Method for monitoring state of plurality of battery cells in energy storage system

Abstract

A method for monitoring a state of an energy storage system (110) connected to a power converter (120) is disclosed. The energy storage system comprises a plurality of interconnected energy storage cells, and a power converter (120) is configured to output power from the energy storage system (110) to a power transfer system (10) and to charge the energy storage system (110) with power from the power transfer system (10). The method includes generating, by the power converter (120), an electrical signal injected into the energy storage system, measuring a response signal generated in response to the injected electrical signal, and determining a state of the energy storage system (110) based on the received response signal.

Description

Technical Field

[0001] 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 Art

[0002] The power transmission system needs to provide power in a reliable manner. Therefore, such technologies usually include energy storage systems to balance and smooth the demand and power swings of the grid. Preferably, the energy storage system can store energy during off-peak hours and release energy to the grid during peak hours, for example to provide fast frequency response and inertial response.

[0003] Examples of such energy storage systems include a plurality of interconnected energy storage cells, such as battery cells or supercapacitors connected in series and / or parallel. The energy storage system may be connected to a power converter that may operate in a bidirectional manner for outputting released energy to a power transmission system and charging the energy storage cells from the power transmission system.

[0004] Monitoring the status of an energy storage system is critical to ensuring reliable performance. This can be done by checking the condition of the energy storage cells during charging and / or discharging of the energy storage system and can indicate, for example, the state of health (SoH) of the energy storage cells.

[0005] However, considering the growing demand for uninterruptible power supplies and support for transmission and distribution systems, it would be beneficial to provide improved techniques for monitoring the status of such systems. Summary of the invention

[0006] This object is achieved in particular by the method and the control unit as claimed in the appended independent claims. Preferred embodiments are specified in the dependent claims.

[0007] Therefore, according to a first aspect, there is provided a method for monitoring the state of an energy storage system connected to a power converter, wherein the energy storage system comprises a plurality of interconnected energy storage cells, and the power converter is configured to output power from the energy storage system to a power transmission system and to charge the energy storage system using power from the power transmission system. The method comprises generating an electrical signal injected into the energy storage system by the power converter, measuring a response signal generated in response to the injected electrical signal, and determining the state of the energy storage system based on the received response signal.

[0008] According to a second aspect, there is provided a control unit configured to monitor the state of the energy storage system according to the first aspect. The control unit comprises an electrical signal generating function configured to cause the power converter to inject an electrical signal into the energy storage system; a state determining function configured to measure a response signal generated in response to the injected electrical signal, and determine the state of the energy storage system based on the response.

[0009] The inventive concept outlined above is based on the realization that a power converter can be used to inject an electrical signal into an energy storage system to determine the state of the energy storage system. This is particularly beneficial than systems in the prior art that rely on load current or charging current to check the energy storage condition, because the present invention also allows the state of the energy storage system to be monitored when the energy storage system is in standby mode (i.e., not charging or discharging). When the energy storage system is used for grid frequency support, it may be advantageous to use a power converter to generate an electrical signal because the time window in which the energy storage system is connected to the load can be very small, such as about 1 second. Since the number of data points may be too limited, it may be difficult to determine the state of the energy storage system based on the load current available in such a short time window. For example, a battery management system (BMS) that updates at a frequency of 10Hz may only generate 10 data points during a 1 second time window, which may result in relatively low accuracy of state monitoring. By using a dedicated electrical signal generated by a power converter, the state of the energy storage system can be monitored even when the energy storage system is not charging or discharging. This in turn allows faults to be detected at an early stage regardless of whether there is a load current.

[0010] The electrical signal may also be referred to as a circulating current flowing between the power converter and the energy storage system. The electrical signal may be understood as an electrical signal having very little or negligible effect on the power transmission system, or in other words, the electrical signal is "invisible" to the external system to which the power converter and the energy storage system are connected.

[0011] Thus, the response signal may be a signal generated by the energy storage system in response to an electrical signal injected into the system, thereby forming part of the circulating current. However, it should be understood that the response signal may also be a signal generated by a sensor or other device (e.g., a BMS) that measures the response of the system to the injected electrical signal.

[0012] Advantageously, the generation of the electrical signal and thereby the determination of the state of the energy storage system may be performed periodically, such as hourly or daily. In other options, it may be performed randomly upon request, such as after each energy storage system operation, or in conjunction with a manual check. Further, the duration of the signal may be selected based on the need for data length.

[0013] In the context of this disclosure, the term "energy storage system" may generally refer to an energy storage solution for storing electrical energy and providing it as a primary or supplemental source to a specified load and for voltage and frequency regulation. Preferably, the energy storage system may be sized for medium to high voltage DC.

[0014] The term "energy storage cell" may generally be 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 and secondary battery cells. Energy storage cells may be interconnected (preferably in series) in a stack configuration to form an energy module. A group of interconnected energy storage modules may be referred to as an energy storage string, and a plurality of strings may form an energy storage group.

[0015] It should be understood that the state of the energy storage system can be determined at different levels, such as determining the state of the energy storage system for a single energy storage cell, a single energy storage module, a string, or an entire energy storage group.

[0016] The state of an energy storage system can refer to the charging condition, such as the system's state of health (SoH), or the connection status (such as cables, busbars, switches, etc.).

[0017] A "power transmission system" generally refers to a structure for transmitting and / or distributing electrical power (eg, AC or DC). In some examples, a power transmission system may be referred to as a power transmission network, a transmission network, a power grid, or a power grid.

[0018] The term "power converter" generally refers to a device that converts electrical energy between AC and DC or DC and DC. A power converter, also known as an inverter, can be configured to convert AC power to DC to charge an energy storage system, and to convert DC power discharged from an energy storage system into active power or reactive power that can be provided to an (AC) power transmission system. Therefore, a power converter can be called a bidirectional converter. A power converter can be, for example, a modular multilevel converter (MMC).

[0019] A control unit or controller may be understood as a device or circuit capable of outputting instructions for controlling the operation of a power converter to inject an electrical signal into an energy storage system, receiving information related to a response signal, and processing the received information to determine the state of the energy storage system. The control unit may be structurally integrated into the energy storage system or arranged to be physically remote from the system. In some examples, some or all of the functions of the control unit may be performed in a cloud-based service. For example, this may apply to analysis of received signals or additional inputs from sensors or battery monitoring systems.

[0020] The electrical signal may preferably be adjusted to reduce the energy exchange between the power converter and the energy storage system. This may be achieved, for example, by using an electrical signal whose average power is zero or at least negligible. Thus, the electrical signal may be AC, for example having a current form conforming to a sinusoidal shape, a square wave, a sawtooth shape, a triangle shape, etc.

[0021] The electrical signal may comprise frequency components below the Nyquist frequency, ie whose highest frequency is less than half the sampling frequency of the control system of the power converter.

[0022] Further, in some embodiments, the electrical signal may include frequencies that are integer multiples of a fundamental frequency of the power transmission system to reduce the effect of the electrical signal on the active power seen by the power transmission system. Second and third harmonics of the fundamental frequency of the power transmission system may be found in power converter control systems for balancing and modulation purposes, so utilizing these frequencies may be advantageous.

[0023] The electrical signal amplitude can be selected to be low enough to have limited or negligible effect on system equipment, but high enough to be measurable. Studies have shown that amplitudes of 10% or less of the energy storage system charging current or load current amplitude are small enough not to cause interference to system equipment.

[0024] According to some embodiments, determining the state of the energy storage system may include calculating an impedance based on the voltage and current of the received response signal, and comparing the calculated impedance with a reference value associated with the energy storage system. For example, the reference value may be an expected value of an energy storage cell or device (e.g., a switch and a bus) of the system, and may be obtained from a previous measurement of the energy storage system, or from a supplier of the energy storage system.

[0025] The state of health (SoH) of the energy storage system can be determined using the real or imaginary part of the impedance. The choice of the real or imaginary part can depend on which parameter has the highest linearity with the SoH and is less frequency dependent. In general, the real part may be more accurate because it has been observed to double at the end of the useful life and vary little with frequency.

[0026] According to some embodiments, the response signal may include information indicating the temperature of the energy storage system. The temperature may be used to determine the state of the energy storage system by comparing the temperature to a reference value (e.g., an expected temperature or an ideal temperature). For example, the reference value may be based on a previous measurement, a predetermined temperature limit, or a predefined thermal model. For example, an increase in temperature may indicate a poor electrical connection, high contact resistance.

[0027] As mentioned previously, processing can be performed by a control unit located within the energy storage system or remotely, such as in a cloud-based service.

[0028] The present invention may be embodied as computer-readable instructions for controlling a programmable computer to perform the above method. Such instructions may be distributed in the form of a computer program product comprising a computer-readable medium storing these instructions. In particular, the instructions may be loaded into a control unit responsible for controlling the control unit as described above in conjunction with the second aspect.

[0029] It is noted that embodiments of the present invention relate to all possible combinations of features recited in the claims. Further, it should be understood that various embodiments described for the method according to the first aspect may be combined with embodiments of the control unit defined according to the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] These and other aspects will now be described in more detail with reference to the accompanying drawings, which show embodiments.

[0031] Figure 1 An energy storage system and a power converter connecting the energy storage system to a power transmission system according to an embodiment of the present invention are illustrated.

[0032] Figure 2 An energy storage system and a modular multilevel converter according to embodiments are illustrated.

[0033] Figure 3 is a flow chart outlining a method for monitoring the status of an energy storage system according to an embodiment.

[0034] All the figures are schematic, not necessarily to scale, and generally show only parts which are necessary to elucidate the embodiments, wherein other parts may be omitted or merely suggested. Throughout the specification, like reference numerals refer to like elements. DETAILED DESCRIPTION

[0035] The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments are shown.

[0036] Figure 1 The energy storage system 110 is schematically illustrated and is operatively connected to a power converter 120 and a control unit 130. The energy storage system 110 includes a plurality of interconnected energy storage cells, such as Figure 2Discussed in more detail, it is configured to output power from the energy storage system 110 and output it to the power transmission system 10 via the power converter 120. The energy storage system 110 is further configured to be charged via the power converter 120 using the power provided by the power transmission system 10, so the energy storage system can be regarded as a bidirectional power converter 120. With this configuration, the energy storage system 110 can be used as an uninterruptible power supply and provide transmission and distribution system support.

[0037] The power converter 120 may be, for example, a modular multilevel converter (MMC), which is known to be associated with relatively low harmonic generation and relatively low energy losses. The power converter 120 may be configured to generate an electrical signal that is injected into the energy storage system 110 for monitoring purposes. The electrical signal may be understood as a circulating current flowing between the power converter 120 and the energy storage system 110, and the response signal returned from the energy storage system 110 may be analyzed to determine the state of the energy storage system. The power converter 120 may, for example, apply a voltage with a controlled frequency and amplitude in the converter arm periodically (e.g., every hour or every day) or randomly upon request (e.g., after each operation or in conjunction with a manual inspection). The resulting current of the same frequency may then be recorded and analyzed to determine the state of the energy storage system 110 or its components. The analysis may, for example, include comparing the voltage and current of the received signal, preferably at a frequency selected for the signal injected into the energy storage system 110. This may give an indication of the impedance, which may be compared with an expected value or a reference value to provide an indication of the state of the energy storage system 110.

[0038] The frequency of the applied voltage is preferably less than 50% of the system sampling frequency. Power converter control systems typically sample at a kHz rate, while the BMS of the battery and the supercapacitors typically sample at a frequency range of a few Hz to a few kHz. Further, in some examples, the frequency of the injected signal can be selected to be an integer of the fundamental frequency of the power transmission system 10 (e.g., the grid), as this has a negligible effect on the active power seen by the power transmission system 10. Further, since second and third harmonics may be found in the power converter control system, it may be beneficial to utilize these frequencies for balancing and modulation purposes.

[0039] The voltage amplitude of the injected signal may be selected so that the resulting current does not cause overcurrent in the energy storage system 110 and further so that the amplitude of the injected current does not generate too high losses but can still be measured with sufficient accuracy.

[0040] With the above in mind, it will be appreciated that the selected frequency and amplitude of the generated electrical signal may vary from case to case, depending on the actual configuration of power converter 120 , energy storage system 110 , and power transmission system 10 supported by energy storage system 110 .

[0041] As shown, the energy storage system 110 may further include a battery management system (BMS) 140 configured to monitor and control the state of the energy storage cells of the energy storage system 110. Advantageously, the BMS 140 may be configured to generate data, for example, indicating the voltage, current or temperature of the entire energy storage system 110, or in particular indicating the voltage, current or temperature of a single energy storage cell.

[0042] Therefore, there are several different options when measuring the response signal generated by the injected electrical signal: by the BMS or power converter controller, or by separate sensors dedicated to measuring, for example, the above mentioned parameters.

[0043] The controller or control unit 130 may be communicatively connected to the energy storage system 110 and the power converter 120. The control unit 130 may include an electrical signal generation function 132 configured to cause the power converter 120 to inject an electrical signal into the energy storage system 110, and a state determination function 134 configured to measure a response signal received at the power converter 120 or provided by a sensor (e.g., BMS 140). Depending on how the response signal is recorded and analyzed, data may be retrieved from different levels of the energy storage system 110. For example, data may be collected at the storage cell level, indicating the state of a single storage cell, or at the module or string level. Further, the state of the entire energy storage system 110 or energy storage group may be determined.

[0044] For example, the response signal may be analyzed to determine the state of health (SoH) of the energy storage system 110 or its components. The SoH may be determined or estimated based on the impedance of the resulting signal. The real part of the impedance may be primarily determined by the internal resistance or equivalent series resistance (ESR), and thus may be used as an indication of the SoH, while the imaginary part of the impedance may reflect the effective capacitance or inductance of the energy storage cell. Further, the current injected by the electrical signal may cause a change in the temperature of the energy storage cell. The temperature response may be compared to a predetermined thermal model to determine the state of the energy storage cell.

[0045] The response signal may be further analyzed to determine the connection status of the energy storage system 110. Poor electrical contact and open points due to, for example, installation faults or human error may cause increased impedance. High contact resistance may also be detected as a temperature increase caused by the injected signal current.

[0046] The control unit 130 may be structurally integrated in the energy storage system 110 or the power converter 120, or provided as a physically separate entity. In an alternative configuration, the control unit 130 may be a remote unit, for example implemented as a cloud-based function. Further, it should be understood that one or more tasks performed by the control unit 130 may be functionally or structurally distributed, for example, the electrical signal generation function 132 may be arranged at the power converter 120 or integrated with the power converter, while the state determination function 134 may be arranged at a remote location, for example in the cloud.

[0047] Figure 2 The power converter 120 is shown as an embodiment of a three-phase MMC (including three converter arms), wherein each arm is connected to a corresponding phase of the power transmission system 10. The MMC 120 is further connected to the energy storage system 110, which configuration may be combined with the above Figure 1 The configuration described is similar. Figure 2 The energy storage system 110 shown in FIG. 1 includes a plurality of energy storage cells 112, which in this example are connected into modules, which are grouped into three strings 114 connected in parallel with an MMC 120. Each energy storage cell 112 may include a secondary cell or capacitor device, which is configured to be repeatedly charged and discharged via the MMC 120.

[0048] Each arm of the MMC 120 may be configured to transmit an electrical signal ( Figure 2 The electrical signal is then injected into the corresponding string 114 of the energy storage system 110 (shown in dashed lines). The response to the electrical signal can then be analyzed as described above, whether based on the response signal looped back to the MMC 120 or based on other parameters recorded by sensors in the energy storage system 110, such as voltage, current or temperature.

[0049] Preferably, the MMC 120 is configured to inject an AC signal into the energy storage system 110 so that the energy exchanged between the energy storage system 110 and the power converter 120 is reduced. The current of the injected signal may be sinusoidal, square, sawtooth or triangular.

[0050] Figure 3 It is a diagram showing the combination of Figure 1 and Figure 2Flowchart of a method for monitoring the state of an energy storage system 110 of a disclosed embodiment. The method may, for example, be embodied as computer executable instructions, distributed and used in the form of a computer program product, the computer program product including a computer readable medium storing such instructions. As shown, the method may include the following actions: generating 210 an electrical signal injected into the energy storage system 110 through the power converter 120, measuring 220 a response signal generated in response to the injected electrical signal, and determining 230 the state of the energy storage system 110 based on the received response signal. These actions are preferably performed by the control unit 130 discussed previously.

[0051] Those skilled in the art will appreciate that computer storage media includes volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. Further, it is known to those skilled in the art that communication media typically embodies computer readable instructions, data structures, program modules or other data in the form of a modulated data signal such as a carrier wave or other transmission mechanism, and includes any information delivery media.

Claims

1. A method for monitoring the status of an energy storage system (110) connected to a power converter (120), wherein: The energy storage system includes a plurality of interconnected energy storage cells (112); and The power converter is configured to output the power from the energy storage system to the power transmission system (10), and to charge the energy storage system with the power from the power transmission system; The method comprises: generating (210) an electrical signal by the power converter for injection into the energy storage system; measuring (220) a response signal generated in response to the injected electrical signal; and A state of the energy storage system is determined (230) based on the received response signal.

2. The method according to claim 1, wherein: The electrical signal injected into the energy storage system is alternating current.

3. The method according to claim 1 or 2, wherein: The electrical signal injected into the energy storage system comprises a frequency that is less than 50% of a sampling frequency of a control system of the power converter.

4. A method according to any one of the preceding claims, wherein: The electrical signal injected into the energy storage system comprises a frequency that is an integer multiple of a fundamental frequency of the power transmission system.

5. The method according to claim 4, wherein: The electrical signal injected into the energy storage system includes a frequency corresponding to a second harmonic frequency or a third harmonic frequency of a fundamental frequency of the power transmission system.

6. A method according to any one of the preceding claims, wherein: The magnitude of the electrical signal injected into the energy storage system is 10% or less of the magnitude of the charging current or load current of the energy storage system.

7. A method according to any one of the preceding claims, wherein: Determining the state of the energy storage system includes calculating an impedance based on the voltage and current of the received response signal and comparing the calculated impedance to a reference value associated with the energy storage system.

8. The method according to claim 7, wherein: A state of health SoH of the energy storage system is determined based on the real part or the imaginary part of the impedance.

9. The method according to claim 7 or 8, wherein: The reference value is obtained from previous measurements performed on the energy storage system or from a supplier of the energy storage system.

10. A method according to any one of the preceding claims, wherein: The response signal includes information indicative of a temperature of the energy storage system.

11. The method according to claim 10, wherein: Determining a state of the energy storage system includes comparing a temperature of the energy storage system to an expected temperature.

12. The method according to claim 11, wherein: The expected temperature is based on previous measurements of the temperature of the energy storage system or a predetermined temperature limit.

13. A method according to any one of the preceding claims, wherein: The state of the energy storage system includes information of at least one of the following: a single energy storage cell, a string of multiple energy storage cells, or the entire energy storage system.

14. A control unit (130) configured to monitor a state of an energy storage system (110) connected to a power converter (120), wherein: The storage system comprises a plurality of interconnected energy storage cells (112), and the power converter is configured to output power from the energy storage system to a power transmission system (10) and to charge the energy storage system using power from the power transmission system, and the control unit comprises: an electrical signal generation function (132) configured to cause the power converter to inject an electrical signal into the energy storage system; and A state determination function (134) is configured to measure a response signal generated in response to the injected electrical signal and determine a state of the energy storage system based on the received response.

15. The control unit according to claim 14, wherein: The state determination function is configured to receive a response signal from a battery management system BMS (140).

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