Environmental Engineering Reference
In-Depth Information
Table 4.3
Grid code requirements for wind turbine generator operation at
off-nominal frequencies
Endurance time
Australia
Denmark
Germany
Ireland
Scotland
Normal limits
( > 1h)
Maximum
50.1 Hz
51 Hz
50.5 Hz
50.5 Hz
50.4 Hz
Minimum
49.8 Hz
49 Hz
49 Hz
49.5 Hz
47.5 Hz
1 h limits
Over
50.1 Hz
51 Hz
50.5 Hz
52 Hz
52 Hz
Under
49.8 Hz
49 Hz
49 Hz
47.5 Hz
47.5 Hz
0.5 h limits
Over
50.1 Hz
51 Hz
50.5 Hz
52 Hz
52 Hz
Under
49.8 Hz
48 Hz
49 Hz
47.5 Hz
47.5 Hz
Minutes
Over
50.1 Hz
51 Hz
51.5 Hz
52 Hz
52 Hz
Under
49.8 Hz
47.5 Hz
(300 s)
48.5 Hz
(180 s)
48 Hz
(120 s)
47.5 Hz
47.5 Hz
Seconds
Over
50.5 Hz
53 Hz
(60 s)
51.5 Hz
52 Hz
52 Hz
Under
49.5 Hz
47 Hz
47.5 Hz
(60 s)
47 Hz
(20 s)
47.5 Hz
< Seconds
Over
52 Hz
(400 ms)
53 Hz
(60 s)
51.5 Hz
52 Hz
52 Hz
Under
47 Hz
(400 ms)
47 Hz
47.5 Hz
47 Hz
47 Hz
(1 s)
Power systems operate ordinarily within a 1% frequency bandwidth around the
nominal frequency (49.5-50.5 Hz) although larger systems may narrow this band,
and this poses no problems for wind generators. Loss of significant generation,
interconnection or load can cause a system to operate, for a time, at up to 104% or
down to 94% of nominal frequency. Load shedding or plant tripping may follow to
restore the frequency to within the normal range. As a result, most grid codes
require that plant is capable of remaining stably connected for defined periods
throughout this frequency spectrum. Table 4.3, drawn from research carried out by
Vestas, summarises the position.
Many wind turbine manufacturers claim that continued performance at these
frequency limits present no significant problems. (Clause CC.S2.3.4 in the sample
grid code in Appendix 2 shows a typical specification of the requirement.) If
capacitors form part of the installation it is important to ascertain whether these are
rated for the higher frequencies. For fixed-speed devices, there may be a significant
increase in mechanical loading at higher frequencies. DFIG machines isolate, to
some degree, the speed of the turbine from the supply frequency, therefore do not
experience extra mechanical forces.
The above discussion relates to performance between frequency limits. On
smaller systems there is a growing problem with rate of change of frequency. Nor-
mally, the rate of change of frequency following a system or system infeed event, or
following a sudden loss of load, is kept to a low level by the rotating inertial mass of
the system. It follows that large systems maintain a very stable frequency profile for
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