STABILITY ANALYSIS OF VARIABLE FREQUENCY TRANSFORMER PARK USING ADVANCED CONTROLLING SCHEME S.VIJAY

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1 STABILITY ANALYSIS OF VARIABLE FREQUENCY TRANSFORMER PARK USING ADVANCED CONTROLLING SCHEME ABSTRACT S.VIJAY Department of EEE, Sasurie College of Engineering, Tamilnadu When HVDC link is used for interconnection, it requires a very costly converter plant at sending end and an inverter plant at receiving end. So a new technology known as Variable Frequency Transformer (VFT) has been developed for transmission interconnections. A VFT is a controllable, bidirectional transmission device that can transfer power between asynchronous networks. VFT park model proposed in this paper allows the simulation of multi unit VFT for increasing power transfer between two electric networks. Each VFT unit consists of round rotor synchronous machine, DC motor and a control system. In order to determine the stability of a VFT, a new approach based on Sequential Continuation Scheme and the Limit Cycle Method are presented. The computation of stability regions of VFT control system provides information for tuning ANFIC. Here PSO based ANFIC and ACO based ANFIC are used for power regulation and speed regulation of VFT control system. Stability diagram are reported for changes of power transfer and frequency on both sides of the asynchronous link. Sequential continuation Scheme determines how solutions vary with a certain parameter and Limit Cycle Method finds the steady state operating point. This will help to carry out the analysis more effectively. The effectiveness of this model is verified by the simulation results concerning the standard IEEE 17 bus system and 118 bus systems. The comparative results will be also shown for variations in power and frequency when PSO based ANFIC and ACO based ANFIC is incorporated in VFT control system. INTRODUCTION The electric power supply systems are widely interconnected, involving connections inside utilities own territories which extend to inter-utility interconnections and then to inter-regional and international connections. This is done to reduce the cost of electricity and to improve reliability of power supply. These interconnections are needed because, the purpose of the transmission network is to pool power plants and load centers in order to minimize the total power generation capacity and fuel cost. There are two ways of transmission interconnection. One is AC interconnection, which is simple and economic but increases the complexity of power system operation and decreases the stabilities of the power system under some serious faults. The other one is known as Back-to-Back HVDC which is asynchronous interconnection. It is easy for bulk power transfer and also flexible for system operation. The HVDC link requires a very costly converter plant at sending end and an inverter plant at receiving end. Recently, a new technology known as variable frequency transformer (VFT) has been developed for transmission interconnection. Thus, power transmission can be controlled within and between power system networks in a desired way [15]. 1

2 Variable frequency transformer is a controllable bidirectional transmission device that can transfer power between asynchronous networks. Stability analysis of VFT has a significant influence on the study of stability of asynchronous links and the power transfer between two electric power networks. Suppose any faults or sudden load change occurs it may cause a voltage dip and in frequency deviation that results in the operation of VFT control system. VFT control system measures the power; shaft speed along with other active functions develops a torque command. Different methods have been proposed for stability analyses of VFT, among them Sequential Continuation Scheme and Limit Cycle are the most popular methods. The PSO and Ant Colony algorithms based ANFIC controllers are used for power regulation and speed regulation. SCOPE OF THE PROJECT When back to back HVDC or other conventional devices are used for power transfer, operation becomes step wise and slow. Also they are complicated and expensive. To overcome these disadvantages VFT has been developed for transmission interconnections. By adding different devices with it, power transmission or power flow can be controlled within and between power system networks in a desired way. When power is transferred through VFT, there is a chance for the occurrence of stability problems. This will affect the synchronous operation of the power system and increases the losses. To analyze stability problem, sequential continuation scheme and limit cycle methods are used. Stability of Variable Frequency Transformer can be analyzed. The steady state operating point of VFT Park is computed with the Limit Cycle Method, while stability analysis of the VFT Park is carried out with a Sequential Continuation Scheme and Floquet multipliers. Sequential Continuation Scheme determines how solutions vary with a certain parameter. To improve the power transfer between two electric networks. Power system interconnection increases the complexity of the operation and decreases the stability of the power system under some serious faults. A stable power exchange between the two asynchronous systems is possible by connecting them with VFT. VFT is a controllable bidirectional transmission device which is capable of exchanging electrical power between two networks, by controlling the speed and torque applied to the rotor. POWER MANAGEMENT Power system engineering is the technical discipline applied to design the electric power system for high power, electric utility applications as well as industrial and commercial power distribution systems. The objective in power systems operation is to serve energy with acceptable voltage and frequency to consumers at minimum cost. All these objectives can be achieved by proper planning, operation and control of power generation and transmission systems. Power management is the engineering aspect of managing the electric loads such that available power sources are not overloaded and power is allocated to the different loads such that the loads are receiving a proper allocation of power. Power management is desired for reducing overall energy consumption and to reduce operating costs for energy. IMPORTANCE OF POWER MANAGEMENT 2

3 Power management is an understanding of how to effectively optimize energy consumption of each system component. Power management in electric systems is desired for many reasons, Reducing overall power consumption to save cost. Reduce overall energy consumption. Reduce operating costs for energy and cooling. The proper use of power management results in, Heat reduction for servers and computing centers. Reduced secondary costs, including cooling, space, cables, generators, and UPS. Extended battery life for laptops. Lower carbon dioxide output. Meeting government regulations or legal requirements. Meeting company guidelines for new systems. Electric-power transmission is the bulk transfer of electric energy, from generating power plants to electrical substations located near demand centers. Transmission lines, when interconnected with each other, become transmission networks, these are typically referred to as power grids [11]. High voltage direct current (HVDC) technology is used for greater efficiency in very long distances, to stabilize against control problems in large power distribution networks where sudden new loads or blackouts in one part of a network can otherwise result in synchronization problems and cascading failures. VFT can be used for bulk system power transfer between large asynchronous power grids. It is a rotating transformer whose torque and speed are adjusted to control the power transmission. VFT is used as a controllable bidirectional power transmission device that can transfer power in-between power system networks. A conventional phase shifting transformer is used to control real power flow along a transmission line. Due to dominant inductive component of the line impedance a variation of the magnitude of the voltages would influence the reactive power flow. In a conventional phase shifting transformer, the introduction of a boost voltage that has a leading phase shift of 90 degree generates line currents in phase with line voltage, thus affecting the real power flow. The VFT when used as a phase shifting transformer all the above difficulties. Controlled power order reduction is required for both back-to-back HVDC and VFT in order to maintain system stability following certain disturbances. VARIABLE FREQUENCY TRANSFOMER INTRODUCTION A Variable Frequency Transformer (VFT) is used to transmit electricity between two (asynchronous or synchronous) alternating current domains. The world s first VFT was recently installed and commissioned in Hydro-Quebec s Langlois substation, where it was used to exchange up to 100 MW of power between the asynchronous power grids of Quebec and Newyork. Most asynchronous grid inter-ties use high-voltage direct current converters, while synchronous grid inter-ties are connected by lines and "ordinary" transformers, but without the ability to control power flow between the systems. 3

4 CONSTRUCTION AND OPERATION A VFT is a controllable, bidirectional transmission device that can transfer power between asynchronous networks. The VFT is a continuously adjustable phase shifting transformer that can be operated at an adjustable phase angle. The function of a VFT is to transfer power in between power system networks. In VFT Park, multiunit s VFTs are operated in parallel in order to increase the power transfer between two electric power networks. The VFT is a doubly-fed wound rotor induction machine (WRIM), the three phase windings are provided on both stator side and rotor side. The two power systems (#1 and #2) are connected through the VFT as shown in Fig.3.1. The power system#1 is connected to the stator side of the VFT, energized by voltage, V S with phase angle,. The power system#2 is connected to the rotor side of the VFT, energized by voltage, V R with phase angle. A drive motor is mechanically coupled to the rotor of WRIM. A drive motor and control system are used to apply torque, T D to the rotor of the WRIM which adjusts the position of the rotor relative to the stator, thereby controlling the direction and magnitude of the power transmission through the VFT [17]. Fig. VFT Model Representation A stable power exchange between the two asynchronous systems is possible by controlling the speed and torque applied to the rotor, which are controlled externally by the drive motor. When the systems are in synchronism, the rotor of VFT remains in the position in which the stator and rotor voltage are in phase with the associated systems. In order to transfer power from one system to other, the rotor of the VFT is rotated. If torque applied is in one 4 direction, then power transmission takes place from the stator winding to the rotor winding. If torque is applied in the opposite direction, then power transmission takes place from the rotor winding to the stator winding. The power transmission is proportional to the magnitude and direction of the torque applied. The drive motor is designed to continuously produce torque even at zero speed. When the two systems are no longer in synchronism, the rotor of the

5 VFT will rotate continuously and the rotational speed will be proportional to the difference in frequency between the two power networks. During this operation the power transmission or flow is maintained. The VFT is designed to continuously regulate power transmission even with drifting frequencies on both grids. PERFORMANCE UNDER FAULTED CONDITION During a fast disturbance like a fault, the VFT reacts like a transformer would. It transfers short circuit from one network to the other with the damping of the overall impedance. The VFT adds about 30%(nominal value)of impedance, this value comes from the summation of impedance for the two power transformers and the rotary transformer.tis transfer of short circuit is a very good and natural way to stabilize a weak network like the one on rotor side of VFT. The stator side as a power of short circuit of about 3000 MVA and the rotor side as only 500 MVA of short circuit. With a better short circuit level the machine maintains a higher electrical power and reduces the acceleration during the fault. A part of the electric power is transferred to the healthy network; this transfer does not cause prejudice and adds damping to the machine after the fault clearance.vft as the ability to increase the capacity limit of a link when the limits come from post contingency stability or voltage collapse limits. A standard phase sifter could only make a readjustment of angle for a post contingency thermal limit [16]. The smooth, analog, changes to power order of the VFT are desirable over steps or discrete changes. The VFT s performance in steady state as well as during grid disturbances as proven it is well suited to such a location. Though the frequency on one side dipped for a moment, the ramp of the power through the VFT was unaffected [16]. The large inertia of the VFT has been shown to be not only compatible with the grid, but also very helpful from a stability standpoint. In order to prevent the addition of a VFT from being a burden to the power system, shunt capacitor banks are supplied with a VFT to provide the necessary reactive power. In the case where a contingency takes place and there is a need for reactive power, the VFT allows reactive power to flow through it to help support that need. Supplying reactive power during times of needs, VFT serves the function of STATCOM by transmitting reactive power naturally to the weaker system. PROJECT ANALYSIS EXSITING METHOD In the existing method, stability analysis is based on a Sequential Continuation Scheme, Floquet theory and the Limit Cycle Method. The steady state operating point of a VFT Park is computed with Limit Cycle Method, while the Continuation Scheme determines how solutions of a system vary with a certain parameter. The Floquet multipliers find the stability of the solution scheme based on the Limit Cycle Method and Floquet multipliers. The computation of Floquet multipliers based on the Limit Cycle Method paves the way to carry out efficient stability studies. The power regulator measures the power flow throughout the VFT and compares it with its reference power. The error signal is fed to a Fuzzy controller and its output represents a speed command. To obtain a reference for the mechanic torque, the speed regulator compares the rotor speed and the speed command determined by the power regulator. A Fuzzy controller computes the torque command using the speed error signal of the speed regulator and 5

6 a torque limiter prevents the torque command to be within the dc motor capability. The computation of the stability regions of the VFT control system provides information for tuning Fuzzy controller. PROBLEMS IDENTIFIED VFT may become unstable because of either power transfer change or variable frequency. Fuzzy controllers are time consuming. The performance robustness trade off is not usually taken into account in case of Fuzzy Controllers. PROPOSED METHOD In this project, VFT is used to transfer power between two asynchronous networks. VFT Park control system compares the VFT Park power capacity with the demanded power transfer and assigns a rated power transfer command. The power through VFT is compared with reference power. The required optimized value of reference power is obtained using ANFIC controller. ANFIC controller computes the speed command and torque command. Sequential Continuation Scheme analyzes the stability and it simplifies clarifies and provides elegant solutions to difficult problems. Limit Cycle Method calculate the steady state operating point and it also computes limit cycle oscillations for potentially large and non linear system of equations. In order to improve the performance of ANFIC controller PSO and ACO algorithms are used. To obtain a reference for the mechanic torque, the speed regulator compares the rotor speed and the speed command determined by the power regulator. An ANFIC controller computes the torque command using the speed error signal of the speed regulator and a torque limiter prevents the torque command to be within the dc motor capability. The continuation parameters chosen in this work are the VFT power transfer and the frequency of asynchronous networks. Advantages of ANFIC controller includes, it incorporates the advantages of both Fuzzy and neural controller, capable of extracting maximum power and can provide fast control actions in comparison to conventional controllers. Advantages of PSO includes it has no evolution operators, easy to implement and there are few parameters to adjust. Advantages of ACO include positive feedback accounts for rapid discovery of good solutions. CONCLUSION AND FUTURE ENHANCEMENT CONCLUSION It is recognized that the use of Sequential Continuation Scheme and Limit Cycle Method for analyzing the stability of VFT finds high quality solution. The former one carries out stability analysis and the later one computes steady state operating point of VFT Park. The advantages of Sequential Continuation Scheme are it simplifies and provides elegant solutions to difficult problems. The merits of Limit Cycle Method includes, it computes limit cycle oscillations for potentially large and non linear system of equations. In phase-i project PSO based Fuzzy Controller was used for power and speed regulation of VFT control system. The computation of stability regions of the VFT control system provides information for tuning Fuzzy controller. The effectiveness of Sequential Continuation Scheme, Limit Cycle Method and PSO based Fuzzy controller has been tested on IEEE 17 bus system. It is recognized that the use of PSO 6

7 based Fuzzy controller in the VFT control system has resulted in efficient power transfer between the asynchronous networks. But disadvantages of PSO based Fuzzy controller are: it requires more fine tuning and simulation before operation. In project Phase-II PSO based ANFIC and ACO based ANFIC are used for regulating the power and speed of VFT control system. The comparative results shows that PSO based ANFIC controller is capable of extracting maximum power than Fuzzy controller. This is because ANFIC finds most suitable regulation depending on current system stability point. The simulation results show that non stable regions have been identified for changes of frequency and power transfer of the asynchronous networks. Instability is very likely to occur in VFTs if the rotor circuit is connected to a weak power network showing variations of frequency. So it can be concluded that PSO based ANFIC has resulted in more efficient power transfer between the asynchronous networks than ACO based ANFIC and PSO based fuzzy controller. FUTURE ENHANCEMENT For future extension of the project work, Von Neumann stability analysis will be introduced to check the stability of VFT Park. The analysis is based on the Fourier decomposition of numerical error. It is a very versatile tool for analyzing stability as it donot need to find eigenvalues, or matrix norms. The effectiveness of Von Neumann method will be tested on IEEE 17 bus and 118 bus systems. Genetic algorithm based ANFIC will be used for power and speed regulation of VFT Control system. REFERENCES 1.A. H. Nayfeh and B. Balachandran, Applied Nonlinear Dynamics: Analytical,Computational and Experimental Methods. NewYork:Wiley, A. Merkhouf, P. Doyon, and S. Upadhyay, Variable frequency transformer Concept and electromagnetic design evaluation, IEEE Trans. Energy Convers., vol. 23, no. 4, pp , Dec A. M. Stankovic and T. Aydin, Analysis of asymmetrical faults in power systems using dynamic phasors, IEEE Trans. Power Syst., vol. 15, no. 3, pp , Aug A. Semlyen and A. Medina, Computation of the periodic steady state in systems with nonlinear components using a hybrid time and frequency domain methodology, IEEE Trans. Power Syst., vol. 10, no.3, pp , Aug B. Bagen, D. Jacobson, G. Lane, and H. M. Turanli, Evaluation of the performance of back-to-back HVDC converter and variable frequencytransformer for power flow control in a weak interconnection, in Proc. IEEE Power Eng. Soc. Gen. Meeting, FL, Jun. 2007, pp C. Kieny, Application of the bifurcation theory in studying and understanding the global behavior of a ferroresonant electric power circuit, IEEE Trans. Power Del., vol. 6, no. 2, pp , Apr C. Kieny, G. Le Roy, and A. Sbai, Ferroresonance study using galerkin method with pseudo-arc length continuation method, IEEE Trans. Power Del., vol. 6, no. 4, pp , Oct D. Mc Nabb, D. Nadeau, A. Nantel, E. Pratico, E. Larsen, G. Sybille, V. Q. Do, and D. Paré, Transient and dynamic modeling of the new langlois VFT asynchronous tie and validation with commissioning tests, in 7

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