44
CHAPTER 3
SINGLE STAGE SOLAR POWER GENERATION USING
BOOST DC-AC INVERTER
3.1
INTRODUCTION
The grid connected and standalone solar power inverters are
generally switched mode power circuits (SMPCs) and their outputs are
derived from coupling one or more basic switch topologies. Inverters are
inverts the variable dc output of photovoltaic (PV) modules (Alajmi et al
2013) into a constant AC voltage with a fundamental utility frequency that
can be fed into the commercial electrical grid or off grid electrical network.
The falling of conventional energy sources, increasing of different pollutions
and the ever increasing demand of the fossil fuels are motivating the
engineering society to involve more investigation in the renewable power
generation and the development of alternative energy sources which are less
or zero pollution and eco-friendly (Bull 2001). Many renewable sources such
as wind energy, biogas and solar are now well developed as the cost effective
solution for many applications. Moreover the solar energy has become one of
the most hopeful sources of energy as they are pollution less and fuel free.
Beside this, solar energy is easy to adopt with existing power converters
(Rong-Jong Wai & Wen-Hung Wang 2008, Rafia Akhter 2007, Brad Bryant
& Marian Kazimierczuk 2005). In conventional solar power system consists
45
of two or more power conversion stages for satisfying the requirement of
single phase ac load or grid.
Therefore the increasing of number of power conversion stages, the
system will result in high power loss, large size, high total harmonic
distortion, more space, more weight and high cost. Also filter is needed for
converting square wave ac voltage into sine wave ac voltage, because output
voltage of the voltage source inverter is square wave.
PV Array
+ D
C
S
, 230V
50Hz ac
Load /
Utility
Grid
Boost Inverter
-
S1 - S4
Gate Driver
VC1
VC2
Controller
Vref
Figure 3.1
iL1
iL1
iref
Block diagram of proposed single stage solar power
generation system
The proposed solar power generation topology overcomes the
drawbacks of the conventional solar power generation system. The system
consists of solar PV array and boost inverter as shown in Figure 3.1. In the
proposed inverter topology, a low dc voltage of photovoltaic array is boosted
46
and inverted into a 220Vrms ac voltage at a fundamental frequency in a single
stage.
Arunkumar Verma et al (2010) have implemented single stage boost
inverter with sliding mode controller previously. Also Pablo Sanchis et al
(2005), developed and proposed a new control strategy for this boost dc-ac
inverter, having a two current bi-directional boost dc-dc converters
functioning in a complementary method. Therefore the main scope of this
research work lies in the designing of a simple controller for boost dc-ac
inverter.
There are four controllers such as Modified Non-Linear State
Variable Structure (MNLSVS) controller, Sinusoidal Pulse Width Modulation
(SPWM) technique based controller, Fuzzy Logic Controller (FLC) and
Comparator based Non-Linear Variable Structure (NLVS) controller are
proposed for boost dc-ac inverter. The results of four proposed controllers are
compared and analysed in the further chapters.
3.2
BOOST DC-AC INVERTER
The design and development of boost dc-ac inverter has been
elaborated in this chapter in detail. The boost inverter is designed and
constructed by the switching device IGBTs, inductors, capacitors and diodes.
The switching devices should be switched at the different switching period.
The boost dc-ac inverter consists of two separate current bidirectional boost dc–dc converters as shown in Figure 3.2 and Figure 3.4
which produce a dc- biased sine wave output so that each source produces
only a unipolar voltage as shown in Figure 3.3.
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+
DC-DC
Converter 1
-
L
o
a
d
+
DC-DC
Converter 2
-
Figure 3.2
Basic arrangements of two current bidirectional dc-dc boost
converters
The modulation on each converter is 1800 out of phase with the
other, which maximizes the voltage available across the load. The load is
connected differentially across the two converters. The dc bias voltage of
each converter appears at each end of the load and differential dc voltage
across the load is zero with respect to ground.
The main advantage of this single stage boost dc-ac inverter is the
reduced number of power conversion stages, because it boosts and inverts in
the single stage itself with smooth sine wave output voltage. The output
voltage of each converter is
V1
Vdc Vm sin t
(3.1)
V2
Vdc Vm sin t
(3.2)
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Vdc Vm sin t Vdc +
V1 in
volts
Vdc
0V
Time in sec
Vdc Vm sin t
V2 in
volts
Vdc
0V
Time in sec
2Vm
2Vm sin t
V0 in
volts
0V
Time in sec
Figure 3.3
Output voltages of each current bidirectional dc-dc boost
converters
Voltage across the load is given by
VO
V1 V2
2Vm sin t t.
(3.3)
The Equation (3.3) describes the output voltage of the boost inverter
which is double the input and also the dc power is boosted and inverted in a
single stage.
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S2
L1
VS
+
S1
D2
C1
VC1
D1
V0
-
S4
L2
D4
+
C2
-
S3
VC2
D3
Figure 3.4 Proposed boost dc-ac inverter
The operation of boost dc-ac inverter can be explained by modes of
operation and each converter operates under two modes such as:
3.3
DC-DC CONVERTER-1 CIRCUIT DESCRIPTION
The operation of the proposed boost dc-dc converter is explained
with the help of electrical equivalent circuit of dc-dc boost converter-1 as
shown in Figure 3.5.
The electrical equivalent circuit consists of dc supply voltage Vs,
input inductors L1 and L2, power switches S1-S4, capacitors C1 and C2
freewheeling diodes D1-D4 and load resistance RL. Converter-1 operates in a
continuous conduction mode when assuming that all the components are ideal
and there are two modes of operation. Figure 3.6 shows two topological
modes for period of operation
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RL
S2
L1
+
+
+
C1
S1
VS
D2
-
VC2
VC1
-
D1
-
Figure 3.5 Electrical equivalent circuit of dc-dc boost converter-1
Mode1:
When the power switch S1 is closed and S2 is open Figure 3.6 (a),
the current iL1 rises quite linearly, diode D2 is reverse polarized, capacitor C1
supplies energy to the output stage and voltage VC1 decreases.
RL
S2
+
L1
ra
+
C1
+
IL1
VS
S1
-
VC1
VC2
-
-
Figure 3.6(a) Operation of the dc-dc boost converter-1; Mode 1: Power
switches S1=CLOSED; S2 = OPEN
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RL
IL1
S2
+
L1
ra
C1
IL1
+
VC2
+
S1
VS
-
VC1
-
Figure 3.6(b) Mode 2: Power switch S1=OPEN; S2 = CLOSED
Mode2:
When the power switch S1 is open and S2 is closed Figure 3.6 (b),
the current iL1 flows through capacitor C1 and the load, the current iL1
decreases while capacitor C1 recharged.
The conduction mode of the converter-1 is given by
the conduction mode of the converter 2 is given by
VC 2
Vs
VC1
Vs
1
1 D
and
1
D
Where D is the duty cycle, VC1 is the voltage across the capacitor of
the converter-1 and VC2 is the voltage across the capacitor of the converter-2,
Vs is the input voltage to the Boost sine wave dc-ac converter.
Since the two converters are 1800 out of phase, the output voltage is
given by
V0 = VC1 - VC2
=
Vs
1 D
Vs
D
(3.4)
(3.5)
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V0
Vs
2D 1
1 DD
(3.6)
The gain characteristics of the boost inverter has been shown in the
form of Equation (3.6) where zero output voltage is obtained for D=0.5.
There is an ac voltage at output terminals, if the duty cycle is varies around
this point. Also the output of the boost inverter is less than or greater than the
input voltage depends on the duty cycle (D) as shown in Figure 3.7.
Figure 3.7 DC gain Characteristics
3.4
CALCULATION
OF
INDUCTOR
AND
CAPACITOR
VALUES FOR BOOST DC-AC INVERTER
Four controllers are proposed for single stage inverter in solar
power conversion system. The inverter consists of four switches, four diodes
two capacitors and two inductors. The capacitors and inductors are to be
53
designed to get the required output voltage. They can be designed based on
the boost dc-ac inverter specifications.
The boost dc- ac inverter specification:Output Power
PO = 100W
Output voltage
VO = 220Vrms
Input voltage
VS = 100 volts
Output frequency
fO = 50Hz
Switching frequency fSW = 20kHz
Output load voltage is determined from the equation (3.3)
VO (t)= V1(t) – V2(t)
325Sin(314t) = Vdc+ 162Sin (314t) – (Vdc- 162Sin (314t)
3.4.1
Calculation of Inductor Current and Inductance (L1)
The inductor current is composed from alternating with switching
frequency and high frequency ripple caused by switching under continuous
conduction mode. Therefore the maximum inductor current is obtained by
using
VS2
VS
4 ra
i Lm
VC 1 t
VC 2 t
VC1 t
RL
(3.7)
2ra
From the figure 3.6(a) the high frequency ripple is obtained and given by
i L1 t
VS
ra i L1 t
L1
t on
(3.8)
The inductors L1 and L2 are designed based on the maximum
inductor current and maximum inductor ripple current. The maximum
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inductor current ripple
i L1m is chosen to be equal to 25% of maximum
inductor current as given in equation (3.7), then from equation (3.8) one can
obtain as L1>726.5µH. Therefore adopted value of the inductor is L1 =
750µH.
3.4.2
Calculation of Capacitor voltage and Capacitance (C1)
The controller operates over the switch to make the voltage V1(t)
follow a low frequency sinusoidal reference. Over V1(t) a high frequency
ripple is imposed which is given by
VC t
V2 t V1 t
t on
C1 R L
(3.9)
The capacitor is designed based on the charge in capacitor voltage given in
equation (3.9). The maximum capacitor voltage ripple
VC1 m is chosen to be
equal to 0.5% of maximum sinusoidal capacitor voltage as given in the
equation (3.9), then C1 obtains C1>11.47µF therefore adopted value of the
capacitor is C1 = 20µF.