INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 11 Digital Modulation Reference and Phase Shifted Triangle Carrier Signal Synthesizer for PWM MultiConverter fed AC Drives G. K. Singh, V. Pant, and Y. P. Singh Abstract-- This paper focuses the design and development of voltage source multi-converters with phase shifted triangular carrier modulation, suitable for application in high rating ac drives; as well as in active power conditioner such as STATCOM /UPFC when used in inverted mode. In multiconverter topology, several voltage source PWM inverters using common sinusoidal modulating signals and phase shifted triangular carrier are operated in tandem with the outputs coupled through current sharing inductors, with an objective of increasing the overall rating to mega-watt range, reducing the harmonic current and torque ripples, increasing the reliability with phase-redundant operation under various fault conditions. Index Terms-- Drive, Multi-Phase, Multi-Converter, UPFC I. INTRODUCTION During early nineties in USA and Japan, Gate Turn Off thyristors (GTO’s) were being used successfully in static power controllers in the range of 80 MVA [1]. The switching rate of GTO’s was only one pulse per 60 Hz periods in order to have minimum switching losses and optimum efficiency. The inherent large on-state losses and large commutation losses due to the existence of tail current were the reasons for such a low rate of switching although the design turn-on and turn-off times set by the manufacturers were quite low i.e. of the order of tens to hundred of micro-second. The survey reveals that the high power converters rated tens of kilo-volt to hundreds of kilo-volt and mega-volt amperes are beyond the capacity of a single solid-state switch of maximum possible kilo-volt and kilo-ampere ratings. The high voltage, large rating PWM inverters, employing power switching devices in series and /or parallel configuration, suffers from the limitations such as an unequal voltage and current sharing, existence of high common mode voltages [2], and corona discharge, voltage surges /dielectric stresses and the resulting motor winding insulation breakdown as well as motor bearing failure, mainly due to the excessive dv/dt. These limitations have been overcome to a great extent through a basic concept of using the devices ingeniously in such away that ultimately lower dv/dt exists in the converter. Based upon converter topologies used in high current-high G. K. Singh is with the Electrical Engineering Department, Indian Institute of Technology, Roorkee-247667, India, (telephone +91-1332-85070, e-mail: gksngfee@iitr.ernet.in). V. Pant is with the Electrical Engineering Department, Indian Institute of Technology, Roorkee-247667, India, (telephone +91-1332-85396, e-mail: vpantfee@iitr.ernet.in). Y. P. Singh is with the Electrical Engineering Department, Indian Institute of Technology, Roorkee-247667, India (telephone +91-1332-85074, e-mail: ypsfee@iitr.ernet.in). voltage power applications, the power converters can be categorized into two major groups with a view to provide a perfect output to input linearity, best efficiency and power quality. Group I: Multilevel Inverters (MLI’s) The multilevel PWM inverter combines the multilevel waveforms and PWM concept to produce high power output with low harmonic contents. In MLI’s, the switching is in between several voltage levels that results in low dv/dt. Hence, the dv/dt related problems are reduced. In order to improve the quality of the output waveform, the most popular subharmonic PWM with one modulating reference and several phase shifted triangular carriers [3] is used. Although the MLI’s have reduced THD, dv/dt stresses and EMI related problems, the inverters of this group have several limitations such as; increased overall KVA rating of the converter module for a given power due to large device count, a problem with voltage balancing of DC bus capacitors in absence of explicit control strategy, complex power converter configurations, reduced power capacity utilization at low modulations indices unless an explicit carrier phase rotation technique is adopted that results in complicated PWM control signal generation. Group II: Multi-Converter /Linear Power Amplifier In multi-converter topology [4, 5], several voltage source PWM inverters using common sinusoidal modulating signal and phase shifted triangular carriers operate as linear power amplifiers with their outputs coupled through current sharing inductors. To obtain an ultimate high voltage and high current rating in overall converter system, the ‘parallel connected series paired modules’ (PCSP) power circuit structure is used. The high voltage rating is obtained through series connection of low voltage converter module structures (LVCM), whereas high current rating is obtained by parallel connection of the multiple LVCM to impart the following specific features: • The devices are operated at reduced switching frequency resulting in better efficiency due to reduced switching loss. • Staggering of switching instants in the individual converters due to the phase shift in triangle carrier creates an overall high frequency switching effect at the ac side of the paralleled module structure of the multi-converter system. • The triangle carrier frequency for the individual converter is selected judiciously so that each module behaves as a ‘linear power amplifier’. NATIONAL POWER SYSTEMS CONFERENCE, NPSC 2002 12 • Due to the relatively higher switching frequency (N times the carrier frequency, N is number of converter modules), the switching noise of the carrier frequency and the lower side band frequencies do not spread over the spectrum of modulation signal (reference sine) and the resulting current unbalances in ac side are avoided [4]. • Voltage stress on the devices are ensured to remain within the range of DC link capacitor voltage, which is set through negative feedback loop around each converter [4]. The multi-converter system exhibits direct control of magnitude and /or frequency; output to input linearity, and power quality with the possibility of implementation of sophisticated modern control theory. Such converter have intensive application in FACTS as shunt or series STATCOM’s and UPFC’s [5] as well as in high rating polyphase ac drives, which have demanding applications in rolling mills, electric tractions; gearless mill drives for cement industry, centrifugal pumps and compressors. As high performance converters, they also have applications in power supplies for super conducting magnets, as linear power amplifiers in high capacity electrodynamic vibrators /shakers and UPS. For high rating ac drives, multi-phase (more than three phase) ac machine [6] fed through multi-converter system seems to be the best proposition provided a detailed study of such a drive is carried out with respect to the following factors: • Improvement in the harmonic currents and torque pulsations, due to the carrier frequency multiplication effects in the multi-converters. • Feasibility of practical realization of mega-volt ampere rating with the available power switching devices particularly IGBT’s. • Switching power loss minimization by selecting suitable switching frequency modulating signals and improved gating circuits for IGBT’s and commutation circuits for GTO’s. • Superior reliability under dynamic loading and overloading due to better current and voltage sharing amongst LVCM’s. • Full utilization of individual LVCM to increase the overall efficiency of the converter system while operating at low modulation index. Hence, a PC based generalized digital modulating reference and phase shifted triangle carrier signal synthesizer capable of producing a variety of modulating and sets of triangular carrier waves for onward PWM signal generation for poly phase multi-converter system is developed and the experimental results are presented in this paper. II. SCHEMATIC DETAILS In this scheme, a master controller, generalized digital modulating reference wave and phase shifted triangular wave synthesizer are used along with the PWM modulator and pulse isolation circuit to drive a multi-phase (more than three phase) induction machine. This drive incorporates two sets of multiinverters termed as group I and II. Each group is producing variable frequency and variable voltage three-phase output with a relative phase difference of 00, 300, and 600 between the two groups. Group I consists of two modules operating as PWM VSI, a common modulating (reference sine) signals and a set of phase shifted triangular carriers fTR ∠0 and fTR ∠1800. The coupling inductors Lxn are used to combine the output of the individual modules, which results in not only the higher power rating but also increased equivalent switching frequency. Thus, there is an improvement in quality as well as capability. It is evident that the controller must provide 12 PWM signals with two inverters and 24 PWM signals with 4 inverters in each converter group. This task is effectively and efficiently handled by the ‘dedicated DMRTCS’ while operating under direct control of DSP and micro controller serving as master controller for the drive. The use of a separate and dedicated DMRTCS virtually relieves the DSP or micro controller from the time consuming and continuous duty of generating the required modulating reference and triangle carrier waves. Thus, the DSP along with micro controller can be effectively used for more useful and essential task of ‘data acquisition, fault detection, diagnosis and related corrective actions as well as computational work related to the operation and control of drive in open loop or closed loop with different control strategies. The master controller/ DSP provides two fSCLK, fTCLK as clock signals and a 12 bit amplitude control command. fSCLK clock governs the base frequency of the reference modulating signals and digital amplitude control command, governs the amplitude of all the reference modulating signals. The fTCLK clock is produced in synchronization in direct proportional relationship to the modulating reference signal. The developed digital modulating reference and triangle carrier signal synthesizer (DMRTCS) module is tested by fTCLK and fSCLK generated through a PC using an internal system clock of 2 Mhz along with two timers Tm1 and Tm2 associated with a timer and I /O add-on card. The fTCLK is generated through Tm1 with a count value of 4 or 2 in order to produce the triangular carriers of 976 Hz or 1952 Hz respectively. The fSCLK is generated through Tm2 with a count value set by an adaptively varying modulation ratio index (MRI). The index MRI is calculated based upon the reference speed NRR (12 bit) data. A typical value of MRI as calculated (NRR = 0FFFH or 4095D; FSR = NRR *50 /4095; MRI = 7812 /FSR) is 156 for a maximum reference modulating signal frequency of 50 Hz. The value of MRI is graded from 156 to 63,960 giving a frequency control range of 410:1 with the minimum possible frequency setting of 0.12 Hz. Such a high resolution or accuracy in modulating signal generation is not possible with available analog schemes. The observations have been recorded in Fig 6 with a time scale of 1 ms /div for RI1 phase and 600 phase shifted RII1 phase sinusoidal reference modulating signals VSRI-1 and VSRII-1 along with the triangle carrier fTR ∠00 and PWM gating signal VSRI-1. The amplitude command set for Fig. 1 is 0FFFH that corresponds to the maximum amplitude of the reference-modulating signal but for the clarity in the recorded PWM gating signals, the amplitude is scaled down through scaling amplifier to a modulation index (MI = AS / AT) of 0.6. However, in actual control it can be set according to the requirement. Fig 2 (a) depicts the VSRI-1 and VSRII-1 signals with INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR 721302, DECEMBER 27-29, 2002 PC based adaptive grading of MRI with an initial (minimum) speed command setting of 019A H to a final speed setting of 0FFFH, with an incremental step of 28H representing an initial, final and incremental modulating reference signal frequencies as 5 Hz, 50 Hz and 0.5 Hz respectively with a time base of 100 ms /div. The amplitude in this case is incremented at each step with a constant v/f relationship. The time base extended version of the waveform of Fig 2(a) is shown in Fig. 2(b), with time base of 5 ms /div confirming that the definite phase relationship between the VSRI-1 and VSRII1 is maintained even under dynamic condition. III. CONCLUSION A detailed study (all the results not included here) confirm that the DMRTCS acting as a dedicated PWM signal generator is capable of producing six modulating reference signals with a relative phase shift of 300, 600 or any arbitrary angle and having a computer /microprocessor based adaptive control of base frequency along with requisite amplitude control of modulating reference signals. It also produces the requisite triangular carrier signals for generating 12 or 24 sets of PWM gating signals for each VSI PWM inverter used in multiconverter system. It is evident that DMRTCS virtually relieves the PC for executing its essential tasks such as data acquisition, computational and other control functions. Finally, it is proposed to use low voltage-high current topology in module structures having the three phase bridge converters operating in voltage source topology and adopting a bipolar sinusoidal pulse width modulation (SPWM) strategy with phase shifted triangular carriers. This structure would impart the following special features: • • • • 13 Best utilization of the power switching devices with respect to their switching speed and volt-ampere rating. Optimum efficiency, performance under steady state and dynamic conditions with improved reliability. Better performance even under fault conditions. Ideally suited for special application drives requiring uninterrupted and redundant operations; STATCOM’s, FACT’s etc. IV. REFERENCES [1] B. Mwinyiwiwa, Z. Wolanski and B. T. Ooi, ‘Current equalization in SPWM FACTS controllers at lowest switching rates’, IEEE Trans. on Power Electronics, Vol. 14, No. 5, pp. 900- 905, September 1999. [2] H. Zhang, A. V. Jouanne, S. Dai, A. K. Walace, Fellow and F. Wang, ‘Multilevel inverter modulation scheme to eliminate common-mode voltages’, IEEE Trans. on Industry Applications, Vol. 36, No. 6, pp. 1645-1653, Nov. /Dec 2000. [3] L. M. Tolbert, F. Z. Peng and T. G. Habetler, ‘Multi-level PWM method at low modulation indices’, IEEE Trans. on Industry Applications, Vol. 16, pp. 719-725, July 2000. [4] B. Mwinyiwiwa, Z. Wolanski and B. T. Ooi, ‘Microprocessorimplemented SPWM for multi-converters with phase-shifted triangle carriers’, IEEE Trans. on Industry Applications, Vol. 34, No.3, pp. 487494, May /June 1998. [5] B. Mwinyiwiwa, Z. Wolanski and B. T. Ooi, ‘UPFC using multiconverter operated by phase-shifted triangle carriers SPWM strategy’, IEEE Trans. on Industry Applications, Vol. 34, No.3, pp. 495-500, May /June 1998. [6] G. K. Singh, ‘Multi-phase induction machine drive research—a survey’, International journal of Electric Power Systems Research, USA, Vol. 61, No. 2, pp.139-147, June 2002.