TECHNICAL FIELD

The invention relates to a distributed method for controlling electrical power in a microgrid and to an arrangement comprising a plurality of distributed generators.

BACKGROUND

The main task of distributed generators in a microgrid is to deliver maximum real power to meet load demands. The system voltage is typically maintained by the main grid, to which it is connected. However, in weak grid and off grid operation, reactive power support from the distributed generators is required in many cases.

Various centralized control approaches for assigning real and reactive power references have been investigated.

EP 1933441 Ai discloses a system for controlling a microgrid including microgrid assets and a tieline for coupling the microgrid to a bulk grid; and a tieline controller coupled to the tieline. At least one of the microgrid assets comprises a different type of asset than another one of the microgrid assets. The tieline controller is configured for providing tieline control signals to adjust active and reactive power in respective microgrid assets in response to commands from the bulk grid operating entity, microgrid system conditions, bulk grid conditions, or combinations thereof.

EP 2518853 Ai discloses systems, methods, and apparatus for providing coordinated volt/VAR control in power distribution networks. According to an example embodiment of the invention, a method is provided for coordinating voltage and volt-amps-reactive (VAR) control (WC) in power distribution networks. The method can include receiving one or more WC objective functions from a distribution management system; determining at least one microgrid conservation voltage reduction factor (CVRF); forecasting microgrid load profile for a predetermined period of time; controlling microgrid WC resources based at least on the one or more received WC objective functions; and communicating the at least one microgrid CVRF and the forecasted load profile to the distribution management system.

SUMMARY

A problem of the centralized control approach is that a central control device has to be provided, adding costs to the control system. Further, a centralized control approach is complex and requires large amounts of control signalling. Another problem of the centralized control approach is that when the central control device fails the entire control system may fail.

An aim of the present invention is to provide a control approach, which alleviates, or at least reduces, the above problems.

According to one aspect of the invention, there is provided a distributed method for controlling electrical power in a microgrid, wherein a plurality of distributed generators supply electrical power to the microgrid, and each of the distributed generators is connected to a controller, such as e.g. a controller of a voltage source converter, for controlling the real and reactive output power from the distributed generator. According to the method there is measured, for each of the distributed generators, a voltage level at a measuring point associated with that distributed generator and the measured voltage level is forwarded to the controller connected to that distributed generator. For each of the controllers, a parameter value related to the received measured voltage level and/or related to a reactive current injection capacity of the distributed generator connected to that controller is

determined, and from each of the controllers, its determined parameter value is communicated to each other ones of the controllers. The reactive current injection capacity may be determined based on the measured voltage level, the real power injection, and/or the current limit. In each of the controllers, a sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid is determined based on the communicated parameter values. Alternatively, the sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid based on the

communicated parameter values is determined in at least one of the controllers, followed by communicating this sequential order to the other ones of the controllers.

Finally, the distributed generators are controlled to inject reactive power within their current limits into the microgrid by means of the controllers in the determined sequential order.

The distributed generators have capabilities for altering their real and reactive power output either by their inherent structure or by being equipped with arrangements such as e.g. voltages source converters. The reactive power support from the distributed generators can be used both in weak microgrids connected to a main grid and in disconnected microgrids.

In one embodiment, the measuring point may, for each of the distributed generators, be at the connection point to the microgrid for that distributed generator.

In another embodiment, the measuring point may, for each of the distributed generators, be at a node neighbouring the connection point to the microgrid for that distributed generator.

In yet another embodiment, there may be measured, for each of the distributed generators, both a voltage level at the connection point to the microgrid for that distributed generator and at a node neighbouring the connection point, and both the measured voltage levels are forwarded to the controller connected to that distributed generator. The parameter value and/or control of each of the distributed generators may be based on one or both of the measure voltage levels for that distributed generator.

The parameter value may, for each of the controllers, be (i) the deviation of the received measured voltage level from a reference voltage, (ii) the reactive current injection capacity of the distributed generator connected to that controller, or (iii) a combination thereof. The reference voltage may be a desired voltage in the microgrid such as e.g. an acceptable minimum voltage. In one embodiment, the controllers may be used to control the distributed generators to inject reactive power into the microgrid in a sequential order such that their parameter values are arranged in decreasing order.

In another embodiment, each of the distributed generators may be controlled to inject reactive power into the microgrid by means of the controller connected to that distributed generator to reduce or eliminate any deviation of the received measured voltage level from the reference voltage. In particular, each of the distributed generators may be controlled to inject reactive power into the microgrid by means of the controller connected to that distributed generator until any deviation of the received measured voltage level from the reference voltage has vanished or until a current limit of the distributed generator has been reached.

In yet another embodiment, an activation signal is sent from each of the controllers but the last one in the sequential order when the controller has finished controlling the distributed generator, to which it is connected, to inject reactive power into the microgrid. The activation signal may be sent to the next controller in the sequential order, which in response thereto starts controlling the distributed generator, to which it is connected, to inject reactive power into the microgrid.

The method of the first aspect of the invention may be initiated in response to an activation signal sent to the controllers from an external device or in response to an activation signal sent from one of the controllers to the other ones of the controllers. In particular, the method of the invention may be initiated in response to a deviation of a measured voltage level in said microgrid from the reference voltage.

According to another aspect of the invention there is provided an

arrangement comprising a plurality of distributed generators for supplying electrical power to a microgrid, and for each of the distributed generators, a controller connected thereto for controlling the real and reactive output power from the distributed generator. The arrangement comprises further, for each of the distributed generators, a voltage sensor configured to measure a voltage level at a measuring point associated with that distributed generator and to forward the measured voltage level to the controller connected to that distributed generator. Each of the controllers is configured to determine a parameter value related to the received measured voltage level and/or related to a reactive current injection capacity of the distributed generator connected to that controller. Further, each of the controllers is configured to

communicate its determined parameter value to each other ones of the controllers, and to determine a sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid based on said communicated parameter values. Alternatively, at least one controller is configured to determine the sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid based on the communicated parameter values, whereafter this sequential order is communicated to the other ones of the controllers.

The controllers are configured to control the distributed generators to inject reactive power into the microgrid in the determined sequential order.

The second aspect may be modified to comprise methods for performing any of the functions disclosed with respect to the first aspect. The present invention provides a distributed or decentralized control approach, wherein the reactive power support comes from distributed power sources in a decentralized fashion. The distributed or decentralized control approach provides for simple, robust, reliable, and cost-efficient control of distributed generators connected to a microgrid.

Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig l illustrates schematically a plurality of distributed generator systems connected to a microgrid in accordance with an embodiment of the invention,

Fig 2 illustrates schematically one of the distributed generator systems of Fig. l in more detail.

Fig 3 illustrates schematically how the controllers of the distributed generator systems of Fig. 1 may be configured to control the distributed generators to inject reactive power into the microgrid in a determined sequential order.

Fig 4 illustrates schematically in a flow chart a method for controlling electrical power in a microgrid according to an embodiment of the invention.

DETAILED DESCRIPTION

Figure 1 illustrates schematically a plurality of distributed generator systems connected to a microgrid 11. Each of the distributed generator systems comprises a distributed generator i3a-d for supplying electrical power to the microgrid 11, and a controller i5a-d, such as e.g. a controller of a voltage source converter, connected to the distributed generator i3a-d for controlling the real and reactive output power from the distributed generator i3a-d.

While Fig. 1 illustrates four generator systems, there is no limitation regarding this with respect to the invention; it is applicable to any

configuration comprising at least two generator systems. Each of the distributed generators has capability for altering its real and reactive power output either by its inherent structure or by being equipped with a power conversion arrangement such as e.g. a voltages source converter. Such arrangement, if required, is in the present description, assumed to be included in the distributed generator.

The controllers isa-d are connected to one another in order to communicate with one another, either via broadcast, multicast, or unicast messages.

Typically, each of the controllers isa-d may broadcast real and reactive power outputs from the distributed generator I3a-d, to which it is connected.

Further, each of the controllers isa-d may receive voltage and device statuses from other devices connected in the microgrid n.

Fig 2 illustrates schematically one of the distributed generator systems of Fig. l in more detail. It shall be appreciated that the other distributed generator systems are configured in a similar fashion. At least one voltage sensor 18a, 18b is configured to measure a voltage level at a measuring point 17a, 17b associated with the distributed generator 13a and to forward the measured voltage level to the controller 15a connected to the distributed generator 13a via signal paths S2 and S3. In the illustrated case, two sensors 18a, 18b are connected at different measuring points: one measuring point is at the connection point 17a of the distributed generator 13a to the microgrid 11 and one measuring point is at neighbouring node 17b. The distributed generator 13a and the controller 15a may communicate with one another via signal paths si and S4. For instance, operation status parameters, such as available power and current limits may be

communicated from the distributed generator 13a to the controller 15a, whereas control information such as recalculated real and reactive power references may be communicated from the controller 15a to the distributed generator 13a.

Each of the controllers isa-d is configured to determine a parameter value related to a measured voltage level and/or related to a reactive current injection capacity of the distributed generator i3a-d connected to that controller isa-d. If measured voltage levels are used for the determination of the parameter values, they may be measured at the connection points 17a of the distributed generators i3a-d or at neighbouring nodes 17b. If

measurements are performed at both locations combinations of the measured voltage levels or the lowest measured voltage levels at each distributed generator i3a-d may be used. The reactive current injection capacity may be determined based on the measured voltage level, the real power injection, and the current limit. The parameter value may, for each of the controllers isa-d, be the deviation of the measured voltage level (or combination of measured voltage levels) from a reference voltage, the reactive current injection capacity of the distributed generator i3a-d connected to that controller isa-d, or a

combination thereof. The reference voltage may be a desired voltage in the microgrid such as e.g. an acceptable minimum voltage.

Each of the controllers isa-d is configured to next communicate its determined parameter value to each other ones of the controllers isa-d, e.g. via broadcast signalling, such that each controller isa-d knows the parameter values for all the controllers isa-d. Each of the controllers isa-d is configured to next determine a sequential order in which the controllers isa-d are to control the distributed generators i3a-d to inject reactive power into the microgrid 11 based on the

communicated parameter values. Each of the controllers isa-d may be configured to then communicate its determined sequential order to each other ones of the controllers isa-d to check that the sequential orders match one another.

Alternatively, at least one controller is configured to determine the sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid based on the communicated parameter values, whereafter this sequential order is communicated to the other ones of the controllers.

Finally, the controllers isa-d are configured to control the distributed generators I3a-d to inject reactive power into the microgrid in the

determined sequential order. During such control, each of the controllers i5a-d may be configured to control the distributed generator i3a-d, to which it is connected, to inject reactive power into the microgrid n until any deviation of the measured voltage level (or combination of measured voltage levels) from the reference voltage has vanished or until a current limit of the distributed generator i3a-d has been reached.

The control of the distributed generators i3a-d to inject reactive power into the microgrid n to thereby change the local or nearby voltage level in the microgrid n may be performed in any manner known to a person skilled in the art. The controllers isa-d may be configured to control the distributed generators I3a-d to inject reactive power into the microgrid 11 in a sequential order, such that their parameter values are arranged in decreasing order.

Fig 3 illustrates schematically how the controllers isa-d of the distributed generator systems of Fig. 1 may be configured to control the distributed generators I3a-d to inject reactive power into the microgrid 11 in a

determined sequential order according to an exemplary embodiment of the invention. Here, the controllers 15c, 15a, isd and 15b have parameter values in a decreasing order, and the sequential order is thus 15c, 15a, isd and 15b. The first controller in the sequential order, i.e. controller 15c may start controlling the distributed generator 13c, to which it is connected, to inject reactive power into the microgrid 11 as described above. Each of the controllers 15c, 15a, isd but the last one 15b in the sequential order is configured to send an activation signal 21a, 21b, 21c when the controller 15c, 15a, isd has finished controlling the distributed generator 13c, 13a, 13d, to which it is connected, to inject reactive power into the microgrid 11, wherein the activation signal 21a, 21b, 21c is sent to the next controller 15a, 15c!, 15b in the sequential order, which in response thereto is configured to start controlling the distributed generator 13a, 13d, 13b, to which it is connected, to inject reactive power into the microgrid 11. The activation signals 21a, 21b, 21c may be broadcast whereupon the intended receiver can act on the signal (since all controllers i5a-d has knowledge of the sequential order). Signals may be broadcast from the first 15c and last 15b controller in the sequential order when the sequenced controlling as performed by the controllers in the sequential order is started and ended. It shall be appreciated that any controller may at any time deactivate the above disclosed reactive power support control function and broadcast such information to the other controllers. Similarly, such controller having the reactive power support control function deactivated or a controller of a newly installed distributed generator system may at any time activate the reactive power support control function and broadcast such information to the other controllers.

In case, a controller is the sole responsible for determining the sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid, and for communicating this sequential order to the other ones of the controllers, and desires to deactivate the above disclosed reactive power support control function, the above functions have to be handed over to one or more of the other controllers which has/have the reactive power support control function activated.

The invention refers also to a microgrid system comprising a microgrid and the distributed generator systems with the above disclosed reactive power support control function implemented.

Fig 4 illustrates schematically in a flow chart a distributed method for controlling electrical power in a microgrid according to an embodiment of the invention. The method is implemented in a microgrid, wherein a plurality of distributed generators supply electrical power to the microgrid, and each of the distributed generators is connected to a controller for controlling the real and reactive output power from the distributed generator, e.g. as the microgrid and the distributed generator systems as disclosed with reference to Figs. 1-3 above. According to the method, there is, in a step 31, measured, for each of the distributed generators, a voltage level at a measuring point associated with that distributed generator wherein the measured voltage level is forwarded to the controller connected to that distributed generator. Next, in a step 32, there is determined, for each of the controllers, a parameter value related to the received measured voltage level and/or related to a reactive current injection capacity of the distributed generator connected to that controller. From each of the controllers, its determined parameter value is, in a step 33, communicated to each other ones of the controllers.

In each of the controllers, a sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid is, in a step 34, determined based on said communicated parameter values. Alternatively, the sequential order in which the controllers are to control the distributed generators to inject reactive power into the microgrid based on the communicated parameter values, in step 34, is determined in at least one of the controllers, followed by communicating this sequential order to the other ones of the controllers.

Finally, the distributed generators are, in a step 35, controlled to inject reactive power into the microgrid by means of the controllers in the determined sequential order. The described control method may be modified to include method steps using any of the devices, modules and/or arrangements as described above with respect to Fig. 1 to perform any of the disclosed functions.

The invention 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 invention, as defined by the appended patent claims. In particular, it shall be appreciated that the different embodiments, features, and alternatives disclosed above may be combined to reach yet further embodiments of the invention.