Investigation of Coupling of EMC Disturbances in Doubly Fed Induction Generators

Transcription

1 PIERS ONLINE, VOL. 5, NO. 8, Investigation of Coupling of EMC Disturbances in Doubly Fed Induction Generators S. Schulz, R. Doebbelin, and A. Lindemann Institute of Electric Power Systems, Otto-von-Guericke-University Magdeburg, Germany Abstract In renewable power generation doubly fed induction generators (DFIG) are used, in which the stator of the generator is directly connected to mains, while the converter for the rotor is rated only for slip power. Due to the interaction of rotor and stator windings in a DFIG, the high-frequency harmonics caused by the pulsed output of rotor power converters are transmitted into the stator current and cause conducted emissions. These high-frequency harmonics have to be considered, taking into account that harmonics which can be limited by filters will also be transmitted into the stator. 1. INTRODUCTION In recent years the exploitation of renewable energy has significantly increased with a major contribution of wind and water energy. One intent of energy production is high yield as achievable by variable speed operation. This requires the use of a converter. However, the high-frequency and fast changing output voltage of the converter can cause conducted emissions and system perturbation. 2. GRID COUPLING 2.1. Possibilities of Grid Coupling In power plants (especially in water-power plants and wind farms) turbines are based on synchronous or induction generators. To supply the energy of the generators into the grid it is common to use a power converter [1]. There are two possibilities: power converter for the full rated power, cf. the topologies in Figures 1(a), (c), (d)) power converter for the rotor of a DFIG only to be rated for slip power, cf. 1(b)) (a) (b) (c) (d) Figure 1: Possibilities of grid coupling in power plants. Generally speaking, care must be taken to comply with the EMC regulations for grid connection. Filters may be used to compensate harmonics; however they reach a considerable size and cause considerable cost at high power ratings of the converter Doubly Fed Induction Machine Using doubly fed induction generators (DFIG), in which the stator of the generator is directly connected to the mains, whereas the rotor of the machine is supplied by a converter. The converter for the rotor is only rated for slip power which is about 30% of the nominal power of the DFIG. Thus, this topology can inherently decrease the EMI level because of the lower power rating of the rotor converter compared to a converter for full rated power in conventional systems. This kind of system according to Figure 2 allows a large speed range of about 70% to 130% of the nominal rotor speed for optimized power generation [2].

2 PIERS ONLINE, VOL. 5, NO. 8, SYSTEM PERTURBATION OF DFIG 3.1. EMI Sources In DFIG the applied rotor converters are designed as indirect power converter with DC link or as cycloconverters. Electromagnetic compatibility aspects concerning system perturbation (up to 2 khz) and conducted emissions (9 khz to 30 MHz) of power converters are regulated in grid connection regulations of the transmission system operator [3], and furthermore in EN [4, 5], EN [6] and EN [7]. The emission limits given by these standards correspond with limits of the fundamental EMC standard EN [8]. System perturbation and conducted emissions are generally determined in the usually non-regulated (2 khz to 9 khz) range of the switching frequency of rotor converters. The harmonics and conducted emissions produced by the rotor converter are compensated by an input filter of the converter. Figure 2 shows several possibilities for the emergence and propagation of conducted emissions in the considered system: High-frequency pulsed rotor voltages cause harmonics of the rotor currents (A) which are transferred to the stator current (B); considering the slip of the generator, interharmonics will occur. The stator current supplies the transformer, thus the harmonics and interharmonics will proceed to the grid (C). The implemented EMC filter is mostly intended to reduce the additional conducted emissions of the rotor converter (E) on transformer side (D). Emissions of the stator current are hardly affected by this filter Simulation Model of the Power System Figure 3 shows the configuration of the simulated power system. The stator shall feed a power of 12 kw into the line sources (u e1, u e2, u e3 ). The rotor current i r of the DFIG is supplied by a converter. It is nearly sinusoidal but will include harmonics given by the modulation of the rotor converter. Mechanically, the rotor is driven by a torque of 76 Nm; it rotates with /min. Note that the relatively low power level has been chosen to facilitate comparison with experimental results according to Section gearbox n 1n2 generato r DFIG f s B 3 690V C grid f r 3 transformer 20kV A L DC-AC converte r AC-DC converte r E EMC filter D Figure 2: Doubly fed induction generator for wind power generation (DFIG). ue1 L = 10 µh i s1 ue2 ue3 U = 400V f e = 50Hz i s2 i s3 i r1 DFIG i r2 i r3 n r = /min j r = -76 Nm L = 300 µh UY = 70V f wr = 0.93Hz f t = 4kHz 3 ~ phase pwm voltage source R = 0.6 Ω Figure 3: Simulation model of doubly fed induction generator (DFIG).

3 PIERS ONLINE, VOL. 5, NO. 8, Simulation Results of the Power System The simulation results correspond excellently to the theoretically expected waveforms. The threephase rotor current and stator current are nearly sinusoidal. While stator frequency corresponds to mains frequency of 50 Hz, rotor frequency is low. The detailed simulation results are shown in Figure 4(a). The harmonics generated by the 4 khz pulsed rotor converter appear in the rotor current i r as well as in the stator current i s. This result illustrates that the harmonics are transmitted by the magnetic field, i.e., the waveform of simulated stator current i s shows the transmitted harmonics into the stator windings. FFT analysis in Figure 4(b) again shows the harmonics caused by pulsed rotor converter Model of Driving a Doubly Fed Induction Machine To verify this effect it is possible to consider the inverse method of operation of the power system. Figure 5 shows the configuration of the simulation of an existing experimental setup. Here, the stator of the 11 kw DFIG is supplied by an AC-DC-AC power converter with a switching frequency of 4 khz, while the wound rotor is short-circuited. The only filter elements are line inductors (L = 810 µh) between converter and stator windings. (a) (b) Figure 4: Simulation of (a) stator current i s, rotor current i r, (b) FFT of the simulated stator current i s, rotor current i r. Figure 5: Simulation and experimental setup for driving a DFIG. (a) Figure 6: Simulation of (a) stator current i s, rotor current i r, (b) FFT of the simulated stator current i s, rotor current i r. (b)

4 PIERS ONLINE, VOL. 5, NO. 8, Simulation Results of Driving a Doubly Fed Induction Machine The simulation results are in very good coincidence with the theoretically expected waveforms. The three-phase stator current is a superposition of a sinusoidal current (50 Hz mains frequency) with high-frequency harmonics caused by the switching frequency of 4 khz of the converter. According to the preceding considerations, harmonics in the stator currents should cause harmonics in the rotor currents as well. Figure 6 illustrates that harmonics of all three stator currents caused by their pulsed supply voltage can be found in rotor currents Circuit Theory Equivalent Circuit of a Doubly Fed Wound Rotor Induction Machine The reason for this behavior can be found in the single-phase equivalent circuit of a doubly fed induction machine which corresponds to the equivalent circuit of a transformer (Figure 7), where the equations of Faraday s law of induction apply: u s = i s (R 1 + jω 1 L 1σ ) + u µ (1) u ( ) r R s = i r 2 s + jω 1L 2σ + u µ (2) ( ) R u s = i s (R 1 + jω 1 L 1σ ) i r 2 s + jω 1L 2σ + u r (3) s Harmonics in rotor voltage u r /s caused by the converter will produce harmonics in rotor and stator currents. Therefore, mains current of the DFIG is a superposition of a sinusoidal current with highfrequency harmonics. The conducted emissions and harmonics depend on the pulse pattern of the rotor converter, frequency, the rotor filter and the line filter of the system. is R 1 L1σ L' 2σ R' 2/s i' r iµ us uµ L h u'r /s Figure 7: One phase equivalent circuit of a DFIG [9, 10]. (a) (b) Figure 8: Measurement of (a) stator current i s, rotor current i r, stator voltage u s, (b) FFT of the measured stator current i s, rotor current i r, stator voltage u s.

5 PIERS ONLINE, VOL. 5, NO. 8, Figure 9: Measurement of rotor current i r with superimposed pulses of the power converter Experiment Experimental Setup To investigate the effect of the transmitted high frequency harmonics, an existing experimental setup according to Figure 5 has been used: Here, the stator of the 11 kw DFIG is supplied by an AC-DC-AC power converter with a switching frequency of 4 khz, while the wound rotor is shortcircuited. The only filter elements are line inductors (L = 200 µh) between converter and stator windings Experimental Results According to the preceding considerations, harmonics in the stator currents of the experimental setup should cause harmonics in the rotor currents as well. The detailed view in Figure 8 with waveforms over time and results of FFT analysis illustrates that harmonics of all three stator currents caused by their pulsed supply voltage can be found in rotor current. 4. CONCLUSION With the increase of power plants with DFIG without line filters of the rotor circuit or system filters also EMC disturbances increase. These high frequency harmonics influence the mains, transformers and line filters of other systems connected to the mains. As a consequence malfunction and damage of electronic equipment can occur. This paper deals with the interaction of rotor and stator winding in a doubly fed induction generator [12]. High frequency harmonics caused by the pulsed output of rotor power converters are transmitted into the stator current and cause conducted emissions. It is possible to reduce them with filters in the rotor circuit, thus requiring a rating of only 30% of the nominal power. REFERENCES 1. Kimura, N., T. Morizane, K. Taniguchi, and T. Hamada, Inverter excited induction machine for high performance wind power generation system, European Power Electronics and Drives Association (EPE), Aalborg, Toufik, B., M. Machmoum, and F. Poitiers, Doubly fed induction generator with active filtering function for wind energy conversion system, European Power Electronics and Drives Association (EPE), Dresden, E.ON Netz GmbH, Grid Connection Regulations for High and Extra High Voltage, Bayreuth, Electromagnetic compatibility (EMC) Part 3-2: Limits for harmonic current emissions (equipment input current 16 A per phase) (IEC :2005); German version EN : Electromagnetic compatibility (EMC) Part 6-4: Generic standards Emission standard for industrial environments (IEC :2006), German version EN : Adjustable speed electrical power drive systems Part 3: EMC requirements and specific test methods (IEC :2004); German version EN : Voltage characteristics of electricity supplied by public distribution networks, German version EN 50160: Industrial scientific and medical (ISM) radio-frequency equipment Electromagnetic disturbance characteristics Limits and methods of measurement (IEC/CISPR 11: A1:2004, modified + A2:2006); German version EN 55011: A2: Sinelnikova, E., Design und optimale Betriebsfuehrung doppelt gespeister Asynchrongeneratoren fuer die regenerative Energieerzeugung, Dissertation, Fakultaet fuer Elektrotechnik und Informationstechnik der Technischen Universitaet Chemnitz, 2004.

6 PIERS ONLINE, VOL. 5, NO. 8, Schroeder, D., Elektrische Antriebe Regelung von Antriebssystemen, Springer Verlag, Giesecke, J. and E. Mosonyi, Wasserkraftanlagen Planung, Bau und Betrieb, Springer Verlag, Schulz, S. and A. Lindemann, Investigation of coupling of EMC disturbances in wind generators with DFIG, 4th PhD Seminar on Wind Energy in Europe, Magdeburg, 2008.