Patent Description:
A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid (macrogrid) via a point of common coupling (PCC). This single point of common coupling with the macrogrid can be disconnected, islanding the microgrid. Microgrids are part of the structure for so called distributed generation (DG) aiming at producing electrical power locally from many small energy sources.

A microgrid (in grid connected mode, i.e. connected to the macrogrid) supplies the optimized or maximum power outputs from the connected DG sites and the rest of the power is supplied by the macrogrid. The microgrid is connected to the macrogrid at a PCC through a controllable switch. This grid connection is lost during grid fault and microgrid is islanded.

<CIT> generally discloses connection between and control of a plurality of microgrids with a PCC to the macrogrid. As an another example of prior art, <CIT> relates to a method for detecting islanding conditions in a lower voltage electric power network. In this method, the electrical power network comprises a plurality of sub-networks and the sub-networks comprise at least one power electrical unit and are separable from each other and a main grid supplying the network by switching devices. The method comprises determining topological information of at least one subnetwork of interest, detecting islanding conditions in at least one sub-network of interest on the basis of the topological information by using separate detecting means for each sub-network of interest, and sending, on the basis of the islanding conditions detected by using the detecting means, a disconnect signal to at least one power electrical unit in at least one sub-network of interest.

<CIT> discloses a method and system for detecting an islanding condition in a grid. The method comprises detecting a potential islanding condition in a grid; and, in response to the detected potential islanding condition, ramping up an amount of reactive power, active power, or a combination of active and reactive power that is generated from a power conversion system until the earlier of the power conversion system shutting down or a threshold condition being reached.

To avoid islanding, multiple PCC can be used. Multiple PCC connection may provide improved grid reliability but power management, protection, power flow control, stability and islanding detection (at one PCC point) become more complex.

It is an objective of the present invention to alleviate the problems of the prior art mentioned above, by reducing the risk of islanding while avoiding the increased complexity of using a plurality of PCCs. The solution of the present invention is to connect at only one PCC, but being able to connect via another PCC when islanding is detected.

The invention is to be found in the appended independent claims <NUM> and <NUM>.

By means of embodiments of the present invention, islanding can be handled by having a plurality of available PCC:s, while control complexity is kept low by not having all the available PCC:s active at the same time.

In some embodiments, the control unit is configured for acting to open the second switch, bringing it to its open position, in response to the detected islanding. This may be convenient to avoid any interference to occur via the second PCC.

In some embodiments, the first network line is comprised in the same network grid as the second network line.

In some other embodiments, the first network line is comprised in a first network grid and the second network line is comprised in a second network grid, different than the first network grid.

In some embodiments, the control unit is configured for receiving information about a voltage of at least one bus in the microgrid and a power of the distributed generator.

In some embodiments, the control unit is configured for periodically receiving information about a power flow in the microgrid.

In some embodiments, the control unit is configured for periodically receiving information about a grid voltage of the first network line when the first switch is in its open position.

In some embodiments, the control unit is configured for calculating a flow reference for the first PCC when the first switch is in its open position, and for using said power reference for controlling power flow over the first PCC when the first switch is in its closed position.

In some embodiments, the control unit comprises a central controller as well as a first interface controller for controlling power flow over the first PCC and a second interface controller for controlling power flow over the second PCC.

In some embodiments, the microgrid comprises a third switch configured for, in a closed position, connecting the microgrid to a third network line at a third PCC, and for, in an open position, disconnecting the microgrid from the third network line at the third PCC; wherein the control unit is configured for acting to close the third switch, bringing it from its open position to its closed position, in response to the detected islanding.

<FIG> schematically illustrates an embodiment of a microgrid <NUM>. The microgrid comprises at least one distributed generator (DG) <NUM>, often several DG:s <NUM>, and possibly one or several loads. The DG <NUM> may typically generate direct current (DC) why the microgrid may typically be a DC grid. The microgrid <NUM> comprises a first switch <NUM> configured for, in a closed position, connecting the microgrid <NUM> to a first network line <NUM> at a first PCC <NUM> and for, in an open position, disconnecting the microgrid from the first network line at the first PCC. The first network line <NUM> is part of a first network grid <NUM>, such as an alternating current (AC) grid e.g. a public AC network or macrogrid. The microgrid <NUM> comprises also a second switch <NUM> configured for, in a closed position, connecting the microgrid <NUM> to a second network line <NUM> at a second PCC <NUM>, and for, in an open position, disconnecting the microgrid from the second network line at the second PCC. The second network line <NUM> may also be part of the first network grid <NUM>, or it may be part of a different (second) network grid <NUM>, such as an alternating current (AC) grid e.g. a public AC network or macrogrid, which is separate from the first network grid <NUM>. The microgrid <NUM> also comprises a control unit <NUM> for controlling the opening and closing of the first and second switches <NUM> and <NUM>, as well as for performing other control of e.g. flow and voltages of the microgrid <NUM> and/or the first and second PCC <NUM> and <NUM> as desired. <FIG> shows the situation where the second switch <NUM> is in its closed position and the first switch <NUM> is in its open position, why only the second PCC <NUM> is active, a flow of electrical power (P, Q), where P is active power and Q is reactive power, passing over the second PCC <NUM>. Depending on whether the power produced by the at least one DG <NUM> is more or less than any power consumed by any loads in the microgrid <NUM>, the flow over the second PCC <NUM> may be from the microgrid <NUM> and into the network <NUM> via the second network line <NUM>, or from the network <NUM> via the second network line <NUM> and into microgrid <NUM>. Although the microgrid <NUM> is disconnected from the first network line <NUM>, the control unit <NUM> may still obtain information relating to the first PCC <NUM> and may calculate reference(s) (Pref and Qref) for the flow over the first PCC <NUM> in case the first PCC needs to be activated (closing the first switch <NUM>) due to a failure (islanding) at the second PCC <NUM>. The power flow (P, Q) before islanding may be used by the control unit <NUM> to calculate the power references (Pref, Qref) for the new (first) PCC <NUM>.

<FIG> schematically illustrates another embodiment of a microgrid <NUM>. The figure is similar to <FIG> above and reference is made to that discussion. <FIG> specifically shows the situation where the first network line <NUM> and the second network line <NUM> both are part of the same network grid, the first network grid <NUM>. Thus, the first network line <NUM> may be comprised in the same network grid <NUM> as the second network line <NUM>.

<FIG> schematically illustrates another embodiment of a microgrid <NUM>. The figure is similar to <FIG> above and reference is made to that discussion. <FIG> specifically illustrates an embodiment where the control unit <NUM> receives measurements of the voltage (Vgrid1 and δgrid1), where Vgrid1 is the voltage magnitude and δgrid1 is the voltage phase angle, at the first PCC <NUM>. These voltage measurements may then be used for calculating the power references (P, Q) for the first PCC <NUM>, for use if the first PCC <NUM> needs to be activated by closing the first switch <NUM>. Thus, the control unit <NUM> may be configured for receiving, e.g. periodically, information about a grid voltage of the first network line <NUM>, e.g. at the first PCC <NUM>, when the first switch <NUM> is in its open position.

<FIG> schematically illustrates another embodiment of a microgrid <NUM>, having more than two PCC:s, here four PCC:s. In addition to the first and second switches <NUM> and <NUM>, the microgrid <NUM> also comprises a third switch <NUM> able to connect, in its closed position, the microgrid to a third network grid <NUM> via a third network line <NUM>, and a fourth switch <NUM> able to connect, in its closed position, the microgrid to a fourth network grid <NUM> via a fourth network line <NUM>. As shown in <FIG>, the microgrid <NUM> may be connected over more than one PCC, and/or may have more than one PCC in reserve in case of islanding. Here, the microgrid has two active PCC:s, the second PCC <NUM> by means of the second switch <NUM> being closed and the fourth PCC <NUM> by means of the fourth switch <NUM> being closed. Further, the microgrid has two inactive PCC:s, the first PCC <NUM> which can be activated by closing the first switch <NUM> and the third PCC <NUM> which can be activated by closing the third switch <NUM>. Thus, the microgrid <NUM> may further comprise a third switch <NUM> configured for, in a closed position, connecting the microgrid <NUM> to a third network line <NUM> at a third PCC <NUM>, and for, in an open position, disconnecting the microgrid from the third network line at the third PCC; wherein the control unit <NUM> is also configured for acting to close the third switch <NUM>, bringing it from its open position to its closed position, in response to the detected islanding. The control unit <NUM> is here shown schematically outside of the microgrid for easier display in the figure, and is called a microgrid central control <NUM>. Although, the control unit <NUM> is herein described as being part of the microgrid <NUM>, it may of course physically be located elsewhere, e.g. in a control room, not necessarily close to the DG(s) and any loads of the microgrid <NUM>. As is schematically illustrated in the figure, the control unit <NUM> may obtain measurements of flow (P, Q) of the active PCC:s, i.e. P<NUM> and Q<NUM> of the second PCC <NUM> and P<NUM> and Q<NUM> of the fourth PCC <NUM> in order to be able to calculate the flow references for the inactive PCC:s, i.e. Pref1 and Qref1 of the first PCC <NUM> and Pref3 and Qref3 of the third PCC <NUM>. Additionally or alternatively, the flow references may be based on voltage measurements of the inactive PCC:s obtained by the control unit <NUM>, i.e. Vgrid1 and δgrid1 of the first PCC <NUM> and Vgrid3 and δgrid3 of the third PCC <NUM>. In some embodiments, the microgrid <NUM> is not grid interfaced via a power flow controller (through power flow control, Pref1 and Qref1). Then the power flow is determined only by the grid voltage (Vgrid1 and δgrid1) which is used to calculate reference voltage for microgrid controllable buses. The control unit <NUM> may also or alternatively obtain information about a voltage of at least one bus in the microgrid <NUM> and a power of the distributed generator <NUM> in order to be able to better control the microgrid. In addition to the microgrid central controller (MGCC), the control unit may comprise interface controllers at each PCC, e.g. for executing opening and closing of the respective switches at each PCC and/or for performing flow and/or voltage measurements at the respective PCC:s as desired.

<FIG> illustrates how the power flow references for inactive PCC:s, here the first PCC <NUM> and the third PCC <NUM>, are calculated by the control unit <NUM>. An input stage <NUM> of the control unit <NUM> receives information about measurements made, based on which measurements the control unit <NUM> calculates the flow references (P, Q) for the inactive PCC:s <NUM> and <NUM>. In addition to the flow references, the control unit outputs a command for closing the first and/or the third switch <NUM>/<NUM> is islanding is detected die to failure at the second PCC <NUM>. If the microgrid <NUM> is not connected through power flow controller (as shown in <FIG>), voltage references for microgrid controllable buses are generated instead of flow references as in <FIG>. As discussed above and as illustrated in <FIG>, input measurment information to the control unit <NUM> may comprise any or all of power flow (P, Q) of active PCC(s) <NUM>; closed switches <NUM> and open switches <NUM>,<NUM>; node voltages in the microgrid <NUM>; power output of the DG(s) <NUM>, power consumption of any loads in the microgrid; and grid voltage (V, δ) at inactive PCC(s) <NUM> and <NUM>. The power references are calculated through load flow imposing acceptable voltage variation at microgrid buses. The power/voltage references and switch closing command are then sent to the inactive PCC(s) which should be activated e.g. to PCC interface controller(s) of the inactive PCC(s). This is an example of how the control unit <NUM> may act to activate a PCC in case of islanding, by acting to close a corresponding switch.

Claim 1:
A control unit (<NUM>) for a microgrid (<NUM>) connecting at least one distributed electricity generator (<NUM>), the microgrid comprising a first switch (<NUM>) configured to, in an open position, disconnect the microgrid from a first network line (<NUM>) at a first point of common coupling, PCC, (<NUM>) and a second switch (<NUM>) configured to, in a closed position, connect the microgrid (<NUM>) to a second network line (<NUM>) at a second PCC (<NUM>);
wherein the control unit is configured to obtain information relating to flow of electrical power (P, Q) passing over the second PCC (<NUM>), and wherein the control unit (<NUM>) is further configured to, when the second switch (<NUM>) is in its closed position and the first switch (<NUM>) is in its open position:
detect an islanding event at the second PCC (<NUM>),
in response to the detected islanding event, act to close the first switch to bring it to its closed position, and act to open the second switch (<NUM>), to bring it to its open position, and
calculate a power reference (Pref, Qref) for the flow over the first PCC (<NUM>) based on the flow of electrical power (P, Q) passing over the second PCC (<NUM>) before the detected islanding event, and use said power reference to control power flow over the first PCC (<NUM>) when the first switch (<NUM>) is in its closed position.