Harmonic Filtering in Variable Speed Drives

Transcription

1 Harmonic Filtering in Variable Speed Drives Luca Dalessandro, Xiaoya Tan, Andrzej Pietkiewicz, Martin Wüthrich, Norbert Häberle Schaffner EMV AG, Nordstrasse 11, 4542 Luterbach, Switzerland Abstract This paper discusses and presents results related to still open controversial issues in passive harmonic filters design, such as the identification of filter resonant frequencies, the effects of interactions of harmonic filters with system impedances and the importance of damping elements. Benefits of passive rectifiers equipped with harmonic filters are compared to those of other rectifier front-ends typically used in VSD applications. An experimental comparative evaluation of harmonic performances of passive and active rectifiers is also provided. 1. Introduction Passive three-phase rectifiers equipped with single tuned (ST) harmonic filters, as shown in Fig. 1, represent the most cost effective, technically simple and robust front end of variable speed drives with low effects on the mains. Harmonic filter design procedures and solutions for complex harmonic filtering application problems have been documented in several publications in the past few decades [1-3]. Every new harmonic mitigation problem still deserves a careful system analysis and special attention needs to be paid on crucial design aspects such as: selection of tuning frequencies and damping factors, interactions between harmonic filter and system impedances, optimal number of harmonic trap branches. Following sections provide further practical insight and considerations on these topics and, additionally, benefits of passive rectifiers equipped with harmonic filters are presented and compared to those of other rectifier front-ends typically used in VSD applications. 2. Single tuned and multi-tuned passive harmonic filters Six pulse current converters are common rectifier interfaces for variable speed drive systems and they are harmonic producing loads [2]. Because grid harmonic current emissions are regulated at both equipment level, e.g. [3, 4] and PCC [5], a typical harmonic mitigation approach is to install single tuned (ST) harmonic filter close to these non-linear loads. An accurate harmonic filter design procedure requires taking into account several aspects, such as estimate of harmonic current injection for different system s loadings, short-circuit level, load demand and harmonic analysis study considering presence in the installation of components which can impact harmonics, as capacitor banks or reactors [2]. A ST filter featuring a sharp tuning close to the 5 th harmonic is typically the first approach to reduce the harmonic distortion, since the 5 th harmonic has higher amplitude for six-pulse converters. If the harmonic content is still above limit, the procedure is to implement further ST filters tuned at higher order harmonics [1, 2, 3]. 148

2 Fig. 1 Effect of a ST harmonic filter tuned at the 5 th harmonic on the mains current i in and on the passive rectifier input current or filter output current i out for a 45 kw, 400V, 50 Hz system. THDi without and with ST harmonic filter is 103.5% and 3.5%, respectively. The attenuation by multiple parallel harmonic traps is achieved at the cost of higher loading of the lower harmonic ST branch, tuned at the 5 th harmonic, and results in higher load current respect to using a ST filter; this effect was also observed in the empirical design procedure presented in [2]. Fig. 2 shows the harmonic spectrum of load or input rectifier current and of the resulting current in the 5 th harmonic trap, corresponding to two different passive harmonic filter (PHF) designs for a 55kW drive: one ST filter tuned at the 5 th harmonic and a filter made out of the combination of four parallel branches each tuned at a different harmonic (5 th, 7 th, 11 th, 13 th ). For both configurations, single trap and multi traps, the passive harmonic filters draw the same level of capacitive current, which is typically 20% of the line current. Furthermore, there is no DC-link choke or rectifier s input choke installed in both cases. The 5th trap inductance in the multi-trap filter results to be three times larger than the one in the single trap model, 4.7mH vs.1.84mh. The 5th branch circuit of the multi-tuned PHF is much more stressed; at the same time it features a larger inductor than a single tuned PHF, which results into a more expensive passive filter solution. The THDi performance is sufficiently good with ST harmonic filter, 5.8% vs 3.3% of multi-tuned PHF. Fig. 2 Harmonic spectrum of load current and the resulting current in the 5 th harmonic trap for the two different PHFs for a 55kW drive: one single filter tuned at the 5 th harmonic and a multi-tuned PHF made out of four parallel branches each tuned to a different harmonic (5 th, 7 th, 11 th, 13 th ). 149

3 3. Interaction with system impedances According to the power rating of the installation and the type of impedances present in the system, PHFs may introduce unwanted distortions and resonances and therefore their design requires a careful system analysis to avoid operational issues. Fig. 3 shows the frequency characteristic of PHFs with different ratings, 1.1kW and 200 kw, installed in input of drives equipped with integrated EMI filters, in particular the X2 capacitor stage. Whilst for the larger rating PHF the interaction with the EMI filter does not cause variations of the frequency characteristic, the curve of the 1.1kW PHF presents a parallel resonance at about 600 Hz, which may result into an amplification of harmonic frequency sources in a range between the 11 th and 13 th order. Such a resonance can hinder the correct functioning of the filter and it is a case of interaction of PHF with EMI filter capacitance which may result into a poorer performance. Fig. 3 Frequency characteristics of PHFs installed in input of drives equipped with EMI filter stage. The interaction of the 1.1kW PHF with the EMI filter capacitance causes an unwanted resonance. The use of adequate damping techniques or even a modification of the filter circuit may preserve correct operation. The frequency and amplitudes values of the first parallel resonance (due to interaction with line impedance L i ) and of the series resonance are given respectively by [2]: f= 1 / 2π (LC + L i C) ½ Q = ((L + L i )/C) ½ / (R+R i ) These two resonances are expected in the ST filter design; the series resonance results from the trap branch LC tuned at about the 5 th harmonic, (L i = R i = 0 in the formula) and has the effect of attenuating the amplitude of the 5 th harmonic source. The parallel resonance, occurring at a lower frequency, is due to the interaction between trap circuit and line-side impedance L i (see Fig. 7a) and it is selected outside the range of harmonic sources; if this condition is not verified, the magnitude of the parallel resonance can be reduced by modifying the filter circuit with additional resistive, damping elements or by varying the value of the line inductor [1]. 150

4 The amplitude of parallel resonance due to interaction of the filter with EMI filter capacitors can be reduced by implementing a further damping branch [12], for instance in parallel to the line reactor. The attenuation effect of such a solution is visible in the plot of Fig. 3. The additional resistance and losses contribute to decrease the value of the Q factor corresponding to the parallel resonance, hence to flatten the shape of the filter frequency characteristic. The accurate tuning of the harmonic filter is crucial to achieve the target attenuation. However, effects like tolerances, component aging and temperature dependent characteristics need to be considered during design and therefore a ST filter is typically tuned to a frequency lower than the harmonic to suppress. The most crucial tolerances are on components of trap branch as depicted in Fig. 4. Fig 4 Plane of possible resonant frequencies for the 3-phase trap inductor in series with the capacitor bank, as function of the component tolerances. In particular, the lines parallel to the z-axis indicate the case of three limbs of the inductor having different values due to tolerance. An overall system harmonic analysis is crucial to define the best filter solution [1-2] to fulfill the regulations [4-6]. Fig. 5 shows the outcome of an analysis realized with the PQS tool [13] for the case of multi-motor system. Fig 5. Harmonic analysis of a multi motor system performed with PQS of Schaffner. In particular, only one motor is equipped with VSD, and the effect of VSD partial loading on the harmonic distortion is considered, with and without PHF. If the rating of the VSD is larger than the demand of both motors, then the resulting TDD at PCC and THDi for the VSD line have different trend. 151

5 4. Figures of merit of rectifiers for VSD applications A state-of-the-art LV VSD is typically constituted by a diode rectifier front-end cascaded by a voltage source inverter which drives the electric motor. In order to comply with harmonic standards [4-6], the diode bridge is commonly equipped by a ST harmonic filter, able to reduce the THDi, typically below 5%. If the application requires regeneration back into the mains, active-front end (AFE) is used. An active rectifier is able to fulfill the harmonic requirements. The enhanced controllability has an immediate impact on cost, reliability and availability of the system. A compromise to improve the input current THDi is the use of multipulse rectifiers, typically 12 or 18 pulses, together with multi-winding input transformers [11]. This kind of rectifier stage increases considerably the complexity of the front-end and has the drawback of reduced performance in presence of imbalance. A further possibility to reduce the THDi of the drive is to actively compensate harmonics through an active harmonic filter (AHF), which is commonly a shunt device installed at the input terminals of a single or multidrive system [3, 11]. Fig. 6 presents figures of merit, derived from [2,3,11] of rectifiers with low harmonic impact on the mains, typically used in VSD applications. a) b) Fig. 6. Figures of merit of rectifiers typically used in VSD applications for nominal loading, a) passive interfaces and b) active interfaces as active harmonic filter (AHF) and active front end (AFE) compared to passive rectifier equipped with PHF. Line reactor in input of diode bridge is a further simple, cost effective solution. 5. Experimental verification The harmonic performances of two rectifiers, typically used as input rectification stage for 2Q variable speed drive applications, were compared; in particular, the following rectifier topologies were considered: A passive rectifier, 3-phase diode bridge, without DC-link choke, equipped with a ST passive harmonic filter tuned at the 5 th harmonic, designed for a target THDi < 5%. An active front-end (AFE), 3-phase, 2-level PWM rectifier equipped with LCL and EMI input filter stages and switching at a frequency of 1.5 khz. For both rectifiers, the input current and voltage are 95Arms and 400Vrms respectively, the mains frequency is 50 Hz, 75 kw the power rating. In addition, the current spectra were derived and current harmonics until the 40 th order shown in Fig

6 Fig. 7. Rectifiers and their respective input filter stages considered for the comparative evaluation of harmonic performance: a) passive rectifier, 3-phase diode-bridge featuring a single tuned (5 th harmonic) passive harmonic filter and b) AFE equipped with LCL and EMI filters. a) b) Fig. 8. Measured input rectifier current waveforms, a) passive rectifier with PHF and b) AFE equipped with LCL and EMI input filters. The amplitude of the measured current waveforms is the same, 95 Arms, the frequency is 50 Hz. PHF L i mh L o mh L t 0.94 mh C t 3x 440 uf wye LCL L i mh L o 0.33 mh C D 3x 40 uf delta EMI C 10 uf L CM1 0.4 mh 26 uh L CM2 Table 1. Parameters of the filters in input to the rectifiers, as shown in the circuits in Fig.7. Source impedance condition is same for both setups. The total harmonic content (THC) and the reference current I ref were assessed using the formulas as defined in [5] for both measured input rectifier currents shown in Fig.8. The THDi defined after [6] is calculated as well to estimate the harmonic performance; in the IEEE 519 the harmonic content is typically up to the 50 th order. 153

7 Fig. 9. Input rectifier current waveforms spectra. Harmonic frequencies are displayed until 40 th, harmonic amplitudes are in a) passive rectifier with PHF and b) AFE equipped with LCL and EMI filters. The amplitude of the fundamental component (h=1) is 95Arms; the enlarged vertical axes cut the representation of these values. Passive rectifier + PHF AFE + LCL, EMI filters THC 4.34 Arms 6.38 Arms I ref 92.2 Arms 94.4 Arms THC/I ref 4.7 % 6.7% THDi 4.7 % 7% Table 2. Harmonic performance of the tested rectifier systems. For this particular case study, the rectifiers and their respective input filters were commercially available equipment and were not modified for the tests. The measured harmonic performance at nominal loading is summarized in Table 2. The passive rectifier equipped with ST filter yields to a better THDi; nevertheless the PHF corresponding to the values in Table 1 is about 50% more expensive than the LCL combined with EMI filter. However, for same target harmonic performance, e.g THDi = 4.7%, and same switching frequency, the cost of LCL and EMI filter stages would exceed the price of a PHF. Furthermore, a passive rectifier equipped with PHF results about 25% cheaper than a AFE featuring LCL and EMI filter, for a power rating ranging from few kw up to several hundreds of kw [7]. The higher contribution to harmonic distortion of an active rectifier is expected to occur at PWM switching frequency f s in the khz range. If f s is lower than 2 khz, corresponding to the 40 th harmonic order, the switching ripple will contribute to increase the harmonic content as defined by the norms [5, 6]. It is good practice to verify for active rectifiers the amplitude of lower harmonic orders [9], which can increase, for instance, if low frequency signals injection techniques are applied to improve modulation [10]. On the other side, if f s is higher than 2.5 khz, the ripple components will be shifted in a range, khz, for which the compatibility limits are still not regulated. The IEC SC77A WG8 has the task of the standardization for khz, which will result in an updated standard, eg. IEC [8]. For an AFE the LC filter stage has to attenuate the ripple current. Further optimization of the filter components is possible and it consists in finding the trade-off between filter volume and overall system efficiency [9]. 154

8 6. Conclusion Passive harmonic filters in combination with three-phase passive rectifiers constitute the most cost effective, technically simple and robust front end of variable speed drives with low effects on the mains. To solve the harmonic mitigation problem and to properly design the harmonic filter, a careful system analysis is needed and dedicated simulation tools, like the PQS [13], help to take into account and verify all the most important design requirements. The paper has shown that special attention needs to be paid on the selection of tuning frequencies and damping factors. Interactions between harmonic filter and system impedances can cause system resonance, which can be avoided by proper shaping of the filter frequency characteristic. Figures of merit for rectifier front-ends typically used in VSD applications were compared to show the superior advantages of passive rectifiers equipped with PHF. Finally, the experimental comparative evaluation of harmonic performances of passive and active rectifiers confirmed the economic and performance benefits of a passive filter solution. 7. References [1] M. Allenbaugh, T. Dionise, T. Natali, Harmonic Analysis and Filter Bank Design for a New Rectifier for a Cold Roll Mill, IEEE Trans. Ind. Appl. Vol 49, No. 3, May 2013 pp [2] J.C. Das, Passive Filters- Potentialities and Limitations, IEEE Trans. Industry Appl. Vol 40, No. 1, Jan./Febr pp [3] H. Akagi, Modern active filters and traditional passive filters, Bulletin Polish Academy of Sciences, Vol. 54, No. 3, 2006, pp [4] IEC EMC - Part 3-2: Limits for harmonic current emissions (equipment input current 16 A per phase)". [5] IEC EMC - Part 3-12: Limits for harmonic currents produced by equipment connected to public low-voltage systems with input current >16 A and 75 A per phase. [6] IEEE , IEEE Recommended Practice and Requirements for Harmonic Control in Electric Power Systems. [7] ABB low voltage drives, ACS800, Product Pricing List, Online Available. [8] IEC :2002, EMC - Part 2-2: Environment - Compatibility levels for low-frequency conducted disturbances and signalling in public low-voltage power supply systems [9] J. Mühlethaler et al. Optimal design of LCL Harmonic Filters for Three-Phase PFC Rectifiers, IEEE Trans. Power Electr., vol. 28, No, 7, July 2013, pp [10] L. Dalessandro et al. Center-Point Voltage Balancing of Hysteresis Current Controlled Three-Level PWM Rectifiers, IEEE Trans. Power Elect., Vol 23, No. 5, Sept. 2008, pp [11] B Bahrani, R Grinberg, Investigation of harmonic filtering for the state-of-the-art variable speed drives, Proc. 13 th European Conf. on Power Electronics and Application EPE 2009, pp [12] Harmonic Filter, Granted patent family US B2. [13] Power Quality Simulator, 155