grupo 3 - trabalho 2

7/30/2019 Grupo 3 - Trabalho 2

1/6

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


Antoni DmowskiInstitute of Electrical Power Engineering


Plac Politechniki 1, 00-661 Warsaw


Kamil KompaInstitute of Information Technology


Nowowiejska 15/19, 00-665 Warsaw


Jerzy SzymanskiFaculty of Transport and Electrical Engineering

Radom University of Technology

Malczewskiego 29, 26-600 Radom


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

978-1-4244-8807-0/11/$26.00 2011 IEEE86


2/6

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.

87


3/6

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.

88


4/6

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

89


5/6

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.

90


6/6

X. ACKNOWLEDGMENTS

This work has been supported from the Grant N N510

325537 of the polish Ministry of Science and Higher Edu-

cation.

REFERENCES

[1] M. Bartosik, Globalny kryzys energetyczny - mit czy rzeczywistosc ?Proceedings of 10th International Conference Nowoczesne Urzadzenia

zasilajace w energetyce, 14-16 March, Zakopane, Poland, 2007, 2007.[2] Survey of energy resources. World energy council, 2004.[3] K. Heinloth, Energy Technologies. Springer Verlag, 2006.[4] D. Foundation, Red Paper - An overview of Desertec Concept - 2nd

Edition. Desertec Foundation, 2009.[5] A. Luque and S. Hegedus, Handbook of Photovoltaic Science and

Engineering. John Wiley & Sons Ltd, 2003.[6] D. Lenardic, Large scale PV power plants - Annual and cumulative

installed power output capacity - Key statistical indicators. AnnualReview 2008, 2008.

[7] S. Pietruszko, Taryfa stala (Feed-in Tariff) motorem rozwoju odnawial-nych zr ode energii. Centre for Photovoltaics of the Warsaw Universityof Technology, 2009.

[8] F. Blaabjerg, F. Iov, R. Teodorescu, and Z. Chen, Power electronicsin renewable energy systems. Proceedings of 12th International PowerElectronics and Motion Control Conference (EPE), 2006.

[9] R. Albarracin and H. Amaris, Power Quality in distribution power

networks with photovoltaic energy Sources. Proceedings of InternationalConference on Environment and Electrical Engineering, 10-13 May,Karpacz, Poland, 2009.

[10] A. Bien and A. Rozkrut, A measurement scale for the light flickeringphenomenon. 6th International Conference, Electrical Power Qualityand Utilisation, 19-21 September, 2001.

[11] A. Bien and Z. Hanzelka, Power Quality Application Guide. VoltageDisturbances.Flicker Measurement. Copper Development Association,October, 2005.

[12] P. Biczel, Optymalne wykorzystanie pierwotnych nosnik ow energii naprzykadzie hybrydowej elektrowni sonecznej z ogniwami paliwowymi.PhD thesis, Warsaw University of Technology, 2003.

[13] M. Rashid, Energy Technologies. Elsevier Inc. 2nd Edition, 2007.[14] D. Sauer, T. Blank, J. Kowal, and D. Magnor, Energy Storage Technolo-

gies for Grids With High Penetration of Renewable Energies and forGrid Connected PV Systems. 23rd European Photovoltaic Solar EnergyConference,1-5 September, Valecia, Spain, 2008.

[15] A. Dmowski, B. Szymanski, and L. Rosaniec, Photovoltaic powerplants as an alternative to conventional power generating systems,

Aktualne Problemy w Elektroenergetyce Conference Proceedings, Jurata3-5 June,Poland, 2009.

[16] B. Szymanski and A. Dmowski, Battery charging system in photovoltaicapplication. X International PhD workshop OWD 2008, 18-21 October2008, Wisa, Poland.

[17] J. Jacobs, Multi-Phase Series Resonant DC-to-DC Converters.Aachen,Germany, RWTH Aachen University, Aachener Beitraege desISEA Band 42, PhD Thesis, 2006.

[18] J. Matysik, Metody sterowania integracyjnego tranzystorowychfalownik ow napiecia klasy D z szeregowym obowodem rezonansowym.Prace naukowe elektryka, zeszyt 114, Oficyna wydawnicza PolitechnikiWarszawskiej, Warszawa 2001.

[19] F. Krismer, J. Biela, and K. J, A comparative evaluation of isolatedbi-directional dc-to-dc converters with wide input and output voltagerange, Fourtieth IAS Annual Meeting Industry Applications Conference

Conference Record of the 2005, Vol.1, pp. 599-606.

91

grupo 3 - trabalho 2

Documents