Environmental Engineering Reference
In-Depth Information
normal operation. It is fair to consider that level of voltage disturbance as a guide.
Again, whilst IEC 60868 and P28 specifically exclude utility switching and fault
finding, it is good practice for utilities to seek to so arrange their affairs that they
minimise adverse quality effects on customers and they may therefore impose
requirements on wind farms during feeder energisation.
Note that a wind farm is often the sole user of a distribution feeder. Since, while
the circuit is being energised, all wind farm circuit breakers would be open, the only
connected apparatus are the wind farm transformers, auxiliaries and site supplies.
The voltage drop should not be such as to cause mal-operation of these units.
4.3
Network voltage management
Network voltage variation is a key design factor in assessing optimum wind farm
connection arrangements. Clearly a wind farm can operate from no load to full load
and may be located at any point in the network. The connection voltage level is
likely to depend on the maximum output of the wind farm. In assessing the output,
due regard should be taken of the short-term overload capability of the wind farm
due to gusting. This could be typically 125% of nominal rating.
A further variable is whether the farm has a dedicated connection or is
embedded in a load serving circuit. If it is in a load serving circuit it might be
located close to the source or close to the remote end. Load served might be point
load or distributed. The circuit source may have no voltage tap changers, manual
tap changers or automatic tap changers. Circuit real-time information may or may
not be available. There may or may not be other embedded generation on that part
of the network and the network may or may not have voltage support. It is the many
permutations of these variables that provide the challenge to integrate wind gen-
eration with its unpredictability and variability. Taken together, these factors mean
that network voltage control is only likely to be successful if fully automatic.
4.3.1 The voltage level issue
The major network components in the impedance path between the wind farm and
grid are transformers, underground cables and overhead lines.
The main impact of transformers on voltage profile, apart from the obvious one
of voltage transformation, is due to leakage reactance - see Section 2.2.5. Winding
resistance is much smaller and less important in this context. The cabling within an
onshore wind farm generally has only a second-order effect on system voltage
profile and will be ignored here. It should be noted, however, that long lengths of
cable within a wind farm installation can give rise to voltage and resonance man-
agement issues for wind farm owners. For offshore installations, cabling linking
turbines is a more significant factor affecting not only voltage profile, but cost and
environmental permission.
The main factors influencing network voltage profile are the parameters of
the distribution or transmission system overhead lines (or underground cables)
adjacent to the wind farm. It was seen in Section 2.5 that transmission lines have an
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