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Operation of Photovoltaic Power Systems
with Energy Storage
Bernard J. SzymanskiELPOL
Electronics and Automation Center
www.elpol.biz
ul. Wrzosowa 10/1, 26-600 Radom
Email: [email protected]
Lukasz RoslaniecInstitute of Electrical Power Engineering
Warsaw University of Technology
Plac Politechniki 1, 00-661 Warsaw
Email: [email protected]
Antoni DmowskiInstitute of Electrical Power Engineering
Warsaw University of Technology
Plac Politechniki 1, 00-661 Warsaw
Email: [email protected]
Kamil KompaInstitute of Information Technology
Warsaw University of Technology
Nowowiejska 15/19, 00-665 Warsaw
Email: [email protected]
Jerzy SzymanskiFaculty of Transport and Electrical Engineering
Radom University of Technology
Malczewskiego 29, 26-600 Radom
Email: [email protected]
AbstractPhotovoltaic (PV) power systems are described inthe article. Power generated from renewable energy sourcesis not stable because of the energy fluctuations caused mainlyby atmospheric conditions. Therefore, in order to improve theparameters and stability of the power system, PV power plantsshould cooperate with electrical energy storage system such aselectrochemical batteries. Moreover, because of the reason thatPV power plants are often located on the terrain which isaccessible by the man, galvanic isolation may be obligatory. Sincethe efficiency of the power conversion in PV power systems has tobe maximized, soft switched resonant power converters, in whichswitching losses are minimized, are utilized in such PV powersystems. Moreover, grid-connected PV inverter which is a crucialelement of the PV power system is presented in the article.
Index TermsSolar power generation, photovoltaic systems,resonant inverters, energy storage
I. INTRODUCTION
Solar energy is a basic energy form among all energy
sources and was processed by means of bioorganic processes
to high concentration form of fossil fuels which are nowadays
used as the main energy source. Figure 1 presents the com-
parison between annual renewable solar energy emitted on the
surface of earth and total available primary energy resources
[1] [2]. Figure 1 shows that annual energy emitted on the
earths surface is much higher than total energy which can be
achieved from all conventional energy sources.
Currently we can convert the solar energy by means ofrenewable energy technologies such as solar thermal power
plants, photovoltaic (PV) sun concentrators and photovoltaic
power systems [3].
The energy conversion in a solar thermal plant starts with
collecting the sunlight, converting it into heat which is then
powering a thermodynamic engine [3]. In the last stage the
engine drives a generator which produces the electricity. The
heat conversion path is similar to any other conventional fossil
or nuclear power plant. The example of such power plant
Fig. 1: World primary energy resources [2]
in Europe is a Sevilles solar power tower located in Spain.
However, the biggest solar thermal power plant is planned to
be founded on the Sahara desert in Africa. The project has the
name DESERTEC [4] and was officially started in July 2009
by consortium of European companies. Produced electricity
is going be transmitted to European and African countries by
means of high voltage DC lines.
Sun concentrators which use lenses or mirrors to concentrate
the sunlight onto PV cells are considered as well. This, in turn,
allows to reduce the cell area which is required for producing
a given amount of power. However, it has turned out, that this
is very difficult [5] in practice.Power plants which use the sun light in order to produce
electricity directly, by means of photovoltaic (PV) cells, are the
next group. Here, sunlight is converted into electrical current
in the process of excitation of electrons in the semiconductor
junction. Standard PV cells have about 10% efficiency, which
is decreasing over the time. Modern technologies such as
the multijunction PV cell or the organic PV cell are used
to increase power conversion efficiency. In the PV systems,
solar energy converted into electricity is fed into the utility
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grid by means of power electronics converter. Nowadays,
the rapid development of PV installations can be observed.
At present, this manner of electrical energy production is
used especially in residential areas as the building integrated
PV power systems. However, large scale (>200 kWp) grid-
connected PV power plants exist as well. The PV power plant
in Braindis (Germany) with 40 MWp or PV power plant
in Puertollano (Spain) with 47 MWp can serve here as an
example [6].
The growth of new PV installations in Europe is mainly
caused by introduction of feed-in tariff [7]. The introduction
of feed-in tariff was the main cause of installation of addi-
tional 2500 MW and 1500 MW photovoltaic power in Spain
and Germany respectively in 2008. Simultaneously in Czech
Republic which also introduced the feed-in tariff, the increase
of 50 MW of new installed photovoltaic power was observed
in 2008. It can be expected that the introduction of the feed-in
tariff in other countries will lead to the same results.
In 2008, approximately 3.8 GWp was gained from large
scale (>200 kWp) photovoltaic power plants [6], whereas in
2006 it was approximately 500 MWp. Since the rated powerof a single photovoltaic power converter has started to exceed
1 MVA, this technology should be taken into consideration as
an alternative to fossil fuels.
I I . PHOTOVOLTAIC POWER SYSTEMS
The photovoltaic power systems can be divided into two
main groups:
Stand-alone PV systems
Grid-connected PV systems
Stand-alone PV systems are in the power range up to several
kW. These systems have no connection to the electrical grid
and are used to supply local loads. Such a solution is mainly
used when the cost of connecting particular localization to thegrid is larger than the cost of the PV power system.
On the other hand, grid-connected PV systems are con-
nected to the electrical grid by means of suitable power elec-
tronics inverter which converts the DC power produced by the
PV cells into alternating current (AC), which is synchronized
with the utility grid. This allows to sell the produced energy
to other users connected to the grid.
Since the PV array is the most expensive element of the
whole PV system, the power extracted from PV array should
be maximized. Therefore, the power converter which serves
as an interface between a PV array and an utility grid has to
track maximal power point (MPP) [8] of PV array.
III. ENERGY FLUCTUATIONS
The energy which comes from the PV power plants is not
stable because it depends on the weather conditions and the
time of the day. The generated current fluctuations in case of
PV and wind power plants are depicted in Figure 2. Therefore,
high power PV plants connected to the utility grid can have
influence on the parameters and stability of the power system.
It can be especially critical in case of high power PV plants
which are installed far away for main power supply point.
Instability of PV power plants can cause the flicker effect [9].
This disturbance is dangerous to the electrical motors supplied
from the utility grid [10] [11]. Described problem can be
minimized by means of proper control of maximum power
output of the PV power plant. Unfortunately, this solution
leads to decrease of efficiency of the PV power plant.
If the PV power plant is connected to the node with stiff
voltage and frequency parameters, than the influence of the
PV power plant on the power system can the minimized by
means of proper amount of ready reserve. Nevertheless, this
solution causes the decrease of the efficiency of power system
because of the utilization of high value of ready reserve and
transport losses.
Usually maintaining system stability (in case of large PV
power plants) requires the continuous contact of PV in-
stallations control system with the local electrical energy
distributor.
Fig. 2: Current generated from photovoltaic (blue) and wind
(red) power plants [12]
IV. PHOTOVOLTAIC POWER SYSTEM WITH ENERGY
STORAGE
The disadvantages which were described above can be
limited to a large extent or even eliminated if the PV power
plants installations are connected with energy storage system.
There are many technical solutions which can be utilized
as the storage elements (e.g. flywheels, supermagnetic coils,
capactiors or electrochemical storage elements [13] [14]). In
case of PV power systems, the best solution is an utilization
of electrochemical storage element. Several types of batteriesare currently available, i.e. lead-acid, nickel-cadium, zinc-
bromide, zinc-chloride, sodium-sulphur , nickel-hydrogen, re-
dox and vanadium batteries. Nowadays, the development of
cost effective electrical energy storage element is one of the
main challenges.
The PV power system with battery storage element should
fulfill following operation modes:
Provide the electrical energy to the utility grid from the
PV generators.
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Fig. 3: Galvanic isolation realized by means of 50 Hz trans-
former
Fig. 4: Galvanic isolation realized by means of high-frequency
transformer
Provide the electrical energy to the utility grid from the
PV generators and battery storage system.
Provide the electrical energy to the utility grid only from
the battery storage system.
Charge the battery storage system from the utility gridwith the excess of the electrical energy produced by other
sources.
Provide a battery backup operation in case when there is
fault in the utility grid and important receivers need to
have power supply.
V. GALVANIC SEPARATION
For the reason that high power PV power plants are mainly
located on the terrain which is accessible for the man, it is
advantageous when PV array is isolated from the utility grid.
The galvanic isolation from the utility grid can be realized
by means of 50 Hz isolating transformer (old solutions).
Such a transformer is located on the grid side (Figure 3).It is also possible to isolate PV array by means of the
modern high-frequency transformer, which is a part of the
PV DC-to-AC converter (Figure 4). The solution with high-
frequency transformer is advantageous in respect to the low-
frequency transformer because of:
much smaller dimensions
higher power density
higher efficiency
If the PV power plant with energy storage system is fully
dispositional, then it helps the grid operator in control of the
power system [15] [16]. In Figure 5, an example of such
a PV power plant is shown. It can be seen that the system
consists of several blocks, i.e: PV array, DC-to-DC converterwhere maximal power point tracking (MPPT) is implemented,
DC-to-AC converter which feeds the energy to the utility grid,
AC-to-DC and DC-to-DC converters which are responsible for
management of battery storage system. The control unit, which
allows to control the power converters according to the grid
operator commands, is crucial. In such a system, the galvanic
isolation is realized by means of high-frequency transformers.
System presented in figure 5 utilizes separate active front
end (gridconnected) converters in the power production and
power storage units. This allows to maintain these units
separately. In case of low power installation, it may be more
cost effective to implement other power flow paths and in
result to minimize power losses and number of converters.
Power plant topology should be always selected according to
particular application.
GRID
OPERATOR
GRID
IMPORTANT
RECEIVERS
PVDC
DC
BATTERY
STORAGE
SYSTEM
DC
DC
CONTROL
UNIT
DC
AC
DC
AC
AC
DC
MPPT
DC
AC
DC
AC
AC
DC
Fig. 5: Fully dispositional PV power plant
VI . RESONANT CONVERTERS IN PHOTOVOLTAIC POWER
SYSTEMS
Nowadays, efficiency and density of processed power is a
crucial factor in case of power converters. In order to decrease
power losses, as well as the volume and weight of the power
converter (increase the processed power density), high switch-
ing frequency of power transistors is used. High frequency
operation results in reduced size and weight of high power
magnetic components (e.g. separation transformers). In case
of hard switching converter, high switching frequency would
result in a very high switching losses. However, utilization
of soft switching methods allows to reduce switching losses
significantly. Thus, high-frequency soft-switching converter ismuch more efficient than typical low-frequency hard-switching
converter. High power DC-to-DC converters are realized as the
multi-phase resonant converters (e.g. three-phase).
The usage example of multiphase series resonant DC-to-DC
converter [17] is presented in Figure 6. In this situation, each
PV panel has its own integrated converter where Maximal
Power Point (MPP) tracking is implemented. Thus, the power
produced by the PV system is maximized. These DC-to-DC
converters are connected in parallel to the single, three-phase
resonant converter which in turn serves as DC-to-DC transfor-
mer providing galvanic isolation and proper DC voltage level
to the DC-to-AC converter.
An example of the topology of the three-phase DC-to-DCresonant power converter is presented in Figure 7. The power
converter has a high frequency isolating transformer and uses
series resonannt circuit in order to convert the energy and
maintain soft-switching of power transistors.
The presented topology, along with unique control algo-
rithm, allows to charge batteries and maintain soft-switching
in the entire operation range. The power converter is con-
trolled by means of frequency and pulse density modulation
techniques [18]. This is depicted in Figure 9.
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Fig. 6: Multiphase series resonant converter in stand alone PV
power system [17]
Bold red and green curves show the battery charging process
(two different possibilities of current and voltage changes on
the converters output over the time). Resonant frequency of
the converters tank circuit is 100kHz. Curves tagged F=
150 kHz and F= 200 kHz show converters output current and
voltage dependences at the particular switching frequency. In
the region below the 200 kHz curve, it is possible to choose
any current level by changing operating frequency (frequency
control mode). In this mode there soft-switching is maintained.
In the presented example, highest possible converters
switching frequency is 200 kHz. Thus, to get low value
of the current on the converters output (especially whenthe voltage is low) it is necessary to utilize pulse density
control method. This method assumes transport of energy in
pulses and therefore any mean current value may be achieved.
Moreover, when the energy is transported in the described
manner, value of current in the resonant tank circuit is always
high enough to maintain soft-switching (ZVS) while power
transistors are operating. Choke on the output of the converter
allows to keep constant current in the battery during pulse
density control.
The 8 kW prototype of described converter was built and
tested. Figure 8 presents experimental results, i.e. the resonant
tank voltage uR (yellow) and current iR (blue). In this case the
resonant current and voltage waveforms have the frequencyof:
f 200 kHz. (1)
Inductors in the resonant tank circuit are realized in form
of leakage inductance of the transformer and small external
chokes. Patent is pending for the described battery charging
method.
VII. BIDIRECTIONAL CONVERTERS IN PHOTOVOLTAIC
POWER SYSTEMS
Cost of the converter, as well as power losses, may be
minimized by integration of power conversions electronics
of the PV power plant and the storage system.
VOUTC
O
LO
TCR
VIN
DO
Fig. 7: Three-phase resonant power converter
Fig. 8: Experimental results - frequency control of three-phase
resonant DC-to-DC converter
Since bidirectional power flow between battery and powersystem is necessary, as well as low loss battery charging from
the PV array, the possible usage of bidirectional resonant
power converters in such systems is an issue which needs
further investigation.
The example of the bidirectional DC-to-DC series resonant
converter is presented in Figure 10 [19]. When bidirectional
power converters are utilized, the topology of the PV power
system depicted in Figure 11 arises. The presented power
plant has two bidirectional converters i.e.: DC-to-DC converter
which cooperates with battery storage system and DC-to-AC
converter which couples PV power plant with utility grid.
U [V]OUT
I [A]OUT
PDMControl
InductanceT ermalLimith
20
D
OutputVoltageHysteresis
240 OFF
ONPDM
F=150k
Hz
280150kHz&200kHzCurvesat565VinDC-Link
F=
0kHz
20
Hard-Switching(EnergyLosses)
BatteryLoadingCurve
Freque cyControlLimit
n
C
30
F enControl
requ cy
B
A
0
Fig. 9: Output characteristic of the three-phase resonant power
converter
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Fig. 10: Bidirectional DC-to-DC series resonant converter
VIII. GRI D-CONNECTED PHOTOVOLATIC INVE RTERS FOR
ENERGY TRANSMISSION
Another crucial part of every grid-connected PV power plant
is a grid-connected inverter which is transmitting energy from
a DC link to the electrical power system. Typically voltage
source inverters (VSI) are used for cooperation with the grid.
Such an inverter has to be properly built and it has to use
modern control techniques to maintain its current quality under
versatile normalized disturbances on the grid side. Control and
construction of such converters is a wide subject. Properly
designed modern inverters are able to fulfill very restrict
requirements.
The grid-connected converter is able to operate as an active
compensator during energy transmission to the grid. This
allows to maintain voltage level in proper range in the place
where inverter is connected. Polish lowvoltage network may
have problems with rapidly growing capacity factor of PV
systems. Production of reactive power by PV power plants
can decrease losses related to reactive power transmission in
the grid.
Typical construction of a singlephase system is shown in
Figure 12 and its simulation model is depicted in Figure 13. Itconsists of DC-side capacitance, IGBT full bridge converter,
grid-side LCL filter, surge arrester, as well as diverse current
and voltage sensors and appropriate control system.
PV
DC
AC
DC
DC
GRID
IMPORTANT
RECEIVERS
BIDIRECTINAL
POWERFLOW
CONTROL
UNIT
GRID
OPERATOR
BATTERY
STORAGE
SYSTEM
Fig. 11: Bidirectional DC-to-DC series resonant converter
Very significant to the system performance are the pa-
rameters of the LCL filter and EMI filters. Electromagnetic
emission, current disturbances and power losses in the system
are minimized by proper design of those filters and the control
algorithm. Control algorithms behavior should be investigated
for wide range of normalized grid disturbances. High power
grid simulators are used in such experiments.
Most of modern grid-connected converters used in PV
systems are hard-switching converters, which use fully con-
trollable power switches such as MOSFETs and IGBTs and
generally use pulse width modulation (PWM) in order to
produce the AC output.
Fig. 12: Single-phase voltage source inverter
Fig. 13: Simulation model of single-phase voltage source
inverter
I X. SUMMARY
In the article issues concerning generation of electrical
energy from PV power plants are described. The difference
between stand alone PV power system and grid connected
power system is explained. Moreover, PV power systems with
energy storage are described, along with the function which
such a system has to fulfill. The issue of galvanic isolation and
resonant power converters utilization in PV power system is
introduced. Furthermore, grid-connected power plant concept
is presented and explained.
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X. ACKNOWLEDGMENTS
This work has been supported from the Grant N N510
325537 of the polish Ministry of Science and Higher Edu-
cation.
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