1. Power system stability
improvement by FACTS
devices: &
a comparison between
STATCOM, SSSC and UPFC

2. INTRODUCTION
The rate of dissipation of transient energy is usually used as a tool to
measure dynamic system damping.
 This presentation applies and discusses the above control strategy for
suppressing undesirable electromechanical oscillations in power system
with a STATic synchronous COMpensator (STATCOM), static synchronous
series compensator (SSSC) and unified power flow controller (UPFC).
To transfer maximum power without loosing synchronism.

3. Definitions:
Stability:
Power system stability is the ability of an electric power system,
for a given initial operating condition, to regain a state of operating
equilibrium after being subjected to a physical disturbance,
with most system variables bounded so that practically the entire system
remains intact.

4. Facts:
“AC transmission systems incorporating power electronic based and
other static controllers to enhance availability and to increase
power transfer capabilities”
Facts controller:
Power electronic based system or other equipment that provides
control of one or more AC transmission system. They can be inserted in
series or shunt and in some cases a combination of both.

5. POWER SYSTEM MODEL
let us consider Single machine infinite bus diagram
Single machine infinite
equivalent circuit diagram.
The output electrical power of the system without any FACTS devices
is given by:

6. MODELLING OF POWER SYSTEM WITH STATCOM
The STATCOM is a voltage-source converter (VSC) capable of generating
or absorbing independently controllable reactive power at its terminal.
It is represented by a shunt reactive current Is source as shown in figure.
Circuit diagram of SMIB with STATCOM.
Equivalent circuit diagram of SMIB with
STATCOM.

7. MODELLING OF POWER SYSTEM WITH SSSC
The SSSC is a solid-state voltage source inverter that generates
a controllable AC voltage.
Circuit diagram of SMIB with SSSC. Equivalent circuit diagram of SMIB with SSSC
In case SMIB with SSSC, e P is rewritten as:

8. MODELLING OF POWER SYSTEM WITH UPFC
 A unified power flow controller (UPFC) is the most promising device
in the FACTS concept.
Equivalent circuit diagram of SMIB with UPFCCircuit diagram of SMIB with UPFC
 UPFC regroups features and properties of both SSSC and STATCOM.

9. RESULTS AND DISCUSSIONS
 A three phase fault occurs at bus m at t=1s and is cleared by
disconnecting line 2 at end. Rotor angle deviation response to this
disturbance for different critical time is shown in figure.
Relative angle position for different critical times

10.  For a critical clearing time tc= 0.088s, the same curves for the SMIB
with and without FACTS devices are shown in figure.
Relative angle position
 Prefault and post fault values of the rotor angle position are not the
same. These values are equal to 41.7714and 54.7260°, respectively.
Figure shows that the uncompensated system is unstable, but when
we added STATCOM and SSSC, the system remains stability, oscillations
are damped out.

11.  Figure shows curves of machine rotor angle velocity plotted with and
without FACTS devices.
Relative angle velocity

12.  Figure shows curves of machine output electrical power for the two
cases: with and without FACTS devices.
The output electrical power in the prefault period is equal
to it’s initial value 0.9 pu.
Generator output power
 At t=1s, output electrical
oscillations are kept constant during
faulted period and post faulted
period for the
uncompensated case.
 For the case with FACTS devices, oscillations are rapidly
damped out.

13. CONCLSIONS AND PROSPECTS FOR FUTUTRE WORK
 The objective of this study was to reach an efficient control of an
electrical power system plus FACTS devices under several perturbations.
Facts is an application of power electronics in power transmission
system.
Facts has a important role in real and reactive power control.
Facts makes a system stable.
 All the above aspects show the effectiveness of these devices to
suppress oscillations and stabilizing the power system.

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II”,IEEE Trans.PAS, vol.
100, pp. 3017–3046. 1981.
[2] M.Noroozian, L.Angquist, M. Ghandari, and G.Anderson, “Use of UPFC for optimal
power flow
control”, IEEE Trans. on Power Systems, vol. 12, no. 4,
pp. 1629–1634. 1997.
[3] Mehrdad AhmadiKamarposhti, MostafaAlinezhad, “Effects of STATCOM, TCSC, SSSC
and UPFC on
Static Voltage Stability”, International Review of
Electrical Engineering, vol. 4, no. 6, pp. 1376–1382.
2009.
[4] E. Z. Zhou,“Application of Static Var Compensators to increase Power System
Damping”, IEEE Transactions
on Power System, vol. 8, no. 2, pp. 655–661. 1993.
[5] P. Kundur, Power System Stability and Control, Example13.1, page 863 McGraw-
Hill, New York, 1994.
REFERENCES