Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 132

Zone 2

Zone 1

Source

Figure 4.16 Distance protection - two circuit breaker arrangement

analysis should be undertaken. It may be, however, that the costs of protecting the
more complex three-ended arrangement correctly are a significant factor in the
decision. While three-ended distance protection schemes are possible, the position
and length of the T-ed circuit will be important in determining how reliable the
scheme will be. The alternative is a full three-ended unit protection scheme with
significant telecommunications needs.
Automatic post-fault circuit re-closures are often allowed since a large number
of faults are caused by transitory phenomena like contact with trees, clashing
conductors and birds. This is seen as an important tool in managing customer
interruption periods. It will however create difficulties as the level of embedded
generation grows. Even where safety considerations prevent auto-reclosure as a
routine event, it is often used during lightning and storm conditions, to help reduce
the demands on repair teams. As a general rule, auto-reclosures are not allowed on
cable or transformer faults because of the low likelihood of transitory phenomena
and the high cost of further damage.
Some wind farms are connected directly into the low voltage side of bulk or
grid supply points. It is a matter of utility policy who owns the circuit breaker for
such a generation connection, but the switchgear may have bus-zone protection that
may need to be extended (or replaced) to accommodate the additional circuit.
Strategic nodes are divided into several bus-sections. This protection detects the
zone in which a fault has occurred and isolates only that zone. At strategic loca-
tions, circuit breaker fail protection may also be included. In the event that a circuit
breaker fails to trip, this protection detects the failure, tripping all other circuits at
the node to prevent continuing energy supply to a fault. This will have to be
modified to account for the new circuit. A further issue is whether the transformers
are fitted with reverse power protection. At times of low load, a strongly generating
large wind farm might cause the flow through the transformers to be reversed. The
issue may only be one of settings, provided the transformers are capable of the level
of reverse flow. The reverse flow protection may have been installed to prevent
unearthed reverse feeding of a fault as shown in Figure 4.17. It will be important to
restore this facility in another way.