ISSN 2348–2370
Vol.07,Issue.01,
January-2015,
Pages:0001-0006
www.ijatir.org
A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost
Inversion for ASD Drives
V. JANAKI RAM1, A. PURNA CHANDRA RAO2
1
PG Scholar, Dept of EEE, Prasad V. Potluri Siddhartha Institute of Technology, Vijayawada, India.
2
Dept of EEE, Prasad V. Potluri Siddhartha Institute of Technology, Vijayawada, India.
Abstract: This paper presents a cascade multi cell Z-source
inverter used to control the speed of an BLDC motor. The
proposed cascade multi cell Z -source inverter (ZSI) can
effectively reduce the voltage stress across the capacitors in
the impedance network. This reduces the voltage range of
the capacitors used, and also the cost of the proposed
topology which is in turn used to control the speed of a
BLDC motor. This concept is used in many industrial
applications. The cascade multi cell Z-Source inverter is a
combined inverter with an additional buck-boost feature and
the proposed topology increases the efficiency of the circuit
by reducing the voltage stress across the capacitors. This
topology finds its applications in a number of renewable
energy sources, where the input voltage is unreliable in
nature and keeps varying from time to time. The simulation
results indicate that the proposed topology is a promising
technique that can be applied to improve the overall inverter
efficiency.
Keywords: Z-Source Inverter, Brushless Dc Motor, Reduce
Voltage Stress.
I. INTRODUCTION
Many modern powers electronic applications usually
demand some amount of voltage boosting, especially those
directly connected to the grid. Traditional voltage-source
inverters (VSIs) alone are, therefore, not satisfactory since
they only step down voltages. To introduce additional boost
functionality, dc–dc boost converters can be placed before
the VSIs or current-source inverters (CSIs) can be used
instead. Both inverters have some amount of boost
inductance added to their dc circuits, which certainly is a
common modification introduced to inverters with boost
functionality (if switched-capacitor technique is not used).
The inductance added to a CSI is usually larger to keep its
dc input current constant. This, together with other
disadvantages like tougher control and lack of standard
semiconductor modules for implementation, usually limits
the use of CSIs, as compared to VSIs. The Z source inverter
is a single stage converter that can either buck or boost the
ac output voltage from a Dc supply. This topology
overcomes the shortcomings of the traditional voltage
source and current source inverters, where the output ac
voltage is either respectively less or more than the input dc
voltage. This combined operation of the z source inverter
eliminates the need of a separate dc-dc converter, thus
reducing the cost and increasing the efficiency of the circuit.
Z source inverter also allows two switches of the same
leg to be gated in the circuit, thus eliminating the shoot
through fault that occurs in traditional converters. This
feature of the inverter provides the elimination of dead time
in the circuit, thus increasing the reliability and reducing the
output distortion of the inverter. Saying that does not free
the Z source inverter from few of its operating problems the voltage across the capacitors in the traditional Z source
inverter is equal to the input voltage which increases the
volume and cost of the capacitors used; and also the start up
current and voltage in the circuit is very much higher which
may destroy the devices at one time or the other. So to
overcome the above said problems, a new topology of the Z
source inverter is used, that can be used to drive a BLDC
motor and speed control. A recently developed new
inverter, the Z-source inverter has a for ASD systems to
overcome the four mentioned problems. A Z-source inverter
based ASD system can:
1. Produce any desired output ac voltage, even greater
than the line voltage, regardless of the input voltage.
2. Thus reducing motor ratings;
3. Provide ride-through during voltage sags without any
additional circuits.
4. Improve power factor and reduce and harmonic current
and common-mode voltage.
This paper presents the Z-source inverter ASD system
configuration, its equivalent circuit, analysis, and control.
Simulation results are included to prove the concept and the
features of the new ASD system.
II. Z SOURCE INVERTER
To overcome the problems of the traditional V-source
and I-source converters, this paper presents an impedancesource (or impedance-fed) power converter (abbreviated as
Z-source converter) and its control method for
implementing dc-to-ac power conversion. Fig. 1 shows the
general Z-source converter structure proposed. It employs a
unique impedance network (or circuit) to couple the
converter main circuit to the power source, load, or another
converter, for providing unique features that cannot be
Copyright @ 2015 IJATIR. All rights reserved.
V. JANAKI RAM, A. PURNA CHANDRA RAO
observed in the traditional V- and I-source converters where
iL>
ii
(2)
a capacitor and inductor are used, respectively. The Zsource converter overcomes the above-mentioned
Again, because of the symmetry of the circuit, capacitor
conceptual and theoretical barriers and limitations of the
currents iC1and iC2 and inductor currents iL1 and iL2 should be
traditional V-source converter and I-source converter and
equal to each other, respectively. In this mode, the input
provides a novel power conversion concept.
current from the dc source becomes
iin= iL1+ ic1= iL1+ (iL2− ii)= 2iL− ii >0
(3)
Therefore, the diode is conducting, and the voltage
across the inductor is
VL = Vo − VC
Which is negative (the capacitor voltage is higher than
the input voltage during boost operation when there are
shoot through states); thus, the inductor current linearly
decreases, assuming that the capacitor voltage is constant.
As time goes on, the inductor current keeps decreasing to a
level wherein the condition of (2) can no longer be met and
the input current iin or the diode current is decreased to
zero; mode 2 ends.
Fig1. General structure of the Z-source converter.
In Fig. 1, a two-port network that consists of a splitinductor L1 andL2 and capacitors C1and C2connected in X
shape is employed to provide an impedance source (Zsource) coupling the converter (or inverter) to the dc source,
load, or another converter. The dc source/or load can be
either a voltage or a current source/or load. Therefore, the
dc source can be a battery, diode rectifier, thyristor
converter, fuel cell, an inductor, a capacitor, or a
combination of those. Switches used in the converter can be
a combination of switching devices and diodes such as the
anti parallel combination as shown in Fig. 1. The inductance
and can be provided through a split inductor or two separate
inductors. The Z-source concept can be applied to all dc-toac power conversion. The diode in series with the fuel cell
in Fig.1is usually needed for preventing reverse current
flow.
III. OPERATING PRINCIPLE OF Z-SOURCE
INVERTER
Mode 1: The circuit is in a switch shoot-through zero
state when the two switches in any of the three phase legs
are turned on at the same time, the sum of the two capacitors
voltage isgreater than the dc source voltage VC1+ VC2> V0,
the diode is reverse biased, and the capacitors charge the
inductors. The voltages across the inductors are
VL1= VC1VL2= VC2
(4)
Fig2. Equivalent Circuit of the Z-Source Inverter
Viewed From The Dc Link When The Inverter Bridge
Is In The Shoot-Through Zero State.
(1)
The inductor current linearly increases, assuming that
the capacitor voltage is constant during this period.
Because of the symmetry L1= L2= L and C1= C2= C of the
circuit, one has vL1=vL2=vL, iL1=iL2=iL, and VC1= VC2= VC.
Mode 2: The inverter is in a non shoot-through state one of
the six active states and two traditional zero states and the
inductor current meets the following inequality:
Fig3. Equivalent Circuit of the ZSI When the Inverter
Bridge Is In One of the Eight Non shoot-Through
Switching States.
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.01, January-2015, Pages: 0001-0006
A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost Inversion for ASD Drives
should also be noted that the smallest capacitance endures
the highest voltage stress, which might unintentionally
create a single point of failure. Balancing resistors for
capacitors (and diodes), together with their losses, are
therefore almost always added to the circuit for long term
usage. To avoid direct series connection, an alternate
cascading technique is discussed after describing the generic
Z-source cell shown in Fig. 5. Moreover, it is intentionally
drawn with an X-shaped structure that resembles the
original Z-source network proposed in . With this X-shaped
cell, the alternate cascading technique can be performed
based on the following few steps:
1. Begin with cell 1 with its windings labeled as W11 and
W2.
2. Duplicate a copy of cell 1, and name it as cell
2.Windings of cell 2 are labeled asw12 andw3 with their
turns ratio marked as γ3.
3. Flip cell 2 vertically and place it below cell 1.
4. Merge cell 1 and cell 2 with W2 of cell 1 replacing W12
Fig4. PWM control with shoot-through zero states.
of cell 2.
5.
Shift W12 of cell 2 to be in parallel with W11 of cell 1.
IV.CMC Z-SOURCE INVERTERS
6. Duplicate cell k with windings W1k and W(k + 1), and
Instead of a transformer with high turns ratio as in
turns ratio γk+1.
multiple smaller transformers with lower turns ratios are
7.
Repeat the flipping and merging until all N cells are
used. Their W1 windings are connected in parallel to share
cascaded (until k = N).
the extreme high instantaneous current stress, while their
W2 windings are connected in series to withstand the higher
The resulting CMC Z-source inverter is shown in Fig. 6,
voltage demanded. Turns ratios of these smaller
which
clearly does not have any direct series connection. No
transformers should be chosen based on available core and
balancing resistors and losses are therefore needed, meaning
wire sizes that can more readily produce better coupling. At
that the inverter in Fig. 6 is likely more efficient than the
times, layout and packaging of the application considered
direct series-connected circuit shown in Fig. 4. The CMC
might also have a role in deciding the transformer sizes.
inverter would however still require parallel connections of
Besides transformers, the circuit multiple diodes and
windings W1k (k = 1 to N) and capacitors (not shown in Fig.
capacitors connected in series and parallel instead of using
6 for clarity) to manage the flow of high instantaneous
single higher rated entities. Such connections are not strictly
current during shoot through. Such parallel connections will
necessary, but might at times be needed if higher rated
not be a concern in practice, unlike series connections.
components are not readily available, are too costly or do
not fit nicely to the layout of an application (e.g., height of
an electrolytic capacitor).
Fig5. Generic trans-Z-source cell.
When attempting series connection though, it is
necessary to be doubly cautious especially for cases where
component parameters drifted greatly. With capacitors, it
Fig6. Proposed CMC Z-source inverter.
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.01, January-2015, Pages: 0001-0006
V. JANAKI RAM, A. PURNA CHANDRA RAO
V. ASD DRIVE CONTROL
VI. SIMULATION RESULT
In servo applications position feedback is used in the
Simulations have been performed to confirm the above
position feedback loop. Velocity feedback can be derived
analysis. Above Figures shows simulation waveforms when
from the position data. This eliminates a separate velocity
the fuel-cell stack voltage is V0 =50V and the Z-source
transducer for the speed control loop. A BLDC motor is
network parameters areL1=L2=L=3e-6H and C1=C2=C=
driven by voltage strokes coupled by rotor position. The
500F. The purpose of the system is to produce a three-phase
rotor position is measured using Hall sensors. By varying
output line-to-line was 400-V power from the fuel-cell stack
the voltage across the motor, we can control the speed of the
whose voltage changes 50~400 V dc depending on load
motor. When using PWM outputs to control the six switches
current. From the simulation waveforms are shown. When
of the three-phase bridge, variation of the motor voltage can
the fuel-cell voltage is low, as shown in Fig.8, the shootbe obtained by varying the duty cycle of the PWM signal.
through state was used to boost the voltage in order to
The speed and torque of the motor depend on the strength of
maintain the desired output voltage. The waveforms are
the magnetic field generated by the energized windings of
consistent with the simulation results With the help of the
the motor, Which depend on the current through them.
designed circuit parameters, the MATLAB simulation is
Hence adjusting the rotor voltage and current will change
done and results are presented here. Speeds are set at 1500
motor speed. Commutation ensures only proper rotation of
rpm and the load torque disturbances are applied at time
the rotor. The motor speed depends only on the amplitude of
t=.06 sec. The speed regulations are obtained at set speed
the applied voltage. This can be adjusted using PWM
and the simulation results are shown. Figure 9 shows the
technique. The required speed is controlled by a speed
inverter input voltage (Vin =50V) waveform respectively.
controller. This is implemented as a conventional
Figure 10 shows the boosting inverter output voltage
waveform (Vout=400V) .The waveforms of the back EMF
proportional-Integral controller.
are shown in Fig.12. The stator current waveforms are
The difference between the actual and required speeds is
shown in Fig 12.Figure.13 shows the speed waveform of
given as input to the controller. Based on this data PI
BLDC Motor.
controller controls the duty cycle of the PWM pulses which
correspond to the voltage amplitude required to maintain the
desired speed. When using PWM outputs to control the six
switches of the three-phase bridge, variation of the motor
voltage can be achieved easily by changing the duty cycle of
the PWM signal. In case of closed loop control the actual
speed is measured and compared with the reference speed to
find the error speed. This difference is supplied to the PI
controller, which in turn gives the duty cycle. PMBLDC
motor is popular in applications where speed control is
necessary and the current must be controlled to get desired
torque. Figure 7.shows the basic structure for closed loop
control of the PMBLDC motor drive. It consists of an outer
speed control loop, an inner current control loop for speed
and current control respectively. Speed loop is relatively
Fig8. Simulation waveform of dc Input voltage of each
slower than the current loop.
cell voltage.
Fig7. Speed Controller.
Fig9. Simulation waveform of combined cell dc Input
voltage.
International Journal of Advanced Technology and Innovative Research
Volume.07, IssueNo.01, January-2015, Pages: 0001-0006
A Renewable Grid for Cascaded Multi Cell Z-Source Inverter with Boost Inversion for ASD Drives
Fig10. Simulation waveform of Inverter Output voltage
(Vout=400v).
Fig13. Controlled Speed of the motor using CMC Zsource inverter.
VII.CONCLUSION
This paper has proposed a cascade multi cell Z-source
inverter used to control the speed of an BLDC motor. The
drive offers the advantages of both Z-source inverter and
BLDC motor. The existing inverter scheme suffers from
shoot-through reliability problem. This topology provides
better performance than the traditional inverter topology for
an identical load and speed conditions. The feasibility of Zsource inverter fed BLDC motor drive is proved by the
simulation results. From the results obtained, it is clear that
the Z-source inverter fed PMBLDC motor drive is very
promising for various industrial applications. The drive
response can be improved by using PWM technique.
Fig11. Simulation waveform of Inverter Output current.
Fig12. Stator Current and Back EMF of BLDC motor.
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Volume.07, IssueNo.01, January-2015, Pages: 0001-0006