Grid Integration and Power Quality - Wind Energy Systems for Electric Power Generation - page 151

7.3.4 Short-Circuit and Fault Ride-Through

New grid codes require WES to contribute to supporting the voltage at the connec-
tion point, similar to a power station. In case of distance short-circuits, leading to a
rest voltage of as low as 15%, the system is not allowed to trip within time intervals
shown in Fig. 7.10 (50 Hz rated frequency, grid voltage level

60 kV), [VDN04].
This requirement covers cases where, due to successful stepping, the voltage is re-
stored in times below 3 s. It is seen that service must be sustained in case the voltage
has been restored to a permanent 80%. Utilities in other countries have established
somewhat different curves, with variants of short-time rest voltage (down to zero,
meaning terminals short circuited), slope of the curve and voltage to be sustained
permanently.

≥

Fig. 7.10 Limiting voltage/time area excluding automatic tripping of wind parks

Also a WES should be capable of delivering a limited contribution to the short-
circuit current. The current to be delivered at short-circuit is subject to agreement.

7.4 Power Quality

7.4.1 Harmonics

Harmonics are mainly generated in systems containing converters. Besides integer
order numbers

ν , non-integer order numbers μ appear in case of PWM controlled