STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNIFIED POWER FLOW CONTROLLER

Transcription

1 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNIFIED POWER FLOW CONTROLLER SATYENDRA KUMAR *, ARVIND KUMAR SINGH ** AND UPENDRA PRASAD *** Abstract: Electrical power systems is a large interconnected network that requires a careful design to maintain the system with continuous power flow operation without any limitations. Flexible Alternating Current Transmission System (FACTS) is an application of a power electronics device to control the power flow and to improve the system stability of a power system. Unified Power Flow Controller (UPFC) is a versatile device in the FACTS family of controllers which has the ability to simultaneously control all the transmission parameters of power systems i.e. voltage, impedance and phase angle which determines the power flow of a transmission line. 1. 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 [1]. 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 * Asst. Professor, EEE Dept., Gurunanadev Engg. College, Bidar, Karnataka 58540, ( Satyendra.satyendra@Gmail.com) ** Elect. Dept., Nerist, Nirjuli, Itanagar, Arnachal Pradesh *** Professor, Elect. Engg. Dept., Bit Sindri, Dhanbad, Jharkhand IJPE, 4:1 (01): 1-11 Research Science Press, New Delhi, India

2 / IJPE, 4(1) 01 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 order to overcome these consequences and to provide the desired power flow along with system stability and reliability, installations of new transmission lines are required. However, installation of new transmission lines with the large interconnected power system are limited to some of the factors like economic cost, environment related issues. These complexities in installing new transmission lines in a power system challenges the power engineers to research on the ways to increase the power flow with the existing transmission line without reduction in system stability and security. In this research process, 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. It provides the ability to increase the controllability and to improve the transmission system operation in terms of power flow, stability limits with advanced control technologies in the existing power systems [3, 4]. The main objective to introduce FACTS Technology is as follows: To increase the power transfer capability of a transmission network in a power system. To provide the direct control of power flow over designated transmission routes. To provide secure loading of a transmission lines near the thermal limits. To improve the damping of oscillations as this can threaten security or limit usage line capacity [5]. FACTS technology is not a single power electronic device but a collection of controllers that are applied individually or in coordination with other devices to control one or more interrelated power system parameters such as series impedance, shunt impedance, current, voltage and damping of oscillations. These controllers were designed based on the concept of FACTS technology known as FACTS Controllers [5].

3 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNITED / 3 FACTS controllers are advanced in relation to mechanical control switched systems that are controlled with ease. They have the ability to control the power flow and improve the performance of the power system without changing the topology. Since 1980s, a number of different FACTS controllers with advanced control techniques proposed as per the demand of the power systems [5]. Unified Power Flow Controller (UPFC) is one among the different FACTS controllers introduced to improve the power flow control with stability and reliability. It is the most versatile device introduced in early 1990s designed based on the concept of combined series-shunt FACTS Controller. It has the ability to simultaneously control all the transmission parameters affecting the power flow of a transmission line i.e. voltage, line impedance and phase angle []. Aim of the Paper: In this Paper, I considered a case study network of a power system with Unified Power Flow Controller (UPFC). The power flow equations derived for the network solved using the Newton-Raphson Algorithm and the simulations of the algorithm carried out in MATLAB.. THE UNIFIED POWER FLOW CONTROLLER Gyugyi in 1991 proposed the Unified Power Flow Controller. It is the most versatile and complex power electronic device and member of third generation FACTS Controller introduced to control the power flow and voltage in the power systems. It is designed by combining the features of second-generation FACTS controllers Series Synchronous Compensator (SSSC) and Static Synchronous Compensator (STATCOM). It has the ability to control active and reactive power flow of a transmission line simultaneously in addition to controlling all the transmission parameters (voltage, impedance and phase angle) affecting the power flow in a transmission line. Figure 1: Unified Power Flow Controller [6]

4 4 / IJPE, 4(1) UPFC Circuit Description The above Figure 1 taken from reference [6] gives a clear description about how UPFC controller connected to a transmission line. It consists of two back-to-back self-commutated voltage source converters - one converter at the sending end is connected in shunt as shunt converter and the other converter connected in between sending and receiving end bus in series as series converter. One end of the both the converters are connected to a power system through an appropriate transformer and other end connected with a common DC capacitor link [6]... Operation of UPFC This arrangement of UPFC ideally works as a ideal ac to dc power converter in which real power can freely flow in either direction between ac terminals of the two converters and each converter can independently generate or absorb reactive power at its own AC output terminal. The main functionality of UPFC provided by shunt converter by injecting an ac voltage considered as a synchronous ac voltage source with controllable phase angle and magnitude in series with the line. The transmission line current flowing through this voltage source results in real and reactive power exchange between it and the AC transmission system. The inverter converts the real power exchanged at ac terminals into dc power which appears at the dc link as positive or negative real power demand [3]..3. Operation of Two Converters Series converter Operation: In the series converter, the voltage injected can be determined in different modes of operation: direct voltage injection mode, phase angle shift emulation mode, Line impedance emulation mode and automatic power flow control mode. Although there are different operating modes to obtain the voltage, usually the series converter operates in automatic power flow control mode where the reference input values of P and Q maintain on the transmission line despite the system changes [3]. Shunt converter operation: The shunt converter operated in such a way to demand the dc terminal power (positive or negative) from the line keeping the voltage across the storage capacitor Vdc constant. Shunt converter operates in two modes: VAR Control mode and Automatic Voltage Control mode. Typically, Shunt converter in UPFC operates in Automatic voltage control mode [3].

5 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNITED / 5.4. Equivalent Circuit Operation of UPFC As shown in Figure, the two-voltage source converters of UPFC can modeled as two ideal voltage sources one connected in series and other in shunt between the two buses. The output of series voltage magnitude V se controlled between the limits V se max V se V se min and the angle θ se between the limits 0 θ se respectively. The shunt voltage magnitude V sh controlled between the limits V sh max V sh V sh min and the angle between 0 θ sh respectively. Z se and Z sh are considered as the impedances of the two transformers one connected in series and other in shunt between the transmission line and the UPFC as shown in the Figure which is the UPFC equivalent circuit [11]. Figure : Equivalent Circuit of UPFC [8] as The ideal series and voltage source from the Figure can written V = V/ (cosθ + j sin) θ (1) se se se se V = V (cos θ + j sin) θ () sh sh sh sh The magnitude and the angle of the converter output voltage used to control the power flow mode and voltage at the nodes as follows: (1) The bus voltage magnitude can be controlled by the injected a series voltage V se in phase or anti-phase. () Power flow as a series reactive compensation controlled by injecting a series voltage V se in quadrature to the line current.

6 6 / IJPE, 4(1) 01 (3) Power flow as phase shifter controlled by injecting a series voltage of magnitude V se in quadrature to node voltage θ m [8]. UPFC power Equations Based on the equivalent circuit as shown in Figure, the active and reactive power equations can be written as follows [7, 7]: At node k: P = V G + V V ( G cos() θ θ sin()) + B θ θ k k kk k m km k m km k m + V Vse( G cos( θ θ + B sin()) θ θ k km k se) km k + V V ( G cos() θ θ sin()) + B θ θ k sh sh k sh sh k sh Q = V B + V V ( G sin() θ θ cos()) B θ θ k k kk k m km k m km k m + V V ( G sin() θ θ cos()) B θ θ k se km k se km k se + V V ( G sin() θ θ cos()) B θ θ k sh sh k sh sh k sh se (3) (4) At node m: P = V mg + V V ( G cos() θ θ sin()) + B θ θ m mm m k mk m k mk m k + V V ( G cos() θ θ sin()) + B θ θ m se mm m se mm m se Q = V mb + V V ( G sin() θ θ cos()) B θ θ m mm m k mk m k mk m k + V V ( G sin() θ θ cos()) B θ θ m sh mm m se mm m se Series converter: P = V seg + V V ( G cos() θ θ sin()) + B θ θ se mm se k km se k km se k + V V ( G cos() θ θ sin() + B θ θ se m mm se k mm se m (5) (6) (7) Q = V seb + V V ( G sin() θ θ cos()) B θ θ se mm se k km se k km se k + V V ( G sin() θ θ cos()) B θ θ se m mm se m mm se m Shunt converter: P = V shg + V V ( G cos() θ θ sin() + B θ θ (8) sh sh sh k sh sh k sh sh k Where Q = V shb + V V ( G sin() θ θcos()) B θ θ (9) sh sh sh k sh sh k sh sh k Y G jb Z Z (10) 1 1 kk = kk + kk = se + sh 1 Ymm = Gmm + jbmm = Z se (11)

7 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNITED / 7 Y Y G jb Z (1) = = + = 1 km mk km km se Y G jb Z (13) = + = 1 sh sh sh sh Assuming a free converter loss operation, the active power supplied to the shunt converter P sh equals to the active power demanded by the series converter P se [10]. P + P = 0 (14) se Furthermore if the coupling transformers are assumed to contain no resistance then the active power at bus k matches the active power at bus m; that is, sh P + P = P + P = 0 (15) sh se The UPFC power equations linearised and combined with the equations of the AC transmission network. For the cases when the UPFC controls the following parameters: (1) voltage magnitude at the shunt converter terminal () active power flow from bus m to bus k and k (3) reactive power injected at bus m, and taking bus m to be PQ bus. 3. NEWTON RAPHSON ALGORITHM AND FLOW CHART From the mathematical modeling point of view, the set of nonlinear, algebraic equations that describe the electrical power network under the steady state conditions are solved for the power flow solutions. Over the years, several approaches have been put forward to solve for the power flow equations. Early approaches were based on the loop equations and methods using Gauss-type solutions. This method was laborious because the network loops has to be specified by hand by the systems engineer. The drawback of these algorithms is that they exhibit poor convergence characteristics when applied to the solution of the networks. To overcome such limitations, the Newton-Raphson method and derived formulations were developed in the early 1970s and since then it became firmly established throughout the power system industry [7]. In this Paper a Newton Raphson power flow algorithm is used to solve for the power flow problem in a transmission line with UPFC as shown in the flow chart in Figure 3 [18] Steps to Solve the Newton-Raphson Algorithm Step 1: Read the input of the system data that includes the data needed for conventional power flow calculation i.e. the number and types of m

8 8 / IJPE, 4(1) 01 buses, transmission line data, generation, load data and location of UPFC and the control variables of UPFC i.e. the magnitude and angles of output voltage series and shunt converters. Step : Formation of admittance matrix Y bus of the transmission line between the bus i and j. Step 3: Combining the UPFC power equations with network equation, we get the conventional power flow equation: n ' ' P + jq = VV Y () θ δ + δ + P i + jq i i i i j ij ij i j j= 1 (16) Where P i + Q i = active and reactive power flow due to UPFC between the two buses.

9 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNITED / 9 Figure 3: Flow Chart for load flow by Newton Raphson with UPFC [18] P i + jq i Active and reactive power flow at the i th bus. V i δ i Voltage and angle of i th bus V j δ j = Voltage and angle at i th bus Step 5: The conventional jacobian matrix are formed ( P k i and Q k i ) due to the inclusion of UPFC. The inclusion of these variables increases the dimensions of the jacobian matrix. Step 6: In this step, the jacobian matrix is modified and power equations are mismatched ( P k i, Qk i for i =, 3,, m and Pk ii, Qk ii ). Step 7: The busbar voltages are updated at each iteration and convergence is checked.

10 10 / IJPE, 4(1) 01 If convergence is not achieved in the next step the algorithm goes back to the step 6 and the jacobian matrix is modified and the power equations are mismatched until convergence is attained. Step 8: If the convergence achieved in Step 7, the output load flow is calculated for PQ bus that includes the Busbar voltages, generation, transmission line flow and losses. REFERENCES [1] R. Billinton, L. Salvaderi, J. D. McCalley, H. Chao, Th. Seitz, R.N. Allan, J. Odom, C. Fallon, Reliability Issues In Today s Electric Power Utility Environment, IEEE Transactions on Power Systems, Vol. 1, No. 4, November [] Jinfu Chen, Xinghua Wang, Xianzhong Duan, Daguang Wang, Ronglin Zhang, Application of FACTS Devices for the Interconnected Line Between Fujian Network and Huadong Network, IEEE. [3] S. Tara Kalyani, G. Tulasiram Das, Simulation of Real and Reactive Power Flow Control With UPFC connected to a Transmission Line, Journal of Theoretical and Applied Information Technology, 008. [4] S. Y. Ge, T S Chung, Optimal Active Power Flow Incorporating Power Flow Control Needs In Flexible AC Transmission Systems, IEEE Transactions on Power Systems, Vol. 14, No., 009. [5] K. R. Padiyar, A. M. Kulkarni, Flexible AC Transmission Systems: A Status Review, Sadhana, Vol., Part 6, pp , December [6] John J. Paserba, How FACTS Controllers Benefit AC Transmission Systems, IEEE. [7] N. G. Hingorani G. Gyugyi Lazlo, Understanding FACTS: Concepts & Technology of Flexible AC Transmission Systems ISBN [8] Nadarajah Muthulananthan, Arthit Sode-yome, Mr. Naresh Acharya, Application of FACTS Controllers in Thailand Power Systems, Asian Institute of Technology, Jan 005. [9] Jody Verboomen, Dirk Van Hertem, Pieter H. Schavemaker, Wil L. Kling, Ronnie Belmans, Phase Shifting Transfomers: Princples and Application, IEEE. [10] M. P. Bahrman, P.E., HVDC Transmission Overview, IEEE. [11] W. Breuer, D. Povh, D. Retxmann, E. Teltsch, Trends for Future HVDC Applications, 16 th Conference of Electric Power Supply Industry, November 006. [1] Rajiv K. Varma, Introduction to FACTS Controllers, Member, IEEE. [13] M. Noroozian, C. W. Taylor, Benefits of SVC and STATCOM for Electric Utility Application. [14] Mark Ndubuka NWOHU, Voltage Stability Improvement using Static Var Compensator in Power Systems, Leonardo Journal of Sciences, Issue 14, p , January-June 009.

11 STUDY THE POWER FLOW CONTROL OF A POWER SYSTEM WITH UNITED / 11 [15] Sidhartha Panda and N. P. Padhy, Thyristor Controlled Series Compensatorbased Controller Design Employing Genetic Algorithm: A Comparative Study, International Journal of Electronics, Vol. 1. [16] Mazilah Binti A Rahman, Overview of Thyristor Controlled Series Capacitor (TCSC) In Power Transmission System. [17] Laszlo Gyugyi, Colin D. Schauder, Kalyan K. Sen, Static Synchronous Series Compensator: A Solid-State Approach To The Series Compensation of Transmission Lines, IEEE Transaction on Power Delivery, Vol.1, January [18] Nitus Voraphonpiput, Teratam Bunyagul and Somchai Chatratana, Power Flow Control with Static Synchronous Series Compensator (SSSC). [19] M. Noroozian, C. W. Taylor, Benefits of SVC and STATCOM for Electric Utility Application. [0] Kalyan K. Sen, STATCOM Static Synchronous Compensator: Theory, Modelling and Applications. [1] Rusejla Sadikovic, Power flow Control with UPFC. [] X.P. Zhang, Robust Modeling of the Interline Power Flow Controller and the Generalized Unified Power Flow Controller with Small Impedances in Power Flow Analysis, Electrical Engineering, Vol. 89, pp. 1-9, 006. [3] Yankui Zhang, Yan Zhang and Chen Chen, A Novel Power Injection Model of IPFC for Power Flow Analysis Inclusive of Practical Constraints, IEEE Transactions on Power Systems, Vol. 1, November 006. [4] Bhanu Chennapragada Venkata Krishna, Kotamarti S. B. Sankar, Pindiprolu. V. Haranath, Power System Operation and Control Using FACT Devices, 17 th International Conference on Electricity Distribution, 1-15 May 003. [5] Bhanu Chennapragada Venkata Krishna, Kotamarti S. B. Sankar, Pindiprolu. V. Haranath, Power System Operation and Control Using FACT Devices, 17 th International Conference on Electricity Distribution, 1-15 May 003. [6] C. Bulac, M. Eremaia, R. Balaurescu and V. Stefanescu, Load Flow Management in the Interconnected Power Systems Using UPFC Devices, 003 IEEE Bologna Power Tech Conference, Bologna, Italy, June 3th-6 th. [7] Samina Elyas Mubeen, R. K. Nema and Gayatri Agnihotri, Power Flow Control with UPFC in Power Transmission System, World Academy of Science, 008. [8] Felix F. Wu, Technical Considerations For Power Grid Interconnection in Northeast Asia.