Controller for a controlling a power grid, computer- implemented method, computer program, non-volatile data carrier and control system

Abstract

A controller (100) obtains a load signal (LS) indicating a frequency (f) of an alternating voltage in the power grid (110). The controller (100) checks the load signal (LS) against at least one regulation criterion (TL1, TL2, TL3, TLR) for the frequency (f) of the alternating voltage in the power grid (110). If at least one of the at least one regulation criterion is fulfilled, the controller (100) transmits at least one control signal (C1, C2, Cn) over at least one communication network (130). The at least one control signal (C1, C2, Cn) causes at least one sauna unit in a set of sauna units (S1, S2, Sn) to be either connected to, or disconnected from the power grid (110). Thus, the frequency (f) is either adjusted down or up.

Description

[0001] Controller for a Controlling a Power Grid, Computer- Implemented Method, Computer Program, Non-Volatile Data Carrier and Control System

[0002] TECHNICAL FIELD

[0003] The present invention relates generally to electric energy systems. Especially, the invention relates to a controller according to claim 1 and a corresponding computer-implemented method. The invention also relates to a computer program, a non-volatile data carrier storing such a computer program and a control system for a power grid.

[0004] BACKGROUND

[0005] In electricity grids there is a general relationship between the relative load and the frequency of the alternating current. This means that if the overall load to which the grid must provide electric power increases, the average frequency decreases. Conversely, if there is a temporary overproduction of electric power relative to the demand from the connected loads, the average frequency increases.

[0006] A failure to quickly balance the power system may result in unwanted frequency deviations. Traditionally, variable renewable generation has been curtailed, or conventional generation has been ramped up to support grid frequency stability. Non-spinning generators may also be quick started to mitigate the problem of temporarily increasing frequency in a power grid.

[0007] Today, there is a relatively large fraction of electric energy that originates from renewable sources, such as wind, solar and wave, whose production is uncontrollable. Consequently, there is an increasing need for methods to support grid stability, such as actively and intelligently controlling loads. EP 2 940 648 shows a refrigerator, freezer or other electrically operated device which only operates at certain, however variable times. The electrically operated device is equipped with an electronic switching unit, which allows the device to be switched on or off by remote control, for instance in response to an hourly varying electricity price, or varying electricity production from renewable sources of energy.

[0008] WO 2012 / 047888 describes a method of dynamically controlling a small-scale electrical load receiving energy from an electricity grid that includes sources of renewable generation causing variations in electricity supply of the electricity grid. The small-scale electrical loads are coupled to a load-matching thermostat having a communication module and a controller that manage electricity load to electrical supply for the electrical load.

[0009] Fl 128 448 shows an apparatus and a method for managing residential electrical loads. A residential electrical current usage is monitored to prevent the peak usage from exceeding the limit. The electrical loads are grouped, and groups are prioritized. The group may have only one electrical load or multiple electrical loads. If the electrical current usage exceeds a predefined limit, the apparatus prioritizes the electrical loads and provides less electrical current to low-priority load. The priority list may be scheduled and dynamic. As one example, during late evenings electrical loads providing residential comfort such as the water heater or sauna heater is prioritized over charging an electric vehicle. If the electric vehicle is scheduled to drive in the morning, early hours are prioritized for charging the electric vehicle.

[0010] Thus, various solutions exist for controlling electric loads inter alia aiming at stabilizing a power grid. However, there is room for improving the efficiency and flexibility of these solutions.

[0011] SUMMARY

[0012] The object of the present invention is therefore to offer a solution that mitigates the problem of grid frequency variations and enab- les quick balancing of the production-to-demand ratio.

[0013] According to one aspect of the invention, the object is achieved by a controller for controlling a power grid. The controller is configured to obtain a load signal indicating a frequency of an alternating voltage in the power grid. The controller is also configured to check the load signal against at least one regulation criterion for the frequency of the alternating voltage in the power grid. If at least one of the at least one regulation criterion is fulfilled, the controller is further configured to transmit at least one control signal configured to cause at least one sauna unit in a set of sauna units to be either connected to or disconnected from the power grid.

[0014] This controller is advantageous because it enables fast control of large electric loads, which are relatively uncritical. In other words, a typical electricity subscriber is not particularly concerned if his / her sauna is automatically switched on or off without his / her consent. Each sauna unit also constitutes a substantial electric load and therefore has a relatively large impact on the power grid.

[0015] According to one embodiment of this aspect of the invention, at least one first regulation criterion of the at least one regulation criterion defines a first frequency threshold level.

[0016] If the load signal indicates that the frequency of the alternating voltage in the power grid exceeds the first frequency threshold level, the controller is configured to transmit the at least one control signal such that a first subset of sauna units in the set of sauna units is caused to be connected to the power grid. The thus increased load may counteract the somewhat elevated frequency level.

[0017] According to another embodiment of this aspect of the invention, at least one second regulation criterion of the at least one regulation criterion defines a second frequency threshold level. Here, if the load signal indicates that the frequency of the alternating voltage in the power grid exceeds the second frequency threshold level, which is above the first frequency threshold level, the controller is configured to transmit the at least one control signal such that a second subset of sauna units in the set of sauna units is caused to be connected to the power grid in addition to the first subset of sauna units. Thereby, the even higher load may retard the continued elevation of the frequency level.

[0018] Preferably, at least one third regulation criterion of the at least one regulation criterion defines a third frequency threshold level, which is above the second frequency threshold level; and if the load signal indicates that the frequency of the alternating voltage in the power grid exceeds the third frequency threshold level, the controller is configured to transmit the at least one control signal such that a third subset of sauna units in the set of sauna units is caused to be connected to the power grid in addition to the first and second subsets of sauna units. This allows yet another load level to be added for reducing the frequency in the power grid.

[0019] According to a further embodiment of this aspect of the invention, at least one criterion of the at least one regulation criterion defines a lower frequency threshold level. If the frequency of the alternating voltage in the power grid falls below the lower frequency threshold level, the controller is configured to transmit the at least one control signal such that at least one subset of sauna units in the set of sauna units being connected to the power grid is caused to be disconnected from the power grid. Consequently, the frequency in the power grid may be controlled to be elevated again.

[0020] According to another aspect of the invention, the object is achieved by a computer-implemented method for controlling a power grid, which method is performed in processing unit in a controller. The method involves obtaining a load signal indicating a frequency of an alternating voltage in the power grid. The method further involves checking the load signal against at least one regulation criterion for the frequency of the alternating voltage in the power grid. If at least one of the at least one regulation criterion is fulfil- led, the method involves transmitting at least one control signal configured to cause at least one sauna unit in a set of sauna units to be either connected to or disconnected from the power grid. The advantages of this method, as well as the preferred embodiments thereof, are apparent from the discussion above with reference to the proposed controller.

[0021] According to a further aspect of the invention, the object is achieved by a computer program loadable into a non-volatile data carrier communicatively connected to a processing unit. The computer program includes software for executing the above method when the program is run on the processing unit.

[0022] According to another aspect of the invention, the object is achieved by a non-volatile data carrier containing the above computer program.

[0023] According to another aspect of the invention, the object is achieved by a control system for a power grid, which control system includes the above-proposed the controller, a frequency metering unit, a set of sauna units and at least one communication network. The frequency metering unit is configured to measure a frequency of an alternating voltage in the power grid and send a load signal indicating said frequency. Each sauna unit in the set of sauna units is selectively connectable to and disconnectable from the power grid in response to the at least one control signal from the controller. The at least one communication network, e.g. represented by the Internet, is configured to communicate the at least one control signal from the controller to the sauna units in the set of sauna units. The advantages of this control system are apparent from the discussion above with reference to the proposed controller and computer-implemented method.

[0024] According to one embodiment of this aspect of the invention, the control system contains a respective gateway unit, which is arranged to control a subset of sauna units in the set of sauna units, e.g. a single sauna unit, via a respective relay control signal to a respective power relay. Specifically, the gateway unit is configured to control the respective power relays such that at least one sauna unit in the subset of sauna units is connected to the power grid, or disconnected from the power grid. This allows for convenient remote control of the sauna units from the controller.

[0025] According to another embodiment of this aspect of the invention, each of the respective gateway units is configured to send a respective position message to the controller via the at least one communication network. The respective position message indicates a respective position in the power grid to which position the at least one sauna unit in the subset of sauna units connectable / disconnectable. This is beneficial, since it enables the controller to select which sauna unit(s) to control based on their position in the power grid.

[0026] Preferably, at least one of the respective gateway units includes a Global Navigation Satellite System (GNSS) receiver that is configured to provide geographic position data as a basis for the geographic position data in the position message.

[0027] According to still another embodiment of this aspect of the invention, each of the respective gateway units is further configured to repeatedly send respective feedback messages to the controller via the at least one communication network. Here, the respective feedback messages indicate a respective connection status for each sauna unit in the subset of sauna units, which subset the respective gateway unit is arranged to control. The respective connection status indicates whether the sauna unit is connected to or disconnected from the power grid. Thus, the controller may confirm which sauna units that are still possible to activate, or deactivate respectively to control the frequency.

[0028] According to yet another embodiment of this aspect of the invention, at least one of the respective gateway units is further configured to repeatedly obtain a respective temperature signal indicating a respective temperature in a room in which the respective sauna is located. The respective gateway unit is further configured to repeatedly include information about the respective temperature in the respective feedback messages sent to the controller. As a result, the controller may for example conclude whether or not a given sauna unit should be continued to be operated, referring to comfort and / or safety factors

[0029] According to another embodiment of this aspect of the invention, at least one of the respective gateway units is communicatively connected to a user interface, which is configured to obtain user commands via the at least one communication network. In response to the user commands, the controller is further configured to control at least one function of a sauna unit in the subset of sauna units. Thereby, the electricity subscriber may also control the sauna unit in a straightforward manner, for instance via a an app in his / her smartphone.

[0030] Further advantages, beneficial features and applications of the present invention will be apparent from the following description and the dependent claims.

[0031] BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

[0033] Figure 1 shows a block diagram of a control system according to one embodiment of the invention;

[0034] Figure 2 shows a graph exemplifying how the frequency in a power grid may vary over time according to one embodiment of the invention; and

[0035] Figure 3 illustrates, by means of a flow diagram, the general method according to the invention. DETAILED DESCRIPTION

[0036] In Figure 1 , we see a block diagram of a control system according to one embodiment of the invention. Figure 2 shows a graph exemplifying how the frequency f in a power grid may vary over time t according to one embodiment of the invention.

[0037] The control system is arranged to control a power grid 110, and the system includes a controller 100, a frequency metering unit 120, a set of sauna units S1 , S2 and Sn respectively and at least one communication network, e.g. the Internet, here exemplified by 130.

[0038] The frequency metering unit 120 is configured to measure the frequency f of an alternating voltage in the power grid 110 and send a load signal LS to the controller 100, which load signal LS indicates the frequency f.

[0039] The controller 100 is configured to obtain the load signal LS from the frequency metering unit 120, and check the load signal LS against at least one regulation criterion for the frequency f of the alternating voltage in the power grid 110. Figure 2 exemplifies such criteria in the form of frequency threshold levels TL1 , TL2, TL3 and TLR respectively. In large power grids, the frequency f should be controlled within tight limits, typically within ±150 mHz. Therefore, assuming that a desired value of the frequency f is 50,00 Hz, the frequency threshold levels TL1 , TL2, TL3 may for example be 50,05 Hz, 50, 10 Hz and 50, 15 Hz respectively; and TLR may for example be 49,95 Hz.

[0040] According to embodiments of the invention, the frequency metering unit 120 is not the sole source of the load signal LS. In addition to the frequency metering unit 120, or as an alternative thereto, the load signal LS may be based on voltage measurements. Namely, there is a general relationship between the voltage in the power grid 110 and the frequency f. This means that if the load situation in the power grid 1 10 is such that the frequency f increases, the average voltage in the power grid 110 also increases. Vice versa, if the frequency f decreases, this means that the average voltage in the power grid 110 also decreases. Consequently, the average voltage in the power grid 110 may serve as partial or complete basis for the load signal LS. Thus, one or more voltage meters (not shown) may be employed as supplements to, or substitutes for, the frequency metering unit 120.

[0041] If at least one of said regulation criteria is fulfilled, the controller 100 is configured to transmit at least one control signal C1 , C2 and / or Cn to one or more of the sauna units S1 , S2 and / or Sn respectively. Each of the at least one control signal C1 , C2 and / or Cn is configured to cause at least one of the sauna units S1 , S2 and / or Sn to be either connected to the power grid 110, or disconnected from the power grid 110. The communication network 130 is configured to communicate the at least one control signal C1 , C2 and Cn from the controller 100 to the sauna units S1 , S2 and Sn respectively.

[0042] According to one embodiment of the invention, two or more sauna units may be jointly controlled via a gateway unit GW1 , GW2 and GWn. Thus, each of the gateway units GW1 , GW2 and GWn may be arranged to control a respective subset of the sauna units. For example, in Figure 1 , a first subset may be represented by S1 , a second subset may be represented by S2, and a third subset may be represented by Sn. A respective relay control signal b1 , b2 and bn from the gateway units GW1 , GW2 and GWn may be used to control a respective power relay R1 , R2 and Rn, such that at least one sauna unit in the subset of sauna units which the respective gateway unit is arranged to control is either connected to the power grid 110, or disconnected there from.

[0043] According to one embodiment of the invention, each of the respective gateway units GW1 , GW2 and GWn is configured to send a respective position message P1 , P2 and Pn to the controller 100 via the communication network 130. The position messages P1 , P2 and Pn indicate a respective position in the power grid 110 to which position the sauna unit(s) in the subset of sauna units in question is(are) connectable / disconnectable. Thus, depending on in which part of the grid 110 additional load or reduced load is required to attain a desired control of the frequency f, the controller 100 may generate the control signals C1 , C2 and / or Cn on the further basis of the position messages P1 , P2 and / or Pn and conveniently add or subtract sauna units via the control signals C1 , C2 and Cn. Indeed, based on the position messages P1 , P2 and / or Pn it is possible to simultaneously control the frequency f differently in different parts of the power grid 110. For example, in a first part of the power grid 110 the frequency may be controlled down by causing at least one sauna unit to be connected to the power grid 110 while, in a second part of the power grid 110, the frequency may be controlled up by causing at least one sauna unit to be disconnected from the power grid 110.

[0044] To produce the position messages P1 , P2 and Pn respectively, one or more of the gateway units GW1 , GW2 and / or GWn may contain a GNSS receiver configured to provide geographic position data. In such a case, the respective gateway unit is configured to generate the respective position message P1 , P2 and Pn based on the geographic position data from the GNSS receiver. This, of course, facilitates installing and reorganizing the gateway units GW1 , GW2 and GWn, since, basically, they may be implemented as plug-and-play units. In the lack of a GNSS receiver, however, applicable geographic position data may instead be entered manually in the gateway unit GW1 , GW2 and GWn, e.g. in connection with configuring the gateway unit in question

[0045] According to one embodiment of the invention, each of the gateway units GW1 , GW2 and GWn is configured to repeatedly send respective feedback messages FB1 , FB2 and FBn to the controller 100 via the at least one communication network 130. Each feedback message FB1 , FB2 and FBn indicates a respective connection status for each sauna unit in the subset of sauna units controlled by the gateway unit in question. The connection status indicates whether the sauna unit is connected to or disconnected from the power grid 110. Thus, the controller 100 may confirm that the sauna units constitute the intended electric loads in the power grid 110. Naturally, any faults and / or unintentional discon- nections / connections of sauna units may also be discovered via the feedback messages FB1 , FB2 and FBn.

[0046] Inter alia for comfort and safety reasons it is beneficial also to monitor the temperature in the room where the respective sauna is located. According to one embodiment of the invention, therefore, at least one of the respective gateway units GW1 , GW2 and / or GWn is further configured to repeatedly obtain a respective temperature signal from a temperature sensor in the room where the respective sauna is located. Thus, the temperature signal indicates a relevant temperature for each sauna unit in the subset of sauna units controlled by the gateway unit GW1 , GW2 and / or GWn in question. Additionally, the respective gateway units GW1 , GW2 and GWn are configured to repeatedly include information about the respective temperature in the respective feedback messages FB1 , FB2 and FBn sent to the controller 100.

[0047] It is generally desirable that each subscriber may control the equipment in his / her own premises in a convenient manner. Consequently, according to one embodiment of the invention, at least one of the respective gateway units GW1 , GW2 and / or GWn is communicatively connected to a user interface IF1 , IF2 and / or I Fn respectively. The user interface IF1 , IF2 and IFn is configured to obtain user commands via the communication network 130.

[0048] In response to the user commands, the gateway unit GW1 , GW2, and GWn respectively is configured to control at least one function of one or more sauna units in the subset of sauna units controlled by the gateway unit in question GW1 , GW2 and GWn. As a result, the subscriber may thereby remotely activate or deactivate his / her sauna unit, or set a desired temperature, e.g. via a smartphone app. Naturally, such remote control does not preclude local wireless control of the sauna units, e.g. via an IR hand control, or similar device. Referring now to Figure 2, we see a graph representing an example of the frequency f of the alternating voltage in the power grid 110 as a function of time t. The diagram of Figure 2 also shows first, second and third frequency thresholds TL1 , TL2, and TL3 respectively, and a lower TLR frequency threshold level representing the regulation criteria for the frequency f.

[0049] According to embodiments of the invention, the controller 100 is configured to control the sauna units S1 , S2 and Sn in relation to the regulation criteria. TL1 defines one such criterion in the form of the first frequency threshold level. TL2 and TL3 define additional criteria in the form of the second and third frequency threshold levels respectively above the first frequency threshold level TL1 , and TLR defines yet another criterion in the form of the lower frequency threshold level below the first frequency threshold level TL1.

[0050] For example, if, at a point in time ti , the load signal LS indicates that the frequency f of the alternating voltage in the power grid 110 exceeds the first frequency threshold level TL1 , the controller 100 may be configured to transmit at least one control signal, say C1 , to a first subset of sauna units S1 , such that the sauna units therein are caused to be connected to the power grid 110. It is expected that the thus increased load in the power grid 110 will reduce the frequency f.

[0051] However, at a point in time t2, the frequency f instead rises above the second frequency threshold level TL2. Therefore, the controller 100 may be configured to transmit at least one control signal, say C2, to a second subset of sauna units S2, such that the sauna units therein are caused to be connected to the power grid 110 in addition to the first subset of sauna units S1. The yet increased load on the power grid 110 is likewise expected to reduce the frequency f. Nevertheless, for various reasons, for instance due to increased production by renewable energy sources and / or because other loads in the power grid 110 decrease their demand, the frequency f continues to elevate. At a point in time ts, the frequency f passes also the third frequency threshold level TL3. Consequently, the controller 100 may be configured to transmit at least one control signal, say C3, to a third subset of sauna units S3, such that the sauna units therein are caused to be connected to the power grid 110 in addition to the first and second subsets of sauna units S1 and S2 respectively. This time, we assume that the total load in the power grid 110 is sufficient to provide the intended effect, namely, to reduce the frequency f. At a point in time t4, the frequency f falls below the third frequency threshold level TL3; at a point in time ts, the frequency f falls below the second frequency threshold level TL2; and at a point in time ts, the frequency f also falls below the first frequency threshold level TL1.

[0052] In fact, at a point time t?, the frequency f even falls below the lower frequency threshold level TLR. According to one embodiment of the invention, this is a trigger for the controller 100 to instigate a reduction of the load added to the power grid 110 in the form of the sauna units in the first, second and third subsets of sauna units S1 , S2 and S3 respectively. For example, after point time t?, the controller 100 may transmit the control signal C1 to the first subset of sauna units S1 , such that the sauna units therein are caused to be disconnected from the power grid 1 10.

[0053] Of course, the controller 100 may transmit a corresponding disconnecting control signal irrespective of which or how many subsets of sauna units that have been connected to the power grid 110, if the frequency f falls below the lower frequency threshold level TLR. However, in order to be disconnected, a sauna unit must first have been connected to the power grid 1 10, either via the controller 100 or by other means, e.g. in response to a usergenerated remote command, or through manual activation by a subscriber.

[0054] It is generally advantageous if the controller 100 is configured to effect the above-described procedure by executing a computer program. Therefore, the controller 100 may include a memory unit 105, i.e. non-volatile data carrier, storing a computer program 107, which, in turn, contains software for making processing circuitry in the form of a processor 103 in the controller 100 execute the actions mentioned in this disclosure when the computer program 107 is run on the processor 103.

[0055] In order to sum up, and with reference to the flow diagram in Figure 3, we will now describe the computer-implemented method according to the invention which is performed in the processing unit 103 of the controller 100.

[0056] In a first step 310, it is checked if a load signal LS has been obtained, which load signal LS indicates the frequency f of the alternating voltage in the power grid 110. If such a load signal LS has been obtained, a step 320 follows; and otherwise, the procedure loops back and stays in step 310.

[0057] In step 320 in is checked if the load signal LS fulfills at least one regulation criterion. If the load signal LS fulfills at least one of the at least one regulation criterion, a step 330 follows; and otherwise, the procedure loops back to step 310.

[0058] In step 330, at least one control signal C1 , C2 and / or Cn is transmitted to at least one sauna unit in at least one set of sauna units S1 , S2 and / or Sn. The at least one control signal C1 , C2 and / or Cn is configured to cause the at least one sauna unit to be either connected to or disconnected from the power grid 1 10 depending on whether the frequency f should be controlled up or down relative to a desired set value, e.g. 50,00 Hz or 60,00 Hz.

[0059] The process steps described with reference to Figure 3 may be controlled by means of a programmed processor. Moreover, although the embodiments of the invention described above with reference to the drawings comprise processor and processes performed in at least one processor, the invention thus also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source code, object code, a code inter- mediate source and object code such as in partially compiled form, or in any other form suitable for use in the implementation of the process according to the invention. The program may either be a part of an operating system, or be a separate application. The carrier may be any entity or device capable of carrying the program. For example, the carrier may comprise a storage medium, such as a Flash memory, a ROM (Read Only Memory), for example a DVD (Digital Video / Versatile Disk), a CD (Compact Disc) or a semiconductor ROM, an EPROM (Erasable Programmable Read-Only Memory), an EEPROM (Electrically Erasable Programmable Read-Only Memory), or a magnetic recording medium, for example a floppy disc or hard disc. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or by other means. When the program is embodied in a signal, which may be conveyed, directly by a cable or other device or means, the carrier may be constituted by such cable or device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant processes.

[0060] Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.

[0061] The term “comprises / comprising” when used in this specification is taken to specify the presence of stated features, integers, steps or components. The term does not preclude the presence or addition of one or more additional elements, features, integers, steps or components or groups thereof. The indefinite article "a" or "an" does not exclude a plurality. In the claims, the word “or” is not to be interpreted as an exclusive or (sometimes referred to as “XOR”). On the contrary, expressions such as “A or B” covers all the cases “A and not B”, “B and not A” and “A and B”, unless otherwise indicated. The mere fact that certain measures are reci- ted in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. It is also to be noted that features from the various embodiments described herein may freely be combined, unless it is explicitly stated that such a combination would be unsuitable.

[0062] The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

Claims1. A controller (100) for controlling a power grid (110), which controller (100) is configured to: obtain a load signal (LS) indicating a frequency (f) of an alternating voltage in the power grid (110), check the load signal (LS) against at least one regulation criterion (TL1 , TL2, TL3, TLR) for the frequency (f) of the alternating voltage in the power grid (110), and if at least one of the at least one regulation criterion is fulfilled, transmit at least one control signal (C1 , C2, Cn) configured to cause at least one sauna unit in a set of sauna units (S1 , S2, Sn) to be either connected to or disconnected from the power grid (110).

2. The controller (100) according to claim 1 , wherein at least one first regulation criterion of the at least one regulation criterion (TL1 , TL2, TL3, TLR) defines a first frequency threshold level (TL1 ), and if the load signal (LS) indicates that the frequency (f) of the alternating voltage in the power grid (110) exceeds the first frequency threshold level (TL1 ), the controller (100) is configured to transmit the at least one control signal (C1 , C2, Cn) such that a first subset of sauna units (S1 ) in the set of sauna units is caused to be connected to the power grid (110).

3. The controller (100) according to claim 2, wherein at least one second regulation criterion of the at least one regulation criterion (TL1 , TL2, TL3, TLR) defines a second frequency threshold level (TL2), and if the load signal (LS) indicates that the frequency (f) of the alternating voltage in the power grid (1 10) exceeds the second frequency threshold level (TL2), which is above the first frequency threshold level (TL1 ), the controller (100) is configured to transmit the at least one control signal (C1 , C2, Cn) such that a second subset of sauna units (S2) in the set of sauna units is caused to be connected to the power grid (110) in addition to the first subset of sauna units (S1 ).

4. The controller (100) according to claim 3, wherein at least one third regulation criterion of the at least one regulation criterion (TL1 , TL2, TL3, TLR) defines a third frequency threshold level (TL3), and if the load signal (LS) indicates that the frequency (f) of the alternating voltage in the power grid (1 10) exceeds the third frequency threshold level (TL3), which is above the second frequency threshold level (TL2), the controller (100) is configured to transmit the at least one control signal (C1 , C2, Cn) such that a third subset of sauna units (S3) in the set of sauna units is caused to be connected to the power grid (110) in addition to the first and second subsets of sauna units (S1 ; S2).

5. The controller (100) according to any one of claims 1 to 4, wherein at least one criterion of the at least one regulation criterion (TL1 , TL2, TL3, TLR) defines a lower frequency threshold level (TLR), and, if the frequency (f) of the alternating voltage in the power grid (110) falls below the lower frequency threshold level (TLR), the controller (100) is configured to transmit the at least one control signal (C1 , C2, Cn) such that at least one subset of sauna units in the set of sauna units being connected to the power grid (110) is caused to be disconnected from the power grid (110).

6. A computer-implemented method for controlling a power grid (110), which method is performed in processing unit (103) in a controller (100), the method comprising: obtaining a load signal (LS) indicating a frequency (f) of an alternating voltage in the power grid (110), checking the load signal (LS) against at least one regulation criterion (TL1 , TL2, TL3, TLR) for the frequency (f) of the alternating voltage in the power grid (110), and if at least one of the at least one regulation criterion is fulfilled, transmitting at least one control signal (C1 , C2, Cn) configured to cause at least one sauna unit in a set of sauna units (S1 , S2, Sn) to be either connected to or disconnected from the power grid (1 10).

7. A computer program (105) loadable into a non-volatile data carrier (107) communicatively connected to a processing unit (103), the computer program (105) comprising software for executing the method according claim 6 when the computer program (105) is run on the processing unit (103).

8. A non-volatile data carrier (107) containing the computer program (105) of the claim 7.

9. A control system for a power grid (1 10), characterized in that the system comprises: the controller (100) according to any one of claims 1 to 5, a frequency metering unit (120) configured to measure a frequency (f) of an alternating voltage in the power grid (110) and send a load signal (LS) indicating said frequency (f), a set of sauna units (S1 , S2, Sn) in which each sauna unit is selectively connectable to and disconnectable from the power grid (1 10) in response to the at least one control signal (C1 , C2, Cn) from the controller (100), and at least one communication network (130) configured to communicate the at least one control signal (C1 , C2, Cn) from the controller (100) to the sauna units in the set of sauna units (S1 , S2, Sn).

10. The control system according to claim 9, further comprising a respective gateway unit (GW1 , GW2, GWn) which is arranged to control a subset of sauna units in the set of sauna units (S1 , S2, Sn) via a respective relay control signal (b1 , b2, bn) to a respective power relay (R1 , R2, Rn) such that at least one sauna unit in the subset of sauna units which the respective gateway unit is arranged to control is connected to the power grid (110), or the at least one sauna unit in the subset of sauna units which the respective gateway unit is arranged to control is disconnected from the power grid (110).

11. The control system according to claim 10, wherein each of the respective gateway units (GW1 , GW2, GWn) is configured to send a respective position message (P1 , P2, Pn) to the controller (100) via the at least one communication network (130), which respective position message (P1 , P2, Pn) indicates a respective position in the power grid (110) to which position the at least one sauna unit in the subset of sauna units connectable / disconnect- able.

12. The control system according to claim 11 , wherein at least one of the respective gateway units (GW1 , GW2, GWn) comprises a GNSS receiver configured to provide geographic position data, and said at least one of the respective gateway units is configured to generate the respective position message (P1 , P2, Pn) based on the geographic position data.

13. The control system according to any one of claims 11 or 12, wherein the controller (100) is configured to generate the at least one control signal (C1 , C2, Cn) on the further basis of the position messages (P1 , P2, Pn)14. The control system according to any one of claims 10 to 13, wherein each of the respective gateway units (GW1 , GW2, GWn) is further configured to repeatedly send respective feedback messages (FB1 , FB2, FBn) to the controller (100) via the at least one communication network (130), which respective feedback messages (FB1 , FB2, FBn) indicate a respective connection status for each sauna unit in the subset of sauna units, which subset the respective gateway unit (GW1 , GW2, GWn) is arranged to control, which respective connection status indicates whether the sauna unit is connected to or disconnected from the power grid (110).

15. The control system according to claim 14, wherein at least one of the respective gateway units (GW1 , GW2, GWn) is further configured to: repeatedly obtain a respective temperature signal indicatinga respective temperature in a room in which the respective sauna is located for each sauna unit in the subset of sauna units, which subset the respective gateway unit (GW1 , GW2, GWn) is arranged to control, and repeatedly include information about the respective temperature in the respective feedback messages (FB1 , FB2, FBn) sent to the controller (100).

16. The control system according to any one of claims 10 to 15, wherein at least one of the respective gateway units (GW1 , GW2, GWn) is communicatively connected to a user interface (IF1 , IF2, IFn) configured to: obtain user commands via the at least one communication network (130), and in response to the user commands control at least one function of a sauna unit in the subset of sauna units, which said at least one gateway unit (GW1 , GW2,GWn) is arranged to control.

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