The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

DEVELOPMENT ON HYBRID FILTER FOR HIGH-PRECISION POWER
SOURCE TO ACCELERATOR ELECTROMAGNETS
Eiichi Ikawa, Teruo Yoshino, Toshiba Corporation, Japan
Yoshiharu Mori, Masayuki Muto, Koji Noda, KEK, Japan

Abstract
This paper describes a development targeting an idealpower supply to the synchrotron system rated at tens ofGeV (deca-GeV) planed jointly by KEK and JAERI inJapan. The developed power supply offers solutions bothin the power capacity and in the precision at the sametime. The power supply consists of the advanced selfcommutate~ current-source type converter (ACSC) andthe "hybrid filter", which has spun out of intensive
development efforts. The configuration and performanceof the hybrid filter are introduced followed by reports on
the successful evaluation results from mini-model tests
where the precision requirement of I o-6 is achieved.

1 INTRODUCTION
In Japan, KEK and JAERI are proceeding a joint
project of a high intensity Proton accelerator facility
including a deca-GeV synchrotron. The synchrotron
requires large-power and high-precision trapezoidal
current pattern. Furthermore, the power supply should
minimize the disturbances to the electric network because
the power is extremely large compared with the power
requirement of the preceding projects.
Corresponding to these requirements, Toshiba hasdeveloped a new DC power supply concept fully utilizing
cutting-edge power electronics technology. For the power
supply, the most advanced power semiconductor device,
IEGT, is proposed, which can tum off the current by its
gate control and is superior to the power thyristors used in
the conventional power supply. The IEGT is also rated at
high-voltage and large-current and has potential to offer

Electromagnet(a) Series compensation
Parallel compensator
Electromagnet(b) Parallel compensation
Figure 1: Ripple compensation methods

solutions to satisfy the requirements of the deca-GeV
project.
The friendliness to the AC system of the advanced
converter with the self-tum-off device is described inanother paper. The advanced converter also offers better
performance in the trapezoidal current profile with lessDC ripple and better tracking response to the patternorder. This paper describes new ripple current
compensation method applied with the advanced
converter.

2 RIPPLE CURRENT COMPENSATION
2.1 Feasibility ofparallel compensation
To make a ripple-free and precisely shaped trapezoidal
current pattern, series compensators, the active filters or
the droppers have been applied combined with theconverters as shown in Figure 1 (a). As a different
approach to this method, the parallel compensator is
proposed in this paper as shown in Figure 1 (b).
The series compensator suppresses the voltage ripple tomake the DC voltage constant. Since it is connectedbetween the converter and the electromagnet, it shouldconduct all .of the DC current. For a large current
application, the series compensator should consist of
many small semiconductor devices connected in parallel.
However, the semiconductor technology now yields thehigh-voltage self-tum-off semiconductor devices, which
can be connected in parallel with the electromagnet. Then,the parallel compensator becomes now feasible tomanufacture. Furthermore, the parallel compensatorcurrent rating can be fairly small since it conducts only
the ripple components. The small current rating requiresfewer parallel devices and does not affect the converter
capacity practically.

2.2 Operation principle of the parallel
compensator
The parallel compensator operates in the following
manner by monitoring the electromagnet current.
• When the current increase, the compensator pulls out
the excessive current from the electromagnet.
•When the current decreases, the compensator pushes
the current flowing into the electromagnet.
In other words, the parallel compensator bypasses theripple components and the smooth current is supplied to-286-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

the electromagnet. Furthermore, the parallel compensatorcan adjust the difference between the current order andthe actual current. The feature contributes to improve thetracking response to the current order.

Equivalent voltage
sources of the converter

Parallelcompensator

3 HYBRID FILTER DESIGN
Ripple voltage
source3.1 Requirements from large-capacity
electromagnets

Bypassing the ripple components
through the compensator

Figure 2 Operation of the parallel compensator

From the basic idea of the parallel compensation, thecompensator might be realized by a current source madeof an analogue amplifier or a transistor current controller.
However, in the deca-GeV synchrotron .with highacceleration speed, the electromagnet voltage ratings willreach several kV. To be connected in parallel with theelectromagnet, the compensator should operate at highvoltage. Thus, the amplifiers or the transistor equipment
are not feasible for that purpose since their voltage ratings
are usually too small.
On the other hand, high-voltage semiconductor devices
for switching use are now commercially available. One of
the most advanced devices is the IEGT rated now up to4.5kV with a single device. Therefore, it is considered tobe reasonable to apply the switching device for theparallel compensation purpose.

3.2 Hybrid filter concept
Based on the preceding consideration, the "hybridfilter" has been innovated. The hybrid filter consists of aswitch and resistor as shown in Figure 3. The simple
configuration operates as a variable resister in principle.
The switch operates at a frequency of several kHz and theduty ratio d is controlled. Then, the equivalent resistanceR.,q of the hybrid filter can be calculated by the followingequation.
R.,q=d X Ro ... (I), where Ro is the resistance of the resistor.

3.3 Prototype hybrid filter
In order to evaluate the performance of the hybridfilter, a prototype was manufactured for a converterelectromagnet mini-model rated at 1600A and 88V driven
by the ACSC described in another paper. The prototype
consists of eight strings of an IGBT switch and resistors
connected in series. The IGBT was used in place of theIEGT considering the voltage rating of the mini-model.
The switching interval was 0.5ms (2kHz) and theswitching phase was displaced 1/8 of the interval one
another to make the equivalent switching frequency to be
16kHz (2kHz x 8). This configuration can change theequivalent resistance in every 0.0625ms (1116kHz). The
sub-mili-second response time means that the hybrid filter
can suppress the current ripple frequency at several Hz oreven kHz.

Passive filter

Hybrid filter··on/off pulse_n_Hybrid filter
control

Electromagnet

precision
DC-CT

Ripplecomponent
detection
Switching duty control

Figure 3 Prototype hybrid filter for mini-model
The resistance value is selected co~sidering theelectromagnet impedance value. If the · values arecomparable, the current sharing ratio between the hybrid
filter and the electromagnet is not sufficient quickly topull or to push the current from or to the electromagnet.
The equivalent resistance value R.,q is designed to besmaller than the ratio between the voltage across theelectromagnet Vm and the expected current ripple
amplitude I,.
R.,q<V n/I, ... (2)The R.,q is synthesized by the eight strings. The
resistance R.,q' of one string will be determined by thefollowing equation.
R.,q'=8R.,q ... (3)If the average duty ratio is assumed to be 0.5, then, theresistance R0' of each string is calculated as follows fromequations (1) and (3).
Ro'=2R.,q'=16R.,q ... (4)3.4 Hybrid filter control
A high-precision DC-CT measures the current in theelectromagnet. From the DC-CT signal, the ripplecomponents are detected. The ripple signal is amplified
and used to determine the duty ratio d of the switch as
shown in Figure 3.
For the mini-model tests, the control is implemented infully digital controller driven by the 32-bit CPU and DSP.

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

4 HYBRID FILTER PERFORMANCE
4.1 Hybrid filter characteristics
The characteristics of the hybrid filter characteristics
was calculated in the frequency domain considering themini-model main circuit characteristics and the controlfunctions. The DC circuit impedance characteristics isshown in Figure 4. The impedance seen from theconverter takes small amplitude in the wide range from 1°
Hz to 102 Hz, where the major ripple current components
stay. The characteristics is difficult to obtain only by theconventional passive filters.
In parallel with the analysis in the frequency domain,
the time domain analysis is also performed. TheEMTDC™ is used as the simulation tool. In the digital
simulation, all of the components and the control
functions are simulated as is in the mini-model. In thedigital simulation, the hybrid filter parameters are finallyconfirm6d.
· ··The authors are now developing the IEGT switch usedin the actual high-voltage system. Combined with thehybrid filter concept and the high-voltage switch
hardware, the authors believe to realize a highperformance DC power supply and to contribute todevelopment of the deca-GeV synchrotron project.
OCcurrent
1600A

On-rent error
2x 104

Without hybrid filter
1.

\20Ga m o(dB )r-..~-2040r;r--80~:~~·111··6 1~:£~11111111111111 I Ill~&With hybrid filter-60
l 0(b) With hybrid filter
Figure 5 Current ripple waveforms'r-.

l .0.o I 1.11 f~~~.o I

0. 0

Frequency (Hz)Figure 4 Frequency characteristics of hybrid filter

2.5000[Hz]

2.0000k.

(a) Without hybrid filter

4. 3 Experimental results .
Some of the mini-model test results are shown in
Figures 5 and 6. Figure 5 visually indicate that the current
ripple is clearly suppressed with the hybrid filter
operation. The results are in well consistent with thedigital simulation.
Figure 6 shows the FFT analysis of the DC voltage.
The DC voltage profile resembles the characteristics
obtained by the frequency domain analysis. From theprofile, the ripple current is calculated to be Jess than 10·6
when it is applied to the actual deca-Ge V synchrotronsystem. The improvement in the tracking performance isalso confirmed in the mini-model tests as shown in Figure

2.5000

OCcurrent
1600A

On-rent emr
2 X J(J'07.

·;!' I'0,., II\k eLl 2A

1. hingHylrid filter SV\IItc~J~·l -i- rJ..~ "+~0

4 CONCLUSION
A new power supply is proposed and developed fordeca-GeV synchrotron planned in near future. The power
supply applied the most advanced and innovative powerelectronics technology. The concept of the hybrid filter isintroduced and the performances are successfullyevaluated through the simulations and the mini-modeltests.

2.0000k[Hz)(b) With hybrid filter
Figure 6 DC voltage spectrum::,!•~t~

If;r •I~1$\ _;·Ifv.,±O 5xJo-• :--....,. i ..,I'' Ill"~,.

~Figure 7 Tracking performance improvement

REFERENCES[ 1] Takami Sakai et.al, "Consideration on SelfCommutated Convertor Characteristics for DC Power
Supply", IPEMC'2000 Beijing, Aug. 2000-288-