Cascaded Doubly Fed Induction Generator with a Back-to-Back Converter Connected to a Small Distributed Generation System

Transcription

1 March Cascaded Doubly Fed Induction Generator with a Back-to-Back Converter Connected to a Small Distributed Generation System Marek Adamowicz Power Electronics, Drives & Control Group Gdynia Maritime University, Morska 81-87, Gdynia, Poland madamowi@am.gdynia.pl Ryszard Strzelecki Power Electronics, Drives & Control Group Gdynia Maritime University, Morska 81-87, Gdynia, Poland rstrzele@am.gdynia.pl Copyright 2009 MC2D & MITI Abstract: The significant benefit of distributed generation (DG) is the production of energy near the load. Moreover DG system can be applied for reactive supply by injecting and absorbing reactive power to grid. In the paper an application of variable speed cascaded doubly fed induction generator (CDFIG) in a small DG system is considered. It requires lower maintenance than single DFIG due the absence of slip rings and brushes. A small DG system with CDFIG and back-to-back converter is discussed. Back-to-back converter of CDFIG is here a common grid interface both for CDFIG and the remained units of DG system. CDFIG power flow is simulated using PSIM software and CDFIG performance is investigated. Keywords: Cascaded doubly fed induction machine, variable speed generator, distributed generation system, power generation. 1. Introduction Variable speed cascaded doubly fed induction generators (CDFIG) [1, 2, 3] named also double stator induction generators [4] or twin stator induction generators [5, 6] can be an attractive alternative to conventional double output wound rotor induction generators. Cascaded induction generators require lower maintenance due the absence of slip rings and brushes. For small power plants, i.e. mini and micro hydropower plants, unidirectional power converters can be employed at the secondary side of the cascaded generator to provide simple power electronics, reduce of the maintenance and increase availability [2, 7]. In such simple and low-cost applications a capacitor bank is always required at the grid side to provide the reactive power compensation. In more advanced systems the secondary stator currents and voltages of the CDFIG can be manipulated using four quadrant converter which ensures ability of power flow control. Some recent papers propose integration of conventional doubly fed induction generators (DFIG) with the fuel cell generation unit [8] or existed small distributed generation systems. In [4] the study of an autonomous Wind Energy Conversion System (WECS) oriented to water pumping in remotes areas has been considered. The WECS discussed in [4] consists of a wind turbine and a CDFIG electrically coupled with a

2 squirrel cage induction machine moving a centrifugal-type water pump. The integration of CDFIG with general secondary energy source is proposed in [4]. The DC-link of the back-toback converter is hence the common point of connection of the integrated system. Following [4] the present paper discusses an idea of integration of CDFIG and a back-toback converter with a small distributed generation (DG) system. The analysis of CDFIG during the speed changes and generated power changes are the key points of the paper and the first step to analysis of whole small DG system integrated with CDFIG. 2. Cascaded Doubly Fed Induction Generator The CDFIG analyzed in the paper consists of two identical 2.2kW 4-pole wound rotor induction machines and is shown in figure 1. Machine parameters are given in Table 1. Two induction machines have their rotors mechanically and electrically coupled and the slip rings are unnecessary. It can be done using two different arrangements of rotor wires interconnections as it is shown in figure 1a and figure 1b. Appropriate signs of shaft speeds and stator and rotor frequencies should be maintained. Figure 1. Two equivalent realizations of cascaded doubly fed induction generator The absence of slip rings and brushes is the main benefit of CDFIG over the conventional DFIG. Table 1: parameters of unit induction machine Rated power (motoring) Rated power Factor Rated stator Voltage Rated stator Current Rated rotor Voltage Rated rotor Current Rated frequency Pole number Stator resistance Stator leakage inductance Rotor resistance Rotor leakage inductance Mutual inductance Hz (kw) (-) (V) (A) (V) (A) (Hz) (-) (Ω) (mh) (Ω) (mh) (mh) Steady state equivalent circuit of CDFIG is schematically shown in figure 2. All phase quantities are referred to the first stator side. With the stators supplied in parallel and the rotor blocked, the two machines will be similar to two back-to-back transformers. However, in the case of the machine operation, there is an additional degree of freedom represented by the relative positions of the stator and rotor windings. The rotor power of cascaded generator is magnetically transferred to secondary stator terminals and it can be further transferred to grid through the secondary side back-to-back converter. The power handling requirements of secondary stator, relative to first stator connected to grid, are equal to the stators frequencies ratio. As it can be seen on simplified diagram of lossless CDFIG presented in figure 3 the relative phase and magnitude of the rotor voltages determine the circulating current in the rotor circuits and, therefore, the torque. From fig. 3 it can be written for the CDFIG rotor voltages U r1, U r2, rotor current I r1 and the total electromagnetic torque T e : Ur1 = U r2 / s jxσ rir1 (1) ( + ) 3 p1 p2 Ur2Ur1 Te = sinδ (2) ω s X s s1 1 σ r Figure 2. Steady state equivalent circuit of the cascaded doubly fed induction generator.

3 When the shaft speed equals twice natural speed of the CDFIG n s = 2n nat, the coupling of two machines in the cascade is lost. The secondary stator voltage and the generated power are then equal zero. 3. Small Distributed Generation System Figure 3. Simplified diagram of lossless CDFIG where T e denotes electromagnetic torque, X σ r - rotor leakage inductance, and ωs1 - first stator pulsation. Natural speed n nat is defined for CDFIG. It can be obtained at zero frequency of the secondary stator f s2 from n nat ( ) fs1+ fs2 60 fs1 60 = = p + p p + p 1 2 fs2= (3) The CDFIG under consideration consists of two identical induction machines (p 1 = p 2 ) and the natural speed n nat =n s /2 =750 rpm. This is a half of the synchronous speed of the single induction machine (IM). The slips s and s 1 in figure 2 and figure 3 can be obtained from: s f f p n /60 r1 s = = (4) fs1 fs1 s 2 f f = = f f p n s2 s2 r1 s2 2 2 /60 f (5) s2 = 1 2 = (6) fs1 s s s where the signs of the secondary stator and rotor frequencies f s2, f r2 and sign of secondary shaft speed n 2 have to be chosen in accordance with the configuration described in figure 1. Distributed generation (DG) is related with the use of small generating units installed at strategic points of the electric power system or locations of load centres [9]. These small generating units DG can be also used in an isolated way operating with connection to common DC or AC buss forming small DG system supplying the consumer s local demand. DG mainly provides primary power. Different small generating units can provide back-up power other improving the reliability of whole DG system and the load. The benefit of DG is the production of energy near the load. Moreover DG can be applied for reactive supply and voltage control of generation by injecting and absorbing reactive power to grid voltage using a locally available mix of prime fuel sources [10]. DG technologies mainly include engines, small wind and hydro turbines, fuel cells and photovoltaic systems. Figure 4 shows the idea of interconnection of small DG system with CDFIG using common DC-link of back to back converter. A small DG system connected to grid consisting of cascaded induction generator, fuel cell and PV and with a common DC-link of the front-end three-phase power inverter is similar to those described in [11, 12]. Only one DC-AC grid inverter is necessary in order to connect the CDFIG and other small DG units to grid. Figure 4. Proposed small DG system with CDFIG and multiple power sources.

4 The small DG units shown in figure 4 operate as a power sources. They could inject an active power into the grid through grid-side inverter or support reactive power injection into CDFIG through machine side converter. In the case of fuel cell and PV the isolated dc-dc converters [13] are used to boost the primary source voltage to appropriate level of DC-link voltage. An important issue for the fuel cell power converter design is the fuel cell current ripple reduction [13]. 4. Control Objectives The main objective of the DG superior control is to maximize the energy efficiency. In the case of wind or solar power generation, the goal is to produce as much energy from the system as possible to recover the installation cost. In present paper an additional objective is to assure a constant DC link voltage. CDFIG is characterized by the lowest efficiency in the whole DG system shown in figure 4. It is always lower than efficiency of two single DFIGs. Next control objective is to maintain the zero amount of reactive power absorbed by CDFIG from grid at primary stator side. All reactive power should be supplied to CDFIG from secondary stator side via the machine converter. The power flow control is accomplished through the secondary stator of CDFIG, changing the frequency and voltage of its excitation [4]. Simple control based on torque estimator and speed measurement has been applied. Sensorless grid side converter [15] has been applied in the proposed DG system as the grid interface. It provides very precise grid sensorless operation insensitive to current measurement errors and unbalances [15]. At synchronous speed n syn = 2n nat the coupling of both machines is lost. The operation region of CDFIG is much lower than above mentioned twice of natural speed and hence a reduced ratings machine converter can be applied. For the lossless CDFIG the balance of generated power can be written as follows: p1 Ps1 = Ptotal (7) p1 + p2 n p 1 Ps2 = Ptotal (8) n syn p1 + p 2 where P s1, P s2 denotes powers generated from primary and secondary stator and P total total nominal power of CDFIG 5. Simulation Results Simulation of CDFIG was carried out using PSIM simulation software [16]. Several simulation models of DG systems can be found in the literature. The investigations in present paper mainly concern the CDFIG behaviour. The building block of simulated CDFIG is single DFIG. Ready PSIM machine models with parameters from table 1 have been utilized to simulate the cascaded generator. For more advanced modelling of single DFIG connected to back-to-back converter using DLL in PSIM software the reader is referred to [17, 18]. The PV unit has been modelled in simply way as a current generator dependent on solar radiation. Detailed PSIM model of the fuel cell and DC- DC converter unit can be found in [19]. Figure 5. CDFIG performance during the shaft speed change: speed (a), primary power P s1,q s1 (b), secondary power P s2, Q s2 (c), grid voltage (d), primary current i S1 and rotor current i R (d), secondary current i S2 (f).

5 Figure 6. Power transient at constant speed n n =1125rpm: primary stator power P s1 and Q s1 (a), primary stator current i S1,rotor current i R and first machine torque T G1 (b), secondary stator power P s2 and Q s2 (c), secondary rotor power P R and Q R (d), secondary stator current i S2 (d), secondary machine torque T G2 (f). Figure 5 illustrates the performance of the CDFIG during the shaft speed change from 1125 rpm to 1050rpm. The synchronous speed of single CDFIG is 1500 rpm and the natural CDFIG speed is 750 rpm. Negative power values denote generated power while positive values denote power absorbed by CDFIG. The primary stator reactive power Q s1 is maintained zero during the speed change and primary active power P s2 also remains almost constant and equal to its rated value. The secondary stator active power P s2 decreases according to (8). Also secondary reactive power Q s2 slightly decreases as the speed decreases. Figure 6 shows the step decrease of reference reactive power Q s1 to 0Var at constant shaft speed 1125 rpm. The secondary reactive power Q s2 varies then from 0kVar up to 2.7kVar. The zeroing of primary reactive power Q s1 is achieved. The decrease of Q s1 do not significantly affects the amplitude of i S1 and it has near rated value all the time. It can be seen from figure 6 that the direction of rotor power Q R flow is changed. The negative Q R means the direction of power flow from the secondary machine to primary grid connected machine. 6. Conclusions The aim of the paper was to find the answer whether CDFIG with back-to-back converter can be connected to small DG system and cooperate with other small DG units. The simulation study of dynamic performance of CDFIG was presented. The grid side inverter of back to back converter can play the role of common grid interface for CDFIG and other small DG units. Open simple control based on speed measurement and torque estimator has been applied. The decoupled control of active and reactive power of CDFIG was not considered in this paper. Future work will concern decoupled control of CDFIG and detailed analysis of whole small DG system interconnected with CDFIG. Acknowledgement This work was financially supported by Polish Ministry of Scientific Research and Information Technology ( ). References [1] B. Hopfensperger, D.J. Atkinson, R.A. Lakin, Steady State of the Cascaded Doubly-Fed Induction Machine ETEP, Vol. 12, pp. 1 12, Nov./Dec [2] Kato S., Hoshi N., Oguchi K., "Small scale hydro-power", IEEE Industry Applications Magazine, pp , July/Aug [3] N. Patin, E. Monmasson, J.P. Louis, Analysis and control of a cascaded doublyfed induction generator in Proc. of 31st Annual Conf. of IEEE Industraial Electronics Society IECON 05, 6 pp., 2005.

6 [4] Camocardi P., Battaiotto P., Mantz R.,"Wind generator with double stator induction machine. Control strategy for a water pumping application", in Proc. 43rd Int. Universities Power Engineering Conference UPEC2008, pp. 1-5,2008. [5] Smith B.H, Theory and Performance of a Twin Stator Induction Machine, IEEE Trans. Power Apparatus And Systems, vol. 85, pp , Feb [6] Basic D., Zhu J.G., Boardman G., "Transient Performance Study of a Brushless Doubly Fed Twin Stator Induction Generator", IEEE Trans. Energy Conv., vol. 18, pp , Sept [7] Adamowicz M., Strzelecki R., "Cascaded Doubly Fed Induction Generator for Mini and Micro Power Plants Connected to Grid", in Proc. Conf. EPE PEMC 2008, Poznan, Poland, CD-ROM, p. 5, [8] Palle, B.; Simoes, M.G., "Dynamic Integration of a Grid Connected DFIG Wind Turbine with a Fuel Cell", in Proc 42nd IEEE Conf. Industry Applications IAS 2007, Sept. 2007, pp [9] L. L. Lai and T. F. Chan, Distributed Generation: Induction and Permanent Magnet Generators, John Wiley & Sons, Ltd [10] Strzelecki R., Benysek G., Power Electronics in Smart Electrical Energy Networks, Power Systems Series, Springer, [11] Gaiceanu M., Fetecau G., "Grid Connected Wind Turbine-Fuel Cell Power System Having Power Quality Issues" in Proc. 9th Int. Conf. Electrical Power Quality and Utilisation, p. 6, Barcelona [12] Kwak M.S., Sul S.-K., Control of an Open- Winding Machine in a Grid-Connected Distributed Generation System, IEEE Trans. Industry Appl., Vol. 44, pp , [13] J. Wang, F.Z. Peng, J. Anderson, A. Joseph, R. Buffenbarger, Low Cost Fuel Cell Converter System for Residential Power Generation, IEEE Trans. Power Electronics, Vol. 19, pp , Sept [14] Protsenko K., Xu D., Modeling and Control of Brushless Doubly-Fed Induction Generators in Wind Energy Applications, IEEE Trans Power Electronics, vol. 23, pp , [15] Wojciechowski D., Novel Estimator of Distorted and Unbalanced Electromotive Force of the Grid for Control System of PWM Rectifier with Active Filtering, in Proc. European Conf. EPE 2005, CD- ROM. [16] Kaminski, B.; Wejrzanowski, K.; Koczara, W., An application of PSIM simulation software for rapid prototyping of DSP based power electronics control systems, in Proc. 35th Annual Power Electronics Specialists Conference PESC 04, Vol.1, pp , [17] Iwanski, G. Koczara, W., Sensorless stand alone variable speed system for distributed generation, in Proc. 35th Annual Power Electronics Specialists Conference PESC 04, Vol.3, pp , [18] Iwanski, G. Koczara, W., Simple autonomous sensorless generation system with wound induction machine, in Proc. IEEE International Symposium on Industrial Electronics ISIE2004, vol. 2, pp , [19] Hui Li; Danwei Liu, Power Distribution Strategy of Fuel Cell Vehicle System with Hybrid Energy Storage Elements Using Triple Half Bridge (THB) Bidirectional DC- DC converter, in Proc 42 nd IEEE Industry Applications Conference IAS2007, pp , 2007.