Transcript
CHAPTER-2,LITERATURE SURVEY
8
2. LITERATURE SURVEY
2.1. Wind Energy Conversion System
Wind power contributes a significant proportion of consumers’ increasing electrical power
demands. Wind power generation has grown at alarming rate in the past years and will continue to
do so as power electronic technology continues to advance. A number of power converter
techniques have been developed for integrating with the electrical grid. The use of power
electronic converters allows for variable speed operation of the wind turbine. A wide range of
control schemes, varying in cost and complexity, integrated with the power electronic converter
are designed to maximize power output at all possible wind speeds. A review is done based on the
possible combinations of converter and generator topologies for different drive systems like
permanent magnet generators, caged rotor induction generators, synchronous generators and
doubly fed induction generators are discussed and different possible control strategies so far
developed are touched upon.
The amount of power captured from a wind turbine is specific to each turbine and is governed by
𝑃𝑡 =1
2𝜌𝐴𝐶𝑃𝑣𝑤
3
Where 𝑃𝑡 is the turbine power, ρ is the air density, A is the swept turbine area, CP is the coefficient
of performance and υw is the wind speed. The turbine power output can be plotted versus the
turbine rotational speed for different wind speeds, an example of which is shown in Figure 2.1.
The curves indicate that the maximum power point increases and decreases as wind speed rises
and falls. In the following sections, the various generator–converter combinations that are able to
obtain maximum power output for varying wind speeds are discussed.The wind turbine
technology can basically be divided into three categories: the systems without power electronics,
the systems with partially rated power electronics and the systems with full-scale power electronic
interfacing wind turbines. First category is the wind turbine systems using induction generators
independent of torque variation keep an almost fixed speed (variation of 1–2%). The power is
limited aerodynamically either by stall, active stall or by pitch control
CHAPTER-2,LITERATURE SURVEY
9
Figure 2.1 Turbine power vs. generator speed
Figure 2.2 Wind power conversion system
The second category is wind turbines with partially rated power converters and much more
improved control performance can be obtained. Figure 2.2 shows two such solutions. Figure
2.2(a) shows a wind turbine system where the generator is an induction generator with a wounded
rotor. A power converter for the rotor resistance control is used with low voltage but high
currents. This also needs a soft-starter and a reactive power compensator.
CHAPTER-2,LITERATURE SURVEY
10
2.1.1. Doubly fed induction generators (DFIG)
Another solution of using a medium scale power converter is with a wounded rotor induction
generator. A significant advantage in this doubly fed induction generators (DFIG) is the ability to
output more than its rated power without becoming overheated. It is able to transfer maximum
power over a wide speed range in both sub- and super-synchronous modes. The DFIG along with
induction generators are excellent for high power applications. Different DFIG proposed are as
follows:
2.1.2. Static Kramer drive
The static Kramer drive consists of a diode rectifier on the rotor side and a line commutated
inverter connected to the supply side Figure 2.3. With this converter, a sliding mode control is
developed which provides a suitable compromise between conversion efficiency and torque
oscillation smoothing. The controller regulates the thyristor inverter firing angle to attain the ideal
compromise. This converter is only able to provide power from both stator and rotor
circuits,undersuper-synchronous operation. With a diode rectifier.
Figure 2.3 DFIG Wind energy conversion system
The inclusion of a second SCR allows the generator reactive power demand to be satisfied by the
rotor-side converter system. When connected to the wind turbine, it is shown that optimum
performance is obtained by adjusting the gear ratio [5].In comparison to the Kramer drive, this
system produces more power output due to the lack of reactive power available More detailed
control of the two rectifiers is given in [6]A range of both firing angles for each mode of operation
CHAPTER-2,LITERATURE SURVEY
11
(sub- and super-synchronous modes) is given as a plot showing the optimum firing angle at
different wind speeds giving greatest power transfer. But, major drawbacks of this approach
include firing and commutation problems with the rotor-side converter and harmonic distortion to
the grid, created by the supply-side thyristor converter.
2.1.3. Back-to-back PWM converters
One advanced method using back-to-back converters is shown in Figure 2.4 and lot of work
has been presented using this type of converter [7],[8],[9]and [10].The converter used in
those works is almost similar; differences lie in the control strategy. One option is to apply
vector control to the supply-side converter, which is controlled to keep the DC-link voltage
constant through regulation of the d-axis current. It is also responsible for reactive power
control through alteration of the q-axis current [7]and [8] .As for the rotor side, the choice of
decoupled control of the electrical torque and the rotor excitation current is presented
[7].The machine is controlled in a synchronously rotating reference frame with the d-axis
orientated along the stator-flux vector, providing maximum energy transfer. In case of [8],
the rotor current was decomposed into d–q components, where d-axis current is used to
control the electromagnetic torque and the q-axis current controls the power factor. Both
types of rotor-side converter control employ PI controllers. Space vector modulation (SVM)
is used in order to achieve a better modulation index [8]. The speed sensors have been
introduced in [9]and [10] in place of speed encoder. To accompany the capacitor in the DC-
link, a battery may be used as a storage device. With the extra storage device, the supply-
side converter now controls the transfer of real power between the grid and the fixed
voltage battery [9]. The supply side controller is made up of three PI controllers - one for
outer loop power control, and the rests for the d–q-axis inner current control loop. Energy is
stored during high winds and is exported to the grid during low wind conditions to
compensate. The control algorithm is modified to regulate the bus voltage for the low-high
wind conditions. Here, the rotor-side converter is gated in order to control the real and
reactive power of the machine. Another different option for rotor control is presented in
[10] where the algorithm searches for the peak power by varying the rotor speed, and the
peak power points are recognized as zero slopes on the power-speed curves. In this
continuous control a significant shift in power causes the controller to shift the speed which
in turn causes the power to shift once again. This control theory is based on voltage space
vectors (VSV). Another control under back-to-back PWM converter scheme uses
CHAPTER-2,LITERATURE SURVEY
12
information on shaft speed and turbine output power to estimate the wind speed [11] . The
turbine output power is stated as a function of TSR. The roots of the equation are solved to
find the optimum TSR within a particular range. With the estimated wind speed and optimal
TSR, the new reference of the generator output power and shaft speed is obtained. The
control is applied to a brushless DFIG, which gives less cost in comparison to machines with
brushes and slip-rings.
Figure 2.4 Back to back converter
2.1.4. Matrix converter
The matrix converter is capable of converting the variable AC from generator into constant AC to
the grid in one stage only Figure 2.5. Another major advantage of this topology is that the
converter requires no bulky energy storage or DC-link. The utilization of matrix converter with a
DFIG has been explored in [12] and [13]. In [12] , the stator–flux oriented control is used on the
rotor matrix converter. The d-axis current was aligned with the stator–flux linkage vector and
controlled by simple PI controllers. The regulation of the d-axis current allows for control of the
stator-side reactive power flow, where as the q-axis current regulates the stator-side active power.
Another option [13] is to control the rotor winding voltage, which consequently manipulates the
power factor of the DFIG. The matrix converter is consisted of nine bi-directional switches
arranged in a manner such that any input phase can be connected to any output phase at any time.
Therefore, each individual switch is capable of rectification and inversion. The matrix converter is
controlled using double space vector PWM, employing the use of input current and output voltage
CHAPTER-2,LITERATURE SURVEY
13
SVM. One of the major drawbacks of a matrix converter is that use of a large number switches,
causing an increase in converter semiconductor cost.
Figure 2.5 Matrix converter
The third category is wind turbines with full-scale power converter between the generator and
grid, which gives extra losses in the power conversion but it, will gain the added technical
performance.
2.1.5. Induction generators
The use of induction generators (IG) is advantageous since they are relatively inexpensive, robust
and require low maintenance. Induction generator need bi-directional power flow in the generator-
side converter since it requires external reactive power support from the grid. The use of back-to-
back PWM converters Figure 2.6, along with the implementation of one or more fuzzy logic
controllers is a consistent converter-control combination [14] , [15] and [16]. The advantages of
fuzzy logic control are parameter insensitivity, fast convergence and acceptance of noisy and
inaccurate signals. A PI type fuzzy logic controller takes in the DC voltage error and controls this
error [15]. The controller outputs the d-axis reference current used in real power flow control.
Similarly, the q-axis current is kept zero to maintain unity power factor.
CHAPTER-2,LITERATURE SURVEY
14
Figure 2.6 Induction generator with back to back PWM converter
Another control scheme using three fuzzy logic controllers has also been presented in [14] .The
first one tracks the generator speed with the wind velocity to extract maximum power. The second
controller programs the machine flux for load efficiency improvement. More specifically, the
machine rotor flux can be reduced from the rated value to reduce the core loss and thereby
improve the efficiency. The third controller gives robust speed control against wind gust and
turbine oscillatory torque. Unlike second controller, the third fuzzy logic controller is always
active. In other work [16] , a PI fuzzy controller is used where rotor slot harmonics (RSH) are
used for speed estimation. The rotor slots interact with the magnetizing component of the air-gap
magneto-motive force (MMF), generating harmonics that are dependent on the machine rotational
speed. Once the algorithm locates the frequency of the RSH through a look-up table, the rotational
speed is found through a series of calculations. Along with the use of RSH, the control system
also utilizes sensor-less control through a model reference adaptive system (MRAS) observer to
estimate the rotational speed. More information on this system is located in [16] , as the details
will not be discussed here. A control option for the supply-side converter includes real and
reactive power control. A reference frame orientated along the supply voltage rotating vector
allows for real power control through d-axis current control and q-axis manipulation controls the
reactive power. The aforementioned control is proven to track fast changes in rotational speed
with high accuracy, a favorable characteristic for systems employing a stall controlled wind
turbine. This control algorithm can react quickly to wind gusts and may be utilized to control the
amount of mechanical power and torque input to the generator. These are common concerns for
stall controlled wind turbines as operation over rated power may cause damage to the generator
CHAPTER-2,LITERATURE SURVEY
15
and power electronic converter. A comparison between the use of a wound rotor induction
machine and a caged rotor induction machine, both of identical size, has been performed [17].
Both the induction machines are six poles and having rated voltage of 415 V, 300kW and have a
rated speed of 1000 RPM. The comparison ensures validity with the use of identical converter
types in each of the systems. The separate designs were each tested under identical variable wind
conditions. It is shown, under the same wind conditions, that the wound rotor induction machine
outputs 35kWh of energy over 10 min, where as the caged induction machine only outputs
28.5kWh in 10 min. The higher cost of the wound rotor induction machine, due to possible need
of slip rings, is compensated by the reduction in the sizing of the power converters and the
increase in energy output.
2.1.6. Synchronous generators
The application of synchronous generators (SG) in wind power generation has also been
researched. A brief description of one possible converter-control scheme is given for a small wind
energy conversion system. The use of diode rectifier along with a DC/DC boost stage and inverter
as a power electronic interface for grid connection has been discussed [18]. In this scheme, the
DC-link voltage is controlled by using the amplitude of the three-phase inverter voltages and the
phase displacement angle of the inverter. Controller performance improvements are achieved over
the traditional power angle control. For low power systems, the existence of a field winding
circuit in the rotor is a drawback as compared with PMSG. In large systems, the energy from the
SG are most commonly converted through back-to-back PWM voltage source inverters. The
supply side PWM inverter allows for control of real and reactive power transferred to the grid.
The generator side converter is used for electromagnetic torque regulation [19] and [20] .The
controllers used in these systems are designed to achieve maximum power transfer to the grid.
These generators are of high efficiency since the whole of the stator current is employed during
torque production. Another advantage is the minimization of stator current through the direct
control of generator power factor, In comparison to IG, the use of SG is advantageous since they
are self-excited machines and the pole pitch of the machine can be smaller. As a result both DFIG
and SG are preferred for high power applications.
2.1.7. Permanent magnet synchronous generators (PMSG)
The permanent magnet synchronous machine (PMSM) based direct wind turbine drive is one of
the more promising technologies especially for advantages such as small size, robustness, less
weight and simple flexible design structure. Since the PMSM wind power generator operates at a
CHAPTER-2,LITERATURE SURVEY
16
variable speed, a back-to-back (ac-dc-ac) converter as a grid interface becomes essential. At the
generator side, either a controlled rectifier or a diode-rectifier can be employed to convert the
generated energy to DC, as illustrated in Figure 2.7. A grid-side inverter is interconnected through
a dc-link element, which can be a capacitor in voltage-source converters. A major cost benefit in
using the PMSG is a diode bridge rectifier used at the generator terminals since no external
excitation current is needed. Many researchers have performed using diode rectifier [21]- [22] ,
some of which is described in details.
Figure 2.7 Thyristor based inverter
2.1.8. Thyristor based inverter
Thyristor-based inverter is used in [21] to allow continuous control of the inverter firing angle,
regulating turbine speed through the DC-link voltage to obtain optimum energy capture [21]
.Advantages of this scheme include lower device cost and higher available power rating than
hard-switched inverters. A major drawback to this inverter is the need for an active compensator
for the reactive power demand and harmonic distortion created, as shown in Figure 2.7. A voltage
source converter (VSC) is used for the compensator and the error signal between the reference
and actual compensator current is used to drive the pulse width modulated (PWM) control.
2.1.9. Hard-switching supply-side inverter
Various control strategies that can be applied to the converter in Figure 2.8 have been discussed
[23] . A proposed control involves the manipulation of the modulation index of the reference
sinusoidal signal applied to the PWM generator. This is achieved by determining the DC-link
CHAPTER-2,LITERATURE SURVEY
17
voltage by a power mapping technique that contains the maximum power versus DC voltage
characteristic. The control system is further improved by using a derivative control on the stator
frequency, since it also changes with change in DC-link voltage. This control is compared with
maximum power point tracking (MPPT), which includes an anemometer, a wind prediction
control scheme and a fixed-voltage scheme. The anemometer measures the wind speed and aids in
providing the wind power reference to the MPPT controller. The reference power is compared
with the actual DC power extracted in which the result is used to determine the new operating DC
voltage. The current control loop of the inverter receives the new operating DC voltage and
outputs an instantaneous driving signal for the PWM. Under fixed-voltage control, the voltage of
the inverter is fixed at a targeted optimum wind speed. In comparing the four control methods, the
fixed voltage scheme was used as the reference since it was least efficient. The MPPT with
anemometer setup proved to be superior, obtaining 56–63% of energy available. However, the
proposed method using sensor-less control was not far behind, obtaining same level of energy
available from the wind.
Figure 2.8 Hard switching Inverter
2.1.10. Intermediate DC/DC converter stage
The use of a voltage source inverter (VSI) accompanied by a DC/DC converter is investigated in
[24] and [25] , depicted in Figure 2.9. This setup is also compared to the converter shown in
Figure 2.10 [26]. Incorporating an extra DC/DC converter gives the following advantages:
1. Control of generator-side DC-voltage through variation of the switching ratio,
2. Maintains appropriate inverter-side DC-voltage,
3. Allows for selective harmonic elimination (SHE) switching, giving reduced losses,
4. Inverter no longer needs to control DC-voltage, and has more flexible control.
The inverter power control can be achieved by regulating the magnitude of the fundamental line
current and the phase angle between the line current and line voltage [24]. The controller is
CHAPTER-2,LITERATURE SURVEY
18
configured such that the VSI is switched at the frequency of the triangular carrier signal and its
output harmonics are well defined. For every shaft speed, optimum values of DC voltage and
current can be identified corresponding to the maximum available turbine power [25] . The
DC/AC voltage ratio and power angle are used as control variables that are tuned to control the
power, and ultimately the speed of the generator. The inverter control can also be implemented to
keep the DC-link constant and vary the reactive power in a manner that attains maximum real
power transfer to the grid. Results show that the thyristor-based inverter with active compensator
is best suited for strong AC systems since it relies on the system to ensure commutation [25] .
However, both the VSI and DC/DC–VSI systems are capable of integrating with both strong and
weak AC systems. Other control strategies have been discovered for this converter in [27] and
[28]. The DC/AC inverter can control the active and reactive power delivered to the grid via
control of the q-axis and d-axis current, respectively [27]. The q-axis reference current is
determined by the error in the DC-link voltage, and is then compared with the actual current. The
phase angle of the utility, used in power factor control, is detected using software phase lock loop
(PLL) in a d–q synchronous reference frame. Power factor control creates the d-axis reference
current allowing it to be compared with the actual d-axis current. The error in both reference
frame currents are used to create the d–q-axis reference voltages used in space vector PWM
control. Using the voltage equation governing a boost-up DC/DC chopper and a proportional–
integral (PI) controller, the duty ratio of the chopper switch may be determined for any particular
optimum point [27] .The inverter-side DC voltage remains constant set by the grid voltage giving
the advantage of flexible transfer of active and reactive power to the grid. A slight modification to
the DC-link is made by including a battery. The battery allows charging during night time when
load demand is usually lower. An immediate advantage is a constant DC-link voltage, therefore,
controlling the chopper output current to its maximum value giving maximum output power [28]
.To perform the control, a relationship between the output power and duty cycle of the chopper is
used. Starting from an arbitrary point, the duty cycle can be continuously and slowly adjusted
between a specific ranges searching for the maximum power point. It is found that the control
system began losing efficiency at high speeds; this was due to the phase lag between the DC
current and duty ratio [28] .A faster sampling rate would help correct this problem.
2.1.11. Back-to-back PWM converters
The use of two, 6-switch, hard-switched converters, with a DC-link capacitor, Figure 2.10 has
been explored [29]. The generator side rectifier is controlled through a PI controller such that the
d-axis current is held to zero to obtain maximum electrical torque with minimum current. A
CHAPTER-2,LITERATURE SURVEY
19
MPPT is used in determining the optimum rotor speed for each wind speed to obtain maximum
rotor power. In contrast, the grid side inverter controls the line current to be sinusoidal through a
hysteresis controller.
Figure 2.9 Converter with DC chopper
Figure 2.10 Back to back PWM converter
.
The DC-link voltage is also controlled by a PI controller, via the grid side inverter. More recently,
a converter using two B-4 converters and two DC-link capacitors has been developed, shown in
Figure 2.10 [22] . Again MPPT calculates the output power of the generator by measuring the
DC-link current and voltage, and then alters the operating point by increasing or decreasing the
reference current magnitude. The MPPT control is performed on the generator side rectifier
The grid-side control sets the inverter current through a PI controller and the DC-voltage error.
The current error is used to drive the inverter switching signals. Multilevel VSCs such as three-
level neutral-point-clamped converter [30] and [31] and multi-modular cascaded H-bridge
CHAPTER-2,LITERATURE SURVEY
20
converter [32] have been also proposed as alternative solutions. The multilevel converters enable
the use of low-voltage devices and provide better waveform quality.
2.1.12. Review Summary of wind energy conversion systems
From the above review of different converter topologies used in combination PMSG, DFIG, IG
and SG, along with different control schemes, it is clear that the cost of the overall system
increases as the complexity of the power electronic converter increases. The controller design also
affects cost; for example, the use of MPPT techniques will cost more than a simple lookup table
method. However, higher order control and converter designs may increase efficiency of the
overall system.
Table 2.1 Advantages and Disadvantages of PMSG for wind power conversion
Advantages Disadvantages
Flexibility in design allows for smaller and
lighter designs
Higher output level may be achieved
without the need to increase generator size
Lower maintenance cost and operating
costs
No significant losses generated in the rotor
Generator speed can be regulated without
the need for gears or gearbox
Very high torque can be achieved at low
speeds
Eliminates the need for separate excitation
or cooling systems
Higher initial cost due to the
magnets used
Permanent magnet costs
restricts such generators for
large scale grid connected
turbine designs
High temperatures and sever
overloading and short circuit
conditions can demagnetize
permanent magnets
The inclusion of a DC-boost stage reduces the control complexity of the grid inverter and also
replacing the diode rectifier with a controlled rectifier allows for a wider range of control of both
the generator and grid real and reactive power transfer. In order to maximize the benefits of the
wind energy conversion system, a compromise between efficiency and cost must be obtained.
Comparative study [33] shows that DFIG offers a vast reduction in converter size but are
CHAPTER-2,LITERATURE SURVEY
21
susceptible to grid disturbances since their stator windings are directly connected to the grid.
Again, SG allows for independent control of both real and reactive power. As for the IG, they are
relatively inexpensive and robust however, they require external excitation circuitry and reactive
power from the grid. A major disadvantage is that they require a synchronizing relay for grid
connection. But, PMSG offer the highest efficiency and are self-exciting machines and are
therefore suitable for small-scale designs. The details Advantages and disadvantages compared to
other generators is given as Table 2.1
2.2. Z-Source Inverter
Z-source Inverter was first proposed by Fang Zheng Peng in [2] . The paper presents an
impedance source power converter and its control methods for implementing DC-AC, AC-DC,
AC-AC and DC-DC power conversion. The paper focuses an example of Z-Source inverter for
DC-AC power conversion needed in fuel cell application. Simulation and experimental results
have been presented to demonstrate the theoretical result. The voltage source and current source
inverter’s conceptual and theoretical barriers have been removed by employing an impedance
network between the DC source and the Inverter. Z-source inverter is a buck-boost inverter that
has a large range of obtainable output voltage. Due to EMI noise, misgating of inverter switches is
one of the major problems in the voltage source inverter. Also, the dead time provided for VSI to
protect simultaneous turning on of same leg switches, distorts the output voltage waveform.
Again, in VSI the ac output voltage can never be greater than the equivalent DC-rail input voltage.
In case of current source converter, the ac output voltage has to be greater than the dc voltage
which feeds the dc inductor connected with the source. Overlap time for safe current commutation
is required in CSI, otherwise an open circuit may take place which can destroy the circuit. A
comparison between current source inverter, voltage source inverter and impedance inverter has
been well presented in [34]. For current source inverter an inductor is used in the dc link having
high source impedance. It acts as constant current source. In VSI a capacitor is used in the DC
link which acts as a low impedance voltage source. And in impedance source inverter inductor
and capacitor both are used so as to work as a high impedance voltage source. The z-network
components serve as the energy as well as the filtering element. Harmonic content in the
traditional and z-source inverter have also been compared. Due to switching losses and
considerable EMI generation, there is decrease in efficiency in VSI and CSI. Dead time is allowed
in VSI which provides voltage distortion increasing the filter size. For light load operations or
small drives with no significant inductance the line current becomes discontinuous.
CHAPTER-2,LITERATURE SURVEY
22
2.2.1. Applications of Z-Source inverter.
Several papers have been published on the application of Z-Source Inverter. The z-source inverter
application in adjustable speed drive system has been proposed in [35] .The ZSI produces any
desired output voltage even greater than the line voltage. It provides ride through during voltage
sag without any additional circuit like transformer or dc-dc boost chopper and reduces the inrush
and harmonic current. The output capacitor of the diode bridge rectifier is used to suppress
voltage surge that may occur due to the line inductance during diode commutation. By controlling
the shoot through zero state intervals, a desired dc voltage can be maintained. But no experimental
results are shown in support of the simulation result and the details of the switching technique is
not explained in the paper. Another proposed motor drive system (ASD) proposed as shown in
Figure 2.11, describes the benefits of using z-source inverter including the minimization of motor
ratings to deliver a required power and reduction of in-rush/ harmonic current.
Figure 2.11 Z-Source Inverter based ASD system
The z-source inverter application in PV-cell has been presented in [36]. Simulation and
experimental results are shown to verify the features of the renewable energy resources. Here only
PV residential power has been considered as one of the renewable energy resource. Two cases
have been considered. First when the radiation happens 1000 W/m², and temperature is 60°C,
input voltage is 230 V, the maximum PV output power is 7200 W. The second one is when
radiation happens 250 W/m², temperature is 0 °C , input is 450 V, the maximum PV output
power is 3360W. In the first case, the shoot-through states are needed, and the shoot-through duty
cycle was set to 0.245. While in the second case, the shoots through states are not needed due to
CHAPTER-2,LITERATURE SURVEY
23
the higher input voltage. Inductors and capacitors used in the network are said to be optimally
designed to lower the cost and size but no design consideration have been chosen here. No control
method other than simple boost control has been considered. Even the efficiency, size and cost of
the proposed system is not compared with the traditional one.The paper [37], presents the
implementation of fuel cells with low output voltage range for supplying the high voltage loads.
For matching the different voltage levels and for the providing of galvanic insulation the isolated
DC/DC interface converter is required. For increasing of power density, efficiency and flexibility
the interface DC/DC converter with three-phase intermediate AC-link is proposed. The impedance
source inverter utilized in the input stage of the converter. The paper verifies the theoretical
background of the proposed topology by the simulations.presents a simulation work of impedance
source inverter applied for Fuel cell. It does not present any controller details as well as no clear
direction of implementation. In another work [38] a fuel cell based hybrid electric vehicles
(FCHEV) system is proposed to control power from the FC, power to the motor, and the battery,
using the Z-source inverter.
Figure 2.12 Configuration of Z-Source Inverter for Fuel Cell based Hybrid Electric vehicles
The Z-source inverter can be effectively used for this application shown in Figure 2.12 for the
advantages of simplicity, low cost, reliability and boosting capability. The Z-source inverter
utilizes an exclusive Z-source network to link the main inverter circuit to the FC (or any dc power
source). By substituting one of the capacitors in the Z-source with a battery and controlling the
shoot through duty ratio and modulation index independently. The Fuel cell power, output power,
and state of charge of the battery can be controlled. Because of these, the Z-source inverter highly
desirable for use in FCHEVs, as the cost and complexity is greatly reduced when compared to
traditional inverters. This proposed topology is verified by simulation and experimental results
2.2.2. Analyzing the characteristics of ZSI
Various papers have been presented in the literature to analyze the characteristics of ZSI. To
understand the characteristics of Z-source inverter, dynamic and steady state modeling of the
CHAPTER-2,LITERATURE SURVEY
24
inverter is required. The transient modeling of a voltage type z-source inverter is described in
[39]. Where its non minimum phase response caused by a dc-side and ac side is shown. The dc–
side phenomenon is associated with the unique z-source impedance network and is studied using
small signal and signal flow graph analysis The small signal analysis is done on the z-network by
introducing a small disturbance in the shoot through and non shoot through duty ratio and is
proved that the z network is a non minimum phase system. Signal flow graphical analysis has
been used to find the control to output and disturbance to output transfer function. When the
capacitor value is increased then the pole of the system is shifted vertically toward the real axis
which will increase the system damping with reduced overshoot and undershoot but an increased
rise time. When the z-source inductance L is increasing then the poles and RHP zero moves
towards the imaginary axis. The shifting of zero increases the nonminimum phase undershoots
while the shifting of pole increases the system settling time and oscillatory response. The main
objective of the paper [40] is to control the capacitor voltage linearly. An algorithm is suggested
in order to improve the transient response for DC boost control of the ZSI. A modified pulse
width modulation scheme is applied to control the shoot through time for boosting dc voltage. The
SVPWM technique gives lower current harmonics and higher modulation index. The proposed
method can achieve the good transient responses of variations of both the reference capacitor
voltage and the reference output voltage. The application of Z-Source inverter in uninterruptable
power supply is presented in [41].Inductor current and output voltage are controlled by the dual
loop in the proposed UPS.A comparison plot has been shown which proves the proposed UPS is
more efficient than traditional UPS. The paper [42] presents five different operating modes and
characteristics of the z-source inverter considering low inductor value or very low load power
factor. The basic operating principle described in different literatures assumes that the inductor
current is relatively high and almost constant. When the inductance is small or the load power
factor is low, the inductor current can have high ripple or even become discontinuous. Instead of
having the two operating modes described in various literatures the Z-source inverter may have
five different operation modes. Modes 1 and 2 is same as the previous modes described whereas
modes 3–5 are new modes that may exist for small-inductance and low power-factor cases is
described here.The paper [43] analyzes the single phase H-bridge topology, three-phase leg and
four-phase-leg topologies with the modulation concepts. Carrier-based reference equations has
been derived and verified in Simulation and experimental results have been shown for a three-
phase-leg Z-source inverter. It presents the modulation analysis on Z –source inverter in
continuous and discontinuous mode. The inverter side of the Z-source network is represented by
an equivalent current source. This current source sinks a finite current when in a nonshoot-
CHAPTER-2,LITERATURE SURVEY
25
through active state and sinks zero current when in a non-shoot-through null state. Through the
proper placement of shoot through states, Z-source inverter modulation can be made to reproduce
the desired performance features of various reported conventional PWM strategies. The paper
[44] presents the use of high frequency sinusoidal signal in place of triangular signal for PWM
pulse generation to get higher boost factor and output peak voltage for the same modulation index.
In the conventional simple boost control method, to achieve the high output voltage, it is required
to increase the shoot through duty ratio, which can only be achieved with the reduction of
modulation index. But small modulation index results in greater voltage stress on the device,
hence it restricts the obtainable gain because of the limitation of device voltage rating. For the
same modulation index, the sine carrier PWM gives high boost factor and hence high peak output
voltage. Also it gives higher fundamental output voltage. To increase the fundamental amplitude
in the triangular PWM technique the only way is increasing modulation index beyond 1.0, which
is called as an over modulation. Over modulation causes the output voltage to contain many lower
order harmonics and also it makes the fundamental component – modulation index non-linear.
From the simulation analysis the proposed PWM gives less magnitude of lower order harmonics
and hence lower values of total harmonic distortion (THD) in the output voltage. The paper [45]
presents the expression of the output voltage, output current, and input current of the inverter in
the form of switching function from which the characteristics of Z-Source inverter can be easily
understood. The switching function in the shoot through state is considered as 0 and in the non-
shoot through state as 1. The parameter analysis, simulation and experimental results verify the
various advantages of Z-source inverter over both traditional inverter topologies. Advanced and
computation-intensive space vector pulse width modulation Z-Source inverter based approach is
presented in [46].Operation in over modulation region is not possible in any other PWM
technique since the inverter behaves as a square wave inverter when the modulation index exceeds
one. Reduced harmonics low switching stress power and low common mode noise .Therefore
PWM modulation allows Z-source inverter to be operated at over modulation region.
2.2.3. Switching techniques of Z-Source inverter
In previous citations, only simple boost control (SBC) switching technique has been used. Though
it has several advantages but it also consists of some demerits like voltage stress across the
devices is very high. To remove the demerits of SBC, maximum boost control of the z-source
inverter has been proposed in [47]. Voltage boost and modulation index relationship has been
derived. Relation between voltage gain and modulation index as well as voltage stress verses
voltage gain are analyzed. Experimental results are presented in support of that. As in simple
CHAPTER-2,LITERATURE SURVEY
26
boost control method, greater output voltage can be obtained by reducing the modulation index.
But the main problem is the voltage stresses across the devices that increase to high value. This
restricts the obtainable voltage gain because of the limitations of the device voltage rating. In
Maximum boost control (MBC) method all the zero states are utilized as shoot through zero states
so that the maximum shoot through duty ratio and the boost factor is obtained for a constant
modulation index without destroying the output voltage waveform. In this method shoot through
duty ratio varies throughout the cycle. The modulation index range greater than one is obtained by
third harmonic injection method which can be used to get the higher range of modulation index.
The method produces low frequency ripple present in the inductor current and capacitor voltage,
so the method is not applicable for low frequency application. To remove these low frequency
ripples another control method known as constant boost control method is proposed in [48]. The
paper proposes a switching technique to obtain maximum voltage gain at any given modulation
index without producing low frequency ripple in the inductor current and capacitor voltages. In
maximum boost control method the shoot through duty ratio varies at six times the output
frequency so the ripple in the shoot through duty ratio will result in ripple in the inductor current
and capacitor voltage. The technique removes the demerits of the maximum boost control method.
The paper [49] presents two control methods for the Z-source inverter which produces higher
voltage gain without low frequency ripple related to the output frequency. The relationship of
voltage boost and modulation index as well as voltage stress of the devices is offered. The main
advantage of the technique is that the Z-network requirement is independent of the output
frequency and determined only by the switching frequency. The paper [50] presents the review of
four PWM control methods SBC, MBC, MCBC and MSVPWM (modified space vector pulse
width modulation). It presents the comparisons of different ZSI topologies (basic, bidirectional
and high performance) under the same input voltage ,shoot through duty ratio ,peak dc voltage
across the inverter ,switching frequency and the output load .The paper also presents the review
for optimal impedance network parameter design.
2.2.4. Improvement in the traditional Z-Source inverter
Since 2004 there were stacks of improvements in the Z-source inverter topology till now. A
modified z-source inverter topology proposed in [51] is shown in Figure 2.13.The modified
Inverter has soft start capability that means high inrush current is suppressed at the starting with
lesser voltage across the capacitor.
CHAPTER-2,LITERATURE SURVEY
27
Figure 2.13 Improved Z-source Inverter
The soft start topology suppresses the inrush surge and the resonance of z-source capacitor and
inductors.It has presented a new Z-source inverter topology. Compared to the previous Z-source
inverter. The proposed topology has several merits. The Z-source capacitor voltage stress is
reduced greatly to perform the same boost ability, thus low-voltage capacitors can be utilized to
reduce the system cost and volume.
The inrush current and resonance of Z-source capacitors and inductors in traditional topology can
be suppressed with a proper soft-start strategy. Simulation and experimental results have been
presented in support of the enhanced merits.The paper [52] presents high performance based z-
source inverter adjustable speed drive system. It removes the different limitations of z-source
inverter. Light load operation is the problem in the z-source iverter based ASD system. The DC
link voltage is increasing infinetely when the system operated with light load. The z-network
inductor has the limited value to guarantee the input current greater than zero. In this paper the
performance of z-source inverter ASD system configutration, its operating mode, voltage
relationship and a partly PAM/PWM control method are described. Modulation index plays an
important role on the motor iron loss and the ripple current below base speed. The conclusion of
the paper is that the partly PAM/PWM control methods of the inverter lowers voltage stress
across the inverter switches, higher modulation index compared to the traditional PWM control.
CHAPTER-2,LITERATURE SURVEY
28
Figure 2.14 High performance Z-source Inverter
An improved topology with soft start strategy having relatively lower size compare to the
traditional z-source inverter is found in [53]. The new improved topology reduces the capacitor
voltage stress greatly and has limitations to inrush current. We conclude from this paper that the
low voltage capacitor size and volume reduces. Suppresses the inrush current and resonance
between the capacitor and the z inductors. The paper is similar to the paper [51].The paper [54]
explains the different configuration of Quasi Z –source inverter (QZSI). These inverters are
similar to the traditional z-source inverters presented in previous works, but have several
advantages and remove the disadvantages of z-source inverter like discontinuous input current in
boost mode; the capacitor should sustain a high voltage. It has some additional advantages like
lower component ratings, reduced source stress, reduced component count and simplified control
strategies.
The paper guarantee that QZSI have lower component rating reduced switching stress, larger
range of gain. Input current is continuous and no need to sustain high voltage and high current for
capacitor and inductor compared to traditional ZSI. The family of quasi z-source inverter for four
quadrant operation is described in [55]. The topology consists of minimal number of switches and
passive devices. These converters employ a Z-source or a quasi-Z-source network with two active
switches to provide four quadrant operation which means bipolar output voltage and bidirectional
current operation. Bipolar output operation can also be realized by using one active switch and a
diode. They also own buck and boost characteristics when the duty cycle is changed from zero to
one. At 0.5 duty cycle, these converters can output either zero or infinity voltage. Experimental
results are given to demonstrate the validity and features of these circuits. The topology is utilized
for bidirectional output voltage and bidirectional output current for motor operation. The paper
CHAPTER-2,LITERATURE SURVEY
29
[56] presents a class of Trans Z-source Inverter for voltage fed and current fed DC-AC inversion
system. Explanation in support of the increase in motoring operation range by the use of current
fed Trans z-source inverter is illustrated. The literature confirmed that the voltage gain has been
increased compared to traditional Z-source Inverter, lower component count and size of the
inductor reduces. In quasi Z-source inverter, the boost ratio cannot exceed two because the diode
will conduct in the active state. Due to the fact that PV-cell output varies widely because of the
change in temperature and solar irradiation, the QZSI is most suitable inverter for its obtainable
large gain range. The paper [57] presents an advanced topology of Z-source inverter for
photovoltaic power generation application. Continuous DC current is drawn from the source. For
spwm modulation index range is between 0 to 1. And for SVPWM it is between 0 to 1.1547.The
main point focused in the paper is that the two capacitors in ZSI sustain the same high voltage
while the voltage on capacitor C2 in QZSI is lower which require lower capacitor rating .The ZSI
has discontinuous input current in the boost mode while the input current of the QZSI is
continuous due to the input inductor L1 which will significantly reduce input stress. There is
common dc rail between the source and inverter which is easier to assemble and causes less EMI
problems. The paper [58] describes the current source rectifier –Quasi z-source inverter based
ASD system. The operating principle and analysis has been explained. The proposed PWM CSR-
qZSI ASD system have various advantages like providing a bidirectional power flow,
maintaining the unity power factor, reducing the input line current harmonics, providing ride-
through during the voltage sags and voltage swells without any additional circuits, and makes the
inverter modulation index high during low-speed operation. The Q-ZSI can operate with constant
modulation index during the output low speed and low voltage.
2.2.5. Comparison of different switching technique applied in Z-Source inverter
The paper [59] presents a comparative study of different switching techniques applied in Z-source
inverter. For common boost factor and modulation index, the output voltage, output current,
output line harmonics profile of the inverters with different PWM schemes powered by the same
dc power supply and three phase RL load is described. The paper confirms that the modified
space vector pulse width modulation provides better operation with effective dc boost and lowest
harmonics. The simple boost control switching technique applied to z-source inverter with
independent relation between modulation index and shoot through duty ratio is explained in [60].
It shows that better performance would be obtained if modulation index (M) and shoot-through
CHAPTER-2,LITERATURE SURVEY
30
duty ratio are set to a high value. The work assures that the proposed topology reduces the inverter
dc link voltage overshoot so that power delivery capacity of the inverter increases. In [61] Z-
source inverter and traditional two-stage buck-boost inverter are compared with considering the
voltage stress and maximum current stress of semiconductor devices and the inverter efficiency in
grid-tied renewable energy generation application. The literature [62] presents the review of four
PWM control methods and comparisons of different ZSI topologies (basic, bidirectional and high
performance) under the same input voltage, shoot through duty ratio, and peak dc voltage across
the inverter, switching frequency and the output load. This also presents the review for optimal
impedance network parameter design. The comparison shows that the maximum constant boost
control is the most suitable PWM control method for all ZSI topologies. Again, another
publication [63] compares the simple boost control method and maximum constant boost control
method with third harmonic injection applied across the Z-source inverter. The paper [64] also
presents the application of z-source inverter in PV cell .Inductor and capacitor design has been
explained. The ZSI self boost phenomenon is represented with its operating mode. For analysis
open loop condition has been considered. Different techniques and control methods use the
maximum power of the PV panel at all times by utilizing the maximum power point tracking
(MPPT) which uses the maximum power transfer theory by keeping the impedance of both the
inverter bridge and the load, very close to the internal resistance of the PV Panel.
2.2.6. Control of Z-Source Inverter
However, for some energy storage applications such as ultra capacitor banks, battery banks etc.,
bidirectional power flow is essential for the operation but in the traditional z-source inverter
power flow is unidirectional. A bidirectional Z-Source Inverter is presented in a literature [65] for
reverse conduction of the current because of the need to charge and discharge all types of energy
storage. A transistor is connected in parallel to the DC Link diode in traditional Z-source
inverter.The methods to generate switching signals for the new transistor as well as those in the
conventional VSI were described for the simple-boost control. It can be easily modified for any
other switching strategy. The process of design was demonstrated through an example where a
battery bank is connected to the grid through the bidirectional ZSI. The accuracy of the design is
verified through computer simulations and the proper operation of the inverter was demonstrated
for both directions of power flow. In [66], a state space average based model with inductive load
for the z-source inverter and a controller is designed using gain scheduling by continuously
varying the control gain according to the operating point. Small signal perturbation is done at a
given equilibrium point and transfer function of capacitor voltage with shoot through duty ratio
CHAPTER-2,LITERATURE SURVEY
31
and modulation index. The literature [67] proposes a closed loop control system for ZSI
application in DG system for quality output waveform with good disturbance rejection properties
.The system is modeled with state space averaging and small signal modeling technique. Proposed
controllers would minimize the effects of non-minimum phase characteristics present in the DC
side and a cushioning method was employed to mitigate the transferring of DC side disturbances
into the AC side. The paper [68] presents the use of resonant regulator in ZSI for UPS application.
This paper presents a control method for obtaining sinusoidal output voltage regardless of the
nonlinear and unbalanced loads. A modeling and design of a closed loop controller for a z source
converter is described in [69]. Z-source impedance network due to non minimum phase
characteristics shows a significant over shoot and undershoot following a step change in the input
due to energy resettling .The modulation index shoot through time and saturation level are
appropriately selected so that the dc side effects are prevented from propagating in to the ac side.
The main objective of this is to consider the dynamics of the closed loop system in designing the
closed loop controller. Design of impedance network is explained in very few papers. The value
of Inductor and capacitor value is chosen without any clear justifications. The study of operating
states of the impedance network and providing guidelines in designing the impedance network are
described in the publication [70]. The change in capacitor voltage and inductor current is assumed
linear in this design procedure instead of sinusoidal for simpler analysis. Another literature [71]
presents the three dynamic operating states along with the three static operating states of the Z-
source inverter. The steady states of a Z-source inverter are identified and analyzed with the
objective of deriving design guidelines for the symmetrical impedance network. This paper shows
that, in addition to the desired three dynamic states, an operating cycle can contain another three
static states that do not contribute to the power conversion process. These three static states can be
avoided by selecting suitably large capacitors and inductors. Computer simulations and laboratory
experiments are used to verify the design method and to demonstrate the appearance of static
states when the capacitors and inductors are sized lower than their critical values. A design
method for the impedance network of the z-source inverter has also been proposed here. The
paper [72] shows the design procedure of Z-Source converter as a single phase PV Grid
connected transformer less inverter. Low frequency ripple across the output is considered in
designing the components. Unipolar technique is preferred since it produces fewer harmonics
compared to bipolar method. A switching pattern for the modulation of the converters has been
proposed. The proposed topology reduces the switching loss and common mode EMI.[73],
describes how a single-phase Z-Source inverter can be built for grid-connection of a photovoltaic
array. Significant changes are needed in sizing the components and generating switching signals.
CHAPTER-2,LITERATURE SURVEY
32
The paper confirms that the voltage ratio is dependent on the capacitor and inductor used in the
DC link.
2.3. Z-Source ac-ac converters
Again, in line with the z-source inverter, inclusion of z-network in ac-ac converter is proposed for
a single phase ac system by researchers in [74]. The z network consisting of two inductors and
two capacitors are connected as shown in Figure 2.15. Buck boost capability of this type converter
is verified by simulation and experimental results. Therefore, by PWM duty ratio control they
become solid state transformer with continuously variable turn’s ratio. For only voltage regulation
PWM ac-ac converter is better choice to achieve smaller size and lower cost. Switches are turned
on and off in complement to each other. Depending upon the operating region of the duty cycle,
the output voltage can be in phase or out of phase with the input voltage.
Figure 2.15 Voltage fed ac-ac Z-Source converter topology
By controlling the duty ratio the output voltage of the proposed ac-ac converter can be bucked or
boosted. When the duty cycle is greater than 0.5, the output voltage is out of phase and operates in
buck mode. Similarly, for less than 0.5 the output voltage is in phase with the input voltage and
operates in boost mode. This can be justified from the voltage transform ratio of voltage fed
converter which is expressed as 1−d
1−2d for duty cycle = d.z-source ac -ac converter has some
drawbacks like the input voltage and output voltage is not sharing the same load which creates
problem in maintaining the phase angle between the output voltage and input voltage or the output
voltage reverses. In addition, the input current is operated in the discontinuous current mode.
When the converter operates in discontinuous current mode the waveform of the input current is
non-sinusoidal which increases the THD of the input current. Moreover, no switching technique
and controlling method of duty cycle is mentioned here in the paper. Careful adjustment of duty
cycle is necessary as around 0.5 duty cycle the system may be unstable due to infinite voltage
CHAPTER-2,LITERATURE SURVEY
33
transform ratio. A QZSI based ac-ac converter proposed in [75] with a different topology of z-
network, the input voltage and output voltage are sharing the same ground. The operation is in
continuous current mode. The proposed QZSI can control to shape the input current to be
sinusoidal and in phase with the input voltage. The paper [76] presents the application of Z
network in three phase ac-ac converter .The converter , the topology of which is shown in Figure
2.16 can be used for boost mode in duty cycle range between 0 to 0.5 and in buck mode between
0.5 to 1. Voltage gain is unity in case of d=0.5. In this proposed converter, there is a limit of
maximum boost factor which is about 1.15 at d = 0.33. The voltage transfer ratio is estimated
as1−𝑑
3𝑑2−3𝑑+1.
Figure 2.16 Three phase ac-ac Z-Source inverter
2.4. Conclusion
The different z-source converters review reveals that it is an emerging technology with the
potential of replacing conventional converter especially for alternate energy sources such as solar,
fuel cell as well as wind having wide voltage variation range. Photovoltaic cell voltage varies with
temperature and irradiation and fuel cell stack voltage drops greatly with load current. Also, wind
generator voltage varies with wind speed and control. The traditional voltage source inverter that
has been the power conversion technology for those energy sources cannot cope with the wide
voltage change and requires voltage boost by additional dc-dc converter, which increases cost,
system complexity, and power loss. The z-source converter/inverter systems have great potential
to solve this problem and can provide a great alternative with lower cost, higher reliability, and
higher efficiency.
Z-source conversion concept has become a hot research topic of the field and there are lot of
literatures published by many peers from around the world. Though this concept has been
extended to the entire power conversion spectrum, few works have been done on the area of wind
CHAPTER-2,LITERATURE SURVEY
34
power conversion. The application of z-source converter on wind power conversion needs wide
exploration. Different new configuration of z-source converters are not even taken up by any
researcher for the application of wind power conversion.
top related