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Transcript
Ben-Gurion University of the Negev
Department. of Electrical and Computer Engineering
System for Feeding Energy to Power Lines
Supervisor: Shmuel (Sam) Ben-Yaakov
Students: Serge Yakushevsky, Dima Vilensky
Abstract:
In a world of limited energy resources, there is a strong need to finding alternative
energy sources which will provide a clean, efficient and convenient solution over
time. One of the promising technologies utilizes the energy of the sun and converts it
into electrical energy and, in contrast to other technologies, does not pollute the
environment.
People have invented many ways to exploit solar energy, one of which employs
photovoltaic panels that collect the solar energy and convert it into electrical energy
using electrical circuits. This kind of energy conversion produces cheap and clean
energy over a long period of time. There are many settings employing solar energy
such as: boilers, private houses, factories, thermo-solar stations and even satellites. In
our project we examine photo-voltaic (PV) panels. One of the main disadvantages of
PV panels is their low efficiency (10-15%), due to limited insolation and to reception
interruption because of clouds, dusts etc. The main aim of our project is to study ways
to increase PV string efficiency, making our project very relevant and at the forefront
of this research area.
In this project we will investigate a PV panel field subject to inhomogeneous
insolation caused by shading changes. The project’s main goals are to find new
power- effective and cheap solutions and to build a system that will maximize the
efficiency of a PV string by using electrical circuitry.
PV modules are generally connected in series in order to produce the significant
voltage required to efficiently drive an inverter and the grid. However, if even a very
small part of the PV module is prevented from receiving light, the generated power of
the PV module is decreased disproportionately. This excessive decrease occurs
because PV modules that do not receive adequate light cannot operate at the normal
operation point, but instead operate as load. As a result, the total power from the array
of PV modules is substantially decreased even if only a small part of the PV modules
is shaded.
The success of this project and its results are likely to impact the energy consumption
approach worldwide, make the usage of PV technology more profitable, help in
2
making our planet cleaner and decrease dependency on coal, gas and other fossil
fuels.
Research Proposal:
The main goal of this project is to develop a new system that will maximize power
from an array of PV panels by tracking the MPP (Maximum Power Point) and
changing the operation point accordingly. Our system uses a new concept based on an
inductor and switches where the voltage and the current are measured and the current
is changed according to desired characteristics.
Today, the PV systems are marginally in use, because of the low efficiency capability
(10-15%). We are planning to change this by continually updating the choice of the
right operation point that will improve the efficiency and lower the costs.
Due to the movement of clouds, the ground is always partially shaded in a timevarying manner. Thus, it is very hard to determine the operation point that will lead to
the MPP. Our system enables achieving changes in the MPP faster than existing
systems.
Fig.1. Schematic of current inverter unit circuit.
3
The Basic Idea and Main Goals:
(1) The proposed current inverter was designed and built based on one-level
architecture, with two solar cells and controlled by a micro controller that changes
the duty cycle of two switches, depending on the output powers of the solar cells.
The inverter’s work frequency is 100 kHz.
(2) The architecture of the inverter includes a pair of solar panels and a pair of
switches: ( P-MOS and N-MOS transistors that works complimentarily in a
positive dead-time mode), a switched inductor that accumulates energy from one
panel in the first half of the cycle and transmits it to another panel in the second
half of the cycle, a micro controller that changes duty cycle and two probe
systems: the first for voltage and the second for current, that provides inputs for
the micro controller and the other components.
(3) The proposed system with a micro controller improves the inverter’s efficiency –
to 86%.
(4) The desired output is achieved using a digital system control (dsPIC33fj16gs502)
microcontroller,- which is responsible for switching the transistors.
(5) A long lifetime is obtained due to (avoid the use of electrolytic capacitors).
(6) A low-cost solution for a current inverter is proposed.
(7) Design of appropriate MPPT algorithms is presented.
Schedule Changes:
We successfully met the time schedule targets of the project except examination of
the final code of micro-controller with the MPPT algorithm. However, in the
simulations the micro-controller worked satisfactorily.
Basic Principle of the Solution:
The solution principle is based on using a current inverter built on a switching circuit
model. The switching is controlled by a digital controller, which varies the duty-cycle
to control the switching. In this way we obtain optimal power at the output with
minimum losses at the solar panel array. Our circuit is based on a buck-boost inverter
and behaves as a buck or a boost inverter depending on the part of the duty cycle. Our
4
micro inverter is naturally synchronized to the grid and its ADC is synchronized to
two different PWMs in order to achieve maximum efficiency.
System Specifications:
Input values (DC):
Requirement number
Definition
Values
1
Recommended input power
180W
2
Maximum input DC voltage
VOC=60V
3
Maximum input DC current
ISC=8A
Table1. System specifications.
Fig.2 Schematic of current inverter with micro controller interface
The topology of the inverter has been designed to minimize power losses and includes
a minimal number of components with minimal resistance. However, there remain a
few points that can be dealt with to improve the efficiency.
5
Fig.3 The PSIM model of our system
Current Invertor Based on Switched Inductor:
In order to transmit energy from one solar panel to another we will connect a current
inverter based on buck-boost inverter with a switched inductor for each pair of panels.
Fig.4 Current inverter with micro controller interface
6
Work Principle of Current Inverter:
Each cycle includes two different states: ON and OFF
In state ON:

Transistor Q1 is open and transistor Q2 is closed

Current goes to inductor and inductor accumulates energy
In state OFF:

Transistor Q1 is closed and transistor Q2 is open

Current of inductor flows via transistor Q1
7
Components:
1) The circuit is based on buck-boost converter. The circuit comprises 100uH
inductor, a pair of balanced electrolytic capacitors Cpv1 andCpv2, 500uF each,
N-MOS transistor SUD40N08-16 and P-MOS transistor SUD50P08-25L.
2) As drivers for the transistors, we chose MIC4426 that are able to drive two
signals to a pair of switches. Note: the output signals of the drivers are in
opposite phase relative to the input signals of the drivers.
3) Protective resistors are connected to the drivers. A voltage divider guarantees
that the driver will give a constant control signal and a resistor of 1kOhm limits
the current for driver safety.
4) For solving the problem of floating point between the switches we chose in DC
restorer help circuit. For correct operation of the DC restorer, we use a
Schottky diode (1N5822), sufficiently fast and protect VGS voltage of the PMOS from overvoltage.
Current Inverter System (before adding the probe circuits):
Fig.5 Photograph of our system before adding probe circuits
8
Results of systems checking with Duty Cycle = 0.5
Equipment for experiments:

Current inverter

Rheostat 24Ω

DMMx2

Oscilloscope
 Energy source 30V 2A
In order to check the system we connected a voltage source of 20V instead of the Pv2
and instead of Pv1 we connected a rheostat so that we could check for different loads
(R). The tests were actually performed for a buck-boost circuit and the results are
shown in the following tables.
RL
ESR
Rdson_N Rdson_P
(Ohm)
(Ohm)
(Ohm)
0.04
0.145
0.016
Cdss_N
Cdss_P
(Ohm)
(F)
(F)
0.021
3.7E-10
3.2E-10
Table2. Values of resistance and capacitance
𝑅𝐿 - Resistance of inductor
ESR - Resistance of balancing capacitors
𝑅𝑑𝑠𝑜𝑛_𝑁 - Conduction resistance of N-MOS switch
𝑅𝑑𝑠𝑜𝑛_𝑃 - Conduction resistance of P-MOS switch
Cdss - Output capacitance of transistor
Vin
R
(Vpv2)
Iin
P_loss
2*Psw
(Ohm)
(V)
(A)
(W)
(W)
P_CdssP P_CdssN P_RdsonP P_RdsonN
(W)
(W)
(W)
(W)
P_L
(W)
2*P_ESR Eff_calc
(W)
(%)
23.1
20.33
0.64 0.56341 0.31462 0.02175
0.025149
0.0180782
0.0137738
0.0688 0.10117
95.66
20.5
20.34
0.74 0.67475 0.36368 0.021739 0.025135
0.0238742
0.0181898
0.0909 0.13118
95.51
15.8
20.22
0.98 0.97456 0.47784 0.021398 0.024741
0.0412118
0.0313994
0.1569 0.2209
95.08
13.7
20.23
1.14 1.2044
0.021316 0.024647
0.0554582
0.0422538
0.2112 0.29474
94.77
11.4
20.2
1.36 1.55699 0.65932 0.021153 0.024458
0.0785582
0.0598538
0.2992 0.41437
94.33
0.5548
Table3. Expect calculated losses for different values of R.
9
Power losses from transistor capacitance:
𝐶𝑑𝑠𝑠 × 𝑉𝑚𝑎𝑥 2
𝑃_𝐶𝑑𝑠𝑠 =
𝑓𝑆𝑊
2
Switching losses:
𝐼𝑠𝑉𝑠
𝑃𝑠𝑤 =
𝑡 𝑓
3 𝑟 𝑆𝑊
Conduction losses:
𝑃𝑟 = 𝐼𝑅𝑀𝑆 2 × 𝑅
R
Vin(Vpv2)
Vo
Vo(Vpv1)
Iin
(Ohm)
(V)
(V)
(V)
(A)
Pin (W)
Loses
Efficiency
Pout (W)
(W)
(%)
23.1
20.33
36.87
16.54
0.64
13.0112
11.84293
1.168274
91.02102
20.5
20.34
36.86
16.52
0.74
15.0516
13.3127
1.738898
88.44709
15.8
20.22
36.57
16.35
0.98
19.8156
16.91915
2.896454
85.38296
13.7
20.23
36.5
16.27
1.14
23.0622
19.32211
3.740091
83.78259
11.4
20.2
36.36
16.16
1.36
27.472
22.90751
4.564491
83.38493
Table4. Measured losses in the experiments
10
P_loss f(Iin)
2
1.5
1
P_loss f(Iin)
0.5
0
0
0.5
1
1.5
Graph1. Calculated power losses as a function of input current
Practical loses f(Iin)
5
4
3
Practical loses
f(Iin)
2
1
0
0
0.5
1
1.5
Graph2. Measured power losses as a function of input current
Eff f(R)
100.0
90.0
80.0
70.0
Eff f(R)
60.0
50.0
0
5
10
15
20
25
Graph3. Measured system efficiency as a function of output resistance
11
Eff f(Iin)
100.0
90.0
80.0
Eff f(Iin)
70.0
60.0
50.0
0
0.5
1
1.5
Graph4. Measured system efficiency as a function of input current
Fig.6. Two PWMs with a dead time
12
Fig.7.Two probe circuits
Fig.8. PWM and ADC
Fig.9. ADC accrues between switching noises
13
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20) http://www.vishay.com/
21) http://sharp-world.com/
22) "Analog Electronic Circuits" Lecture notes of the course by Prof. S. BenYaakov
http://www.ee.bgu.ac.il/~angcirc/frames.html
23) "Switch Mode DC-DC Converters "Lecture notes of the course by Prof. S.
Ben-Yaakov
http://www.ee.bgu.ac.il/~dcdc/
15