CONTROLLING ON-TIME OF ENERGY MODULES OF AN ENERGY STORAGE

Abstract

The invention relates to a method of controlling the on-time of a plurality of energy modules of an energy storage. The energy storage comprising a plurality of series connected energy modules forming an energy module string. A string controller is controlling which of the individual energy modules that is part of a current path through the energy module string, by control of the status of a plurality of switches. The string controller is controlling the frequency of the energy module string voltage according to an electric system reference related to a system to which the energy storage is connected. And wherein the string controller is controlling the switches of the individual energy modules so that each of the individual energy modules that are required to be included in the current path to establish the energy modules string voltage are included in the current path for at least a minimum on-time.

Claims

1-29. (canceled)

30. A method of controlling the on-time of a plurality of energy modules of an energy storage, the energy storage comprising a plurality of series connected energy modules forming an energy module string, wherein each of the individual energy modules are connected to the energy module string by a plurality of switches configured in an H-bridge, wherein a string controller is controlling which of the individual energy modules that is part of a current path through the energy module string, by control of the status of a plurality of the switches, wherein the string controller is controlling the frequency of the energy module string voltage according to an electric system reference of a system to which the energy storage is connected, and wherein the string controller is controlling the switches of the individual energy modules so that each of the individual energy modules that are required to be included in the current path to establish the energy modules string voltage are included in the current path for at least a minimum on-time.

31. A method according to claim 30, wherein the string controller is establishing the on-time of the individual energy modules dynamically according to a dynamic performance evaluation of the plurality of energy modules of the energy module string.

32. A method according to claim 30, wherein the string controller performs the dynamic performance evaluation prior to each turning on of an energy storage module.

33. A method according to claim 30, wherein the dynamic performance evaluation includes sorting the plurality of energy modules into a dynamic performance list.

34. A method according to claim 30, wherein sorting the plurality of energy modules into a dynamic performance list is based on at least one energy module parameter of the list comprising: on-time, state of charge, state of health, temperature and internal resistance.

35. A method according to claim 30, wherein the dynamic performance evaluation includes sorting the plurality of energy modules according to at least one of the list comprising: state of charge, state of health, temperature of the plurality of energy modules.

36. A method according to claim 30, wherein the dynamic performance evaluation further includes that the selection of which energy module that is to be connected next to the current path complies with at least one of the conditions selected from the list comprising: minimum on-time, minimum temperature, able to be charge and able to be discharged.

37. A method according to claim 30, wherein the string controller is furthermore controlling the amplitude of the energy module string voltage according to input received from controllers external to the energy module string.

38. A method according to claim 30, wherein the performance evaluation includes a wear evaluation established by the string controller based on historic data of use of the energy modules.

39. A method according to claim 30, wherein the energy storage comprises at least two energy module strings, each controlled by a string controller.

40. A method according to claim 30, the energy storage comprises an energy storage controller communicating with the string controller, wherein the energy storage controller is configured for establishing an active power reference or a reactive power reference based on measured electric system reference and provide the established active or reactive power reference to the string controller.

41. A method according to claim 30, wherein the string controller is configured to calculate a sequence in which the energy modules are turned on and turned off based on the dynamic performance list of the plurality of energy modules.

42. A method according to claim 30, wherein the string controller is configured to control the sequence in which the energy modules are turned on and turned off so that each energy module comprised by the sequence complies with at least one of the conditions selected from the list comprising: above a minimum on-time and below a maximum temperature.

43. A method according to claim 30, wherein the energy storage is a high powered energy storage for supplying stationary loads.

44. An energy storage comprising an energy module string, the energy module string comprising a plurality of energy modules, each of the plurality of energy modules comprises four switches forming an H-bridge, wherein one midpoint of the H-bridges of at least two energy modules is electrically connected, thereby establishing the energy module string, and wherein a string controller is configured for controlling the status of the switches of the H-bridge and thereby a current path through the energy module string so that the individual energy modules are turned on for at least a minimum on-time.

45. An energy storage according to claim 44, wherein the string controller is configured to control the on-time of the individual energy modules different in two subsequent periods of an AC voltage output from the energy storage string.

46. An energy storage according to claim 44, wherein the string controller is configured to calculate the sequence in which the energy modules are turned on and turned off based on a performance evaluation of the plurality of energy modules.

47. An energy storage according to claim 44, wherein the string controller is configured to determine a sequence in which the energy modules are turned on and turned off based on the dynamic performance list of the plurality of energy modules.

48. An energy storage according to claim 44, wherein the energy storage is a high powered energy storage for supplying stationary loads.

49. An energy storage according to claim 44, wherein the energy storage comprises at least two energy module strings, for example, at least three energy module strings.

Description

THE DRAWINGS

[0082] For a more complete understanding of this disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts:

[0083] FIG. 1 illustrates energy modules of a string of an energy storage,

[0084] FIG. 2a illustrates an energy storage module,

[0085] FIG. 2b illustrates the switches of an energy storage module,

[0086] FIG. 3a illustrates on-time of an energy storage module in an AC scenario,

[0087] FIG. 3b illustrates on-time of an energy storage module in a DC scenario and

[0088] FIG. 4 illustrates a flow charge of the method of controlling the energy storage.

DETAILED DESCRIPTION

[0089] The energy storage 7 of the present invention, can be used in several applications and for several reasons. To list a few, the energy storage 7 could be connected to the output of a generator of a wind turbine. Such generator is connected to a first end of an electric current path, the second end of which is connection to the utility grid. Between the generator and the utility grid a converter is typically located in the electric current path. Such converter may comprise a generator side converter connected to a grid side converter via a direct current (DC) link. Other configurations of a wind turbine may also be suitable for use with the present invention.

[0090] The energy storage 7 can be used in relation to all types of energy systems including wind turbine converters including DFIG (DFIG; Doubly Fed Induction Generator) converters, Full power 2 level back-to-back, Full power 3 level back-to-back, MMC (MMC; M Modular Multi-Level Converter), etc. The energy storage 7 can be located between the converter and the grid, in fact, it can be connected either in the dc link or between the converter and the transformer including a stator path of a DFIG configuration, in fact, it can be place on any AC or DC power line. Further, the energy storage 7 can be used in relation to all types of wind turbine generators including Induction Generator, Permanent Magnet Sync. Generator, Doubly Fed Induction Generator, Synchronous Generator, etc.

[0091] Further, the energy storage 7 can be used external to a wind turbine or other renewably energy generation systems as energy storage or grid support. One or more energy storages can be used as power supply to ships either when these are in harbour or between harbours to reduce use of fossil fuel generators and to reduce load on the electric grid of the harbour. In the following only one string of one energy storage is illustrated for simplicity, but the described principles could be used with several serial or parallel strings and several serial or parallel energy storages.

[0092] It should be noted, that the energy storage 7 including energy storage modules 8 preferably is located inside an electric cabinet. The electric cabinet protects the energy storage from environmental impact and may help maintaining a desired temperature, direct flow of cooling air, etc. Locating the energy storage in an electric cabinet is advantageous in that it is possible to located in sites of e.g. a wind turbine or other extreme sites.

[0093] FIG. 1 illustrates the principles of the design of an energy storage 7 including the minimum elements of the energy storage 7. The energy storage 7 is built of a plurality of energy storage modules 8. Each of the energy storage modules 8 comprise at least two semiconductor switches 10a, 10b and at least one energy storage element 9. The energy storage element 9 is preferably a battery cell, but could also be other alternatives such as capacitors. The status of the semiconductor switches 10 is controlled by a string controller 12 and thereby, the string controller 12 is controlling a current path 13 through the energy storage modules 8 of the energy storage 7. It should be mentioned, that in embodiments, the current path 13 is also considered passing through an energy storage modules 8 even though the energy storage element 9 hereof is by-passed.

[0094] The way of the current path 13 through the energy storage 7 is determined by the status of the semiconductor switches 10 and is therefore controlled by the string controller 12. The status of the semiconductor switches 10 is determined based on availability of energy storage modules/energy elements 8, 9, health status of the energy storage module/energy elements 8, 9, state of charge of the energy storage elements 9, charging voltage available, desired/required voltage across/from the energy storage 7, health/wear of switches 10, temperature, internal resistance and/or historical on-time of the energy storage elements or energy storage modules etc. The status of a semiconductor switch 10 is changed between a conducting mode (switch closed) and a non-conducting mode (switch open). A deadtime between change from one status of the switch to another status is preferably adjustable between 10 nanoseconds and 1 microsecond, typically the value is a couple of 100 nanoseconds.

[0095] The availability of an energy storage element 9 may refer to a defect element such as a battery cell, in this case a battery module 8 will not be available. The health status of an energy storage element 9 may refer to the number of times the particular energy storage element 9 has been charged/discharge. The high number, the closer to end of life time of the energy storage element 9 hence, the string controller 12 may keep track of this number and activate energy storage module 8 trying to keep this number more or less the same i.e. balanced for all energy storage elements 9 of the energy storage 7. In the same way, health of switch 10 can also be estimated based on the number of times it has been switching.

[0096] The energy storage 7 illustrated in FIG. 1 comprises a first energy storage module 8a and a second energy storage module 8b each including a plurality of energy storage elements 9a, . . . , 9n. The energy storage elements 9a-9n of the first energy storage module 8a are bypassed because of the non-conducting status of switch 10a and the conducting status of switch 10b. The energy storage elements 9a-9n of the second energy storage module 8b is included in the current path 13 because of the conducting status of switch 10a and the non-conducting mode of switch 10b.

[0097] The status of the switches 10 is as mentioned controlled by string controller 12 communicating with the switches 10 via a wired control signal path 14 or a wireless communication protocol. The string controller 12 is preferably also connected to an external controller 15. The external controller may be a wind turbine controller, wind park controller, photovoltaic controller, grid controller, etc. providing references for output of the energy storage 7 in terms of frequency, voltage level, etc. to the string controller 12 and/or an energy storage controller 6. Further, as illustrated the string controller 12 preferably also receives input from a current sensor 1 which is implemented and measuring current conducted in the current path 13. On FIG. 1, one control signal path 14 is illustrated between the string controller 12 and the battery monitoring module 2 and on between the string controller 12 and the switch board 11. It should be mentioned that only one signal path 14 may be used to these two modules/boards 2, 11. Such alternative design may be advantageous in that it is possible for the string controller to verify, that the boards are physically mounted correct in line with the software of the string controller 12.

[0098] It should be mentioned, that FIG. 2 illustrates an example of series connected energy storage modules 8 which would be referred to as a string. A energy storage 7 may comprise more strings and in this case preferably each string has its own string controller 12. In this case these string controllers 12 may communicate with the energy storage controller 6 which again may communicate with the external controller 15.

[0099] The number of strings of an energy storage 7 may vary between 1 and 25 or even more, typically the number of stings reflects the number of phases and/or consumption of the system to which the energy storage is connected. In the strings, the energy storage modules 8 are series connected and each string typically comprises between 1 and 20 energy storage modules 8, preferably between 5 and 15. The number of energy storage modules 8 and thereby energy storage elements 9 is determined by the desired voltage over the energy storage 7 which is preferably higher than the peak voltage of the electric network to which the energy storage 7 is connected. The storage capacity of the energy storage 7 is determined by the application in which the energy storage 7 is used. Further, the number of energy storage elements 9 of the energy storage modules 8 may vary like the energy storage modules 8 does not have to be identical within the energy storage 7 and even not within the strings. Just as long as the string controller 12 is updated with information of what is behind the individual PCB (PCB; Printed Circuit Board) switch boards 11.

[0100] Preferably, the switches 10 are semiconductor switches 10 of the IGBT (IGBT; Insulated Gate Bipolar Transistor), MOSFET (MOSFET; Metal-Oxide-Semiconductor Field-Effect Transistor) type, GaN transistors (Gan; Gallium Nitride) or SiC transistors (SiC; Silicon Carbide), however other types of switches can also be used.

[0101] Preferably, commodity switches 10 are chosen in that they are well tested and low in price. The commodity switches are typically not designed for operation in high voltage (e.g. above 1000V) and with high currents (e.g. above 500 A) so the number of this type of switches is higher compared to designs using switches designed for higher voltage and currents. However, the increased number is compensated by the lower price of the commodity switches. A preferred type of switch 10 for use in the present invention is designed to currents of 100 A and voltages of 50V. At higher voltages of the preferred types of switches, the on-resistance of the semiconductor switch 10 is increasing and thereby the power loss in the switch 10.

[0102] Preferably, a reference to energy storage element 9, is a reference to a plurality of battery elements connected in series. The number of battery elements may vary, between 2 and 25 or even more in one column of series connected battery elements in an energy storage module 8. A typical column comprises between 10 and 20 series connected battery elements 9. The number of battery elements 9 in a column depends on requirements to the energy storage 7 and to a compromise between few cells 9 leads to low price and reduced power loss while many cells 9 reduces the harmonic current contribution and leads to a more reliable system in that the redundancy/flexibility in control is increased.

[0103] The energy storage elements 9 are preferably of the li-ion type since the characteristics of this battery type complies with the requirements of the energy storage 7 and the environment of e.g. a wind turbine. With this said, other battery types may also be used. As an example, one battery element 9, may be a 3.2V element which when connected with e.g. 14 similar elements 9 leads to a 48V battery pack within one energy storage module 8. Hence in this example, the energy storage 8 comprise one 48V battery which can be controlled by the switches 10 of the energy storage module 8. The capacity of the battery elements 9 is preferably between 10 Ah and 200 Ah or even higher, but as mentioned this is a design choice based on requirements to the energy storage 7 and prices of the system. Especially in the preferred embodiment where the switches 10 are mounted on a PCB, the maximum current is determined as the lower of the maximum current allowed through the current path 13 of the PCB 11 and the maximum battery current.

[0104] FIG. 2a schematically illustrates an energy storage 8. The switches 10 are implemented on a PCB 11. It is illustrated, that the PCB includes all four switches 10 together with gate drivers 5 controlling the switches 10. The gate drives 5 may be galvanic isolated from the current path 13. The galvanic isolation may be implemented as part of the gate driver 5.

[0105] FIG. 2b illustrates an electric diagram of the switch configuration according to an embodiment of the invention, where the diode of the semiconductor switch 10 is a body diode of a MOSFET. The energy storage module 8 illustrated on FIG. 2b includes four semiconductor switches 10 in an H-bridge. This is because the energy storage 7 is able to comply with AC current and voltage i.e. both negative and positive polarity and still be able to bypass the energy module 8 as described above. FIG. 2b only illustrate one battery element 9 in the energy storage module 8, however as understood from the above description, there may be several battery elements 9 in an energy storage module 8.

[0106] The energy storage 7 described with reference to FIGS. 1 and 2 is an example of a type of energy storage that can be controlled according to the inventive method described below with reference to FIGS. 3a and 3b.

[0107] It should be mentioned, that the string controller 12, in case the energy storage 7 comprises a plurality of strings may be communicating with the energy storage controller 15. If the energy storage only comprises one string, the energy storage controller may be superfluous. Hence, either the energy storage controller or the string controller communicates with an external controller 15 from which is received current, voltage, frequency etc. references for the delivery of energy from the string i.e. based on the received information, the string controller controls the output from the string. Further, the string controller 12 may receive information from sensors and be configured to, based here on, control if the energy storage should deliver energy to or receive energy from the electric system to which it is connected. The string controller knows the capacity of the energy storage modules and if it receives sensor input that energy is available the string controller may control the current path 13 (modules connected thereto) to charge energy storage modules that may need to be charged. The external controller may be a wind turbine controller, grid operator controller, etc.

[0108] Further, in an exemplary embodiment, the string controller are communicating with a battery monitoring system of each of the energy storage modules 8 comprising battery elements 9. The battery monitoring system knows hardware details of the battery elements 9 such as type of battery, operation temperature, capacity, internal resistance, historical on-time etc. Hence, at least based on this information, the string controller is able to calculate the state of charge, state of health, etc. and thereby the current path through the energy module string.

[0109] The battery monitoring system further may measure current by means of a current sensor 1 and the temperature by means of a temperature sensor 4 and module voltage by means of a voltage sensor 3. These sensors may be part of a battery monitoring module 2 comprising information of hardware configuration of the battery module and based on the sensors provide real-time information of the battery module to the string controller. Information from these sensors may also be used by the string controller to establish e.g. state of charge of the battery elements 9. Especially, information of which of the individual module 8 are connected to the current path along with a measurement of current in the current path can be used by the string controller to optimize control of the output voltage according to a desired overall control strategy including load distribution of the individual modules 8. In addition, replacement of a module 8 does not interrupt operation in that instantly, the string controller is aware of new type of battery elements 9, there capacity hereof, etc.

[0110] Information of temperature can be used to determine capacity of a battery element in case the battery element capacity is sensitive to ambient temperature. Thus, the string controller may consider the temperature of an energy module or battery element to determine if a given battery element or energy module should be switched in or out of the string, even at normal operating temperatures. Thus, while complying with the minimum on-time, the string controller may reduce the on-time of energy modules or battery elements with highest temperature, or even determine to switch these off of the string, even if they are within safe operating temperature

[0111] As mentioned, in an embodiment, the battery monitoring module 2 may also provide information of the battery cells 9 of the battery module 8. Hence, in a memory of the battery monitoring module 2 at least some of the following is stored, type of energy storage (battery, capacitor, etc.), type of e.g. battery cell 9, number of battery cells 9, capacity of such battery cell 9 (e.g. 25 Ah and 50 Ah) and thereby of the entire battery module 8, producer of the battery cell 9, production date of print 11, 18 and/or battery cells 9, installation date of battery module 8 in energy storage 7, switching information such as type, number of cycles, etc. It should be mentioned that the battery monitoring module 2 may be implemented as a PCB.

[0112] Summing up, the string controller establish a performance evaluation based on the information received from the different sensors and from information of energy module hardware configuration. A result of this performance evaluation may be one or more lists, so called dynamic performance lists, including all energy modules sorted according to SOC, SOH, voltage, temperature, number of switching of switches, number of times the energy modules has been connected to the current path, time the energy modules has been connected to the current path times, internal resistance, historical on-time of the energy modules, etc.

[0113] The energy modules may be sorted into the one or more dynamic performance lists based on a linear or non-linear function of the above mentioned parameters. Examples hereof may comprise a weighted average or a weighted sum of several of the above mentioned parameters. In an example, the first dynamic performance list may be sorted based on SOC with the energy modules having the highest SOC placed first on the dynamic performance list and the energy module having the lowest SOC placed last on the dynamic performance list. In this example, a second list may sort the energy modules based on on-time, for example historical on-time, and a third dynamic performance list may be sorted based on temperature of the energy modules and so forth.

[0114] Based on one or more of these lists, the string controller(s) and/or the energy storage controller may determine which of the energy modules that should be used to establish the energy storage output. In this example, the sting controller may be configured to give largest weight to the state of charge, second largest weight to SOH, while a smaller weight is given to temperature, when establishing which energy modules to turn on to establish the energy storage output. The string controller may further manages the on-time of each of the modules that is turned on, so that it complies with the minimum on-time, in order to minimize transients and thereby EMC, EMI and high frequency disturbances in the energy storage. In an embodiments, this determination may include considering an overall control strategy of e.g. maintaining a certain level of SOC, peak capacity, etc. In an embodiment the overall control strategy may be overruled by the minimum on-time, to reduce the above mentioned disturbances that may occur when the on-time of an energy module is short, for such as for example below the minimum on-time.

[0115] In a different example according to the invention, instead of generating a dynamic performance list for each of the mentioned parameters, for example SOC, the energy modules are simply sorted into one dynamic performance list, based on a linear combination of measurements of a selection of the above mentioned parameters, for example again, SOC, on-time and temperature. The string controller then utilize this list to select which energy modules to turn on and turn off in order to establish the energy storage output. In this example, the list is sorted so that the energy modules with highest SOC, lowest temperature and lowest on-time is placed first in the list. The string controller then switches on the energy modules, starting with the energy module placed first on the dynamic performance list, then it turns on the second energy module on the dynamic performance list, then the third energy module on the dynamic performance list etc., to establish the output of the energy storage.

[0116] FIG. 3a illustrates part of a voltage output curve from one string of an energy storage 7 as described above according to an exemplary embodiment. It can be seen, that the energy storage needs five energy storage modules 8 (8a-8e) to establish the illustrated voltage curve. Further it can be seen, that each of the energy storage modules 8a-8e adds 50V to the output voltage and that they are connected to the current path through the string in the numeric sequence with order 8a, 8b, 8c, 8d and 8e. Finally, it can be seen, that they are disconnected from the string in the numeric sequence 8c, 8d, 8b, 8e and 8a.

[0117] Notice that the sequence in which the energy modules are disconnected are following a different order compared to the sequence in which the modules where turned on. In a preferred implementation of the invention, and with reference to FIG. 3a, it may be preferred that the first energy module to be turned off, is different from the last energy module to be turned on. With reference to FIG. 3a, this means that 8e, which is the last energy module to be turned on, should not be the first energy module to be turned off. This is advantageous in that the frequency of the energy module output may be high, while the switching frequency of the energy modules may be kept lower to minimize wear on the switches and reduce transients of each of the energy modules, because each energy module is never switched on longer than the minimum on-time.

[0118] To avoid an on-time of an energy module which is below a predetermined minimum on-time, in this example, the modules 8 are not just turn off in an ordered sequence that is opposite the sequence in which they are turned on i.e. turn on (ordered): 1, 2, 3, 4 and turn off (ordered) 4, 3, 2, 1. If the sequence of which the modules are turned on is ordered (e.g. 1, 2, 3, 4) the sequence of which the modules are turned off is un-ordered (e.g. 4, 2, 3, 1 or 1, 3, 2, 4) or vice versa. Further, if the sequence in which the modules are turned on is unordered, they should be turned off in an alternatively unordered sequence. The sequence is determined based on a sorted list, for example a dynamic performance list, of the modules and one or more conditions as explained below.

[0119] FIG. 3a only illustrates a first half period of a sinusoidal curve. Typically, the second half period mirrors the first half period with respect to sequence in which the modules are turned on and off.

[0120] It should be mentioned that in an exemplary embodiment not illustrated, on FIG. 3a, if the temperature of the module 8a turns out to be too high during the first half period. Then the string controller will detect this and replace its contribution with a contribution from another module. Then, maybe within the same period, the temperature drops below the temperature threshold and the string controller may swap back and use module 8a again. An example of a maximum temperature is of an energy storage could be between 40° C. and 60° C., preferably between 45° C. and 55° C. Yet, it is within the scope of the invention, to also switch energy modules out of the string even if temperatures of the battery modules are within a normal safe operating range. Thus, according to the invention, temperature may not only be used to switch out energy modules with temperature above a specified operating temperature range.

[0121] In should be mentioned that by on-time should be understood the time in which the an energy module is connected to the string.

[0122] The total contribution from the energy storage modules 8a-8e is the same no matter the sequence of disconnection, as long as the sum of time, the energy storage modules are connected to the string does not change. More particularly, each of the levels of 50V has to be connected to the current path a time period specified by the requirements output voltage. Therefore, an energy storage module has to be connected throughout the time between time T1 and time T2. It does not need to be one particular energy storage module the whole time, but the time could be divided in contributions from several energy storage modules 8. In this way, the output stays the same, but what changes is the on-time of the individual modules 8. In other words, the on-time of the individual energy storage modules can be better balanced leading to a much better distribution of the wear among the switch module/switches of the energy module 8.

[0123] FIG. 3b illustrates a DC output curve of 125V. Since the energy storage modules are of 50V each, two energy storage modules would have to always be turned on and one energy storage module would have to be turned on 50% of the time. In the illustrated embodiment, energy storage module 8a is always turned on, whereas energy storage 8b and 8e supplies 50V shifting at time T5 and therefore together with module 8a supplies 100V. The remaining 25V is provided by turning on one module 50% of the time, in this embodiment, this is delivered partly by module 8c and partly by module 8d. The on-time between time T3 and T4 and between T4 and T6 is above the minimum on-time and thus no problems with respect to switching losses and EMI and EMC. However, if the control strategy is to balance the SOC of the individual storage modules 8, different modules can be connected to the current path 13. In exemplary embodiments of the invention, the control strategy may be overruled by the minimum on-time.

[0124] In the situation, where the string controller is asked to deliver a current to an AC load or AC grid, the string controller is controlling the individual modules to establish an output voltage complying with the system frequency of the system to which the current is to be delivered. Typically, the system frequency is 50 Hz or 60 Hz in AC systems.

[0125] The string controller controls the on-time of the individual modules 8 and as illustrated on FIG. 3a several modules 8 are needed to establish a desired amplitude of the output voltage. The frequency with which the string controller turns on or off the individual modules is in this document referred to as control frequency.

[0126] The on-time of an individual module could be referred to as module frequency. The on-time for the individual modules 8 is controlled by the string controller based on information received from all of the individual modules and in addition, maybe also an overall control strategy on how to establish the desired output voltage from the energy module. Hence, the on-time is determined in consideration of e.g. state of charge, state of health, system frequency and other requirements from the system to which the energy storage is connected to, etc. Hence, the string controller may be instructed to deliver 250 VAC and at least 10 A and it is then up to the string controller based on its knowledge of the individual modules 8, control strategy, current sensor input, etc. to determine how many modules that is needed and when these are to be connected to the current path 13. The string controller may in a preferred embodiment of the invention, further control the on time of individual energy modules so that the on-time of energy modules of a string is always above the minimum on-time.

[0127] The distribution of which energy storage modules 8 that has to be connected at which voltage levels (at FIG. 3 at 0V, 50V, 100V, 150V and 200V) is determined by the string controller 12. In an exemplary embodiment, this is done according to the flow chart of FIG. 4.

[0128] In the first step S1, an output reference is provided to the string controller 12. The output is typically received from an external data processor 15 such as a controller of the electric system to which energy storage 7 is connected. Such system could e.g. be a wind turbine, a solar system, utility grid and the like. Typically, the energy storage 7 is designed to a particular system and therefore optimized to deliver e.g. backup power to an auxiliary system of a wind turbine or solar plant. The energy storage may also be used as storage of surplus energy and to support utility grid. In such an exemplary embodiment, when needed, the wind turbine controller communicates to the energy storage controller 6, if the energy storage comprises more than one string (more than one phase) or to the string controller 12. Either a start signal is communicated if the energy storage/string controller knows which output is required by the “load” (in this example auxiliary system) or an output reference is provided. The output reference could be one or more of a voltage reference or a frequency reference.

[0129] In step S2, the string controller 12 is establishing a performance evaluation of the majority of the plurality of energy storage modules. An existing performance evaluation may be updated or a new may be made based on received input form battery monitoring module, sensors and/or information storage regarding previous use of the individual energy modules i.e. historic data.

[0130] In step S3, the sting controller 12 is using e.g. the received output reference together with the determined control strategy and the performance evaluation to establishes gate signals for the switches 10 of the energy storage modules 8. One control strategy could be balancing SOC or SOH equally between the energy storage modules 8 another control strategy could be the opposite namely using one or more of the storage modules 8 more than others and yet another could be a lower limit of switch time of switches 10 or a combination of these and others. The strategy of using one module more than others could be chosen if one storage module 8 seems to be close to end of life and service is planned shortly and the last capacity is to be squeezed out of it. The other way around, e.g. if service is not planed, there might be a desire to use such battery module 8 a little as possible.

[0131] No matter which control strategy that is chosen, the string controller establishes a turn on and a turn off sequence for the energy storage modules 8 that is required to comply with the required output reference and complies with the control strategy in light of the performance evaluation. It should be mentioned, that more energy storage modules 8 than needed may be included in the string in that it adds flexibility to how to establish the energy storage output.

[0132] The establishing of the turn on/turn off sequence includes the test in step S3 where the switching time i.e. time between a switch is turned on (closed) and turned off (opened) i.e. the time in which the energy storage module 8 controlled by the switches 10 is connected to the current path 13. To reduce switching loss, EMI and EMC disturbances in the energy storage 7, the on-time is preferably above a lower limit in the range of 80 us to 150 us e.g. 100 us or for example in the range of 200 to 300 us. This lower limit may be a predetermined minimum on-time. If the switching sequence according to the overall control strategy result in one energy module turns out to be below this lower limit, the lower limit overrules the overall control strategy and thus the sequence is adjusted accordingly. If for example the overall control strategy dictates that energy modules should be turned on sequentially according to SOC, starting with the energy module with the highest SOC, this results in that energy modules with the highest SOC will be turned on longest, and the last energy module to be turned on may be turned on in less than the minimum on-time at the peak of an AC wave form. In this example, the string controller may modify the sequence dictated by the overall control strategy, to prolong the on-time of that energy module having an on-time below the minimum on-time, while reducing the on time of one of the other energy modules that are turned on, even if this means that such energy module is then turned on for longer time than another energy module that is turned on having a higher SOC.

[0133] Step S3 is illustrated as an independent step and opposite, step S2 includes both establishing SOC or the like and calculate a pattern (sequence) thereon. It should be mentioned, that the presentation of the present method in a flow diagram is only to help understand and describe the steps and as the described order and content of the steps may be preferred, it is not absolutely necessary to follow strictly.

[0134] In step S4, gate signals according to the determined sequence is provided to gate rivers of the individual energy storage modules 8.

[0135] As mentioned, the energy storage module may comprise energy storage elements of different types. The elements 9, may be different battery types and capacitor types. Typically, only one type of battery/capacitor is used in one battery storage module 8, however this is not always the case. Two energy storage modules 8 in the same string may have different types of energy storage elements 9 i.e. a first may comprise batteries, a second may comprise a different type of batteries and a third may comprise capacitors.

[0136] This is possible to control in that each of the energy storage modules 8 preferably comprises a battery monitoring system module 2 that provide information of the status of the energy storage elements 9 of the energy storage module 8. Further, it comprises information of the hardware element comprised by the energy storage elements 9 including the type and number of battery or capacitor cells comprised by the energy storage element 9.

[0137] As can be understood from the above, the present invention relates to an energy storage 7 and the control of on-time of the energy storage module 8 hereof to establish a desired energy storage output voltage while remaining the system bandwidth and reducing switching loss. The output voltage may require several strings of energy modules 8. The control is made by one or more string controller 12 based on input from a controller 15 of the electric system to which the energy storage 7 is connected to, based on input from sensors of the electric system, trigger signals from the electric system, current sensor 17, the performance evaluation of the energy modules, etc.

[0138] The energy storage modules 8 comprises an energy module monitoring module 2 (referred to as battery monitoring module if the energy storage elements are batteries) via which the string controller 12 receives information of hardware configuration of the battery module 8 as well as real-time status of the battery elements 9. The status may include temperature and voltage measured from sensors 3, 4 which may be implemented on the battery monitoring module PCB.

[0139] The control according to the present invention is advantageous in that wear of the battery modules can be better distributed in that conduction of current from the modules can be controlled with a reduce noise occurring from the switches when turning on and off. Further, it is advantageous in that the switching time of the switches 10 can be controlled to be above a lower limit for example above a minimum on-time, which may be predetermined This reduces the EMC, EMI and high frequency noise in the energy storage system.

[0140] The energy storage may be used as local grid, backup, storage of surplus energy and grid support including supporting with respect to reactive or active power, frequency, etc.

[0141] More particularly according to an exemplary embodiment, the control of the charging or discharging i.e. the current/voltage of the sting is controlled according to the following steps.

[0142] First, a discrete electric reference (frequency, voltage, current or power) is provided to the string controller. The reference may be received from a controller of the load or from the energy storage controller and is via an algorithm transformed to a continuous electric reference such as a sinusoidal waveform.

[0143] Second, one or more electric values are measured of the energy storage module sting. If the electric reference is a voltage, then the voltage of the string is measured.

[0144] Third, the string controller is calculating a voltage reference based on the continuous electric reference and the measure electric values. This voltage reference determines the number of energy modules that is needed from the string to go from the current voltage to the next voltage level determined by the continuous electric reference. It should be note that the voltage refence instead of a voltage reference could be a frequency, current or power reference in other exemplary embodiments.

[0145] Fourth, this voltage reference is then used to determine the number of energy modules that needs to be connected to the current path. The energy module that is to be connected is selected from a list, for example a dynamic performance list, which preferably comprises each of the energy modules of the string. The energy modules of the list is sorted according to one or more of SOC, SOH, temperature or other relevant electric parameters, including for example internal resistance. This and the below is referred to as performance evaluation or dynamic performance evaluation.

[0146] The sorted list of energy modules may be established and updated with time intervals. The minimum time between two updates of the list is the frequency with which the string controller is receiving measurements from the battery monitoring system (if the battery elements are batteries) i.e. the sampling frequency of the battery monitoring system. Alternatively, the time intervals can be determined based on the frequency of the system to which energy storage is connected i.e. every period or half period. Alternatively, the time interval could be a predetermined time of 1 ms, 1 second, 1 minute or any times therebetween. Accordingly, the time interval may be determined by the application in which the energy storage is used.

[0147] As an example, if the energy modules are sorted according to SOC and the energy modules are to be charged, the energy module having the lowest SOC i.e. the bottom module of the list is selected first. In contrary, if the energy module is to be discharged the energy module having the highest SOC i.e. the top module of the list is selected first. As illustrated on FIG. 3a, the energy module that is connected first 8a, is the one that is charged/discharged the most.

[0148] Fifth, before string controller send turn-on signal to the switches of the energy module selected from the list, the sting controller examines if this energy modules complies with one or more conditions. These conditions may include maximum/minimum temperature, minimum on-time, minimum off-time, charge/discharge, etc.

[0149] The minimum on-time is as mentioned to avoid switching losses due to high module frequency. To comply with the minimum on-time, the string controller can control the time an individual energy module is turned on, turned off or a combination thereof.

[0150] Further, to avoid transients, the string controller can ensure a minimum time between one module is turned off and then turned on again or vice versa.

LIST

[0151] 1. Current sensor [0152] 2. Battery monitoring module [0153] 3. Voltage sensor [0154] 4. Temperature sensor [0155] 5. Gate drivers [0156] 6. Energy storage controller [0157] 7. Energy storage [0158] 8. Energy storage modules [0159] 9. Energy storage elements [0160] 10. Semiconductor switches [0161] 11. PCB switch board [0162] 12. String controller [0163] 13. Current path [0164] 14. Control signal path [0165] 15. External controller