Page 269 - Fluid Power Engineering

236 Chapter Eleven

circuit, power can still flow from all turbines, in most cases.
Such redundancy may be warranted when using overhead
cables for intrawind farm MV circuit because in strong wind
areas, overhead cables reduce reliability of the MV circuit.

Protection Systems
One of the key elements of electrical design is a protection system.
There are three main components of this type of system:

Switch or circuit breaker that is operated electromechanically

or electronically that can open or close connections in a circuit
to protect the generator, transformer, grid, and other systems.
The speed of switches is very important; the time of operation
is of the order of 0.1 s after a fault is detected.
Monitoring system that measures quantities like current, volt-

age, frequency, and others. Quantities are measured in each of
the three phases and may be measured at the input and out-
put of the generator, input and output of transformer, input
to the grid, and flow in the grid. The measured quantities are
used in the logic described below.
Logic system that compares measured quantities against pre-

set threshold levels, compares two measured quantities and
performs other computations following which the logic sys-
tem computes the action required. The signal about the
action is sent to the switch, which executes the action. In a pro-
grammable protection system, the logic, the threshold levels,
and other parameters are programmed.
Newer wind farms also have a fourth component, which is a so-
phisticated communication system that connects the different protec-
tionunitsinawindfarmusingafastfiber-basedEthernettoeachother,
a centralized controller, and SCADA system. Such protection systems,
with the three or four components, are referred to in the industry by
the generic name of intelligent electronic device (IED). IED is defined
as 14 “device with versatile electrical protection functions, advanced
local control intelligence, monitoring abilities, and the capability of
extensive communications directly to a SCADA system.”
Several quantities are monitored by protection systems:
Over and under voltage

Over current and direction of current

Over and under frequency

Directional and nondirectional earth fault protection

For the purposes of illustrating the stringent time requirements,
consider the following example. A protection system has detected a