DC VOLTAGE MICRO GRID IN THE DISTRIBUTED GENERATION

Piotr BICZEL*, Józef PASKA**

* Warsaw University of Technology, Institute of Electrical Power Engineering00-662 Warszawa, 75 Koszykowa St., Poland, e-mail: biczel@ee.pw.edu.pl

** Warsaw University of Technology, Institute of Electrical Power Engineering00-662 Warszawa, 75 Koszykowa St., Poland, e-mail: jozef.paska@ien.pw.edu.pl

Abstract: One of the EU energy policy goals is develop-ment of utilization of renewable and small loac-al power sources. Local balancing of powerproduction and consumtion is aimed due topower transmission loses minimazation. In oth-er words all energy consumed in separatedsmall area (cottage, town, commune) should beproduced within this area. Research it has beendone so far shows that it is very hard, in partic-ular using unstable renewable power based onsolar and wind energy, to reach the aim withkeeping very high quality factors, high eff-ciency and low costs. One of the solutionscould be local microgrid connected or not topower system. The microgrids can be realizedas AC or DC current. There is DC current mi-crogrid idea presented in the paper. In authorsopinion such microgrid using power electronicsconverters and advanced control systems al-lows easier effective connection of differentpower sources as AC microgrid. In particularthere are no problems like reactive power con-trol, voltage and current distortion, reactiveloses etc.

Keywords: distributed generation, renewable powersources, microgrids, power electronic convert-ers.

1. INTRODUCTION

Strategic aims of EU clearly describe the role of dis-tributed generation (DG) and renewable energy re-sources (RES) in power production. Experts are unanim-ous that DG participation in production will be raisingalso due to economical reasons. Unfortunetly it willcause necessity of some changes in power system. Cur-rent centralized solutions have to be transformed [5].The transformation is forced by another EU aim – powerquality raising.

From opposite direction EU main goal is sustainabledevelopment. In case of power it means increasing of ef-ficiency, RES utilization and local energy balancing.

Local balancing means that almost all energy con-sumed in separated area needs to be produced withinthis area using local primary carriers. Such solution al-lows to minimize power loses connected with power

transmission and power distribution and encorages in-vestment in local small production units.

Goals and development directions described aboverequire transforamtion of current distribution networksof medium and low voltage. It is necessary to introducepossibility of local control of power flow. Protectionsystems need to work with bidirectional energy flows.There is no data communication subsystem necessary tocontrol all components in current distribution network.In consequence the idea of microgrids was come intobeing. Microgrid means small, balanced power subsys-tem which connect distributed power stations and con-sumers located on not big area. Description of differentAC microgrids can be found in references [4][6].

The principle of mcrogrid operation is balancing ofproduction with consumption. In case of stand-alone mi-crogrid system has to be balanced all time. In case ofsubsystem connected to power system has to be con-structed in the way the power interchange should beplannned like in case of big power plants.

Unfortunetly, a disadvantage of microgrids is prob-lem of voltage and frequency control (reactive and act-ive power flow). It is clear that there will also problemswith voltage distortion due to wide utilization of powerelectronic converters.

Authors propose to use DC current microgrids as asolution which avoids most of problems desribed above.The solution allows to keep low costs in many cases andmakes easier some issues connected with control ofquality parameters. Problem of quality in DC systems isreduced to keep voltage or current in required range.

2. AC CURRENT MICROGRIDS

Microgrid conceptions described in references con-cern AC current systems. Two interesting examples canbe found in [4][6].

The [6] concerns technical solutions allowing wideand safty introduction of uncontrolled energy sourcesinto public grid. The aim of project was to analyse of re-liability and cohension of AC microgrids. There weresignificant amount of installed power in renewablepower sources. The next aims were research on systemproperties with decentralized control and collecting ex-perience in exploitation.

To reach the aims fragment of 20 kV distributionnetwork was chosen. Structure of the network is shownin fig. 1.

Fig.1. AC microrid structure [6].

There were two main conclusions. The first one wasthere is no significant technical difficulties to constructsuch system. But an adequate level of reliability andavailbility of the microgrids eleents have to be kept. Thesecond one was it is necessary to develope data commu-nication subsystem which will connect all sources intovirtual power plant (fig. 2).

Fig. 2. Concept of virtual power plant [6].

Different in its idea is system described in [4]. It isstand-alone microgrid built on an island (fig. 3).

Fig. 3. AC stand-alone microgrid presented in [4].

Unfortunately AC current based solutions have manydisadvantages. Wide usage of not big sources extractsintroducing of many power electronic converters – in-

verters – to convert power into AC 50 Hz current at re-quired voltage level from DC or AC frequency vriablecurrent. As a result system of many inverters connectedin parallel will be coming into being. Assurance o prop-er parameters of energy supplied by microgrid will de-pend on strategy of control of the inverters and on howthe problem of changing the leading inverter will besolved. Leading inverter needs to be changed in case thecurrent leading source losts its production capacity.

There are not trivial problems and more research isstill required [3][5]. It is particularly important introdu-cing such control strategy which allows to aviod voltagedrops and frequency changes. Other difficulties arecaused by problems with reactive power and shape con-trol.

3. DC CURRENT MICROGRID STRUCTURE

An idea, which is a base of DC current microgridconception, is realization of postulate of local balancingof production with consumption like in case of AC mi-crogrid. In addition high quality parameters will be kept.Thanks to introduction of DC current there will not besuch problems like frequency, shape, reactive powercontrol. Block diagram of the system proposed by au-thors is shown in fig. 4. DC current microgrid principleof operation is based on convertion of all kind of pro-duced power into DC current. In case of generators withvariable frequency the convertion is always applied.Connection with power grid is also realized by retifierand inverter. All power balancing and control functionsare performed in DC circuit.

Fig. 4. DC microgrid concept.

Consumers are connected by inverters. If connectionneeds to be bidirectional it has to be done by bidirec-tional converter or parallel connection of rectifier andinverter.

Problem of power quality in the microgrid is reducedto voltage level control. It is obvious consumers need tobe joined to the AC 50 Hz sources. So, there are invert-ers which produce 50 Hz sinusoidal voltage. Due to thepower control is executed in DC circuit, if the DCvoltage are kept higher then minimum threshold value,the output AC voltage will meet all requirements. It isparticulary interesting that voltage in DC circuit does

not have to be kept with very high precision. Modern in-verters can keep stable sinusoidal waveform with relat-ively wide changes of DC input voltage.

Described solution can have a few variant dependingon number of inverters and their power and localization.The border solutions are the one inveter supplying allconsumers on one side and individual inverters for eachconsumer on the second side. The second possibility al-lows for instance to measure energy in DC circuit. Itcould be interesting approach in the light of current“trade” problems in Poland.

As in microgrids presented in [4][6], the criticalproblem is data communication subsysem and propercontrol strategy. The strategy will have direct impact onpower quality, power sources utilization, balancing leveletc. In consequence it will have crucial influence on en-ergy costs in the microgrid.

4. AUTHORS’ RESEARCH

Team leaded by authors is experienced in hybridpower production systems [1]. Particularly in DC cur-rent systems. We have also very good experience in re-mote supervising and control systems [2]. A model ofDC current microgrid is constructed (fig. 4) in laborat-ory of Chair of Power Plants and Power Economy.

Intensive research is done on all components of themicrogrid. Fig. 5 shows an example of results. It is achart of power flow in hybrid production system basedon solar source and PEM fuel cell. As it can be seenboth sources cvers energy demand together dependingon current production in photovoltaic array. The experi-ment were done on solar power plant and PEM fuel cellsystem developed at Warsaw University of Technology(fig. 6 & 7).

Fig. 5. Power flow in DC hybrid power plant.

Fig. 6. Solar power plant developed at Wasaw University ofTechnology.

Fig. 7. PEM fuel cell supply system developed at WarsawUniversity of Technology.

5. CONCLUSIONS

Common utilization of small power units, often un-stable and unpredictible, will be causing necessity ofbuild networks subsystems called microgrids. Mi-crogrids, according to EU strategic goals, will cover allenergy demand in area they will built using localprimary carriers. It is political aim which has to be real-ized. It will increase power safety and life level by in-creasing of power availability, efficiency and quality.

Intensive research is performed on microgrids allover Europe. Authors propose, as their own contribu-tion, DC current microgrid. The DC microgrid allows:• avoid many difficulties with control of energy

parameters,• simplfication of control strategy and control units,• costs reduction,• transmission loses reduction,• developing of new method of energy measurment,• introducing load active control methods.

Technical realization of the DC microgrid is possibleand quite simple at present timewith relatively low costs.It is the result of sudden development of power electron-ic converters and data communication equipment andsignificant their costs reduction.

6. REFERENCES

1. Biczel P.: Opitmal utilizan of primary energy car-ries on example of hybrid solar and fuel cell pow-er plant. PhD Dissertation. Warsaw University ofTechnology Department of Electrical Engineering.Warsaw 2003.

2. Drążkiewicz J.: Microprocessor control and su-pervising systems for stand-alone photovoltaicand wind turbine power plant. PhD Dissertation.Warsaw University of Technology Department ofElectrical Engineering. Warsaw 2002.

3. Engler A.: Applicability of Droops in Low VoltageGrids. International Journal of Distrbiuted Ener-gy Resources. Vol. 1 No. 1 (2005) p. 3 – 15. Tech-nology and Science Publishers, Kassel, Germany.

4. Hatziargyriou N.: More Microgrids: Advanced Ar-chitectures and Control Concepts. 2nd EuropeanSeminar “Distributed Energy Resources and Fu-ture Electricity Networks”, Warsaw 8th October2004

5. Parol M.: Zaawansowane systemy sterowania wprzyszłościowych strukturachelektroenergetycznych sieci rozdzielczych. VIIIMiędzynarodowa Konferencja Naukowo-Techniczna “Nowoczesne urządzenia zasilające wenergetyce”. Elektrownia Kozienice S.A., ŚwierżeGórne 2 – 4 marca 2005.

6. Styczyński Z., Bucholz Z.: Planowanie ieksploatacja sieci elektroenergetycznych zgeneracją rozproszoną i zasobnikami energii. VIIMiędzynarodowa Konferencja Naukowo –Techniczna “Nowoczesne urządzenia zasilające wenergetyce”. Elektrownia Kozienice S.A. ŚwierżeGórne 10 – 12 marca 2004.