Page 314 - Decision Making Applications in Modern Power Systems

Decision-making-based optimal generation-side Chapter | 11 275

Voltage control
Voltage control of power systems is mainly provided by the generat-
ing units. Terminal voltage of generating units is controlled and main-
tained within specific limits by means of the automatic voltage regulator
(AVR). This control loop is local, and the set point of the terminal volt-
age can be adjusted through the SCADA system or WAMS. Besides the
AVR, other methods of voltage control include tap-changing transfor-
mers, static Var compensators, and synchronous condensers.
Frequency control
Frequency of an electric system depends on the active power balance.
If a generation-load active power imbalance occurs, the frequency will
either increase or decrease, thus moving away from its nominal operating
value. In order to control the frequency the active power balance must be
restored. Since some generating units, which can quickly increase or
decrease their active power production, are therefore mainly used in fre-
quency control.
The frequency control is mainly divided in three control levels: pri-
mary; secondary, that is, automatic generation control (AGC) or LFC;
and tertiary.
Primary frequency control (PFC), here, a local proportional controller,
is used to measure frequency deviations and adjust the active power pro-
duction of the corresponding generating unit. The response of the PFC is
usually quick and in the scale of seconds and fast generating units partici-
pated in this control action.
In the AGC/LFC scheme, it is performed through the WAMS, and
only preselected generating units participate in the control area. The aim
is to make sure that frequency deviation is restored back to zero and also
to maintain the power flow on the tie-lines that connect it with the other
control areas at its prescheduled value. The response generally takes a
few minutes; therefore it is possible for slower units to participate. The
units that participate in this scheme must have predisturbance production
set point where a sufficient margin from their capacity limits can be used
once the frequency control is activated. Each generating unit can have a
reserve capacity margin for primary and/or secondary frequency control.
Tertiary frequency control, after the primary and secondary activation
of the automatic frequency control loops, tertiary frequency control takes
place. This is a manual process, and the main purpose is to release the
deployed primary and secondary control reserves while performing an
economic dispatch.
Security assessment
A commonly used security criterion measure is the N 2 1 security cri-
terion, where the system is supposed to be secured if it can withstand a
predefined set of credible single contingencies. The contingencies include
outages that are likely to occur with higher probability, such as a single