Simulink Models of Power Electronic Convertersfor DC Microgrid SimulationPiotr Biczel

Warsaw University of TechnologyInstitute of Electrical Power Engineering

ul. Koszykowa 75, 00-662 Warszawa, PolandEmail: biczel@ee.pw.edu.pl

Łukasz MichalskiWarsaw University of Technology

Institute of Electrical Power Engineeringul. Koszykowa 75, 00-662 Warszawa, Poland

Email: michalsl@ee.pw.edu.pl

Abstract—In the time of rapid changes in power industrythe need to develop new power systems arises. The newlydeveloped systems should aim at wide utilisation of renewablepower sources and CO2-neutral technologies. This goal can beachieved by e.g. dissipated generation, or integrating DG intomicrogrids. The microgrids can be built using AC or DC currentnetworks. Authors propose DC microgrid as a new idea for asmall, balanced distribution power system. It has been designedto supply small towns and villages with electricity with highpenetration of renewable energy resources. However, designing anoptimal microgrid is not an easy task, due to the fact that primaryenergy carriers are changeable and uncontrollable, as is thedemand. Traditional design and optimisation tools, developed forcontrolled power sources, cannot be employed here. Simulationmethods seem to be the best solution. The microgrid consists ofseveral devices, with most important among them - the powerelectronic devices.

Schematic simulation tools, like PSpice, PSIM or TCad, areusually used for power electronic converters simulation. Howeverthe above mentioned simulation method is slow and does notallow modeling the microgrid’s network. Therefore new models ofconverters have been developed. The authors employed Simulinkto simulate power flow and static voltage behaviour in themicrogrid. The models developed on the example of half bridgeconverter are presented in the paper, as is the comparison ofPSIM and authors’ models.

The conclusion from the research is that new static and powerflow models are interesting solutions for designing small powersystem simulation like the DC microgrid.

Index Terms—DC power systems, Power distribution, Powerelectronics, Power system simulation

I. INTRODUCTION

Nowadays new ways of power delivery need to be developed.Several methods of power generation exist. All of them have someadvantages and disadvantages. When we take into considerationworldwide fossil fuels resources, political problems and possiblescenarios [11] of further changes in power demand, dissipatedpower resources (DER) and dissipated generation systems (DG)seem to be the most attractive. Systems based on renewable powersources like solar irradiation, wind and water flow, biogas andother biofuels are particularly interesting. On the other hand,power generation based on DG sources allows power delivery tobe fossil fuels independent.

Still, some claim that the most important problems now arethe global warming and climate change. Although it is easy tofind periods in the history of Earth when climate was significantlywarmer, people almost all over the world worry about their futureexistence. Hence, politicians have established an aim for furtherdevelopment - climate protection.

The most important sources of pollution, which can have animpact upon climate, are warming gases like methane, freon andcarbon dioxide (CO2). The most problematic of them is CO2.Emission of either freon or methane can be easily reduced byimproving certain technologies, i.e. changing working media incooling systems or capturing gases from waste dumps. CO2 isreleased in the process of burning, which is the base of our today’spower generation. Approximately 30% of all global CO2 emissionis caused by heat power plants [9]. As a result, to solve theproblem of CO2 emission new technologies of power generationneed to be developed.

No one can imagine producing all the energy consumed allaround the world without burning fossil fuels. Therefore, inorder to reduce the emission of CO2 new burning technologies,like integrated gasification combined cycle (IGCC) and carboncapture and storage (CCS) are being developed [9].

Another way to decrease the emission of CO2 is developingCO2-neutral technologies. The goal is very hard to achieve inpower generation on mass scale. Still, it becomes much easier inthe case of DER and RES. Even now small scale power generationcan be CO2-neutral. However instability and unpredictability ofpower generation from solar irradiation or wind cause manyproblems with their integration to present power systems [7].

Another problem with integrating DER into modern powersystems is power quality control. DER are usually connectedto low voltage distribution networks, which can cause someproblems with voltage regulation and stabilization.

The third issue, by many recognized as the most important,is the economy of this solution. New DER systems seem to bemore expensive then huge power plants employed at present. As aconsequence, new systems have to be carefully designed to avoidsignificant increase in the cost of power.

Taking into consideration all the above mentioned points, theauthors have propose DC microgrid as a possible solution to theproblem [2]. However, if one wants to use the microgrid as anew solution for power generation and delivery, one has to takeinto consideration several issues, i.e.:• power plant capacity optimisation,• control strategy,• power generation and delivery costs optimisation,• storage system sizing.

As the result of the fact that power generation of RES isunpredictable and uncontrollable, there is no simple analyticalmethod for DC microgrid optimisation. A simulation methodshould be employed here, i.e. the microgrid simulation modeland models of all its components need to be developed. Amongthe components of the microgrid, power electronic converters areone of the most important. In this paper authors present theirown suggestion for their effective modelling.

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Fig. 1. Concept of DC microgrid

II. IDEA OF DC MICROGRID

One of the ideas of DC microgrid was presented duringprevious CPE conference [2]. Other suggested solutions of DCdistribution systems can be found in [1] [8].

Suffice it to say that a microgrid is a small power systemcontaining all subsystems present in a regular power generationsystem:• power plants,• storage system,• transmission (distribution) network,• control system,• loads.

Microgrid can operate connected to the power system or off-grid.All devices in the microgrid operate in a way which ensures thatload and demand are equal in all time. The microgrid’s networkhas typically only one voltage level - low voltage. Hence, voltageregulation is performed only by power sources and storagesystem. The network can be an AC or a DC system. Fig. 1presents the authors’ idea of DC microgrid.

Fig. 2. Typical structure of the power plant

Authors’ research focuses on power sources construction andoperation. Power plants used in the research are usually basedon solar and wind energy, fuel cells and combustion enginegenerators. Battery storage system can also be employed in themicrogrid. A general diagram of the power sources employablein a microgrid is shown in fig. 2.

III. POWER ELECTRONIC CONVERTERSIN DC MICROGRID

As can be seen in fig. 1 and 2, DC microgrid contains numerouspower electronic converters. From the point of view of applicationof power electronic devices, power sources used in the microgridcan be divided into [3]:• DC sources (solar batteries, fuel cells),• AC sources of variable frequency (turbines, some engine

generators),• AC sources of constant frequency (turbines, engine genera-

tors).

Separate category consists in devices for two way energy flow -storage systems and couplings with power systems EPS. The lastgroup consists in inverters for supplying AC loads.

As has been mentioned above, the only control and regulationdevices employed in the microgrid are the power plants. Theplants behaviour from point of view of the output terminalsdepends only on power electronic devices. Hence, one can assumethat the converters are the most important part of the microgrid.The converters allow, among others:• voltage control,• power flow control,• system balancing,• fault protection,• power sources MPP tracking.

Although almost all known converters can be applied in thesystem, some are more popular then others. It depends mainlyon industrial experience. The most important group of convertersare, in case of DC system, DC/DC converters. The most commonDC/DC converters in power range of kilowatts are [5]:• buck converter,• boost converter,• half bridge converter,• bridge converter.

Further considerations are made at the example of half bridgeconverter, as it consists of all the most important simple partswhich form all the other devices (including grid inverters):• input filter,• high frequency inverter,• high frequency transformer,• high frequency rectifier,• output filter.

The diagram of half bridge converter is shown in fig. 3.

Fig. 3. Half bridge converter

IV. MICROGRID MODELLING

The most important issue, which has an impact upon technicaland economical behaviour of the microgrid, is the system design.Unfortunately, there are no tools allowing complex design andoptimisation of all components of the microgrid. New ones needto be developed. This chapter describes authors’ suggestion onhow to do that.

The following phenomena should be investigated throughdesign process:• power plants production,• power flow and balance,• storage system behaviour,

162 2009 COMPATIBILITY AND POWER ELECTRONICS CPE2009 6TH INTERNATIONAL CONFERENCE-WORKSHOP

• voltage stabilisation,• voltage transient states.

There are three periods of time in which the phenomena occursand should be investigated:• years - power generation and balance,• days, hours, minutes - voltage static behaviour and stabili-

sation, storage system behaviour,• milliseconds - voltage transient states and faults.

Hence, three different simulation tools need to be developed.There are three groups of devices in the microgrid which have

to be designed according to the presented philosophy:• microgrid network,• power plants and storage devices,• power electronic devices.One year period comes from power demand and power sources

profiles. They change periodically and one year is the shortestcharacteristic period, which allows only some technical andeconomical planning. The most important parameter in the yearsimulation is the power flow. One can optimise power production,as well as the capacity of power plants and storage capacity. Asa result, overall power costs can be optimised. The most variableparameters here are solar irradiation, wind and power demand.Simulation step time is usually set up at from 10 seconds to 10minutes. In the case the microgrid network is modelled as powerbalance equation

∆p(t) =∑

pp(t)−∑

pl(t)−∑

ps(t) (1)

where:pp(t) - power of power plants,pl(t) - loads power,ps(t) - power of storage system (less then zero if storage ischarged).

A few day period is taken into consideration for simulation ofstatic voltage behaviour. It allows investigating the behaviour ofnetwork, power plants, and storage system during fast changesof the input quantities. The most common simulation time hereis one day or week, according to solar irradiation and demandprofiles. The step time reaches from milliseconds to seconds. Theadmittance network’s model (2) is used in such simulations.

I = YU (2)

where:I - currents vector,U - voltages vector,Y - networks admittance matrix.

The fastest phenomena are connected to rapid changes andfaults of load or power sources. Dynamic voltage behaviour isinvestigated to avoid sudden overvoltages and power shortages.Dynamic behaviour of microgrid’s components has to be takeninto consideration. Typical simulation time takes from a few todozen seconds and step time amounts to ca. microseconds.

V. MODELLING OF THE CONVERTERS

All in all, voltage quality and microgrid behaviour dependon the behaviour of power electronic converters. As a result,modelling of the converters is very important for simulation ofthe whole microgrid. The most popular converters’ simulationmethod uses such tools as PSIM or TCad. However, those toolswere designed for converters simulation and it is not easy tomodel a network with them. The aim of the simulation is toinvestigate the internal converter’s quantities. Hence, simulationtimes are usually lower then a second and the step time is about10−8 to 10−7 of second. As a consequence, it is impossible to

simulate longer periods, e.g. minutes, hours and years. Therefore,modelling of the microgrid network is also difficult.

In the case of the microgrid simulation it is assumed thatwell-known converters are applied. Therefore, no convertersresearch takes place during the design, and internal states ofthe converters can be simplified or completely neglected. Insteadexternal behaviour of the converter should be modelled and othersimulation tools should be used.

The most common simulation tool for modelling dynamicsystems is Mathworks Matlab/Simulink. Authors have proposedthree simulation techniques for the three above-mentioned timehorizons implemented in Simulink. They are described in thefollowing subchapters, on example of half bridge converter(fig. 4).

Fig. 4. Half bridge converter scheme

A. Power Flow ModelPower flow simulation has been performed to establish the

power flow between power plants and load and to check if poweris balanced at all times. The following assumptions have beenmade:• power plants work optimally (i.e. at MPP),• network losses are neglected,• converters’ regulators work as it was designed.

To cut long story short, the microgrid operates optimally with ob-servable changes in power balance, primary power and demandchanges.

In case of such simulations, power electronic converters aremodelled only as losses. All converters in the range of kilowattshave similar efficiency versus normalised load characteristic(fig. 5) [10]. Hence the characteristics are applied in SimulinkLook-Up Table block (fig. 6). Output power is calculated with(3).

Pout = Pin · η(Pin

PN

)(3)

where:PN - converter’s nominal power.

Fig. 5. Typical converter’s efficiency characteristic

POWER QUALITY, ALTERNATIVE ENERGY AND DISTRIBUTED SYSTEMS 163

Fig. 6. Power flow converter model

It is obvious that the model is considerably simplified. Still, it israther fast and hence useful in long term simulations. Efficiencycharacteristics of the modelled device can be easily changed byemploying of producer data or measurements. The model can besimply improved as described in e.g. [8], or [12].

B. Static ModelConverters’ static model was designed to simulate the micro-

grid’s voltage behaviour. Voltage can change in accordance withsolar irradiation, wind speed or power demand fluctuations. Thequantities of fluctuations are relatively slow and in the range ofseconds. From the point of view of power electronic devices, theycan be assumed as almost constant.

In fact one of the following assumptions should be met:• the fluctuations are significantly slower then converter’s

filters time constants,• converter’s output voltage stabilisation are less then 1%,• possible voltage or current ripples can be neglected.

Hence, the converter can be always considered internally insteady state with voltage and current changes only caused byregulators. Converter’s power losses can be neglected as well.

Taking into consideration all the above points, converter canbe considered as a voltage gain (4).

Vout = k · Vin

Iin = k · Iout(4)

In case of half bridge converter k coefficient is repre-sented by (5):

k = Dz2z1

(5)

where:z2z1

- transformer turns ratio;D ∈ 〈0, 0.5) - pulse width.

The Simulink converter’s static model designed in line withthe above suggestions is shown in fig. 7.

Fig. 7. Half bridge converter’s static model in Simulink

The converter’s controller is modelled as parallel voltage andcurrent regulator. Simple PI regulator is used. Typically outputvoltage and current are regulated. That is why such a controllerhas been implemented in the described model. Other quantitiescan also be controlled by power electronic converter, i.e. inputcurrent (in the case of fuel cell) or input power (in the case of

solar battery). The regulator is modelled as follows: (6), (7) (seefig. 8).

D = min (DV , DI) (6)

where:DV - output voltage regulator signal;DI - output current regulator signal.

DV = (Vref − Vout)KPIU

DI = (Iref − Iout)KPII

KPIX = kpix(1 + 1sTix

)(7)

where:Ti - regulator integral time constant;kpi - regulator gain;Vref - voltage reference value;Vout - voltage measured value;Iref - current reference value;Iout - current measured value.

Fig. 8. Simulink regulator’s model

VI. COMPARISON OF SIMULATION RESULTS

To proof the method authors have performed simulation ofthe half bridge converter with elements and parameters listedbelow:• input filter capacitance 470 µF,• voltage divider capacitance 940 µF,• output filter inductance 0.1 mH,• output filter capacitance 470 µF,• load resistance 10 Ω/2 Ω,• input voltage 100 V,• voltage regulator gain 1,• voltage regulator integration time constant 0.001,• current regulator gain 10,• current regulator integration time constant 0.001,

First, PSIM simulation was performed as a point of reference.The results are shown in fig. 9.

Fig. 9. PSIM half bridge converter simulation results

164 2009 COMPATIBILITY AND POWER ELECTRONICS CPE2009 6TH INTERNATIONAL CONFERENCE-WORKSHOP

Then Simulink simplified static model has been designed andtested. The results of comparison of both simulation techniquesare shown in fig. 10 and 11. As can be seen, simplified model doesnot accurately reflect dynamic behaviour of the converter. Butit was not the aim of the simulation. Nonetheless, static voltagelevel and response time are modelled accurately.

Fig. 10. Comparison of voltage

Fig. 11. Comparison of current

VII. CONCLUSION

A simulation method for accurate modelling of static behaviourof power electronic converter has been presented. The compar-ison of the authors’ method and schematic simulation in PSIMhas also been presented. As has been shown, the model is notaccurate in transient states but it has not been the aim of themodelling.

The most important advantage of the authors’ model is thespeed of the simulation. The modelling technique has been de-signed to simulate complicated power systems, like DC microgrid,which consist of several power electronic devices. The half bridgeconverter’s simulation using authors’ model takes less then onesecond. The same converter simulation in PSIM takes about 30seconds.

Due to the fact that the model is very fast in reproducingstatic behaviour of the converter, it can be used for designingcomplicated models of power systems with the limitation to staticsimulation where all quantities change slowly and converters canbe assumed to always operate in a steady state. The aim of theresearch has been reached.

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