Operation of power systems - Wind Power Integration: Connection and System Operational Aspects - page 145

with the integral element eliminating the area control error and the proportional
element providing improved regulation. b and T are the regulator parameters.
Subsequently, D P i ( ref ) , the change in reference power for each unit i , is determined
by proportioning the total change in power output with respect to the stiffness
(governor gain) of each generating unit. Updated P i ( ref ) signals are then commu-
nicated directly to the individual generating units (in a similar manner to economic
dispatch). Identical actions will be performed for each region, with individual
generators regulating their output, such that interconnecting tie-line flows are
maintained at contract levels.
5.2.2 Emergency frequency control
So far it has been assumed that any change in the system load or generated output
of units occurs relatively slowly, over minutes and hours, and that the magnitude
of the change is small, compared to the system demand, so that the impact on
individual generating units is minimal. If, instead, a significant imbalance occurs,
such as a large load being connected to the system, or the loss of an interconnecting
tie line, then there may be difficulty in stabilising the system.
Consider what could happen, without forward planning , following the sudden
disconnection of a large generating unit from the system. Initially, demand will
exceed prime mover power and the system frequency will begin to fall. This will
cause the governor control system of each unit to react, in an attempt to negate the
imbalance between generation and demand. Within a conventional power station,
there are various fans and pumps controlling the flow of air, feedwater, etc. As the
frequency falls, the performance of these fans and pumps will deteriorate, but for
small deviations in frequency this can be more than accounted for by the governor
control system, so that the electrical output of the station can be increased as
desired. However, for larger changes in frequency, the former effect will begin to
dominate and the electrical output may actually decrease as the frequency falls.
Continued operation at reduced frequency will also impose severe vibratory stres-
ses on the steam turbine sections. If the frequency falls too far, power station
protection schemes will safeguard the unit by isolating individual generators from
the system grid. Clearly, this will cause the system frequency to fall even further,
before leading quickly to a system blackout.
For example (Andersson et al. , 2005), during the night of 28 September 2003
the Italian power system was importing approximately 6,700 MW (representing
24 per cent of the load demand) from Switzerland, France and other nearby
countries (UCTE, 2004). At 3.01 am, during storm conditions, a tree hit the 380 kV
Mettlen-Lavorgo transmission line, one of the main feeds into Italy, causing it to be
isolated. Subsequently, Gestore del Sistema Elettrico (GRTN SpA), the Italian grid
operator, lowered the requested power import to minimise overloading on the
remaining infeeds. Then, at 3.25 am, the second main 380 kV Soazza-Sils inter-
connector was also hit by a tree, and similarly isolated. The resulting massive
generation-demand imbalance within Italy caused the system frequency to plum-
met and within 10 seconds all connection with the European UCTE grid was lost,
due to line overloading and subsequent tripping. As the frequency fell below 49 Hz,