3rd International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2013) April 29-30, 2013 Singapore
A New Step –Up Photovoltaic Inverter with a
Active Filter for Grid Applications
S.Saravana Kumar, and J.Rekha
source , other sources with inherent dc behaviour are relevant.
Any dc load might be of interest, although the emphasis in this
paper is on sources and loads with long lifetimes that would
benefit from a power converter with similar life. The basic
design challenge in Fig. 1 is that energy storage internal to the
switching converter is required to support conservation of
energy between two ports. In Fig. 1, the dc port has a constant
power flow
(1)
Pdc = Po = VdcIdc
Abstract— This paper presents minimum energy storage
requirements for converters with a dc port and a single phase ac port.
The minimum energy storage required to isolate the power ripple
from the dc port is presented, so that the converters in both dc and ac
port uses active filter. This paper presents a active filter port to
manage minimum energy storage and differentiates capacitor ripple
from power ripple. The designer can be allowed to choose vales of
the capacitor such that the system differs from capacitor voltage.
The combination of an ac link converter and a active filter leads to a
dramatic increase in reliability: it is shown that converters with
nominal ratings up to 230 W can be designed. This large increase in
life is achieved with minimal extra cost.
Keywords— Active filters, inverters, photovoltaic power
generation, power conditioning, power converter reliability, rectifiers,
single-phase converters.
I. INTRODUCTION
I
N PHOTOVOLTAIC (PV) applications, the source (in the
solar case) is a simple semiconductor device that lasts for
decades. These applications require a power conversion
interface between the utility grid and the device. In small-scale
applications, the utility grid is a single-phase connection.
Single-phase situations have the distinct disadvantage that the
grid power flow is time varying, with a double-frequency
variation about a nonzero mean [1].In PV applications, the
flow from the panels must be dc. Any variation requires extra
operating headroom and reduces the available average output.
The net result is that a PV application, requires a dc energy
port, while the grid interface requires a double frequency port.
The inherent dc power on the device side must interface to
single-phase time-varying grid power, and the power
converter must provide energy storage to manage the
difference.
The general problem can be considered in terms of a twoport converter, as in Fig. 1. The dc port interfaces to the dc
source or load. While the context of this paper is a PV dc
Fig. 1.Block diagram of PV inverter
The ac port has time-varying power flow
Pac (t) = vac (t) iac (t)
(2)
For sinusoidal voltage, with
vac (t) = Vo cos(ω t)
iac (t) = Io cos (ω t − φ)
(3)
this yields
(4)
Pac (t) = VoIocos (φ)/2 + VoIocos (2ω t − φ)/2
The fundamental challenge of time-varying energy storage
for single-phase power conversion applications is well
established. Two conventional solutions exist: passive filters
and active filters. Passive filters can include the conventional
approach of an electrolytic capacitor attached to the dc port, or
series inductance, or coupled filter designs [2]. In the most
common passive solution, time-varying energy flow is
provided by allowing a small ripple voltage on the dc port
connected to an electrolytic capacitor. The ripple voltage
produces a ripple power.
II. PASSIVE FILTER SIZING
The simplest form of a passive filter is a large capacitor
connected at the dc port, as in Fig. 2, reflecting conventional
design practice. With arbitrarily large capacitance, the voltage
and dc side current at the dc port become approximately
constant, while the power converter current takes whatever
form is necessary to resolve the double-frequency ripple
power issue. The challenge is that time-varying energy flow
from a capacitor requires time-varying voltage, so
requirements to maintain constant dc conflict with the need for
S.Saravana Kumar is with B.S.Abdur Rahman University, TamilNadu,
India. He is now with the Department of Electrical and Electronics
Engineering, B.S.Abdur Rahman University,TamilNadu, India (e-mail:
srkmtech23@gmail.com).
J.Rehka. is with B.S.Abdur Rahman University,TamilNadu, India. She is
now working in the Department of Electrical and Electronics Engineering,
B.S.Abdur Rahman University, TamilNadu, India (e-mail: jrekha @ bsauni v .
ac.in ).
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3rd International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2013) April 29-30, 2013 Singapore
power variation. A coupled inductor filter as in [2] breaks part
of this trade off. It can be employed to divert ac ripple away
from the dc port and into a capacitor [3].In that case, the
capacitor voltage can be allowed to vary over a wide range
with less direct impact on the dc port.
absolute value function rather than a true double frequency.
The authors report that, for a 42V bus and a power level of
110W, the expected capacitance is 100 μF if the dc bus is
allowed to vary between 50 and 85V. This is reported as a
good compromise for efficiency. While this method decouples
the double-frequency ripple from the dc bus voltage switching
ripple, the authors of [6] report that in practice a dc voltage
ripple on the order of 8% remains with their approach. In
[7],the circuit of [6] is modified to allow the ac sources to
enforce a minimum capacitor voltage by connecting the ac
side through a transformer and rectifier to the internal storage
capacitor.
Fig. 2.Conventional dc bus capacitor provides ripple power interface.
For example, a 100W inverter with a 48V dc bus and an
allowed voltage ripple of 1% requires 5760 μF in a 60 Hz
system. The capacitor surely will be electrolytic to meet this
need. In the passive case, energy storage levels are
fundamental. Capacitor values can drop as voltage increases,
but a drop in stored energy requirements can be accomplished
only by allowing more voltage ripple. For example, if total
peak-to-peak ripple of 15% ripple can be tolerated, the
necessary capacitance in the 100W application drops to about
770 μF at 48V, and the stored energy is “only” 3.33 times the
minimum amount. The trouble in a PV application is that
voltage ripple at the dc source reduces effective available
power: the PV source should be held at an optimum voltage to
deliver maximum power [4].At mains frequency, ripple
reduces the output nearly linearly, such that 10% peak-to-peak
ripple reduces average power by about 5%, and so on.
Fig. 3. Active filter configuration with dc control on the bus replaces
the large dc-bus capacitor
IV. RIPPLE PORT CONFIGURATION
This paper considers an alternative active filter
configuration, shown in Fig. 4, in which a third energy
management port is added to the converter. This port can be
physical or conceptual.
III. ACTIVE FILTER SIZING
The small signal approach in which capacitor voltage
variation is limited was employed in [5], which sought to
reduce capacitance requirements, but remained limited by
because of the small voltage variation. In [6], the filter draws
energy from the dc port and acts to divert double-frequency
current from the dc bus. In this configuration, shown in Fig. 3,
an inductor permits current control. Now the capacitor can be
allowed to have voltage variation without direct impact on the
dc bus, so a smaller value can be used compared to a passive
filter. It is important that the aforementioned analysis was
predicated on a nearly constant capacitor voltage ,but the
higher voltage variation does indeed lead to a lower capacitor
value. In Fig. 3, the capacitor voltage must be higher than the
dc bus voltage in general, but substantial voltage variation
above this value can be permitted. The control must act to
force a double-frequency current in the inductor to follow the
double-frequency power ripple, since the dc bus is to remain
approximately fixed.
In the circuit in Fig. 3, the capacitor value can be made
small if the capacitor average voltage is allowed to increase
while wide voltage variation is maintained. In the
configuration shown, it would be preferable to keep the
capacitor voltage close to the dc bus voltage value to keep
efficiency high in the active filter stage. In [6], a current
hysteresis control is employed to force the current to follow an
Fig.4. Three-port configuration and power flow associated with ripple
port
Depending on the specific operating strategy, the hardware
associated with this port can employ either the ac port or the
third port. In this case, the voltage variation on the capacitor is
large by design, and the power ripple is controlled directly to
achieve the desired double-frequency value.
A notable attribute in Fig. 4 is that, in contrast to [5]–[7],
direct control of the ripple power implies control over the
capacitor voltage and current rather than an injected bus
current. On one hand, this avoids voltage limitations of the
dc–dc topology in Fig. 3. On the other hand, the frequency
behaviour is markedly different: the energy is to be controlled
directly rather than indirectly. In [8], it is pointed out that
since the capacitor energy is 1/2CV2, it is necessary to provide
a voltage given by the square root of the desired time-varying
power to achieve the necessary result. The authors of [8] do
not discuss how this dynamic square root might be
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3rd International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2013) April 29-30, 2013 Singapore
accomplished, although well-known analogue circuits and
obvious digital computational processes certainly could be
employed. There are multiple square roots, and the time when
is negative would impose problems.
C.AC to AC Converter Stage
Single phase cycloconverter is used in this converter stage,
from DC-AC voltage.Cycloconverters are more commonly
used because of the following reasons:
i. Single phase AC power is readily available.
ii. It is economical to provide AC supply to grid of capacity
230v, 50HZ rather than single phase rectifier.
iii. The ripple frequency of the output current of the single
phase cycloconverter is higher than that for three-phase
cycloconverter.
Single phase cycloconverter are commonly used in grid
applications to produce a AC voltage and current for
consumer loads. Like three phase cycloconverter, single
phase cycloconverter also have two types that are step
down and step up.
As mentioned before in single phase cycloconverter,
thyristors are used as switch and the total number of switches
are eight, each thyristor conducts for 120o single phase
cycloconverter into two groups which are top group and
bottom group. For top group, thyristor with its anode at the
highest potential will conduct at one time. The other three will
be reversed. Thus for bottom group, thyristor with its cathode
at the lowest potential will conduct. The other three will be
reversed.
TABLE I
MINIMUM CAPACITANCE VALUES FOR VARIOUS VOLTAGES
0% RIPPLE, 100W APPLICATION
Peak Voltage
Capacitance Required
Before(2% ripple)
24V
921µF
23000µF
48V
230µF
5760µF
100V
53µF
1330µF
380V
3.7µF
92µF
600V
1.5µF
37µF
V. CIRCUIT DESCRIPTION
VI. MODELLING OF INVERTER
The modelling of this inverter is done using PSIM software
and the modelling diagram is shown in the Fig 6. Here as
described in the previous section, the left hand side is the
inverter (DC-AC)fed by PV supply which comprising of two
switches connected to dc link and right hand side comprising
of a eight switch thyristor bridge cycloconverter (AC-AC
converter) with a ripple port. Both sides are linked by a single
phase four winding transformer.
Fig. 5. Circuit diagram of single phase DC-AC inverter
The circuit diagram of single-phase step-up DC-AC
inverter is shown in the Fig.5. Left hand side is a single phase
inverter (DC-AC) fed by PV supply which comprises of active
filter and two switches connected to dc link. Right hand side is
a eight thyristor bridge cycloconverter (AC-AC converter)
with a ripple port. Both sides are linked by a single phase step
up transformer.
A. Inverter Stage
A simplified schematic diagram of a single-phase (DC-AC)
inverter is given in Fig 5. Here the switches will conduct for
120o. Only one switch is on at any instant of time. The gating
signals are applied for the conduction sequence of the switches
is S1, S2. Due to the converter’s input-voltage-source
characteristic, at least one switch must always be on, there
exist two topological stages of the inverter’s operation.
B. StepUp Stage
The step-up stage is using a transformer to increase the
voltage of the output of single-phase DC-AC (PP) inverter and
the inverter’s output as input to the transformer and its output
is again fed to a single-phase AC-AC converter. A singlephase two winding transformer connected in star-star mode
acts as a step-up transformer for this inverter approach.
The transformer ratio is set to a value according to its
required supply voltage. While choosing the ratio, secondary
winding should be greater than the primary.
Fig.6 Simulation diagram of inverter
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3rd International Conference on Electronics, Biomedical Engineering and its Applications (ICEBEA'2013) April 29-30, 2013 Singapore
VII. SIMULATION RESULTS
J Rekha received the B.E degree in Electrical and Electronics from
Government Engineering College, Thrissur, Kerela,India in 2008 and Master
of Technology(Power Electronics) from National Institute of Technology
Calicut in 2010.She is currently working as Assistant Professor in the
Department of Electrical and Electronics Engineering in B.S.Abdur Rahman
University, Chennai,Tamil Nadu. Her area of interest include Power
Converters, Power Electronics in Battery Charging Applications and Power
Quality Improvement..
The modelling of this inverter is done using PSIM software
and the modelling diagram is shown in the Fig.6. The output
voltage and current of the inverter are depicted. According to
the switching signals of the converter, the duty cycle D is
varied, a PWM technique is used for switches S 1 , S 2 . The duty
ratio D is defined by D= t ON/ T S , where t ON is the on-time
interval of the switches and T S is the switching period. The
inverter model is simulated for the following:
Input Voltage = 24V DC Voltage
Output Voltage ≈ 230V AC Voltage
Transformer Ratio, n=N P /N S (n=2/4 i.e., n=1:2 Ratio)
The waveforms for the input voltage and output voltage and
for the model are shown below in the following Fig. 7
Fig. 7 Waveform of input voltage and output voltage
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
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S.Saravana Kumar received the B.E degree in Electrical and Electronics
from Kamaraj College of Engineering, Virudhunagar, Tamil Nadu, India in
2010.He is currently pursuing M.Tech in Power Electronics & Drives in
B.S.Abdur Rahman University, Chennai,Tamil Nadu. His area of interests are
power electronic converters and electrical drives.
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