Control Strategy of Four Single Phase AC-DC Converter
in Auxiliary Block for High Speed Train
1
Yuen Chung Kim, 1Tae Hwan Kim, 1Kyung Hyun Jang, 2Jong Mook Choi
Senior Research Engineer, Electric Equipment Development Team, R&D Center, Hyundai-Rotem
Company, Ui-Wang Shi, Korea1;
Chief Research Engineer, Electric Equipment Development Team, R&D Center, Hyundai-Rotem
Company, Ui-Wang Shi, Korea2
Abstract
To increase power capacity and stability, reduce harmonics in the input line and achieve a unit
power factor, four single AC-DC converters in a auxiliary block of high speed train is applied. To
improving a redundancy and stability, four single phase AC-DC converter is divided to two groups.
The paper describes the control method and performance characteristics of the applied system.
1. Introduction
Auxiliary block of high speed train consist of AC-DC converter for generating DC power, VVVF
inverters for ventilation and air compressor, battery charger for emergency light and controller power,
and CVCF(Constant Voltage Constant Frequency) inverter for AC load. Among this configuration, a
thyristor phase controlled rectifier is applied mainly as AC-DC converters which convert AC power to
DC power at high power system, because of the limitation of switching device capacity. A thyristor
phase controlled rectifier has a problem with respect to power factor and control performance[1-3]. In
contrast to thyristor phase controlled rectifier, PWM converter which consist of high speed switching
device such as IGBT can obtain unit power factor by controlling converter input voltage and supply
high power by parallel configuration. Therefore, introduction of PWM converter gradually increase[4].
This paper presents a description of a PWM converter control method for obtaining unit power factor
and parallel operation of two groups with two controllers for redundancy.
2. Control Strategy
2.1 Configuration of Auxiliary block
The AC-DC converter converts the AC power to DC power and obtains unit power factor by
controlling the power flow between the mains and DC link. In order to achieve this, the converter
produces a voltage which is applied to its applied transformer winding. The voltage consists of voltage
blocks of a width which has to be determined by a controller and a PWM.
The specification of the available IGBT does not allow the auxiliary block to be built with input
converter which consists of only one PWM converter. That is the main reason why usually several
converters are connected in parallel in order to achieve the desired power rating. And with two or
more converters in parallel, not only the power rating of the converter can be increased but also the
harmonic currents which are injected in the mains can be reduced.
Fig 1 shows the four single phase AC-DC converter configuration which is applied in this paper. To
improving a redundancy and stability, four single phase AC-DC converter is divided to two groups and
each group have one controller for controlling output voltage and unit power factor.
Fig 1 : Configuration of auxiliary block on high speed train with the two group two parallel single phase
AC-DC converters
2.2 Single Phase AC-DC Converter
is
vs
Ls
vcon
V dc
Fig 2 : Single Phase AC-DC converter
A simplified schematic of the converter circuit and it’s operation is shown in Fig 2. The ac voltage vs
from the secondary main transformer winding is connected to the input of the converter via an
inductance Ls . The input ac voltage vs is related to the input converter voltage vcon with the equation.
vs = Ls
dis
+ vcon
dt
(1)
is is converter input current.
The converter output voltage Vdc across the capacitor bank is essentially a constant dc voltage by
where,
switching the IGBTs in the converter circuit in an appropriate manner.
2.3 Unity Power Factor Control
In the electric train with an ac supply it is very important that the power factor seen by the supply is
high. This is because a low power factor leads to a low effectiveness in drawing power from the
catenary, and will require that the current drawn for the same power to be higher. This increases the
volt-ampere requirement of the main transformer, power drive components, and the catenary line.
Therefore, in order to improve the compatibility of the converter with the ac supply, a real time unity
power factor control scheme has been implemented. To achieve this, real time control of the converter
voltage and current is used, in order to control the phase difference between the ac supply voltage
and current.
φ
Is
Vs
jϖ Ls Is
Vcon
Fig 3 : Phase Diagram for unity power factor
The phasor diagram for the case when the angle between the current and voltage are zero can be
seen in Fig 3. It can be easily derived that in this unity power factor case, the converter voltage
amplitude can be found from
vcon = Vs2 + (ϖ Ls I s )
2
(2)
Therefore, if the converter voltage can be controlled to this value then unity power factor will be
maintained. In the implemented real time controller, a feedback control system is used to maintain
unity power factor. In addition, the real time controller maintains a constant dc output voltage. The
block diagram of the converter controller can be seen in Fig 4.
2.4 Control method of two group two parallel AC-DC converters
VS
Vref +
-
Phase
Detection
Voltage
Controller
sinӨ
Iref +
-
Current
Controller
Signal
Generator
Converter #1
Current
Controller
Signal
Generator
Converter #2
Ireal1
VDC
+
Ireal2
High Speed
Communication
To Slave Controller
(a) Control block diagram of master controller
(b) Control block diagram of slave controller
Fig 4 : Control block diagram for two group parallel operation
The function of the control system is to regulate the DC link voltage by manipulating the magnitude
of the input current to accommodate the output loading while controlling the power factor. The current
to demand signal is generated by the output voltage controller. The demanded current is
synchronized in phase with the input voltage and compensated calculating offset for parallel operation.
Each current control module regulates the current drawn by one converter from one secondary
winding. The input voltage to the converter is controlled by the duty ratio of the PWM via the uni-polar
modulator. The duty ratio signal is compared with two 180° phase shifted triangle carrier signals. The
uni-polar modulators for both converter bridges are identical excepting that the carrier groups are
phase shifted by 1/2 of a cycle from each other. This is resulted in a large degree of ripple current
cancellation in the converters primary side. Fig 4 shows control block diagram for a four parallel ACDC converters with two controllers.
To improve stability and redundancy, four parallel AC-DC converters is divided to two groups with
controller. High speed communication is applied for exchanging the information about load sharing
with controllers.
3. Simulation
Transformer primary voltage
Transformer primary current
DC output voltage and reference
Fig 5 : Output voltage and Input current at 1[MW] resistor load
Transformer primary voltage
Input current of converter #1
Input current of converter #2
Input current of converter #3
Input current of converter #4
Fig 6 : Input current of two group two parallel AC-DC converter at 1[MW] resistor load
Shown in Fig 5 is the transformer primary voltage, current and output voltage with a power output of
1[MW]. The output voltage is set at 670[V]. Fig 6 shows the input current of each converter which flow
approximately equivalent.
4. Experiment
The system specification and parameters are shown in Table 1.
1[MW]
250[kW]
AC 25000[V]
Transformer
AC 383[V]
Output Voltage
DC 670[V]
Inductance
0.25[mH]
Filter Capacitor
Each group
30000[uF]
Switching Frequency
900[Hz]
Table 1 : System specification and parameter
Capacity
Total
Each group
Primary Voltage
Secondary Voltage
Fig 7 : Implemented system configuration for two group two AC-DC converter
Fig 7 is block diagram of implemented system configuration for two group two converters. High
speed communication between two groups is applied for load sharing.
Fig 8 through Fig 11 shows some experimental results.
The input current of two converters(Converter #1, Converter #3) and output current of two groups
are shown in Fig 8. Both converters have the same fundamental input current and output current of
two groups is approximately equivalent.
Fig 9 shows the output current of two groups and output voltage. The ripple of output voltage is
about ±25[V].
Fig 10 shows the scale-down primary voltage of main transformer by ACPT(AC Potential
Transformer) and input current of converter. It is operated with near unit-power factor by converters.
Output current of group 1
Output current of group 2
Input current of converter 1
Input current of converter 2
Fig 8 : Input and Output current waveform at 1[MW] resistor load
CH1 : 1000[A/div], CH2 : 1000[A/div], CH3 : 2000[A/div], CH4 : 2000[A/div]
Output current of group 1
Output current of group 2
Output voltage
Fig 9 : Output voltage and current waveform at 500[kW] resistor load
CH1 : 200[V/div], CH3 : 500[A/div], CH4 : 250[A/div]
Voltage
Current
Fig 10 : Scale-down primary voltage of main transformer and input current of converter at 200[kW]
resistor load
CH1 : 100[V/div], CH3 : 500[A/div]
5. Conclusion
This paper has presented a control method of four single AC-DC converters which is applied in
auxiliary block of high speed train. To improve the performance of load sharing, high speed
communication between controllers is applied. The two controllers exchange information for
controlling two groups with two converters. Experiment results show that the implemented method for
controlling two groups is fit.
References
[1] A. I. Maswood, M. H. Rashid. “Input Current Harmonic Reduction in High Power AC/DC Rectifier”,
IECON’91, pp. 593-599, (1991).
[2] Ned Mohan, Tore M. Undeland, and William P.Robbins. “Power Electronics Converters,
Applications and Design”, (1995).
[3] L. J. J. Offringa and W. A. G. de Jager. “Modelling and Control of a 4-Quadrant Pulse Modulated
Line-Side Converter for 25kV, 50Hz-Supplied Traction Equipment”, EPE Proc., Volume 1, pp.
105-110, (1991).
[4] J. Shen and A. D. Mansell. “The Simplified Analysis Design of a Converter System for a ThreePhase Traction Drive”, RAILTECH, (1994).