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

DEVELOPMENT ON ADVANCED CURRENT-SOURCE-TYPE SELFCOMMUTATED CONVERTER FOR ACCELERAT OR
ELECTROMA GNETS
Eiichi Ikawa, Toshiba Corporation, Japan
Koji Noda, NIRS, Japan

Abstract
A new power supply system for accelerator
electromagnets is proposed based on the advanced
current-source-type self-commutated converter (ACSC).
The ACSC offers simple AC to DC conversion system
resulting in economical and reliable power supply for theaccelerator. Furthermore, the ACSC operates in unitypower factor and with little harmonics to the AC network.
An ACSC mini-model using IGBT was made andevaluated at the maximum output current of 1600A .Theexperimental results showed the current ripple less than 1
X 10-6 and the tracking error less than ±0.5 X 104 •2 SELF-COMMUTATED CONVERTERS
2.1 Configuration of the self-commutatedconverters
The self-commutated converters are constructed withself-commutated switching device such as IGBT for low
voltage, IEGT (Insulated Gate Enhanced Gate bipolarTransistor) for high voltage. By controlling the switchingdevice as intended, the self-commutated converter offers
advantages as follows.
•1 INTORODUCTION
The ACSC system has a capacitor in the bridge arm,
which softens the turn-off phenomena, while theconventional current-source-type converter (CSC) places
large filter capacitors at the AC input. Thus, theconventional esc sometimes yields resonance between
AC network and the converter although the ACSC
inherently does not [1][2].
As one of the applications, the ACSC is considered
suitable to the accelerators, which are used in the field ofHIMAC (at NIRS, Japan). The power supply is required
to suppress ripples including ringing less than I o· 6 at theflat top of the current pattern in the DC circuit. It is also
required precisely to follow the trapezoidal current patternat the same time. The ACSC can flexibly control its DC
voltage output by its self-commutation ability. Thus, theACSC is expected to achieve such requirements for highquality. In addition to the performance, the ACSC isexpected to offer better cost-performance for suchapplication since the system consists of the converter anda special filter with variable impedance [3].
In this paper, the ACSC is explained from the aspectsof the configuration, the principle of operation and theadvantages. The ACSC performance is evaluated by theexperimental results from ACSC mini-model tests.

•Fast control response( 1) Fast response to the DC current pattern(2) Immunity to AC network disturbance(voltage variation, flicker and so on)
Small disturbances to AC network(1) Unity power factor(2) Suppressing harmonics current

Two types of converter system can be applied foraccelerator electromagnets. One is the ACSC system andthe other is the voltage-source-type converter (VSC) withthe chopper system as shown in Figure 1 (a), (b),respectively. The ACSC system configuration is simple
since it converts AC power to DC power directly.

- 60-ACSC(a) ACSC system(b) VSC with chopper system
Figure 1 Power supply systems

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

2.2 Operation principle of the ACSC
The feature of the ACSC is the arm capacitor as shownin Figure 2. The operation of the ACSC consists of fourmodes as shown in Figure 3.
(1) On-state: The switch devices S 1 and S2 conductparallel through the diode Dl and D2, respectively.
(2) Turn-off: The arm capacitor Cs is charged by thecurrent flowing through Dland D2.
(3) Off-state: The bridge arms are all in off-state.
(4) Turn-on:The charges in Cs is discharged throughS 1 and S2 and regenerated to the AC input.
Since the capacitor is separated from the AC input byswitches, the capacitor capacity can be easily increased
and the AC capacitor can be eliminated. Thus the ACSC
has the advantages compared with the conventional esc
as follows.

••

Figure 2 ACSC configuration

No resonance between AC network and theconverter due to no AC capacitor(1 )On-state

Small switching loss by suppressing di/dt and dv/dt
with the arm capacitor(2)Turn-off(charge)(3 )Off-state (4 )Turn-on(discharge)

Figure 3 Operation principle of the ACSC

3 ADVANTAGES OF ACSC

4 ACSC MINI-MODEL EVALUATIONThe ACSC system has two advantages. One is thenumber of devices and the second is the protection. TheACSC consists of only the converter while the VSC
requires chopper to convert the DC voltage to the DC
current in addition to the converter as shown in Figure 1(b). Thus, the ACSC can be manufactured with smaller
number of devices than the VSC with the chopper. The
smaller number will result in smaller loss and better costperformance. Since the ACSC is the current-source-type
converter, the established protection system used in thethyristor converter can also be applied.
The features and performances of the power supply
systems are summarised in Tablel.

4.1 Mini-model
In order to evaluate the ACSC system, a mini-model
was made as shown in Figure 4, with a variable
impedance filter introduced in another paper. Theparameters of mini-model are listed in Table 2.
Passive filter

Id~ 1600Arnax

Vd~88Vmax

Test load
7mH(electromagnet)

T able 1 Companson of ACSC an d VSC Wl"th c h opper

SystemConfiguration

ACSC
Converter

DeviceNumberControl
response
Power factor
Dimensions,
Loss & Costs
Protection

Rather small

VSC with chopper
Converter+chopper
Rather large

GoodGoodUnity
Rather small

Unity
Rather large

Simple(conventional)

Need for highresponse fuse

19.6m-ohm

Figure 4 ACSC mini-model
Table 2 Parameters of ACSC mini-model
Ratings
14lkVA-88V-1600A
DeviceIGBT
Configuration
Four bridges in parallel
Switching frequency
450Hz (9pulse PWM)
The control is implemented in full digital controller
driven by the 32-bit CPU and DSP. The PWM control isadapted for the converter as shown in Figure 5.

- 61-

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

CtnTent reference
Id* --...~

Magnitude signal

MF(Hz)~.DillL

50100200300
400500

CtnTent feedback

Q Reactive power feedback
Figure 5 Control method

4.2 Experimental results
The experimental results from the ACSC mini-model
tests are shown in Figures from 6 to 8.
Figure 6 shows the current ripple waveforms at DC
operation with the maximum output current of 1600A.
Figure 7 shows the FFT analysis of the DC voltage. The
current ripple was calculated with the load impedance andthe DC voltage spectrum.
The calculation results of the ripple current are listed inTable 3. For HIMAC Bending Magnet (BM) system, theripple should be less than 10-6 • The ripple is estimated byEquation (1), considering the voltage and current ratings
and the load impedance of the mini-model and HIMAC
BM. From Table 3, the precision requirement is proved tobe achieved.

T able3 C urrent npp e at DC 16OOADC Voltage DC Current DC Current
RippleRipple{p-p)Ripple(p-p)(dBVr)*calculation *expect forfor7mH
HIMAC-98.174.962E-6
9.469E-7-91.68
5.237E-6
9.995E-7-88.283.873E-6
7.392E-7-86.71
3.D94E-6
5.904E-7-88.43
1.904E-6
3.633E-7-86.65
1.869E-6
3.567E-7
Note: DC voltage probe 500: 1Figure 8 shows the tracking error, the error between the
Reference current and the DC current, which isnormalized by the maximum DC current. At a trapezoidal
current pattern (flatbase:7%, flattop: 100%, nsmg
time: ls), the tracking error was within 0.5* w·•. From theexperimental results, the response requirement is also
proved to be achieved.

. .\ I

DCCtnTent~ ~~~·~&Reference ~~l·~.,...1600A
f-.

v

Tracking error
2x 1040,.._ LL~ 2A

TJ~C; ITt"'i •:;I[\\_ _)/dv

ISI

1-~ ·-:~osxw•.~~i I I

Id_ripple= Vd_max *dt/(R+sL )/Id_max( 1)
where, R:load resistance, L:load inductance,
dt: ripple frequency period

Figure 8 Tracking performance

4 CONCULUSION
A new power supply system is proposed and developed
for accelerator electromagnets. The system is based on
the ACSC. The ACSC system has the advantage ofconfiguration, dimensions, and reliability. The ACSC
mini-model using IGBT was made and the experimental
results showed successful performances.

REFERENCES
Figure 6 Current ripple waveforms
0.01-mdBVr~

20.0dB/-160.0
2.5000[Hz)[1] Takami Sakai et al, "SMES system with snubber loss
free type current source converter", IECON'97,Louisiana, Nov. 1997
[2] Takami Sakai et al, "Consideration on SelfCommutated Converter Characteristics for DC PowerSupply", IPEMC'2000, Beijing, Aug. 2000
[3] Teruo Yoshino et al, "Development on Hybrid Filter
for High-Precision Power Source to Accelerator
Electromagnets" ,SAST' 01 ,Osaka,Oct.2000

2.0000k

Figure 7 DC voltage spectrum- 62-