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A model for the design and development of smart micro grids.

Authors: Antonella Scaglia, Claudio Brocca, Giordano Torrigiordano.torri@asiansaldo.com, antonella.scaglia@asiansaldo.com,claudio.brocca@asiansaldo.com

Ansaldo Sistemi Industriali - S.p.A. viale Sarca 336, Milano, Italy.www.asiansaldo.com

Key words: smart micro grid, active front end converter, renewable energies.

Abstract:

Energy means secure sources, independence of provisions, reduction of CO2 emissions,efficiency, controllable costs. The diffusion of renewable energies means distributedgeneration, or generators installed anywhere, connected to grids in several points, randomproduction. This paper describes a model of smart micro grids suitable for limited areasalready served by existing networks and for remote zones where electricity is not available.This design integrates distributed generators, load controls and main grid exchange by using apower management system. Maximum energy efficiency and saving is the scope of the designas well as promotion of renewables. Some application cases are shown.

1 Forewords.

Nowadays, energy is the main topic under discussion all around the world. Many issues arerelated to “energy” simply because our ways of life depends on “energy”. So, this word callsto mind a lot of questions: how to get secure sources of energy, how to achieve independentprovisions of the energy we need, how to reduce the CO2 emissions, how to get a cleanerworld, how to get lower and controllable costs. All these issues are cross-correlated and it isnot easy to give a global answer to all these questions. Meanwhile, a strategy is beingimplemented based on so-called renewable energies. Our world is changing while we arediscussing how to change it and the introduction of renewable sources in a random wayimplies facing new problems we have not had to deal with in the past. However, at the sametime, this is a chance to experiment new ways to introduce solutions for solving even if onlypartially, the issues listed above.The progressive extensions of the renewable “green” energies, mainly wind and sun, are defacto modifying the transmission and distribution network. Energy is no longer going to beproduced in one big plant but in many “distributed” small-to-medium generating systems,owned in many cases by the consumers themselves. Promoting wind and sun has a drawbackbecause these clean energies depend in a random way on climate conditions and on the dayand night cycle. We have to cope with this unusual supply of energy availability.Unfortunately energy cannot be stocked in a large quantity and an electrical network is stableonly when production and usage are well balanced at any given point of time. So, the realquestion is how to integrate these distributed, random sources (DG) in the users’ systemwithout degrading the quality of the service we are used to up to now.An answer to all these questions can be found in a distribution energy system called Smartmicro Grid. This paper shows the basic concepts of this network and how the newtechnologies of the power electronics, and ICT can optimize the integration of the DG in anexisting or new network. The concept developed is related to medium and small scale

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distribution systems and has the benefit of integrating itself in larger networks withoutrequiring modifications of the main distribution backbones.The development of the smart micro grids is based on some fundamentals ideas: providingincentives to increase the use of green energies, pushing for energy saving policies, reducingtransmission losses, controlling the cost of the energy, arranging functional redundancy of thesmart micro grid from the main network (islanding).

2 The Smart micro Grid.

Distributed generation (DG) and energy efficiency are the basic concepts for conception ofthe model of a smart micro grid. This grid is conceived for limited areas and/or limitednumber of users and it must manage the local production of energy, the usage of energy andthe exchange with the main network in order to maximize energy efficiency and the usage ofrenewable resources. The future development will incorporate new means for energy storage.The smart micro grid is an electric system that works either in parallel with or disconnectedfrom the main grid. In the latter case it can work in the so-called “island” mode. Thechangeover between the two operational modes is simply and immediate, thanks to its abilityto manage energy flows.A simple schematic of the smart micro grid is shown in fig. 1, where the main components arefound: the local generating units, the loads with their controllers, the power interface to the

main grid and the powermanaging system.The usage of powerequipment that canguarantee high standardfor the power quality,high reliability andefficiency is of primaryimportance. Powerquality means providinglow distortion of the gridvoltage and reduced

electromagneticemissions in order toavoid extra losses in theusers’ equipment andunwanted disturbances.A reliable and safe dutyrequires powerequipment capable of

managing every normal and transient situation without tripping or loosing their functionality.

3 The Distributed Generation.

Different equipment can be used for the so-called distributed electric generation. In order todefine a model for the smart micro grid a classification of these sources can be doneaccording to two distinct parameters: the kind of duty they can provide and the kind ofequipment used for delivering energy to the grid.The first one makes a distinction between sources that can produce energy on demand andcontinuously from the ones that can produce energy only in a “random” way, because of their

Figure 1 –Simplified schematic of a smart micro grid.

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dependence from uncontrollable factors (like the climate). The most important renewablesources can be included into this group of random generators because they take power fromwind and sun. Due to this dependency these sources by themselves cannot be considered asthe primary sources of a grid, even if of small dimensions. So, the smart micro grid must befed by other generators capable of providing a controllable and continuous duty or it must beconnected to the main grid, in order to compensate this random production of energy.It is of primary importance the calculation of the amount of the energy production comparedto the total required power by the users connected to the grid. In case the power from windand sun is relevant, as we can expect from the promotion of renewable sources, it isimmediate to think that the smart micro grid must have an equivalent back-up productionsystem or can draw energy from the main grid when the sun and the wind systems are not ableto work at the required level of power.This approach is simple but it is expensive because renewable energies require greatinvestments and at the same time the back-up capability costs are added too. This approach isbased on the assumption that the usage of the electric energy can be allowed without anycontrol. A less expensive solution, more aligned with the CO2 reduction strategy, is to providethe smart micro grid with an intelligent system that can control not only the production ofenergy but also its usage, in order to get a reasonable balance between production anddemand, minimizing, as a consequence, the usage of conventional sources of energy.So, the design of a smart micro grid must take into consideration the balance between theinstalled capacity from renewable energies over the total installed power and the types ofloads connected to the grid.As a comment, it must be noted that there are renewable sources that have the capability ofproviding energy on a continuous basis, like hydraulic, biomass, fuel-cells. However, due tothe great interest and expansion of DG from wind and sun, the economic terms said before donot change.Going back to the classification of the energy sources and considering the second parameter, adistinction is made between static and rotating generators. The renewable sources requireextensive usage of static power equipment instead of the classic rotating machinery. Both ofthem must work in parallel on the same grid.The rotating machinery is well-known for some relevant factors: they produce sinusoidalvoltage, provide reactive and active power to the loads and in case of fault provide therequired short-circuit current for tripping the safety breakers.At present, the most important norms of the utilities requires that DG can deliver energy tothe grid in a “following” mode instead of working as any other rotating generator of the

existing powerstations [3]. It meansthat they can deliveractive power onlywhen the grid isalready fed by thegenerators of thepower stations andare not intended tocontribute to thefrequency stability.Further more, they

cannot deliver reactive power because there are stringent limits concerning the power factor.

Figure 2- Power converter for photovoltaic system.

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However, in a smart micro grid the power delivered by the DG using static power convertersis relevant compared to the total power installed. As a consequence, the functionality of thesestatic power converters should be changed as it is expected to change for the main grids too.At present, the static power converters used for the renewable energies are designed assumingthat the grid is of infinite capacity. If so, the criteria of delivering sinusoidal voltage (lowdistortion), of contributing to the voltage and frequency stability as well as the management ofthe short-circuits can be neglected.These specific topics have been studied and improved during the last years and new solutionsfor a better integration of these static power generators are available. The most importantsolution is based on the configuration of the static power converter called “Active Front End”.

4 Generating Energy by Active Front End power converters.

The Active Front End (AFE) solution can be seen in its basic hardware elements in fig. 2 and3, where two main cases are illustrated. The first one (fig. 2) provides a link between a dcsource to an ac grid, as in case of photovoltaic systems. The AFE converter has a continuousvoltage at its dc input and produces an output voltage with an amplitude and frequency equalto the voltage of the grid.The second case (fig. 3) is related to the method used for connecting a variable speed rotatinggenerator to a grid. It’s the case of some wind generators driven at variable speed or usingnew technologies like the permanent magnet generators. In such cases a converter acting intwo steps is used. The frequency and amplitude of the variable voltage produced by therotating machine is converted in dc voltage, first. Then, from the dc voltage the power sectionconnected to grid produces a voltage equal to the one of the grid.

It is easy to notethat bothsolutions sharethe same AFEpower topologyfor the conversiondc/ac to the grid.The power

equipmentdefined AFE canbe equipped with

some special SW features that can make it works as any other rotating generator used forpowering the grid. Doing so, the AFE converter can be seen as a generalized solution fordelivering energy to the grid from any renewable source, taking into account the situationswhere the grids has a limited short-circuit power (weak networks) and the contribution fromrenewable sources is of great importance.In the following section the features of the AFE solutions are explained.

Production of sinusoidal voltage and control of emc emissions.The AFE is equipped with a special “T” low-pass” power filter in order to suppress anyvoltage harmonics having an order greater than the fundamental frequency. It is well-knownthat any power converter (and among them the pwm inverters) produces a voltage with highharmonic contents.The “T” filter is inserted between the three phase output of the AFE converter and the grid.High power converters (like the ones used for powers of hundred or thousands of kWs)requires a special approach for the design of this filter, due to its physical size. An optimal

Figure 3- AFE converter for wind generator

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criterion of design dictates to maintain a ratio of at least 3 between the frequency of the pwmcarrier and the resonance frequency of the filter. In such a case the attenuation of the filterallows the reduction of the amplitude of the voltage harmonics to a value less than 1,5%. Of

course, specialattention mustbe taken for thedesign of thepower sectionbecause thehigher the pwm

carrierfrequency thesmaller thefilter and thebetter the

behaviour.Assuming

typical valuesfor the carrierfrequencies inthe range 2 – 5kHz a good

trade-offbetween the

size of the filterand the need of avoiding a too lower resonance frequency is obtained. It must also beremembered that a too low resonance frequency of the filter might hit other harmonicfrequencies already existing on the grid.The fig. 4 shows how a clean power filter works on a 5 MW converter on an isolated grid.The voltage produced is quasi sinusoidal, with a distortion factor of less than 1,5%.

Delivering power to the grid in parallel with other generators.This issue is related to the weak grids, fed by generators of relative small power.

In such cases the AFEconverter might be requiredto contribute to the controlof both its active andreactive energy components.Dedicated regulationalgorithms are implementedinto the control system ofthe AFE converter, and thesimplified schematic of suchsolution is shown in fig. 5and 6. The AFE converter isable to detect by means oftransducers the active and

reactive components of the power delivered to the grid. Two separate control loops areprovided for independent regulation of those two quantities.

Figure 4- Filtering action of the Clean Power Filter

Figure 5- Control system of the AFE converter

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The active power control is dependent by the capability of the upstream generator to produceactive power, having as a limit the capability of the grid of receiving it. The upstreamgenerator is controlled in a way that allows the maximum usage of the renewable source.

Then, the AFE converterdelivers the active power tothe grid using a control lawthat links the grid frequencyand the active power with alinear curve with a negativecoefficient, as shown in fig.7. Doing so, the amount ofactive power is delivered tothe grid according to the loaddemand, while maintaining astable frequency and anautomatic load sharingbetween the various sources.The demand of reactivepower can also be satisfiedby the AFE converter. Insuch a case, a control law forthe regulation loop of thereactive power is set with alinear relationship of thisquantity vs. the grid voltage

with a negative slope as shown in fig. 8. This control law makes the AFE converter participateto the stability of the grid voltage.

Management of the grid transients.The AFE converter can also be equipped with special control functions in order to manage the

transients of the grid, mainly the short-circuitconditions.This is a vital function because it prevents theAFE converter from tripping and contributes theclearing of the fault occurred on the grid. Whensuch a fault occurs the AFE converter does nottrip and it changes its mode of operation. For alimited span of time it works in a current mode,controlling its output current supplied to the gridin short-circuit, in order to let the protectionbreakers to open. Fig. 9 shows the working curveof the AFE converter under normal and transientconditions.The output current, delivered in short circuit bythe AFE converter, must be greater than itsnominal one by a factor between 3 and 5. Forthermal reasons this mode of operation is limited

to the time strictly necessary for the protection breakers to clear the fault.The fig. 10 shows how an AFE converter rated 5MW reacts on an isolated grid in case of asudden short circuit at its output. The line voltage goes to zero and the current regulator of the

Figure 6- Voltages and currents for the AFE converter

Figure 7- Frequency vs. active power control.

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AFE converter takes over, limiting its output current to a value of 3xIn, until the breakeropens.

Thanks to the functionalities shown, the AFEconverter can be seen as a generalized solution forconnecting distributed generators to a grid(especially to weak grids or micro grids) where therotating machine is not applicable and allowing anoperational mode emulating the rotating machine.[1], [2], [4].

5 Power control of the loads.

The electric energy efficiency requires anextensive usage of power converters in order tocontrol the power absorbed by the loads in anoptimal way. The power converters are widelyused in motor controls and in lighting controls. Itis well known how the usage of power convertersfor feeding motors coupled with fans, pumps,compressors can reduce the amount of energy used

by the process, increasing the efficiency and saving energy. [5].Much different equipment is known under the words “drive”, “inverter” or “dc controller”.However, their usage when the grid is weak or has a limited installed power (like the microgrids), implies some further considerations.

For instance, the mostcommon and lessexpensive solution forinverters feedinginduction motors makesuse of an incomingrectifies diode stage in6-pulse configuration.This solution impliesthe emission of currentharmonics of order 5th,6th, 11th, 13th, …. .These currentharmonics produce

distortion of the gridvoltage and its amount

depends on the line impedance. The higher the total inverter power installed the larger thevoltage distortion.A mitigation of the current harmonics emitted by these power converters is required. Twomain options are available: using an inverter with a front end diode bridge having aconfiguration with higher “pulse number” (e.g., 12 or 18) or using again an AFE converter.The first one is shown in fig. 11, where the solutions with 6, 12 and 18 pulse are found.

Figure 8- Voltage vs. reactive power control

Figure 9- Normal operation and short circuit area for the AFE converter

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The latter one can be seen again in fig. 3, because the AFE converter is inherently a 4-quadrant powerconverter, allowing apower flow in bothdirections. It can be usedeither for connecting aDG source to the grid orfor driving a motor atvariable speed.Fig. 12 shows thedifferent behaviouroffered by the solutiondescribed before. Thepower quality given bythe AFE inverter is ofsuperior level. In anycase, when designing a

smart micro grid accurate calculations must be made in order to select the most suitablesolutions for the control of the loads by inverters.

6 Power interface to the main grid.

The most easy and common way for linking a smartmicro grid to the main network is by a substationwhere dedicated switchgear is installed. However, analternative solution can be found by using a staticpower converter which acts as a frequency changer.Such a solution is viable when the smart micro gridworks at a different frequency than the main grid(e.g., 50 Hz vs. 60 Hz) or when the frequency of thesmart micro grid is not synchronous with thefrequency of the main grid.In such cases a frequency converter is used forlinking the two grids and the power convertersolution like the one shown in fig. 3, is the mostsuitable. The same considerations given for the AFEconverter apply to this case. This solution has theadvantage of decoupling the two grids in terms offrequency and reactive power. The frequencyconverter can transmit the active power between thetwo grids in both directions, while the reactivepower on each side can be separately managed bythe control system.

7 The Energy Management Program of the Smart micro Grid.

As anticipated in par. 2, the smart micro grid must be equipped with a power managementsystem and a supervision system, capable of implementing a well defined EnergyManagement Program. [6], [7], [8].

Figure 10- AFE current under short circuit condition.

Figure 11- Inverter in 6-12-18 pulseconfiguration.

Figure 10- AFE current under short circuit condition.

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“Energy management embodies engineering, design, applications, utilization, and to someextent the operation and maintenance of electric power system of the micro grid in order toprovide an optimal use of the electrical energy” [Cit., 10].An effective Energy Management Program can be organized on the following basis:

economics incentives such as savings, realized by reducing (saving) energy use minimizing the probability of energy supply interruption promoting the use of renewable energies promoting the energy efficiency

An Energy Management Program is based on an Energy Audit.

8 Energy Audit.

The energy audit consists of a detailed examination of how the energy is currently used in anexisting area of users (or installations) or it will be used in a new application, of the cost ofthe energy and of the changes in operating practices or in equipment in order to reduce (oroptimize) the energy consumption. The audit will be focused on:• the identification of energy loads and costs of energy usage.• the analysis of the energy usage inside that area/application.• the provision of an action plan to realize the better solution to maximize the energyefficiency.In order to understand the energy consumption, the usage of the energy can be classified intosome major groups: Lightning HVAC - Energy used directly for heating or cooling areas of the plant for comfortconditioning. Motors and drives - Energy used for Motors in ventilation systems, pumps, and otherindustrial applications. Processes - Energy used to heat the product being processed or to maintain the productionprocess. Other energy consumption devices – Energy used to supply other devices, not directlyinvolved in the production process, such as computers, alarms systems, welders, drying ovensor any type of infrequently used machine.

For each of the definedenergy applicationtypes, the energy auditwill report theassociated costs andthe analysis of thepotential savingstogether with arecommended ActionPlan.The audit will alsoidentify the loads interms of their duty,

priority and emission ofharmonics in order to be able of determining a strategy for their control, according to theenergy production and availability inside the smart micro grid. The classification in terms ofduty will define the loads as Continuous, Intermittent and Stand-by. The classification interms of priority will identify the loads as Critical, Necessary, Deferrable, Unnecessary. The

Figure 12- Typical voltage distortion caused by 6, 12, 18 and AFE

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classification in terms of harmonic emission will state the fundamental rules for managing thequality of the energy delivered, avoiding excessive noise on the grid.

9 Power Management System (PMS).

The Power Management System is based on a flexible and high performance hardware andsoftware platform that provides the monitoring functions and the tools for managing andoptimizing either the energy production or the consumption.The aim of the system is to carry out the better performance of the controlled equipment,optimizing both the energy produced by generators (internal) or coming from utilitycompanies (external) and the energy used by loads.Information coming from equipment and field sensors allows quantifying present and shortterm energy production capacity of energy source and, on the other side, to quantify presentand short term energy consumption from the various load typologies.The definition of a production strategy will result, able to detail the drawing or the handoverof energy from/to the external grid or the necessity to decrease the power consumption fromsome non vital loads. A main objective remains the maximization of the contribution ofrenewable energy production, to avoid energy peak requested to the external utility.The most relevant features of this monitoring and control system are given here below.

Control and management of the electric power flows.The control system must regulate the energy production, the exchange of energy with themain grid and the usage of the energy by the loads in order to get the best performance for thesmart micro grid in terms of efficiency, energy saving and usage of the renewable sources. Inorder to perform such tasks, many data must be collected by the equipment installed and bythe field sensors. Using these data the production of energy can be planned on short andmedium time as well as the usage of the energy by the loads.As a consequence, the production strategy can be defined in terms of internal production (withspecial care to the renewable sources) and exchange with the main grid, as well as theconsumption strategy taking into account the need of limiting the power peaks by the loads.

Power meteringIn order to provide a strategy for the management of the power flows it is essential themeasurements and analysis of the current active and reactive power required by the smartmicro grid as a whole and by its equipment as well as of other data coming from the fieldsensors.All these data are then compared to the desired levels, in order to avoid excessive peakdemands of energy, to maintain the best efficiency and to get the maximum usage of therenewable sources.The power metering is also used for determining the rule for the tariffs applicable during thevarious hours of the day to the loads, informing at the same time the customers. The meteringof the quality of power in order avoid situation where the distortion factor can reach excessivelevels.The power metering also includes the continuous monitoring of any disturbances that mayaffect the power quality (measured in terms of THD) delivered by the smart micro grid. Thesedisturbances may come either from the equipment inside the grid or from the main externalgrid. The analysis of these disturbances will require the activation of the actions defined bythe energy audit in order to recover the guaranteed quality of the electric energy.

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Supervision of the whole smart micro grid.A real-time monitoring system is required by the smart micro grid in order to provide theoperators with the necessary information about the status of the smart micro grid and of itsequipment, the need of immediate actions for the resolution of any problem that may occurand the actuation of the maintenance plan. The central supervision system must be able ofassuring an adequate “throughput” towards the connected peripheral systems, from where thefield data and the equipment data are taken and to where the necessary commands andregulation levels are sent.In details, the relevant functions performed by the supervision system:

Access to real-time and historical power data. PMS offer both offline data access andonline (real-time) data access. The first is used to get immediate decisions on thecurrently processed production, the latter is used to plan an efficient long time forecaststrategy all over the plant

Graphical plant synoptic. A PMS synoptic display view allows the operators to get thecurrently evolving data along the whole controlled area. The operators can have bothan overview as well a detailed view of the single controlled equipments from the sameworking station. Different type of data are grouped and displayed coherently whereneeded.

Centralized visualization of all the electric distribution system. A unique access pointproviding all the needed tools to interact an analyze the electric distribution system isanother key feature of a PMS, a must if the primary need is to take a global plant-scoped decisions about power management. This will be performed by the usual HMIworking station.

Monitoring of electrical parameters. Faults and alarms intervention display. Acquired data must also be used to monitor

critical (physical, electrical, etc.) values. From a PMS perspective data must reflect thecorrect application of the planned energy strategy. Malfunctioning equipment orunpredictable event can occur making the real PMS variable evolution to differ fromthe forecasted PMS variable evolution.

Remote access through browser. By providing a “browser access” to the PMS system,the operators can remotely monitor the system. A remote access is very useful from ahigh-level user perspective, to deal with the reporting statistics and to get a closer lookat the plant evolution.

The typical monitoring pages that appear to the operators are shown in fig. 20. As anexample a substation is shown in order to prove how easy and detailed the informationcan be displayed.

Historical data collection.Historical data collection provides the systematic collection of data related to past events andthe storage in a dedicated data-base. The following features are performed.

Database historical collection. The data are efficiently collected and then stored in asafe and reliable database. The storing of many different parameter values sampledalong the time requires an efficient and fast process for the data retrieval, and aflexible process for the data query part (retrieval of data from the user perspective andpost-process analysis).

Graphical presentation. Collected data must also be displayed in a user friendlyrepresentation in order to be interpreted and used by the operators. Different graphicalrepresentations can be used to better understand the data:

o geometrical layouts help to visualize the physical “positions” of the datao electrical layouts help on keep the focus over the single loads

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o diagram block layouts helps to better understand the flow of the information Real-time and historical trending display. A single Process Data continuously acquired

makes understand its evolution along the time. A graphical display of such values willhelp to determine strategic choices from a grid efficiency perspective.

Preconfigured and customized reports, including :o “summaries” of the collected data in order to build simple KPI (Key

Performance Indicators) of the smart micro grido focus over time-bound trend of subsets of Process Datao immediate overview of the main consumption-intensive areas by comparing

them at various levelso maintenance planso energy production schedule and demand

Management of the external fault conditions.A relevant function for the smart micro grid is its ability of working without being connectedto any other external grid. This functionality assures a redundancy in case a fault occurs onthe main network. In such a case, the power management system must detect the external faultand must actuate the so-called “islanding mode”. As a consequence a strategy for balancingthe internal production capability with the energy consumption by the loads will be put inplace in order to maintain the grid in a stable state and providing energy according to thepriority of the loads.

10 Structure of the power management system.

Although the smart micro grid is a limited electrical system the amount of data to be treatedand the number of control function required for running the entire system are huge. Much

equipment isinstalled and all ofthem must beregulated andmonitored. Thiscomplex situationsuggest to use anarchitecture for thePMS that is basedon a main centralcontrol andmonitoring system,connected to “some”distributed (local)control systems.In order to get astructured hierarchyof the entire PMS it

is necessary organizing the local subsystems according to their functionalities. These can beidentified as distributed generators, loads and interface to the external grid.The DGs can be considered as single units or grouped in array, according to their size andplace of installation, while the loads can be grouped according to their functionality orinstallation order.

Figure 13- Global control and monitoring system for the smart micro grid.

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Each power subsystem (either DG or load) will be equipped with its own control andmonitoring system, able of being connected to the main central control system of the smartmicro grid with a fast, reliable and redundant data network. These local control systems willbe connected to the single equipment and field sensors, in order to provide a reliable and fastdata collection. The fig. 13 shows the entire control system of the smart micro grid.

11 The local control systems.

The local control system must actuate the control actions and the data collection required bythe central PMS for each single equipment.

DistributedGenerators.When dealing withthe DG, the localcontrol systemmust perform the

functionalitiesrecalled in par. 3and 4 in order tobe able to stay inparallel on the gridand sharing theload with the othergenerators. Inaddition to thesefunctionalities, the

renewable sourcesrequire otherspecific functions,mainly the searchof the maximumpower point andthe collection ofdata from the fieldsensors for theestimate of thefuture productioncapability. Thelatter is a vitalfeature for thesmart micro gridbecause the DGthat take energy

form wind and sun produces power in a random way. Using special algorithms it is possibleto elaborate the data from the field sensors and from historical data in order to provide theshort and medium term evolution of the energy production for each single DG. Doing so, therenewable sources can be exploited at their maximum extent and the central PMS of the gridhas enough information for planning either the exchange of energy with other grids or theactivation of other conventional generators, installed inside the smart micro grid. The fig. 14

Figure 14- Local control and monitoring system for DG.

Figure 15- Local control and monitoring systems for loads

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shows a typical control system for wind and photovoltaic DG, in order to perform thefunctions stated above.

LoadsWhen dealing with the loads, the local control and monitoring system is required to managethe energy demand according to the energy strategy planned during the energy audit. In fig.15 a typical schematic for such load control and monitoring is shown. The load control can beperformed on the basis of the availability of the energy given by the central PMS and by thetype of duty that the loads are required to perform, in order to avoid peaks of energy demandthat might overcome the production capability or that might require an excessive cost of theenergy.The local control system for these loads must also collect data of the historical consumptionof energy in order to provide future trends for short and medium terms. These data are sent tothe central PMS of the smart micro grid for a global strategy of the demand of energy.Another important function performed by the local control system consists of providing theusers with information about its consumption, the availability of energy, the price of theenergy and the parameters of the energy efficiency.

12 Application cases.

Smart micro Grid for power generation and distribution for on board application.The electric power generation for marine on board applications is normally made by diesel-

generatorsequipment. Inorder to improvethe efficiency ofthe energyproduction, theso-called shaft-alternators aresometimes usedin addition tothese diesel

generators.A shaft-alternator is asynchronous machinemounted on the mainpropulsion shaft andtakes its power fromthe main propulsionengine. When the shipis in navigation it ismore efficient to usethe shaft alternator toits maximum extentfor producing theelectric energy insteadof maintaining all thediesel generators onduty. The efficiency

Figure 16- Shaft generator system for on-board energy production using AFEconverter.

Figure 17- Smart micro grid for on board power generation and distribution.

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of the propulsion engine is higher than the efficiency of the single diesel groups.However, the shaft-generator is driven at variable speed by the main propulsion shaft and itsac terminals can provide variable voltage and frequency only.So, the connection of this generator to the grid is possible only if a static frequency changer isused. The most suitable solution in such a case is represented by the AFE converter, as shownin fig. 16.This power conversion system works as explained in chapter 4. In particular, it is essentialthat the AFE converter must be able to work either in parallel with the other diesel generatorsor as a single power source and it must also guarantee all the relevant functions of providing:● low distortion harmonic voltage● short circuit capacity in order to allow the protection systems to clear any fault● capacity of separate control of active and reactive power.

The complex architecture of the on-board powergeneration and distribution grid is done by a powermanagement system. The main task of this system isto perform the optimum usage of the powerresources in order to get the best efficiency and loadsharing between the various generators. It makes thechoice of how many groups must be on duty inorder to provide energy according to the on-boarddemand. The power management system must alsohave a monitoring capability in order to allow acomplete diagnostic for the operators. Any normaland transient condition must be detected in real timein order to provide a safe and reliable operation. Thecomplete power generation and distribution systemis shown in fig. 17 where a solution for a LV gridworking at 0,44 kV – 60 Hz and fed by 4 dieselgroups rated each one 1,2 MW and by one shaft-alternator of 1,4 MW is shown.This on-board power generation and distributionsystem is an example of a smart micro grid capableof working in island mode for most of the time. Thissmart micro grid can also be fed by a power sourcewhen the ship is docked in port.In order to provide a reliable a safe solution in everyconfiguration, the AFE converter has been designed

with a short-circuit capability of 3 x In for 5 seconds. This value has been stated taking intoaccount the required current for letting the circuit breakers to open in case of fault. The designof the AFE converter also takes into account the need of limiting the distortion factor of thegrid voltage. The power filter has been chosen in order to limit the THD to a value less than2,5%. During the commissioning several measurements have been done for assessing theTHD value and the results are shown in fig. 18.

Power generation and distribution for remote areas.

The smart micro grid design fits the needs of developing the electric energy production inremote areas where the transmission lines do not come.

Figure 18- THD measured on board

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In areas where the solar irradiation is good and the territory offers wide installation zones, thephotovoltaic panels can contribute to the energy production in order to optimize the usage offuel and limiting the CO2 emissions.A study for a remote area where a pumping station is needed has been developed according tothe schematic shown in fig. 19. This solution is strictly designed according to the experiencedone for the on board marine application, mentioned above. Here again, the AFE converterplays an important role because it can be used as the main power source when the sunlightgives its maximum irradiation. So, the AFE converter must perform all the functions alreadycited in par. 4.

The AFE converter can also beput in parallel with the otherdiesel generators according tothe demand of power when thephotovoltaic system is no longerable to deliver its full power.This happens every day, whenthe sun goes down and achangeover between theproduction modes is required inorder to maintain the grid fedduring the night.The system has been conceivedfor a pumping station equippedwith pumps rated 500 kW. So,the power source has installedfour diesel groups rated each one2 MVA, letting at least one ofthem as a back-up in case ofemergency. The photovoltaicfield is rated 2 MW and it canprovide the full power for theentire pumping station when theirradiation is at its maximum.Due to the relative long distance(approx. 10 km) between thearea where the pumps areinstalled and the area where thephotovoltaic panels are placed, atransmission line at high voltage

(11 kV) has been used for theenergy transmission, in order to

reduce the losses. The control of the generation system and of the loads is done by a powermanagement system according to the schematic of fig. 11.A centralized control and monitoring system performs the entire grid management. Localsubsystems are dedicated to the control of the generators’ side and the load side.

13 Conclusion.

This paper introduced some techniques for an efficient management of the energy either interms of production by DG or consumption. All these techniques have been tested and proved

Figure 19- Micro grid for remote area.

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in many industrial applications and now they can contribute to the complex application of asmart micro grid, where production and usage of energy is controlled by an intelligent controland supervision system, coordinating the both.The micro grid becomes “smart” and is able to overcome the issues given by the introductionof the DG with random production capability, such as wind and sun. The smart micro grid is

able to attenuate thefluctuation of therandom production ofenergy, because it cancontrol the loads aswell as the othersource of energy at thesame time.The smart micro gridis also able either toget an efficient usageof the energy or toperform the energysaving thanks to themost advancedtechniques of thecontrol of the loads.The smart micro gridmodel is flexiblebecause it can beintegrated into morecomplex grids withoutany special

requirements. This model can be also used for the electrification of remote lands where theelectricity is not available due to the distance from the main distribution network. The smartmicro grid can avoid the installation of new transmission lines, because it can rely on its ownresources.

14 Bibliography.

[1] Marina G., Gatti E., 2004. Large Power PWM IGBT Converter for shaft alternatorsystems. PESC 04, Aachen giugno 2004. Vol. 5 pagg: 3444-3450.[2] Numeroli R., Gatti E., Torri G., Kranenburg R, 1995. Four quadrant, large power, igbtvector controller adjustable speed drive. Design and test. EPE 95, Sevilla. 1-508, 512.[3] AA. VV. Enel. 2007. DK5940 ed. 2.2[4] Torri G., 2007. Power Quality ed immissione in rete di energia da fonte rinnovabile. 2²Giornata sull’efficienza energetica nelle industrie. Fondazione Megalia. Milano, 30 Maggio2007.[5] Pagano R., Torri G., 2004. Variable Speed Drives and Energy Saving. Congresso Energyand Environment. Fondazione Megalia. Sorrento, 30 Settembre - 02 Ottobre 2004.[6] Flueck A., Zuyi Li, 2008. Destination: perfection. IEEE Power & Energy Magazine.November/December 2008. pp 36-47.

Figure 20- Typical monitoring page of the micro grid. The example shows thesubstation.

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[7] Yeager K., 2008. Striving for perfection. IEEE Power & Energy Magazine.November/December 2008. pp 28 – 35.[8] Magni P., 2009. L’ICT per l’efficienza energetica e il risparmio energetico. MondoDigitale n°2 – Giugno 2009. pp 17 – 29.[9] A.Scaglia, C. Brocca, G. Torri. Un modello per lo sviluppo delle micro reti intelligenti.Forum Telecontrollo. XI edition. Rome, 14-15 October, 2009.[10] IEEE Std 739. Ed. 1995 – Recommended Practice for Energy.