a new pv/fuel cell based bidirectional converter for microgrid applications
TRANSCRIPT
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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