Controlling a virtual power plant to avoid energy drift

Abstract

A computer implemented method for controlling a virtual power plant, VPP, to provide (401) symmetric power reserve for an electric grid, wherein the VPP comprises a plurality of battery units. The method includes detecting (402) a need to activate the power reserve for up regulation or for down regulation, wherein the up regulation comprises drawing energy from one or more battery units of the VPP and the down regulation comprises storing energy to one or more battery units of the VPP; obtaining (403) information about energy levels of the battery units of the VPP; selecting (404) battery units for up regulation or down regulation based on the information about energy levels of the battery units; continuously monitoring (305) total energy level of the VPP; detecting (406) drift in the total energy level of the VPP; and compensating (407) the drift in the total energy level of the VPP.

Description

[0001] CONTROLLING A VIRTUAL POWER PLANT TO AVOID ENERGY DRIFT

[0002] TECHNICAL FIELD

[0003] The present disclosure generally relates to controlling a virtual power plant.

[0004] BACKGROUND

[0005] This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

[0006] A virtual power plant (VPP) is formed of a network of energy producing or storage devices, such as solar panels and / or batteries, that are pooled together to serve the electricity grid. In this way a larger capacity may be built by pooling together smaller scale resources.

[0007] For example, a distributed energy storage (DES) system including spatially distributed battery units may be used for forming a VPP. The battery units of the VPP may be resources maintained for example for emergency energy backup purposes, such as backup batteries of a wireless communication network. Additionally or alternatively, the battery units may be resources owned by households or small and medium sized companies or other smaller scale operators.

[0008] VPPs may participate in balancing of electric grid or in intraday trading market. Transmission system operators (TSO) offer reserve markets where reserve providers, such as VPP, can offer energy capacity for grid balancing purposes.

[0009] Now, there are provided some new considerations for controlling VPPs.

[0010] SUMMARY

[0011] The appended claims define the scope of protection. Any examples and technical descriptions of apparatuses, products and / or methods in the description and / or drawings not covered by the claims are presented not as embodiments of the invention but as background art or examples useful for understanding the invention.

[0012] According to a first example aspect there is provided a computer implemented method for controlling a virtual power plant, VPP, to provide symmetric power reserve for an electric grid, wherein the VPP comprises a plurality of battery units. The method comprises detecting a need to activate the power reserve for up regulation or for down regulation, wherein the up regulation comprises drawing energy from one or more battery units of the VPP and the down regulation comprises storing energy to one or more battery units of the VPP; obtaining information about energy levels of the battery units of the VPP; selecting battery units for up regulation or down regulation based on the information about energy levels of the battery units; continuously monitoring total energy level of the VPP; detecting drift in the total energy level of the VPP; and compensating the drift in the total energy level of the VPP.

[0013] In some embodiments, the compensation of the drift in the total energy level is performed by drawing energy from one or more battery units of the VPP to the electric grid or by storing energy from the electric grid to one or more battery units of the VPP.

[0014] In some embodiments, the compensation of the drift in the total energy level is performed by using intraday markets as a source or sink of energy for the compensation.

[0015] In some embodiments, the symmetric power reserve is provided according to Frequency Containment Reserve, FCR.

[0016] In some embodiments, the symmetric power reserve is provided according to Frequency Containment Reserve for Normal Operation, FCR-N.

[0017] In some embodiments, drift in the total energy level of the VPP is detected based on detecting that a difference between the total energy level of the VPP and a predefined energy level setpoint exceeds a predefined threshold.

[0018] In some embodiments, the threshold is 50-75% of the predefined energy level setpoint.

[0019] In some embodiments, using intraday energy markets for compensating the drift in the total energy level of the VPP comprises offering to sell surplus energy to the intraday market during a next possible bidding time period or offering to buy missing energy from the intraday market during the next possible bidding time period.

[0020] In some embodiments, the amount of offered surplus or missing energy depends on a difference between the total energy level of the VPP and a predefined energy level setpoint.

[0021] In some embodiments, the amount of offered surplus or missing energy is spread over more than one bidding time period.

[0022] In some embodiments, the battery units are selected so that battery units with higher energy level are selected for up regulation and battery units with lower energy level are selected for down regulation. In some embodiments, the battery units are selected so that battery units with energy level above a battery unit specific target level are selected for up regulation and battery units with energy level below the battery unit specific target level are selected for down regulation.

[0023] In some embodiments, the method further comprises assigning the battery units to an up regulation list or to a down regulation list based on the information about energy levels of the battery units; and selecting battery units from the up regulation list for up regulation and selecting battery units from the down regulation list for down regulation.

[0024] In some embodiments, the method further comprises prioritizing the battery units on the up regulation list based on one or more of: an amount of available energy to be discharged, an amount of energy available before reaching a battery unit specific target level, absolute capacity of the battery unit; and / or prioritizing the battery units on the down regulation list based on one or more of: an amount of unit space for storing energy, an amount of unit space available before reaching a battery unit specific target level, absolute capacity of the battery unit.

[0025] According to a second example aspect of the present invention, there is provided an apparatus comprising means for performing the method of the first aspect or any related embodiment. The means may comprise a processor and a memory including computer program code, and wherein the memory and the computer program code are configured to, with the processor, cause the performance of the apparatus.

[0026] According to a third example aspect of the present invention, there is provided a computer program comprising computer executable program code which, when executed by a processor, causes an apparatus to perform the method of the first aspect or any related embodiment.

[0027] According to a fourth example aspect there is provided a computer program product comprising a non-transitory computer readable medium having the computer program of the third example aspect stored thereon.

[0028] Any foregoing memory medium may comprise a digital data storage such as a data disc or diskette; optical storage; magnetic storage; holographic storage; opto-magnetic storage; phase-change memory; resistive random-access memory; magnetic random-access memory; solid-electrolyte memory; ferroelectric random-access memory; organic memory; or polymer memory. The memory medium may be formed into a device without other substantial functions than storing memory or it may be formed as part of a device with other functions, including but not limited to a memory of a computer; a chip set; and a sub assembly of an electronic device.

[0029] Different non-binding example aspects and embodiments have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in different implementations. Some embodiments may be presented only with reference to certain example aspects. It should be appreciated that corresponding embodiments may apply to other example aspects as well.

[0030] BRIEF DESCRIPTION OF THE FIGURES

[0031] Some example embodiments will be described with reference to the accompanying figures, in which:

[0032] Fig. 1 schematically shows a system that supports one or more example embodiments of present disclosure;

[0033] Fig. 2 shows a block diagram of an apparatus that supports one or more example embodiments of present disclosure;

[0034] Fig. 3A shows an example of electric grid frequency deviation and corresponding VPP activation in FCR-N;

[0035] Fig. 3B shows logical components of an arrangement that supports one or more example embodiments of present disclosure; and

[0036] Figs. 4-5 are flow charts of methods of example embodiments.

[0037] DETAILED DESCRIPTION

[0038] In the following description, like reference signs denote like elements or steps.

[0039] Various embodiments of present disclosure provide mechanisms to control a virtual power plant (VPP) to provide symmetric power reserve for an electric grid, wherein the VPP comprises a plurality of battery units. The VPP may be controlled to provide up regulation for the electric grid by drawing energy to the grid from one or more battery units of the VPP and down regulation by storing energy from the grid to one or more battery units of the VPP. That is, the battery units can be used for temporarily feeding energy to the electric grid and / or for temporarily storing surplus energy from the electric grid.

[0040] The battery units may be spatially distributed battery units that are centrally controlled to form the VPP functionality. The battery units may be resources maintained for example for emergency energy backup purposes, such as backup batteries of a mobile network. Additionally or alternatively, the battery units may be resources owned by households or small and medium sized companies or other smaller scale operators. As an alternative nonlimiting example, the battery units may be intended for storing energy from local renewable sources such as solar panels and / or wind generators or even from a fuel-operated genset. As yet another alternative or additional non-limiting example, the intended use of the battery units is optimization of self-consumption. The VPP may be a hybrid system using multiple energy sources and / or multiple different types of battery units. In an example embodiment, the VPP is formed of centrally controlled backup batteries of a mobile network.

[0041] In general, the battery units in this disclosure refer to battery units that are able to handle regular charge and discharge cycles. For example, lithium based batteries are such battery units. In more detail, one or more of the following battery technologies may be represented in the batteries of present disclosure: lithium-nickel-cobalt, NCA, lithium-iron-phosphate, LFP, lithium-nickel-manganese-cobalt, NMC, flow batteries, and solid-state batteries. The battery units may have different properties with regard to price, durability, physical size and wear depending for example on the battery technology and storage capacity.

[0042] In general, lithium based batteries should not regularly exceed extreme low or high charge values. For example, state of charge below 5% or above 95% should be avoided. Such limitations should be taken into account in usage of the lithium-ion batteries to avoid increased wear of the batteries.

[0043] Fig. 1 schematically shows a system that supports one or more example embodiments of present disclosure. The scenario shows a VPP formed of battery units 121-125, each including one or more batteries. In a real life system, there may be thousands of battery units in the VPP. The battery units 121-125 may be located at different geographical locations, but equally there may be plurality of battery units at the same location. Fig. 1 shows the battery units 123-125 at the same location and the battery units 121 and 122 individually at different locations. The battery units 123-125 are co-located battery units that may be owned for example by a small company. In practice, each battery unit may be associated with an individual IP address to enable remote control of the battery units. In an example, co-located battery units may require multiple co-located IP addresses for sending control messages to the battery units. Alternatively, co-located battery units may be controlled through one IP address that can be used for distributing control messages to the co-located battery units. In a further alternative, there may be additional hardware providing control of the battery units. The additional hardware may comprise a gateway box or the like, for example. Such additional hardware may require one IP address as well. The battery units 121-125 may be intended for emergency backup purposes, but this is not mandatory. In an example embodiment, the battery units are backup batteries of a wireless communication network. In another example embodiment, the battery units are batteries of households or batteries of buildings. In an example embodiment, the battery units are co- located with an energy production unit, such as solar or wind farm. It is to be noted that this is only a non-limiting illustrative example and in practical implementations many different setups are possible.

[0044] Further, the scenario shows a control system 111. The control system 111 may control the battery units 121-125 to operate as a virtual power plant. Still further, Fig. 1 shows electric grid 151.

[0045] The control system 111 is configured to implement at least some example embodiments of present disclosure to provide VPP operation. For this purpose, the control system 111 is operable to interact with the battery units 121-125 and / or equipment associated thereto. The control system 111 may comprise a first interface 112 for such interaction. Communication over the first interface 112 may be implemented for example using Simple Network Management Protocol (SNMP). Other examples of possible control protocols include modbus and Rest API, for example. Additionally, the control system 111 is operable to interact with the electric grid 151 or equipment associated thereto to coordinate participation in frequency balancing of the electric grid. The control system 111 may comprise a second interface 113 for this purpose.

[0046] The operator of the VPP may receive compensation based on the frequency balancing carried out for the electric grid. The compensation may depend on actual activation of frequency balancing and / or on reserving capacity for the possible frequency balancing needs. Further, there may be penalty if the VPP fails to fulfil the frequency balancing commitments. The VPP may fail to fulfil the frequency balancing commitment by not being able to activate enough capacity in up or down direction and / or by not being able perform the necessary activations fast enough. In general, there may be an incentive to fulfil the commitments made. Sometimes failing to fulfil the commitments made may be acceptable, though. For example, if it is clear, it is not possible to fully fulfil the commitment, it may be chosen not to activate at all. In principle, it may be considered that it is better to activate 0% of the resources and fail than to activate 80% of the resources and fail.

[0047] Fig. 2 shows a block diagram of an apparatus that supports one or more example embodiments of present disclosure. The apparatus 20 is for example a general purpose computer, cloud computing environment or some other electronic data processing apparatus. The apparatus 20 can be used for implementing at least some embodiments of present disclosure. That is, with suitable configuration the apparatus 20 is suited for operating for example as the control system 111 of Fig. 1 or for providing at least some functionality of the control system 111 of Fig. 1 or other functionality according to one or more example embodiments of present disclosure.

[0048] The apparatus 20 comprises a communication interface 25; a processor 21 ; a user interface 24; and a memory 22. The apparatus 20 further comprises software 23 stored in the memory 22 and operable to be loaded into and executed in the processor 21 . The software 23 may comprise one or more software modules and can be in the form of a computer program product.

[0049] The processor 21 may comprise a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a graphics processing unit, or the like. Fig. 2 shows one processor 21 , but the apparatus 20 may comprise a plurality of processors.

[0050] The user interface 24 is configured for providing interaction with a user of the apparatus. Additionally or alternatively, the user interaction may be implemented through the communication interface 25. The user interface 24 may comprise a circuitry for receiving input from a user of the apparatus 20, e.g., via a keyboard, graphical user interface shown on the display of the apparatus 20, speech recognition circuitry, or an accessory device, such as a headset, and for providing output to the user via, e.g., a graphical user interface or a loudspeaker.

[0051] The memory 22 may comprise for example a non-volatile or a volatile memory, such as a read-only memory (ROM), a programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), a random-access memory (RAM), a flash memory, a data disk, an optical storage, a magnetic storage, a smart card, or the like. The apparatus 20 may comprise a plurality of memories. The memory 22 may serve the sole purpose of storing data, or be constructed as a part of an apparatus 20 serving other purposes, such as processing data.

[0052] The communication interface 25 may comprise communication modules that implement data transmission to and from the apparatus 20. The communication modules may comprise a wireless or a wired interface module(s) or both. The wireless interface may comprise such as a WLAN, Bluetooth, infrared (IR), radio frequency identification (RF ID), GSM / GPRS, CDMA, WCDMA, LTE (Long Term Evolution) or 5G radio module. The wired interface may comprise such as Ethernet or universal serial bus (USB), for example. The communication interface 25 may support one or more different communication technologies. The apparatus 20 may additionally or alternatively comprise more than one of the communication interfaces 25.

[0053] A skilled person appreciates that in addition to the elements shown in Fig. 2, the apparatus 20 may comprise other elements, such as displays, as well as additional circuitry such as memory chips, application-specific integrated circuits (ASIC), other processing circuitry for specific purposes and the like.

[0054] Frequency balancing of electric grid may be arranged for example by using automatic Frequency Restoration Reserve, aFRR, or Frequency Containment Reserve, FCR, capacity market. aFRR is a centralized automatically activated reserve. Its activation is based on a power change signal calculated on the basis of the frequency deviation in the Nordic synchronized area. Its purpose is to return the frequency to the nominal value. FCR is an active power reserve that is automatically controlled based on the frequency deviation. FCR may be Frequency Containment Reserve for Normal Operation, FCR-N, or Frequency Containment Reserve for Disturbances, FCR-D. Their purpose is to contain the frequency during normal operation and disturbances.

[0055] FCR and FCR-N are examples of symmetric power reserve, wherein the VPP providing the power reserve needs to be able to adjust the power level to both directions. That is, it is required that the VPP is able to feed energy to the grid and to draw energy from the grid depending on the frequency balancing requirement. Moreover, symmetric power reserve requires that the VPP is able to provide the same volume of energy to be fed to the grid and to be drawn from the grid. That is, if symmetric power reserve commits to providing certain volume of energy for up regulation, there is a need to commit to providing the same volume of energy needs for down regulation, too.

[0056] In FCR-N, there is a need to react directly to the frequency of the grid and frequency balancing needs to be activated always when the frequency of the grid deviates from the nominal 50 Hz. The activation is symmetric and in case of battery units it means that the battery units need to recharge when the frequency of the grid is > 50 Hz and discharge when frequency of the grid is < 50 Hz. The required power depends on the magnitude of the deviation.

[0057] Some geographical regions may have multiple TSOs. The TSOs may also have different power reserve or regulation systems. Grid frequency may also vary, for example 60Hz or 50Hz, depending on the region.

[0058] Fig. 3A shows an example of electric grid frequency deviation and corresponding VPP activation in FCR-N.

[0059] Line 330 shows frequency deviation over time and line 331 shows corresponding cumulative activated energy (i.e. cumulative activation of battery units of VPP). During time period 335, down regulation is activated and the VPP is storing energy to one or more battery units of the VPP (i.e. storing energy from the grid). During time period 336, up regulation is activated and the VPP is drawing energy from one or more battery units of the VPP (i.e. feeding energy to the grid).

[0060] The activation of the battery units may be relatively symmetric over time, i.e. about the same amount of energy is drawn from the VPP and stored into the VPP from the grid over time, but sometimes there may be more deviation in one direction. For example in Fig. 3A, the activations over time periods 335 and 336 may cancel each other out and the total energy level of the VPP may remain relatively stable over time. However, there may be more deviation in one direction, e.g. significantly more up regulation than down regulation or vice versa, whereby the total energy level of the VPP may drift to relatively low or relatively high levels. Such situation is not desirable as too low or too high energy levels may cause a risk that the battery units cannot be used as intended for example for later grid balancing actions or for providing a backup energy reserve or for some other purpose.

[0061] Various embodiments of the present disclosure provide solutions to improve VPP control in the context of symmetric power reserve. At least some embodiments of the present disclosure may help in avoiding drift of energy levels to too low or too high levels. Various embodiments of the present disclosure provide e.g. solutions to avoid excessive drift in the total energy level of the VPP. Some embodiments may further provide avoiding excessive drift in energy levels of individual battery units.

[0062] The solution provided herein is to controllably compensate energy drift in the total (or aggregate) energy level of the VPP. This may be performed e.g. by drawing energy from one or more battery units of the VPP to the grid or by storing energy from the grid to one or more battery units of the VPP. For example, intraday energy markets may enable this. In more detail, the solution is to continuously monitor the total energy level of the VPP, to detect drift in the total energy level of the VPP, and to compensate the drift in the total energy level of the VPP. Intraday markets may be used as source or sink of energy for the compensation. There may be a feedback loop that continuously monitors the overall state of charge (SoC) of the VPP system and injects intraday market bids if the total SoC starts to drift too much towards one or the other direction (i.e. to low or high SoC). There may be a predefined energy level setpoint defining a desired total energy level or an acceptable amount of deviation in the total energy level. The drift may be detected based on detecting that a difference between the total energy level of the VPP and a predefined energy level setpoint exceeds a predefined threshold. In an embodiment, the intraday market that is used for compensating the energy drift is a market called "Picasso" where participants bid on energy and capacity. If the SoC starts to drift too much towards one or the other direction (i.e. to low or high SoC), the energy levels of the bids to the Picasso market can be adjusted correspondingly.

[0063] Fig. 3B shows logical components of an arrangement that supports one or more example embodiments of present disclosure.

[0064] Fig. 3B shows a control system 111 formed of a bidder entity 301 and a control entity 302. Further, Fig. 3B shows plurality of battery units 121-125. The bidder entity may run in a cloud environment 310, whereas the control entity may be running in a local computing environment (a real time environment). The control entity 302 comprises an intraday bidder 305, a SoC monitor 304 and a controller 303.

[0065] In an example embodiment, the arrangement of Fig. 3B operates as follows. The bidder 301 bids on the FCR-N market the day before and creates an operating schedule for the VPP for the following day. The schedule is delivered to the control entity 302.

[0066] The controller 303 may allocate the battery units 121-125 separately for up and down activations to drive their respective SoC to a battery unit specific target level (e.g. optimal value). If a battery unit has very high SoC it will be selected for up regulations and if a battery unit has very low SoC it will be selected for down regulation. Respective activation commands are conveyed from the control entity 302 to the battery units 121-125. The controller 303 obtains information about the SoC levels of the battery units 121-125 for making the selections. The information may be obtained directly from the battery units 121- 125 or elsewhere.

[0067] The SoC monitor 304 monitors the total SoC of the VPP. In case the total SoC drifts too much up or down, there is a feedback mechanism from the SoC monitor 304 to the intraday bidder 305. The intraday bidder 305 makes intraday market bids based on the feedback from the SoC monitor 304 to fine tune the total SoC balance of the VPP system. Information about the intraday market bids is fed to the controller 303 to command the battery units 121- 125 accordingly.

[0068] Figs. 4-5 are flow charts of methods of example embodiments. Figs. 4-5 illustrate processes for controlling a VPP comprising various possible steps including some optional steps while also further steps can be included and / or some of the steps can be performed more than once. The processes may be implemented in the control system 111 of Fig. 1 , in the control entity 320 of Fig. 3A, and / or in the apparatus 20 of Fig. 2. The processes are implemented in a computer program code and does not require human interaction unless otherwise expressly stated. It is to be noted that the processes may however provide output that may be further processed by humans and / or the processes may require user input to start.

[0069] The process of Fig. 4 comprises the following steps:

[0070] 401 : The VPP is controlled to provide a symmetric power reserve for an electric grid. The symmetric power reserve may be provided for example according to Frequency Containment Reserve, FCR, or according to Frequency Containment Reserve for Normal Operation, FCR-N or according to some other system. For example, in some countries also secondary power reserve (such aFRR) could be symmetric.

[0071] 402: A need to activate the power reserve for up regulation or for down regulation is detected. This need may be detected e.g. based on the deviation in frequency of the electric grid. In this context, the up regulation comprises drawing energy from one or more battery units of the VPP and the down regulation comprises storing energy to one or more battery units of the VPP.

[0072] 403: Information is obtained about energy levels of the battery units of the VPP. This is performed in real time in order to always be aware of current status of the battery units.

[0073] 404: Battery units are selected for up regulation or down regulation based on the information about energy levels of the battery units.

[0074] 405: Total energy level of the VPP is continuously monitored.

[0075] 406: Drift is detected in the total energy level of the VPP.

[0076] 407: The drift in the total energy level of the VPP is compensated for example by using intraday markets.

[0077] The process of Fig. 5 comprises the following steps. One or more of the steps disclosed in Fig. 5 may relate to further example embodiments associated with features of one or more steps of Fig. 4.

[0078] 501 : Drift in the total energy level of the VPP is detected (e.g. in step 406 of Fig. 4) based on detecting that a difference between the total energy level of the VPP and a predefined energy level setpoint exceeds a predefined threshold. The predefined energy level setpoint may be defined for example as percentage of total aggregated storage capacity of the VPP. The percentage may vary depending on an operating plan for a particular day and other usage needs of the resources of the VPP. Further, the predefined energy level setpoint may vary depending on time of day. For example, the predefined energy level setpoint may be 40% during some hour and 70% during some other hour of the day. In yet another example the percentage may be 50%. In general, the percentage may be for example a value selected from range 30-80%.

[0079] Also the threshold may vary depending on the implementation and may be defined as a percentage or as an absolute energy level value. The threshold may be for example a value selected from range 50-75% of the predefined energy level setpoint. In yet other alternatives, the threshold may be for example 55%, 60%, 65%, or 70% % of the predefined energy level setpoint. By using the threshold, one may avoid ping pong effect of continuously detecting drift in either direction.

[0080] 502: The intraday energy market for compensating the drift in total energy level of the VPP is used (e.g. in step 407 of Fig. 4) by offering to sell surplus energy to the intraday market during a next possible bidding time period or offering to buy missing energy from the intraday market during the next possible bidding time period.

[0081] The amount of offered surplus or missing energy may depend on a difference between the total energy level of the VPP and the predefined energy level setpoint. In an embodiment, the offered amount is at least a little bit less than difference between the total energy level of the VPP avoid ping pong effect of continuously detecting drift in either direction. The amount of offered surplus or missing energy may depend on determined risk levels and operating conditions. Further, observed price points could be taken into account, but this is not mandatory.

[0082] In an embodiment, the offer to the intraday market is made for next possible bidding time period, e.g. hour. In this way the energy level may be compensated as fast as possible. In another alternative, the offer to the intraday market is spread over more than one bidding time period. The offer may be spread over 2-3 hours, for example. For example, if there are some power limits and / or the energy drift is big, there may be a need to spread the offer to the intraday market for a couple of hours to ensure sufficient power and stability of the system.

[0083] 503: The battery units are selected (e.g. in step 404 of Fig. 4) so that battery units with higher energy level are selected for up regulation and battery units with lower energy level are selected for down regulation.

[0084] 504: The battery units are selected (e.g. in step 404 of Fig. 4) so that battery units with energy level above a battery unit specific target level are selected for up regulation and battery units with energy level below the battery unit specific target level are selected for down regulation.

[0085] 505: The battery units are assigned to an up regulation list or to a down regulation list based on the information about energy levels of the battery units.

[0086] 506: The battery units are selected (e.g. in step 404 of Fig. 4) by selecting battery units from the up regulation list for up regulation and selecting battery units from the down regulation list for down regulation.

[0087] 507: The battery units on the up regulation list are prioritized based on one or more of: an amount of available energy to be discharged, an amount of energy available before reaching a battery unit specific target level, absolute capacity of the battery unit.

[0088] 508: The battery units on the down regulation list are prioritized based on one or more of: an amount of storage space for storing energy, an amount of storage space available before reaching a battery unit specific target level, absolute capacity of the battery unit.

[0089] Steps 503-508 relate to different alternatives that provide effect that energy levels of the battery units of the VPP are individually controlled instead of treating all battery units the same way. In this way energy is internally balanced within the VPP and the energy levels of the battery units may be driven desired state of charge (SoC) level even if the SoC levels vary between the battery units.

[0090] In an embodiment, the energy sold to or bought from the intraday energy market (e.g. in step 502 of Fig. 5), is spread to the individual battery units of the VPP based on the information about energy levels of the battery units. In this way energy drift in individual battery units may be effectively compensated. For example, if energy is sold to the intraday energy market, the sold energy may be drawn from battery units that benefit from or are suited for discharging energy (i.e. battery units that have higher energy level or battery units that have energy level above a battery unit specific target level or battery units that are on the up regulation list). For example, if energy is bought from the intraday energy market, the bought energy may be stored to battery units that benefit from or are suited for charging energy (i.e. battery units that have lower energy level or battery units that have energy level below a battery unit specific target level or battery units that are on the down regulation list).

[0091] Without in any way limiting the scope, interpretation, or application of the appended claims, a technical effect of one or more of the example embodiments disclosed herein is improved control of a virtual power plant, VPP. Various embodiments help taking into account the heterogeneous characteristics of the battery units of the VPP as battery units are selected for up or down regulation tasks based on individual energy levels of the battery units. By means of detecting and compensating the drift in total energy level of the VPP reliability of the VPP may be improved. Combination of compensating the drift in total energy level of the VPP and selecting battery units based on individual energy levels one achieves further improvements as the compensation of the drift in the total energy level is spread to individual battery units based on individual energy levels.

[0092] Any of the afore described methods, method steps, or combinations thereof, may be controlled or performed using hardware; software; firmware; or any combination thereof. The software and / or hardware may be local; distributed; centralised; virtualised; or any combination thereof. Moreover, any form of computing, including computational intelligence, may be used for controlling or performing any of the afore described methods, method steps, or combinations thereof. Computational intelligence may refer to, for example, any of artificial intelligence; neural networks; fuzzy logics; machine learning; genetic algorithms; evolutionary computation; or any combination thereof.

[0093] Various embodiments have been presented. It should be appreciated that in this document, words comprise; include; and contain are each used as open-ended expressions with no intended exclusivity.

[0094] The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

[0095] Furthermore, some of the features of the afore-disclosed example embodiments may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

CLAIMS1. A computer implemented method for controlling a virtual power plant, VPP, to provide (401 ) symmetric power reserve for an electric grid, wherein the VPP comprises a plurality of battery units; the method comprising detecting (402) a need to activate the power reserve for up regulation or for down regulation, wherein the up regulation comprises drawing energy from one or more battery units of the VPP and the down regulation comprises storing energy to one or more battery units of the VPP; obtaining (403) information about energy levels of the battery units of the VPP; selecting (404) battery units for up regulation or down regulation based on the information about energy levels of the battery units; continuously monitoring (405) total energy level of the VPP; detecting (406) drift in the total energy level of the VPP; and compensating (407) the drift in the total energy level of the VPP.

2. The method of claim 1 , wherein the compensation of the drift in the total energy level is performed by drawing energy from one or more battery units of the VPP to the electric grid or by storing energy from the electric grid to one or more battery units of the VPP.

3. The method of any preceding claim, wherein the symmetric power reserve is provided according to Frequency Containment Reserve, FCR.

4. The method of any one of claims 1-2, wherein the symmetric power reserve is provided according to Frequency Containment Reserve for Normal Operation, FCR-N.

5. The method of any preceding claim, wherein drift in the total energy level of the VPP is detected based on detecting (501 ) that a difference between the total energy level of the VPP and a predefined energy level setpoint exceeds a predefined threshold.

6. The method of claim 5, wherein the threshold is 50-75% of the predefined energy level setpoint.

7. The method of any preceding claim, wherein the compensation of the drift in the total energy level is performed by using intraday markets as a source or sink of energy for the compensation.

8. The method of claim 7, wherein using intraday energy markets for compensating the drift in the total energy level of the VPP comprises offering to sell surplus energy to the intraday market during a next possible bidding time period or offering to buy missing energy from the intraday market during the next possible bidding time period (502).

9. The method of claim 8, wherein the amount of offered surplus or missing energy depends on a difference between the total energy level of the VPP and a predefined energy level setpoint.

10. The method of claim 8 or 9, wherein the amount of offered surplus or missing energy is spread over more than one bidding time period.

11. The method of any preceding claim, wherein the battery units are selected so that battery units with higher energy level are selected for up regulation and battery units with lower energy level are selected for down regulation (503).

12. The method of any preceding claim, wherein the battery units are selected so that battery units with energy level above a battery unit specific target level are selected for up regulation and battery units with energy level below the battery unit specific target level are selected for down regulation (504).

13. The method of any preceding claim, further comprising assigning (505) the battery units to an up regulation list or to a down regulation list based on the information about energy levels of the battery units; and selecting battery units from the up regulation list for up regulation and selecting battery units from the down regulation list for down regulation (506).

14. The method of claim 11 , further comprising prioritizing (507) the battery units on the up regulation list based on one or more of: an amount of available energy to be discharged, an amount of energy available before reaching a battery unit specific target level, absolute capacity of the battery unit; and / or prioritizing (508) the battery units on the down regulation list based on one or more of: an amount of unit space for storing energy, an amount of unit space available before reaching a battery unit specific target level, absolute capacity of the battery unit.

15. An apparatus (20, 111) comprising means for performing the method of any one of claims 1-12.

16. The apparatus (20, 111 ) of claim 13, wherein the means comprise a processor (21 ) and a memory (22) including computer program code, and wherein the memory and the computer program code are configured to, with the processor, cause the performance of the apparatus.

17. A computer program comprising computer executable program code (23) which when executed in an apparatus causes the apparatus to perform the method of any one of claims 1-12.

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