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Trends of power Electronics on Renewable

Energy systemS.Santhosh ,M.Iyappan

Department of electrical and electronics engineering

Arunai engineering college,TiruvannamalaiEmail id:iyps036@gmail.com,salemsanthosheee@gmailcom

Abstract The paper focuses on the power electronics usedin renewable energy systems and especially in wind and

photovoltaic (PV) applications. During the last years there was abroad development in the field of power electronics which led tomore efficient systems and reduction of the cost per installed kW.

The inverters became more efficient and reached efficiencies inexcess of 98%, while commercial solar modules reached almost17% efficiency. Furthermore, the wind turbines use inverters

of improved efficiency, reliability and regulation capabilities. In

this paper the recent trends of power electronics topologies used insuch systems are presented.

KeywordsInclude at least 5 keywords or phrases1. Introduction

The Kyoto agreement renewed interest in renewable energysystems worldwide. Many renewable energy technologies today arewell developed, reliable, and cost competitive with conventional

generators. The cost of renewable energy technologies is on afalling trend and is expected to fall further as demand andproduction increases [1], [4]. There are many renewable energysources such as biomass, solar, wind, mini-hydro, and tidal power

[6]. Power electronics find applications in most RES technologies,solar and wind energy systems being the most importantapplications. During the last years, there is a constant effort toimprove each part of a photovoltaic (PV) and wind turbine (WT)application. The efficiency of commercial PV modules nowexceeds 17%, inverters have reached almost 99% European

efficiency and there are new topologies found which make WTsmore efficient and flexible in their operation. Due to the increaseddemand, each manufacturer is trying to find new concepts in orderto achieve better system yield, which results in increased economicreturns for the investor. Most of the systems used in such

applications produce DC current, so inverters are required toconvert this power to AC, which is needed in most applications anddefinitely for grid connection. There are two types of PV systems:stand alone and grid connected. The first is used on remotelocations, where the utility grid is not present. The grid connected

systems inject power and energy directly to the utility grid. Thesesystems have different structure and the inverters which are usedhave different methods to synchronize and produce clean AC power.

2. Power Electronics for PV applicationsThe PV modules and the power electronics that convert the

produced electric power by the PV modules are the basicparts of a PV installation. The PV modules comprise severalsolar cells which convert the energy of the sunlight directlyinto electricity, and are connected in a proper way (typically

in series), to provide desired levels of DC current and

voltage. They produce electricity due to a quantum-mechanic process known as the photovoltaic effect [1]. Apresentation of this conversion is shown on Figure 1a.There are many semiconductor materials suitable for solar

cells manufacturing. The most commonly used aremonocrystalline Si cells, polycrystalline Si cells andamorphous Si cells, although several other thin filmtechnologies exist in the market. The efficiency of

monocrystalline Si cells is almost 17%, for polycrystalline

cells reaches almost 15%, while an efficiency of 10% is

achieved in the case of amorphous Si PV cells. All PVmodules have a typical current-voltage characteristic curve,used to make all necessary calculations, as shown on Figure

1b [4].

Figure 1a: Photovoltaic effect

PV module curve

Off-grid PV systems are used in cases, where the grid is not

present and the use of batteries to store energy is required,in order to cover the demand during the night or wheneverenergy is needed. Blocking diodes are used to prevent the

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batteries to discharge on the modules during the night, whilethey also protect the batteries from short circuit. If more

than one string is used, they also provide over-currentprotection of the strings in case of short circuits. Chargeregulators control the charging of the batteries [1], [6]. Inoff-grid systems, there is the need to use dc voltage and

current with stable characteristics, independent fromirradiance fluctuations. Therefore, a DC DC conversiontopology is used. Switch mode DCDC converters [1] are

used to match the dc output of a PV generator to a variableload. Three different topologies are mostly used; step downconverters, step up converters and a combination of thesetwo. In Figure 2 simplified diagrams of these threetopologies are presented.

Simplified diagrams of DC/DC converter topologies.

In order to maximize the performance of the string, in most

charge regulators a maximum power point tracker (MPPT)

controller is used. The MPPT applies heuristic algorithms totrack the array voltage which results in maximum power,given a solar irradiance level. The efficiency of modern MPPTs

is between 92-97%, getting a typical 20-45% power gain inwinter and a 10-15% in summer. Actual gain can vary widelydepending on temperature, battery state of charge, and

other factors.The MPPT is often performed via a high

frequency DC to DC converter. Its input is the output of the

solar panels strings. It converts the DC input to high frequencyAC, and then back to a different DC voltage and current in orderto match the panel voltage to that of the batteries. MPPTs

operate at very high (audio range) frequencies, usually in the 20-80 kHz range. The advantage of high frequency circuits is that

they can be designed with high efficiency and small volumetransformers and other components. The design of high

frequency circuits can be a very difficult task because ofEMI/EMC considerations (e.g. problems with circuit parts that

act as antennas, causing radio and TV interference). Noiseisolation and suppression issues become very important [6],

[4].Inverters convert DC to AC. In off-grid systems, stand alone

self-commutated inverters producing AC current withoutsynchronisation with a reference signal are used. These invertershave the responsibility to produce AC voltage and current

characteristics (sinusoidal 230V/50Hz) same as those of a

typical grid in order to supply off-grid loads. Otherwise, theinverter is not suitable for most electronic devices [1]. Severaldifferent semiconductor devices such as MOSFETs and IGBTsare used. The first are used in units up to 3KW, because they

have the advantage of low switching losses at higher frequencies.At higher voltages and powers IGBTs are used [2]. Theseinverters can be single phase or three phases. A commonswitching technique in order to eliminate higher frequencies is

the SPWM method. A general layout of a single phase inverterwith half bridge and full bridge topology is shown in figure 3a,b. In the half bridge topology, the two switches S1 and S2,

the capacitors C1 and C2 are connected in series with the dc

source (batteries). The center point between the two capacitors isat mid-potential. The voltage across each capacitor is VDC /

2. The switches S1 and S2 switch on and off periodically

to produce the ac voltage. A filter (Lf and Cf) is used to

reduce high switching frequency components and to producesinusoidal output from the inverter.

Single phase half bridge inverter

Single phase full bridge inverter

The output of the inverter is connected to the load through a

transformer. The full bridge inverter has a similar function, but theoutput voltage is higher than the half bridge inverter. In figure 4 we

can also see the layout of a three phase stand alone inverter [3], [4].

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Three phase inverterIn grid-connected applications the energy is provided directly

to the grid and the necessary parts are the PV modules and theinverters. This reduces the cost of the system and it also reducesthe necessary maintenance, as the batteries are the most

maintenance-demanding components. The inverters for grid

connected applications may have different topology andoperation than off-grid ones. They have to produce excellentquality sine wave output, follow the frequency and voltage ofthe grid and extract maximum power from the PV modules

through the MPPT. The inverter input scans the IV curve of thestring until the maximum power point is found. [2], [1] The gridinverter always monitors the grid and the output voltage andfrequency must be controlled. The most common modulation is

the PWM modulation and operates at a range of 2 to 20 KHz [1].Grid connected inverters are classified as voltage source

inverters (VSI) and current source inverters (CSI). However, inPV applications VSI inverters are used. The layout of the VSI

topology is shown in Figure 5.

VSI inverterDuring the years, the inverter topologies and the technology ofpower electronics has improved. Until some years ago, the commonpractice was to use central inverters for most PV applications.

The PV modules were divided into series connections (calledstrings), each generating a sufficiently high voltage to avoid furtheramplification. Then all the strings were connected in parallel

through string diodes in order to reach high power levels. The useof central inverter has many drawbacks such as MPPT power losses,losses from differentiations between the modules and high voltageDC cables from the PV panels to the inverter. In the beginning, linecommutated inverters using thyristors were applied, characterized by

poor harmonic performance. Today, the most popular inverter

topology is the string inverter. One or two strings of crystallinemodules are connected to each inverter which has its own MPPtracker and the power losses are significantly lower [11]. The use

of more than one MPP trackers in the power plant is necessary inthe case of different module orientation and shading. This

maximizes each strings I-V curve power output [10]. From tests

that have been performed on string inverters, the developmentthrough the years is obvious. According to [H. Haeberlin,Berner Fachhochshule], the inverter efficiency in 1988 was in the

order of 85.5 90%, in the mid 90s was increased to 90 92%

and nowadays it has reached 98% [12]. The most popular stringinverter is the transformerless one, because the transformers thatoperated at grid frequencies are bulky, expensive and cause losses.Furthermore, the transformers impose limitations in the control

of grid current by the inverter. Especially at low load, the reactivepower for the magnetization of the transformer leads to a lowerpower factor [10]. Another important factor that affects the systemdesign optimization is the maximum input voltage (Vmax-in) of the

inverters as well as their input voltage (Vin) bandwidth. These two

characteristics (Vmax-in and Vin) are getting wider and still rise, a

fact that allows the designer to perform more efficient and flexiblecombinations in order to achieve the desired power. The Vmax-in

of the string inverters kept rising from 600V upt o 900V in2009, while the in 2010 inverters with Vmax-in=1000V allowing

even bigger strings are already in the market. The higher the Vmax-

in is, the less strings of more modules are used, so the losses are

further decreased as less cables are used [11].The IGBTs andMOSFETs with high pulsing frequencies provide improved power

quality in compliance with the regulations of the utility grid. Thehigh frequency used has led to the use of high frequencytransformers with lower weight. This fact reduced the total weight

of the inverters significantly (up to 20%). The todays string

inverters vary from 22 - 65 kg. The lower the weight is, theeasier the installation and the lower the transportation costsare.According to the power categorization, the string inverters arenow available in the range of 2 to 30 kWp. Until 2008, string

inverters were not produced at more than the 5kWp. Single phase

ones are used in PV stations of even 2MWp. However, a morerecent trend is the development of the three phase string inverters,at almost the same power range. In 2010 the first 3-phase inverters

became available in the market providing easier design andelectrical connections , as well as a completely symmetric power

output, an important factor for the utility grid operator.Themultistring inverter is a development of the string inverter. Acombination of strings are connected to separate DC/DC converters

and then to a common DC/AC converter. This is beneficial incomparison to the central inverter because each string iscontrolled individually. This results to higher efficiency andflexibility of the system [11], [9].Central inverters are used in

larger scale applications, offering Operation and Maintenancecontracts for the plant owners. The operation availability ofsuch inverters is warranted up to 99% throughout a complete year ofoperation.Until 2008 the power range of the central inverters was

from 100kW but not more than 500kW. However, the Invertermanufacturers developed larger inverters mostly using scalable

techniques up to 1,25MWp.The efficiency of Central Inverters hasclimbed from 92% since the 1990s to 98.8% in 2010. Their main

advantage is their high reliability providing maximumoperational life. Recently, the trend is to offer Central StationInverters (CSI), which include the house, the transformer, themedium voltage switchgear, the monitoring system, the cooling

concept and the wiring and they arrive on the installation pre-

assembled, which minimizes all the required tasks andconnections.Such inverters are used mostly in PV parks higher than2MWp, however lower size installations are also common.

3. Power Electronics for WT applications

There are two types of wind turbines (WT), the horizontal

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axis and the vertical axis. They are both shown on Figure6. However, the horizontal axis WT is by far the most

popular design [4].

Horizontal axis WT

Vertical axis WTA large number of designs are available, ranging from 50W up

to 7MW size. The number of blades can vary but the most

commonly seen are with 2 or 3 blades. There are three types ofwind power systems, the stand alone, the hybrid and the grid

connected.The stand alone ones are mostly used for householdapplications and require batteries in order to store the producedenergy and an inverter to convert to AC current. This system

requires the use of a charge regulator which will feed thepower from the wind generator to the battery bank in a controlledmanner. The most common turbine which is used in such

applications is the permanent magnet generator and the charging

control is made through controlled rectifiers. The charge regulatorshould be programmed to limit the current into the batteries, toreduce current when the batteries are charged and to maintain atrickle charge during full of charge periods [1], [4]. The hybrid

systems could also include other renewable sources such as PV

systems or even a diesel generator and feed the energy to ahousehold or even to the grid.The grid connected WT areconnected to the utility grid either directly or through power

electronics, feeding the produced energy to the grid. On this typeof WT all manufacturers are trying to increase the size andefficiency of the machines. Many studies have been made on the

speed control part and on ways to reduce the cost of the unit.

There are several types of inverters which are used on wind turbineinstallations, such as PWMVSI converters and matrix converters.However the matrix converter is not widely used. The back toback PWM VSI is a bi-directional power converter consisting of

two PWM VSI inverters. The topology of this inverter isshown in Figure 7a.To achieve full control of the grid current, the

DC link voltage must be boosted to a level higher than theamplitude of the grid line voltage. The power flow of the grid sideconverter is controlled in order to keep the DC link voltage

constant, while the control of the generator is set to suit the

magnetization demand and the reference speed.The matrix inverterscan efficiently convert the three phase electrical output of the WTto the requested electrical grid characteristics for a properconnection. They use an array of controlled bidirectional

switches to convert AC power from one frequency to another.They produce a variable output voltage with unrestrictedfrequency. The privilege compared to other topologies is that thematrix converters do not have a dc link circuit and they do not use

large energy storage elements. A figure of the matrix inverter isshown in Figure 7b where the switching devices, the input filter

and the clamp circuit are presented

Back to back PWM - VSI converter

Matrix converter

MOSFETs for low power and IGBTs for higher power enablethe implementation of bidirectional switches, which makethe inverter topology very attractive for AC powerhandling.The input filter minimizes the high frequency

components in the input currents and reduces the disturbances

of input power. The input filter can be designed with acombination of inductors and capacitors with paralleldamping resistors where on the other hand the clamp circuit

provides overvoltage protection and uses fast recovery diodes.The general operation of the inverter is to convert the AC

current of the WT to the AC current that the utility griddemands. One example of this taken from L.H.Hansen is the

conversion of 300 Hz current coming from the WT to 60Hzwhich is the grid frequency for American utility. A 7200 Hzswitching frequency is applied on the switches and the

result of the conversion is shown on Figure 8. There is a

duty cycle factor that can be adjusted to regulate the ratio ofoutput to input voltage, up to a maximum value. Finally, theoutput is passed through a filter to eliminate high frequencyharmonics [8].

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Matrix convertersteady state simulations

The driving circuit of a typical IGBT is the same as MOSFET,

and the linkage capacitors between an IGBTs terminals arerather low. In PWM type rectifiers, when the switchingfrequency increases, the power loss becomes high during the

deactivation of the switching element and the commutation diode.

This case limits the usage of an IGBT with 50 kHz as theswitching element. Thus, in high frequency resonance-typeinverters, it can be practically used at the frequency of 250 kHz.For 50 kHz, the power loss during conduction of an IGBT is

approximately 6 watts where for 250 kHz it is approximately 0.8watts.Apart from the matrix converters there are also other three

suggested topologies, namely the tandem converter, the multilevelconverter and the resonant converter (NCC). The advantage of thematrix converter is that it consists only of active components on

the power part where the others consist of passive components too.The matrix converter can omit the transformer without requiringhigh voltage ratings for the components. However, on the

harmonic performance the best topology is the multilevel

converter which shows the best spectra on both the grid and

generator side. Finally, the NCC topology is the most efficientfollowed by the multilevel and the tandem converter. Furtherinformation can be found on [7].Apart from the above topologies,

what really makes the difference between the manufacturers is the

control concept. Constant research and development is beingperformed on this field in order to develop bettermodulation concepts and to eliminate harmonics, so as to

minimize the cost for the filters.A common application of thestatic converters is the switching to the grid of WT equipped withinduction generators (soft starting). Direct connection of the

WT to the grid causes high inrush currents which are

undesirable especially for weak grids, also severe torquepulsations and damage to the gearbox. For this reason the softstarter is used which regulates the applied stator voltages.Thecommutating devices are two anti-parallel thyristors per phase. The

relationship between the firing angle () and the resulting

amplification of the soft starter is non- linear and dependsadditionally on the power factor of the connected element. In thecase of a resistive load, may vary between 0 (full on) and 90

(full off) degrees, in the case of a purely inductive load between90, (full on) and 180 (full off) degrees. For any power factor

between 0 and 90 degrees, will be somewhere between thelimits sketched in Figure 9b.Usually, the turbine accelerates under

pitch-control to the synchronous speed through the wind poweralone and only then is it switched onto the grid. ACcontrollers have been also reported for the connection to the grid atzero speed and subsequent fast acceleration to the operating speed,

which is particularly useful in stall controlled WT [13].When thegenerator is connected to the grid a contactor bypasses the softstarter to reduce losses. This is also shown in figure 9a (Kbyp). It

is possible to utilize the controller in normal operation in order to

reduce the static voltage and resulting to reduce magnetizinglosses when operating under light wind, at the expense of highharmonic distortion [7].A recent trend in WT technology isthe use of multi-pole low speed generators which permit the

direct coupling of the turbine rotor to the electrical generator.This eliminates the gearbox, the interconnection axes and thecoupling and results to price and weight reduction and improvedreliability [13].

Soft starter

Control characteristic for a full controlled soft starter.

4. Conclusion

In this paper the main trends of the powerelectronics used in PV and WT applications are presented.

Due to the high demand for renewable energy sourcesapplications, there is a continuing research for

improving the total efficiency of these applications andby improving each electronic part included. As far as thePV systems are concerned the inverters efficiency iscontinuously improving and ways to minimize the weight

of the devices are tested so as to decrease transportationcosts and ease the installation. Moreover, the power andvoltage range of the string and central inverters is

increased, so that more efficient and cheaper PVinstallations can be realized using a relatively low numberof inverters. The power electronics for WT systems are

subject to extensive R&D, especially about more efficientcontrol concepts and even more efficient converters.

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REFERENCES

[1] S. M. Metev and V. P. Veiko,Laser Assisted Microtechnology, 2nd ed.,R. M. Osgood, Jr., Ed. Berlin, Germany: Springer-Verlag, 1998.

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Statistics. Berlin, Germany: Springer, 1989, vol. 61.

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M. Wegmuller, J. P. von der Weid, P. Oberson, and N. Gisin, Highresolution fiber distributed measurements with coherent OFDR, inProc. ECOC00, 2000, paper 11.3.4, p. 109.

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[6] (2002) The IEEE website. [Online]. Available: http://www.ieee.org/[7] M. Shell. (2002) IEEEtran homepage on CTAN. [Online]. Available:

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[8] FLEXChip Signal Processor (MC68175/D), Motorola, 1996.[9] PDCA12-70 data sheet, Opto Speed SA, Mezzovico, Switzerland.[10] A. Karnik, Performance of TCP congestion control with rate feedback:

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