The new pumped storage power plant with variable speed generator

Transcription

1 The new pumped storage power plant with variable speed generator SeyedMohammadHassan Hosseini 1, Sina Eslami 2 Electrical Engineering Dep. 1, 2 Islamic Azad University, South Tehran Branch Tehran, Iran 1 smhh110@ azad.ac.ir Abstract: - It is important, to optimize of clean electrical energy by employing of variable Speed pumped storage power plant (VSPSP). Variable speed machines are used extensively in wind power plants and pumped storage power plants. Therefore, the advantages of this technology are including: stability, reliability, fast dynamic responses, frequency control and high efficiency of power system. By using doubly fed asynchronous machine (DFAM) technology and the modern equipment (Cycloconverter or multilevel voltage source converters) the mentioned purposes will be achieve. In this paper, technical investigation& comparison between the whole of PSP s drives methods is presented then a VSPSP s unit is modeled and simulated by Direct Torque& Flux Control (DTFC) technique and two level voltage source converters (2LVSC). The results of simulation for one unit of 250 Mw pumped storage power plants have been presented. Key-Words: Doubly Fed Asynchronous Machine (DAFM), Two-Level Voltage Source Converter (2LVSC), Direct Torque& Flux Control (DTFC), Variable Speed Pumped Storage Power Plant (VSPSP) 1 Introduction Pumped storage power plants are one of the most efficient methods to restore large amounts of energy. By developing of power electronic components, high power converters such as multilevel voltage source converters (MLVSC) including two level voltage source converters (2LVSC) and three level voltage source converters (3LVSC) can be applied in variable speed pumped storage power plants (VSPSP) [1]. DFAM technic can be employed in variable speed pumped storage power plants to provide higher efficiency and stability. Direct torque control is one of the strategies to control DFAM. This strategy controls the machines torque and the rotor flux amplitude, directly. In [2], the drive system of a variable speed pump-storage power plant consisting of a doublyfed induction machine with a 3-level voltage source inverter feeding the rotor is presented. A stator power factor adjustable direct torque control of doubly-fed induction machine is proposed in [3]. a static frequency converter (SFC) used in pumpedstorage plants for rapid responses and soft connected to power network. New predictive DTC strategy of the DFAM, proposed by the authors in [4], the benefits of this method is: low electromagnetic torque and rotor flux ripples, rapid dynamic responses. Also, new predictive DPC strategy of the DFAM proposed by the authors in [5]that are able to operate at considerably low constant switching frequencies with good power ripple. In [6] a rotor flux amplitude reference generation strategy is proposed for doubly fed induction machine that applied in wind turbines. In [7] a new control strategy for a grid connected DFAM based wind energy conversion system is presented. In this paper, first the control method of DFAM is introduced, and modeling of DFAM considering, then DTFC strategy with 2LVSC simulation of a variable speed pump-storage power plant is studied in generating mode according to 250 Mw units by MATLAB Software. 2 Control Strategies for PSP There are different types of control methods strategy in pumped storage hydropower plants. Some of the most important and useful strategies have been mention as below: 2.1 Static Frequency Converter (SFC) Soft starting of the PSP and rapid start of the motorpump are the main features of using SFC equipment. Availability of equipment's related to SFC's topology (grid side rectifier; harmonic eliminator inductor, and rotor side alternator), easy synchronization to grid where use the governor for speed control, fast response to sudden load changes, and preventing power flow from inversing back into generator, are some of the additional Benefits for this topology. Fig 1 illustrates SFC equipment's in a PSP. ISBN:

2 restrictions, due to its direct connection to grid. These restrictions are: 1) Limited performance in the presence of grid disturbances such as faults and etc. 2) The effect of inductive load on angle phase AC controllers (cycloconverter) 3) Effect of the grid power side 4) harmonics output converter existence Fig.1. SFC equipment in a PSP schematic It must be stated, that although the system stabilization will be improved by SFC equipment's, but synchronous machines are more effective and economic as stabilizers [9]. This strategy is more applicable than pole change method. Using DFAM technology instead of synchronous generators is one of the most efficient methods, due to decrease the frequency converter's capacity, and increase the efficiency specially in pumping mode. 2.2 Doubly-Fed Asynchronous Machine This configuration consists of the frequency converter connected to the rotor of the induction machine. With this configuration, variable speed can be obtained with a partially rated frequency converter (less than the generator rating). This is the preferred system in large scale implementations of pumped storage, since the converters rating does not limit the total system rating. Another advantage is that the reactive power to and from the grid can be controlled. This can be utilized for voltage control in the grid and contribute to improve the stability and the operating conditions in the rest of the power system [10] DFAM with Ac-Ac Convertors In this strategy, the stator connects to grid directly and the rotor will connect to grid by means of a cycloconverter. Cycloconverter is a frequency converter by using thyristor (SCR) as switching elements. The cycloconverter includes two 3-phases unparalleled current poles, as positive and negative, in each phase. Therefore, the rotor speed, active and reactive power are controlled by means of a cycloconverter's control set. Despite of the cycloconverter's advantage, such as high reliability, low maintenance cost, high stability in steady state and dynamic modes, etc. it has DFAM with Back to Back Convertors (VSC) The back-to-back converter system is able to generate power both above and below synchronous speed. The configuration is often called doubly-fed induction generator (DFIG), which emphasizes the ability to transfer power into or out of the rotor, as well as out of the stator. The VSC s topology is similar to cycloconverters. This strategy includes both rectifier and inverter modes. and uninterrupted performance high frequency makes these converters size small. 3 DTFC Strategies in DFAM The DTFC is based on a direct control of two properties of the machine, the electromagnetic torque and the rotor flux of the machine. This method is an alternative control solution for AC drives, in general, the present control principles and performance features are different from vector control techniques, as shown in previous research such as: FOC, FLC and etc. This control technique has the following general benefits: 1.Reliability 2.Good perturbation and disturbance rejection 3.fast dynamic response and so on. The DTFC topology for VSPSP has been shown In Fig.2. Fig.2. DTFC topology for VSPSP ISBN:

3 Torque and the rotor flux amplitude of DFAM employed as main parameters in this method. Comparative evaluation on modern mentioned driving methods with technical & economic considerations are illustrated in Table DFAM Modeling In this section the equations of the DFAM are presented. Hence, the voltage and flux equations in the stator and the rotor reference frame are: Rotor voltage vector Stator flux linkage vector Rotor flux linkage vector Stator self-inductance Mutual-inductance Total electromagnetic torque { } Stator resistance 3.2 Voltage Vectors Effect on DTFC In the study of the space vector diagram, creating a circular trajectory of the rotor flux space vector is needed, as shown in Fig 3. By injecting the available rotor voltage vectors, the amplitude of the rotor flux space vector and its movement are controlled, in order to locate it to a specific distance from the stator flux space vector. Thus, with this simple control strategy, it is possible to modify the electromagnetic torque of the machine according to equation (6).When the speed of the machine is higher than the synchronous speed, the space vectors rotate clockwise. In the opposite, the effect of the zero vectors on the electromagnetic torque is at lower speeds than the synchronous speed. Rotor resistance Stator current vector Rotor current vector in stator side Stator current vector in rotor side Rotor current vector Stator voltage vector Fig 3. Effect of voltage vectors on regulation of torque and flux Amplitude Driving Methods Synchronous Machine Drive Efficiencies DFAM Drive 80% Equipment Type 70% SFC Table 1 Comparison of modern control methods in PSP Primary Costs Average Reliability Cycloconvertor MLVSC PWM-MLVSC Very Very O&M Costs Driving speed More Advantages Average Braking Ability Uninterrupted Operation in Voltage Dropping Switching at Frequency Uninterrupted Operation in SCC ISBN:

4 4 Case Study The Siahbisheh PSP located in Karaj province of Iran has 4 250MW synchronous units. In this paper, one of these units considered as our case study. The proposed strategy (DFAM) applied to one unit of this power plant and modeled and analyzed. 5 Simulation Results The simulation results are presented for the proposed strategy in generating mode (by DTFC Technology with 2LVSC) by using the MATLAB software for SiahBisheh PSP. Fig. 4 shows the Simulation Results of DTFC in 250MW with 2LVSC column. The first figure shows the torque, the second shows the rotor fluxes, the third shows rotor current, the fourth shows the stator current and the fifth shows rotor mechanical speed. Fig 5, 6 show the rotor current torque and the torque generated by induction machine. Stator active power increased 30%, approximately after applied changes according to torque s diagrams in 2LVSC and flux s diagrams and stator reactive power decreased the same quantity. 6 Conclusions In this paper, technical evaluation& comparison between the whole PSP s drives methods performed. Investigation results show that the VSPSP has more efficiency than fixed speed PSP. Among of drive& control topologies for VSPSP, DTFC with 2LVSC is proposed. By this topology, despite the correction and elimination the some of the other equipment s restrictions, good dynamic response, uninterrupted operation in fault optimum control between active& reactive power have been provided. No need to reactive power compensator in grid side is one of the other VSPSP s benefits, too. The behavior of a VSPSP s unit with capacity of 250MW has been modeled and simulated in generating mode. It has been shown with this strategy all of mentioned advantages, such as: extremely high dynamic performances in full compatibility with safety requirements have been achieved. Indeed, a 30% active power change can be achieved in 0.1s by proposed strategy, due to the flywheel effect. Also, efficiency in generation mode has been increased. Fig.4 Simulation Results of DTFC in 250MW with 2LVSC column: first shows the torque Second shows the rotor fluxes Third shows the rotor currents Fourth shows the stator currents Fifth shows the rotor mechanical speed or reference set points ISBN:

5 [3] J. Janing, A. Schwery, Next generation variable speed pumped-storage power station IEEE Trans. Power Electron, Vol. 25, pp , Feb [4] V. Yaramasu, Bin Wu, Predictive Control of a Three-Level Boost Converter and an NPC Inverter for -Power PMSG-Based Medium Voltage Wind Energy Conversion Systems, IEEE Trans. Power Electronics, Vol 29, pp , Oct Fig 5. The rotor current generated by induction machine [5] Mohammed O. A., Liu Z. and Liu S., Stator power factor ajustable direct torque control of doubly-fed induction machines, in IEEE International Conference on Electric Machines and Drives, pp , [6] J. Poza, and J. M. Canales, Direct Torque Control for Doubly Fed Induction Machine- Based Wind Turbines under Voltage Dips and Without Crowbar Protection, IEEE Trans. Energy Conversion, Vol. 24, No. 3, pp , [7] Van-Tung.Phan, Hong-Hee. Lee, Control Strategy for Harmonic Elimination in Stand- Alone DFIG Applications with Nonlinear Loads, IEEE Trans. Power Electronics, Vol 26, pp , Sept References: Fig 6. The torque generated by induction machine [1] Y. Pannatier, B. Kawkabani, C. Nicolet,, J. J. Simond, A. Schwery, P. Allenbach, Investigation of Control Strategies for Variable-Speed Pump-Turbine Units by Using a Simplified Model of the Converters, IEEE Transactions on Industrial Electronics, Vol. 57, No. 9, September [8] M.C. Di Piazza, A. Ragusa, G.Vitale, Effects of Common-Mode Active Filtering in Induction Motor Drives for Electric Vehicles. IEEE Trans, Vehicular Technology, Vol 59, pp , July [9] Do-Hyun Jang, Duck-Yong Yoon, Spacevector PWM technique for two-phase inverterfed two-phase induction motors, Industry Applications, IEEE Transactions on, Vol 39, pp , [10] H. Fang, L. Chen, N. Dlakavu, and Z. Shen, Basic modeling and simulation tool for analysis of hydraulic transients in hydroelectric power plants, IEEE Trans Energy Convers, Vol. 23, no. 3, pp , Sep [2] Van-Tung.Phan, Hong-Hee. Lee, Control Strategy for Harmonic Elimination in Stand- Alone DFIG Applications with Nonlinear Loads, IEEE Trans. Power Electronics, Vol 26, pp , Sept ISBN: