2. literature survey

CHAPTER-2,LITERATURE SURVEY

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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


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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.


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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


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(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


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


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.


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


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


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


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


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


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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


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


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.


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


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


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-


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


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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.


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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.


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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


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


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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


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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.


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


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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


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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.

2. literature survey

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