Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 242

resort. The main advantages of a circuit breaker over a fuse are its ability to be
reset, and the ease of turning it off for testing or maintenance purposes.
Fuses and circuit breakers have different specifications when used for AC or DC
because of differences in arcing. With AC, any electrical arc will self-extinguish as
the current reverses direction. However, DC arcs can establish plasma and hence
continue conducting and arcing, potentially with a low voltage drop across the arc
and little effect on the current. Any prolonged arc will cause heating, which can
melt and ignite the switch, and then spread to the connecting wires.
Table 11.3 shows the tripping characteristics of the ABB pro M compact
series of circuit breakers at marginal over-current conditions and severe over-
current conditions. Note that these circuit breakers have two ways of releasing—
heat and electromagnetic, with different characteristics. The circuit breakers have
a nominal tripping current, but also a breaking current capacity. This is the
maximum current that the breakers are guaranteed to break (interrupt), typically
during a short circuit. The breaking capacity typically ranges from 6000 to
15,000 A.
The tripping characteristics labelled B, C, D, K and Z in Table 11.3 relate to the
duration of the currents, the current magnitude, and the length of time it takes for
tripping to occur. Of particular interest is the value of the ''current hold surges''.
Surges often occur as equipment is either turned on or connected to the electrical
supply from the turbine (e.g. inverters). The circuit breakers should not trip under
these conditions, hence higher holding current surge rated circuit breakers may be
needed. Class D breakers are commonly used for electric motors.
Fuses are often cheaper then circuit breakers but can be inappropriately
replaced by lengths of fencing wire, nails etc. High rupture capacity (HRC) fuses
are recommended. Many have very high current breaking capacities of around
80,000 A. All fuses will start to melt at currents above its rating. However, the
rupturing of the fuse will take some time, depending on the current and the design
of the fuse. This relationship is usually given as an I 2 t value, where t is the time to
rupture.
The rupture capacity fuse is the maximum current (usually under short circuit
conditions) that the fuse is guaranteed to interrupt safely. HRC fuses are usually
sand filled to assist with the extinguishing of any arc. Such fuses typically have
rupture capacities in the tens of thousands of Amps, even if they are rated at only a
few Amps. Hence fuses have the advantage of very high rupturing capacity, low
cost, and the ability to maintain shut down until the fault is investigated.
Remote power systems usually have a dump load in the form of a resistive
element which may be used for water heating. Normally the dump must be capable
of absorbing the continuous maximum power output of the turbine. Despite being
crude, dump loads are simple, cheap, and reliable. They can protect the more
expensive power electronics especially in systems which are often complex and
poorly integrated. Dump loads can also be used for ''soft braking''; by dumping
part (but not all) of the generated power, the blades can be slowed down for
protection or for parking. Few grid connected turbines have separate dump