Page 270 - Fluid Power Engineering

Deploying W i nd T urbines in Grid 237

drop in frequency on the grid of 6% and, in such situations, the utility
requires that the wind farm be taken offline. The sequence of events
is: (a) Sensor will detect the situation, (b) controller will analyze data
and specify the action, (c) action is sent to the circuit breaker, and
(d) circuit breaker will “throw the switch.” There are two time periods
of a protection system: Relay time to detect and analyze the condition,
and circuit-breaker time to “throw the switch.” Relay times are in the
range of 15 to 30 ms and circuit breaker times are in the range of 30 to
15
70 ms. Therefore, the total time from detection of fault on the grid to
isolation of system is of the order of 50 to 100 ms. On a 60-Hz system,
this is three or six cycles.
Readers may ask why not use other means to take the wind
farm offline, like pitch the blades to stop energy production? The an-
swer is, mechanical components do not react as swiftly as electronic
components. The mechanical components can take minutes to react:
The process of turning gears to change the pitch from operating to
feather position, and the process of losing all the inertia of a rotating
machine.

Grounding for Overvoltage and Lightning Protection
Grounding or earthing design is crucial for safe operation of a wind
project. Most turbine manufacturers provide a grounding system and
supervise its installation. A common grounding system consists of a
ring installed in or around the foundation of the turbine with long cop-
per rods driven into the earth. 16 The ring is bonded to the reinforced
concrete foundation and then connected to a neighboring turbine us-
ing conductors that emanate out radially from the two sides of the
ring (see Fig. 11-10). The radial earthing conductor is placed in the
same trench that carries power or fiber for SCADA communication.
The same grounding system is used for grounding the wind turbine
and all its components, and grounding lightning strikes. Grounding
system must provide: (a) Low impedance path for current to flow and
(b) equipotential bonding, which is to ensure that all the metal com-
ponents like tower, electrical cabinets, and foundation are at the same
potential.
The length of the electrodes depends on the resistivity of soil (see
IEC 61400-24). 16 In place of a ring, reinforcements of the concrete
foundation may be used if the foundation is of 15 m or larger dia-
meter.
Design of grounding systems is a very detailed process that will
not be described here. It must be emphasized that good grounding
is necessary for safe operation of a wind plant. It is not only the
grounding of individual components, but good grounding also en-
sures that ground fault relays are triggered promptly in case of ground
fault.