND ENERGY SYSTEMS - Cd3wd

Chapter 5—Electrical Network 5–54
The power pool operating center will also receive status information on the large power
plants and major transmission lines from the dispatch centers for emergency use. If a large
power plant trips off line due to equipment malfunction, this information is communicated to
all the utilities in the pool so that lightly loaded generators can be brought up to larger power
levels, and one or more standby plants can be started to provide the desired spinning reserve
in case another plant is lost. Spinning reserve refers to very lightly loaded generators that are
kept operating only to provide emergency power if another generator is lost.
Power flows may also be coordinated between power pools when the necessary transmission
lines exist. The southern states have a peak power demand in the summer while the northern
states tend to have a winter peak, so power flows south in the summer and north in the winter
to take economic advantage of this diversity.
Since the wind resource is not uniformly distributed, there may be significant power flows
within and between power pools from large wind turbines. These power flows can be handled
in basically the same way as power flows from other types of generation. The challenge of
properly monitoring and controlling wind turbines is primarily within a given utility, so we
shall proceed to look at this in more detail.
Most utilities have some form of Supervisory Control And Data Acquisition (SCADA)
system. The SCADA system will provide the appropriate dispatch center with information
about power flows, voltages, faults, switch positions, weather conditions, etc. It also allows
the dispatch center to make certain types of adjustments or changes in the system, such as
opening or closing switches and adjusting voltages.
The exact manner of operation of the SCADA system depends on the operating state of
the power system. In general, a power system will be found in one of five states: normal,
alert, emergency, in extremis, and restorative. The particular operating state will affect the
operation of the wind generators on the system, so we shall briefly examine each state[1].
In the normal operating state, generation is adequate to meet existing total load demand.
No equipment is overloaded and reserve margins for generation and transmission are sufficient
to provide an adequate level of security.
The alert state is entered if the probability of disturbance increases or if the system security
level decreases below a particular level of adequacy. In this state, all constraints are satisfied,
such as adequate generation for total load demand, and no equipment is overloaded. However,
existing reserve margins are such that a disturbance could cause overloads. Additional
generation may be brought on line during the alert state.
A severe disturbance puts the system in the emergency state. The system is still intact but
overloads exist. Emergency control measures are required to restore the system to the alert
or to the normal state. If the proper action is not taken in time, the system may disintegrate.
When system disintegration is occurring, the power system is in the in extremis state. A
transmission line may open and remove most of the load from a large generator. The generator
speeds up and its over-frequency relay shuts it down. This may make other generation inad-
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001