ABSTRACT

The penetration of renewable energy in modern power system, microgrid has become

a popular application worldwide. In this project bidirectional converters for AC and DC hybrid

microgrid application are proposed as an efficient interface. To reach the goal of bidirectional

power conversion, both rectifier and inverter modes are analyzed in order to achieve high

performance operation and single-phase bi-directional inverter with dc-bus voltage regulation

and power compensation in dc microgrid applications. This concept proposes a control design

methodology for a multi-functional single-phase bidirectional PWM converter in renewable

energy systems. There is a generic current loop for different modes of operation to ease the

transition between different modes, including stand-alone inverter mode, grid-tied inverter

mode, ac voltage regulation is of importance because of the sensitive loads in dc microgrid

applications, a power distribution system requires a bi-directional inverter to control the power

flow between dc bus and ac grid, and to regulate the dc bus to a certain range of voltages, in

which dc load may change abruptly. This will result in high dc-bus voltage variations; the bi-

directional inverter can shift its current commands according to the specified power factor at

ac grid side. Parallel-connected bidirectional converters for AC and DC hybrid microgrid

application are proposed as an efficient interface. To reach the goal of bidirectional power

conversion, both rectifier and inverter modes are analyzed. The Simulation results are carried

out by Matlab/Simulink to verify the performance of the proposed method.

II

LIST OF FIGURES

Figure. No Title of the Figure Page No

Figure 1.1 Large scale renewable energy system

interacted with utility grid 2

Figure 1.2 Single phase AC/DC interactive renewable

energy system 3

Figure 1.3 Single phase full bridge converter topology

and it’s wavforms 4

Figure 1.4 Typical Microgrid 6

Figure 1.5 Single line diagram of Distributed system

with multiple DGs 8

Figure 1.6 Scenario of islanding operation 11

Figure 1.7 Islanding detection techniques 13

Figure 1.8 Distributed generation power line signaling

islanding detection 14

Figure 1.9 Distributed Generation Multi Power Line

Signaling Islanding Detection Issue 15

Figure 1.10 Distributed Generation Transfer Trip

Islanding Detection 16

Figure 2.1 Ideal switch representation 22

Figure 2.2 Ideal phase leg switches representation 23

Figure 2.3 Ideal model of stand-alone inverter mode 23

Figure 2.4 Ideal model of grid-tied inverter mode 24

Figure 2.5 Ideal model of grid-tied rectifier mode 24

Figure 2.6 Ideal model of grid-tied charger/discharger

mode 26

Figure 2.7 Unipolar and bipolar modulation scheme 27

Figure 2.8 Stand-alone inverter mode control structure 28

Figure 2.9 Grid-tied inverter mode control structure 28

Figure 2.10 Grid-tied rectifier mode control structure 29

Figure 2.11 Grid-tied charger/discharger mode control

structure 29

Figure 2.12 Single-phase converter control structure 30

Figure 2.13 Single-phase ac-dc renewable energy systems 31

Figure 2.14 Current loop block diagram 32

Figure 2.15 Voltage loop block diagram 33

Figure 2.16 Input and output of rectifier mode 34

Figure 3.1 Single-phase AC/DC interactive renewable

energy system 35

Figure 3.2 Control structures under different modes of

operation 39

Figure 3.3 Multi-functional converter control system 41

III

LIST OF TABLES

Table No. Title of the Table Page No.

Table 1.1 Switching combination for single-phase

full-bridge converter

4

IV

ABBREVATIONS

1. PWM - Pulse Width Modulation

2. DHPS - Distributed hybrid power systems

3. CPES - Centre for Power Electronics Systems

4. UPS - Uninterruptable power supplies

5. NDZ - Non detection zone

6. DG - Distributed Generation

7. ROCOF - Rate of change of frequency

8. THD - Total harmonic distortion

9. PCC - Point of common coupling

10. PF - Positive feedback

11. VU - Voltage imbalance

12. ARPS - Adaptive reactive power shift

13. SPST - Single-pole-single-throw

14. SPDT - Single-pole-double-throw

15. SAM - Stand-Alone Mode

16. GCM - Grid-Connected Mode

17. GDM - Grid-Disconnected Mode

18. API - Application Program Interface

V

CONTENTS

ACKNOWLEDGEMENT I

ABSTRACT II

LIST OF FIGURES III

LIST OF TABLES IV

ABBRIVATIONS V

CHAPTER NAME Page No

CHAPTER-1

INTRODUCTION 1

1.1 Thesis Background 1

1.2 Renewable Energy Systems Overview 1

1.2.1 Stand-alone inverter mode 3

1.2.2 Grid-tied inverter mode 3

1.2.3 Grid-tied rectifier mode 3

1.2.4 Grid-tied charger/discharger mode 4

1.3 Single Phase Converter 4

1.4 Current Work on Single-phase Full-bridge Converter 5

1.5 What is Microgrid 5

1.6 The Need Of Microgrid 6

1.7 Microgrid Advantages 6

1.8 Microgrid Disadvantages 7

1.9 Islanding Protection for Distributed Generation 7

1.10 Types of Distributed Generation 8

1.11 Distribution system with multiple DGs 8

1.12 Advantages of distributed generations 9

1.13 Technical Challenges Facing Distributed Generation 9

1.13.1 Different Technical challenges for distributed generations 10

1.14 Islanding 10

1.15 Issues with Islanding 11

1.16 Utility Perspective of Distributed Generator Network Islanding 12

1.17 Review of Islanding Detection Techniques 12

1.17.1 Remote islanding detection techniques 13

1.17.2 Local detection techniques 16

1.17.2.1 Passive detection techniques 16

1.17.2.2 Active detection techniques 16

1.17.3 Hybrid detection schemes 19

CHAPTER-2

2. MODELING AND DESIGN OF MULTI FUNCTION SINGLE-PHASE

CONVERTER 22

2.1 Converter Modelling 22

2.1.1 Modelling of Full-bridge Switches 22

2.1.2 Modelling of Stand-alone Inverter Mode 23

2.1.3 Modelling of Grid-tied Inverter Mode 23

2.1.4 Modelling of Grid-tied Rectifier Mode 24

2.1.5 Modelling of Grid-tied Charger/Discharger Mode 25

2.2 Modulation Scheme 25

2.2.1 Modulation Strategy 25

2.3 Control Structure 28

2.4 Generic Inner Current Loop Design 30

2.4.1 Stand-alone Inverter Mode 31

2.4.2 Grid-tied Inverter Mode 31

2.4.3 Grid-tied Rectifier/Charger/Discharger Mode 31

2.4.4 Current Loop Design 32

2.5 Outer Loop Controller Design 33

2.5.1 Stand-alone Inverter Mode 33

2.5.2 Controllability & Observability 33

2.5.2.1 Control Design 33

CHAPTER-3

PROPOSED CONCEPT 35

3.1 Introduction 35

3.2 System Modes Of Operation: Modeling And Control Structure 37

3.3 Generic Inner Ac Current Loop Design 39

3.4 Outer Loop Controller Design 41

3.4.1 Stand-Alone Inverter Mode 41

3.4.2 Rectifier Mode 42

CHAPTER-4

SIMULATION RESULT

4.1 Simulation Model Of Bi-Directional PWM Converter In Grid Connected

Inverter Mode 43

4.1.1 Measurement 43

4.1.2 Control Circuit 44

4.1.3 Simulation Output Of Bi-Directional PWM Converter In

Gcm-Inverter Mode 44

4.2 Simulation Model Of Bi-Directional PWM Converter In Grid Connected

Rectifier Mode 45

4.2.1 Measurement 45

4.2.2 Simulation Output Of Bi-Directional PWM Converter In

Gcm Rectifier Mode 46

4.3 Simulation Model Of Bi-Directional PWM Converter In Stand Inverter Mode 47

4.3.1 Measurement 47

4.3.2 Simulation Output Of Bi-Directional PWM Converter In

Stand Alone Inverter Mode 47

4.4 Simulation Model Of External Voltage Current Control 48

4.4.1 Measurement 48

4.4.2 Simulation Output Of External Voltage Current Control 49

CONCLUSION&FUTURE SCOPE 51

REFERENCES 52

1

CHAPTER 1

INTRODUCTION

1.1 THESIS BACKGROUND:

With pressing demand for and limited production of chemical energy resources, such

as petroleum and natural gas, energy problems have become more urgent, and have become a

focus of people around the world. According to the latest reports from British Petroleum, the

world reserves of petroleum now represent 41 years of production at the current rate [1]. So-

called “energy crisis” has been a hot topic recently.

The direct consumption of chemical energy resources has significant environmental

impacts. In particular, the emission pollution from vehicles, which leads to global warming,

pushes governments to make efforts towards the development of clean renewable energy

resources, such as wind, bio-gas, and solar energy to address energy and environmental

problems.

Renewable energy technologies such as solar power, wind power, hydroelectricity,

micro hydro power, biomass and biofuels could be used to replace conventional chemical

energy resources. In 2006, about 18% of global energy consumption came from renewable

resources. Wind power is growing at the rate of 30% annually, with a worldwide installed

capacity of over 100 GW [3], and is widely used in several European countries and the United

States.

1.2 RENEWABLE ENERGY SYSTEMS OVERVIEW:

Currently, industrial countries generate most of their electricity in large centralized

facilities, such as coal, nuclear, hydropower or natural gas-powered plants. These plants have

excellent economies of scale, but usually transmit electricity over long distances. Most plants

are built this way due to a number of economic, healthy, safety, logistical, environmental,

geographical and geological factors. For example, coal power plants are built away from cities

to prevent their heavy air pollution from affecting the populace; in addition, such plants are

often built near collieries to minimize the cost of transporting coal.

2

Distributed generation is another approach to the manufacture and transmission of

electric power. It reduces the amount of energy lost in transmitting electricity because the

electricity is generated very near where it is used, perhaps even in the same building. This

reduces the size and number of power lines that must be constructed [9]. If renewable energy

resources are used as distributed generation resources, the distributed hybrid power systems

can also be referred as renewable energy systems. Figure 1.1 illustrated a typical renewable

energy system with a conventional utility grid.

Figure 1.1: Large scale renewable energy system interacted with utility grid

Distributed hybrid power systems (DHPS) consist of ac and dc sub-systems connected

to various load types, where the DG resources can be either dc or ac sub-system-based [10]. A

self-sustainable energy system has been built in the lab of Centre for Power Electronics

Systems (CPES), and was interconnected with a solar converter, a utility grid, and load in both

3

the power and communication sense; in addition, a wind converter was wired to be part of the

system.

The critical component of this system is the bi-directional converter, which connects

the dc and ac sub-systems together, and connects the system with the utility grid. Figure 1.2

shows a probable single-phase DHPS with energy storage on the dc side and other renewable

energy resources through the system.

Figure 1.2: Single phase AC/DC interactive renewable energy system

As we can see in Figure 1.2, the bi-directional single-phase converter in a distributed

hybrid power system should fulfil the following modes of operation:

1. Stand-alone inverter mode: When the grid is lost, the converter regulates the ac bus voltage

and frequency to sustain the ac load, while the renewable energy resources or energy storage

on the dc side provide power. The ac-side renewable energy resources would act as current

sources.

2. Grid-tied inverter mode: When the grid is connected, the converter acts as a current source

to source or sink power to the grid to balance the power flow between the dc and ac subsystems,

while one of the dc resources regulates the dc bus voltage.

4

3. Grid-tied rectifier mode: When the grid is connected, the converter regulates the dc bus

voltage to sustain the dc load while all the dc-side renewable energy resources run as current

sources.

4. Grid-tied charger/discharger mode: When the grid is connected, the converter charges

energy storage elements, such as a battery pack. When the grid is lost, the battery discharges

supplying power to sustain both the dc and ac load.

1.3 SINGLE PHASE CONVERTER:

The full-bridge pulse-width-modulation (PWM) single-phase converter is widely used

in uninterruptable power supplies (UPS), wind and solar power dc-ac interfacing, stand-alone

voltage regulators in distributed power systems, and many other industry applications. Single-

phase converters are used where transformation between dc and ac voltage is required; more

precisely where converters transfer power back and forth between dc and ac. The single-phase

full-bridge converter in Figure 1.3 shows the basic circuit topology. Ac output voltage is

created by switching the full-bridge in an appropriate sequence. The output voltage of the

bridge, vac, can be either +Vdc, -Vdc or 0 depending on how the switches are controlled.

Figure 1.3: Single-phase full-bridge converter topology and its waveforms

5

Notice that both switches on one leg cannot be ON at the same time; otherwise there would be

a short circuit across the dc source, which would destroy the switches or the converter itself.

Table 1.1 summarizes all the possible switching combinations for the single-phase inverter and

their corresponding created full-bridge voltage, vac.

Table 1.1: Switching combination for single-phase full-bridge converter

1.4 CURRENT WORK ON SINGLE-PHASE FULL-BRIDGE CONVERTER:

Many grid-tied converter control strategies have been proposed in recent work, such as

hysteresis control and predictive control for the grid-tied inverter mode; fuzzy control, sliding

mode control, repetitive control for the stand-alone inverter mode; and predictive control for

the rectifier mode. Few papers have addressed the seamless transition between different modes

[14-16]. Many of the existing methods actually require a different control system for each

mode of operation, which increases the complexity and affects the reliability of the system, as

well as causing difficulty in the transition between modes. Moreover, the existing work has

not explored all of the possible modes of operation; the battery charger mode in particular has

been neglected.

Among these different modes of operation, the ac voltage control under stand-alone

mode presents the most difficulty since the traditional control design for a dc/dc converter

cannot be applied directly to a dc/ac inverter. The main goal of a full-bridge converter control

system is to achieve a fast dynamic ac voltage and frequency regulation during transients, while

maintaining zero steady-state error under different types of loads. Many control schemes for

this converter have been proposed during the past years.

6

1.5 WHAT IS MICROGRID:

It is a small-scale power supply network that is designed to provide power for a small

community.

It enables local power generation for local loads.

It comprises of various small power generating sources that makes it highly flexible

and efficient.

It is connected to both the local generating units and the utility grid thus preventing

power outages.

Excess power can be sold to the utility grid.

Size of the Microgrid may range from housing estate to municipal regions.

Fig 1.4 Typical Microgrid

1.6 THE NEED OF MICROGRID:

Microgrid could be the answer to our energy crisis.

Transmission losses gets highly reduced.

Microgrid results in substantial savings and cuts emissions without major changes to

lifestyles.

Provide high quality and reliable energy supply to critical loads

7

1.7 MICROGRID ADVANTAGES:

A major advantage of a Microgrid, is its ability, during a utility grid disturbance, to

separate and isolate itself from the utility seamlessly with little or no disruption to the loads

within the Microgrid.In peak load periods it prevents utility grid failure by reducing the load

on the grid.

Significant environmental benefits made possible by the use of low or zero emission

generators. The use of both electricity and heat permitted by the close proximity of the

generator to the user, thereby increasing the overall energy efficiency.Microgrid can act to

mitigate the electricity costs to its users by generating some or all of its electricity needs.

1.8 MICROGRID DISADVANTAGES:

Voltage, frequency and power quality are three main parameters that must be

considered and controlled to acceptable standards whilst the power and energy balance is

maintained.

Electrical energy needs to be stored in battery banks thus requiring more space and

maintenance.Resynchronization with the utility grid is difficult.Microgrid protection is one of

the most important challenges facing the implementation of Microgrids.Issues such as standby

charges and net metering may pose obstacles for Microgrid.

1.9 ISLANDING PROTECTION FOR DISTRIBUTED GENERATION:

This thesis presents a novel method of islanding detection for the protection of

distributed generator fed systems that has been tested on power distribution busses of 25 kV

and less. Recent interest in distributed generator installation into low voltage busses near

electrical consumers has created some new challenges for protection engineers that are

different from traditional radially based protection methodologies. Therefore, typical

protection configurations need to be re-thought such as re-closures out-of-step monitoring,

impedance relay protection zones with the detection of unplanned islanding of distributed

generator systems. The condition of islanding, defined as when a section of the non utility

generation system is isolated from the main utility system, is often considered undesirable

because of the potential damage to existing equipment, utility liability concerns, reduction of

power reliability and power quality.

8

Current islanding detection methods typically monitor over/under voltage and

over/under frequency conditions passively and actively; however, each method has an ideal

sensitivity operating condition and a non-sensitive operating condition with varying degrees

of power quality corruption called the non detection zone (NDZ). The islanding detection

method developed in this thesis takes the theoretically accurate concept of impedance

measurement and extends it into the symmetrical component impedance domain, using the

existence of naturally and artificially produced unbalanced conditions. Specific applications,

where this islanding detection method improves beyond existing islanding detection methods,

are explored where a generalized solution allows the protection engineer to determine when

this method can be used most effectively. To start, this thesis begins with a brief introduction

to power systems in North America and the motivation for the use of distributed generation.

Further chapters then detail the background and specifics of this technique.

1.10 TYPES OF DISTRIBUTED GENERATION:

Distributed Generators can be broken into three basic classes: induction, synchronous

and asynchronous. Induction generators require external excitation (VARs) and start up much

like a regular induction motor. They are less costly than synchronous machines and are

typically less than 500 KVA. Induction machines are most commonly used in wind power

applications. Alternatively, synchronous generators require a DC excitation field and need to

synchronize with the utility before connection. Synchronous machines are most commonly

used with internal combustion machines, gas turbines, and small hydro dams. Finally,

asynchronous generators are transistor switched systems such as inverters. Asynchronous

generators are most commonly used with micro turbines, photovoltaic, and fuel cells.

1.11 DISTRIBUTION SYSTEM WITH MULTIPLE DGS:

Distributed or dispersed generation may be defined as generating resources other than

central generating stations that is placed close to load being served, usually at customer site. It

serves as an alternative to or enhancement of the traditional electric power system. The

commonly used distributed resources are wind power, photo voltaic, hydro power.

9

Figure 1.5: single line diagram of Distributed system with multiple DGs

1.12 ADVANTAGES OF DISTRIBUTED GENERATIONS:

Flexibility - DG resources can be located at numerous locations within a utility's service

area. This aspect of DG equipment provides a utility tremendous flexibility to match

generation resources to system needs.

Improved Reliability - DG facilities can improve grid reliability by placing additional

generation capacity closer to the load, thereby minimizing impacts from transmission

and distribution (T&D) system disturbances, and reducing peak-period congestion on

the local grid. Furthermore, multiple units at a site can increase reliability by dispersing

the capacity across several units instead of a single large central plant.

Improved Security - The utility can be served by a local delivery point. This

significantly decreases the vulnerability to interrupted service from imported electricity

supplies due to natural disasters, supplier deficiencies or interruptio6ns, or acts of

terrorism.

Reduced Loading of T&D Equipment - By locating generating units on the low-voltage

bus of existing distribution substations, DG will reduce loadings on substation power

transformers during peak hours, thereby extending the useful life of this equipment and

deferring planned substation upgrades.

Reduces the necessity to build new transmission and distribution lines or upgrade

existing ones.

Reduce transmission and distribution line losses

Improve power quality and voltage profile of the system.

10

In fact, many utilities around the world already have a significant penetration of DG in

their system. But there are many issues to be taken into account with the DG and one of the

main issues is islanding.

1.13 TECHNICAL CHALLENGES FACING DISTRIBUTED GENERATION:

Distributed Generation (DG) is not without problems. DG faces a series of integration

challenges, but one of the more significant overall problems is that the electrical distribution

and transmission infrastructure has been designed in a configuration where few high power

generation stations that are often distant from their consumers, ”push” electrical power onto

the many smaller consumers.

1.13.1 Different Technical challenges for distributed generations:

Voltage Regulation and Losses

Voltage Flicker

DG Shaft Over-Torque During Faults

Harmonic Control and Harmonic Injection

Increased Short Circuit Levels

Grounding and Transformer Interface

Transient Stability

Sensitivity of Existing Protection Schemes

Coordination of Multiple Generators

High Penetration Impacts are Unclear

Islanding Control

1.14 ISLANDING:

Islanding is the situation in which a distribution system becomes electrically isolated

from the remainder of the power system, yet continues to be energized by DG connected to it.

As shown in the figure1.6. Traditionally, a distribution system doesn’t have any active power

generating source in it and it doesn’t get power in case of a fault in transmission line upstream

but with DG, this presumption is no longer valid. Current practice is that almost all utilities

require DG to be disconnected from the grid as soon as possible in case of islanding. IEEE

929-1988 standard requires the disconnection of DG once it is islanded .Islanding can be

11

intentional or Non intentional. During maintenance service on the utility grid, the shutdown of

the utility grid may cause islanding of generators. As the loss of the grid is voluntary the

islanding is known. Non-intentional islanding, caused by accidental shut down of the grid is

of more interest. As there are various issues with unintentional islanding. IEEE 1547-2003

standard stipulates a maximum delay of 2 seconds for detection of an unintentional island and

all DGs ceasing to energize the distribution system,

Figure 1.6: Scenario of islanding operation

1.15 ISSUES WITH ISLANDING:

Although there are some benefits of islanding operation there are some drawbacks as

well. Some of them are as follows:

Line worker safety can be threatened by DG sources feeding a system after

primary sources have been opened and tagged out.

The voltage and frequency may not be maintained within a standard permissible

level. Islanded system may be inadequately grounded by the DG

interconnection.

Instantaneous reclosing could result in out of phase reclosing of DG. As a result

of which large mechanical torques and currents are created that can damage the

generators or prime movers Also, transients are created, which are potentially

damaging to utility and other customer equipment. Out of phase reclosing, if

occurs at a voltage peak, will generate a very severe capacitive switching

12

transient and in a lightly damped system, the crest over-voltage can approach

three times rated voltage

Various risks resulting from this include the degradation of the electric

components as a consequence of voltage& frequency drifts. Due to these

reasons, it is very important to detect the islanding quickly and accurately.

1.16 UTILITY PERSPECTIVE OF DISTRIBUTED GENERATOR NETWORK

ISLANDING:

Utilities have a more pragmatic point of view of distributed generation islanding. Their

goal is to improve the distribution level (25 kV and below) customer service reliability

especially in regions where the reliability is below customer’s needs. It is believed that

customer reliability could improve with the addition of DG sources and that the DG may be

able to sell electricity back to the utility. However, without complex studies and frequent

expensive system upgrades DG islanding is not allowed. Some examples of these studies are:

real and reactive power profile and control, planning for islanding, minimum/ maximum feeder

loading, islanding load profile, minimum/maximum voltage profile, protection sensitivity and

DG inertia. One more specific example is how substation auto-reclosers of circuit breakers and

main line reclosers may be disabled and other protection devices may need to be removed to

allow proper coordination of utility sources and DG sources.

Maintenance times might also increase as utility workers will not only need to lockout

the utility lines but they will need to take additional time to lockout all the installed DG lines.

Some of the required installation studies of an Independent power producer must complete to

be able to island are: 1. inadvertent islanding and planned islanding study, 2. reliability study,

3. power quality study, 4. utility equipment upgrade assessment, 5. safety and protection

reviews, and 6. commercial benefit study. Clearly the costs of designing a DG to be capable of

islanding or to simply be installed into the main utility owned network requires extensive and

costly engineering and business reviews which may be outside the financial range of smaller

DG suppliers.

13

1.17 REVIEW OF ISLANDING DETECTION TECHNIQUES:

The main philosophy of detecting an islanding situation is to monitor the DG output

parameters and system parameters and/ and decide whether or not an islanding situation has

occurred from change in these parameters. Islanding detection techniques can be divided into

remote and local techniques and local techniques can further be divided into passive, active

and hybrid techniques as shown in Figure 1.7

Figure 1.7: Islanding detection techniques

1.17.1 Remote islanding detection techniques:

Remote islanding detection techniques are based on communication between utilities

and DGs. Although these techniques may have better reliability than local techniques, they are

expensive to implement and hence uneconomical .Some of the remote islanding detection

techniques are as follows:

a) Power line signaling scheme:

These methods use the power line as a carrier of signals to transmit islanded or non-

islanded information on the power lines. The apparatus includes a signal generator at the

substation (25+ kV) that is coupled into the network where it continually broadcasts a signal

as shown in figure (1.8). Due to the low-pass filter nature of a power system, the signals need

to be transmitted near or below the fundamental frequency and not interfere with other carrier

14

technologies such as automatic meter reading. Each DG is then equipped with a signal detector

to receive this transmitted signal. Under normal operating conditions, the signal is received by

the DG and the system remains connected. However, if an island state occurs, the transmitted

signal is cut off because of the substation breaker opening and the signal cannot be received

by the DG, hence indicating an island condition.

Figure 1.8: Distributed Generation power line Signaling Islanding Detection

This method has the advantages of its simplicity of control and its reliability. In a radial

system there is only one transmitting generator needed that can continuously relay a message

to many DGs in the network. The only times the message is not received is if the

interconnecting breaker has been opened, or if there is a line fault that corrupts the transmitted

signal.

There are also several significant disadvantages to this method, the fist being the

practical implementation. To connect the device to a substation, a high voltage to low voltage

coupling transformer is required. A transformer of this voltage capacity can have prohibitive

cost barriers associated with it that may be especially undesirable for the first DG system

installed in the local network. Another disadvantage is if the signaling method is applied in a

non radial system, resulting in the use of multiple signal generators. This scenario can be seen

in Figure 1.9 where the three feeder busses connect to one island bus. The implementation of

this system, opposed to a simple radial system, will be up to three times the cost.

15

Figure 1.9: Distributed Generation Multi Power Line Signaling Islanding Detection Issue

Another problem for power line communication is the complexity of the network and

the affected networks. A perfectly radial network with one connecting breaker is a simple

example of island signaling; however, more complex systems with multiple utility feeders may

find that differentiation between upstream breakers difficult.

b) Transfer trip scheme:

The basic idea of transfer trip scheme is to monitor the status of all the circuit breakers

and reclosers that could island a distribution system. Supervisory Control and Data Acquisition

(SCADA) systems can be used for that. When a disconnection is detected at the substation, the

transfer trip system determines which areas are islanded and sends the appropriate signal to the

DGs, to either remain in operation, or to discontinue operation. Transfer tip has the distinct

advantage similar to Power Line Carrier Signal that it is a very simple concept. With a radial

topology that has few DG sources and a limited number of breakers, the system state can be

sent to the DG directly from each monitoring point. This is one of the most common schemes

used for islanding detection This can be seen in figure 1.10.

16

Figure 1.10: Distributed Generation Transfer Trip Islanding Detection

The weaknesses of the transfer trip system are better related to larger system

complexity cost and control. As a system grows in complexity, the transfer trip scheme may

also become obsolete, and need relocation or updating. Reconfiguration of this device in the

planning stages of DG network is necessary in order to consider if the network is expected to

grow or if many DG installations are planned. The other weakness of this system is control.

As the substation gains control of the DG, the DG may lose control over power producing

capability and special agreements may be necessary with the utility. If the transfer trip method

is implemented correctly in a simple network, there are no non-detection zones of operation.

1.17.2 Local detection techniques

It is based on the measurement of system parameters at the DG site, like voltage,

frequency, etc. It is further classified as:

1.17.2.1 Passive detection techniques

Passive methods work on measuring system parameters such as variations in voltage,

frequency, harmonic distortion, etc. These parameters vary greatly when the system is

islanded. Differentiation between an islanding and grid connected condition is based upon the

thresholds set for these parameters. Special care should be taken while setting the threshold

value so as to differentiate islanding from other disturbances in the system. Passive techniques

are fast and they don’t introduce disturbance in the system but they have a large non detectable

zone (NDZ) where they fail to detect the islanding condition.

17

There are various passive islanding detection techniques and some of them are as follows:

(a) Rate of change of output power: The rate of change of output power, 𝑑𝑝

𝑑𝑡 , at the DG side,

once it is islanded, will be much greater than that of the rate of change of output power before

the DG is islanded for the same rate of load change. It has been found that this method is much

more effective when the distribution system with DG has unbalanced load rather than balanced

load.

(b) Rate of change of frequency: The rate of change of frequency, 𝑑𝑓

𝑑𝑡, will be very high when

the DG is islanded. The rate of change of frequency (ROCOF) can be given by

Where,

∆P is power mismatch at the DG side

H is the moment of inertia for DG/system

G is the rated generation capacity of the DG/system

Large systems have large H and G where as small systems have small H and G giving

larger value for 𝑑𝑓

𝑑𝑡 ROCOF relay monitors the voltage waveform and will operate if ROCOF

is higher than setting for certain duration of time. The setting has to be chosen in such a way

that the relay will trigger for island condition but not for load changes. This method is highly

reliable when there is large mismatch in power but it fails to operate if DG’s capacity matches

with its local loads. However, an advantage of this method along with he rate of change of

power algorithm is that, even they fail to operate when load matches DG’s generation, any

subsequent local load change would generally lead to islanding being detected as a result of

load and generation mismatch in the islanded system.

(c) Rate of change of frequency over power:

𝑑𝑓

𝑑𝑡 in a small generation system is larger than that of the power system with larger

capacity. Rate of change of frequency over power utilize this concept to determine islanding

condition. Furthermore, test results have shown that for a small power mismatch between the

DG and local loads, rate of change of frequency over power is much more sensitive than rate

of frequency over time.

18

(d) Voltage unbalance:

Once the islanding occurs, DG has to take change of the loads in the island. If the

change in loading is large, then islanding conditions are easily detected by monitoring several

parameters: voltage magnitude, phase displacement, and frequency change. However, these

methods may not be effective if the changes are small. As the distribution networks generally

include single-phase loads, it is highly possible that the islanding will change the load balance

of DG. Furthermore, even though the change in DG loads is small, voltage unbalance will

occur due to the change in network condition.

(e) Harmonic distortion:

Change in the amount and configuration of load might result in different harmonic

currents in the network, especially when the system has inverter based DGs. One approach to

detect islanding is to monitor the change of total harmonic distortion (THD) of the terminal

voltage at the DG before and after the island is formed..The change in the third harmonic of

the DG’s voltage also gives a good picture of when the DG is islanded.

1.17.2.2 Active detection techniques:

With active methods, islanding can be detected even under the perfect match of

generation and load, which is not possible in case of the passive detection schemes. Active

methods directly interact with the power system operation by introducing perturbations. The

idea of an active detection method is that this small perturbation will result in a significant

change in system parameters when the DG is islanded, whereas the change will be negligible

when the DG is connected to the grid.

(a) Reactive power export error detection:

In this scheme, DG generates a level of reactive power flow at the point of common

coupling (PCC) between the DG site and grid or at the point where the Reed relay is connected.

This power flow can only be maintained when the grid is connected. Islanding can be detected

if the level of reactive power flow is not maintained at the set value. For the synchronous

generator based DG, islanding can be detected by increasing the internal induced voltage of

DG by a small amount from time to time and monitoring the change in voltage and reactive

power at the terminal where DG is connected to the distribution system. A large change in the

terminal voltage, with the reactive power remaining almost unchanged, indicates islanding.

19

The major drawbacks of this method are it is slow and it cannot be used in the system where

DG has to generate power at unity power factor.

This detection scheme can be used in a system with more than one inverter based DG. The

drawback of this method is that the islanding can go undetected if the slope of the phase of the

load is higher than that of the SMS line, as there can be stable operating points within the

unstable zone.

1.17.3 Hybrid detection schemes:

Hybrid methods employ both the active and passive detection techniques. The active

technique is implemented only when the islanding is suspected by the passive technique. Some

of the hybrid techniques are discussed as follows:

(a) Technique based on positive feedback (PF) and voltage imbalance (VU):

This islanding detection technique uses the PF (active technique) and VU (passive

technique). The main idea is to monitor the three-phase voltages continuously to determinate

VU which is given as

V +Sq and V-Sq are the positive and negative sequence voltages, respectively. Voltage spikes

will be observed for load change, islanding, switching action, etc. Whenever a VU spike is

above the set value, frequency set point of the DG is changed. The system frequency will

change if the system is islanded.

(b) Technique based on voltage and reactive power shift:

In this technique voltage variation over a time is measured to get a covariance value

(passive) which is used to initiate an active islanding detection technique, adaptive reactive

power shift (ARPS) algorithm.

20

The ARPS uses the same mechanism as ALPS, except it uses the d-axis current shift

instead of current phase shift. The d-axis current shift, idk or reactive power shift is given as

kd is chosen such that the d-axis current variation is less than 1 percent of q-axis current in

inverter's normal operation. The additional d-axis current, after the suspicion of island, would

accelerates the phase shift action, which leads to a fast frequency shift when the DG is islanded.

There is no single islanding detection technique which will work satisfactorily for all systems

under all situations. The choice of the islanding detection technique will largely depend on the

type of the DG and system characteristics. Recently, hybrid detection techniques have been

proposed and it seems that the hybrid detection technique is the way to go with passive

technique detecting the islanding when change in system parameter is large and initiating the

active technique when the change in system parameter is not so large for the passive technique

to have an absolute discrimination.

1.17.4 Advantages Islanding Detection Techniques

1 Remote Techniques - Highly reliable

2 Local Techniques :

a. Passive Techniques - 1) Short detection time

2) Do not perturb the system

3) Accurate when there is a large mismatch in generation

and demand in the islanded system

21

b. Active techniques - Can detect islanding even in a perfect match between generation

and demand in the islanded system (Small NDZ).

c .Hybrid Techniques - 1) Have small NDZ.

2) Perturbation is introduced only when islanding is

suspected

1.17.5 Disadvantages Islanding Detection Techniques:

1 Remote Techniques - Expensive to implement especially for small systems.

2 Local Techniques

a. Passive Techniques - 1) Difficult to detect islanding when the load and generation

In the islanded system closely match.

2) Special care has to be taken while setting the thresholds

3) If the setting is too aggressive then it could result in

nuisance tripping

b. Active Techniques - 1) Introduce perturbation in the system

2) Detection time is slow as a result of extra time needed to

See the system response for perturbation

3) Perturbation often degrades the power quantity and if

significant enough, it may degrade the system stability

even when connected to the grid.

c .Hybrid Techniques - Islanding detection time is prolonged as both passive and

active technique is implemented.

22

CHAPTER 2

MODELING AND DESIGN OF MULTI FUNCTION SINGLE-PHASE

CONVERTER

2.1 CONVERTER MODELING:

2.1.1 Modeling of Full-bridge Switches:

The standard single-phase full-bridge converter topology consists of two phase legs.

Each phase leg has two solid-state devices in series. Based on the functionality of the

semiconductor switch, the solid-state switch can be represented as a single-pole-single-throw

(SPST) ideal switch [43] when losses, parasitics and the interior structure are ignored, which

is shown in Figure 2.1

Figure 2.1: Ideal switch representation

The switching function of the SPST switch is provided below.

Thereby, one phase leg switches can be represented as a single-pole-double-throw (SPDT)

ideal switch with the same modeling method as shown in Figure 2.2.

23

Figure 2.2: Ideal phase leg switches representation

2.1.2 Modeling of Stand-alone Inverter Mode:

Applying the ideal SPDT instead of one phase leg switch gives the simplified inverter

topology illustrated in Figure 2.3.

Figure 2.3:Ideal model of stand-alone inverter mode

2.1.3 Modeling of Grid-tied Inverter Mode:

The grid-tied inverter topology illustrated in Figure 2.4 is actually the same as the stand-alone

inverter mode. The difference between them is that the ac capacitor is removed because its

dynamics are ignored by the connection with the strong grid source.

24

Figure 2.4: Ideal model of grid-tied inverter mode

The switching model of the grid-tied inverter mode is as follows

2.1.4 Modeling of Grid-tied Rectifier Mode:

The topology of this mode is illustrated in Figure 2.5. It is clear to see that the capacitor is

moved from the ac side to the dc side to stabilize the dc voltage and improve the transient

response.

Figure 2.5: Ideal model of grid-tied rectifier mode

25

2.1.5 Modeling of Grid-tied Charger/Discharger Mode:

Figure 2.6 shows the circuit of the charger/discharger mode. The difference between

this mode and the rectifier mode is that with the charger/discharger mode, a charging inductor

is hooked up on the dc side in series with an energy storage element, such as a battery.

Figure 2.6: Ideal model of grid-tied charger/discharger mode

2.2 MODULATION SCHEME:

2.2.1 Modulation Strategy:

Pulse-width modulation (PWM) is used to create proper gating signals for four

switches, the two main advantages of PWM are the control of the output voltage amplitude

and fundamental frequency, and the filter requirements are decreased for minimizing the

harmonics. The switches must be controlled in a certain sequence to create a sinusoidal output

voltage in a single-phase inverter. So, a reference sinusoidal waveform is required. The

reference waveform is also called the modulation or control signal and it is compared to a

carrier signal. The carrier signal is usually a triangular signal that controls the switching

frequency while the reference signal controls the output voltage amplitude and its fundamental

frequency.

This section describes two common methods of modulation, bipolar and unipolar

modulation. Fig 2.7a illustrates bipolar pulse-width modulation.

26

Unipolar modulation has the particular attraction that it can be implemented with very

simple circuitry since one phase leg maintains the inverse switch state of the other. However,

unipolar modulation generates substantial carrier frequency and sideband harmonics, unlike

the double carrier frequency and sideband harmonics generated by bipolar modulation. This is

because unipolar modulation doesn’t cancel the harmonics between the two phase legs and the

reduced roll-off in magnitude of the baseband harmonics that occurs with this modulation

strategy. Therefore, the bipolar modulation strategy is the natural selection.

(a) Unipolar modulation scheme

27

(b) Bipolar modulation scheme

Figure 2.7: Unipolar and bipolar modulation scheme

28

2.3 CONTROL STRUCTURE:

The control structures for each mode are shown in Figures 2.8 to 2.11.

Figure 2.8: Stand-alone inverter mode control structure

Figure 2.9: Grid-tied inverter mode control structure

29

Figure 2.10: Grid-tied rectifier mode control structure

Figure 2.11: Grid-tied charger/discharger mode control structure

30

We can see that all modes have an inner-line inductor current loop. This paper proposes

that all modes share the same inner current loop. In order to combine the inner current loops,

the current loop dynamic response should be checked particularly at the crossover frequency.

The proposed control structure is selected as double loop feedback controller as shown in

Figure 2.12. The inner loop is selected as the ac current of the line inductor to achieve fast

dynamic response for input disturbances, and the outer loop is designed with different

compensators to regulate the desired control variables, such as ac voltage, dc voltage and dc

charging current.

Figure 2.12: Single-phase converter control structure

2.4 GENERIC INNER CURRENT LOOP DESIGN:

Figure 2.13 presents a simplified topology with all probable modes of operation of the

passive components. Our main objective is to see the possibility of building a generic inner

current loop for all modes of operation.

31

Figure 2.13: Single-phase ac-dc renewable energy systems

2.4.1 Stand-alone Inverter Mode:

From the small-signal model, we know that the control-to-current small-signal

transfer function at stand-alone inverter mode is:

The plant transfer function is a two-order system which has double-pole, left half-plan zero.

2.4.2 Grid-tied Inverter Mode:

The control-to-current small-signal transfer function at grid-tied inverter mode is

2.4.3 Grid-tied Rectifier/Charger/Discharger Mode:

For the grid-tied rectifier mode, since there is no fixed operating point and the dc-link

voltage changes, the dynamics of both sides of the bridge should be considered. This results in

32

a lack of a complete small-single model covering from dc to half of the switching frequency

[50]. However, we can model the current loop for only the high-frequency range and the

voltage loop for only the low-frequency range. Therefore, the quasi-static modeling [50] is

used to approximate the current loop.

2.4.4 Current Loop Design:

After the sensor, we implement a low-pass filter to filter out the noise picked up by

the environment. The low-pass filter for the current sensor is a two-order low-pass filter.

Figure 2.14: Current loop block diagram

33

2.5 OUTER LOOP CONTROLLER DESIGN:

2.5.1 Stand-alone Inverter Mode:

After designing the current compensator, the current-to-voltage transfer function is

The voltage sensor filter is the same as current sensor filter, which is

Figure 2.15: Voltage loop block diagram

2.5.2 Grid-tied Rectifier Mode:

2.5.2.1 Control Design:

The voltage loop is designed under the presumption that the current loop has already

been designed and it can track the current reference very well. This means, the inductor

dynamics can be ignored. Since the voltage loop is very slow, we can use power balance [52]

to model the voltage loop.

34

Figure 2.16: Input and output of rectifier mode

35

CHAPTER 3

PROPOSED CONCEPT

3.1 INTRODUCTION:

In Recent years, due to the growing concern with energy shortage and network stability,

the concepts of distributed generation (DG), microgrid systems [1], [2], dc nanogrid systems

[3], [4], and ac/dc hybrid power systems [5]–[7] have all become progressively more popular;

especially with the decreasing costs of various clean renewable energy sources (RES), such as:

wind, solar, and fuel-cells to name a few and more adoption of dc powered residential loads,

such as solidstate lighting. These DG systems would be connected to the utility grid under

normal operating conditions, but also have the additional capability to sustain a local system

(micro- or nanogrid) by sourcing power directly from the renewable energy sources and energy

storage devices if necessary to make grid transmission level black- and brownouts seem

transparent to the local system loads.

Fig. 3.1. Single-phase AC/DC interactive renewable energy system

Distributed hybrid power systems (DHPS) consist of ac and dc subsystems connected

to various load types, where DG resources can be connected on the ac or dc systems [6]. The

critical component for such a system is the ac/dc bidirectional, pulsewidth-modulation (PWM)

36

converter that connects the ac and dc subsystems together and to the utility grid. The diagram

in Fig.3.1 illustrates an example of a single-phase , DHPS with renewable energy sources

throughout the system. Using this type of system configuration, the ac/dc converter of a DHPS

should operate in the following modes and sub modes:

1) Stand-Alone Mode (SAM): When the grid is lost, the converter regulates the ac bus voltage

and frequency feeding the ac loads while drawing energy from dc-side, supported by the

renewable energy sources or energy storage on the dc-side. The RES on the ac side act as

current sources in this case.

2) Grid-Connected Mode (GCM): When the grid is present, the converter acts as an ac current

regulator, injecting or sinking power from the grid to achieve:

a) Inverter sub mode: Regulate the power flow (active and reactive) between the dc and

ac subsystems, while other dc sources regulate the dc bus voltage ,

b) Rectifier sub mode: Regulate the dc bus voltage and performs energy balancing to

sustain dc bus integrity while other dc side energy sources operate as current sources.

3) Grid-Disconnected Mode (GDM): When the grid is lost, the converter still operates as GCM

inverter/power flow control supplying ac loads. Normally, GDM is the transient state in mode

transition between GCM and SAM; however, given the nondetection zones (NDZ) [8], GDM

could exist for a while.

Islanding refers to the condition of the single-phase converter operating at SAM or

GDM continuing to power a local ac load even though power from the electric utility is no

longer present. Many control strategies have been proposed recently, such as hysteretic and

predictive control for GCM (acting as an inverter) [10], [11], fuzzy-logic/neural-network,

sliding mode, and repetitive control for SAM [11]–[13], and predictive control for GCM

(acting as a rectifier) [14]. Some work has covered the capability and control behind the

seamless transitions between different modes [11]–[13].

Many of the existing methods require a different control system for each mode of

operation, which increases the complexity and decreases the reliability of the system, as well

as increasing the difficulty in transitioning between modes. Moreover, existing papers did not

37

cover all of the probable modes of operation, neither the overall functionality nor the modeling

and control design of all the aforementioned modes. As it will be shown, some of them are

very different requiring different control approaches.

In terms of system level control reliability, compared to the above, multi loop current-

voltage PID controls based on frequency-response analysis provides several advantages in

terms of design and implementation simplicity, achieving good regulation, excellent

performance, but most importantly, a well-defined region of predictable stability for the

converter operation.

All of the basic modes of operation for GCM and SAM are defined and modeled. A

generic inner current loop based multi loop control structure is proposed to integrate and

simplify the various modes of operation. A novel single-phase PLL and associated islanding

detection algorithm is proposed for system level operation. The proposed control system is

very simple, as well as reliable, to implement and seamless transfers between the modes of

operation is easy to achieve.

3.2 SYSTEM MODES OF OPERATION:MODELING AND CONTROL

STRUCTURE:

Before the control structure is designed and implemented, the converter system needs

to be modeled. Specifically, the full bridge, multifunctional PWM converter with different

possible ac or dc configurations is shown in Fig. 3.2, where idg is the current flowing from the

dc DG resources, and its average model of full-bridge is described in (3.1) and (3.2).

(3.1)

(3.2)

As seen in (3.1) and (3.2),VAB ,idc and dab are the average terminal voltage of the full

bridge, average dc-link current, and average duty-cycle varying between -1 and 1, respectively.

Notice that if vdc is constant, the terminal voltage VAB is only a function of the duty-cycle dab .

38

The differential equations describing the average model of the full-bridge converter may then

be derived as follows:

(3.3)

(3.4)

(3.5)

The average and small-signal models for the different modes can be derived by

combining (1)–(5). Notice that in GCM, the dynamics of the ac capacitor can be ignored due

to the stiff grid, just as the dynamics of the dc-link capacitor can be ignored because of the

constant dc-link voltage during SAM. With the control architecture selected to be a double-

loop feedback system, as shown in Fig. 3.3, the inner loop is used to regulate the ac line

inductor current. In order to achieve fast dynamic responses from a wide array of disturbances,

the inner loop will need to be designed with high bandwidth, while the outer loops regulate

different control variables depending on the operating mode.

Inclusion of all control features, such as digital delays and sensor filters should also be

included. Each sensor filter is assumed to be a second-order, low-pass-filter, Hfilter(6). A one

switching-cycle (Tsample) delay, Hdelay(7), is modeled in the modulator to approximate the

digital computation and A/D conversion delay. The modulator gain is assumed to be unity.

(3.6)

39

Fig.3.2 Control structures under different modes of operation

(3.7)

3.3 GENERIC INNER AC CURRENT LOOP DESIGN:

It is worthwhile to demonstrate that the same current loop compensator can be designed

and shared by all modes. Specifically, for GCM inverter mode the small-signal transfer

function from control-to-current can be simply obtained from the average model given by

(3.1)–(3.3) and applying a small-signal perturbation, of which the result is as follows:

(3.8)

Notice that (3.8) is obtained by assuming the small-signal grid voltage is zero due to

the stiff grid condition. For the SAM, the transfer function from control-to-current is obtained

from the average model described by (3.1)–(3.4):

40

(3.9)

Although SAM does not have a constant operating point for the “output”, the small-

signal model is linear time-invariant when dc-link voltage is constant. For the GCM rectifier

mode however, since there is no fixed operating point and no constant dc-link voltage, the

dynamics of both sides of the bridge have to be considered. The result is that there is not a

complete accurate small-signal model that can describe the entire dynamics from dc up through

the Nyquist frequency [15]. However, if the current loop is modeled in a higher frequency

range and the voltage loop in a lower frequency range separately, following the quasi-static

modeling approach proposed in [15], the high-frequency small-signal current loop can be

modeled since the all the operating points (60 Hz) and varying dc-link voltage (120 Hz) can

be assumed to be in steady state because of the high current loop control bandwidth. Hence,

(3.3) and (3.5) can be perturbed as usual for all operating points, and the control-to-current

transfer function is then derived as below.

The detailed derivation can be found in [17].

(3.10)

With the transfer functions described in (8)–(10), the current compensator can be

designed; but the varying nature of the resultant transfer functions must be considered given

that it changes under varying line and load conditions, actual operating mode, and even the

converter operating point itself. A comprehensive linear design can still be carried out

however, if the control-to-current transfer function is closely examined under these different

conditions.

The current loop compensator is designed under SAM light load conditions based on

(3.9). The current PID compensator has the following structure in (3.11) to compensate the

current loop gain in (3.12).

41

(3.11)

(3.12)

The zeros in (3.11) are placed around the resonant frequency to compensate the drop

in phase, and two poles are used to attenuate the high frequency resonance from grid

impedance. The gain, K , is used to achieve the desired loop-gain and bandwidth for the inner

current loop.

The designed current loop bandwidth is around 2 kHz. As seen, by using a generic

current controller one control system can be used for different modes of operation, as shown

in Fig. 3.4. Per (8)–(10), a feed-forward loop is also applied to measure the dc-link voltage

(Fig. 3.4) and decouple the dc gain from vdc.

Fig. 3.3 Multi-functional converter control system

3.4 OUTER LOOP CONTROLLER DESIGN:

3.4.1 Stand-Alone Inverter Mode:

The outer loop in this case is an ac voltage loop; as such, the ac-current to ac-voltage

transfer function is given by (3.13) assuming a resistive load.

42

(3.13)

The open-loop gain to design the outer voltage compensator is shown in(3.14),where

Gi-c and Gvi are the closed-loop current transfer-function, and ac-current to ac-voltage transfer

function, respectively.

(3.14)

3.4.2 Rectifier Mode:

Dc-link voltage loop controller is designed under the assumption of the previously

designed high-bandwidth current loop. Since the dc-link voltage loop bandwidth must be much

lower than the frequency of the dc-link voltage ripple (120 Hz). Thus, taking the rms value of

vac as the steady-state operating point, the power balance between the ac and dc side of the

converter is used to model the voltage loop, which can be found in [16], [17] for detailed

derivation. If a resistive load is connected, the outer loop transfer function becomes (3.15),

where is the scaling factor of the PLL.

(3.15)

The outer loop compensators of stand-alone mode and rectifier mode are shown in (3.16) and

(3.17), respectively.

(3.16)

(3.17)

43

CHAPTER 4

SIMULATION RESUTS

4.1 SIMULATION MODEL OF BI-DIRECTIONAL PWM CONVERTER IN GRID

CONNECTED INVERTER MODE

4.1.1 MEASUREMENT

44

4.1.2 CONTROL CIRCUIT

4.1.3 SIMULATION OUTPUT OF BI-DIRECTIONAL PWM CONVERTER IN

GCM-INVERTER MODE

45

4.2 SIMULATION MODEL OF BI-DIRECTIONAL PWM CONVERTER IN GRID

CONNECTED RECTIFIER MODE

4.2.1 MEASUREMENT

46

4.2.2 SIMULATION OUTPUT OF BI-DIRECTIONAL PWM CONVERTER IN GCM

RECTIFIER MODE

47

4.3 SIMULATION MODEL OF BI-DIRECTIONAL PWM CONVERTER IN STAND

INVERTER MODE

4.3.1 MEASUREMENT

4.3.2 SIMULATION OUTPUT OF BI-DIRECTIONAL PWM CONVERTER IN

STAND ALONE INVERTER MODE

48

4.4 SIMULATION MODEL OF EXTERNAL VOLTAGE CURRENT CONTROL

4.4.1 MEASUREMENT

49

4.4.2 SIMULATION OUTPUT OF EXTERNAL VOLTAGE CURRENT CONTROL

50

51

CONCLUSION&FUTURE SCOPE

This project proposed a complete modeling and control system for a bidirectional, single-

phase, multifunctional PWM converter for residential power level distributed renewable

energy and grid connected microgrid system applications. A simple control structure was used

to cover all of the required operating modes, including stand-alone (SAM), grid-connected

(GCM) inverter, and rectifier mode. A new single-phase PLL and active islanding detection

algorithm was also proposed for system-level operation in order to meet IEEE standard 1547.

The resulting control structure is very simple and presents robust, low transient responses even

for extreme load steps between no-load and heavy-load conditions. The transition between

modes was also seamlessly achieved, as predicted, due to the common inner current-loop that

all operating modes have.

In future the proposed concept can be implemented with artificial neural

network (ANN) for voltage and current control, and the proposed concept can be implemented

in real time with different loading conditions.

52

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