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并联三相PWM变换器连接系统环流控制的带宽扩展方法的详细资料说明
资料介绍
采用无隔离变压器的公共直流并联三相EPWM变换器会引起零序环流问题。以往的工作已经证明，循环电流主要受各次PWM周期所采用的零矢量的影响。提出了一种抑制环流的新方法。基于推导出的平均模型，详细分析了零序环流产生的原因。提出了零矢量前馈控制策略与传统PI控制相结合的方法来抑制零轴电流系统中的扰动。此外，还采用了双电流采样和双电流比例更新方案来扩展零轴电流环的带宽。结果表明，在不同的滤波电感和变换器输出电流条件下，可以获得较好的循环电流抑制性能。
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| 并联三相PWM变换器连接系统环流控制的带宽扩展方法的详细资料说明.pdf | 1M |
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（完整内容请下载后查看）IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 12, DECEMBER 2014
6847
Bandwidth Expansion Method for Circulating
Current Control in Parallel Three-phase PWM
Converter Connection System
Zhang Xueguang, Member, IEEE, Chen Jiaming, Ma Yan, Wang Yijie, and Xu Dianguo, Senior Member, IEEE
Abstract—The use of common dc-link parallel three-phase PWM parallel module and degrade the overall performance of the par-
converters without isolating transformers will cause zero-sequence
circulating current problem. Previous works have proven that the
circulating current is mainly affected by zero vectors employed in
each PWM cycle. This paper proposes a novel method to suppress
allel system. Consequently, circulating current suppression has
become a focus in this field [4].
The zero-sequence circulating path can be cut open by using
an isolating transformer at ac side [5], but the parallel system will
the circulating current. Detailed analysis is presented on the causes
of zero-sequence circulating current based on a derived average become costly and bulky. Using separate dc power supply will
model. A zero vectors feed-forward control strategy in combina-
tion with traditional PI control method is proposed to reject dis-
turbances in zero-axis current system. In addition, a dual current
sampling and dual PWM duty ratio update (DSDU) scheme is used
encounter similar problems [6]. For parallel systems with both
common dc-link and ac bus to reduce costs and size, interphase
reactors may be used to provide high zero-sequence impedance
[7], [8]. Nevertheless, the reactors cannot prevent low-frequency
to expand the bandwidth of zero-axis current loop. As a result, bet-
ter circulating current suppression performance can be achieved in components in the circulating current.
different filter inductance and converter output currents condition.
The mechanism of circulating current has been analyzed
Compared with the PI control method, the converters operated in
parallel can be switched on and switched off separately with small
current impact. Experimental results confirm the performance and
effectiveness of the proposed method.
in pervious works [9], [10]. The paralleled converters can be
centrally controlled as one converter [11], for example, a two-
converter parallel system can be controlled as a six-phase con-
verter, however, whereas design of this control system will
be complicated. Special PWM techniques are proposed to in-
hibit the circulating current [12]–[14]. The discontinuous space
modulation-based interleaved PWM method would effectively
reduce the circulating current, but it would result in high cur-
rent ripple in converter output currents, and this drawback may
be alleviated by the phase-shifted scheme proposed in [13]. The
proposed HEPWM method of Chen [14] would effectively elim-
inate both the high-frequency and low-frequency components
of circulating current, but it suffers from high switching losses.
Due to the fact that the zero-sequence circulating current is
mainly affected by zero vectors employed in each PWM cycle,
control method for decreasing common mode voltage distur-
bance with multicarrier technique is proposed by Hou [15]. The
multicarrier can effectively mitigate the circulating current by
means of nullifying the use of zero vectors. On the other hand,
the circulating current control method for SVPWM technique
was discussed in [16]. A control variable was introduced to ad-
just the distribution of zero vectors in each PWM cycle with
a proportional integral (PI) controller. However, the limitations
to the bandwidth of zero-axis circulating current loop is rarely
discussed. Nonlinear control methods were also presented to
resist the circulating current [17], [18], yet the algorithms are
too complicated to implement. An open-loop control method
is proposed in [19] and the control methods on zero-sequence
circulating current caused by dead time effect [20] has been
developed.
Index Terms—Bandwidth expansion, circuiting currents control,
DSDU, module parallel connection, three-phase PWM converter,
zero vectors feed-forward.
I. INTRODUCTION
HREE-PHASEPWM converter hasbeenwidelyemployed
in low-voltage high-current applications [1]–[3] owing to
T
its advanced features. As the capacity of converter is increased,
the use of parallel three-phase converters has become more pop-
ular due to its simplicity, low cost, and high flexibility. However,
when the common dc-link converters are connected in parallel
without using of isolating transformers, the zero-sequence cir-
culating current problem will occur. Circulating current can be
produced by the differences in inductors and converter currents,
other aspects such as dead time, measuring errors, analog to digi-
tal conversion, discretization, loose of synchronism between the
carriers cause the circulating current problem. The circulating
current will result in current distortion and harmonic loss in
Manuscript received December 15, 2013; revised February 17, 2014; accepted
February 23, 2014. Date of publication March 11, 2014; date of current version
August 13, 2014. This work was supported in part by the National Natural
Science Foundation of China under Grant 51107020 and Grant 51237002.
Recommended for publication by Associate Editor D. Vinnikov.
The authors are with the Department of Electrical Engineering, Harbin
Institute of Technology, Harbin 150001, China (e-mail: ;
; ; ; xudiang
@hit.edu.cn).
In this paper, an average model of the parallel system is de-
rived to analyze the mechanism of zero-sequence circulating
current with SVPWM modulation technique. It is found that the
circulating current suppression performance can be enhanced by
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/TPEL.2014.2311046
0885-8993 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
6848
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 12, DECEMBER 2014
expanding the bandwidth of zero-axis current loop. However,
both the difference in filter inductance and output currents would
cause disturbances to the zero-axis current system. More im-
portantly, in low-voltage high-current applications, low switch-
ing frequency is usually required as to reduce the switching
losses [21], [22]. Unfortunately, the time delayed by the digital
control system and disturbances will all make it difficult to ex-
pand the bandwidth of zero-axis current loop and even degrade
the suppression effect. Hence, it is very meaningful to improve
the suppression performance by simultaneously decreasing the
time delayed by the digital control system and eliminating the
impacts of disturbances without increasing switching frequency.
Based on the analysis, a feed-forward control method in com-
bination with a dual current sampling and dual PWM duty ratio
update (DSDU) scheme on the basis of traditional PI control
method is proposed. As a result, the bandwidth of zero-axis
circulating current loop can be effectively improved with dis-
turbances rejected by the feed-forward strategy. Better circu-
lating current suppression performance can be obtained both
in transient state and steady state. Compared to the PI method
combined with DSDU scheme, online converter module switch-
ing on when another converter is running and switching off (by
inhibiting pulse of the switched converter) with small current
impact can be realized. As a result, the reliability of the parallel
system is greatly enhanced.
Fig. 1. Topology structure of parallel connection system of three-phase PWM
converter.
effects would also contribute to the generation of circulating
current. The circulating current of the parallel system can be
defined as
izx = iax + ibx + icx
(1)
where x = 1, 2 represents the number of converters 1 and 2,
respectively. For the sake of convenience, choose the dc negative
side as reference point, and assume the duty ratio of the top
switch in phase k (k = a,b,c) of converter x(x = 1,2) to be
dkx,then the average model of the parallel system in three-phase
stationary frame can be obtained as
The remainder of the paper is organized as follows.
Section II reviews the theoretical background of the parallel con-
verter system with SVPWM modulation technique. The analysis
of zero-axis current loop bandwidth with PI method is detailed
in Section III and the feed-forward strategy for eliminating the
restrictions in combination with DSDU scheme for expanding
zero-axis current loop bandwidth is proposed in Section IV.
Section V presents the experimental results and Section VI con-
cludes the paper.
⎡
⎤
⎡
⎤
⎡
⎤
⎡
⎤
⎡
⎤
iax
iax
ea
eb
ec
uN
uN
uN
dax
dbx udc
dcx
d
⎣
⎦
⎣
⎦
⎣
⎦
⎣
⎦
⎣
⎦
Lx
ibx =−Rsx ibx
+
+
−
dt
icx
icx
(2)
where Lx and Rsx are the inductance and equivalent series
resistances of converters 1 and 2, respectively.
II. THEORY OF CIRCULATING CURRENT SUPPRESSION
This model canbe transformedintothe synchronous reference
frame by (3) with a 3-D nonsingular matrix defined as
A. Model of Three-Phase Parallel-Converter System
⎡
⎤
In this paper, parallel converters are assumed to be con-
nected to the ac gird through filter inductance as illustrated
in Fig. 1. As Fig. 1 shows, the switches of parallel modules
in the same phase are connected in series. Consequently, cir-
culating current path is formed. Ideally, if the parallel con-
verters with equal output currents are completely identical in
parameters, uniformly modulated, no circulating current would
be generated as the output voltages in the ac side have the
same frequency, phase, and amplitude [23]. However, in prac-
tical applications, while parallel modules are usually designed
to have the same parameters, tolerance in parameter disper-
sion is very common, which in return would result in differ-
ent filter inductance [24] and unequal measured values of out-
put currents. Moreover, the synchronism between the carriers
of different converters may be interfered. As a consequence,
differences will occur in the output voltages of the parallel
modules during the short switching process, both in amplitude
and phase. Consequently, circulating current would be gener-
ated as asynchronous action emerges. Besides, the dead time
cos ωt
cos(ωt − 2π/3)
cos(ωt + 2π/3)
2
3
⎢
⎣
⎥
⎦
− sin ωt − sin(ωt − 2π/3) − sin(ωt + 2π/3)
Ts−r
=
1
2
1
2
1
2
(3)
(4)
xdqz = Ts−r.xabc
where ω is the grid fundamental angular frequency, and xdqz
is the transformed variable in synchronous frame of xabc. With
this understanding, the average model of the parallel system in
synchronous frame d–q–z can be obtained as
ꢀ
ꢁ
ꢀ
ꢁ
−ω
ꢀ
ꢁ
idx
iqx
d
idx
iqx
idx
iqx
ed
eq
Lx ·
= −Rsx
− Lx ·
+
dt
ꢀ
ꢁ ꢀ
ꢁ
ꢀ
ꢁ
0
ω
udx
uqx
·
−
(5)
(6)
0
diz2
dt
(L1 + L2) ·
= −(Rs1 + Rs2) · iz2 + Δdz · udc
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