Page 266 - Fluid Power Engineering

Deploying W i nd T urbines in Grid 233

at V g , voltage at PCC is V cc , line impedance is Z cc . Short-circuit power
is computed as if short-circuit were created at PCC:
2
S cc = V /Z cc (11-3)
cc
ρ cc = S cc /P (11-4)
where ρ cc is the ratio of short-circuit power to the amount of planned
injected power (P) at PCC by a wind farm (or any other source); note
S cc and P must have the same units of kVA and kW or MVA and MW.
ρ cc is used during sizing of a wind farm.

Weak grid. When ρ cc is 8 or lower, a grid is likely to experience

disturbing voltage variations. In such situations, a grid is con-
sidered a weak grid with respect to the amount of power P
that is being injected into it.
Strong grid. In design situations when ρ cc is 20 or higher, the

grid is considered to be strong with respect to the amount
of power that will be injected to the grid at the PCC. Under
strong conditions, the variations in voltage are minor and pre-
dictable.
As the distance of the interconnection point increases from the
nearest substation, the short-circuit power decreases at the point of
interconnection. Values of S cc andV cc are determined by power-flow
or load-flow studies under normal operations and short-circuit con-
ditions. The grid operator normally does these studies using network
analysis and simulation software.

Wind Farm Topologies
The layout of a wind farm is a function of wind speeds and wake
effects. Once a layout of turbines has been determined, an intrawind
farm collector system is designed. Considerations include:

Topology of the collector system include the following, op-

tions: radial, feeder-subfeeder, and looped feeder. 13 Most
wind farms operate on a feeder-subfeeder topology.
Voltage level for the collector system

Mix of underground and overhead cables in the wind farm

Location of substation

Design of the neutral and grounding systems

Consider a wind farm with 20 turbines. A simple radial topol-
ogy in Fig. 11-6 that strings the 20 turbines is low cost, but high-risk
topology with no redundancy. If there is a break in the feeder link, all