A RENEWABLE GRID FOR CASCADED MULTI CELL Z-SOURCE INVERTER
WITH BOOST INVERSION USING PWM TECHNIQUE
V. Janaki Ram1, A. Purna Chandra Rao2
1
(M Tech student EEE Department, Prasad V. Potluri Siddhartha Institute of Technology,Vijayawada,India)
(Asst Professor EEE Department, Prasad V. Potluri Siddhartha Institute of Technology,Vijayawada,India)
2
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 which is used in many valuable 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. A speed sensor is connected to a BLDC motor to sense the
actual speed, which is compared with a reference speed. A PI controller is designed which
helps to run the motor to the reference speed. 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
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 an BLDC motor and to control its operating speed.
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, thus reducing motor ratings;
2) Provide ride-through during voltage sags without any additional circuits.
3) 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 and experimental results are included to prove the concept
and demonstrate 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 impedance-source (or impedance-fed) power converter (abbreviated as Zsource converter) and its control method for implementing dc-to-ac, ac-to-dc, ac-to-ac, and
dc-to-dc power conversion. Fig. 3 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
observed in the traditional V- and I-source converters where a capacitor and inductor are
used, respectively. The Z-source converter overcomes the above-mentioned conceptual and
theoretical barriers and limitations of the traditional V-source converter and I-source
converter and provides a novel
power conversion concept.
Figure 1: General structure of the Z-source converter.
In Fig. 1, a two-port network that consists of a split-inductor L1 andL2 and capacitors
C1and C2connected in X shape is employed to provide an impedance source (Z-source)
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-to-ac, ac-to-dc, ac-to-ac, and dc-to-dc power conversion.
1. OPERATING PRINCIPLE
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 is greater 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
(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.
Fig.2: Equivalent Circuit of the Z-Source Inverter Viewed From The Dc Link When
The Inverter Bridge Is In The Shoot-Through Zero State.
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:
iL > 1/2ii ---------------(2)
Again, because of the symmetry of the circuit, capacitor currents iC1and iC2 and inductor
currents iL1 and iL2 should be equal to each other, respectively. In this mode, the input 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----------------(4)
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.
Fig.3: Equivalent Circuit of the ZSI When the Inverter Bridge Is In One Of The
Eight Non shoot-Through Switching States
Figure 4: PWM control with shoot-through zero states
III.
PRECAUTIONS FOR HIGHER VOLTAGE GAIN
The reason for recommending the Z-source circuits (and others) is mainly to produce a
higher voltage boost using a higher transformer turns ratio .With transformer, some
components would then experience higher steady-state voltage or current stresses depending
on the windings to which they are connected. The transformer itself might be tougher to
design too with perfect magnetic coupling, low-leakage inductances and resistances still
being the goals to achieve. Any deviation from perfect coupling will lead to large transient
over voltages caused by the breaking of currents. That happens with current through W1
when entering a non shoot-through null state and current through W2 when entering a shootthrough state. Avoiding the over voltages would require interrupted energy from one winding
to be transferred to the other through perfect coupling. A compactly coupled transformer is,
therefore, a basic necessity for Z-source inverters.
Besides over voltages, extreme high instantaneous current will flow through winding
W1 and capacitor C when in the shoot through state (not experienced by W2 and diode D).
Such high current is caused by the sudden transfer of energy from W2 to W1 when the input
diode D reverse biased. A simple way to resolve it is to connect multiple capacitors and
windings in parallel which, unlike series connection, will not overly complicate the circuit
operation in practice. More elaborated discussion about multiple components is continued in
the following section with both series and parallel connections taken into consideration.
IV.
CMC Z-SOURCE INVERTERS
Instead of a transformer with high turns ratio as in multiple smaller transformers with
lower turns ratios are used. Their W1 windings are connected in parallel to share the extreme
high instantaneous current stress, while their W2 windings are connected in series to
withstand the higher voltage demanded. Turns ratios of these smaller transformers should be
chosen based on available core and wire sizes that can more readily produce better coupling.
At times, layout and packaging of the application considered might also have a role in
deciding the transformer sizes.
Besides transformers, the circuit multiple diodes and capacitors connected in series
and parallel instead of using single higher rated entities. Such connections are not strictly
necessary, but might at times be needed if higher rated 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). When attempting series connection though, it is necessary to be doubly cautious
especially for cases where component parameters drifted greatly. With capacitors, it
shouldalso 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.
Figure 5:Generic trans-Z-source cell.
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 of cell 2.
5) Shift W12 of cell 2 to be in parallel with W11 of cell 1.
6) Duplicate cell k with windings W1k and W(k + 1), and turns ratio γk+1.
7) Repeat the flipping and merging until all N cells are cascaded (until k = N).
The resulting CMC Z-source inverter is shown in Fig. 6,which clearly does not have
any direct series connection. No balancing resistors and losses are therefore needed, meaning
that the inverter in Fig. 6 is likely more efficient than the direct series-connected circuit
shown in Fig. 4. The CMC inverter would however still require parallel connections of
windings W1k (k = 1 to N) and capacitors (not shown in Fig. 6 for clarity) to manage the flow
of high instantaneous current during shoot through. Such parallel connections will not be a
concern in practice, unlike series connections.
Figure 6: Proposed CMC Z-source inverter.
V. SPEED CONTROL
Commutation ensures proper rotor rotation of the BLDC motor, while the motor
speed depends only on the amplitude of the applied voltage. The amplitude of the applied
voltage is adjusted by using the PWM technique. The required speed is controlled by a speed
controller. The speed controller is implemented as a conventional PI controller. The
difference between the actual and required speed is input to the PI controller and, based on
this difference, the PI controller controls the duty cycle of PWM pulses, which corresponds to
the voltage amplitude required to keep the required speed. For applications requiring the
motor to operate with a specified torque regardless of speed (e.g., in-line Tensioning), a
current controller can be used, since torque is directly proportional to current. In this mode,
the speed will be held at the value set by the speed reference signal for all loads up to the
point where the full armature current is needed. If the load torque increases further, the speed
will drop because the current-loop will not allow more armature current to flow. Conversely,
if the load attempted to force the speed above the set value, the motor current will be reversed
automatically, so that the motor acts as a brake and regenerates power to the mains. The
current controller is implemented as a conventional Proportional-Integral (PI) controller. The
Output from the speed controller will be input into the current controller, along with
measured DC Bus current. The output of the current controller will control the duty cycle of
the PWM pulses. The combination of both speed and torque controllers is shown in figure.7
Figure 7: Speed Controller
VI.
SIMULATION RESULT
Figure 8: Input voltage
Figure 9: Output voltage
Figure 10: Stator Current and Back EMF
Figure 11: Speed of the motor
VII.
CONCLUSION
This paper has proposed a new electric drive system with permanent magnet BLDC
motor fed by Z-source inverter. 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 Z-source inverter fed BLDC motor
drive is proved by the simulation results. From the results obtained, it is clear that the Zsource inverter fed PMBLDC motor drive is very promising for various industrial
applications. The drive response can be improved by using PWM technique.
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