CONTROL STRATEGY OF FLYBACK
MICROINVERTER WITH HYBRID MODE FOR PV AC
MODULES USING FUZZY LOGIC
ACKNOWLEDGMENTS
I dedicate this work to my project guide Dr. Xingguo Xiong who supported me to work
in the field of my interest and has guided me all along. I would like to thank University of
Bridgeport for providing a platform to pursue my Masters’ program and helping me through
every step of it and for providing all the infrastructure I have ever needed.
I wish to express my gratitude to my parents without whose support my idea of pursuing
a Masters’ degree would not have been possible. I would also like to thank my friends and family
who have been a strong support system.
1
ABSTRACT
A New fuzzy control strategy of a flyback microinverter with hybrid operation mode
for photovoltaic (PV) AC modules is proposed in this paper. In this paper, we are using the fuzzy
controller. Because, it has many advantages comparing to another controller. The fuzzy
controller is the most suitable for the human decision-making mechanism, providing the
operation of an electronic system with decisions of experts. The proposed control strategy
consists of two components: the proportional-resonant (PR) controller with the harmonic
compensator (HC), and the hybrid nominal duty ratio. The PR controller with HC provides
higher system gain at the fundamental and harmonic frequencies of the grid without using high
proportional gain in both operation modes. In FLC, a set of linguistic rules determines basic
control action. The system determines these rules Then, it enhances tracking speed and
disturbance rejection performances satisfying the desired stability. Moreover, by applying the
hybrid nominal duty ratio yielded from the proposed operation mode selection, the disturbance
rejection is achieved more effectively, and the control burden is reduced. By using the fuzzy
controller for a nonlinear system allows for a reduction of uncertain effects in the system control
and improve the efficiency. By using the simulation results we can verify the tracking speed and
disturbance rejection performances of the proposed control strategy.
2
LIST OF TABLES
TABLE
5.a.
Page
FUZZY RULES
9.b PARAMETERS AND COMPONENTS OF THE PROTOTYPE
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3
4
Error!
LIST OF FIGURES
FIGURE
Page
2.1.1. Buck Converter .............................................................................................. Error!
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2.1.2 Voltage and current changes ......................................................................... Error!
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2.2.1 Buck Converter at Boundary .........................................................................
3
2.3.1
6
Buck Converter - Discontinuous Conduction ...............................................
2.3.2 Output Voltage vs Current............................................................................. Error!
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2.4.1 Fig Boost Converter Circuit ........................................................................... Error!
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2.4.2
Voltage and current waveforms (Boost Converter) ......................................
3
2.5.1. schematic for buck-boost converter............................................................... Error!
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2.5.2 Waveforms for buck-boost converter ............................................................ Error!
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2.6.1
Comparison of Voltage ratio .........................................................................
3
2.7.1. CUK Converter .............................................................................................. Error!
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2.7.2
CUK “ON-STATE” ......................................................................................
6
2.7.3 CUK “OFF-STATE” ..................................................................................... Error!
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2.9.1
Buck-Boost Converter ..................................................................................
3
2.9.2. Replacing inductor by transformer ................................................................ Error!
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4
2.9.3 Flyback converter re-configured .................................................................... Error!
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2.10.1 Forward converter .........................................................................................
3
2.10.2 Forward converter with tertiary winding ....................................................... Error!
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3.0
Solar farm......................................................................................................
6
3.1.1 Diagram of the possible components of a photovoltaic system .................... Error!
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3.2.1
Schematics of a typical residential PV system .............................................
3
4.0
Schematic of a flyback converter .................................................................. Error!
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4.2.1. Two Configurations of a flyback converter in operation on state and off stateError!
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4.2.2
Waveform - using primary side sensing techniques - showing the 'knee point' 6
4.5.1 PID controller ................................................................................................ Error!
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4.6.1
Plot of PV vs time, for three values of K (K and K held constant) .............
p
i
d
3
4.7.1 Plot of PV vs time, for three values of K (K and K held constant) ............ Error!
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d
5.1.1
p
i
Fuzzy logic controller ...................................................................................
3
5.1.a Input error as membership functions ............................................................ Error!
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5.1.b
Change as error membership functions.........................................................
3
5.1.c
Output variable Membership functions .........................................................
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5.3.1
fuzzy description ..........................................................................................
3
5.6.1 The FIS Editor .............................................................................................. Error!
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5.6.2 ‘Save to workspace as...’window ................................................................. Error!
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5
5.6.3
The updated FIS Editor ................................................................................ 6
6.0
Circuit diagram of the flyback micro-inverter ............................................. Error!
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6.1.1
Key waveforms in DCM and CCM ..............................................................
3
6.2.1 Operation regions of the flyback inverter during a half-cycle of the grid voltageError!
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7.1.1. Equivalent circuit of the grid-connected flyback microinverter……………..Error!
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7.1.2 Bode plots of PI controller (a) In DCM. (b) In CCM…………………………
8.1
6
Open-loop Bode plots of the compensated system by the proposed controller.
(a) In DCM.
(b) In CCM.
8.1.2 The block diagram of the proposed system ................................................. Error!
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9.1.1
Block diagram of simulation.........................................................................
3
9.1.2 Simulation results for conventional control system ...................................... Error!
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9.2.1
Block diagram of proposed system ...............................................................
3
9.2.2
Simulation results for the proposed system ..................................................
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TABLE OF CONTENTS
CHAPTER-1: INTRODUCTION
CHAPTER-2: DC-DC CONVERTER
2.1 Buck Converter Step-Down Converter
2.2 Transition Between Continuous and Discontinuous
2.3 Voltage Ratio of Buck Converter (Discontinuous Mode)
2.4 Boost Converter Step-Up Converter
2.5 Buck-Boost Converter
2.6 Converter Comparison
2.7 CUK Converter
2.8 Isolated Dc-Dc Converters
2.9 Flyback Converter
2.10
7
Forward Converter
CHAPTER-3: PHOTOVOLTAIC (PV)
3.1 Overview Of PV
3.2 Grid Connection
CHAPTER-4: FLYBACK CONVERTER
4.1 Structure and Principle
4.2 Operations
4.3 Controllers
4.4 Concept of PI Controller
4.5 Tuning PI Controller
4.6 Proportional Term
4.7 Integral Term
CHAPTER-5: FUZZY LOGIC CONTROLLER
5.1 Fuzzification
5.2 Interference Method
5.3 What Is Fuzzy Logic
5.4 Why Use Fuzzy Logic?
5.5 When not to Use Fuzzy Logic?
5.6. The FIS Editor
CHAPTER-6: OPERATION PRINCIPLE
6.1 Steady-State Analysis of DCM And CCM Operations
6.2 Flyback Microinverter With Hybrid Mode
CHAPTER-7: OPERATIONAL ANALYSIS (HYBRID MODE)
7.1 Control Issues
CHAPTER-8. PROPOSED CONTROL STRATEGY
8
CHAPTER-9. SIMULATION RESULTS
9.1. Case:1 When Both PI and PR Controllers Are Present
9.2. Case:2 When Both Fuzzy and PR Controllers Are Present
CHAPTER-10. CONCLUSION
CHAPTER-11. REFERENCES
1. INTRODUCTION
Nowadays, in renewable energy sources the PV energy has been widely utilized with the
increasing demand of power. The PV power systems can be categorized into 3 systems such as,
centralized, string, and ac module systems. Among them, the ac module system has many
advantages. it can track the individual maximum power point (MPP), and so reduces power
losses by PV module mismatch and partial shading. Moreover, the ac module system has higher
reliability and easier maintenance than those of other PV systems. The ac module PV system has
been used mostly with these advantages, as a trend of the future PV power systems. The worth of
the microinverter is evaluated by its power conversion efficiency, the shape of output current,
power density, reliability, and cost. Due to its simple circuit structure and potential for high
efficiency and reliability, a single-stage flyback inverter topology has been adopted. Moreover,
the flyback inverter topology has both step-down and step-up functions; for the PV applications,
this characteristic is suitable where the inverter should operate in wide voltage range.
9
Under the constant switching frequency conditions the flyback inverter operates, the
operation modes can be classified as the discontinuous current mode (DCM) and continuous
current mode (CCM). The PV inverter called as the CCM flyback inverter has both operation
modes; it inevitably operates in DCM at the low instantaneous power level or low solar
irradiation level although it operates in CCM at all instantaneous power levels for rated average
power. Then, it can be regarded that the flyback inverter has hybrid operation mode over whole
ac-line period. Compared to a flyback inverter only with DCM, the flyback inverter with hybrid
mode has the numerous merits such as higher efficiency with lower current stress, higher power
capability, and easier filter design. However, the control input-to-output current transfer function
of the flyback inverter in the CCM region has a right half-plane (RHP) zero which results in the
limitations of increasing the system gain and controller bandwidth. Since the operating point
varies widely in the PV inverter applications, especially, the controller should cover the
minimum RHP zero. When a conventional PI controller is applied to the flyback inverter with
hybrid mode, the proportional gain is designed to be relatively low for ensuring stability in all
operating points.
The system gain of the flyback inverter in the DCM region is inherently much low. To
achieve fast reference tracking and disturbance rejection performances, the high gain feed-back
controller is required in the DCM operation. However, when the conventional PI controller is
applied, the control gain is limited by the RHP zero in CCM. As a result, it causes unacceptable
power quality and high total harmonic distortion (THD) by the poor control performance in
DCM. This is the reason, the use of the flyback inverter with hybrid mode is limited despite its
many advantages.
To avoid the mentioned problem, some previous studies in control the primary
current instead of controlling the output current because there is no RHP zero in the transfer
function for the control input to the primary current. The control approach bypasses the
difficulties posed by the RHP zero. However, the power quality is low because this approach
controls the output current indirectly.
The PR controller is an alternative of the PI controller. It provides an infinite gain
without using high proportional gain at a selected resonant frequency. Moreover, for
compensating the harmonics of the selected fundamental frequency because the controller has
10
flexibility of selecting the resonant frequency, adding multiple PR controllers such as the
harmonic compensator is possible
The current control strategy of the flyback microinverter with PR and fuzzy
controller is proposed in this paper. The proposed control strategy consists of two components:
the PR controller with HC and the hybrid nominal duty ratio. At the fundamental and harmonic
frequencies of the grid the PR controller with HC provides high gain and achieves the zerotracking error in both operation modes. The proposed operation mode selection the hybrid
nominal duty ratio performs as a feedforward control input and is determined. According to
proper operation region, it can achieve more effective disturbance rejection and faster dynamics
by applying the hybrid nominal duty ratio. By using the simulation results the proposed control
strategy gives the higher tracking performance and better disturbance rejection in both operation
modes and strengthens the many advantages of the flyback inverter with hybrid mode.
2. DC-DC CONVERTER
A DC-to-DC converter is a electrical device it will accepts the input voltage as DC and
produces a DC output voltage. At a different voltage levels of input the output will be
produced. And, to provide noise isolation, power bus regulation, etc. DC-to-DC
converters are used.
2.1 Buck Converter Step-Down Converter
In this circuit two modes of operation will be considered. Firstly, the transistor turning ON will
put voltage Vin on one end of the inductor. This voltage will tend to cause the inductor current to
rise. Secondly, When the transistor is OFF, the current will continue flowing through the
inductor but now flowing through the diode. We initially assume that the current through the
inductor does not reach zero, thus the voltage at Vx will now be only the voltage across the
conducting diode during the full OFF time. The average voltage at Vx will depend on the average
ON time of the transistor provided the inductor current is continuous.
11
Fig: 2.1.1 Buck Converter
Fig: 2.1.2 Voltage vs Current
To analyze the voltages of this circuit let us consider the changes in the inductor current
over one cycle. From the relation
di
Vx − V0 = L dt………………. (1)
the change of current satisfies
i = ∫ON(Vx − V0 )dt + ∫OFF(Vx − V0 )dt ………. (2)
For steady state operation, the current at the start and end of a period T will not change.
To get a simple relation between voltages we assume no voltage drop across transistor or diode
while ON and a perfect switch change. Thus, during the ON time Vx=Vin and in the OFF Vx=0.
Thus
t
t
0 = di = ∫0 on(Vin − Vo )dt + ∫t on
on
12
+toff
(−Vo )dt…………………. (3)
This simplifies to
(Vin − Vo )t on − Vo t off = 0 …………… (4)
or
Vo
Vin
=
ton
T
…………… (5)
and defining "duty ratio" as
D=
ton
T
………. (6)
The voltage relationship becomes Vo=D Vin Since the circuit is lossless and the input
and output powers must match on the average Vo* Io = Vin* Iin. Thus, the average input and
output current must satisfy Iin =D Io These relations assume that the inductor current does not
reach zero.
2.2 Transition between continuous and discontinuous
When the current in the inductor L remains always positive then either the transistor T1
or the diode D1 must be conducting. For continuous conduction, the voltage Vx is either Vin or 0.
If the inductor current ever goes to zero then the output voltage will not be forced to either of
these conditions. At this transition point the current just reaches zero as seen in Figure (buck
booster boundary). During the ON time Vin-Vout is across the inductor thus
IL (peak) = (Vin − Vout ).
tout
(7)
L
The average current which must match the output current satisfies
IL (average at transition) =
13
IL (peak)
2
dT
= (Vin − Vout ) 2L = Iout (transition)
(8)
Fig: 2.2.1 Buck Converter at Boundary
If the input voltage is constant the output current at the transition point satisfies
Iout (transition) = Vin
(1 − d)d
T
2L
(9)
2.3 Voltage Ratio of Buck Converter (Discontinuous Mode)
As for the continuous conduction analysis, we use the fact that the integral of voltage
across the inductor is zero over a cycle of switching T. The transistor OFF time is now divided
into segments of diode conduction ddT and zero conduction doT. The inductor average voltage
thus gives
(Vin − V0 )DT + (−Vo )id T = 0
14
(10)
Fig: 2.3.1 Buck Converter - Discontinuous Conduction
Vout
d
=
Vin
d + δd
for the case
. To resolve the value of
(11)
consider the output current which is half the
peak when averaged over the conduction times
Iout =
IL (Peak)
d + δd
2
(12)
Considering the change of current during the diode conduction time
V0 (δo T)
L
(13)
V0 δ0 T(d + δ0 )
L
(14)
Vin dδd T
2L
(15)
IL (Peak) =
Thus from (6) and (7) we can get
Iout =
using the relationship in (5)
Iout =
and solving for the diode conduction
δd =
2LIout
Vin dT
(16)
The output voltage is thus given as
V0
d2
=
Vin d2 + (2LIout )
Vin T
(17)
defining k* = 2L/(Vin T), we can see the effect of discontinuous current on the voltage ratio of
the converter.
15
Fig: 2.3.2 Output Voltage vs Current
As seen in the figure, once the output current is high enough, the voltage ratio depends only on
the duty ratio "d". At low currents the discontinuous operation tends to increase the output
voltage of the converter towards Vin.
2.4 Boost Converter Step-Up Converter
The schematic below shows the basic boost converter. This circuit is used when a higher
output voltage than input is required.
Fig: 2.4.1 Boost Converter Circuit
While the transistor is ON Vx =Vin, and the OFF state the inductor current flows through
the diode giving Vx =Vo. For this analysis, it is assumed that the inductor current always remains
flowing (continuous conduction). The voltage across the inductor is below and the average must
be zero for the average current to remain in steady state
Vin t on + (vin − Vo )t off = 0
This can be rearranged as
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V0
T
1
=
=
Vin
t off 1 − D
and for a lossless circuit the power balance ensures
I0
= 1−D
IIN
Fig: 2.4.2 Voltage vs Current (Boost Converter)
Since the duty ratio "D" is between 0 and 1 the output voltage must always be higher than
the input voltage in magnitude. The negative sign indicates a reversal of sense of the output
voltage.
2.5 Buck-Boost Converter
17
Fig: 2.5.1 Schematic for buck-boost converter
With continuous conduction for the Buck-Boost converter Vx =Vin when the transistor is
ON and Vx =Vo when the transistor is OFF. For zero net current change over a period the
average voltage across the inductor is zero.
Vin t ON + V0 t OFF = 0
which gives the voltage ratio
V0
D
= −
Vin
1−D
and the corresponding current
I0
1−D
=−
IIN
D
Since the duty ratio "D" is between 0 and 1 the output voltage can vary between lower or higher
than the input voltage in magnitude. The negative sign indicates a reversal of sense of the output
voltage.
18
Fig: 2.5.2 Waveforms for buck-boost converter
2.6 Converter Comparison
The voltage ratios achievable by the DC-DC converters is summarized in Fig. 10. Notice
that only the buck converter shows a linear relationship between the control (duty ratio) and
output voltage. The buck-boost can reduce or increase the voltage ratio with unit gain for a duty
ratio of 50%.
Fig: 2.6.1 Comparison of Voltage ratio
2.7 CUK Converter
19
The buck, boost and buck-boost converters all transferred energy between input and
output using the inductor, analysis is based of voltage balance across the inductor. The CUK
converter uses capacitive energy transfer and analysis is based on current balance of the
capacitor. From DUALITY principle on the buck-boost converter the circuit in Fig. below (CUK
converter) is derived.
Fig: 2.7.1 CUK Converter
If we assume that the current through the inductors is essentially ripple free we can
examine the charge balance for the capacitor C1. For the transistor ON the circuit becomes
Fig: 2.7.2 CUK "ON-STATE"
and the current in C1 is IL1. When the transistor is OFF, the diode conducts and the current in C1
becomes IL2.
20
Fig: 2.7.3 CUK "OFF-STATE"
Since the steady state assumes no net capacitor voltage rise, the net current is zero
IL1 t ON + (−IL2 )TOFF = 0
which implies
IL2 1 − D
=
IL1
D
The inductor currents match the input and output currents, thus using the power conservation
rule
V0
D
= −
Vin
1−D
Thus, the voltage ratio is the same as the buck-boost converter. The advantage of the
CUK converter is that the input and output inductors create a smooth current at both sides of the
converter while the buck, boost and buck-boost have at least one side with pulsed current.
2.8 Isolated DC-DC Converters
In many DC-DC applications, depending on the application multiple outputs are required
and output isolation may need to be implemented. Additionally, to meet safety standards and / or
provide impedance matching input to output isolation may be required. To provide isolation
between input and output DC-DC topologies can be adapted.
21
2.9 Flyback Converter
The flyback converter can be developed as an extension of the Buck-Boost converter. Fig
(a) shows the basic converter; Fig (b) (replacing inductor by transformer) replaces the inductor
by a transformer. The buck-boost converter works by storing energy in the inductor during the
ON phase and releasing it to the output during the OFF phase. With the transformer, the energy
storage is in the magnetization of the transformer core. To increase the stored energy a gapped
core is often used. In Fig (c) removal of the common reference of the input and output circuits
clarify the isolated output.
Fig:2.9.1 Buck-Boost Converter
Fig: 2.9.2 Replacing inductor by transformer
22
Fig: 2.9.3 Flyback converter re-configured
2.10
Forward Converter
The concept behind the foward converter is that of the ideal transformer converting the
input AC voltage to an isolated secondary output voltage. For the circuit in Fig. (forward
converter), when the transistor is ON, Vin appears across the primary and then generates
Vx =
N1
V
N2 in
The diode D1 on the secondary ensures that only positive voltages are applied to the
output circuit while D2 provides a circulating path for inductor current if the transformer voltage
is zero or negative.
Fig:2.10.1 Forward Converter
23
The problem with the operation of the circuit in Fig above (forward converter) is that
only positive voltage is applied across the core, thus flux can only increase with the application
of the supply. The flux will increase until the core saturates when the magnetizing current
increases significantly and circuit failure occurs. The transformer can only sustain operation
when there is no significant DC component to the input voltage. While the switch is, ON there is
positive voltage across the core and the flux increases. When the switch turns OFF we need to
supply negative voltage to rset the core flux. The circuit in Fig. below shows a tertiary winding
with a diode connection to permit reverse current. Note that the "dot" convention for the tertiary
winding is opposite those of the other windings. When the switch turns OFF current was flowing
in a "dot" terminal. The core inductance act to continue current in a dotted terminal.
Fig:2.10.2 Forward converter with tertiary winding
24
3. PHOTOVOLTAIC (PV):
A photovoltaic device, also sun PV power system, or PV system, is a strength system
designed to supply usable solar strength via photovoltaics. It includes an arrangement of
numerous additives, along with solar panels to soak up and convert daylight into electricity, a
sun inverter to exchange the electric modern-day from DC to AC, in addition to mounting,
cabling and other electric add-ons to install an operating gadget. it could also use a sun tracking
machine to improve the system's typical overall performance and encompass an integrated
battery solution, as costs for garage gadgets are expected to say no. A solar array simplest
encompasses the ensemble of sun panels, the seen a part of the PV system, and does now not
include all of the different hardware, regularly summarized as balance of system (BOS).
Moreover, PV structures convert mild without delay into energy and should not be
confused with different technologies, which includes concentrated sun energy or solar thermal,
used for heating and cooling.
PV systems variety from small, rooftop- mounted or building-incorporated structures
with capacities from some to several tens of kilowatts, to big software-scale energy stations of
hundreds of megawatts. These days, maximum PV systems are grid-linked, even as off-grid or
stand-on my own systems handiest account for a small part of the market.
Operating silently and without any shifting elements or environmental emissions, PV
structures have advanced from being area of interest marketplace packages right into a mature
era used for mainstream energy generation. A rooftop gadget recoups the invested power for its
production and set up within zero.7 to 2 years and produces about ninety five percent of internet
clean renewable strength over a 30-year provider lifetime.
25
Fig:3.0 solar farm
3.1 Overview of photovoltaic:
Diagram of the viable components of a photovoltaic system. A photovoltaic system
converts the sun's radiation into usable electricity. It incorporates the sun array and the stability
of system additives. PV systems may be labeled by way of numerous elements, along with, gridconnected vs. stand-alone systems, constructing-included vs. rack-mounted structures, residential
vs. utility structures, disbursed vs. centralized systems, rooftop vs. ground-established systems,
tracking vs. fixed-tilt structures, and new built vs. retrofitted systems.
Fig:3.1.1 Diagram of the possible components of a photovoltaic system
26
3.2 Grid-connection:
Grid-related photovoltaic strength system Schematics of a normal residential PV device
A grid related system is connected to a larger impartial grid (usually the public power grid) and
feeds power without delay into the grid. This strength can be shared by way of a residential or
commercial constructing before or after the sales size factor. The distinction being whether or
not the credited energy manufacturing is calculated independently of the purchaser's strength
consumption (feed-in tariff) or most effective at the distinction of power (net metering). Grid
related systems range in length from residential (2–10 kWp) to sun strength stations (as much as
10s of MWp). this is a form of decentralized strength technology. The feeding of energy into the
grid calls for the transformation of DC into AC with the aid of a unique, synchronising grid-tie
inverter. In kilowatt-sized installations the DC facet device voltage is as high as accredited
(commonly 1000V except US residential 600 V) to restrict ohmic losses. most modules (60 or 72
crystalline silicon cells) generate 160 W to three hundred W at 36 volts. it is once in a while
important or proper to connect the modules partially in parallel rather than all in series. One set
of modules related in series is referred to as a 'string'.
Fig:3.2.1 Schematics of a typical residential PV system
27
4. FLYBACK CONVERTER:
The flyback converter is utilized in both AC/DC and DC/DC conversion with galvanic
isolation among the enter and any outputs. The flyback converter is a buck boost converter with
the inductor split to form a transformer, so that the voltage ratios are accelerated with a further
benefit of isolation. When driving as an instance a plasma lamp or a voltage multiplier the
rectifying diode of the boost converter is left out and the device is known as a flyback
transformer.
Fig:4.0 Schematic of a flyback converter
4.1 Structure and principle:
The operation of storing energy in the transformer before transferring to the output
of the converter allows the topology to easily generate multiple outputs with little additional
circuitry, although the output voltages should be able to match each other through the turns ratio.
Also, there is a need for a controlling rail which should be loaded before load is applied to the
uncontrolled rails, this is to allow the PWM to open up and supply enough energy to the
transformer.
4.2 Operations:
The flyback converter is an isolated power converter. The two prevailing manage
schemes are voltage mode manage and current mode control (usually current mode manipulate
28
needs to be dominant for balance all through operation). It requires a signal related to the output
voltage. There are 3 common methods to generate this voltage. The primary is to use an
optocoupler at the secondary circuitry to send a signal to the controller. The second is to wind a
separate winding at the coil and depend upon the go law of the layout. At the primary side, it
composed on sampling the voltage amplitude, throughout the discharge, referenced to the
standing primary DC voltage.
The first method related to an optocoupler has been used to reap tight voltage and current
regulation, while the second one approach has been developed for cost- sensitive applications
where in the output does not need to be as tightly managed, but up to 11 components inclusive of
the optocoupler will be eliminated from the overall layout. also, in programs wherein reliability
is crucial, optocouplers can be unfavorable to the MTBF (suggest Time among Failure)
calculations. The 0.33 approach, primary-facet sensing, can be as correct as the first and more
within your budget than the second one, yet calls for a minimum load so that the dischargeoccasion maintains happening, offering the possibilities to pattern the 1: N secondary voltage on
the primary winding (while Tdischarge,)
Fig:4.2.1 Two configurations of a flyback converter in operation on state and off state
A variation in primary-side sensing technology is where the output voltage and
current are regulated by monitoring the waveforms in the auxiliary winding used to power the
control IC itself, which have improved the accuracy of both voltage and current regulation. The
auxiliary primary winding is used in the same discharge phase as the remaining secondary’s, but
29
it builds a rectified voltage referenced commonly with the primary DC, hence considered on the
primary side.
Fig: 4.2.2 Waveform - using primary side sensing techniques - showing the 'knee point'.
Previously, a measurement was taken across the whole of the flyback waveform
which led to error, but it was realized that measurements at the so-called knee point (when the
secondary current is zero, see Fig. allow for a much more accurate measurement of what is
happening on the secondary side. This topology is now replacing ringing choke converters
(RCCs) in applications such as mobile phone chargers.
30
4.3 Controllers
Proportional & Integral Controllers Proportional + Integral (PI) controllers
were developed because of the desirable property that systems with open loop transfer functions
of type 1 or above have zero steady state error with respect to a step input.
4.4 Concept of PI Controller:
The PI regulator is:
𝑈(𝑠)
𝐸(𝑠)
= 𝐾𝑃 +
𝐾𝐼
𝑠
But can be realized easily in the following form:
4.5 Tuning PI Controllers
General approach to tuning:
1. Initially have no integral gain (TI large)
2. Increase KP until get satisfactory response
3. Start to add in integral (decreasing TI) until the steady state error is removed in satisfactory
time (may need to reduce KP if the combination becomes oscillatory)
To eliminate the steady state error P-I controller is principally used. However, it's a
negative impact in terms of the speed of the response and overall stability of the system. In areas
wherever speed of the system isn't a problem this controller is usually used. To predict the future
31
errors of the system P-I controller has no ability it cannot decrease the increase time and
eliminate the oscillations. If applied, any quantity of I guarantees set point overshoot.
Fig: 4.5.1 PID controller
In industrial control systems, a proportional–integral–derivative controller (PID
controller) may be a control loop feedback mechanism(controller) usually used. A PID controller
because the difference between a measured process variable and a desired setpoint continuously
calculates an error value. By adjustment of an impact variable the controller makes an attempt to
reduce the error over time, similar to the position of an impact valve, adamper, or the ability
supplied to a component, to a new value determined by a weighted sum
𝒕
𝒅
. 𝒖(𝒕) = 𝑴𝑽(𝒕) = 𝑲𝒑 𝒆(𝒕) + 𝑲𝒊 ∫𝟎 𝒆(𝝉)𝒅𝝉 + 𝑲𝒅 𝒅𝒕 𝒆(𝒕)
Equivalently, the transfer function in the Laplace Domain of the PID controller is
𝐿(𝑠) = 𝐾𝑝 +
Where : complex number frequency
32
𝐾𝑖⁄
𝑠 + 𝐾𝑑 𝑠
4.6 Proportional Term:
The proportional term produces an output value that is proportional to the current error
value. The proportional response can be adjusted by multiplying the error by a constant Kp,
called the proportional gain constant.
The proportional term is given by:
𝑃𝑜𝑢𝑡 = 𝐾𝑝 𝑒(𝑡)
In a large change in the output for a given change in the error is the high proportional
gain results. The system can become unstable, if the proportional gain is too high (see the section
on loop tuning). In contrast, to a large input error a small gain results in a small output response,
and a less responsive or less sensitive controller. when responding to system disturbances if the
proportional gain is too low, the control action may be too small.
Fig: 4.6.1 Plot of PV vs time, for three values of Kp (Ki and Kdheld constant)
4.7 Integral Term:
The integral in a PID controller is the sum of the instantaneous error over time and gives
the accumulated offset that should have been corrected previously To both the magnitude of the
error and the duration of the error the contribution from the integral term is proportional.. The
accumulated error is then multiplied by the integral gain (
33
) and added to the controller output.
The integral term is given by:
𝑡
𝐼𝑜𝑢𝑡 = 𝐾𝑖 ∫ 𝑒(𝜏)𝑑𝜏
0
The integral term eliminates the residual steady-state error that occurs with a pure
proportional controller and accelerates the movement of the process towards set point. However,
since from the past the integral term responds to accumulated errors, to overshoot the setpoint
value it can cause the present value.
Derivative Term
By determining the slope of the error over time and multiplying this rate of change
the derivative of the process error is calculated by the derivative gain Kd. The magnitude of the
contribution of the derivative term to the overall control action is termed the derivative gain, Kd.
The derivative term is given by:
𝐷𝑜𝑢𝑡 = 𝐾𝑑
𝑑
𝑒(𝑡)
𝑑𝑡
Derivative action improves settling time and stability of the system. An ideal derivative is
not casual, so that implementations of PID controllers include an additional low pass filtering for
the derivative term,. Derivative action is seldom used in practice to limit the high frequency gain
34
and noise - by one estimate in only 25% of deployed controllers - because of its variable impact
on system stability in real-world applications.
Fig: 4.7.1 Plot of PV vs time, for three values of Kd (Kp and Kiheld constant)
35
5. FUZZY LOGIC CONTROLLER
In FLC, a set of linguistic guidelines decides fundamental control action. These rules are
determined by using the gadget. Since the numerical variables are transformed into linguistic
variables, mathematical modeling of the gadget isn't required in FC. The FLC incorporates of
three elements: fuzzification, interference engine and defuzzification. The FC is characterized as;
i. seven fuzzy units for every enter and output. ii. Triangular club features for simplicity. iii.
Fuzzification using non-stop universe of discourse. iv. Implication using Mandeni’s „min‟
operator. v. Defuzzification the usage of the „top‟ method.
5.1 Fuzzification:
Membership function values are assigned to the linguistic variables, using seven fuzzy
subsets: NB (poor large), NM (negative Medium), NS (bad Small), ZE (0), PS (Positive Small),
PM medium (Positive Medium), and PB (Positive big). The partition of fuzzy subsets and the
form of membership CE(K) E(K) feature adapt the shape as much as appropriate system. The
cost of input errors and alternate in mistakes are normalized by way of an enter scaling
component.
Fig:5.1.1 Fuzzy logic controller
In this system, the input scaling factor has been designed such that input values are
between -1 and +1. The triangular shape of the membership function of this arrangement
presumes that for any E(k) input there is only one dominant fuzzy subset. The input error for the
FLC is given as
36
E(k) =
𝑃𝑝ℎ(𝑘) −𝑃𝑝ℎ(𝑘−1)
𝑉𝑝ℎ(𝑘) −𝑉𝑝ℎ(𝑘−1)
CE(k) = E(k) – E(k-1)
(a)
(b)
TABLE: FUZZY RULES
.
Fig:5.1. a input error as membership functions
37
Fig:5.1. b change as error membership functions
Fig:5.1. c output variable Membership functions
5.1 Interference Method:
There are various composition techniques which contents of Max–Min and Max-Dot are
proposed in this literature. In this paper Min technique is used. The output membership feature
have various rule are given with the aid of the minimal operator and most operator. Table.1
suggests rule base of the FLC.
38
5.2 Defuzzification:
The plant normally needed a non-fuzzy value of control, which is defuzzification stage is
required . To compute the output of the FLC, height method is used along with the FLC output
modifies the control output.
Fuzzy logic having completely different which means in 2 means they're in slender sense,
symbolic logic may be a system of rules, which may able to expand of multivalve logic. a lot of
over its employed in massive sense symbolic logic (FL) is sort of substitutable beside the
speculation of fuzzy sets, a theory which may able to relates to the categories of objects on with
un-sharp boundaries within which membership may be a matter of degree. during this
perspective, symbolic logic in its slender sense may be a branch of Everglade State. Even in its a
lot of slender definition, symbolic logic differs each in conception and substance from ancient
multivalve logical systems.
As the plant typically needs a non-fuzzy price of management, a defuzzification stage is
required. To work out the output of the FLC, „height‟ technique is employed and therefore the
FLC output modifies the management output. Further, the output of FLC controls the switch
within the electrical converter. In UPQC, the active power, reactive power, terminal voltage of
the road and electrical device voltage square measure needed to be maintained. so as to regulate
these parameters, they're detected and compared with the reference values. to attain this, the
membership functions of FC are: error, modification in error and output
The set of FC rules are derived from
u=-[α E + (1-α)*C]
(6)
Where α is self-adjustable factor which can regulate the whole operation. E is the error of
the system, C is the change in error and u is the control variable.
39
5.3 What Is Fuzzy Logic?
fig 5.3.1 fuzzy description
Fuzzy logic is used in two ways they are old and new ,because from the modern and
methodical science , fuzzy logic is the young concept ,this concept is related to the fuzzy logic
relies on age-old skills of human reasoning.
5.4 Why Use Fuzzy Logic?
Fuzzy logic may be a convenient thanks to map AN input house to AN output house.
Mapping input to output is that the place to begin for everything. contemplate the subsequent
examples:
• With data concerning however smart your service was at a eating place, a symbolic logic
system will tell you what the tip ought to be.
• With your specification of however hot you wish the water, a symbolic logic system will
change the tap valve to the correct setting.
• With data concerning however remote the topic of your photograph is, a symbolic logic system
will focus the lens for you.
• With data concerning how briskly the automotive goes and the way exhausting the motor is
functioning, a symbolic logic system will shift gears for you.
40
To determine AN acceptable quantity of tip needs mapping inputs to the proper outputs.
Among the input and also the output, the previous verify suggests a recording equipment which
will embody any amount of things: fuzzy structures, linear structures, knowledgeable systems,
neural networks, differential equations, interpolated third-dimensional search tables, or maybe a
spiritual consultant, merely to decision variety of the possible choices. Clearly the listing have to
be compelled to cross on and on.
5.5 When not to Use Fuzzy Logic?
Fuzzy logic is not a remedy-all. while should you not use symbolic logic? the foremost
secure Announcement is that the initial one created during this advent: fuzzy logic may be a
convenient manner to map AN enter area to an output area. If you discover it's not convenient,
attempt some factor else. If a easier answer already exists, use it. symbolic logic is that the
codification of commonplace sense — use commonplace feel once you place into result it and
you may in all probability build the correct call. several controllers, as AN instance, do a good
task while not victimization symbolic logic. However, just in case you are taking the time to
grow to be accustomed to symbolic logic, you will see it is a really powerful device for dealing
speedy and with success with impreciseness and nonlinearity.
5.5 The FIS editor:
The following discussion walks you thru building a replacement fuzzy abstract thought
system from scratch. If you wish to store time and adjust to on fast, you'll load the already made
machine by means that of writing fuzzy tipper this can load the FIS associated with the report
tipper.fis (the .fis is implied) and unharness the FIS Editor. but, just in case you load the pre-built
device, you may not be building pointers and building club options. The FIS Editor shows most
well-liked facts just about a fuzzy abstract thought machine. there is a simple diagram that shows
the names of every input variable on the left, and people of every output variable at the proper.
The sample membership capabilities shown inside the bins area unit simply icons and don't
depict the particular shapes of the membership capabilities.
41
Fig:5.6.1 The FIS Editor
We'd like to change the variable names to reflect that, though:
•
In the white edit field on the right, change input1 to service and press Return.
•
Click once on the left-hand (yellow) box marked input1 (the box will be highlighted in
red).
•
Click once on the right-hand (blue) box marked output1.
•
In the white edit field on the right, change output1 to tip.
•
In the white edit field on the right, change input2 to food and press Return.
•
Click once on the left-hand (yellow) box marked input2 (the box will be highlighted in
red).
•
From the File menu select Save to workspace as.. and a window appears as below
•
Enter the variable name tipper and click on ok.
We can show the diagram updated to reflect the new names of the input and output
variables. There is now a new variable in the workspace called tipper that contains all the
information about this system.
42
Fig: 5.6.2 ‘Save to workspace as...’ Window
By saving to the workspace with a new name, you also rename the entire system. Your
window will look like as shown in Fig.5.
Fig:5.6.3 The updated FIS Editor
43
6. OPERATION PRINCIPLE
Below is a circuit diagram of the flyback microinverter; it consists of an input
capacitor Cin, a flyback converter with turn ratio n (Ns / Np), a full-bridge type unfolding circuit
(S2 − S5), and an output filter.
Fig : 6.0 Circuit diagram of the flyback microinverter
The flyback converter operates under the high switching frequency to convert PV power
into rectified sinusoidal waveform. The unfolding circuit works under the grid frequency f to
inject sinusoidal ac current into the grid; switches S2 and S5 are turned on during the positive
half-cycle of the grid voltage vg, while S3 and S4 are turned on during the negative half-cycle.
6.1 Steady-state analysis of DCM and CCM operations :
Under the constant switching frequency fs, the operation modes are classified into DCM
and CCM. In DCM, the magnetizing current im becomes zero within each switching period Ts,
and the transformer T is completely demagnetized as shown in Fig. 2. When S1 is turned on, the
primary current Ipri is stored in the magnetizing inductance Lm, and its peak value is expressed
as follows:
𝐼𝑝𝑟𝑖.𝑝𝑘−𝐷𝐶𝑀 (𝑡) =
𝑉𝑃𝑉
𝐿𝑚
𝐷𝐷𝐶𝑀 (𝑡)𝑇𝑠
(1)
where DDCM is the duty ratio in DCM. The energy ELm stored in Lm is expressed as
44
1
(𝑉𝑃𝑉 𝐷𝐷𝐶𝑀 (𝑡)𝑇𝑠 )2
2
2𝐿𝑚
𝐸𝐿𝑚 = 𝐿𝑚 𝐼𝑝𝑟𝑖.𝑝𝑘 2 (𝑡) =
(2)
Assuming lossless operation in the inverter, the power balance equations can be obtained as
𝑉𝑔 𝐼𝑔
𝑉𝑃𝑉 𝐼𝑃𝑉 =
= 𝑃0
(3)
2
𝑉𝑃𝑉 𝐼𝑃𝑟𝑖 (𝑡) = 𝑣𝑔 (𝑡)𝑖𝑔 (𝑡) = 𝑣𝑔 𝐼𝑔 𝑠𝑖𝑛2 𝜔𝑡 (4)
where VPV and IPV are the average values of the voltage and current for a PV
module. vg and ig are the grid voltage and grid current; Vg and Ig are the peak values of vg and
ig, respectively. Po is the average output power. Ipri is the average primary current. ω is the
angular frequency of the grid voltage. If there is no loss, the energy stored in Lm is equal to the
energy transferred to the grid. Assuming |vg |≈vo, from (2)-(4), DDCM can be derived a
𝐷𝐷𝐶𝑀 (𝑡) =
2
𝑉𝑃𝑉
√𝑃0 𝐿𝑚 𝑓𝑠 |𝑠𝑖𝑛𝜔𝑡| = 𝑑𝐷𝐶𝑀.𝑝𝑘 |𝑠𝑖𝑛𝜔𝑡|
(5)
where dDCM,pk is the peak value of DDCM. In CCM, Lm is applied to VPV during
the turn-on time of S1, while the voltage across Lm is reflected the output voltage during the
turn-off time. Using the voltage-seconds law for Lm, the duty ratio DCCM in CCM is
calculation as
𝐷𝐶𝐶𝑀 (𝑡) =
𝑉𝑔 |𝑠𝑖𝑛𝜔𝑡|
𝑛𝑉𝑃𝑉 +𝑉𝑔 |𝑠𝑖𝑛𝜔𝑡|
(6)
The duties DDCM and DCCM determine the relationship between the input voltage
VPV and output voltage vo.
45
Fig:6.1.1 Key waveforms in DCM and CCM.
As mentioned above, the peak value of the primary current in DCM is expressed
as the equation (1). In case of CCM, as shown in Fig. 2, the peak value of the primary current
Ipri, pk_CCM is equal to the sum of the average magnetizing current Im and the ripple
component. Because the magnetizing current is the same as the primary current when S1 is
turned on, its average value can be expressed using the average value of the primary current Ipri
and the duty ratio DCCM as
𝐼𝑚 (𝑡) =
𝐼𝑝𝑟𝑖 (𝑡)
(7)
𝐷𝐶C𝑀 (𝑡)
Thus, from Fig. and the power relationship in (2), the peak value of the primary
current Ipri, pk_CCM is calculated as
𝐼𝑝𝑟𝑖.𝑝𝑘−𝐶𝐶𝑀 (𝑡) =
46
𝐼𝑝𝑟𝑖 (𝑡)
𝐷𝐶𝐶𝑀
+
(𝑡)
𝑉𝑃𝑉 𝐷𝐶𝐶𝑀 (𝑡)𝑇𝑆
2𝐿𝑚
=
𝑖0
𝑉𝑃𝑉
(𝑛𝑉𝑃𝑉 + |𝑣𝑔 (𝑡)|) + 2𝐿
1
𝑚 𝑓𝑠
𝑉𝑃𝑉
|𝑣𝑔 (𝑡)|
𝑛𝑉𝑃𝑉 +|𝑣𝑔 (𝑡)|
(8)
The peak value of the secondary current is the same as the peak value of ipri for each
mode divided by the turn ratio n.
6.2 Flyback microinverter with hybrid mode :
Under the DCM operation, the turn-off time toff is divided into the falling time tf and the
zero time Δt. The time tf is constant and is given by
𝑡𝑓 =
𝑛𝑉𝑃𝑉 𝑑𝐷𝐶𝑀,𝑝𝑘
𝑣𝑔
𝑇𝑠 = 𝑛𝜆𝑑𝐷𝐶𝑀,𝑝𝑘 𝑇𝑠
(9)
where VPV/Vg is denoted as λ. Because im is zero before end of each switching period
Ts, the following condition (10) is satisfied in DCM
𝑡𝑜𝑛 (𝑡) + 𝑡𝑓 = 𝑑𝐷𝐶𝑀,𝑝𝑘 (|𝑠𝑖𝑛𝜔𝑡| + 𝑛𝜆)𝑇𝑠 ≤ 𝑇𝑠
(10)
With an increase of the output power, DDCM also increases, and the sum of the turn-on
time and falling time becomes Ts. Thus, from (5) and (10), the critical duty ratio Dcri can be
obtained as follows:
𝐷𝑐𝑟𝑖 (𝑡) =
t𝑜𝑛 (𝑡)
𝑇𝑠
=
𝑑𝐷𝐶𝑀.𝑝𝑘 𝑇𝑠 |𝑠𝑖𝑛𝜔𝑡|
𝑑𝐷𝐶𝑀.𝑝𝑘 (|𝑠𝑖𝑛𝜔𝑡|+𝑛𝜆)𝑇𝑠
=
|𝑣𝑔 (𝑡)|
𝑛𝑉𝑃𝑉 +|𝑣𝑔 (𝑡)|
(11)
From (11), the fact that the duty ratio DDCM is equal to DCCM under boundary
condition is verified, and the flyback operates in the DCM region when DDCM is smaller than
DCCM. Fig. shows the operation regions of the flyback inverter in a half-cycle of the grid under
conditions given in Table I. As shown in Fig. 3, the flyback inverter operates in DCM at the low
instantaneous power level or low solar irradiation level although it operates in CCM region
above a certain power level in ac line period. Because the flyback inverter has both operation
modes over whole ac line period, it performs as the flyback inverter with hybrid mode.
47
Fig:6.2.1 Operation regions of the flyback inverter during a half-cycle of the grid voltage.
The boundary between DCM and CCM regions varies according to the magnetizing
inductance Lm. Lower Lm results in larger DCM region at the given output power. The critical
magnetizing inductance Lm,cri is derived from (5) and (6) as follows:
𝐿𝑚,𝑐𝑟𝑖 =
1
(
𝑉𝑃𝑉 𝑉𝑔
4𝑃0 𝑓𝑠 𝑛𝑉𝑃𝑉 +𝑉𝑔
2
)
(12)
To make the flyback inverter only perform within the DCM region, Lm should to
be lower than Lm,cri at a certain output strength. The flyback inverter only with the DCM region
suffers from the high current strain which reasons high strength losses and bounds the boom of
the electricity ability. As Lm increases, the CCM region increases, and the maximum cuttingedge stress step by step decreases. as a result, the high Lm complements the performance and
electricity capability. But, while placing the cost of Lm, there is a alternate-off between the
performance and transformer length; higher Lm offers the decrease current strain however a
larger transformer size. Therefore, the design of Lm for the flyback inverter with hybrid mode
must be above Lm,cri, and consider the desirable current stress and transformer size.
48
7. OPERATIONAL ANALYSIS (HYBRID MODE)
7.1 Control issues :
In the flyback microinverter with hybrid mode, the current controller should
ensure the reference tracking and disturbance rejection performances in both operation regions.
Fig. shows the equivalent circuit of the grid-connected flyback micro- inverter.
Fig:7.1.1 Equivalent circuit of the grid-connected flyback microinverter.
Using the control input-to-output current transfer function introduced in the transfer
function Gid_DCM in DCM can be expressed as follows:
𝐺𝑖𝑑−𝐷𝐶𝑀 =
𝑉𝑃𝑉
𝑉𝑔,𝑟𝑚𝑠
𝑃𝑜
√2𝐿
(13)
𝑚 𝑓𝑠
where Vg,rms is the rms value of the grid voltage. Eqn. (13) is noted that the system
gain in DCM is constant and very low at all frequency ranges.
Using a small signal modeling, the transfer function Gid_CCM in CCM can be represented as
𝐺𝑖𝑑−𝐶𝐶𝑀 =
49
𝐴𝑠+𝐵
2
𝑅𝑖𝑛 𝐶𝑖𝑛 𝐿𝑚 𝑠 2 +𝐿𝑚 𝑠+𝐷𝐶𝐶𝑀
𝑅𝑃𝑉
−
𝐼𝐿𝑚
𝑛
(14)
Where
(1 − 𝐷𝐶𝐶𝑀 ) (𝑉𝐶𝑖𝑛 +
𝑣𝑔
𝑛)
𝐴 = 𝑅𝑃𝑉 𝐶𝑖𝑛
𝑛
𝑣𝑔
1 − 𝐷𝐶𝐶𝑀
𝐵=
(𝑉𝐶𝑖𝑛 + − 𝐷𝐶𝐶𝑀 𝐼𝐿𝑀 𝑅𝑃𝑉 )
𝑛
𝑛
From (14), it is observed that the control input-to-output current transfer function in
CCM has an RHP zero. The RHP zero varies according to the operating points, and its minimum
value is at the peak of the grid voltage under maximum output power. Thus, the minimum RHP
zero should be considered when the controller for the flyback inverter with hybrid mode is
designed.
In the conventional manipulate system [17], the PI controller is used to ensure the
reference tracking and disturbance rejection performances. Fig. indicates open-loop Bode plots
of the compensated systems by way of the traditional PI controller. Used parameters are listed in
Table-2. The working factor in DCM is at the instant energy 25W below the rated common
output energy. while, the point in the CCM is the height of vg beneath the rated average output
power wherein it's miles the minimal RHP is zero. The proportional benefit kp of the PI
controller is tuned to be low to make sure the steadiness within the operation factor with the
minimum RHP 0. Accordingly, the profits of the conventional PI controller are set as follows:
The proportional crucial benefit kp = 0.08 and ki = 64. As shown in Fig, the device advantage on
the fundamental and its harmonic frequencies of the grid in DCM is lots decrease than that
during CCM although the imperative action of the PI controller gives excessive DC benefit on
the beginning. This makes the flyback inverter in the DCM area be unable to make sure
monitoring the reference and rejecting disturbances by means of the PV and grid voltages effect.
To increase the device advantage at the ones frequencies, a high proportional gain is needed.
However, it raises the gadget gain in any respect frequencies and so ought to make the flyback
inverter within the CCM region become unstable. Consequently, whilst applying the traditional
PI controller to the flyback inverter with hybrid mode, there may be a trade-off among
manipulate overall performance in DCM and stability in CCM.
50
(a)
(b)
Fig:7.1.2 Open-loop Bode plots of the compensated system by the conventional PI controller. (a)
In DCM. (b) In CCM
51
8. PROPOSED CONTROL STRATEGY
To satisfy the desired control performance and stability in both operation modes, the
PR controller can be developed, and its transfer function is expressed as
𝐶𝑃𝑅 (𝑠) = 𝑘𝑝 +
𝑘𝑟 𝑠
2
𝑠 +𝜔2
(15)
where kr is the resonant gain. The PR controller in (15) has an infinite gain at the grid
frequency. However, the infinite gain would degrade the control performance and even cause the
system to become unstable. In the practical implementation, the following form of the PR
controller can be adopted as
CPR (s) = k p +
2kr ωc s
2
s +2ωc s+ω2
(16)
In which ωc is selected to widen the controller bandwidth and determines the -three dB
cutoff frequency of the controller. That is, the value of the compensator will become kr / 2 at ωωc or ω+ωc. In the PR controller, kp is selected inside the same manner as that of a PI controller.
this is, it is designed with the aid of the steadiness of the compensated structures thinking about
the minimal RHP zero. The benefit kr is tuned notably high to decrease constant-kingdom error
but it's far restricted by way of the stability. In case of the proposed PR controller, the gains kr
and ωc are 20 and 16, respectively; To ensure the stability kr is selected to make the system
have the segment margin above 45 degrees. Similarly, the harmonic compensator is able to
alleviate errors for the selective harmonic frequencies, and its transfer function is represented as
2k ωc s
2
c s+ω
CHC (s) = ∑h=3,5,7… s2 +2ωr
52
(17)
(a)
(b)
Fig:8.1 Open-loop Bode plots of the compensated system by the proposed controller. (a) In
DCM. (b) In CCM.
where h is the harmonic order, and krh is the resonant gain for each harmonic
frequency. The open-loop Bode plots of compensated system by way of the PR controller with
the third to seventh harmonic compensators is shown in Fig. 6. The third, fifth, and seventh
harmonics are the most prominent harmonics below the grid environment.
Like the PR
controller, HC affords the excessive advantage at selected harmonic frequency additives, which
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allows eliminating consistent-kingdom errors and the disturbance by way of the chosen
frequency components. Compared to the compensated machine by means of the conventional PI
controller proven in Fig., it is apparent that the PR controller with harmonic compensators brings
the higher benefit on the essential and its harmonic frequencies of the grid without excessive
proportional advantage. in addition, the bandwidth in DCM is extended. Accordingly, it
enhances the reference tracking velocity and disturbance rejection performances with satisfaction
of the favored stability in both operation modes.
Fig:8.1.2 The block diagram of the proposed control system.
The general proposed control device for the flyback inverter with hybrid mode is
proven in Fig .it includes the PR controller with HC and the nominal obligation ratio Dn. Ig * is
the peak fee of the reference grid current (or output current). As a kind of the feedforward
control inputs, the nominal duty ratio Dn gets rid of the disturbance results and reduces the load
of the comments controller. the responsibility ratio Dccm in (6) is carried out to entire ac line
period. In this situation, the duty ratio Dccm reasons the voltage mismatch in the DCM location,
which increases the burden of the feedback controller. For the reason that system gain in DCM
is relatively lower, this burden will become heavier. To overcome the mismatch in DCM, the
duty ratio DDCM have to be carried out while the flyback inverter operates inside the DCM
region; it method that the nominal duty ratio must be determined according to the operation
region. To categorized the section of operation modes without an extra modern sensor, the vital
duty ratio in may be used; it is stated that the flyback inverter operates within the DCM region
while the subsequent condition is satisfied as
DDCM (t) ≤ DCCM (t)
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(18)
Thus, the hybrid nominal duty ratio Dn in the proposed control strategy is determined as
follows:
Dn (t) = {
DDCM (t) ifDDCM (t) ≤ DCCM (t)
DDCM (t), ifDDCM (t) ≥ DCCM (t)
(19)
Finally, the proposed hybrid nominal duty ratio can significantly reduce the disturbance effect in
both operation modes, and so improve the performance of the feedback controller.
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9. SIMULATION RESULTS
To verify the feasibility and performance of the proposed control strategy, the
simulation by a simulator Psim and experiment using the prototype for the flyback microinverter
shown in Fig. 1 were conducted. The nominal PV voltage and rated power were set up to 60V
and 200W, respectively. The detail system parameters and parts are listed in Table II.
TABLE II
PARAMETERS AND COMPONENTS OF THE PROTOTYPE
Parameters
PV voltage
Grid Voltage
Grid frequency
Rated average output power
Switching frequency
Primary winding turns
Secondary winding turns
Magnetizing inductance
Leakage inductance
Input capacitor
Output capacitor
Output inductor
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Symbols
VPV
Vg
f
Po
fs
Np
Np
Lm
Llk
Cin
Co
Lo
Value
40-80V
210Vrms
60HZ
200w
60HZ
14 turns
51turns
50µH
0.6 µH
6.6mF
0.68 µF
400 µF
CASE:1
When both PI and PR controllers are present
Fig:9.1.1 Block diagram of simulation
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(a)
(b)
Fig:9.1.2 Simulation results for the grid current ig and its reference ig_ref when the
conventional control system is applied. (a) quarter-load condition. (b) full-load condition.
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Fig. shows the waveforms for the grid voltage and current when the proposed control
strategy is applied. As shown in Fig , regardless of load conditions, the grid current has an almost
perfect sinusoidal form and desired power level. The THD on the grid current is measured as
2.4% under full-load condition. Fig. shows the dynamic performance under the load variation.
From Fig it is verified that the proposed control system makes output current well track its
desired value under the load transient-state.
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CASE:2
When both Fuzzy and PR controllers are present
Fig:9.2.1 Block diagram of simulation of proposed system
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Fig:9.2.2 Simulation results for the grid current ig and its reference ig_ref when the proposed
control system is applied. (a) quarter-load condition. (b) full-load condition.
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10.CONCLUSION
In this paper proposes the current control strategy of the flyback microinverter with
hybrid mode for PV ac module has been introduced and verified by simulation results. In the
proposed control strategy, the PR controller with HC provides the high system gain at
fundamental and harmonic frequencies in both operation modes without using high proportional
gain. Here we are using the fuzzy controller compared to other controllers. The proposed hybrid
nominal duty ratio yielded from the proposed operation mode selection eliminates the
disturbance more effectively and reduces the burden of the feedback controller. In FLC, basic
control action is determined by a set of linguistic rules. These rules are determined by the
system. Since the numerical variables are converted into linguistic variables, mathematical
modeling of the system is not required in FC. By using the simulation results we can verify the
proposed method having faster reference tracking and better disturbance rejection than those of
the conventional strategy. The proposed control strategy has the many advantages of the flyback
inverter with hybrid mode and makes it to be used in the industrial field. By using the simulation
results we can analyze the proposed method.
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