Co-Ordination of Multiple
Controllers using Linear – Control Techniques
Ø The term coordination does not imply centralized
control; rather, it implies the simultaneous tuning of the controllers to
attain an effective, positive improvement of the overall control scheme.
Ø It is
understood that each controller relies primarily on measurements of locally
available quantities and acts independently on the local FACTS equipment.
2. The Basic Procedure for
controller-design procedure involves the following steps:
of the system model;
of the system-performance specifications;
of the measurement and control signals;
4. Coordination of the controller design; and
of the design and performance evaluation.
Step 1: Derivation of System Model
Ø First, a
reduced-order nonlinearsystem model must be derived for the original power
system and this model should retain the essential steady-state and dynamic
characteristics of the power system .
Ø Then, the
model is linearized around an operating point to make it amenable to the
application of linear-control design techniques. If a controllermust be
designed for damping electromechanical oscillations, a further reducedlinear
model is selected that exhibits the same modal characteristics over the
relevant narrow range of frequencies as the original system.
situations where linearized-system models may not be easily obtainable,
identification techniques are employed to derive simple linear models from
Step 2: Enumeration of the System – performance
Ø The damping controller is expected to satisfy
the following criteria.
1. It should
help the system survive the first few oscillations after a severe system
disturbance with an adequate safety margin. This safety factor is usually
specified in terms of bus-voltage levels that should not be violated after a
2. A minimum
level of damping must be ensured in the steady state after a disturbance.
deleterious interactions with other installed controls should be avoided or
objectives over a wide range of system-operating conditions should be met
(i.e., it should be robust).
Step 3: Selection of the Measurement and Control
choice of appropriate measurement and control signals is crucial to controller
signals must have high observability and controllability of the relevant modes
to be damped, and furthermore, the signals should only minimally affect the
other system modes.
selection of these signals is usually based on system-modal magnitudes, shapes,
and sensitivties—all of which can be obtained from small-signal-stability
Step 4: Controller Design and Coordination
Ø The FACTS
controller structures are usually chosen from industry practice. Typically, the
controller transfer function, Hj(s), of controller j is assumed to be
transfer function consists of a gain, a washout stage, and a pth-order leadlag block, as well as
low-pass filters. Alternatively, it can be expressed as
the basic structure of different controllers is assumed as from the preceding
text, the coordination of controllers involves the simultaneous selection of
gains and time constants through different techniques.
Ø Doing so
permits the system-operating constraints and damping criteria to be satisfied
over a wide range of operating conditions.
coordination techniques may use linearized models of the power system and other
embedded equipments, capitalizing on the existing sparsity in system
model may be further reduced by eliminating certain algebraic variables yet
still retaining the essential system behavior in the frequency range of
analysis–based controller-optimization and -coordination techniques are
applicable to power systems typically with a thousand states occurring when
full modal analysis must be performed. However, sometimes a limited number of
electromechanical modes must be damped; hence the eigenvalue analysis of a
selected region can be performed even for relatively larger power systems.
Step 5: Validation of the Design and performance
though the controller design is performed on the simplified system model, the
performance of the controller must still be established by using the most
detailed system model.
Ø The controller
should meet the specifications over a wide range of operating conditions and
consider all credible contingencies.This validation is generally performed with
of the system.