The invention relates to a method of controlling power in a microgrid and a microgrid arrangement comprising a plurality of distributed generators connected to at least one load by means of a feeder.

Background

The present invention especially concerns the problem of sudden changes that affect a microgrid abruptly. For example, when the microgrid is connected to a main grid the microgrid is usually stable, but a sudden loss of a load may lead to an abrupt change of power flow in the microgrid. Another example is when the connection to the main grid is lost and the microgrid goes through a process of islanding, where the sudden loss of power from the main grid cause an abrupt change of the microgrid that may lead to instability. A further example is in the so called island mode when the microgrid operate without support from a main grid. In island mode the risk of problems with power stability is high since the distributed generators usually do not have a high level of inertia to stabilize sudden changes. The sudden loss of a distributed generator or the sudden connection, or disconnection, of a load may cause an abrupt change of system stability. The microgrid concept of a small distribution grid with distributed generators and loads have been discussed within the prior art literature. A list of references is provided at the end of this description.

Document R1 ("Integration of Distributed Energy Resources - The CERTS MicroGrid Concept") describes a microgrid arrangement, called the CERTS concept, and illustrates a typical microsource (in fig. 5.1 of Ref. 1) and a typical microgrid (in fig. 6.1 of Ref. 1). The microsource includes a distributed generator ("prime mover") connected to a feeder via an inverter that includes an energy storage ("DC interface") and controls the power supplied from the distributed generator. Document R1 suggests decentralized control of the d istributed generators and also mentions controll ing the power flow through the feeders.

Document R2 ("Autonomous Control of M icrogrids") is also related to the CERTS concept. Document R2 further describes (in chapter IV in R2) using a so called "un it power control" configuration and a "feeder power control" configuration . In un it power control mode, the microsource controls the d istributed generator to supply available power in an efficient way. The microsources also uses droop control for controll ing the voltage and frequency of the microg rid . In the feeder power control mode, the microsource controls the supply of power in view of the power flow through the feeder. Document R2 recommends using a combination of un it power control and feeder power control , which is referred to as "m ixed control configuration". One m icrosource may for example operate in un it power control mode, wh ile another microsource operate in feeder flow control mode . A microsource may switch between these two operating modes .

Document R3 ("Power Management Strateg ies For A Grid-Connected PV-FC Hybrid Systems") refers to document R2 (ref. [1 4] in R3). Document R3 describes using a mixed control method of the d istributed generators, wh ich includes power un it control and feeder flow control (see page 4-6 in R3). In the feeder flow mode, the d istributed generators regulate the voltage magn itude at the connection point to the main grid and the power that is flowing in the feeder at connection point. With th is control mode, extra load demands are picked up by the distributed generators, wh ich maintain a constant load from the viewpoint of the main grid . Document R3 suggests switch ing to feeder flow control of the connection point if the power flow increases to its maxi mu m (see page 5, first paragraph in R3). The purpose of controll ing the power flow at the connection point to the main grid is to provide a constant load for the main grid .

Patent document US201 4/01 88300 also relates to control of a microgrid wherein microsources ("d istributed power supply") can operate in a feeder flow control mode. The power flow is measured at the con nection point of the m icrogrid to a main g rid , and one or more microsources can operate simultaneously in the feeder flow control mode. Thus in US201 4/01 88300 the power flow from the main grid is measured and the microsources control led on the basis of the power flow from the main grid . The control apparatus measures the power from each m icrosource and the consumption of power of the loads and regulates the power flow from the main grid to the microgrid by selecting operating mode of one or more microsources, wh ich operating modes include a feeder flow control based on the power flow from the main grid (see §0035 of US2014/01 88300).

The prior art provides l ittle information on how to select operating mode for controll ing the power flow in the respective feeders with in the microgrid .

Summary of invention

An aim of the present invention is to provide a better stabil ity with in a microgrid , especial ly in feeders between microsources and loads of the microgrid .

An aim of the present invention is to util ize the power produced by d istributed generators in an effective way, but also contribute to the stabil ity in the microgrid upon changes of the power supply or consumption .

An aim is to provide fast acting stabil ization that can be implemented in a comparably easy and cost-effective way.

Accord ing to a first aspect, the present invention provides a method for control l ing power in a microgrid . The m icrogrid comprises:

- at least one m icrosource, each m icrosource comprising a d istributed generator,

- at least one feeder, and

- at least one load , wherein each of the at least one feeder is arranged to transfer power from at least one of the at least one microsource to at least one of the at least one load . The method comprises: control l ing each microsou rce alternately in a first control mode and a second control mode.

For each microsource said first control mode comprises:

- controll ing the microsource to supply power from the d istributed generator of the m icrosource to the m icrogrid, wh ich power is supplied at a respective first power level in accordance with a reference power level of that d istributed generator,

- mon itoring the power flow through one feeder l ine of the at least one feeder l ine wh ich feeder l ine is associated with the m icrosource, - switch ing to control the microsource in the second control mode when the power flow through the associated feeder l ine moves outside a predeterm ined power range.

For each microsource said second control mode comprising :

- controll ing the su pply of power from the microsources based on the power flow of the feeder associated with the m icrosource, and

- switch ing to control the microsource in the first control mode when the power flow through the associated feeder l ine has re-entered the predeterm ined power range and remained with in the predeterm ined power range for a predefined time period .

In th is way one, or more, of the microsources may be operatively connected to and associated with one feeder in order to control the power flow through th is associated feeder, so that upon a sudden change when the power flow of the associated feeder moves out of the predefined operating range, the microsource immed iately may counteract the in-appropriate power flow. Other changes may occur or be in itiated in the microgrid during the second mode and the microsource wil l revert back to the first mode when the power flow has stabil ized , i .e. remained with in the predefined operating range for a predefined time period . Each feeder of the microgrid may be associated with one, or more, of the microsources so that the power flow of each feeder may be controlled in a d istributed fash ion by means of each associated microsource. An advantage is also that using the microsources for stabil ization of the microgrid is comparatively cost- effective in comparison to add ing separate means for stabil ization , such as standalone energy storages and/or power converters. The reference power level may be the nominal operating level of the d istributed generator in question , but may someti mes be set to a lower level . For d istributed generators able to provide a constant power, such as micro gas turbines or fuel cells, the first power level would normal ly be equal to the reference power level . For d istributed generators having a varying capacity, such as wind turbines and photovoltaic voltage arrays, the first power level will be the maximum available power, and whenever possible the first power level wil l be equal to the reference power level .

In an embodiment of the first aspect of the present invention , the second mode comprises verifying that the number of attempts to return to the first mode has not exceeded a maxi mu m allowed nu mber of attempts.

In th is way unwanted switch ing back and forth between the first and second mode can be avoided . When the predefined number of attempts has been made, the microsource may continue in the second mode and await a command from for example a centrally arranged microgrid control ler before reverting back to the first mode.

In an embodiment of the first aspect of the present invention , the method incl udes mon itoring the power of the microsources of the microgrid and the power consumption of the loads of the microgrid and adjusting the reference power level of each m icrosource based on the mon itored power levels of the m icrosources and loads.

Such mon itoring may be provided by a central , or main , microgrid controller, and the reference power level of one or more m icrosources may be changed for example during control of one or more of the microsou rces in the second mode. A central microgrid control ler may also be provided to d isconnect loads based on such monitoring in order to stabil ize the m icrogrid . In an embodiment of the first aspect of the present invention , the controll ing of each microsource in the second mode incl udes :

- controll ing the microsource to supply power to the microgrid at a second respective power level , wh ich second respective power level corresponds to the first respective power level together with a respective deviation from the first respective power level , in order to re-enter the predeterm ined power range, and

- monitoring the power flow through the associated feed ing l ine. The deviation may be changed based on the mon itored power flow in the second mode. An advantage is that the orig inal reference power level need not, but may, be changed during the second mode, and the second mode can act independently from the reference power level of the first mode.

Accord ing to a second aspect, the present invention provides microgrid comprising at least one microsource, at least one feeder, and at least one load , wherein each feeder is arranged to transfer power from at least one of the at least one microsources to at least one of the at least one loads, wherein each of the at least one microsource comprises a d istributed generator and is configured to control the d istributed generator in a first mode that incl udes supplying power up to a reference power level , said m icrogrid further comprising at least one feeder flow controller, wherein each feeder flow controller is configured to mon itor the power flow through one of the at least one feeder, each feeder flow control ler being operatively connected to at least one of the at least one microsou rce and configured to in itiate a switch to a second mode of operation of the associated at least one microsource when the monitored power flow of the monitored feeder depart from a predefined operating range, wherein the second mode incl udes supplying power at a level deviating from the reference power level in order to adjust the power flow to the predefined operating range, and wherein the feeder flow controller is configured to in itiate a switch back to the first mode when the monitored power flow has re- entered the predefined operating range and remained with in the predefined operating range during a predefined time period . In an embodiment of the second aspect of the present invention, the at least one microsource comprises an energy storage configured to receive energy from the distributed generator of the microsource and transfer energy to the associated feeder of the microgrid.

In an embodiment of the second aspect of the present invention, the at least one microsource comprises a converter for controlling the power from the distributed generator of the microsource, and transferring the power to the microgrid.

The converter may be an AC/DC converter, or inverter, or an AC/AC converter adapting the AC or DC power of the distributed generator to the AC power of the microgrid.

An embodiment of the second aspect of the present invention, comprises a microgrid controller configured to adjust the reference power level for the first mode for at least one of the at least one microsource.

In an embodiment of the second aspect of the present invention, the microgrid controller is configured to monitor the power supplied from the at least one microsources and the power of the at least one load and adjust the reference power level on the basis of the power of the microsources and the loads of the microgrid.

In an embodiment of the second aspect of the present invention, said at least one feeder flow controller being a plurality of feeder flow controller, wherein each feeder flow controller is incorporated in the microgrid controller.

In an alternative embodiment of the second aspect of the present invention said at least one feeder flow controller being a plurality of feeder flow controllers, wherein each feeder flow controller is incorporated in at least one of the at least one associated microsource. Accord ing to a th ird aspect, the present invention provides microgrid control ler for controll ing a microg rid . The microgrid controller is configured to mon itor the power consumption of the loads, the power production of the power sources, and the power flow through each feeder. The microgrid controller comprises a first mode controller that is operatively configured to connect and disconnect each of the loads, and commun icatively connected to each microsource and configured to control each microsource in a first control mode by provid ing a respective power reference level to each of the microsources. The first mode controller is configured to control the connection and d isconnection of loads and provide the respective reference power levels to the microsources based on the power consumption and production . The microgrid control ler comprises a second mode controller comprising a respective feeder flow control ler for each feeder. Each feeder flow controller is configured to:

- determ ine whether the power flow through the associated feeder deviates from a predefined operating range of that feeder,

- instruct one or more of the microsources, which microsource is, or microsources are, ded icated to controll ing the power flow of the associated feeder, to switch to a second control mode, wherein the output power of the microsource or microsources is adjusted in relation to the power reference level when the associated feeder has a deviating power flow, in order to control the power flow through the feeder, and

- instruct the ded icated m icrosource or m icrosources to revert back to the first mode, and output power in accordance with the power reference level ( PSET, i) , when the power flow has returned to the predefined operating range and remained with in the operating range for a predefined time period .

In an embod iment of the th ird aspect, each feeder flow controller is configured to keep track of the number of attempts to revert back to the first mode, configured to l imit the number of attempts to revert back to the first mode, such as to a predefined number of attempts. Preferably, each feeder flow controller is adapted to await connection , or d isconnection , of a load or adjustments of the power reference level made by the first controller before instructing the microsource, or microsources, to revert back to the first mode when the number of attempts has reached a predefined maximum number of allowed attempts.

Brief Description of the Drawings

Figure 1 shows part of a microgrid in accordance with an embodiment of the invention;

Figure 2 shows part of a microgrid in accordance with another embodiment of the invention;

Figure 3 shows a part of a microgrid including four microsources, three feeders and two loads arranged in accordance with an embodiment of the invention;

Figure 4 illustrates an embodiment of a method according to the invention;

Figure 5 illustrates embodiments of further method steps of the method of figure 4.

Figure 6 illustrates an embodiment of a microgrid controller.

Detailed Description

Figure 1 illustrates an embodiment of a microgrid 1 in accordance with the invention. The microgrid 1 comprises a microsource 10, a load 6 and a feeder line A. The feeder line A is arranged to transfer electric power from the microsource 10 to the load 6. The microgrid 1 also comprises a microgrid controller 3 configured to control the overall functioning of the microgrid. The microgrid controller is configured to monitor the microsources, feeders and loads of the microgrid, such as the illustrated microsource 10, load 6 and feeder A, and controlling power levels of the microsource 10, or microsources, and connection and disconnection of the, or each, load 6. The microgrid controller 3 is further configured to monitor a connection 2 to a main grid (not illustrated) and configured to balance the power produced in the microgrid with the loads of the microgrid, especially during island mode when the microgrid 1 is not connected to the main grid. The microgrid 1 is arranged downstream on a distribution side of the main grid and the grid connection 2 is located on, for example, the low voltage side of a transformer, which connects the microgrid to the main grid, for example via a transmission line to the main grid. In general, the microgrid 1 also comprises measuring means, control units and communication means for monitoring voltages, currents, determine power levels at connection points, power flows through the feeders and communicate measurements and control signals between the units. However, these means that correspond to units normally used for controlling electrical grids are not illustrated in detail in the figures.

Figure 1 illustrates a typical microsource 10 that comprises a distributed generator 11 and an interface 12 that connects the distributed generator 11 to a feeder A of the microgrid, which interface 12 is adapted to control the supply of power to the microgrid 1. The interface 12 comprises a converter 13, an energy storage 14, such as an electric battery storage, and a microsource controller 15. The microsource controller 15 is configured to control the distributed generator 11, control the supply of power from the distributed generator 11 to the microgrid 1 and the power transfer to and from the energy storage 14.

The distributed generator 11 comprises for example a DC power source such as solar cells in a photovoltaic array, or fuel cells. Alternatively, the distributed generator 11 comprises an AC power generator, such as a wind turbine or a small gas turbine.

The converter 13 is illustrated as an AC/DC converter, often called inverter, but may in case of an AC power source include two AC/DC converters in a back-to-back configuration with a DC step between the AC/DC converters, which DC step is connected to a DC storage, such as an electrical battery storage. The energy storage 14 is illustrated as an electric battery storage, but may alternatively be a mechanical-electrical fly-wheel storage. The microsource controller 1 5 is configured to control the d istributed generator 1 1 to supply power up to a nominal operating level . The operating level may be regulated and restricted to a lower level . Preferably, the microsource control ler 1 5 is adapted to receive a reference power level PSET from the m icrogrid controller 3 and control the d istributed generator 1 1 to supply power at, or up to, the received reference level PSET. The power available from for example solar cells may vary and the microsource control ler is, in case of a solar power d istributed generator 1 1 , su itably configured to perform a maxi mu m power point tracking of the solar cells to provide power to the microgrid 1 up to the maxi mu m power level of the solar cells, or to the restricted power level . The power available from a gas turbine will usually be constant at the nom inal operating level of the gas turbine, and the microsource controller 1 5 is in such a case adapted to provide power in accordance with the reference power level PSET, either at the nominal power or a set lower level .

Thus, the microsource control ler 1 5 is configured to control the d istributed generator to supply power to the microgrid at, or up to, a reference level PSET, wh ich way of controll ing herein is referred to as the power supply control mode or the first mode. In case of more than one microsource, each microsource wil l have an ind ividual power reference level PSET, i .

The microsource controller 1 5 of figure 1 also comprises a feeder flow controller 5A arranged to mon itor the power flow in the feeder A, and control the microsource 1 0 to regulate the power flow through the feeder A based on the monitored power flow. The regulation of the power that is based on the power flow through the feeder A is referred herein as the feeder flow control mode or the second mode.

The feeder flow controller 5A is operatively connected to the microsource 1 0 incl ud ing the d istributed generator 1 1 , so that the microsource 1 0 incl ud ing the d istributed generator 1 1 is adapted to regulate the power flow of the feeder A in response to the monitored power flow in the feeder A.

The microsource controller 15 is preferable configured to utilize the energy storage 14 to store energy from the distributed generator 11 and inject energy into the microgrid 1.

The feeder flow controller 5A is especially configured to select operating mode of the microsource 10, i.e. the first mode and second mode. The feeder flow controller 5A is for this purpose configured to switch control mode in dependence on whether the power flow through the feeder A is within a predefined operating range or not, so that the microsource 10 operate under power supply control when the power flow of the feeder is within the predefined operating range and change to operate under feeder flow control when to power flow of the feeder moves out of the predefined operating range.

Figures 2 and 3 illustrate further examples of a microgrid 1, wherein each microsource 10, 20, 30, 40 are configured to operate alternatingly in the first mode and the second mode. The microsources 10, 20, 30, 40 may be equal to the microsource 10 of figure 1, but are numbered differently for ease of description. For clarity purposes, the microgrid controller 3 is not illustrated in figures 2 and 3, but is suitably arranged to monitor the microsources 10, 20, 30, 40, feeders A, B, C and loads 6, 7 and provide reference power levels PSET, i for each microsource 10, 20, 30, 40.

In figure 2 a first 10 and a second microsource 20 are connected to the same feeder A, which feeder A connects the microsources 10, 20 to a load 6. Each microsource 10, 20 comprises a respective distributed generator 11, 21 and has a respective interface 12, 22 that connects its distributed generator 11, 21 to the microgrid 1, i.e. to the feeder A of the microgrid 1. A feeder flow controller 5A is arranged to monitor the power flow of the feeder A and to control the microsources 10, 20 to regulate the power flow through the feeder A. The feeder flow controller 5A is illustrated as a separate device, but may be integrated in either interface 12, 22 or be a distributed device integrated into both interfaces 12, 22. Thus, figure 2 illustrates a configuration were two microsources 10, 20 are dedicated to the control of the power flow of one common feeder A.

In figure 3 a first and a second microsource 10 and 20, respectively, are dedicated to controlling the power flow through one first feeder A, in the same way as in figure 2, and a first feeder flow controller 5A is arranged to monitor the first feeder A. Figure 3 also illustrates a third 30 and a fourth microsource 40. Each one of the third microsource 30 and the fourth microsource 40 is connected to two feeders, the second feeder B and the third feeder C. A respective feeder flow controller, i.e. a second feeder flow controller 5B and a third feeder flow controller 5C, is arranged to monitor the power flow through the second B and third feeder C. Each feeder flow controller 5A, 5B, 5C is configured to provide a deviation Δ, for the power flow control, in order to adjust the supplied power of the second mode from the reference power level PSET, i. In case two microsources 10, 20 share responsibility for the power flow through one feeder A, the feeder flow controller 5A provides a fraction Δ, to each microsource 10, 20 of the total deviation necessary for adjusting the power level.

It should be noted that each microsource (10, 20, 30, and 40 in figures 1 to 3) is dedicated to control the power flow of only one feeder A, B, or C. More than one, for example two or three, microsources (10, 20 in figure 2 and 3) can be dedicated to control the power flow of the same one feeder (A in figures 2 and 3).

Figures 1 to 3 also illustrate arrangements of distributed feeder flow controllers 5A, 5B, 5C configured for monitoring and controlling the power flow of one respective feeder A, B, C. As an alternative, one or more, such as all, of the feeder flow controllers 5A, 5B, 5C can be arranged centrally, for example in the microgrid controller 3. In such a case the feeder flow control provided by the feeder flow controllers 5A, 5B, 5C are suitably configured as a separate control function from the overall control of regulating the sources of power and loads 6, 7 in the microgrid 1 as provided by the microgrid controller 3. Thus, the feeder flow control function should suitably be provided to act as fast as possible, especially faster than the balancing of power sources 10, 20, 30, 40 and loads 6, 7 as provided in the overall control loop of the microgrid controller 3. Thus, when the power flow through any feeder A, B, C deviates from its predefined operating range, the power flow should be regulated immediately, while the microgrid controller 3 determines the necessary steps for stabilizing the microgrid 1 by for example load shedding or power injections.

In addition to the illustrated microsources 10, 20, 30, 40 that includes distributed generators 11, 21, 31, 41, and preferably energy storages 14, the microgrid 1 may also be provided with one or more standalone energy storages, such as a battery storage and/or a fly-wheel energy storage for injecting power in order to stabilize the microgrid 1 upon, for example, a disconnection from the main grid.

Figure 4 illustrate a method for controlling the power in a microgrid 1 in accordance with the invention. Especially the first mode (steps 100- 103) and the second mode (steps 200-203) are illustrated. In the first mode, the microsource controller 15 controls 100 the microsource 10 to supply power from the distributed generator 11 in accordance with the reference power level PSET, such as at the nominal operating level or the maximum available power level. The method further includes monitoring 101 the power flow through a feeder A associated with the microsource 11 , e.g. the feeder flow controller 5A monitors feeder A. The method continues with determining 103 whether the power flow of the feeder falls within the desired operating range of that feeder, or not. If the power flow is within the operating range, the method continues in the first power supplying mode. If the power flow moves out of the predefined operating range of the feeder, the method continues with switching 103 to the second mode.

The second mode comprises regulating 200 the power flow through the feeder by adjusting the injected power in accordance with a deviation Δ that is selected in view of the amount of excessive or deficient power in the feeder. The second mode continues determining (201, 202) whether to revert back to the first mode. Thus, the second mode continues with monitoring 201 the power flow and determining 202 whether the power flow returns to the desired operating range and stabilizes within the operating range for a certain time, i.e. during a predefined time period. The method includes switching back 202 to the first mode 100 if the power flow has stabilized, or continue in the second mode 200 if the power flow has not stabilized. In case the microsource continues in the first mode, the power is supplied in accordance with the reference power level PSET. This may lead to an unwanted power flow, in which case the method switches to the second mode again. However, during the stabilization provided by the second mode, the conditions of the microgrid may have changed, or the central microgrid controller 3 may have initiated a change, so that the feeder power flow may continue within the predefined operating range in the first mode.

Figure 5 shows a further embodiment of the second mode of operation. In this further embodiment, the second mode includes monitoring the number of attempts that are made to switch back to the first mode when counteracting a change of the power flow outside the operating range. This further embodiment counteracts changing back and forth between the first and second control mode in cases were the power flow does not stabilize. In this further embodiment a maximum number of attempts to move back to the first mode is allowed when counteracting an unwanted power flow of a feeder. Thus, the method includes determining 203 whether the number of attempts to change back to the first mode have reached the maximum allowed number of attempts before switching back to the first mode. If the predefined allowed number of attempts has been reached, the method continues with controlling the microsource in the second mode including regulating 204 the power flow of the feeder and awaiting a command from for example a central microgrid controller to for example return to the first mode and/or adjust the reference power level. Figure 6 illustrates an embodiment of a microgrid controller 3 that is adapted for controlling the power flow of the feeders A, B, C of the microgrid 1. The power flow control described in relation to the other embodiments of figures 1-5 is incorporated in this microgrid controller 3. The microgrid controller 3 comprises means for monitoring and controlling 60, and is for example communicatively connected the microsources and switches for connecting and disconnecting loads, as well as connected to a communication and control system of the microgrid and main grid, and/or to measurement units, status monitoring etc. disposed within the microgrid.

The microgrid controller 3 comprises a first mode controller 61, which is arranged to provide the reference power levels PSET, i to the microsources of the microgrid, in order to control the power production and consumption in the microgrid based on the power levels of the microsources and the load levels in the microgrid.

In this embodiment, the microgrid controller 3 further includes a second mode controller 62 configured for providing adjustment commands, such as a respective deviation Δι, to each microsource 10, 20, 30, 40 when the feeder A, B, C of the respective microsource moves out of the predefined operating range of the feeder. Thus, the second mode controller 62 includes one power flow controller 5A, 5B, 5C dedicated for controlling the power flow of each feeder A, B, C.

Thus, instead of using distributed power flow controllers, the power flow controller is centrally arranged in the microgrid controller 3. The function and operation of the feeder flow controllers is the same as those described with reference to figure 1-3, and the microgrid controller 3 of figure 6 is adapted to perform the corresponding method steps illustrated in figures 4-5.

The first mode controller 61 and the second mode controller 62 is configured to operate independently from each other. Especially, the second mode controller 62 is configured to provide an independent power flow control when an unwanted power flow is detected in any of the feeders A, B, C. Thus, the second mode controller 62 is utilized by the microgrid controller 3 to control the power flow of any feeder that deviates and revert back to the first mode of control when the power flow has re-entered the predefined operating range and remained within this range for a predefined time period.

Embodiments of a method for controlling active power in a microgrid 1, a microgrid 1 and a microgrid controller 3 has been described. The microgrid 1 comprises at least one microsource 10, 20, 30, 40 comprising a distributed generator 11 , 21 , 31 , 41 , at least one feeder A, B, C, and at least one load 6, 7, wherein the feeder A, B, C is arranged to transfer power from at least one microsource 10, 20, 30, 40 to at least one load 6, 7. The microsources 10, 20, 30, 40 are controlled alternately in a first control mode and a second control mode. In the first control mode, a microsource 10, 20, 30, 40 supply power in accordance with a reference power level P _se t, based on the power production and power consumption of the microgrid 1. In the second control mode the power from the microsource 10, 20, 30, 40 is controlled based on the power flow of an associated feeder A, B, C, for example the microsource is controlled to deviate from the power level of the first mode by a power deviation Δ, in the second mode in order to regulate the power flow. The second mode is used when the power flow of a feeder moves out of a predefined operating range of the feeder A, B, C. When the power flow has re-entered the predefined power range and remained within this power range for a predefined time period, the control reverts to the first control mode, and supplies power in accordance with the reference power level P _se t, \.

The person skilled in the art realises that the present invention is not in any way restricted to the embodiments described above. On the contrary, several modifications and variations are possible within the scope of the invention as defined in the appended claims. List of Documents

R1: Lasseter et al., "Integration of Distributed Energy

Resources— The CERTS MicroGrid Concept,"

White Paper Consortium for Electric Reliability

Technology Solutions, Apr. 1, 2002.

R2: P. Piagi and R. H. Lasseter, "Autonomous Control of Microgrids," in 2006 IEEE Power Engineering Society General

Meeting, PES, Montreal, QC, Canada, 2006

R3: B. Vishnu Priya, G. (2012). Power-Management

Strategies for a Grid-Connected PV-FC Hybrid

Systems. International Journal of Engineering

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(October - 2012)(Vol.1 - Issue 8 (October - 2012).

Available at:

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