Microsoft Word - 6-695-ed.doc Engineering, Technology & Applied Science Research Vol. 6, No. 5, 2016, 1139-1148 1139 www.etasr.com Mohamadian and Khanzade: A Five-Level Current-Source Inverter for Grid-Connected or High-Power… A Five-Level Current-Source Inverter for Grid- Connected or High-Power Three-Phase Wound-Field Synchronous Motor Drives Sobhan Mohamadian School of Engineering Damghan University Damghan, Iran s_mohamadian@iust.ac.ir Mohammad Hosein Khanzade Department of FAVA Engineering Imam Hossein Comprehensive University Tehran, Iran mhokhanzade@gmail.com Abstract—Simple converter structure, inherent short-circuit protection and regenerative capability are the most important advantages of current-source inverters (CSI’s) which have made them suitable for medium-voltage high-power drives. Usually in grid-connected gas turbine generators or pumped storage hydro power plants, efficient and reliable current-source load- commutated inverters (LCI’s) with thyristor switches are employed. Also, this type of CSI is widely used in very large drives with power ratings of tens of megawatts to supply wound- field synchronous motors (WFSM’s). However, LCI’s suffer from some disadvantages such as large torque pulsations, poor power factor, and start-up criticalities. In this paper, a novel multilevel- based CSI is proposed. The proposed converter consists of one LCI and one CSI bridge with self-turn-off switches along with a voltage clamping circuit. The CSI switches are forced commutated; hence, a voltage clamping circuit is employed to limit voltage spikes caused by current variations in inductive paths during commutation transients. Drastic reduction in harmonic distortion of stator current and improved fundamental power factor are achieved by the proposed topology. In addition, torque pulsations are reduced remarkably for normal and starting operating conditions. Comprehensive analysis of the proposed structure is presented and the design of converter components is elaborated. Keywords-current-source inverter (CSI); five-level current waveform; load-commutated inverter (LCI); voltage clamping circuit; wound-field synchronous motor (WFSM) drive I. INTRODUCTION In industrial current-source inverter (CSI)-fed drives, self- turn-off power devices such as gate turn-off thyristors (GTO’s) or insulated gate bipolar transistors (IGBT’s) are switched by pulsed-width modulation (PWM) schemes such as trapezoidal PWM (TPWM), selective harmonic elimination (SHE) and space vector modulation (SVM) to attenuate the low-frequency harmonic components of the machine stator current [1]1-3]. In these structures, a three-phase capacitor bank is mandatory at the CSI output to absorb the high-voltage spikes produced due to the switching of the current in inductive paths. The capacitor bank also serves as a filter for high-frequency current harmonics created by the PWM switching. However, this capacitor bank has some drawbacks such as the possibility of the resonance risk between output capacitors and machine magnetizing inductance at the fundamental frequency or machine leakage inductance at higher harmonic orders [4]. Thus, the capacitor bank must be designed so that the lowest harmonic order of the stator current is much higher than the resonance frequency. Accordingly, the bank size will typically range between 0.3 per unit (pu) to 0.5 pu for switching frequencies of approximately 400 Hz [1]. Furthermore, the effect of the capacitor bank on the motor current and air-gap flux specifically at high operating speeds must be taken into account in the control system so as to get adequate response and avoid instability of the drive at transients [5]. In addition to these disadvantages, for some high-power applications (usually higher than 10 megawatts [6]) using PWM switching is not usual in CSI-fed drives since the converter efficiency is reduced due to the switching and conduction losses of the self-turn-off power devices. Therefore, load-commutated inverter (LCI)-fed wound-field synchronous motor (WFSM) drives are the preferred choice in this power range because of the higher efficiency and reliability of the converter. Also, the initial investment cost of the converter is low due to the inexpensive silicon-controlled rectifier (SCR) devices [1]. In these drives, WFSM operates at over-exciting mode resulting in natural commutation of the thyristors [7]. Therefore, there is no need for capacitor banks. LCI-fed drives are widely used in high-power applications specifically in pumped-storage power plants and gird- connected synchronous machines for star-up purposes [8, 9]. Due to the great increase in social electricity consumption level and the proliferation of the capacity of nuclear power and renewable energy resources, it was expected that the capacity of pumped-storage power plants would gain more than 200 GW worldwide by 2014 [10, 11]. There is also a great trend in utilizing LCI’s for driving high-power compressors used for liquefying natural gas, pumps, fans [12-14], grinding mills in mining applications [15] and ship propulsion systems [1]. Engineering, Technology & Applied Science Research Vol. 6, No. 5, 2016, 1139-1148 1140 www.etasr.com Mohamadian and Khanzade: A Five-Level Current-Source Inverter for Grid-Connected or High-Power… However, the quasi-square waveform of the stator phase current of these drives is a major problem. This waveform is rich in low order harmonics, specifically 5th and 7th orders, which result in large torque pulsations and increased winding and iron losses. The torque ripples can interact with the torsional natural frequencies (TNF’s) of the mechanical system and lead to accelerated shaft fatigue, lifetime reduction, gear- box damage, and system failures [12-14]. The most common method to reduce the torque pulsations is to use dual three- phase WFSM in which each three-phase winding set of the machine is independently supplied from a three-phase LCI [6, 16]. In this configuration, the 6th order of the electromagnetic torque is cancelled out. Although, this solution is not always possible, e.g. it does not cover the cases where LCI’s are utilized to feed grid-connected (three-phase) generators for start-up before synchronization or in pumped storage hydro power plants [7]. Another solution is to use multi-pulse converters. In these structures, two series or paralleled 6-pulse (three-phase) LCI’s are connected to a three-phase three- winding phase-shifting transformer feeding a three-phase WFSM to eliminate the 6th order harmonic of the torque. However, the transformer is bulky and expensive and increases system losses [17]. On the other hand, recent development in in high-power semiconductor devices such as IGBT’s has drawn the attention to use voltage-source inverters (VSI’s) in these applications. In [11], a multilevel cascaded H-bridge (CHB) VSI is used for the start-up of synchronous motor in a pumped storage power station. In this configuration the harmonic distortion of the stator current is improved remarkably, however, the volume of the converter is increased since there are nine power cells connected in series each phase. Another problem regarding to LCI-fed WFSM drives is the start-up operating conditions. At standstill or low speeds, the generated back electro-motive forces (EMF’s) of the machine are not large enough to turn off thyristors and the load commutation process, as will be explained in III.A, is not accomplished. To cope with this problem, dc-link pulsing method is generally used [7, 18]. In this method, the dc-link current is forced to zero by the operation of the rectifier at the commutation instants. Therefore, the previous conducting thyristors in the LCI is turned off since its current reaches below the holding current [19, 20]. After a short interval, the next incoming thyristor is fired and the dc-link current is again built up to its reference value by the rectifier. However, this method produces high torque ripples. In this paper, a novel multi-objective structure for LCI-fed three-phase WFSM’s is proposed based on the five-level current-source configuration. In addition to several advantages related to high-power multilevel converters such as reduction of the voltage and current stresses of the switching devices and lower harmonic distortion of the load current [21][21], the machine fundamental power factor is improved significantly. Furthermore, the start-up criticalities of the LCI-fed WFSM drives are removed with the proposed structure. II. STRUCTURE OF THE PROPOSED FIVE-LEVEL CONVERTER The proposed five-level converter is shown in Figure 1 and consists of two converters (that supply the main power of WFSM): an LCI with thyristors T1-T6 and a CSI with self-turn- off power devices S1-S6. In CSI-fed WFSM drives, each motor phase can be modeled as a sinusoidal back EMF in series with the commutation inductance, LC, which is approximately equal to (Lʺd+Lʺq)/2 [19], as shown in Figure 1. Lʺd and Lʺq are the machine sub-transient inductances along rotor d and q axes, respectively. It should be mentioned that stator resistances are neglected in this circuit. Fig. 1. Proposed five-level CSI for WFSM drives. Both converters are switched at the machine fundamental frequency. Since the CSI switches are forced-commutated, the variation rate of current in machine inductances must be limited at the commutation transients. On the other hand, the capacitor bank cannot be utilized because its size would be very large due to the low switching frequency of the converters. In this paper, a voltage clamping circuit is used to inhibit voltage spikes. Voltage clamping circuits usually consist of a diode rectifier to transfer the reactive energy of the machine to an electrolytic capacitor at the commutation intervals [21]. In order to avoid the continuous increment of the capacitor voltage, an energy recovery circuit such as a DC/DC chopper or an inverter is used to pump the capacitor energy into the dc-link or ac-grid, respectively [21]. In this paper, a voltage-source inverter (VSI) is utilized with six self-turn-off devices K1-K6 and six free-wheeling diodes D1-D6 as shown in Figure 1. The same VSI is introduced in [22] to reduce voltage spikes in a thyristor-based CSI-fed induction motor drive. In [22], the VSI is employed for two main tasks: 1) forced-commutation of the thyristors and 2) clamping the voltage spikes. While, the VSI in the proposed multilevel converter serves only as the voltage clamping circuit during commutation transients and thyristors are turned off naturally. For this reason, as will be explained in Section III.B, the conduction states of the switches in the proposed multilevel are different and simpler than those presented in [22]. III. OPERATION PRINCIPLES OF THE PROPOSED FIVE-LEVEL CONVERTER The desired stator phase current waveform, e.g. ia(t) in Figure 1, of the proposed five-level converter is shown in Figure 2a. This waveform results from the summation of two quasi-square waveforms, i.e. iLCIa(t) and iCSIa(t), at node a Engineering, Technology & Applied Science Research Vol. 6, No. 5, 2016, 1139-1148 1141 www.etasr.com Mohamadian and Khanzade: A Five-Level Current-Source Inverter for Grid-Connected or High-Power… (Figure 1). As in the case of conventional LCI’s, iLCIa(t) and iCSIa(t) have a pulse-width of 120 electrical degrees in each half cycle and iLCIa(t) leads iCSIa(t) by 30̊. The resultant waveform of ia(t)=iLCIa(t)+iCSIa(t) has five levels of 0, Idc/2 and Idc. The phase displacement between iLCIa and iCSIa, i.e. ∆t where  is the motor speed in electrical rad/sec, is set to 30̊ so as to minimize the THD of ia as shown in Figure 2b. According to this figure, the amplitude of the 5th and 7th harmonic components of the desired phase current is equal to 5.36% and 3.83% of the fundamental for ∆t=30̊, respectively, while they would be 20% and 14.35% if the usual LCI topology with quasi square-wave currents were employed. A. Operation principles of LCI The switching pattern for the LCI thyristors is shown in Figure 3. Since the thyristors of the LCI are naturally commutated, currents iLCIa(t), iLCIb(t) and iLCIc(t) must lead the phase voltages ea(t), eb(t) and ec(t) with a minimum angle of LCImin=µ+tq [19], where µ is the commutation angle and tq is thyristors turn-off time. Considering phase ‘a’ as an illustration, practically negative and positive zero-crossing instants of eac(t) are used as the reference for firing T1 and T4, respectively. Thus defining  as the firing angle, T1 will be fired at point A in Figure 2a and the commutation interval ‘1’, shown in Figure 3, is started. Conduction of T1 produces a short-circuit loop between the incoming phase ‘a’ and the outgoing phase ‘c’ as shown in Figure 4 in which eac(t) drives iLCIc(t) to decrease from Idc/2 to 0 and iLCIa(t) to increase from 0 to Idc/2. The current variations are almost linear since the stator phase resistances are practically negligible. The commutation phenomenon lasts for µ electrical degrees. Consequently, the current in T5 is diminished to zero and the commutation interval is finished. After commutation, the voltage across T5 (eac(t) in this case) must be negative for a time interval of at least equal to tq in order to be able to withstand positive voltages [20]. Usually in high-power LCI-fed drives the maximum firing angle is set to 150̊ to avoid commutation failure. In this case, the fundamental component of the current leads stator phase voltage by LCI =30̊. It should be mentioned that in a conventional three-phase three-level LCI (with levels of 0, +Idc and -Idc in the output waveform of phase current), the fundamental power factor angle, LCI, is equal to . At higher loading conditions, the firing angle is lower than 150̊ which can be translated into a relatively high reactive power. B. Operation principles of CSI and VSI The switching instants and sequences for S1-S6 are the same as T1-T6 except for a 30̊ displacement as shown in Figure 5. For instance, S1 must be switched on at point B (Figure 2a) which is 30̊ later than the firing instant of T1. Contrary to the situation explained for LCI at point A, the voltage eac(t) might be positive at point B and, consequently, the current in phase ‘c’ cannot be naturally commutated to phase ‘a’ by switching of S1. Therefore, an external negative and positive voltage must be applied to phases ‘c’ and ‘a’, respectively, to force ic(t) to decrease from Idc/2 to 0 and ia(t) increase from Idc/2 to Idc. In this way, the currents in the outgoing and incoming phases should not abruptly change in order to limit the voltage spikes over the machine inductances. The external voltage source can be a dc capacitor which is applied by proper switching of VSI at commutation instants. Fig. 2. (a) iLCIa(t), iCSIa(t) and ia(t) waveforms and their corresponding fundamentals and WFSM phase and line voltages, (b) 5th and 7th harmonic amplitudes (relative to fundamental) and THD for the phase current ia as a function of ∆t. Fig. 3. Switching pattern of LCI.  corresponds to point A in Figure 2a Fig. 4. The loop created by the simultaneous conduction of T1 and T5 (commutation interval ‘1’ in Figure 3) and (b) current variations. Engineering, Technology & Applied Science Research Vol. 6, No. 5, 2016, 1139-1148 1142 www.etasr.com Mohamadian and Khanzade: A Five-Level Current-Source Inverter for Grid-Connected or High-Power… Fig. 5. Switching pattern of CSI and VSI devices. +/6 corresponds to point B in Figure 2a. Fig. 6. CSI and VSI equivalent circuits for: (a) normal conduction before commutation between phases ‘a’ and ‘c’ starts, (b) mode 1, (c) mode 2 and (d) mode 3. Fig. 7. Waveforms related to commutation between phases ‘a’ and ‘c’, (a) iCSIa and iCSIc, (b) D1 and D2 currents and -icap, (c) vcap. In order to explain the operation of CSI and VSI at commutation instants, consider switches S5 and S6 of the CSI and T1 and T6 of the LCI are conducting with the equivalent circuit shown in Figure 6a and S1 is switched on at t=t0 (point B in Figure 2a). Thanks to the symmetrical operation of the circuit, we confine our analysis to one sixth of a cycle which can be divided into three operating modes. The equivalent circuits of these three modes and their corresponding waveforms are shown in Figs. 6 and 7, respectively. In the following subsections, details are given on how forced commutation is obtained in the CSI bridge using the VSI. The description includes mathematical details which are however required to obtain some practical dimensioning rules regarding capacitors Cdc as well as VSI switches K1-K6 and D1- D6. 1) Mode 1 (t0