WIND ENERGY SYSTEMS - Cd3wd

Chapter 9—Wind Power Plants 9–29
a synchronism check relay, might only be located at one circuit breaker location. The relays
need to be carefully coordinated so the windfarm operation will be both safe and economical.
Several of the possible relays will be briefly discussed here.
Overcurrent relays are very important in preventing damage to equipment due to equipment
failure or faults. They have two types of overcurrent operation. One is for moderate
overcurrent conditions of perhaps five or six times rated current for a short period of time
(a second or so). This could be experienced during normal operation, such as the starting
of an induction motor, and should not cause the circuit breaker to open. If this current is
sustained for several seconds, however, the circuit should be opened. A relay circuit involving
the product of time and current is used, so that a larger overcurrent will cause relay operation
in a shorter time.
The other operating mode is the so-called instantaneous trip mode. Under fault conditions,
when conductors have shorted together, the current may be 20 times the rated current or more.
This very large current is never a part of normal operation, so the relay is built to operate as
quickly as possible under such conditions.
Overfrequency and underfrequency relays will operate when the windfarm is disconnected
from the utility grid. The utility grid operates at a very precise 60 Hz in the U.S.A. so that
any significant deviation from this frequency means the windfarm is not connected to the
frequency controlled grid. It is possible that the utility lines could open at some distance from
the windfarm, leaving some utility load attached to the windfarm. Depending on the load,
the wind speed, and the presence of power factor correcting capacitors, wind driven induction
generators could supply this load for some time, but at frequencies probably quite different
from 60 Hz. This could result in damage to utility customer equipment and also in physical
harm to linemen repairing the utility transmission system. Therefore the main circuit breaker
to the windfarm must be opened when the frequency is outside some range (perhaps 59 to 61
Hz), and not reclosed until the utility lines again have 60 Hz present on them.
It is not obvious that all the circuit breakers need utility quality overfrequency and underfrequency
relays connected to them. One set at the main transformer may be adequate, with
perhaps some less expensive relays set for a wider frequency range at the individual turbines,
as a backup for the main circuit breaker.
Overvoltage and undervoltage relays will probably also be required. If the windfarm is
disconnected from the utility, both voltage and frequency will shift away from the proper
values. It is conceivable that frequency would stay in the proper range while the voltage went
either higher or lower than what is acceptable. It is also possible that a voltage regulator
system would fail on the utility side, so that frequency is still controlled by the utility but
the voltage is incorrect. Again, a sophisticated set of relays at the main circuit breaker and a
crude set at the turbines may be all that is required.
Power directional relays indicate whether power is flowing from the utility to the windfarm
or from the windfarm to the utility. The induction generators will automatically operate as
motors in light wind conditions, driving the turbines as fans, a condition which obviously must
Wind Energy Systems by Dr. Gary L. Johnson November 21, 2001