SE1751562A1 - Node controller for switches in microgrids - Google Patents

Abstract

11 Abstract Method (100) of determining current breaking and current limiting requirementsfor at least one node (41-44) of a microgrid (40), and node controller (30) formonitoring current breaking and current limiting requirements of a node (41 -44)in a microgrid (40) is provided. The node controller (30) performs said methodincluding determining current breaking and current limiting requirements forserial connections as well as shunt connections to the node (41 -44) and communicating said requirements to at least one other controller in the microgrid, such as neighboring node controllers or a central microgrid controller. Figure 8

Description

Title: Node Controller for Switches in Microgrids Technical FieldThe present invention concerns microgrids, and especially adding assets, suchas power sources of loads, to a microgrid.

Background and Prior Art There are multiple sources and loads in a microgrid, connected through thecables/lines and other interfacing equipment (power electronic and/or galvanic).The system operates under normal condition to supply the loads and duringabnormal situations (faults) part of the system is isolated with controllableswitches as breakers, disconnectors, sectionalizing breakers or disconnectorsetc. These switches may be used for connecting a power source or load as wellas to protect a line. The selection of the switches depends on the need ofcurrent breaking and isolation at different locations. The requirement changes ina microgrid depending on the connection of DGs (distributed generators), loadsor energy storages to the microgrid. During building a microgrid or microgridexpansion, each of the power sources or loads must be connected with acontrollable switch for self-isolation. Moreover, adding/subtracting devices, e.g.DGs, loads, cable/lines, will change the collection of controllable devices at theconnection bus. And so, the requirement on the isolation and/or protectiondevices on connected neighboring buses.

Each of these controllable switches have different controllability in terms ofcurrent breaking, current making, isolation and phase wise operations. Also,such controllable switches differ in costs.

To be on the safe side, a new sufficiently dimensioned breaker can be selectedfor every new connection, however, the costs will rise even when such abreaker would not be needed, and a less expensive alternative could be employed.

Summary of lnventionAn aim of the present invention is to facilitate planning, building, expansion, reconfiguration and/or operation of microgrids in a cost-effective manner.

According to a first aspect, the present invention provides a node controller for amicrogrid in order to overcome the deficiencies of the prior art. The nodecontroller is configured to - determine current breaking and/or current limiting requirements for new shuntconnected devices and assets for connection to the node, - determine current breaking and/or current limiting requirements for new seriesconnected devices and assets for connection to the node, wherein the determining of requirements is based on the electrical data of theassets and switches currently connected to the node, and on electrical dataconcerning connections from other nodes. The electrical data of the devicesand switches includes fault current data and switching capability.

With the requirements on how high current breaking capacity and currentlimiting capacity a new device and the switch that connect the device shouldhave, the rating of the device and/or switch can be selected. lf, for example, anenergy storage should be connected to a node of the microgrid, which energystorage interfaces the microgrid node with a converter, the requirementscalculated by the node controller will specify the current breaking ability and thecurrent limiting ability the converter, including the switch that connects theconverter to the node, should fulfil.

Preferably, each node of the microgrid is provided with a dedicated nodecontroller, said node being preferably a bus bar to which assets and distributionlines/cables are connected.

The node controller is further configured to communicate the determined requirements, i.e. current breaking and current limiting ability, on series connected and shunt connected devices and assets to the other nodes of the microgrid, or alternatively or additionally to a (central) microgrid controller.

The node controller is further configured to perform a dynamic scheduling ofcurrent limiting devices, such as converters or power electronic breakers, basedon the calculated node requirements information exchange among the nodecontroller. Thus, when the configuration of the microgrid changes, the nodecontroller updates the requirement for current limiting ability for connections tothe node.

According to a second aspect, the present invention provides a methodperformed by the node controller; including the steps of determining the currentbreaking and/or limiting requirements for serial as well as shunt connections tothe at least one node and exchanging the determined requirements with at least one other node controller.

When new elements, such as DGs, loads, lines, are connected, the switches forthe new elements should fulfil the requirements of the node to which theelement is connected, and the requirements of the neighboring nodesconnected to the node the element is connected to. ln this way expansion of the microgrid is facilitating by providing requirementsfor connection of elements to each node of the microgrid.

Important further aspects of such a node controller are: A. Dynamic scheduling of current limiting devices based on the calculated noderequirements information exchange among the node controllers.

B. Reconnection of previously tripped DG, Loads or network reconfiguration atsystem level with the node controller information available at a microgridcontroller.

C. Facilitating microgrid expansion with new lines and elements.

The invention proposes an operation and expansion philosophy of microgrid(regarding controllable switches) where the ability of controllable switches at abus is communicated to the neighboring buses. The invention providesimportant information during reconfiguration, expansion or connection withanother microgrid. A main idea of the invention is to provide information on therequirement on the controllable switch when an element (such as a new line or a DG) is added to a microgrid.

Brief Description of Drawings Figure 1 illustrate a microgrid were new devices should be added; Figures 1A and 1B illustrate two power sources for a microgrid, each havingcurrent controlling and current limiting ability; Figure 2A illustrates an example of integrating a DG with current limiting abilityto a bus by means of a lower rated switch, said bus having current breakingdevices for connection to a first and a second line, such as a distribution line.Figure 2B illustrates connecting a load to the bus of figure 2, which load may beconnected to said bus by means of a comparably slow isolating device; Figure 3A-3B exemplifies different types of switches and their use whenconnecting a DG; Figure 4 illustrate a DG and two electric lines connected to a node; Figure 5 illustrates radial expansion in a microgrid; Figure 6 illustrates adding a node with a DG to a microgrid; Figure 7 illustrates interconnecting two microgrid segments, or two microgrids;Figure 8 illustrates communication among a plurality of node controllers;Figure 9 illustrates the functionality and configuration of a node controller; andFigure 10 illustrates a method performed by a node controller. ln the figures; controllable switches are in general denoted and switcheswith current breaking ability are denoted lower rated switches “2” andisolating devices Description of EmbodimentsFigures 1 to 8 shows some examples of when and how the present inventioncan be beneficial.

Figure 1 illustrate a typical situation were a DG 11 and a distribution line 20should be added to a microgrid 40. For example, when a DG 11 should beadded to a node 41 in the microgrid 40; the node controller of that node 41provides the requirements for current limiting and breaking capability that theDG 11 so that the controllable switch 9 for connection of the DG 11 to the node41 can be selected. Similarly, the node controllers of nodes 42 and 43 of themicrogrid 40 provides the requirements for adding a transmission or distributionline 20 between the nodes 42 and 43 so that proper controllable switches 9 canbe selected for connecting the line 20 between the nodes 42, 43 of the microgrid 40.

Figures 2A illustrate in general a DG 11 connected to a bus 41, i.e. node of themicrogrid, by means of a controllable switch 9, and figure 2B illustrate how a PVsystem, including PV DC generator 12 and converter 13 connected via acontrollable switch 9 to a bus, i.e. node of the microgrid. Depending on theselection of the controllable switches 9, different possibilities for configuring themicrogrid exist. The controllable switches 9 may be selected as currentbreaking switches (or breakers 1), lower rated switches (2), with current limiting ability, and isolating devices (3).

Figure 3A-3B provides examples of the use of various switches (1, 2, 3) andillustrate a DG 11 that has current limiting capability, and therefore may beconnected to a bus 41 by means a lower rated switch 2 to provide currentbreaking, provided that the other connections to said bus 21 are provided bymeans of current breaking switches 1. A load 14 can be connected to said busby means of a (simple) isolating device 3 as illustrated in figure 3B.

Figure 4 illustrate how a DG 11 with current limiting capability can be connectedto a node 41 by means of a lower rated switch 2, and how two electrical lines,each with current breakers 1 in there ends opposite the node 41, can beconnected to the node 41 with isolating devices 3.

Thus, information of the breaking capability of the other end of the line isvaluable for selecting switches for a node 41. The value of information fromneighboring nodes is further illustrated in figures 5-7.

Radial expansion from a node can be provided either with or without currentbreaking switches. Figure 4 illustrate an example on expansion without currentbreaking capability, where a DG 11 with current limiting ability, to the left, isconnected to a node 41 by means of a lower rated switch, wherein an electric line 20 is connected to the node 41 by means of an isolating device 3, and, wherein the other end of the electric line 20 is provided with a current breaker 1.

The right side of figure 4 illustrate expansion with current breaking capability,wherein the line 20 is connected to the node 42 by means of a current breaker1, and a DG 11 and a load 14 is connected to the node 42 by means ofa lowerrated switch 2, and an isolating device 3, respectively.

Figure 6 illustrates expansion of a microgrid 40 with a DG 11 having currentlimiting capability. The DG 11 is connected to the new node 41 with a lowerrated switch and the node 41 is connected to the microgrid 40 by means of anelectric line 20, wherein the microgrid 40 provides current breaking capability,breaker 1, at its end of the line whereas the node 41 is connected to the line 20by means of an isolating device 3.

Figure 7 illustrates interconnection of two microgrids 40, or microgrid segments40A, 40B, by means of an electric line 20. The microgrid segment 40A, to theleft, does not have current breaking capability and is connected to the electric line 20 by means of a current breaking switch 1. The microgrid 40B, on the right, is provided with current breaking capability and is connected to the interconnecting electric line 20 by means of an isolating device 3.

Figure 8 illustrate a microgrid 40 provided with four nodes 41-44. A nodecontroller 30 is arranged at each node 41, 42, 43, and 44, respectively. Eachnode controller 30 is configured to determine its node (e.g. 41) requirements forshunt connections and series connections, and communicate theserequirements to its neighboring nodes, i.e. 42, 43 for node 41. The microgrid 40preferably also includes a central microgrid controller 49, and each nodecontroller 30 is further adapted to communicate the requirements of itsassociated node 41, 42, 43, and 44, respectively, to the microgrid controller 49.

Figure 9 illustrates a node controller 30 comprising a communication unit 36 forexchange of requirements with other node controllers, including nodecontrollers 30 of neighboring nodes and preferably a central microgrid controller49. The node controller 30 is provided with computing means for determiningthe requirements for series devices 31, 32 and means for determiningrequirements for shunt devices 33, 34, 35. The determining means for serialconnection is preferably divided into a determiner 31 of requirements inavailable electrical lines and a determiner 32 of requirements for new electricalline connections. The determining means for shunt connections may suitably bedivided in a determiner 33 for requirements of power sources, e.g. DGs, withcurrent limiting ability, a determiner 34 for requirements of power sources, e.g.DGs, with high fault current, and a determiner 35 for requirements for load connections.

Figure 10 is a flow diagram of a method of the invention performed by eachnode controller 30. The method for determining node requirements 100 includesexchanging 110 requirements with other nodes, especially neighboring nodes,and preferably also exchanging 120 requirements with a microgrid controller.These exchanges (110, 120) can be made and updated regularly, and especially upon changes being made in the configuration of the microgrid 40.The method further includes determining 130 requirements for seria|connections to the node, and determining 140 requirements for shuntconnections of the node. The determining of seria| requirements (130) mayinclude the sub-steps of determining 130A requirements of existing electricallines and cables and determining 130B requirements of new line or cableinstallations. The determining of shunt requirements (140) may be divided intosub-steps of determining 140A requirements for power sources with currentlimiting capability, determining 140B requirements for power sources withoutcurrent limiting capability (or high fault current power sources) and determining140C requirements for load connections.

The invention has been described in embodiments including a method 100 ofdetermining current breaking and current limiting requirements for at least onenode 41-44 of a microgrid 40, and node controller 30 for monitoring currentbreaking and current limiting requirements of a node 41-44 in a microgrid 40.The node controller 30 performs said method including determining currentbreaking and current limiting requirements for seria| connections as well asshunt connections to the node 41-44 and communicates the requirements to atleast one other controller in the microgrid, such as neighboring node controllersor a central microgrid controller. The invention is however not limited to theseembodiments, and a person skilled in the art may perform further variations ofthe invention within the scope of the appended claims.

Claims (7)

Claims

1. A node controller (30) for monitoring current breaking and current limitingrequirements ofa node (41-44) in a microgrid (40), said node controller beingconfigured to - determine (140) current breaking and/or current limiting requirements for newshunt connected devices and assets for connection to the node, - determine current (130) breaking and/or current limiting requirements for newseries connected devices and assets for connection to the node, wherein the determining of requirements is based on the electrical data of theassets and switches currently connected to the node, and on electrical data concerning connections from other nodes.

2. The node controller (30) of claim 1 further configured to communicate (110, 120) the determined requirements to other controllers.

3. The node controller (30) of claim 2, said communication (110, 120)comprising exchanging (110) requirements with neighboring node controllers.

4. The node controller (30) of claim 2, said communication (110, 120)comprising sending (110) the determined requirements of its associated node(41-44) to a microgrid controller.

5. The node controller (30) of any of claims 1 to 4, further configured to performdynamic scheduling of current limiting devices upon changes in theconfiguration of the microgrid (40).

6. Method (100) of determining current breaking and current limitingrequirements for at least one node (41-44) of a microgrid (40) including: - determining (130) current breaking and/or limiting requirements for serialconnections to the at least one node (41 -44); and - determining (140) current breaking and/or limiting requirements for shuntconnections to the at least one node (41-44).

7. The method (100) according to claim 6, said determining (140) of currentbreaking and/or current Iimiting requirements for shunt connections includes atleast one of: a) determining (140A) requirements for power sources, such as DGs, withcurrent Iimiting capability, b) determining (140B) requirements for power sources without current Iimitingcapability; c) determining (140C) requirements for load connections.