J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

The Discharge Operation Analysis and Excitation Control of Pulse Motor
Generators on HL-2A Tokomak
Xiaolong Liu1,*, Kazuo Nakamura2, Mingnan Bu1, Weimin Xuan1, Huajun Li1, Yingqiao Wang1
1

Southwestern Institute of Physics, Chengdu, China
2

RIAM, Kyushu University, Fukuoka, Japan(Received: 3 November 2009 / Accepted: 9 March 2010)

The HL-2A is a magnetic fusion confinement device. The power of its Toroidal Field coils is supplied by foursets of parallel diode three-phase bridge rectifiers, which are connected with the outputs of two identical 80MVA
pulse generators. The Poloidal Field power supplies, including Ohmic Heating, Vertical Field, Radial Field andMulti-Pole Field, and auxiliary heating power supplies are thyristor converters and connected to the 125MVA pulse
generator by their transformers respectively. The loads of two 80MVA generators can be balanced, since the twogenerators with the same excitation current have the same output voltage. The excitation AC power source of80MVA generators comes from the coaxial exciting generator. During the discharge, the output voltage frequencyof the exciting generator decreases as the rotational speed of the shafting decreases. The PLL (Phase Locked Loop)on the excitation gate controller is used to track the frequency change, and PID feedback control program makes theDC excitation power supply stable.
Keywords: Excitation Control, Pulse Motor Generator, Discharge, PLL (Phase Locked Loop), HL-2A

main generator (SG) and exciting generator (EG) are all
coaxial. The excitation power supply of exciting generator
is AC power grid; the outputs of exciting generator aredesigned as the excitation power supply of main generator.
The rectifier outputs of 1# main generator and 2# main
generator are parallel to supply power to Toroidal Fieldcoils.
3# motor generator has similar structure with 1# and2# motor generator except its power supply of excitation
system. The schematic diagram of the Poloidal Field power
supply is shown in Fig. 2. AC power grid is used as theexcitation power supply for 3# generator, and the loads of3# generator include the OH, RF, VF and MPF coils [3].

1. Introduction

The total pulsed output power of magnetic field coil
power supplies of HL-2A is around 25MW with duration
5s. However, it is not conducive to get power directly fromthe power grid because of huge load in short period andlong whole discharge cycle, so flywheel with large mass isused for energy storage [1, 2]. It takes a longer period oftime to store energy from power grid by induction motordragging a huge flywheel, and then the stored energy isreleased in a short period of time by pulse synchronousgenerator to supply power for the magnetic field. The
schematic diagram of the Toroidal Field power supply isshown in Fig. 1.
1# M-G
FlywheelM

2# M-G
Flywheel
SGEG

EG

SG

AC

3# M-G
Flywheel

M

M

SG

AC

Switching cabinet

TF

RFOH

Fig. 1 Schematic diagram of Toroidal Field power supply

VF

MPF

Fig. 2 Schematic diagram of the Poloidal Field power
supplies

The pulse capacity of 1# generator and 2# generator is80MVA; wound rotor asynchronous motor (M), flywheel,*Present address: IGSES, Kyushu University Fukuoka, Japanauthor’s e-mail: liuxl@triam.kyushu-u.ac.jp

300©2010 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchX. Liu et al., The Discharge Operation Analysis and Excitation Control of Pulse Motor Generators on HL-2A Tokomak

de-excitation.

The main parameters of 1#, 2# and 3# motor
generator are given out by Tab. 1.
Tab. 1 Main parameters of three sets of MG
Main parameters
1# & 2#3#Generator

Motor

Shafting

Phase voltage
Phase current
Pulse capacity
Synchronous
speed
Rated capacity
Rated voltageSynchronous
speedStored energy

1500 V
11 kA
90 MVA
1500 rpm

3000 V
12 kA
125 MVA
3600 rpm

2500 kW
6000 V
1500 rpm

2500 kW
6000 V
3000 rpm

500 MJ

214 MJ

2. Discharge Procedure and Analysis on Motor
Generator

Fig. 3 Waveform of 1# motor generator for Toroidal
Field power supply during discharge

Before discharge order is given out, there is no load
for generator, by decreasing the slip resistance of woundrotor asynchronous motor, motor generator is uniformly
accelerated approximately to rated rotating speed of motor,
and energy is stored by flywheel at the same time, the slip
resistance is set to 0. After the shafting rotating speed
achieves the rated rotating speed of motor, the slip
resistance is set to operation again before discharge toreduce the impact on power grid, the generator begins to
supply power to Toroidal Field coils as excitation order isgiven out by CCC (Central Control Computer) [4],meanwhile the slip resistance is adjusted to 0 again during
discharge, the stored energy is released. The excitation
order can be divided into four stages: force excitation,
inversion, flat topping and de-excitation.
1# and 2# generator supply power to Toroidal Fieldcoils through the non-controlled diode rectifiers, theadvantage is that the investment of rectifier equipments isabout half of SCR devices and a complex trigger system is
not needed. The terminal voltage of 1# and 2# generator iscontrolled by the excitation output current; thereby theToroidal Field current waveform can be regulated byexcitation system. To Poloidal Field power supply, thestable output voltage of 3# generator is required.
The waveform of the main exciter current (1#If),
exciter voltage (1#Vf), input on exciter (1#Vin), outputs ofgenerator (1#IA1: current of phase A in first Y winding,
1#UAB1: line voltage between phase A and B in first Ywinding), rotational speed of 1# motor generator (1#N) andToroidal Field current (It) is shown in Fig.3. It takes morethan 1 second to establish the terminal voltage ofgenerator; the load current still exists when the input on
exciter is changed to 0 because of remanence, whichgravely affects the cooling system of generator and theload coils. Therefore fast de-excitation is needed when flat
topping is over. In addition, internal fault of generator andshort circuit between the outputs of generator also need fast

As all the three controlled rectifiers of excitation
system work in full-bridge mode, completed SCR inverter
can be used for de-excitation. The energy of excitation
windings from the DC side can be delivered to the AC sidethrough completed SCR inverter. By adding a constantnegative voltage to the excitation windings, the rotor
current decreases basically in straight line, thus it spends
less time on de-excitation.

3. Excitation System of Motor Generator on
HL-2A

The outputs of generator are adjusted by excitation
system, in order to accommodate the requirement ofdischarge; excitation system must perform suchfunctionalities: the input waveform order from CCC must
be changed into trigger pulse on exciter by excitation
system [4], PID feedback control on exciter andmeasurement protection. The typical excitation system
can be divided into three sections, the excitation control
program, the gate controller and the exciter.
1# and 2# exciting generator is coaxial with main
generator and supply power to 1# and 2# excitation
system, the output voltage frequency of exciting
generator is changing irregularly as shafting rotational
speed is falling down during discharge; the exciter power
supply of 1# and 2# exciting generator is AC grid.
The power supply of 3# excitation system is also AC
grid, the same as 1# and 2# exciting generator. The
frequency and amplitude of AC grid can be considered as
fixed.
Fig. 4 shows the diagram of the power supply of 1#and 2# generator. The excitation control program of thethree sets of motor generator is similar with each other.
Especially, the outputs of 1# and 2# excitation system are
not only determined by the trigger angle on main exciter301

X. Liu et al., The Discharge Operation Analysis and Excitation Control of Pulse Motor Generators on HL-2A Tokomak

the same way.

but also by the output of exciting generator. The output
amplitude of exciting generator must be kept constant by
adjusting exciter of exciting generator during discharge,
and then output of the whole excitation system can becontrolled by adjusting the trigger angle on main exciter.

1# Excitinggenerator

ACgrid

3.2 Gate Controller of Excitation SystemThe gate controller acquires the voltage signal fromthe AC input side, and then transforms the control signal
into trigger angle to control the excitation rectifier. Its
structure schematic diagram is shown in Fig. 6.
Controlsignal


1# MainExciting coil
VinF
PLLControlControl

6F

Count value
CPU

Driver2# MainExciting coil

A/DCounter group

AC synchronoussignalDigital givenvalue

Status Filter2# Excitinggenerator

ACgridTriggerPulse

8192F Fig. 4 Power supply of 1# and 2# generator

Fig. 6 Structure schematic diagram of gate controller

3.1 Excitation control program

The phase-shift trigger pulse is generated by counting
function of the counter group. At natural commutation
point of three-phase AC, the counter group is started withan initial value which is decided by the digital input value;
when the counter value becomes equal to zero, the countergroup will give out pulse signal to driver to trigger theexciter. RC filter is used for elimination of interference
from harmonic wave and EMI (Electro MagneticInterference) of AC synchronous signal, the comparison
circuit transforms the signal into square-wave frequency F.
PLL (Phase Locked Loop) with its frequency divider andsignal F composes the closed-loop circuit. By theclosed-loop circuit, the output frequency can be locked infrequency of N times of the input signal with no phasedifference; in the trigger phase-locked loop, the input
signal F is square wave from the main circuit with thesame frequency, the output signal includes 6F, 8192F andpower status signal S. There are a total of six counters,
their serial number is from 0 to 5, the counters arerespectively Counter 0, Counter 1, Counter 2, Counter 3,Counter 4, Counter 5, which are used for time delay togenerate the trigger pulses corresponding to +A, -C, +B,-A, +C, -B phase, where symbols +A, -C, +B, -A, +C, -B
are the six corresponding thyristors of three-phase (phase A,phase B and phase C) rectifier bridge; CPU determinesworking time and initial number of the six counters.
The relationship among the PLL input signal F, output
signal 6F, and the power status signal S is shown in Fig.7.
The signal 6F describes the natural commutation point ofthree-phase AC; the power status signal S (S2S1S0)
describes the status of AC input, determines which countermust be started. As the role of PLL, when the input AC
synchronous signal frequency fluctuation occurs, the signal
6F, 8192F, and S is adjusted by the PLL to follow thefrequency fluctuation.

The 1# and 2# excitation control program isprogrammed by object-oriented language. Each program
must control two objects: the exciter of the exciting
generator and main exciter. Every object includes
waveform parameters from CCC, local control
parameters which input is included in and acquired
parameters from output. The code of every object remains
the same with different parameters.
PID control is used on every object. The specificfeedback control diagram of Toroidal Field power supply
is simplified and shown in Fig. 5, where Tf is the time
constant of excitation system, Tg is the time constant ofgenerator and Tt is the time constant of Toroidal Fieldcoils.
Vin + ě-PID

1T r s +1

1Tg s +11
ItT t s +1

Fig. 5 Control diagram of power supply for Toroidal
Field coils
Based on the parameters of the whole system wherethe excitation inductance is 600mH, excitation resistanceis 161mΩ, the inductance of Toroidal Field coils is230mH and the resistance of Toroidal Field coils is 60mΩ, so Tf is 3.727s, Tt is 3.833s; and Tg is 5.8s from thefactor parameters of main generator. The feedback gain
and stability can be analyzed from the control diagram,based on Nyquist Stability Criterion, the feedback gain
can’t be over 72.296.
Similar to the control diagram of Torodal Field
power supply in Fig. 5, the control diagram of exciting
generators for 1# and 2# main generator, excitation
system of 3# motor generator also can be normalized, andmeanwhile feedback gain can be required accuracy in302

X. Liu et al., The Discharge Operation Analysis and Excitation Control of Pulse Motor Generators on HL-2A Tokomakcurrent in Poloidal Field coils. The load current iscontrolled not only by the excitation system, but also
controlled by the convertors which are connected withoutput of 3# generator.

UacF6F
S0
S1
S2UacS000
S2S1S0

Ubc001

Uba010

Uca011

Ucb100

Uab101

Fig.7 The relationship among the PLL, power status andAC synchronous signal
No matter how the natural commutation pointchanges, the angle between the adjacent naturalcommutation point and the trigger pulse resolution remain
constant, which can make the orderly trigger pulse begenerated and not affected by AC frequency.

Fig. 9 Experimental results of 3# motor generator
discharge with load5. Conclusions

The structure of HL-2A power supply and discharge
procedure is analyzed. Based on that, in order to control
power supply, the typical excitation system of motor
generator is introduced, which consists of control program,
gate controller and exciter. PLL on gate controller is used
to trace the output frequency of exciting generator, so thatthe thyristors of main exciter can be triggered at correcttime point. The output coils of 1#, 2# excitation system areconnected in series. With the same input value on theexcitation control system, there is the same exciting current
on 1# and 2# generator. Therefore 1# and 2# generator giveout the same voltage and the load is balanced.

4. Experimental Results of Excitation System

Experimental results of 1# and 2# motor generator
discharge with their load are shown in Fig. 8. Where theoutput coils of the two exciters are connected in series,
there is only small difference between the output currents,
and the stored energy is released equally.

6. References
[1]

[2]
[3]

Fig. 8 Experimental results of 1# and 2# motor
generator discharge with their load

[4]

Experimental results of 3# motor generator discharge
with load are shown in Fig. 9, where 3#If is the excitation
current, 3#Vf is the excitation voltage, 3#Vin is the inputof excitation system, 3#IA1 is the current of phase A of thefirst winding, 3#UAB1 is the line voltage between phase A
and B of first winding, 3#N is the rotating speed, Ip is the303

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