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International Journal of Advanced Research in Computer Science and Electronics Engineering
Volume 1, Issue 2, April 2012

A study on Angle Stability Solution Using
coordinated Facts Devices
Dr. K.T. Chaturvedi, Assistant Professor Dept. of Electrical Engineering UIT RGPV Bhopal
Kripashankar Pandeya UIT RGPV Bhopal

 this model an iterative linear stability analysis is
Abstract— This paper presents a Angle Stability Solution performed. This approach analyzes all possible
Using coordinated Facts Devices located in different combinations of loads and the sustainable electricity
areas of a power system. Analysis of initial conditions and generations. Eventually, this method reveals the most
a contingency analysis to determine the voltage stability vulner- able operating points of the system and
margin and voltage variations at different critical nodes consequently can be used as an indicator of small
are held. The response is carried out by the coordination disturbance angle stability.
of multiple-type FACTS devices due to which the reactive
power is compensated, improving the voltage stability
margin at the critical nodes. II. PROBLEM FORMULATION

Index Terms— FACTS Devices, Transmission Postfault rotor angle oscillations lead to power swings. Both
Loadability,Angle stability, Voltage Stability, Continuation unstable and stable swings can induce distance relay tripping.
Power Flow, Tangent Vector Technique, Real Power Flow For unstable swings, a new computational procedure to
Performance Index Sensitivity Factor locate all the electrical centers is developed. It simplifies the
work associated with visual screening of all the R-X plots.
For stable swings, a generic three tier hierarchy of stability
I. INTRODUCTION related norms defined by branch norm, fault norm and system
Currently, electric power systems are experiencing struc- norm is proposed. Ranking by branch norm leads to ranking
tural changes due to the growing incorporation of renewable of power swings. Ranking by fault norm leads to ranking of
sources of energy. Another determinant factor in modifying faults or contingencies. Magnitude and rate of change of
the configuration of electric power systems is the system norm can be used to detect an out-of-step condition.
liberalization of electricity market and restructuring the Results on a 10-machine system and a utility system with
trade of energy. Also, in near future, the number of detailed models are also presented. Voltage stability is
interconnections in electric power systems will increase. concerned with the ability of a power system to maintain
Thus, the future electric power systems will operate closer acceptable voltage at all nodes in the system under normal
to their limits. Considering the aforesaid reasons, together conditions or after being subjected to a disturbance [1]. In
with the major switching actions concerning the connection order to detect the system conditions and to predict voltage
of renewable energy sources, the stability of future power instability, it is necessary to conduct a voltage stability study
systems is undertaking a more highlighted role. Being large at all nodes. To prevent or correct voltage instability, solution
scale and complex, are features of modern power methods based on the results of the power system study must
systems. Also, because of deregulation, the configuration be applied; these methods allow to improve the voltage
of interconnected networks is routinely in a state of change. stability margin and to avoid voltage collapse.
Therefore, an indicator of stability, which covers the vast
spectrum of states, is of interest. Such a methodology Study of Voltage Stability
should reconcile the stochastic behavior of the The methods for studying voltage stability are used to find
renewable energy sources and the deterministic approach the operation state, the voltage stability margins and limits
of stability and to study the system variation and element responses. The
analysis. voltage stability study can be conducted using analytical or
A methodology to indicate the small disturbance angle monitoring methods.
stability in future power systems is to make use of an
1) Analytical Methods: These methods allow a detailed study
iterative- stochastic approach. By applying such an
of the variables, parameters and elements behavior of the
approach, the probabilistic nature of sustainable energy
power system, in order to find design solutions and operation
sources is modeled and subsequently, for each sample of
criteria that allow the system to work far from the instability
point. Each of these methods uses a mathematical technique,
which is implemented in a computational tool with a great
number of nodes, lines and loads. These methods arebased on

All Rights Reserved © 2012 IJARCSEE
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conventional power flows, progressive power flows and
dynamic analyses. Conventional power flow techniques are 2) Control of Elements: these techniques are used to avoid
based on mathematical calculations made for each load voltage instabilities and collapses in the power system by
condition of the power system and represent voltage variation controlling elements that change the operation conditions as
at all nodes due to the change of the active and reactive power protection relays [14], current limiters [15] and TAP
of the load. These techniques allow the calculation of the changers.
system operation state and the voltage stability limits and
margins, in normal operation conditions and after 3) System Changes: these techniques are based on the
contingencies. Some of these techniques are: sensitivity entrance of elements or the shedding of loads to avoid
analysis [4], reduction of the Jacobian matrix (matrix voltage instability in the system. They are used to increase the
singularity [5] and modal analysis [6]), network equivalent, power transmission capacity and to alleviate the power
vectorial difference [7] and energy-based techniques. system overloads. These methods are divided in undervoltage
Progressive flow techniques are static analysis methods load shedding [16], and system configuration changes [17].
based on consecutive power flow results used to find the
voltage stability margins and limits of the nodes with higher 4) Mixed: this is a technique based on the combination of the
numerical accuracy [8]; they use a prediction tangent vector above mentioned techniques to create a voltage stability
to estimate a solution to another load value, which is then prevention and correction scheme using different elements
corrected [9]. These methods have been widely used because [18].
of their precision in the voltage stability limit
calculation.Dynamic analysis techniques are based on the
solution ofalgebraic equations in time-domain [1] and they
are used for transient and small signal stability events IV. VOLTAGE STABILITY PROBLEM
analysis. These techniques allow to create different scenarios
Each node of a power system initially operates with a voltage
that include contingencies or normal operation states in order
stability margin as shown in Fig. 1(a). Once a contingency
to determine the variables behavior and response of power
occurs, voltage decreases to a stable point as shown in Fig.
system elements in the occurrence of an event [10].
1(b) or it becomes unstable as shown in Fig. 1(c). Also when
the node operates near the voltage stability limit, Fig. 1(b), a
2) Monitoring Methods: these methods are based on data voltage collapse can occur after other events or operations
measurements of the power system variables such as that decrease the reactive transfer capacity to the critical
voltages, current, active power, reactive power and vector nodes; therefore it is necessary a prompt response to increase
angles, to find the operation state, voltage stability limit and the voltage stability margins as shown in Fig. 1(d).
margin as well as the critical nodes of the system. They can
be used as a tool for the on-line and off-line voltage
stability detection and prediction

III. SOLUTION OF VOLTAGE STABILITY
The methods used for the solution of the voltage stability are
based on the prevention and correction of the problem.
Prevention methods are aimed at maintaining voltage
stability at all nodes, avoiding them to get close to the limit;
correction methods are aimed at restoring the unstable
conditions to return voltage stability to the nodes. These
methods are divided in: reactive compensation, control of
elements and power system changes. Control actions can be
automatic or manual and must be used according to the time
of response to improve in a short, middle or long-term
stability.

1) Reactive Compensation: these techniques are based on the
compensation of reactive power to the system, from power
generation sources [11], transmission lines [12], transformers
Fig. 1. PV curves and Voltage stability margins
[13] and load nodes. The line compensation can be carried
out with a constant value of reactive power, called static
Line compensation using FACTS devices compared with
compensation, using switched elements to increase the
other techniques, allows a fast response after disturbances,
voltages at the nodes; also, a variable compensation can be
allows reactive power injection, does not change the system
carried out, controlled by power electronic devices as
configuration or sheds loads, does not interfere with the
FACTS; this is called dynamic compensation and is used to
availability of generators and can change the direction of the
respond during transitory and small signal variations that
power flow from one area to another. These techniques can
occur in the power system due to disturbances.
be used to respond to the events occurred after a contingency,

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with a static or dynamic increasing of the voltage at the
nodes. Some methods seek to solve voltage instability by the This methodology is based on a fast injection and the
optimal location of FACTS devices close or in the critical direction of power through the transmission path of lower
nodes [19]; other methods are based on the coordination of losses to increase the reactive power supply to the critical
FACTS devices, as in 1998, when a method to coordinate nodes of a power system. This method avoids load shedding,
thyristor controlled series and shunt compensators (TCSC increases the time for other slower reactive control elements
and SVC) was presented in order to improve angle and to operate, can be applied to global power systems and does
voltage stability, using a disturbance response method based not need to relocate FACTS devices so the costs are not
on the Disturbances Auto-Rejection Control (DARC) theory increased. The coordination of FACTS devices to improve
[20]. In 2003, a secondary voltage control method was
voltage stability is made in several stages: FACTS selection,
proposed to eliminate voltage violations at the nodes after a
design and implementation.
contingency, using the coordination of SVC and STATCOM
devices to provide reactive power; the secondary voltage A. FACTS Selection
control is implemented by a learning fuzzy logic controller The FACTS controllers used in power systems are classified
[21]. In 2005, the development of a control system and in: shunt controllers (SVC, STATCOM, BESS, SMESS,
control strategies capable of governing multiple flexible AC SSG, TCBR, TCR, TSC, TSR, SVG, SVS), series controllers
transmission system (FACTS) devices in coordination with (SSSC, TCSC, TSSC, TCSR, TSSR, TCPST), combined
load shedding was proposed to remove overloads caused by shunt and series connected controllers (UPFC, TCPST, IPC)
lines outages in transmission networks, based on linearized and other types (TCVL, TCVR) [23]. These devices allow
expressions in steady state [21]. In 2005, a method for controlling power flow, increasing transmission line
coordination of FACTS devices as SVC, TCSC and TCPST capacity, improving stability and reducing reactive losses.
was presented, based on the optimal power flow to avoid The devices that should be used for voltage stability solution
congestion, to give greater security and to minimize the are those that allow reactive power injection and power
active losses in transmission lines [22]. FACTS devices transfer with lower losses.
location and coordination techniques mentioned above have
been based on the voltage stability improvement in an area
near the devices, improving voltages in steady state, B. Design
preventing violations of the voltage limits, coordinating few The design stage is based on the determination of the
quantity and types of FACTS devices, they do not allow to coordinated control strategies of the FACTS devices. The
handle reactive power in the lines and aim at relocating the steps to find the control strategies are shown in Fig. 3.
devices increasing the costs. Table I shows the comparison
among the techniques used for the solution of voltage
stability problems after a contingency. The rating of the
indexes was done with numbers that indicate the low (1),
middle (2) and high (3) levels of the objective functions of
the proposed method.

V. PROPOSAL
The proposed methodology consists on compensating the
reactive power in an interconnected power system of several
areas as shown in Fig. 2, by a coordinated control strategy of
a wide quantity of different FACTS devices located in
several parts of an interconnected power system in order to
increase the voltage stability margin during critical voltage
variations created by a contingency.

1) Initial Conditions: the initial conditions of a system are
calculated to determine the operation state and the voltage
stability margin of each node.

2) Contingency Analysis: a contingency analysis to determine
nodes with voltage instability or near the limit is carried out.
The selection of the

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determine the type of stability and the magnitude of the
disturbance.

2) Determination of Instability Type and Magnitude: one of
TABLE I the most important parts of the control strategy is the
COMPARISON OF THE TECHNIQUE FOR THE SOLUTION OF determination of the type of instability and the magnitude of a
VOLTAGE STABILITY disturbance occurred in the system, because it is necessary to
establish the time for the operation of the devices and the
reactive power quantity to be transferred to the critical nodes.

3) Determination of the Coordinated Control: after
determining the type and magnitude of the event, the
most critical contingencies and the nodes for the coordinated actions of the available devices are determined,
measurement of the entrance variables of the control strategy transmitting through the transmission path of lower losses to
are carried out. reduce reactive losses and to allow a higher reactive supply to
the nodes.
3) Voltage Analysis: based on the selection of the most
critical contingencies and the nodes for the variables 4) Sending Signals to Controllers: the operation signals for
measurement, a dynamic study of the voltages after the FACTS controllers are sent according to the obtained control
contingency to determine the system response is carried out. decisions; they control the operation of capacitors and
inductors of each FACTS device, allowing the reactive
4) Coordination of FACTS: the control strategy of FACTS injection and power flow direction to the critical nodes.
devices is developed to improve voltage stability at critical
nodes of the system. These devices should allow the reactive 5) Devices Operation: the devices operate based on the
power supply to the weak nodes and the directing of the signals sent and inject reactive power or allow the pass of the
reactive power flow. power flow to a specific node based on the transmission path
of lower losses.
5) Operation Verification: after determining the control REFERENCES
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