Energy storage system with resistor circuit for bidirectional high power applications

Abstract

An energy storage system (100) and a method for controlling such a system are disclosed. The energy storage system includes a plurality of interconnected energy storage cells (112) forming an energy storage module (110), a power converter (120) connected to the energy storage module, a resistor circuit (130) connected in parallel with the energy storage module, a switch (140) connected in series with the resistor circuit and configured to disconnect the resistor circuit from the energy storage module, and a control unit (150) operably connected to the switch. The control unit is configured to receive information regarding the transfer of energy in the energy storage system and to control the switch based on the information.

Description

[Technical field]

[0001] FIELD OF THEINVENTION The present invention relates to the field of power transmission systems, and in particular to energy storage devices for use in such power systems. [Background technology]

[0002] 2. Background of the Invention Power systems need to deliver power in a reliable manner. Therefore, such technologies often include energy storage systems to balance frequency fluctuations, shortages, etc. Preferably, the energy storage system may store energy during off-peak periods and discharge energy to the power grid during on-peak periods, for example to provide fast frequency response and inertial response.

[0003] An example of such an energy storage system includes an energy storage bank having a number of series and / or parallel connected battery cells or supercapacitors arranged in modules. The modules may be connected in series to form an energy storage string. Several such strings may be connected to a converter system, which may operate in a bidirectional manner to output discharged energy to the AC power transmission system and to charge the battery cells from the AC power transmission system.

[0004] Thus, energy storage systems may be successfully employed to balance and smooth demand and power oscillations on the power grid. However, this requires operation at relatively high power levels, and the energy storage system may be required to absorb and release relatively large amounts of energy, for example, over a relatively short period of time. At the same time, the system should ensure that individual battery cells are maintained at safe voltage levels during operation. Meeting the capacity requirements for such bidirectional use has proven to be quite costly and requires relatively large energy storage systems. Summary of the Invention [Problem to be solved by the invention]

[0005] Summary of the Invention It would therefore be advantageous to have an energy storage technique that overcomes or at least mitigates the above-mentioned shortcomings. In particular, it would be desirable to provide improved means for stabilizing and improving the quality of power transmission systems. [Means for solving the problem]

[0006] These objects are achieved, inter alia, by an energy storage system and method as claimed in the independent claims.

[0007] Thus, according to a first aspect, there is provided an energy storage system configured to be connected to an alternating current (AC) power transmission system. The energy storage system comprises a plurality of interconnected energy storage cells forming an energy storage module, and a power converter connected to the energy storage module. The power converter is configured to output power from the energy storage module to the AC power transmission system and charge the energy storage module with power from the AC power transmission system. The energy storage system further comprises a resistor circuit connected in parallel to the energy storage module, a switch connected in series with the resistor circuit and configured to disconnect the resistor circuit from the energy storage module, and a control unit operatively connected to the switch. The control unit is configured to receive information regarding the transfer of energy in the energy storage system and to control the switch based on the received information.

[0008] According to a second aspect, there is provided a method for controlling an energy storage system according to the first aspect, the method including receiving information relating to a transfer of energy in the energy storage system and controlling a switch based on the received information.

[0009] During operation, an energy storage system may be required to absorb a relatively large amount of energy in a relatively short period of time. This may be the case, for example, when the energy storage system is used for frequency regulation, where the energy storage system injects or absorbs power into the power grid to maintain and regulate the power of the power grid. Typically, this requires a relatively large power capacity of the energy storage system to allow a relatively large amount of energy to be absorbed in a relatively short period of time. Traditionally, this may be achieved by selecting a high charge / discharge rate (C-rate) energy storage, or by increasing the number of energy storage cells so that the absorbed energy can be distributed across a larger number of cells.

[0010] The resistor circuit according to the present invention provides an alternative solution where peaks in the absorbed energy may be absorbed or "burned" by the resistor circuit. In this way, the high storage capacity requirements placed on the energy storage cells may be alleviated and the size and C-rate of the entire energy storage system may be reduced. Thus, by connecting the resistor circuit to the energy storage module of the system, the system may be able to absorb increased power levels while maintaining the energy storage cells within safe voltage levels without increasing its storage capacity. Furthermore, the resistor circuit can be employed to limit or balance the voltage across the energy storage module and discharge energy from the energy storage module as a fast way to maintain the voltage at a safe level.

[0011] The control unit is used to control a switch that connects and disconnects the resistor circuit to the energy storage module. The switch may be opened or closed based on information about the transfer of energy in the energy storage system, such that the resistor circuit may consume at least a portion of the energy input to the energy storage system depending on specific operating requirements, which are discussed in more detail below.

[0012] According to one embodiment, the control unit may be configured to control a switch to disconnect the resistor circuit from the energy storage module when energy is output from the energy storage module, allowing the energy stored in the energy storage module to be released from the energy storage module and transferred to the power converter.

[0013] In one embodiment, the control unit may be configured to control the switch to connect the resistor circuit to the energy storage module when energy is transferred to the energy storage module. By connecting the resistor circuit to the energy storage module, the resistor circuit may consume or burn at least a portion of the energy transferred to the energy storage module. Preferably, the resistor circuit may be used to lower the voltage of the energy storage module and to allow the energy storage system to absorb more power than would otherwise be possible, given the capacity of the energy storage module. In other words, the resistor circuit may be used to increase the energy absorption capacity of the energy storage system.

[0014] In an embodiment, the control unit may be configured to control the switch to connect a resistor circuit to the energy storage module when the energy transferred to the energy storage module exceeds a predetermined power threshold. The resistor circuit may thus be employed to handle peaks in the power transferred to the energy storage module and thus help protect the energy storage module from overload.

[0015] According to an embodiment, the control unit may be configured to control the switch to connect the resistor circuit to the energy storage module when the level of energy stored in the energy storage module exceeds a predetermined storage threshold. In this way, the resistor circuit may absorb the transferred energy when the remaining storage capacity of the energy storage module is limited or when the energy storage module is fully charged.

[0016] According to an embodiment, the energy storage system may further comprise an additional switch connected in series with the energy storage module and operably connected to the control unit. The additional switch, which may also be called a disconnector, may be configured to disconnect the energy storage module from the power source, i.e., from the power converter, and further to allow the resistor circuit to operate independently without being connected to the energy storage module.

[0017] According to an embodiment, the control unit may be configured to control an additional switch to disconnect the energy storage module from the power converter if energy is (or will be) transferred to the energy storage module and the level of energy stored in the energy storage module exceeds a predefined storage threshold. The disconnector thus allows the energy storage module to be disconnected if its remaining storage capacity is limited or if the energy storage module is fully charged. This is a way to ensure that the energy storage system is still able to absorb energy even if the energy storage module is fully charged.

[0018] In one embodiment, the energy storage system may include multiple energy storage modules, and the resistor circuit is connected in parallel to the multiple energy storage modules. The energy storage modules may be connected in series and / or in parallel with each other and in parallel with one or more resistor circuits, depending on, for example, the desired voltage to be achieved.

[0019] In one embodiment, a plurality of series connected energy storage modules may form an energy storage string, with a resistor circuit connected in parallel to the energy storage string.

[0020] It should be noted that various configurations and combinations of energy storage modules and resistor circuits are contemplated within the scope of the claims, and several detailed examples are also discussed in connection with the accompanying drawings.

[0021] According to one embodiment, the energy storage module and the resistor circuit may be structurally located within a common physical entity. From this, it may be understood that the resistor circuit may form part of the energy storage module or may be integrated or collocated with the energy storage module. This configuration may be contrasted with other configurations in which the resistor circuit is provided at a distance from the energy storage module, for example in a different physical location or in a separate structure that is separate from the structure in which the energy storage module is located.

[0022] In the context of this disclosure, "energy storage system" may generally refer to an energy storage solution for storing electrical energy and supplying it to designated loads as a primary or supplemental source, as well as for voltage and frequency regulation. Preferably, the energy storage system is sized for medium to high voltage DC.

[0023] "AC power transmission system" generally refers to a structure for transmitting and / or distributing electrical power. An AC power transmission system may be referred to as a power transmission network, a transmission network, a power system, or a power grid, in some examples.

[0024] The term "energy storage cell" may be generally understood as a device that can accept electrical energy, store electrical energy, and release electrical energy. Thus, an energy storage cell may refer to a device that can be repeatedly (re)charged. Examples of such devices include supercapacitors, flywheels, and batteries.

[0025] The energy storage cells may be interconnected, preferably in series, in a stacked configuration to form an energy storage module. A group of interconnected energy storage modules may be referred to as an energy storage string, and multiple strings may be grouped into an energy storage bank.

[0026] The term "power converter" generally refers to a device for converting electrical energy between AC and DC. A power converter, which may also be called an inverter, may be configured to convert AC power to DC to charge an energy storage module and convert DC power from the energy storage module into one of multiple phases of AC energy at a frequency determined by user requirements. Thus, a power converter may be called a bidirectional converter.

[0027] The term "resistor circuit" may refer to a passive or active electrical component that may dissipate at least a portion of electrical power as heat. In some examples, a resistor circuit may be referred to as a power resistor.

[0028] The resistor circuit may be provided at the energy storage module level, i.e., connected to one or more energy storage modules. However, the resistor circuit may be provided at the energy storage cell level, where the resistor circuit is connected to an individual cell rather than to a module of multiple cells. For this purpose, an energy storage module may, in some embodiments, be considered to include only one energy storage cell.

[0029] A "switch" may be realized in the context of this application as an electrical component that may disconnect and connect the resistor circuit and / or the energy storage module from a power source. In its disconnected state, the switch may be referred to as "open" and in its connected state as "closed". Preferably, the operation of the switch is controlled by a control unit, which may send control signals to the switch, which, when received at the switch, may cause the switch to change between an open state and a closed state, or vice versa. The switch may in some instances be structurally integrated with the resistor circuit, and thus the term may also include the switch.

[0030] The switches are preferably semiconductor switches such as MOSFETs or IGBTs. MOSFET switches may be suitable for voltages up to 1000 volts, while IGBT switches may be suitable for voltage levels above 200 volts.

[0031] A control unit or controller may be understood as a device capable of receiving information regarding the condition, state or operation of the energy storage system and outputting instructions for controlling the operation of the components of the energy storage system. In particular, the control unit may be capable of controlling the operation of the switches as well as the power converter and further components included in the energy storage system. The control unit may be structurally integrated into the energy storage system or may be located physically separate from the system.

[0032] The control unit may thus be configured to issue control commands based on the received information. The received information may relate to an actual state or condition of the system, such as an actual transfer of energy to / from an energy storage module or an actual absorption of energy. However, the received information may also refer to a desired state or condition of the system, such as a desired absorption or release of energy. The information may thus relate to the (actual or desired) charging, discharging or absorption of electrical energy in the energy storage system as a whole, at the level of individual energy storage modules, or at the level of individual energy storage cells.

[0033] The present invention may be embodied as computer readable instructions for controlling a programmable computer to perform the method outlined above. Such instructions may be distributed in the form of a computer program product comprising a computer readable medium storing the instructions. In particular, the instructions may be loaded into a control unit responsible for controlling components of the energy storage system, such as switches.

[0034] It is noted that embodiments of the present invention relate to all possible combinations of the features recited in the claims. Moreover, it will be understood that all the different embodiments described for the energy storage system according to the first aspect can be combined with the embodiments of the method defined according to the second aspect.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects will now be described in more detail with reference to the accompanying drawings, in which embodiments are shown. [Brief description of the drawings]

[0036] [Figure 1] 1 illustrates an energy storage system according to one embodiment. [Figure 2a]1 illustrates a plurality of interconnected energy storage modules and resistor circuits forming an energy storage bank according to one embodiment. [Figure 2b] 1 illustrates an energy storage module, a resistor circuit, and a switch according to one embodiment. [Diagram 3] 1 illustrates an energy storage module, a resistor circuit, and a switch according to some embodiments. [Figure 4] 1 illustrates an energy storage module, a resistor circuit, and a switch according to some embodiments. [Diagram 5] 1 is a flow chart outlining a method according to one embodiment; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] All figures are not necessarily to scale and generally show only parts necessary to elucidate the embodiments, other parts may be omitted or merely suggested. Like reference numbers refer to like elements throughout the description.

[0038] Detailed Description This aspect will now be described more fully hereinafter with reference to the accompanying drawings, in which presently preferred embodiments are shown.

[0039] An energy storage system 100 according to one embodiment will be described with reference to FIG. 1, which shows an energy storage bank 102, a power converter 120, and a control unit 150. The energy storage bank 102 may include a plurality of energy storage cells (not shown) and one or more resistor circuits that may be connected to the energy storage cells to burn at least a portion of the energy provided to the energy storage cells. The configuration of the energy storage cells and resistor circuits will be described in more detail below in relation to FIGS. 2-5.

[0040] The energy storage bank 102 may be charged by electrical energy provided by the power converter 120 and may also release energy to the power converter 120. The converter may then be connected to an AC power transmission system, also referred to as a power grid. Thus, the power converter 120 may be a bidirectional AC / DC converter capable of transferring power to and from the energy storage bank 102.

[0041] A control unit 150 may be operatively connected to the energy storage bank 102, and preferably also to the power converter 120, to control the operation of the energy storage system 100. The control unit may preferably receive information indicative of power transfer in the system, such as charging or discharging power of the energy storage bank 102, and use the received information to control the charging / discharging of the energy storage cells of the bank 102 and the absorption / discharging of energy of the energy storage system as a whole.

[0042] The energy storage system 100 may be incorporated into a separate physical entity, such as one or more physical enclosures (not shown), that can be transported to a desired location and connected to an AC power transmission system.

[0043] 2a shows a pair of energy storage strings 104, each including a number of energy storage modules 110. The energy storage modules 110 in each string 104 may be connected in series, however, two energy storage strings 104 may be connected in parallel to form an energy storage bank 102 similar to that described above in connection with FIG 1. In this example, each storage module 110 may further be connected in parallel to a respective resistor circuit 130, which may be disconnected by a switch (not shown in this figure).

[0044] During charging, energy may be transferred to the energy storage modules 110 via the conductors 106. Preferably, the conductors 106 may serve as a common bus bar connecting the strings 104 to a power converter. If the power transferred to a particular energy storage module 110 exceeds a predetermined threshold, the resistor circuit 130 may be connected to burn at least some of the energy, thereby reducing the voltage of the energy storage module 110.

[0045] The resistor circuit 130 may be disconnected from the energy storage module 110 during discharging to avoid energy being consumed by the resistor circuit 110.

[0046] The connection / disconnection of the resistor circuit 130 may be made possible by switching means, which may for example form part of the resistor circuit 130, be integrated or collocated with the resistor circuit 130, or be provided as a separate component connected to the resistor circuit 130. Examples of implementations of the connection / disconnection means of the resistor circuit 130 are described below.

[0047] Figure 2b illustrates an example of an energy storage module 110 according to an embodiment that may form part of an energy storage system similar to that described in relation to Figure 1. The energy storage module 110 may be connected in parallel with a resistor circuit 130, which may be disconnected from the energy storage module 130 by a switch 140 connected in series with the resistor circuit 130. Furthermore, an additional switch 160 may be provided to allow the energy storage module 110 to be disconnected from power.

[0048] As shown in this figure, the energy storage module 110 may include a plurality of interconnected energy storage cells 112. In this embodiment, the energy storage cells 112 may be interconnected in series in a stack configuration, and the number of cells 112 may be selected to achieve a desired voltage level. The energy storage cells 112 may be, for example, battery cells or other types of rechargeable cells, such as capacitors or supercapacitors. Examples of battery cells 112 may include lithium ion batteries, lead acid batteries, nickel cadmium batteries, and sodium nickel chloride batteries.

[0049] The energy storage module 110 may be connected to an additional switch or disconnector 160 for selectively connecting the energy storage module 110 to other energy storage modules 110 and / or a power converter (not shown). The disconnector 160 may be a semiconductor switch, including, for example, a MOSFET or IGBT device with an anti-series connection, or a single MOSFET or IGBT with an anti-parallel diode. In one example, the energy storage module 110 may be connected in series with a configuration 160 that includes an anti-charging diode with a controllable switch for charging, which has the advantage of allowing the voltage of the resistor circuit 130 to be clamped, allowing the charging current to be limited as necessary and overcharging to be avoided. Preferably, the switch 160 may be opened when the energy storage module 110 is fully charged, thereby reducing the risk of overcharging.

[0050] As mentioned above, the resistor circuit 130 may be connected in series with a switch or disconnect 140 for selectively connecting the resistor circuit 130 to the energy storage module 110. The switch 140 may be a semiconductor switch, such as, for example, a MOSFET or IGBT device with an anti-series connection, or a single MOSFET or IGBT with an anti-parallel diode.

[0051] The switch 140 and / or the additional switch 160 may be operatively connected to a control unit (not shown), which may be configured to selectively control the operation of the switches based on a current or desired state of the energy storage system. In one example, the switch 40 may be closed to allow the resistor circuit 130 to discharge the energy storage module 110 for maintenance purposes. In that case, the energy stored in the energy storage module 110 may be dissipated through the resistor circuit 130 instead of being released to the utility grid. The switch 130 may also be used to discharge the energy storage module 110 during steady-state operation of the energy storage system, thereby allowing energy to be balanced among different modules of the system.

[0052] 3 illustrates a configuration in which multiple energy storage strings 104 including series-connected energy storage modules 110 are connected in parallel to a switch-controlled resistor circuit 130. The energy storage modules 110, resistor circuit 130, and switch 140 may be similar to those described in connection with the previous figures, with the difference that the resistor circuit 130 is common to multiple energy storage modules 110 arranged in at least two parallel strings 104.

[0053] It will be appreciated that further configurations are contemplated in which one or more energy storage modules 110 are connected in parallel and / or in series with each other and with one or more resistor circuits 130 and switches 140 .

[0054] 4, the energy storage module 110 may be connected in parallel to multiple resistor circuits 130, three in this example. As shown in the figure, the resistor circuits 130 may be connected in parallel with each other to increase power burning capacity, and may further be connected in series with three switches 140 arranged in series with each other to increase their ability to handle higher voltage levels. The switches 140 may include, for example, MOSFET devices controlled by the control unit.

[0055] The energy storage module 110 and the resistor circuit 130 may be structurally disposed within a common physical entity, such as a common housing, according to some embodiments. Thus, it should be understood that the resistor circuit 130 may be disposed in physical proximity to the energy storage module 110 or may be structurally integrated into the energy storage module 110.

[0056] 5 illustrates a method for controlling an energy storage system that may include an energy storage module 110, a resistor circuit 130, and a switch 140 according to some embodiments of the present invention. The energy storage system may be configured similarly to any of the embodiments described above in connection with FIGS. 1-5. In some embodiments, the energy storage system may further include an additional switch 160 or disconnector 160 connected in series with the energy storage module 110 and operable to disconnect the energy storage module 110 from the power converter. The disconnector 160 may be connected to and controlled by the control unit 150.

[0057] The method may include a step 10 of receiving, by the control unit 150, information regarding the transfer of energy in the energy storage system 100. This information may indicate, for example, the level of power transferred to the energy storage module 110 of the system, the level of energy charged to the energy storage module 110, or the energy storage capacity of the module 110. This information may include parameters such as the voltage or current of the electrical energy delivered to the energy storage module 110 and / or the resistor circuit 130, as well as the charge / discharge rate capabilities of the energy storage module 110. Additionally, this information may indicate whether energy is being charged to or discharged by the energy storage module 110.

[0058] If the received information indicates that energy is being output (or is intended to be output) from the energy storage module 110, i.e., the energy storage module 110 is discharging energy into the AC transmission system, the control unit 150 may control the switch 140 to disconnect the resistor circuit 130 from the energy storage module 110 (20). Thus, the switch 140 may be opened (20) to allow energy to flow from the energy storage module 110 to the power converter and further to the AC transmission system or grid.

[0059] In addition, if the energy storage system also comprises a disconnector 160, the control unit 150 may be configured to control the disconnector 160 to keep the energy storage module 110 connected to the power converter. Thus, by closing 22 the additional switch 160, the energy stored in the energy storage module 110 is allowed to be released to the power grid.

[0060] If the received information indicates that energy is to be input to the energy storage module 110, i.e., that the energy storage module 110 absorbs (or is intended to absorb) energy from the power grid, the control unit 150 may control the switch 140 to connect (30) the resistor circuit 130 to the energy storage module 110. In an embodiment, the switch 140 may be closed (30) when the energy transferred to the energy storage module 110 exceeds a predetermined power threshold, preferably to protect the energy storage module 110 from overload. In an embodiment, the switch 140 may be closed (30) during charging when the level of energy stored in the energy storage module 110 exceeds a predetermined storage threshold, for example indicating that the energy storage module 110 is fully charged. If the energy storage system includes a disconnect switch 160, it may be opened (32) so that the input energy is dissipated by the resistor circuit 130 and the energy storage module 110 is protected.

[0061] If the control unit 150 receives information indicating that the power transferred to the energy storage module 110 is below a predetermined power threshold, for example, indicating that the energy storage module 110 is operated at a safe voltage or power level, and further indicating that the energy storage module 110 is not fully charged, the control unit may control the switch 140 to open (40) and, optionally, the disconnect switch 160 to close (42). This allows the transferred energy to be stored within the energy storage module 110 instead of being consumed or burned by the resistor circuit 130.

[0062] If the information indicates that the energy storage module 110 is exposed, or at risk of being exposed, to excessively high voltages that could damage the module 110, the control unit 150 may operate to close (50) the switch 140 to protect the energy storage module 110. Additionally, the control unit 110 may open (52) the disconnect switch 160 to further reduce the risk of damage to the energy storage module 110.

[0063] As outlined above, the method illustrated by FIG. 5 may be embodied as computer-executable instructions that are distributed and used in the form of a computer program product that includes a computer-readable medium that stores the computer-executable instructions. By way of example, the computer-readable medium may include computer storage media and communication media. As is well known to those skilled in the art, computer storage media includes both volatile and non-volatile media, as well as removable and non-removable media, implemented in any method or technology for storing information, such as computer-readable instructions, data structures, program modules, or other data. In addition, communication media typically embodies computer-readable instructions, data structures, program modules, or other data in a modulated data signal, such as a carrier wave or other transport mechanism, and is known to those skilled in the art to include any information delivery media.

Claims

1. 1. An energy storage system (100) configured to be connected to an AC power transmission system, comprising: a plurality of interconnected energy storage cells (112) forming an energy storage module (110); a power converter (120) connected to the energy storage module, the power converter configured to output power from the energy storage module to the AC power transmission system and charge the energy storage module with power from the AC power transmission system, the energy storage system further comprising: a resistor circuit (130) connected in parallel with the energy storage module; a switch (140) connected in series with the resistor circuit and configured to disconnect the resistor circuit from the energy storage module; a control unit (150) operatively connected to the switch; the control unit is configured to receive information regarding a transfer of energy in the energy storage system and to control the switch based on the information; The control unit is configured to control the switch to connect the resistor circuit to the energy storage module when energy transferred to the energy storage module exceeds a predetermined power threshold.

2. 2. The energy storage system of claim 1, wherein the control unit is configured to control the switch to disconnect the resistor circuit from the energy storage module when energy is output from the energy storage module.

3. 2. The energy storage system of claim 1, wherein the control unit is configured to control the switch to connect the resistor circuit to the energy storage module when energy is transferred to the energy storage module.

4. 2. The energy storage system of claim 1, wherein the control unit is configured to control the switch to connect the resistor circuit to the energy storage module when a level of energy stored in the energy storage module exceeds a predetermined storage threshold.

5. 10. The energy storage system of claim 1, further comprising an additional switch (160) connected in series with the energy storage module and operably connected to the control unit.

6. 6. The energy storage system of claim 5, wherein the control unit is configured to control the additional switch to disconnect the energy storage module from the power converter when energy is transferred to the energy storage module and a level of energy stored in the energy storage module exceeds a predetermined storage threshold.

7. 10. The energy storage system of claim 1 comprising a plurality of energy storage modules, the resistor circuit being connected in parallel to the plurality of energy storage modules.

8. 10. The energy storage system of claim 1 comprising a plurality of series connected energy storage modules forming an energy storage string, the resistor circuit being connected in parallel to the energy storage string.

9. The energy storage system of claim 1 , wherein the energy storage module and the resistor circuit are structurally disposed within a common physical entity (170).

10. The energy storage system of claim 1 , comprising a plurality of resistor circuits (130) connected in parallel to the energy storage module.

11. 13. A method for controlling an energy storage system according to claim 1, comprising: receiving information regarding a transfer of energy in the energy storage system; and controlling the switch based on the information.

12. opening the switch when energy is output from the energy storage module; and closing the switch when energy transferred to the energy storage module exceeds a predetermined power threshold.

13. 12. The method of claim 11, wherein the energy storage system further comprises an additional switch connected in series with the energy storage module and operably connected to the control unit, the method including opening the additional switch when energy is transferred to the energy storage module and a level of energy stored in the energy storage module exceeds a predetermined storage threshold.

Images