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Modeling and Control Coordination of Power Systems with FACTS Devices in Steady-Sta
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Modeling and Control Coordination of
Power Systems with FACTS Devices in
Steady-State Operating Mode


by

Van Liem NGUYEN
This thesis is presented for the degree of Doctor of Philosophy
of The University of Western Australia
Energy Systems Centre
School of Electrical, Electronic and Computer Engineering


ABSTRACT

This thesis is devoted to the development of new models for a recently-implemented
FACTS (flexible alternating current transmission system) device, the unified power
flow controller (UPFC), and the control coordination of power systems with FACTS
devices in steady-state operating mode. The key objectives of the research reported in
the thesis are, through online control coordination based on the models of power
systems having FACTS devices, those of maximising the network operational benefit
and restoring system static security following a disturbance or contingency.

Based on the novel concept of interpreting the updated voltage solutions at each
iteration in the Newton-Raphson (NR) power-flow analysis as dynamic variables, the
thesis first develops a procedure for representing the unified power flow controllers
(UPFCs) in the steady-state evaluation. Both the shunt converter and series converter
control systems of a UPFC are modeled in their dynamical form with the discrete time
variable replaced by the NR iterative step in the power-flow analysis. The key
advantage of the model developed is that of facilitating the process of UPFC constraint
resolution during the NR solution sequence. Any relative priority in control functions
pre-set in the UPFC controllers is automatically represented in the power-flow
formulation.

Although the developed UPFC model based on the dynamic simulation of series and
shunt converter controllers is flexible and general, the number of NR iterations required
for convergence can be large. Therefore, the model is suitable mainly for power system
planning and design studies. For online control coordination, the thesis develops the
second UPFC model based on nodal voltages. The model retains all of the flexibility
and generality of the dynamic simulation-based approach while the number of iterations
required for solution convergence is independent of the UPFC controller dynamic
responses.

Drawing on the constrained optimisation based on Newton s method together with the
new UPFC model expressed in terms of nodal voltages, a systematic and general
method for determining optimal reference inputs to UPFCs in steady-state operation is

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developed. The method is directly applicable to UPFCs operation with a high-level line
optimisation control (LOC) for maximising the network operational benefit. By using a
new continuation technique with adaptive parameter, the algorithm for solving the
constrained optimisation problem extends substantially the region of convergence
achieved with the conventional Newton s method.

Having established the foundation provided by the comprehensive models developed
for representing power systems with FACTS devices including the UPFC, the research,
in the second part, focuses on real-time control coordination of power system
controllers, with the main purpose of restoring power system static security following a
disturbance or contingency.

At present, as the cost of phasor measurement units (PMUs) and wide-area
communication network is on the decrease, the research proposes and develops a new
secondary voltage control where voltages at all of the load nodes are directly controlled,
using measured voltages. The new secondary voltage control avoids the possible
degradation of the performance of the existing coordinated secondary voltage control
which is based on the direct voltage control at only a limited number of load nodes. The
control strategy developed is fully adaptive to any changes in loads and/or system
configuration.

However, to achieve the lowest possible system operating cost, real-time corrective
control rather than preventative control is required. Depending on the nature of the
disturbance or contingency, secondary voltage control might not be able to provide the
necessary corrective control to restore power-flow security. In order to provide a
comprehensive control scheme which has the capability of restoring power system static
security in its entirety, the final research contribution made in the thesis is that of
developing a coordinated secondary control scheme for restoring voltage and/or power-
flow security subsequent to a disturbance/contingency, and, simultaneously, minimising
the network active- or reactive-power loss. The active- or reactive-power loss
minimisation leads to optimal reactive-power schedule for generators and compensators
together with system voltage profile while only a limited number of load nodes referred
to as the pilot nodes are selected for direct control. In addition to the voltage control
function, the new scheme includes FACTS devices of the series form or UPFC to

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achieve the corrective control for removing transmission circuit overloading. For
enhancing the accuracy in control and coordinating the time responses of the power
system primary controllers and secondary control, each secondary control cycle is
subdivided into a number of steps which is adaptive to the nature of the
disturbance/contingency.

State-of-the-art computer systems for implementing the comprehensive secondary
control law developed are referred to, and discussed in the thesis.
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