Page 297 - Mathematical Models and Algorithms for Power System Optimization

Local Decoupling Control Method for Transient Stability of a Power System 289

calculation only based on local measurements, by which norm reduction control criterion is
derived for local stability control. The control criterion consists of local observable decoupled
state variables, so it requires local feedback only. Theoretically, if the operation mode exists
after the fault happened, and the system could make each local stability control always meet the
norm reduction control criterion in an online manner, the overall system is asymptotically
stable in the sense of Lyapunov.

8.2.3 Two Stage Countermeasures in Local Stability Controls

In summary, the countermeasures in local stability control in emergency situations can be
divided into two stages:

In the first stage: Implement emergency control over the instability of local single generator in
the first swing period, so as to brake the synchronous generator, which is required to estimate or
determine the excess energy obtained during the fault as soon as possible, so as to withdrawal or
absorb the excess energy.

To achieve this, the switching of the braking resistor is desirable because it has a certain degree
of fastness and the required flexibility for the operation. In addition, fast valve can also be
considered if it can be opened and closed quickly enough.

It is known from the equal area criterion that when the sum of the energy absorbed during the
braking period and the excess energy obtained during the fault period is zero, then the impact of
switching off the brake on the subsequent oscillation is minimal. Therefore, this criterion can be
carried out for the first stage control. Because this criterion is based on the energy equilibrium,
it can be called the first stage control criterion (energy equilibrium).
In the second stage: Control multiple swings of multiple generators in each part of the
system by following the norm reduction control criterion, based on no loss of synchronism of
the local single generator in the first swing at the first stage. Because this criterion is based
on the value of critical power, it can be called the second stage critical power control
criterion.

However, it should be pointed out that, in the actual local stability control of the system, it is
difficult to rigidly meet the norm reduction control criterion because the value of critical power
needs to be calculated online and is thus time-varying. Furthermore, the actual control measures
based on the critical power value could be only switched on and off in a step-wise form in
general.

In addition, the norm reduction control criterion requires that each part of the system must be
controlled in line with the derived respective criterion value. To this end, even if it is not
difficult, it will be costly, so only a limited number of stability control devices are set at several
critical local parts for the actual system.