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
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•
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,
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61,
No.
2,
pp.139-147,
June
2002.