A series-resonant converter used as an amplitude and
frequency function generator
Citation for published version (APA):
Huisman, H., & Gravendeel, B. (1988). A series-resonant converter used as an amplitude and frequency
function generator. In Proceedings of the 3rd International Conference on Power Electronics and Variable Speed
Drives, 13-15 July 1988, London (pp. 146-148). Institute of Electrical Engineers.
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Published: 01/01/1988
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146
A SERIES-RESONANT CONVERTER USED AS AN AMPLITUDE AND FREQUENCY FUNCTION GENERATOR
H.Huisman and B. Gravendeel
Delft University of Technology, Delft, the Netherlands.
Abstract
A series-resonant power converter system is presented which allows generation of multiphase output voltages with very low distortion at high efficiency. The selfcommutated resonant operation mode ensures the converter to be short-circuit proof. After a discussion of the control concept some typical output voltages and efficiency measurements are shown.
Introduction
Current trends in electrical drive techniques, such as the well-known vector control method (for an overview see Leonhard (1)) for servo applications, have nurtured the demand for fast and accurate multiphase power converters with high efficiency. The established work- horse in the field appears to be the transistor- or GTO-ized PWM voltage source inverter (VSI). The advantages of the VSI technique, being its relative simplicity and low part count, are in some applications countered by its main drawbacks: restriction to low switching frequencies at medium power levels, high levels of EM1 if power cables are not properly shielded and low ruggedness as regards short-circuit behaviour. Work in our group has led to the development of a novel philosophy of power converter design by using resonant techniques. The smooth internal waveforms and absence of HF common-mode voltages at both input and output terminals of the converter provide for low EMI, while the discrete-charge operation mode guarantees the converter to be short-circuit proof. Low values of di/dt at switching instants provide for low switching losses (Schwarz (Z)), thereby allowing a high internal frequency.
Power circuit
A simplified schematic of the power circuit of the converter system is depicted in fig. 1.
Main components are thyristors, a resonant tank consisting of L1 and C1, filtering capacitors (Cr-Cw), commutation inductances (Lr-Lw), and the three-phase source and load.
The drawing suggests the circuit to be build up as two three-phase thyristor bridges. Topologically, however, the circuit can be viewed as a six-phase thyristor bridge as well, and for modularity it is controlled as such. The separation of functions between input and output bridges is made only in the most outer control loop. A significant feature of this power part is that every (input or output) terminal is connected by means of filter capacitors Cr-Cw to a common neutral, which can but needs not be the neutral of one or both connected three-phase systems. This configuration guarantees the line-to-neutral voltages to be free from HF common-mode components, such as can be present in other series-resonant converter configurations (Klaassens and Smits ( 3 ) ) and in PWM converters.
--
Low-level (fast) control
The six-phase symmetry of the power circuit can be used to a great extent in the control system. For every terminal of the six-phase circuit a voltage or current reference signal is defined. These signals are compared
to the actual terminal voltage or current in order to obtain six error signals err-r - err-w. The function of the common control circuit will then be to minimize a "total" error signal, thereby forcing every actual terminal voltage or current to match its reference. The "total" error signal err-tot in our setup is
constructed from the individual error signals err-r - err-w as follows:
Every time the controller senses that the total error exceeds a predefined level, a selection procedure is started which marks those three out of the six terminals which have the largest value of error signal. Only those three terminals are then used to generate one resonant pulse with the right polarity to reduce the magnitude of the three error signals involved. The resonant pulse effectively flows in a reduced converter, whose power circuit is shown in fig. 2.
The operation of this three-terminal converter has been described in extent in recent literature (Tilgenkamp et a1 ( 4 ) ) and will not be treated here.
It is important to note that the actual terminals used for the reduced converter in general are different for every subsequent cycle, catering to the exact needs of load and source, and can be composed as follows:
a: three input terminals b: three output terminals
c: two input terminals and one output terminal d: two output terminals and one input terminal In classical terms, modes a and b would be generating or absorbing reactive power, and modes c and d would be transferring power between input and output. This distinction is not relevant for the operation of the converter, however.
High-level (slow) control
Up to this point we have assumed that we could operate the converter with six degrees of freedom (or reference signals). Unfortunately, physics allows less freedom than we assumed and some controllers are needed in order to reduce this number. Physics restricts us in two ways:
- Kirchhoff's current law (we can't generate net - law of conservation of energy (we can't generate
charge) net power)
Both restrictions can be taken care of by using an adequate controller: one which forces the sum of reference currents for all terminals to zero and one which does the same for the net power consumed by the converter (disregarding losses). These controllers are more fully described in Huisman and de Haan ( 5 ) ,
Huisman ( 6 ) , and Huisman (7). Both controllers effectively reduce our number of degrees of freedom with 1, which leaves us four degrees which we can use to our liking. In the prototype converter we have used three degrees to control the three individual output
147
voltages, and the fourth and last degree to set the reactive current which is generated or consumed at the input side of the converter. Note that other
configurations are possible: we could use the fourth degree to set one input current to zero, thereby reducing the system to a two-phase to three-phase converter. Note also that from the configuration it follows that the number of output terminals is not restricted to three. In fact any number of outputs from zero to infinity can be handled.
Results
The control philosophy has materialized in the construction of a three-phase to three-phase 15 kW prototype converter operating on the 50
Hz 380
Vrms utility grid. Internal frequency was chosen to be 10 kHz.
Photo
at an output frequency of 100 Hz and at full load. The characteristic voltage waveform which is composed of a low-frequency signal with a superimposed HF ripple due to the modulation process is clearly visible. To illustrate the generation of the waveforms in photo taken before the capacitor filter and the current in the resonant tank circuit. Here the process of selective rectification of the resonant current, which produces the output and input waveforms, is visible. The speed of operation is shown in photo
output voltage reference signals are modulated stepwise between zero and rated voltage. The slope of the stepresponse was measured to be ca. 540 V/ms. It needs to be stressed that to the converter this peculiar operation mode looks the same as the "steady state'' shown in photo 1.
As a last item, efficiency of the converter system was measured for four different output voltages using wideband wattmeters. The results are shown in fig. 3a, b, c, and d.
Efficiency at full power and rated voltage reaches a nice 92 %, remaining high over a large part of the operating area.
1 depicts the output voltages of the converter
1, photo 2 shows the current in one output phase
3. Here the
Conclusion
A multiphase power converter system was presented which allows efficient transformation of AC or DC power to
the voltage and current level and shape desired by the multiphase (possibly active) load. Smooth internal and external waveforms allow operation at low EMI levels and easy, lowcost measurements in control systems. The speed of response will allow medium-power servo drives to reach very fast dynamical response, while retaining accuracy.
Acknowledgment
The research presented in this paper has been made possible due to a grant of PEO, the coordinating office for energy research of the Dutch Government.
Literature
( 1) W. Leonhard, Microcomputer Control of High Dynamic Performance AC-Drives
-
A
Survey. Automatica, Vol. 22, No.1, pp. 1-19, 1986.( 2) F.C. Schwarz,
A
Method of Resonant Current Pulse Modulation for Power Converters, IEEE Transactions on Industrial Electronics and ControlInstrumentation, Vol IECI-17, No. 3, pp. 209-21, May 1970.
( 3) J.B. Klaassens and Eugenio J.F.M. Smits,
A
series- resonant AC power interface with an optimal power factor and enhanced conversion ratio, Proceedings of Pesc-86, pp. 39-48, Vancouver 1986.( 4) N. Tilgenkamp, S.W.H. de Haan, and
H.
Huisman, A novel series-resonant converter topology, IEEE Transactions on Industrial Electronics, Vol. IE- 34, No. 2, pp. 240-246, May 1987.( 5) H. Huisman and S.W.H. de Haan, General control method for high-frequency multiphase power converters, Proceedings of Power Conversion International, Munich, pp. 201-219, May 11-13 1987.
( 6 ) H. Huisman,
A
three-phase to three-phase series- resonant power converter with optimal input current waveforms, Part 1: Control Strategy. IEEE Transactions on Industrial Electronics, may 1988.( 7) H. Huisman,
A
three-phase to three-phase series- resonant power converter with optimal input current waveforms, Part 2: Implementation and results. IEEE Transactions on Industrial Electronics, may 1988.Ur
uu
us
u v
ut
uw
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148
L - -~
photo 1: Output voltages of the converter at output frequency of 100 Hz, output rms line voltage of 380 V , full load.
photo 2: Buildup of output voltage from discrete resonant current pulses.
photo 3: Output voltages of the converter with stepwise modulation between zero and full power.
0
-
155 Volt 0 - 220 Volt 201
I
0 - 380 Volt
--
00 2 4 8 8 1 0 1 2 1 4 1 8 0 2 4 8 8 1 0 1 2 1 4 1 8 0 2 4 8 8 1 0 1 2 1 4 1 8 0 2 4 8 8 1 0 1 2 1 4 1 6 output power (kw output pomr ( k m output pomr (kW
output pomr (kll)
fig. 3: Efficiency of the converter system measured at 55 Hz. output frequency. Output rms line voltages of a) 110
