Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 95

event is not a steady-state phenomenon. Take a simple device like a water heater.
It is thermostatically controlled and a constant impedance load, and therefore will
remain connected for longer at times of lower voltage. Consider also an induction
motor connected to a constant load. It will increase its current requirement as the
voltage falls. In places with high levels of air conditioning or heat pump load, this
phenomenon makes voltage stability a serious issue, and the effects of wind farms
need to be considered carefully. It may also be important to record whether WTGs
or the wind farm as a whole are able to generate or must absorb reactive power at
the most critical period after a system event. During that period network voltage
may be much reduced. Typically, replacing traditional generation with wind farms
will change this requirement to the extent that some wind generators may introduce
a further demand for reactive power under reduced voltage conditions, hence
increasing the dynamic burden. Also, some wind farm plant will trip off if the
voltage falls, causing lengthier energy flows to supply load and a consequent
increase in dynamic burden. Therefore, different penetration levels and different
generation types should be treated as distinct development scenarios. These studies
could lead to a table which shows the amount of reactive power required for dif-
ferent levels of penetration and types of wind generator for, say, a 15 year forecast.
For worst case investigation, it is necessary to assume that traditional plant will be
switched off to accommodate wind farms. It will therefore make no contribution to
serving reactive requirements. This leads to a design requirement for static and
dynamic reactive power for a given wind penetration and wind generator type mix,
leaving the system safe against voltage instability. NERC Report IVGT Task Force
1-3 recommends the following under Specification of Dynamic Reactive Capability :
'The standard should clearly define what is meant by ''Dynamic'' Reactive Cap-
ability by specifying the portion of the reactive power capability that is expected to
be dynamic. A prospective standard should specify the minimum performance
characteristic of the response in terms of response time, granularity (maximum step
size), and repeatability (close-open-close cycling capability).' The report, under
Definition of Control Performance , further recommends that 'Expected volt/VAr
control performance should be specified, including minimum control response time
for voltage control, power factor control and reactive power control.'
A further phenomenon is associated with the recovery period following a
significant system event, e.g. a major fault, and to study this it is important to have
accurate models for wind farm generator, their control and a wind park's overall
control. When a low voltage event occurs close to a wind farm area, the turbines
cannot export wind energy because the local voltage is so low. If unchecked, tur-
bine rotational speed would therefore tend to increase. This is dangerous because of
forces on the hub and gearbox. One strategy would be to control the speed by
immediately feathering the blades of the turbines. When the fault is cleared, the
turbines are not optimally controlled for power output and the turbine control now
needs to readjust. In that period, after fault clearance, there may consequently be a
considerable period of dip in real power output from the turbine and a need to draw
reactive power from the system to facilitate the re-optimisation of the turbine. This
may be a critical period for the system recovery from a credible contingency.