TECHNICAL FIELD

The present disclosure relates to methods and devices for controlling a microgrid comprising at least one distributed generator (DG) which is connected in said microgrid via at least one direct current (DC) to alternating current (AC) converter.

BACKGROUND

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 a structure aiming at producing electrical power locally from many small energy sources, DGs. In a microgrid, a DG is connected via a converter which controls the output of the DG, i.e. the power injected into the microgrid.

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 the microgrid is islanded.

During islanding, there is a risk of imbalance in the microgrid due to the loss of power import from grid as well as loss of voltage control by the grid. For voltage control it is required to change control mode of the DGs. The power balancing is solved by fast storage action and immediate load shedding schemes.

The change in control mode of the DGs is from current control to voltage control operation and is initiated to create the voltage and frequency reference for the microgrid without presence of the macrogrid. This switchover from one mode to another is initiated by island detection and thus the set point tracking of the converters depend on the island detection time, mode change signal and settling time of the primary control loop.

Further, a microgrid may be an AC microgrid with an AC bus connected to the different DGs and loads, or a DC microgrid with a DC bus. Hybrid microgrids with both an AC bus and a DC bus are known. For instance, US 8,183,714 discloses DGs connected via a DC bus to an AC side via converters.

SUMMARY

It is an objective of the present invention to provide an improved method of controlling power exchange between a DG and an AC bus of a microgrid.

According to an aspect of the present invention, there is provided a microgrid comprising at least a first distributed generator (DG); a direct current (DC) bus; an alternating current (AC) bus; a switch arranged for connecting the AC bus to a power grid when in a closed position and for disconnecting the AC bus from the power grid when in an open position; a DC to AC converter for connecting the first DG to the AC bus; a first power controller for controlling power exchange between the first DG and the DC bus; and a converter controller of the DC to AC converter for controlling an output of the first DG to the AC bus. The converter controller is configured for controlling the DC to AC converter in a first mode when the first power controller is configured not to allow any power exchange between the first DG and the DC bus, and in a second mode when the first power controller is configured to allow power exchange between the first DG and the DC bus. According to another aspect of the present invention, there is provided a method performed in a microgrid comprising at least one distributed generator (DG). The method comprises exchanging power between the DG and an AC bus via a first DC to AC converter, when a power controller is configured not to allow any power exchange between the DG and the DC bus; detecting an event in the microgrid; and in response to the detected event, exchanging power with the DC bus when the power controller is configured to allow power exchange between the DG and the DC bus, thereby allowing the first DC to AC converter to control an output of the DG to the AC bus, e.g. an output voltage or power factor. By means of the DC bus, the DC to AC converter is able to limit and control the amount of power exchanged with (e.g. injected into or extracted from) the AC bus from the DG, without having to shut down or reduce the power production of the DG. Typically, all power produced by the DG (except regular losses) is injected into the AC bus during normal operations, i.e. in the first mode of the converter controller. However, e.g. in case of a fault, power imbalance or voltage fluctuation, if injecting all available power into the AC bus may make the microgrid unstable, e.g. with regard to frequency and/or voltage, the converter controller may conveniently change mode to the second mode, in which some power is allowed to be injected into the DC bus. In other embodiments, power may be extracted from the DC bus, albeit injection into the DC bus may be most common. For instance, the converter controller may in its second mode control the voltage of the AC bus, i.e. act as a voltage controlling source, e.g. if the power grid (macrogrid) is not able to control the voltage in the microgrid. Thus, the power controller, e.g. a switch or a DC to DC converter, allows exchange of power with the DC bus when the converter controller is in the second mode, and does not allow it, e.g. switch is open, when the converter controller is in the first mode. In some

embodiments, the power controller is controlled by the converter controller, while in other embodiments, the power controller is controlled by a central control unit. By means of the present invention, it is possible to achieve selective grid participation of the DGs in the microgrid while the DGs are still allowed to produce to full capacity.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig l is a schematic circuit diagram of an embodiment of a microgrid in accordance with the present invention.

Fig 2 is a schematic circuit diagram of another embodiment of a microgrid in accordance with the present invention.

Fig 3 a schematic circuit diagram of another embodiment of a microgrid in accordance with the present invention.

Fig 4 is a schematic block diagram illustrating embodiments of a control method in accordance with the present invention.

Fig 5 is a schematic block diagram illustrating an example embodiment of a control method in accordance with the present invention. Fig 6 is a schematic flow chart of an embodiment of a method of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Figure l schematically illustrates an embodiment of a microgrid l of the present invention. The microgrid l is connected to a power grid (macrogrid) 2 via a circuit breaker or other switch 3. The microgrid comprises a plurality of distributed generators (DG) 4, here a first DG 4a, a second DG 4b and a third DG 4c, e.g. wind turbines or solar power arrangements each producing a DC power output Pdcdg. Each DG 4 is connected to a DC bus 6, typically via a respective DC to DC power converter for controlling the DC output of the DG 4. Each DG 4 is also associated with a respective power controller 8, i.e. a first power controller 8a associated with the first DG 4a, a second power controller 8b associated with the second DG 4b and a third power controller 8c associated with the second DG 4c. These power controllers 8 are configured for controlling how much power Pdc, if any, is injected into or extracted from the DC bus 6 from the respective DG 4. Each DG 4 is also connected to an AC bus 5, via a DC to AC power converter 7 for controlling the power P _ac and/or voltage V _ac outputted to the AC bus 5 having the voltage Vgrid. Herein, the DG 4a may e.g. inject power into the AC bus 5 via the first DC to AC converter 7a while the DGs 4b and 4c inject power into the AC bus 5 via the second DC to AC converter 7b. However, provided that the first power controller 8a allows power from the first DG 4a to be exchanged with the DC bus 6, power from the first DG 4a may additionally or alternatively be exchanged with the AC bus 5 via the second DC to AC converter 7b and the DC bus 6. An energy storage 11, e.g. a battery or flywheel, is connected to the DC bus 6, allowing power injected by any of the DGs 4 into the DC bus 6 to be stored in the energy storage. Similarly, power may be extracted from the DC bus 6 by discharging the energy storage 11. Thus, there is no need for each DG 4 to have its own energy storage, since excess power can be stored in the energy storage 11 of the DC bus by means of the power controller 8. Typically, each converter 7 and 10 has its own control unit 9. In figure 1, only the converter controller 9 of the first DC to AC controller 7a is schematically shown. This converter controller 9 is in accordance with the present invention configured to control the amount of power exchanged between the AC bus 5 and the first DG 4a, e.g. injected by the DG 4a. In a first control mode, when no power from the first DG 4a is exchanged with the DC bus 6 (e.g. if the first power controller 8a is a switch, when the switch is open) the converter controller 9 may typically control the converter 7a to inject all power outputted by the DG 4a (and any other power from other DGs which reach the converter 7a). However, in order to maintain stability of the microgrid, e.g. in case of islanding of the microgrid (switch 3 is open), it may not always be suitable to inject all available power into the AC bus 5. In that case, the first power controller 8a is controlled (e.g. if the first power controller 8a is a switch, by closing the switch) to allow exchange of power between the first DG and the DC grid 6, and the converter controller 9 switches to a second control mode wherein the amount of power P _ac exchanged with the AC grid is regulated/limited and any excess power Pdc is instead injected into the DC bus and e.g. stored in the energy storage 11. The second control mode may conveniently be a voltage control mode for controlling the voltage V _gr id in the AC bus 5 in absence of the voltage controlling influence of the power grid 2.

Herein, as an example, it is assumed that power produced by any of the DGs 4, e.g. the first DG 4a, is injected into AC bus 5 and/or the DC bus 6.

However, it should be noted power exchange with the DC bus 6, when allowed by the power controller 8, may in some cases include extraction of power from the DC bus 6, e.g. by discharging the energy storage 11 or by extracting power which is injected into the DC bus 6 by another DG 4, e.g. the second DG 4b. Also the power exchange with the AC bus may be in either direction, both when the converter controller 9 is in its first mode and/or in its second mode, although power may usually be injected into the DC bus, especially in the first mode. Similarly, power and/or voltage outputted to the AC bus 5 may be positive or negative power or voltage, especially in the second mode. Below, however, the present invention is exemplified with the power exchange with both the AC bus and the DC bus being an injection of power, unless otherwise specified. Figure 2 schematically shows another embodiment of a microgrid 1 of the present invention. In this embodiment, the power controller 8a which controls the injection of power into the DC bus 6 from the DG 4a is a switch. When the converter controller 9 is running in its first mode, the switch 8a is open whereby no power is allowed to be injected into the DC bus 6, i.e. all power is injected into the AC bus 5. On the other hand, when the converter controller 9 is running in its second mode, the switch 8a is closed, allowing power to be injected into the DC bus 6 as needed depending on how much power, if any, the DC to AC converter 7a injects into the AC bus 5. Figure 3 schematically shows another embodiment of a microgrid 1 of the present invention. In this embodiment, the power controller 8a which controls the injection of power into the DC bus 6 from the DG 4a is a DC to DC converter, which may or may not be complemented by a switch. When the converter controller 9 is running in its first mode, the converter 8a controls the power exchange with the DC bus 6 such that no power is allowed to be injected into the DC bus 6, i.e. all power is injected into the AC bus 5. On the other hand, when the converter controller 9 is running in its second mode, the converter 8a controls the power exchange with the DC bus 6 such that power is allowed to be injected into the DC bus 6 as needed depending on how much power, if any, the DC to AC converter 7a injects into the AC bus 5. Using a DC to DC converter as the power controller 8 may offer more flexibility than a switch (which on the other hand is less complex), since the power controller 8 may then regulate how much power is injected into the DC bus 6 and is not only an on-off switch. Figure 4 schematically illustrates an embodiment of a control system for controlling the microgrid 1. In this embodiment, the converter controller 9 may comprise a DG controller 42 and may be configured to control, in addition to the operation of the DC to AC converter 7, also the operation of the power controller 8. Alternatively, in other embodiments, the converter controller 9 may be viewed as separate from or comprised in the DG controller 42. A central microgrid controller 41 cooperates with the converter controller 9/DG controller 42. Alternatively, a decentralised control without a central microgrid controller 41 may used, e.g. with cooperating local DG controllers 42 (one for each DG 4). The microgrid controller 41 obtains voltage (v) and/or frequency (f) measurements. Local measurements from where the DG 4 injects power into the AC bus 5 may be obtained from the DG controller 42 and global measurements may be obtained from elsewhere in the microgrid 1 (typically from the AC side of the microgrid). Voltage and frequency regulation control functions in the microgrid controller 41 analyses the measurements and draws the conclusion that the power voltage and/or frequency of the AC bus 5 needs to be regulated. A function in the microgrid controller 41 selects which one or several DG 4 should be used for said regulation and communicates this to the DG controller 42 of the selected DG. The DG controller steers the operation of the power controller 8 such that power exchange with the DC bus 6 is allowed. Then, the DG controller 42/converter controller 9 changes the control mode to the second mode for the control of the DC to AC converter 7. The microgrid controller 41 may additionally (separately) activate change of mode to the second mode for other converter controllers 9 of other DGs 4.

Figure 5 illustrates an example embodiment of control of a microgrid 1 when using a DC bus 6 connected to the DG 4. AC side regulations of voltage and frequency of the AC bus 5 are maintained with DC side maximum power point tracking (MPPT) and connection with DC bus 6 (switch 8 is closed). It can be seen that separate power or voltage control in the AC side is possible with DC bus voltage control while maintaining maximum power extraction from the DG 4. Figure 6 is a flow chart of an embodiment of a method of the present invention. The method may be performed by the converter controller 9 or by a control system of the microgrid 1 comprising the converter controller 9. Power from the DG 4 is injected Si into the AC bus 5 via the first DC to AC converter 7a, when the power controller 8 is configured not to allow any power exchange between the DG 4 and the DC bus 6. Then, an event is detected S2 in the microgrid 1. In response to the detected S2 event, power from the DG 4 is injected S3 into the DC bus 6 when the power controller 8 is configured to allow power exchange between the DG 4 and the DC bus 6, thereby allowing the first DC to AC converter 7a to control an output of the DG 4 to the AC bus 5.

In some embodiments of the present invention, the converter controller 9 in the first mode is configured for controlling the DC to AC converter 7a to inject all power (except any losses) outputted from the first DG 4a into AC bus 5. In the second mode, the converter controller 9 is configured for controlling the DC to AC converter 7a to inject less than all power outputted from the first DG 4a into AC bus 5 thereby allowing a part of the power outputted from the first DG 4a to be injected into DC bus 6. In some other embodiments, the converter controller 9 is, in its second mode, configured for injecting more than all power outputted from the first DG 4a into the AC bus 5 by extracting power from the DC bus 6.

In some embodiments of the present invention, the first power controller 8a is a switch or a DC to DC converter.

In some embodiments of the present invention, the controlled output is a voltage output and/or a power output. Thus, the voltage and/or frequency of the AC bus 5 can be controlled.

In some embodiments of the present invention, the microgrid 1 also comprises a second DG 4b, and a second power controller 8b for controlling power exchange between the second DG 4b and the DC bus 6. The second DG 4b may also be connected to the AC bus 5 via a second DC to AC converter 7b. The control system of the microgrid 1 may also control the second DC to AC converter 7b to regulate the amount of power/voltage outputted into the AC bus 5 from the second DG (when the second power controller 8b allows power exchange with the DC bus 6) and/or from the first DG 4a (when the first power controller 8b allows power exchange with the DC bus 6).

In some embodiments of the present invention, the microgrid 1 also comprises an energy storage 11 connected to the DC bus 6 and arranged for storing or discharging at least some of the power injected into or extracted from the DC bus by the first and/or any second or further DG:s 4.

In some embodiments of the present invention, the detected S2 event is any of an indication that the microgrid 1 has lost its connection to the power grid 2, an indication that the power import to the microgrid 1 from a power grid 2 is below a predetermined threshold and/or an indication that there is a voltage drop in the microgrid 1. These are examples of when the microgrid may need to switch to voltage control, by a converter controller 9 running in its second mode. In some embodiments of the present invention, all available power from the DG 4 is injected Si into the AC bus 5, before detecting S2 the event.

In some embodiments of the present invention, at least a part of the power exchanged S3 with the DC bus 6 is stored in or discharged from an energy storage 11 connected to the DC bus, and/or is exchanged with the AC bus 5 via a second DC to AC converter 7b. These are examples of how to take care of the power injected into or extracted from the DC bus 6, i.e. excess power produced by the DG 4 which may currently not be injected into the AC bus 5, at least not via the first DC to AC converter 7a.

The stability of the microgrid 1 may be improved with DC bus 6 voltage control and interlink connection between the different DGs 4. Multiple DGs and common storage 11 connections at DC bus 6 may reduce DC voltage fluctuations. The stability aspect of parallel converter controllers 9 in the AC side is determined by the controller parameters and control structure which may remains unchanged (the DC voltage may be taken as input and relate the switching functions to the system states). The control scheme is modular in structure and an individual DG 4 failure may only lead to non-participation of that particular DG. The microgrid controller (centralized or decentralized) may be updated based on operating DGs.

Advantages of embodiments of the present invention include: - Selective participation of the DGs 4 in microgrid 1 AC side regulation while outputting maximum available power without individual storages.

- Can be implemented in decentralized or centralized microgrid control systems.

- Controllable connection 8 at DC bus 6 provides possibility in decoupling of DC and AC side power.

- Formation of the DC bus 6 can supply a DC load or be used for a common storage 11.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.