2004 35lh Annual 1EEE Power Electronics Specialists Conference Aochen, G e m n y , 2004

Current Mode %Source Inverter-Fed ASD System

Xu-Peng Fang', Zhao ming Qian', Qi-Gao', Bin-Gut, Fang-Zheng Pengl.', Xiao-ming Yuan'
'Zhejiang University, Hang Zhou, China, 310027
'Michigan State University
Department of Electrical and Computer Engineering
'GE R&D Center, Shanghai, China
Abstraci- The paper proposes a novel topology of Z s o u r c e quality due to six step switching.
inverter system and control for adjustable speed drives (ASD).
The Zsource inverter employs a unique LC network to couple
the inverter main circuit to the diode front end. By controlling
the open-circuit duty cycle, the %source inverter can produce
any desired output ac voltage, even less than the line voltage.
As results, the new %source inverter system provides better
low speed characteristic than the traditional current source
inverter-fed ASD through the simple circuit structure and the
unique control strategy. The simulation results testify the 510vdc 380Vac
rationality of the topology. Fig. 1 Traditional current source inverter-fedASD system configuration
I . INTRODUCTION A recently developed new inverter-Z source inverter
Most of the medium and high power ASD system are [5,6] has a niche for ASD systems to overcome these
based on current-source inverters (I-source inverters), problem mentioned above. A Z-source inverter based ASD
consisting of a diode rectifier front end, a dc link inductor, system can:
and an inverter bridge as shown in Fig.1. Because of the - produce any desired output ac voltage, even less than the
I-source inverter, the ASD system suffers the following line voltage;
common limitations and problems: - reduce in-rush and harmonic current.
- Obtainable output voltage is limited quite greater the input This paper presents the basic idea of a current mode ASD
line voltage. The I-source inverter is a boost (step-up) system using the Z-source inverter, its main circuit
inverter. For example, Fig.1 illustrates voltages of a 3-phase configuration, equivalent circuit, and control will be
380V drive system where the diode rectifier power by the presented too. Simulation results will be included to
380V ac line produces about 5 1OV dc, under which the demonstrate the idea and features of the new ASD system.
inverter can only produce a minimum 380V ac. For a 380V
motor, the great obtainable output voltage significantly
limits the low speed characteristicthat relates to the voltage.
This is a very undesirable situation for many applications
where the motor and drive system need to operate in the
low speed operation.
- I m h and harmonic current from the diode rectifier
can pollute the line. Vac
Another type of current source inverter-fed ASD Rg.2 Current some inverter-fed ASD system configuration that has a
system consisting of a controllable rectifier, which has a thyristor front end
thyristor rectifier front end, a dc link inductor, and an 11. CURRENT MODE Z-SOURCE ASD SYSTEM
inverter bridge, could overcome the fust disadvantage Fig.3 shows the main circuit configuration of the
aforementioned, shown in Fig.2. Its obtainable output proposed Z-source inverter ASD system. Similar to that of a
voltage can he less than the input line voltage through traditional ASD system, the main circuit of Z-source ASD
controlling the fire angle, but it suffers from poor waveform system consists of three parts: a diode rectifier, a dc-link

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 2 w 4 351h Annual lEEE Power Electronics Specialists Conference Aachen, C e m n y , 2004

circuit-2-source network, and an inverter bridge. The only inverter bridge is in the open-circuit zero state, as shown in
difference from the traditional one is the dc link (circuit (or Fig.5. Note that the inverter bridge can be also represented
Z-network C, and CI and L, and L2) and snlall input by a voltage source with zero value (i.e. a short circuit)
inductors (La,Lb, and Lc)connected to the diodt: rectifier. when it is in one of the three traditional zero states. In the
Since the Z-source inverter bridge can boost the dc inductor open-circuit zero state, all six diodes conduct, and input
(LIand L2) current to a value above the average idc current current hold on (detailed current relation is illuminated
of the rectifier, a desired output current is always obtainable later).
regardless the line voltage, by using open-ciicuit zero
Diode-Rectifier 2-source 3-phase inverter
switching states when a less output voltage is needed or
during low speed operation condition. Theoretically, the dc
inductor current can be boosted to any value ;ibove the
average dc current of the rectifier.
HI. EQUIVALENT CIRCUIT, OPERATING PRINCIPLE,
AND CONTROL
The operating principle and control of the 2-source
inverter fed by a dc source such as fuel cells have been
presented in detail in [5,6]. In the proposed Z-source ASD Fig.3 Main circuit configuration of proposed 2-sawce inverter ASD
system in Fig.3, the diode rectifier bridge with input system
inductors (La,Lb, and LJ serves as a dc source feeding the ILI
id 4
2-source network. The input inductors are used to suppress
current surge that may occur due to the line capacitance
during diode commutation, thus requiring a small
inductance. As we know that the traditional current source
inverter has nine permissible switching states (vectors). A b, C . or a
K Y
Among them there are six active vectors when the dc Lb, c. or a
Dnb, c. or B
current is impressed across the load, and three zero vectors
IU
when the input terminals are shorted through both switches t

conducting simultaneously in same phase leg. However, the Fig.4 Equivalent circuit of the 2-some inverter-fed ASD in the non-open
three-phase Z-source inverter bridge has ten permissible circuit switching state
switching states (vectors), it has one extra zero state (or
vector) when all the switches are gated-off, which is named
as open-circuit zero state. It is well known that the open-
circuit zero state (or vector) is forbidden in the traditional
current source inverter because it would cause open circuit - w I I \ '
and destroy the power devices. However, the Z-source
network makes the open-circuit zero state possible, and the
open-circuit zero state provides the unique buck-boost
feature of Z-network for the inverter. IU
Fig.4 and Fig.5 show two equivalent circuits for the t

Z-source inverter-fed ASD based on different operation Fig3 Equivalent circuit of the 2-same inveer-fed ASD in the
state. The inverter bridge becomes an equivalent voltage Open-circuit rem state
source as shown in Fig.4 when the inverter bridge is in one All the traditional pulse width modulation (PWM)
of the six active states and the three zero states, whereas the schemes can be used to control the 2-source inverter-fed
inverter bridge is equivalent to an open circuit when the ASD and their theoretical input-output relationships still

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2004 35rh Annual IEEE Power Elecrronics Specialisrs Conference Aachen, Germany, 2004

hold. Here the switching strategy adopted is on-line

carrier-based PWM pattern generator [1,4]. In every
switching cycle, one of the three zero states (not open-
circuit zero state) is used along with two adjacent active
states to synthesize the desired current. Fig.6 (a) shows
active and zero vectors in one switching interval of the
on-line carrier-based PWM. In Fig.6 (a) Sap, Sbp, Scp are
the switching sequence of upper switch of phase A, B, C
and San, Sbn, Scn are the switching sequence of lower
switch of phase A, B, C respectively, as shown in Fig.6 (a).
When the dc current is high enough to generate the desired
ac current, the on-line camer-based PWM pattem generator
of Fig.6 (a) is used. While the dc current is not enough to (a) mive and zero state in the one switching interval of on-line
directly generate a desired output current, a modified carrier-based PWM
on-line carrier-based PWM pattern generator with open-
circuit zero state will be used as shown in Fig.6 @) to boost
current. It should be noted that each switch still switches on
and off twice in per switching cycle. Without change the
total zero state time interval, open-circuit zero states are
evenly allocated into zero state of each phase. The active
states remain unchanged. However, the equivalent dc-link
current of the inverter is boosted because of the inserted
open-circuit zero states. It is noticeable here that the
equivalent switching frequency viewed from the Z-source
network is four times the switching frequency of the main
inverter, which greatly reduces the required capacitance and
open circuit zero 'states
inductance of the Z-source network.
Assuming that the inductors L1,L2 and capacitors C1, @) active and zero and open circuit zem state in the one switching interval

C2 have the same inductances (L) and capacitances (C) in of modified on-line carrier-based PWM

Fig.4 and Fig.5 respectively, the 2-source network becomes Fig.6 One switching interval of on-line carrier-based PWM and the
symmetrical.From the symmetry and the equivalent circuits, modified on-line carrier-based PWM.
we have Consider that the inverter bridge is in one of the nine
non-open-circuit switching states for an interval of TI,
I,, = I,, = I,, i,, = i,, = i,. (1)
during the switching cycle T. In the operating state, two
Given that the inverter bridge is in the open-circuit zero diodes of two phases conduct, shown in Fig.4, one bas
state for an interval of TO during a switching cycle T, one
i, = I , -I,, id = I o , i i = I , - i , = 21, -I,. (3)
has

i, = I , , id = 2 Z , , i i . = O . (2) where I,, is the dc source current and T= TO+ TI.

The average current through the capacitors over one

switching period (T) in steady state should be zero, from (2)
and (3), thus, we have

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2004 3Srh A n n u l lEEE Power Electronics Specialisa Conference Aachen. Germany, ZW4

appropriate buck-boost factor BE,

I - I - T o . I , + T , . ( I o - I ) =o.
c - c - (4)
T
From (l), ( 5 ) and (8), the inductor current can expressed as
(5) TO
1 --
I,, =I,, = I , = - Io. (12)
Similarly, the average dc-link current into the inverter
1-2- TO
bridge can be found as follows: T
The buck-boost factor BE is determined by the
- To.0+T,.(21,-Io) T,
.I ., = i .
8
= =- Io = I , modulation index M and the boost factor B.
T T, -To Note that the open-circuit zero state does not affect the
(6) PWM control of the inverter, because it equivalently
The peak dc-link current into the inverter bridge is produce the same zero current to the load terminal. The
expressed in (3) and can be rewritten as available open circuit period is limited by the zero-state
period that is determined by the modulation index.
Obviously the instantaneous input and output power
are equal, since the output current could be boosted, the
(7) output voltage could be proportional bucked, then the motor
can operate in the low speed operation.
T 1
where B = -= - (8) IV.SIMULATION VERIFICATION OF THE ASD
T, -To 1-2- SYSTEM
T Simulations have been carried out to confirm the
B in (8) is the boost factor resulting from the operating principle of the proposed 2-source ASD system.
open-circuit zero state that can be controlled by duty cycle In order to show clearly the output voltage obtained from
(i.e. interval, ratio) of the open-circuit zero state over the the inverter, a filter is placed in-between the inverter bridge
non-open circuit states of the inverter PWM. and the motor. The simulationparameters are as follows:
3-phase line voltage: 380V, line impedance: 3%
. The peak dc-link current 6 is the equivalent dc-link
Load 3-phase IO KW R-L load;
current of the inverter. On the other hand, the output peak Input inductors (La,Lb, and LJ: ImH
phase current from the inverter can be expressed as 2-source network LI=L2=4mH,C1%2=2 P F
Switching frequency: lokHz
,Z" = &
-M.z,
, (9) Fig.8 shows the simulation circuit structure. Fig.9 shows
2 simulation waveforms under the nominal line voltage of
where M is the modulation index. Using (7), (9) can he 380 Vac. After the filter, the voltage becomes sinusoidal,
further expressed as indicating a 320 V rms value, which is not obtainable by the
traditional current-source inverter-fed ASD system. It
shows the simulation waveform of the output phase current,
output line voltage, 2-network inductor current and input
For a traditional on-line carrier-based PWh4 I-source phase current of the proposed circuit. It is noted that the line
inverter, we have the well-known relationship: current contains less harmonics because of the 2-source
network and input inductors.
,-i 43
=-M.I0 . Equation (10) shows that the output In this case, the modulation index was set to M=0.7, the
2 open circuit duty cycle was set to 0.26.From the above
current can be stepped up and down by choosing an analysis, we have the following theoretical calculations:

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2004 351h Annunl IEEE Power Electronics Specialists Conference Aachen, Cennany, 2004

1
B=- -
1-2- To
T
= 2.08
w- M........................
.............................................
.................................
.......................................
.......................
;.. ......
................
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...............
___,_____ ...

-i, 43
=--M,B.Z, =24.6A
2
T

w
1 - 0
I,, = I,, = I, =-1, =30.8A.
1-2- To ................... . .....................
...................................................
. . . . . . . . . . . . . . . . . . . . .. .. . . ...- ............
T .....................
............... ......................... .^.. .............
The theoretical values are well consistent with the 0.10 032 0.13 015 0.17 0.18 020
simulation results. The simulation proved the Z-source rn(*I
inverter concept. Fig.9 Simulation waveforms ofthe output phase current, output line
V .CONCLUSION voltage, 2-network inductor current and input phase current ofthe
This paper has presented a novel ASD system based on the proposed circuit
Z-source inverter. The Z-source inverter ASD system has REFERENCES
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