State of the Art Review of PV Module-Level Power
Electronics
Roman Kosenko
Tallinn University of Technology (Estonia), Chernihiv National University of Technology (Ukraine)
mr.roman.kosenko@ieee.org
Abstract— The main goal of this paper is to systematize
existing converter topologies and to determine the perspective
directions of research and development of high efficient wide
operating range module-level-converter. Different converter
topologies are compared in terms of cost, efficiency and
application specific features.
I. INTRODUCTION
Solar photovoltaic (PV) is safe to environment, easy to
install and maintains, can work almost everywhere in the
world, doesn’t violate the tranquility of the people around, as
opposed to other renewables. That’s why solar energy is now
one of the most promising renewable energy source.
Fig. 1 shows the primary types of power converters. There
are different derived types and topologies of converters and
the full classification is not among the tasks of this paper,
many of them are shown in [1-4].
The vast majority of renewables and in particular PV are
dc-based, and almost all modern consumer electronic is
already using dc-voltage as energy source, or can be easily
adopted to do so (personal computers, air conditioners,
mobile phones, LED lightning, domestic appliance), and there
is no need to convert it in to ac-voltage. It leads to activation
of the research efforts towards the use of the dc-grids [5]. But
this doesn’t make the ac-grid less important. The most basic
converters systematization is shown in the Fig. 1.
shaded PV-module on the total power generated are shown in
Fig. 2 – Fig. 5, all numbers are for reference only and do not
take into account the losses in the diodes. From figures it’s
seen that is not enough to use one converter for a few PVmodules. That is because of the main drawback of the PV
modules – their high dependence of the output power on
operational conditions such as the solar irradiance,
temperature, and the electrical load. In case of simple
connecting few modules together into array, different
operational conditions of all of them leads to situation when
the efficiency of all modules in serial or parallel branch of
array is determined by the efficiency of the worst one. This
drawback can be easily avoided when each module is
connected to individual power converter, such converters are
also referred as module level converters or micro-converters.
Fig. 2. The serial connection of PV-modules.
Generally PV-modules are relatively small and low power
(typically up to 300 W) for ease of installation and
maintenance. So one PV-module is not enough to supply
energy even for one household, that’s where the distributed
energy generation comes in view. To meet amount of energy
it is necessary to connect multiple PV-module in series or
parallel with each other, thereby forming the PV-array. The
simplest PV-modules interconnections and impact of the
32V,
5A
32V,
5A
32V,
5A
500W/m2
1000W/
m2
A. Distributed Power Generation
1000W/
m2
1000W/
m2
Fig. 1. Systematization of power converters.
Total power
570W
GRID
DC/
AC
32V,
2.8A
Fig. 3. The parallel connection of PV-modules.
The next systematization principle is by converter size, not
a physical size but the size of array part that is covered. By
this principle power converters can be divided to central (Fig.
6), string (Fig. 7) and module level (Fig. 8) converters.
146
Fig. 4. The parallel connection of PV-with the individual power converters.
Fig. 8. Module level inverters in distributed PV-generation topology.
Fig. 5. The serial connection of PV-with the individual power converters.
Module-level power converters have one more benefit in
terms of power dissipation across the distributed network
wires. Typical solar module has output power about 275 W
and maximum power point voltage of 30 V. To transfer
energy to electrical load at such low voltage the current of
9.17 A is needed, as compared to 0.69 A at voltage of 400V
(typical voltage level for dc-grid). The supply current
magnitude comes on the first places when it comes to power
dissipation on the connection wires. As well-known power
dissipation across the active load is proportional to the current
in the power of two (P= I2R) so for 400 V grid voltage, power
dissipation across the same wires will be in 176.62 times
lower than for 30 V-grid. That’s why it is preferred to use
higher voltage transmission lines to minimize power losses
and to achieve high overall system efficiency.
Each invertor size is optimal for use at its own specific
application field and it’s important to know where exactly [6,
7, 8]. Basically optimal application for specific inverter can
be determined from the comparison of the key parameters
(see Table 1) [9].
Fig. 6. Central inverter in distributed PV-generation topology.
TABLE I.
KEY PARAMETERS COMPARISON OF DIFFERENT SIZE INVERTORS
Nominal
power
Input
voltage
Max
efficiency
Cost
Cost/Watt
<1kW
<100V
96%
Low
High
String
inverter
<30kW
150V1000V
98%
Medium
Medium
Central
inverter
30kW1MW
450V1500V
99%
High
Low
Micro
inverters
Fig. 7. String inverters in distributed PV-generation topology.
Micro inverters are connected to one PV-module and
usually used in the small PV installations, like individual
households. They are best in dealing with partial shading,
different panel types, sizes, characteristics and optimization.
Microinverters are physically located on the panel and their
outputs are connected directly to ac-grid.
147
String inverters are usually used in small to medium PV
installations where the panels characteristics and operational
conditions are almost the same.
Central inverters are usually used in large, utility-scale PV
power plants
String and central inverters are connected to one string and
to a whole array respectively, they connected from one side to
the ac-grid and from another to the panels using long cables.
Despite the fact that central inverters have the highest
efficiency among the others, the overall efficiency of the
system with only central invertor will be lower than in the
system with micro inverters, because panel characteristic are
always slightly differ and only by the use of micro invertors
all of the PV-panels will be working at its MPPT. But in
large-scale plants the use of micro inverters is usually
economically unreasonable, because their cost can reach up to
a half of the PV-module cost itself.
B. Module-Level Power Electronics Classification
The concept of module-level power electronics (MLPE)
covers a great variety of devices, the simplified classification
is shown in the Fig. 9.
Here is a brief description of each group:
(1) – includes the PV-modules fuse-boxes content, like
electrical fuses and protective diodes;
(3) – the module level dc-dc converters;
(4) – the module level dc-ac converters;
(5) – in such converters energy from PV-module is directly
transformed to either ac or dc voltage while keeping the PVmodule at its MPPT. They are simple, easy to control but
cannot achieve high step-up rate and have some problems
when trying to harmonize them with other modules in array.
(6) – micro-converters and micro-inverters are generally
consist of several conversation stages, it helps them to
achieve higher step-up rates. They use more complex control
algorithms but can work in wider range of input voltages
without significant efficiency drop.
(9-12) – the dc-dc converter topology have a greatest
impact on efficiency, provided classification is taken from
[3].
(13-14) – vast majority of micro converters are
unidirectional, and can transfer energy only in one direction
from PV-module to load. In last year’s bidirectional dc-dc
converters appeared, they are showing highest efficiency
when PV-arrays illumination is uneven. Usually they are used
inside serial PV-string and are able to inject current to one of
its neighbor converters to achieve more uniform operational
mode across the string and thus higher efficiency.
by use of transformer which at the same time provides a
major step-up of the voltage level;
– have low price, now for the micro-convertor to be
competitive, it must be worth up to 120 € [10];
– about 25 years of nonfailure operating time (approximate
minimal time that PV-module last). To satisfy this
requirement, some specific measures must be taken when
developing it, like it uses the high-grade elements, avoiding
the using of electrolytic capacitors, fully passive cooling and
others;
– peak efficiency ≥95% but there is a catch, the cost of
further converter efficiency increasing rises quickly, therefore
there is always must be a balance between price and
efficiency. Another requirement - high efficiency over the
entire operating power range, so called “flat efficiency”, it
often described by integral parameter called “Euro
Efficiency”;
– maximum power point tracking of ≥99.5%;
– low nighttime power consumption;
– ability to island in case of the converter or the grid
failures;
– low weight and dimensions so it can be easily installed on
PV-module;
– total semiconductor elements count. The semiconductor
keys switching and conduction losses has the major impact on
the converter efficiency, so the converter topologies with less
semiconductors are preferred. The greatest conduction losses
are caused by the diodes in the inverter stage this is due to
their forward voltage drop. So in to minimize conduction
loses, the diodes in corresponding topologies must be
replaced by active switches;
– passive elements count. Passive elements have no direct
impact on the converter efficiency but they may cause losing
system stability due to the parasitic resonant circuit and
additional poles and zeroes in the transfer function of the
system.
Based on these requirements most module-level converters
are multistage. They consist of invertor stage, isolating
transformer and rectifier stage (Fig. 10). The inverter stage
works with high currents and defines the transformer working
mode thus it has the major impact on the overall microconverter efficiency.
Considering these requirements the list of the power
converters topologies suitable for the module-level
applications is rather limited. Some of the topologies that fit
all of these requirements are listed in the Table II, their names
are based on the inverter stage topology names [4].
TABLE II
II. TRENDING MICRO-CONVERTERS TOPOLOGIES
CONVERTER TOPOLOGIES SUITABLE FOR PV MODULE-LEVEL APPLICATIONS
The micro-inverter is basically the micro-convertor with
one additional stage so they will not be discussed separately.
In general the micro-converter is the low power (up to
300W – typical power of PV-module) dc-dc converter that
has the input voltage around 30 V and the output voltage
about 400 V. To work with high efficiency in such conditions
the micro-converters must fulfill these special requirements:
– must be galvanically isolated, to protect PV-module and
people that are using it from high voltage, this is easily done
148
Name
Ref.
Figure
1
Interleaved flyback
[11]
Fig. 11.
2
Half-bridge resonant
[12],[ 13]
Fig. 12.
3
Full-bridge resonant
[14-17]
Fig. 13.
4
qZS-based
[18][19]
Fig. 14.
5
Current fed half-bridge
[20]
Fig. 15.
6
Current fed full-bridge
[21]
Fig. 16.
Fig. 9. MLPE classification.
Cons: low transformer utilizing factor, two transformers are
required.
B. Half-Bridge Resonant Converter Topology:
Fig. 10. Multi-stage converter structure.
A. Interleaved Flyback Topology:
Fig. 12. Half-bridge resonant converter topology.
Fig. 11. Interleaved flyback converter topology.
Pros: only one switch in the inverter stage (two if active
clamping is used) thus it is easy to control and the conduction
losses
are
lowest
among
all
topologies.
Pros: Can operate over a wide input voltage and a wide
load range. Zero voltage and/or zero current switching
capability over entire operation range. Only one switch is
conducting at a specific time, so conduction losses are low.
High transformer utilizing factor
Cons: Limited output power due to the use of the capacitor
driven “virtual middle point”.
149
F. Current Fed Full-Bridge Converter Topology
C. Full-Bridge Resonant Converter Topology:
Pros: Same as Full-bridge resonant. The boost capability of
the inverter stage, thus lower requirements to the output filter
and higher efficiency at low input voltage.
Cons: Same as in the full-bridge resonant. Additional
diodes in the inverter stage limit the efficiency.
Fig. 13. Full-bridge resonant converter topology.
DC out
Pros: Same as in previous topology. High output power
capability.
Cons: Two switches conducting at a time, so conduction
losses are relatively high. Complex control algorithm.
Fig. 16. Current fed full-bridge converter topology.
D. qZS-Based Converter Topology:
DC out
Fig. 14. qZS-based converter topology.
Pros: The boost capability of the inverter stage, thus lower
requirements to the output filter and higher efficiency at the
low input voltage.
Cons: Additional diode in the inverter stage limits the
efficiency. Additional capacitors and inductors in the inverter
stage, lower system stability and requires complex analysis
during control stage development.
As seen in Fig. 14-16, qZS-based and current fed
topologies have additional inductor in the invertor stage. This
inductance acts like the energy storage tank and allows
additional voltage level step-up without the use of the
additional switching stages. This additional step-up comes in
handy when the PV-module solar irradiation is lower than
nominal and so its output voltage is lower. In the previous
topologies to ensure high efficiency in the whole range it is
required to calculate the transformer to provide the desired
output voltage when the input voltage low and reduce the
duty cycle when the PV-module output voltage is nominal . In
topologies with the voltage boost capability transformer ratio
is selected based on operation at the rated voltage and when
the PV-module voltage falls – the boost capability of the
inverter stage is used. This ensures high efficiency of the
converter at low input voltages without compromising
efficiency at rated conditions.
III. CONCLUSIONS
Several multi-stage power converter topologies that can be
used in micro-converters were discussed. Each topology has
its advantages and disadvantages, and each has a way to
further performance improvement. For PV module level
applications the most promising are qZS-based and current
fed topologies, because they have “flat” efficiency curve in
the whole range of operation, thus achieving high integral
efficiency coefficient (Euro efficiency). Drawbacks of this
topologies can be relatively easy compensated by use of the
new control algorithms, active switches instead of diodes and
by the use of the modern semiconductor devices with high
switching frequencies and low on-state resistances.
E. Current Fed Half-Bridge Converter Topology:
Fig. 15. Current fed half-bridge converter topology.
Pros: Same as half-bridge resonant. The boost capability of
the inverter stage, thus lower requirements to the output filter
and higher efficiency at low input voltage.
Cons: Same as in the Full-bridge resonant.
ACKNOWLEDGMENT
Publication of this paper has been supported by European
Social Fund (project “Doctoral School of Energy and
Geotechnology II”).
150
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