International Journal of Electrical, Electronics and Computer Systems, (IJEECS)
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EFFECTS OF UPFC FOR POWER FLOW CONTROL AND DAMPING OF OSCILLATIONS IN A POWER SYSTEM
1Rudresh S J, 2Kumudeesh K C, 3Neetha H.M, 4Chaitra. C, 5Vishwas.S
1,2,3,4,5Assistant Professors, Department of Electrical and Electronics, PES Institute of Technology and Management,
Shimoga, Karnataka, India Email : [email protected]
Abstract - Electrical power system is a large interconnected network that requires a careful design to maintain the system with continuous power flow operation without any limitations. The technology of power system utilities around the world has rapidly evolved with considerable changes in the technology along with improvements in power system structures and operation.
The ongoing expansions and growth in the technology, demand a more optimal and profitable operation of a power system with respect to generation, transmission and distribution systems. The solution is the use of FACTS devices especially the use of UPFC. The objective of this paper is to study the effect of UPFC with its various modes of operation such as power flow control, voltage injection.
Later, damping of oscillations in a power system is also studied. In this paper the MATLAB/Simulink software is used to model the power system by installing UPFC in transmission link to study its use as power flow controller and damping the power system oscillations. The simulations results were presented and found satisfactory.
I. INTRODUCTION
The technology of power system utilities around the world has rapidly evolved with considerable changes in the technology along with improvements in power system structures and operation. The ongoing expansions and growth in the technology, demand a more optimal and profitable operation of a power system with respect to generation, transmission and distribution systems. In the present scenario, most of the power systems in the developing countries with large interconnected networks share the generation reserves to increase the reliability of the power system. However, the increasing complexities of large interconnected networks had fluctuations in reliability of power supply, which resulted in system instability, difficult to control the power flow and security problems that resulted large
number blackouts in different parts of the world. The reasons behind the above fault sequences may be due to the systematical errors in planning and operation, weak interconnection of the power system, lack of maintenance or due to overload of the network. In the late 1980’s the Electric Power Research Institute (EPRI) introduced a concept of technology to improve the power flow, improve the system stability and reliability with the existing power systems. This technology of power electronic devices is termed as Flexible Alternating Current Transmission Systems (FACTS) technology. The two main objectives of FACTS are to increase the transmission capacity and control power flow over designated transmission routes. FACTS controllers are capable of controlling the network condition in a very fast manner and this feature of FACTS can be exploited to improve the voltage stability, and steady state and transient stabilities of a complex power system.The improvements in the field of power electronics have had major impact on the development of facts controllers. These controllers are based on voltage source converters and include devices such as Static Var Compensators (SVC), Static Synchronous Compensators (STATCOM), Thyristor Controlled Series Compensators (TCSC), the Static Synchronous Series Compensators (SSSC), and the Unified Power Flow Controller (UPFC).
II. UNIFIED POWER FLOW CONTROLLER (UPFC)
UPFC is capable of both supplying to and absorbing from the power system through the excitation converter and transformer a controllable amount of reactive power and inserting a voltage of controllable magnitude and phase angle in series with the transmission system through the converter and transformer. The UPFC can
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provide simultaneous and/or independent control of all basic power system parameters, which are transmission voltage, impedance and phase angle. Such "new"
FACTS device combines together the features of two
"old" FACTS devices: the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC).
In practice, these two devices are two Voltage Source Inverters (VSI’s) connected respectively in shunt with the transmission line through a shunt transformer and in series with the transmission line through a series transformer, connected to each other by a common dc link including a storage capacitor. The shunt inverter is used for voltage regulation at the point of connection injecting an opportune reactive power flow into the line and to balance the real power flow exchanged between the series inverter and the transmission line. The series inverter can be used to control the real and reactive line power flow inserting an opportune voltage with controllable magnitude and phase in series with the transmission line. Thereby, the UPFC can fulfill functions of reactive shunt compensation, active and reactive series compensation and phase shifting.
Besides, the UPFC allows a secondary but important function such as stability control to suppress power system oscillations improving the transient stability of power system. As the need for flexible and fast power flow controllers, such as the UPFC, is expected to grow in the future due to the changes in the electricity markets, there is a corresponding need for reliable and realistic models of these controllers to investigate the impact of them on the performance of the power system.
2.1 Basic operating principles of UPFC
The main function of UPFC is to control the flow of real and reactive power by injection of a voltage in series with transmission line. Both the magnitude and the phase angle of the voltage can be varied independently.
Real and reactive power flow control can allow for power flow in prescribed routes; loading of transmission lines closer to their thermal limits and can be utilized for improving transient and small signal stability of the power system. The UPFC consists of two voltage sourced converters, connected back-to-back and are operated from a common dc link provided by a storage capacitor as shown in the figure 1
. Figure 1. Implementation of the UPFC by two back-to-
back voltage sourced converter
The arrangement shown in the figure 3.1 functions as an ideal ac-to-ac power converter in which the real power can freely flow in either direction between the ac terminals of the two converters, and each converter can independently generate (or absorb) reactive power at its own ac output terminal.
Converter 2 provides the main function of the UPFC by injecting a voltage Vpq with controllable magnitude Vpq and phase angle in series with the line via an insertion transformer. This injected voltage acts essentially as a synchronous ac voltage source. The transmission line current flows through this voltage source resulting in reactive and real power exchange between it and the ac system. The reactive power exchanged at the ac terminal is generated internally by the converter. The real power exchanged at the ac terminal is converted into dc power which appears at the dc link as a positive or negative real power demand.
The basic function of converter 1 is to supply or absorb the real power demanded by converter 2 at the common dc link to support the real power exchange resulting from the series voltage injection. This dc link power demand of converter 2 is converted back to ac by converter 1 and coupled to the transmission line bus via a shunt connected transformer. In addition to the real power need of converter 2, converter 1 can also generate or absorb controllable reactive power, if it is desired, and thereby provide independent shunt reactive compensation for the line.
III. CASE STUDY AND SIMULATION RESULTS
3.1 Case study 1-Power Flow Control with the UPFC To illustrate the study and effects of UPFC the Test system is considered and its Single Line diagram is shown in figure 2.
Figure 2. Single Line Diagram of the Multimachine system
Explanation of Single Line Diagram
In a 500 kV /230 kV transmission system, which is connected in a loop configuration as shown in the figure 4 consists essentially of five buses (B1to B5) interconnected through three transmission lines (L1, L2,
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L3) and two 500 kV/230 kV transformer banks Tr1 and Tr2. Two power plants located on the 230kV system generate a total of 1500 MW which is transmitted to a 500 kV, 15000 MVA equivalent connected at bus B5 and to a 200 MW load connected at bus B3. The plant model includes a speed regulator, an excitation system as well as a power system stabilizer (PSS).
In normal operation, most of the 1200 MW generation capacity of power plant #2 is exported to the 500 kV equivalents through two 400 MVA transformers connected between buses B4 and B5. For this illustration we consider a case where only two transformers out of three are available (Tr2= 2*400 MVA = 800 MVA). The simulation shows that most of the power generated by plant #2 is transmitted through the 800 MVA transformer bank (901 MW out of 1000 MW) and that 92.58 MW is circulating in the loop. Transformer Tr2 is therefore overloaded by 101 MVA. This power congestion can be relieved by placing the UPFC in the transmission line as shown in figure 3.
Figure 3. Single Line Diagram of the Test system with UPFC
The above test system is modeled in MATLAB- SIMULINK environment as shown in figure 4.
Parameters of the UPFC: the series converter is rated 100 MVA with a maximum voltage injection of 0.1 pu.
The shunt converter is also rated 100 MVA. The DC link nominal voltage (Vdc) is 40KV and DC link total equivalent capacitance(C) is 750µF.
The UPFC located at the right end of line L2 as shown in figure 4.4 is used to control the active and reactive powers at the 500 KV bus B3, as well as the voltage at bus B_UPFC. The UPFC consists of two 100 MVA, IGBT-based, converters (one shunt converter and one series converter interconnected through a DC bus).The important keys to note in the block diagram are:
Use of Bypass breaker – Used to connect or disconnect UPFC Block from Power System
The reference power inputs [Pref, Qref] – Reference for power flow control
The reference voltage Vdqref – Reference for voltage injection
The series converter of UPFC can inject a maximum of 10% of nominal line-to-ground voltage (28.87 KV) in series with line L2.
Figure 4. MATLAB- SIMULINK Model of single line diagram with UPFC
SIMULATION RESULTS:
Power Flow Control with the UPFC
Figure 5. P Pref, Q Qref, Voltage Mag (p.u), Voltage phase (deg) of the UPFC
Figure 6. Voltage, Real Power and Reactive power at Buses
In figure 5 it shows how P and Q measured at bus B3 follow the reference values of converter. The Bypass breaker is opened at t=5sec the natural power is diverted from the breaker to the UPFC series branch without noticeable transient. At t=10 sec, the power increases to 684MW at a rate of 1 pu/sec. This increase of 100 MW
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at bus B3 is achieved by injecting a series voltage of 0.091 pu with an angle of 95 degrees. This results in an approximate 100 MW decrease in the active power flowing through Tr2 which now carries an acceptable load i.e. active power decrease from 901 MW to 799 MW at bus 4.
The figure 6 shows the variations of voltages, active powers and reactive powers at buses B1 to B5 reflect the above.
4. Case study 2- Performance of UPFC on damping of power system oscillations
Low frequency electromechanical oscillations are inevitable characteristics of power systems and they greatly affect the transmission line transfer capability and power system stability. PSS and UPFC devices can help the damping of power system oscillations.
In this case the same test system shown in the figure 3 is used to study the effect of UPFC on damping the power system oscillations under fault condition. A three phase fault is considered near to the bus B1 in the test system and is modeled in the simulink.
Figure 7. MATLAB-SIMULINK Model of the single line diagram with Fault
In this case the study of effect of UPFC on damping the power system oscillations under fault condition is studied with and without PSS.
The two machines are equipped with a Hydraulic Turbine and Governor (HTG), Excitation system and Power System Stabilizer (PSS). These blocks are located in the two 'Turbine and Regulator' subsystems.
In this case a 3-phase fault is applied near to the bus B1 using Fault breaker and select the parameters "Switching of phase A, B and C" in the Fault breaker to simulate a three-phase fault.
The magnitude of the fault and the time of application of the fault are kept constant.
The transition time is set as 1s to 1.15s. This means that the fault will be applied at 1 s and is cleared at 1.15s.
The various cases considered under the fault condition are:
Case A - Multimachine system without PSS as well as UPFC
Case B- Multimachine system without PSS but having UPFC
Case C- Multimachine system with PSS and without UPFC
Case D- Multimachine system with PSS and UPFC.
In this multimachine system the damping of oscillations is tested for all the above cases with respect to Rotor Speed (wm) and Load angle (Delta) and simulation results are presented.
Case A- Multimachine system without PSS as well as UPFC
In the figure 8 and 9 it is observed that oscillations in the Rotor Speed (wm) and Load angle (Delta) of Generators G1 and G2 are sustained in the system.
SIMULATION RESULTS:
Figure 8. Rotor speed variations of G1 and G2 without PSS as well as UPFC
Figure 9. Load angle (Delta) variations of G1 and G2 without PSS as well as UPFC
Case B- Multimachine system without PSS but having UPFC
Simulation results are shown below, it is observed that Rotor Speed (wm) and Load angle (Delta) oscillations are damped by the UPFC is shown in figure 10 and 11.
Figure 10. Rotor speed (Delta) variations of G1 and G2 without PSS but having UPFC
Figure 11. Load angle (Delta) variations of G1 and G2 without PSS but having UPFC
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Case C-Multimachine system with PSS and without UPFC
Simulation result for this case is shown in figure 12 and 13 with respect to Rotor Speed (wm) and Load angle (Delta).
Figure 12. Rotor speed (wm) variations of G1 and G2 with PSS and without UPFC
Figure 13. Load angle (Delta) variations of G1 and G2 with PSS and without UPFC
Case D-Multimachine system with PSS and UPFC.
Simulation result for this case is shown in figure 14 and 15 with respect to Rotor Speed (wm) and Load angle (Delta).From the result it is observed that the oscillations are well damped in the system with UPFC.
Figure 14. Rotor speed (wm) variations of G1 and G2 with PSS and UPFC
Figure 15. Load angle (Delta) variations of G1 and G2 with PSS and UPFC
CONCLUSION
In power system transmission, it is desirable to maintain the voltage magnitude, phase angle and line impedance to improve power system operation and stability.
Therefore to control the power from one end to another end, the concept of power flow control and the concept of voltage injection using UPFC are applied. Modeling the system in MATLAB-SIMULINK and studying the simulation have given an indication that UPFC are very useful to control power flow in a transmission line using its various modes of operation
The Low frequency electromechanical oscillations are inevitable characteristics of power systems and they greatly affect the transmission line transfer capability and power system stability. Hence the Damping of oscillations using UPFC is analyzed in the system under fault condition. In brief the Following conclusions are made
Power flow control is achieved and congestion is less.
Improved Voltage Profile
Faster Steady State achievement.
Damping of oscillations in a power system.
REFERENCES
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