Environmental Engineering Reference
In-Depth Information
energy loss in the equipment due to current of magnitude I flowing for time t or I 2 t .
It is therefore important to remove the disturbance as quickly as possible to reduce
the heating effect and consequent disruption. The other effect is an electromagnetic
phenomenon proportional to rate of change of current dI / dt .
The process of current interruption usually involves the separation of physical
contacts. As the contacts start to separate, the contact pressure gradually decreases
and the resistance R of the connection increases. This dissipates power proportional
to I 2 R . As separation continues, there is a tendency for a stream of hot ionised
particles to maintain the arc between the contacts. Insulating gas or vacuum
environments reduce this phenomenon. Extinction may only occur when the cur-
rent passes through a natural cyclic current zero. If the X / R ratio of the network is
very high, this will be almost at the time when the voltage is reaching a maximum
between the separated contacts, tending to re-strike the arc. It is clear therefore that
it is not only the modulus of the current which is significant but its vector rela-
tionship to the system voltage.
Embedded generation changes the situation in two ways.
It introduces a new source of energy at the tail-end of the electricity network
system, hence providing an increased fault level to be dealt with by distribution
switchgear. Induction machines are known to contribute up to six times full
load current as fault level during a close-up three-phase fault. In the case of a
DFIG machine, unless the manufacturer provides a fault level (resulting from
tests as indicated above), it must be assumed that it behaves as an induction
machine plus a converter. A full speed range machine will behave as the con-
verter behaves. For the most part the fault level contribution will be negligible.
It should be noted that, because these machines are generally rated at 690 V, the
impedance of the connecting transformer will reduce this contribution greatly
when viewed from the HV network. Another factor is that the grounding
arrangements of multiple wind turbine transformers need to be considered in
the zero-sequence network for determination of earth fault currents.
It changes the X / R ratio because an almost pure inductive source is introduced.
This is most significant when the wind farm is electrically close to the dis-
tribution switchgear concerned. When the wind farm is at the end of a long
line, a different phenomenon occurs, in that the X / R ratio is reduced and the
sub-transient and transient parts of the fault current are affected. It is important
to check whether the switchgear is trying to interrupt during the presence of
these currents.
In the United Kingdom the Embedded Generation Working Party has, through
its technical committee, sought to better understand the way forward in addressing
the fault level issues on distribution networks. It has looked at a range of solutions.
Network splitting (which results in lower network security by operating the
network with fewer interconnections) which reduces the number of parallel
transformers/network paths and hence reduces fault level.
Fault limiting devices, e.g. fuses and variable impedance links.
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