Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 240

Fig. 11.12 Pulse width
modulated (PWM) voltage
(solid line) and corresponding
sine wave (dashed line)

1

0.5

0

-0.5

-1

0 0.2 0.4 0.6 0.8 1

phase angle/2 π

11.5 System Protection

Modern microprocessor-based controllers can provide extensive protection for the
system and condition monitoring to extend its lifetime. For example, the battery
can be protected from being drawn too low and can also be boost-charged, say
every month. Some possible faults and appropriate responses are listed in
Table 11.2 . However, sophisticated protection should be applied intelligently. For
example, a turbine should not shut down if smoke is detected from a nearby
barbecue.
When constructing or testing a wind turbine, extensive protection is often not
desirable as it may affect the turbine behaviour and hence make it difficult to test.
Protection against most short term severe damage, such as caused by short circuits,
requires the ability to:

1. Disconnect the generator from the rest of the system.
2. Apply and disconnect any dump load.
3. Disconnect the wind turbine from any battery, and to be able to disconnect the
battery from any load.
4. Disconnect the inverter from the wind turbine or battery, and to be able to
disconnect the inverter from any load.

In all of these cases, the disconnection point also requires over-current pro-
tection. Ordinarily, this is done by appropriate circuit breakers, which can be used
for both protection and as isolating switches when required, see Fig. 11.13 . Note
that the figure also shows switches immediately after the generator that can be
used to delay power extraction until the blades have reached a suitable rpm after
starting. Circuit breakers have differing voltage ratings as to whether they are
switching AC or DC currents. Typically, the DC voltage ratings are much lower
than the AC ratings.
Basic electrical protection aims to prevent excessive electrical currents. High
currents typically cause heating and may melt wires, burn through insulation, and
destroy electronic components. In addition, excessive battery-charging current can
cause gassing of the batteries and explosions. Excessive current discharging from