Improving the voltage stability and performance of power networks using power electronics based FACTS controllers 1
S. Fahad Bin Shakil, 2Nusrat Husain, 3M. Daniyal Wasim, 4Shayan Junaid 2 Faculty, 1,3,4 Students
Department of Electronics and Power Engineering, National University of Sciences & Technology (NUST), Karachi, Pakistan.
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Abstract— This paper discusses the modern trend in power networks regarding the applications of Power Electronic devices and how they are replacing the conventional methods of improving the power networks performance. The role of Flexible AC Transmission Systems (FACTS) for voltage stability and reactive power compensation is described and how FACTS controllers are making our power systems more efficient. The main focus is on the role of Static Var Compensator (SVC) and Static Synchronous Compensator (STATCOM) in improving the voltage and power stability of the power networks. In the end a comparison is being made between these FACTS controllers. Key Words— Power System Stability, Power Networks, Voltage Stability, FACTS
I.
INTRODUCTION
Power quality problems have always been a great issue in power transmission and distribution networks. Nowadays with the unplanned expansion of the networks and installation of different kinds of loads, power distribution and transmission system is facing serious power quality issues. These power quality issues include harmonics, excessive reactive power burden, load unbalancing, voltage instability etc [1]. Voltage instability is one of the major problems increasing day by day. Voltage instability problems arise mainly due to the increase in demand of load and inability of the power system to fulfill that demand of reactive power. To address these problems technical advancements have been made and are still in progress. These advancements are made in Power Electronics, their high performance and rapid response is of great importance which enables high end power and voltage stability and other essential benefits. FACTS controllers are basically power electronic devices which enhance the power carrying and transmitting capacity of the power system network. The response of these devices is very fast and that’s why they are very suitable for problems relating dynamic load changes. Voltage collapse is undesirable in the power systems, it occurs when the system is being overloaded. This collapse may cause partial or even full power failure in the system. This overloaded condition arises when power system
is unable to provide the required reactive power. This is where the role of FACTS devices is most important. The inclusion of FACTS controllers at the right location can resolve the problem of voltage instability by injecting or absorbing reactive power needed by the load. FACTS controllers have great impact on the transmitting capacity of the transmission lines. They are also used in Power Distribution Networks for voltage stability and improved power quality [2]. Several types of FACTS controller are available, which include; • Static VAR compensator (SVC) • Thyristor Controlled Series Capacitor (TCSC) • Static Synchronous Series Compensator (SSSC) • Unified Power Flow Controller (UPFC) • Static Synchronous Compensator (STATCOM) In this paper all of these are discussed, but our main focus is on SVC and STATCOM. The reactive power flow and operation of STATCOM is discussed. Voltage Source Converter (VSC) which is an integral part of STATCOM and how much flexibility it gives in the reactive power compensation, are highlighted. II. FUNDAMENTAL CONCEPTS A. Power Flow and Compensation As we know in power networks there are power sources and loads. If we consider a transmission line having a source (Sending end) and a load (Receiving end), it can be seen that the sending end and receiving end voltages are different in magnitude as well as in phase. We don’t want this to occur, we want the voltage to be remain at the rated or the desired value. This difference has occurred because line was unable to supply the required power to the load. Now in order to keep the system voltage stable and at rated value two control strategies are possible: i. ii.
System Compensation Load Compensation
In load compensation, a parallel capacitive load is inserted to account for the reactive power. Due to this the demand of current decreases in the line and as a result, line drops decreases significantly and voltage is improved, but still the voltage does not attain the desired value. Hence, it does not provide total compensation. In case of System compensation, a reactive compensator is inserted in the system which eliminates the variations in voltage. This can be achieved by controlling the reactive power of the compensator over a wide range either manually or by automatic control mechanism using solid state devices. Such compensators can absorb or inject reactive power as per the line requirements. Power Flow in transmission line can be given by the relation
ܲൌ
ೞ ೃ
ߜ
Where ܸ௦ ǡ ܸோ are sending and receiving end voltages respectively, X is line impedance and ߜ is power angle [3].
III.
FACTS DEVICES
FACTS devices are applications of power electronics. FACTS have the principal role to enhance controllability and power transfer capability in power systems. They are mainly thyristor based controllers which are solid state devices and do not involve mechanical parts. These devices can be turned on or off by giving proper signals. Their switching characteristics vary, depending on the device. These FACTS controllers are used for the dynamic control of phase angle, voltage and impedance of the power system networks. These controllers have advantages of quick response, smooth control, superior controllability, higher efficiency and immunity from wear and tear. Below some of them are very briefly discussed, with main focus on STATCOM. Figure-2 shows a simplified diagram showing FACTS controller implemented between bus ‘b’ and ‘c’ which is between sending and receiving end.
Figure-2
Figure-1 Transmission line model
The active power in a system always flows from leading to lagging voltage side. The reactive power flow depends on the magnitude of the voltage. It flows from high to low voltage end. Our desire is to transmit as maximum power (P) as possible. This can be obtained by increasing the power factor and/or decreasing the line impedance (X). Since extensive transmission and distribution systems are already in place so it is neither easy nor feasible to significantly reduce line impedance so our main focus is to improve power factor. B. Power System Stability Stability of a power system is that property of a power system that keeps it in equilibrium in any condition [4] or it can be defined as the tendency of the system to recover from disturbances such as faults or changes of load. [5] We want our system to be very stable. After power systems deregulation it has become more important for the power companies to improve and optimize their output as consumers’ of today are more aware. They would not tolerate any disturbances in the power supply and would not bear any damage caused to their machinery because of voltage instability. It is now a prime objective to make our transmission and distribution networks stable and more reliable.
A. Thyristor Controlled Series Capacitor (TCSC) These devices belong to the first generation of FACTS devices. Thyristor controlled series capacitor are one of the FACTS devices used within the transmission lines for the improvement of power quality by suppressing the oscillations and controlling dynamic power flow. As shown in Figure-3, it basically involves connecting a series combination of oppositely poled thyristors and a reactor, parallel to the capacitance in the transmission lines. Primarily, problems were faced when distributing AC power over long distances due to increasing reactive impedances. Controlling the reactance of the reactor connected in parallel to the capacitor solves the problem, hence improved transmission efficiency. TCSC provides a fixed-series compensation on the parallel path to improve the power capabilities of a transmission line. This concept leads to more advantages like voltage stability, transient stability and damping of power oscillations. The control possessed on varying the firing angle through a control system also allows us to use this idea further for dynamic power flow. The relationship between reactance of the reactor and the firing angle is given below [6][7][8]. ܺ ሺߙሻ ൌ ܺ
ߨ ߨ െ ʹߙ െ ʹߙ
C. Unified Power Flow Controller (UPFC)
Where; ܺ = reactance of the reactor ߙ = firing angle
Figure-3 TCSC
B. Static Synchronous Series Compensator (SSSC) SSSC is a series compensating device. Its principle is similar to that of STATCOM which is discussed in detail, later in the paper. In SSSC the VSC is not connected in shunt but in series to the ac line. It is a synchronous voltage source which can provide controllable voltage magnitude and phase angle in series with the line. The voltage which is injected in the transmission line is almost in Quadrature (90 degree phase shift) with line current. This Quadrature voltage provides the effect of placing a capacitive or inductive reactance in series with the transmission line. [9][10] Figure3 shows a simplified diagram of a SSSC connected in series with a transmission line.
UPFC belongs to the second generation of the FACTS controllers. It is the most complex and flexible among other FACTS controllers. It has got independent control of both the real and reactive power, unlike other controllers. It can adjust almost all the parameters concerning power stability. It can adjust transmission line reactance, bus voltage and phase angle between the connecting buses. It has got a very strong and complex control mechanism through which it controls the inphase, Quadrature voltage and shunt reactive power compensation. It is used for transient, steady state and dynamic stability of the power systems. Figure - 5 shows a simplified diagram of a UPFC connected to a transmission line. It consists of two ac/dc converters and both sharing the same dc link capacitor, one of the converter is connected in parallel and other in series to the transmission line using shunt and series transformer. The series converter injects a voltage of controllable magnitude and phase. It has the ability to exchange real power with the line, which improves the power flow capability of the line and gives it more transient stability. The shunt converter injects current of controllable magnitude and power factor to the transmission system. Its main purpose is to maintain a balance of real power flow to or from the series converter and to minimize the losses by keeping the dc bus voltage at the desired value. [10][11]
Figure-5 Unified Power Flow Controller (UPFC)
D. Static VAR Compensator (SVC)
Figure-4 Static Synchronous Series Compensator (SSSC)
SVC is another application of FACTS which is nowadays used in transmission systems for improving voltage stability. In its starting years SVC was used mainly in heavy steel mills and in electric arc furnaces because load characteristics abruptly change. In recent times SVCs are mainly used in transmission lines at distribution and generation levels for enhancing reliability of system. In Extra High Voltage transmission lines step down transformer is
used with SVC. It is made of capacitance, reactors, thyristor valves and coupling transformers. SVC is doing all this by controlling shunt reactive power sources (capacitive and inductive) with high-tech power electronic devices. It can supply and absorb reactive power by controlling firing angle of thyristor elements. Capacitors and inductors are switched on and off from the circuit by the help of thyristors. In high voltage transmission networks SVC can provide compensation of reactive power hence improving the performance of electric services. It is dynamic because of the use of thyristor devices such as GTO, IGCT. SVCs advantage over compensation schemes which are mechanically switched is its fast response to changes in voltage. Some other benefits are; •
Reduction in transmission loses
•
Less voltage drop in high load areas
•
Improved transient stability
Gating angle
Inductance
Qind
Gating angle
Inductance
Qind
Qnet Qnet
Control of SVC Maintaining desired voltage at the high voltage bus is one of the control objectives of SVC. SVC will provide steady state voltage control for maintaining voltage bus at a set point. Reactive power (Qnet) will be injected by SVC if voltage bus falls below a desired level hence increasing the voltage at a desired level. If the voltage of the bus increases reactive power will be absorbed by SVC, hence voltage will come to its desired level. From Fig-6, +Qcap is fixed because of fixed capacitance value. Hence by varying -Qind amount of reactive power, Qnet injected in the system is controlled. Thyristor valve which controls the thyristor controlled reactor will now be discussed. The thyristor commutates by itself on every zero current, so by firing the thyristor at desired firing angle current, we control the reactance of the reactor. When the gating angle equal 90 degrees, full conduction is achieved. When gating angles are between 90 and 180 degrees partial conduction is achieved. Current’s fundamental harmonic component is reduced when gating angle is increased and it is equivalent to a rise in inductance of inductor. When the voltage bus falls below a desired set point, gating angle is decreased so decreasing the inductance, the effect of which is to decrease Qind and so Qnet increases. Hence, reactive power is being supplied by SVC. If voltage of the bus increases, gating angle is increased automatically by controller, increasing the inductance, the effect of which is to increase Qind. As Qind increases above Qcap direction of Qnet changes so reactive power is now being absorbed by the SVC. [12]
Qnet=Qcap - Qind
Figure-6 Control of SVC
E. Static Synchronous Compensator (STATCOM) Voltage Source Converter (VSC) VSC (Voltage Source Converter) is a solid state device used to produce an AC output using a DC input voltage source. This device basically is used as an inverter (DC to AC converter). A GTO (Gate Turn-Off Thyristor) or IGCT (Integrated Gate-Commutated Thyristor) or IGBT (Insulated Gate Bipolar Transistor) is used for switching purposes. Different sets of combinations of conducting GTOs/IGBTs/IGCTs are used to produce an alternating signal on the output load. Voltage Source Converters are used for many purposes which include Voltage Stability, Reactive Power compensation, elimination of current harmonics and Power Quality improvement. Voltage Source Converter can be used to produce an alternating output signal of the desired frequency, magnitude and phase angle which makes it more versatile. [13][14]
STATCOM STATCOM (Static Synchronous Comppensator) is a solid state device. It comprises of 3 major co omponents. A switching converter i.e. VSC (Voltage Source Converter), a group of step-down transformers and a DC capaacitor. Voltage instability is caused by a number of parameters. This problem can be solved by installing a com mpensator on the receiving end. In the past, Synchronous Conndensers were used as an aid to this problem. Due to mechaniccal switching, the response time was higher and so voltag ge fluctuation problem couldn’t be solved in due time. STAT TCOM can be useful because of its fast switching, and becausee of its ability to generate higher reactive power when the su upply voltage drops too low. The arrangement of a Synchronous Compensator is shown in Figure-7. STATCOM M can be used in both voltage control mode and current conttrol mode. In this case, STATCOM is used in voltage contrrol mode. As shown in the figure, the Voltage Source Conveerter converts the DC voltage input to a set of 3 phase AC vo oltage output, which is coupled with the 3 phase AC system via v a coupling transformer. The phase angle of the AC output from the Voltage Source Converter and the AC supply sh hould be kept same. This is essential or else it might result in active power flow. For understanding what is happening in this t circuit of figure-8 we will be selecting line to neutral vooltages which are of terminal A and U. Lets consider EAN and EUN •
If EAN = EUN current flow will be zero in the reactance y hence compensation will bee nill.
•
If EAN EUN current IA will flow which w will be lagging 90 degree behind EUN. Hence reeactive power will be drawn by the compensato or from the transmission line. Compensator is now like a inductor although no coils are present.
Figure 7: Principle of operation of o a STATCOM
E. Summary If we consider a two area m model i.e. there is a power network having sending end andd receiving end and in between FACTS controllers are connnected in shunt or series. The performance of these controlllers is presented in the Table-1. [15][16] TABLE I
SVC STATCOM TCSC SSSC UPFC
Load Flow Control
Voltage Control
Transient Stability
Dynamic Stability
o o oo ooo ooo
ooo ooo o o ooo
o oo ooo ooo ooo
oo oo oo oo oo
Influence: O Small OO Meddium OOO Strong It can be seen from Table-11 that UPFC provides the best solution to the problems faced by b power networks. SVC and STATCOM are best suited wheen voltage stability is the prime objective. Their performance is usually compared and it can be seen that STATCOM has an edge over SVC as it provides better transient stability. Evven at low voltages SVC fails to compensate but STATCOM M can efficiently provide reactive power even at low voltages.
IV. CONCLUSIIONS •
If EAN EUN current IA will lead EUN by b 90 degree. The value of IB will be same as in prev vious case but it will be negative so in this case conv verter will be delivering reactive power to the transsmission line. Converter will now act like a capaccitor although there are no electrostatic plates.
In this paper a general overrview of FACTS devices is given. It shows how these controlllers can be used in Power Transmission and Distribution netw works to make them more efficient. These controllers increasee the power transmitting capability of the lines with minim mal losses hence, greatly improving the efficiency of the power networks. Although conventional methods of power stabbility are still in operation at many places but these power elecctronics based controllers
have gain attention over the years for their dynamic, steady state and transient operation. These devices have shown remarkable improvement in the power transmission and power distribution networks. Still research is in progress to make most out of these controllers. STATCOM which is discussed in the paper is by far most suitable for reactive power compensation for voltage stability if compared with other FACTS devices. It is normally compared with SVC and results have shown that STATCOM is better in response and it can even deliver power at very low voltages as well, which is deficient in SVC. FACTS devices are not only applicable to Transmission networks but their application in power distribution networks is noteworthy. REFERENCES [1] Rodda Shobha Rani, B. Jyothi, “VSC Based DSTATCOM & Pulse-width modulation for Power Quality Improvement”, International Journal of Engineering Trends and TechnologyVo.l2 Issue 2- 2011. [2] Balwant K. Mehta, P. J. Patel, “Static Voltage Stability Improvement In Power System Using Statcom FACTS Controller”, Journal Of Information, Knowledge And Research In Electrical Engineering- Vol.02,Issue.02,2013. [3] P. S. Kundur, “Power System Stability and Control”, McGrawHill, New York, 1994. [4] E Acha, V G Agelidis, “Power Electronic Control in Electrical Systems”, Newness, 2006. [5] E Acha, V G Agelidis, “Power Electronic Control in Electrical Systems”, Newness, 2006. [6] S. Meikandasivam, Rajesh Kumar Nema and Shailendra Kumar Jain, “Behavioral Study of TCSC Device – A MATLAB/ Simulink Implementation”, World Academy of Science, Engineering and Technology Vol:2 2008-09-24. [7] John J. Paserba, Gregory F. Reed , Masatoshi Takeda and Tomohiko Aritsuka, “FACTS and Custom Power Equipment for the Enhancement of Power Transmission System Performance and Power Quality”. Symposium of Specialists in Electric Operational and Expansion Planning (VII SEPOPE) Curitiba, Brazil, May 21-26, 2000. [8] Parmar Hiren .S, Vamsi Krishna .K and Ranjit Roy, “Shunt Compensation for Power Quality Improvement using a STATCOM controller”. ACEEE International Journal on Electrical and Power Engineering, Vol. 1, No. 2, July 2010. [9] D. Murali, Dr. M. Rajaram, “Active and Reactive Power Flow Control using FACTS Devices”, International Journal of Computer Applications (0975 – 8887) Volume 9– No.8, November 2010. [10] Vireshkumar G. Mathad, Basangouda F. Ronad , Suresh H. Jangamshetti, “Review on Comparison of FACTS Controllers for Power System Stability Enhancement” ,International Journal of Scientific and Research Publications, Volume 3, Issue 3, March 2013. [11] N. G. Hingorani and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible AC Transmission Systems”. New York: IEEE Press, 2000. [12] MD M.Biswas, Kamol K. Das,”Voltage level improving by Using Statcom VAR Compensator”, Global journals Inc (USA), Volume 11 Issue 5 Ver.1, 2011.
[13] Alok Kumar Mohanty, Amar Kumar Barik, “Power System Stability Improvement Using FACTS Devices”, International Journal of Modern Engineering Research (IJMER) Vol.01, Issue.02, pp-666-672. [14] D. Murali, Dr. M. Rajaram, N. Reka, “Comparison of FACTS Devices for Power System Stability Enhancement”, International Journal of Computer Applications (0975 – 8887) Vol.08– No.4, 2010. [15] A. Sode-Yome, N. Mithulananthan, Kwang Y. Lee, “A Comprehensive Comparison of FACTS Devices for Enhancing Static Voltage Stability” IEEE, 2007. [16] Mehrdad Ahmadi Kamarposhti, Mostafa Alinezhad, Hamid Lesani, Nemat Talebi, “Comparison of SVC, STATCOM, TCSC, and UPFC Controllers for Static Voltage Stability Evaluated by Continuation Power Flow Method” IEEE Electrical Power & Energy Conference, 2008.
