Method for detecting earth faults, control unit and energy storage system

Abstract

The invention provides a method, a control unit and an energy storage system for detecting a ground fault. Disclosed herein is a method (100) for detecting a ground fault in an energy storage system (20) comprising at least one string (21) having a plurality of energy storage modules (22). The string is connected between a first direct current (DC) pole (12) and a second DC pole (14). The first and second DC poles are connected to the AC side by respective converters (10A, 10B). The method comprises: determining (110) a first in-phase harmonic component based on at least one voltage to ground (112A-112C) measured at the AC side; determining (120) a second in-phase harmonic component based on at least one pole-to-ground voltage (122A, 122B) measured at the DC side; and determining (130) the presence of a ground fault if the first in-phase harmonic component increases and / or the second in-phase harmonic component decreases. A control unit and an energy storage system are also disclosed herein.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for detecting a ground fault in an energy storage system. The present disclosure further relates to a control unit for an energy storage system and an energy storage system. BACKGROUND

[0002] The continuous electrification of various sectors, such as transportation and heating, together with the demand for non-fossil energy, such as wind and solar energy, results in more complex power systems and higher demand for energy. Non-fossil energy is often intermittent, and in order to balance supply and demand, energy storage systems are implemented on a larger scale in power systems. In order to meet this high energy demand, multiple energy storage units are connected in series and in parallel to meet voltage and current requirements.

[0003] Ground faults pose a significant safety risk, including the risk of electric shock and the risk of fire, and also disrupt the efficient operation of the energy storage system. Thus, detecting and resolving ground faults is crucial to ensure the reliability, safety, and performance of the energy storage facility. Ground faults in or at the poles or near the pole position can be detected based on the DC pole voltage. However, a ground fault near the electrical midpoint of the energy storage system will not affect the pole voltage, and there will be little deviation in the pole voltage from its normal operating conditions, or there will be very small deviations that can be insufficient to detect a ground fault. Therefore, there is a need for a refined method for detecting ground faults. SUMMARY

[0004] It is therefore an object of the present disclosure to provide a method for detecting a ground fault in an energy storage system that alleviates at least part of the above-mentioned drawbacks. For example, it is an object of the present disclosure to provide a method that reduces the risk of a ground fault not being detected and is more efficient in detecting ground faults than existing methods. It is a further object of the present disclosure to provide a control unit implementing the method and an energy storage system comprising said control unit.

[0005] According to a first aspect of the present disclosure, a method for detecting a ground fault in an energy storage system is provided. The energy storage system comprises at least one string having a plurality of energy storage modules. The at least one string is connected between a first direct current, DC, pole and a second DC pole providing a DC side. The first DC pole is connected to an AC side through a first converter and the second DC pole is connected to the AC side through a second converter. The method comprises determining a first in-phase harmonic component based on at least one phase-to-ground voltage measured at the AC side, determining a second in-phase harmonic component based on at least one pole-to-ground voltage measured at the DC side, and determining the presence of a ground fault if the first in-phase harmonic component increases and / or the second in-phase harmonic component decreases.

[0006] According to a second aspect of the disclosure, a control unit for an energy storage system is provided. The energy storage system comprises at least one string of a plurality of energy storage modules. The at least one string is connected between a first direct current, DC, pole and a second DC pole providing a DC side. The first DC pole is connected to an AC side through a first converter and the second DC pole is connected to the AC side through a second converter. The control unit is configured to determine a first in-phase harmonic component based on at least one phase-to-ground voltage measured at the AC side, determine a second in-phase harmonic component based on at least one pole-to-ground voltage measured at the DC side, and determine a presence of a ground fault if the first in-phase harmonic component increases and the second in-phase harmonic component decreases.

[0007] According to a third aspect of the disclosure, an energy storage system is provided. The energy storage system comprises at least one string of a plurality of energy storage modules. The at least one string is connected between a first direct current, DC, pole and a second DC pole providing a DC side. The first DC pole is connected to an AC side through a first converter and the second DC pole is connected to the AC side through a second converter. The energy storage system further comprises a control unit according to the second aspect.

[0008] Ground faults are a common problem in electrical systems and can be challenging to detect and locate in some occasions, which increases the number of maintenance hours to solve the ground fault. In particular, it is challenging to detect a ground fault near an electrical midpoint of a string with conventional methods for detecting ground faults.

[0009] The electrical midpoint can be seen as a neutral point, where the potential with respect to ground or a common reference is zero. This point can exist inside one of the energy storage modules, at a middle position between two poles, or between two energy storage modules at approximately the same electrical distance from each of the first DC pole and the second DC pole.

[0010] With the provided method, it is possible to detect a ground fault near one of the first DC pole and the second DC pole and also a ground fault at or close to the electrical midpoint. By determining the first in-phase harmonic component and the second in-phase harmonic component and determining if a change in the first in-phase harmonic component or the second in-phase harmonic component occurs, the ground fault can be detected. By monitoring if the first in-phase harmonic component increases or the second in-phase harmonic component decreases, it is possible to determine that a ground fault is present in the energy storage system. The first in-phase harmonic component and the second in-phase harmonic component are independent of where in the string the ground fault occurs. Even if the ground fault is located at the electrical midpoint, an increase / decrease will be observed. Thus, the provided method is more efficient than conventional methods, since a ground fault can be detected everywhere in the energy storage system. This reduces the risk that a ground fault goes undetected and thus it also increases the safety of the energy storage system.

[0011] The ground fault can be detected based on only one of the first and second in-phase harmonic components, however, it can be advantageous to use both as this will provide a back-up parameter for determining whether a ground fault is present. With both the first and second in-phase harmonic components, a more accurate identification of a ground fault can be achieved. The first in-phase harmonic component is based on the at least one measured phase-to-ground voltage. The method typically uses all phases on the AC side to determine the first in-phase harmonic component. However, sufficient results can be obtained using only one phase. If a ground fault occurs in the energy storage system, the first in-phase harmonic component determined on the AC side is expected to increase. A ground fault at the DC side can also be detected based on whether a DC component (i.e. DC offset) appears in the measured phase-to-ground voltage at the AC side. The second in-phase harmonic component is based on the at least one pole-to-ground voltage measured at the DC side 2. Typically, both the pole-to-ground voltage of the first DC pole and the pole-to-ground voltage of the second DC pole are measured and used to determine the second in-phase harmonic component. If a ground fault occurs in the energy storage system, the second in-phase harmonic component determined on the DC side is expected to decrease.

[0012] The in-phase harmonic component refers to the n-th harmonic of the fundamental frequency (where n is 1, 2, 3, etc.) and it has the same fundamental as the fundamental frequency. As an example, if the fundamental frequency is 50 Hz in a 3-phase system and the third harmonic is 150 Hz, then a 150 Hz component will be generated in all three phases and they will be aligned with respect to each other (i.e. due to the in-phase component). It should be noted that sub-multiples can also be used. The in-phase harmonic component is an additional layer superimposed on the fundamental voltage, which can be used to reduce voltage peaks. Thus, it can help to reduce the size of the converter. Because these harmonic components occur at different frequencies (essentially in-phase), they are relatively easy to track in the presence and absence of a ground fault.

[0013] Typically, the energy storage system comprises a plurality of strings connected between the first and second DC poles. In each string, there is a plurality of energy storage modules. Depending on the system requirements, the plurality of energy storage modules can be arranged in series and / or in parallel with each other. The energy storage modules can be supercapacitors or batteries connected in series and / or in parallel. Further, each energy storage module can comprise a plurality of energy storage cells arranged in parallel and / or in series.

[0014] The first and second DC poles can also be the positive and negative poles of the DC side, respectively.

[0015] Each of the first DC pole and the second DC pole is connected to the AC side by a respective converter, i.e. a first converter and a second converter. The first converter and the second converter can each have a wye configuration. The first converter and the second converter can be a first modular multilevel converter, MMC, and a second MMC.

[0016] The AC side is intended herein as the side of the first converter and the second converter that is connected to or can be connected to a power grid.

[0017] The method can further comprise determining a pole imbalance voltage based on the voltages measured at the first DC pole and the second DC pole for determining whether the ground fault is located at the electrical midpoint of the string or close to the DC pole.

[0018] By also determining the pole imbalance voltage, it can be determined where the ground fault is located. As an example, if the first DC pole is the positive DC pole and the second DC pole is the negative DC pole, a positive pole imbalance voltage will indicate that the ground fault is located closer to the second DC pole, while a negative pole imbalance voltage will indicate that the ground fault is located closer to the first DC pole. Based on the magnitude and the polarity of the pole imbalance voltage, it can be determined where in the string the ground fault is located approximately. However, if the fault is in the electrical midpoint, there will be no or only a small pole imbalance voltage. Thus, only the pole imbalance voltage will not be able to detect and locate a ground fault at the electrical midpoint. By combining the use of the deviation in the first in-phase harmonic component and / or the second in-phase harmonic component to detect the ground fault and the use of the pole imbalance voltage to locate the ground fault, the method can determine whether the ground fault is located at or close to the electrical midpoint. If the first in-phase harmonic component and / or the second in-phase harmonic component indicates that there is a ground fault in the energy storage system and there is no or only a small (negligible) pole imbalance voltage, the method can determine that the ground fault is located at or close to the electrical midpoint.

[0019] The pole imbalance voltage refers to a voltage imbalance between the first DC pole and the second DC pole of the energy storage system. In a balanced energy storage system, the absolute values of the voltages at the first DC pole and the second DC pole are identical. This means that if the voltages at each pole are measured with respect to a common reference point, such as ground, the voltages will have equal magnitude but opposite polarity. In this case, the pole imbalance voltage will be zero. However, if an imbalance occurs between the first DC pole and the second DC pole, the absolute values of the voltages will not be identical and the pole imbalance voltage will not be zero.

[0020] If the first in-phase harmonic component increases and / or the second in-phase harmonic component decreases for a predetermined operating time, a ground fault can be determined.

[0021] When measuring the current and the voltage, there is a risk that spikes or sudden increases in the measured values can occur. A spike refers to a sudden, short increase in voltage or current that can occur due to, for example, lightning, switching operations, and electromagnetic interference. By determining a ground fault only as a situation in which the first in-phase harmonic component increases and / or the second in-phase harmonic component decreases for a predetermined operating time, it is possible to avoid that the method falsely indicates that a ground fault is present in the energy storage system due to spikes. By using a predetermined operating time for the determination, the accuracy can be further increased, i.e. the risk of false detections can be reduced. The predetermined operating time can be set to be greater than or equal to the fundamental period of the AC voltage.

[0022] The method can further comprise generating a first predetermined in-phase harmonic voltage with the first converter and the second converter and / or generating a second predetermined in-phase harmonic voltage with the first converter and the second converter; and comparing the first predetermined in-phase harmonic voltage with the determined first in-phase harmonic component and / or comparing the second predetermined in-phase harmonic voltage with the determined second in-phase harmonic component.

[0023] The first and second converters can be used to generate a first predetermined in-phase harmonic voltage at the AC side and a second predetermined in-phase harmonic voltage at the DC side. This generated first and second predetermined in-phase harmonic voltages can be used as a reference for comparison with the determined first and second in-phase harmonic components. In other words, the first and second converters are provided with inputs to generate the first and second predetermined in-phase harmonic voltages. Each of the first and second converters generates a fundamental voltage and a DC voltage to exchange active and reactive power between the AC side and the DC side. Along with the fundamental voltage and the DC voltage, the first and second converters can also be controlled to generate the first and second predetermined in-phase harmonic voltages. The generated first and second predetermined in-phase voltages propagate along with the fundamental voltage and the DC voltage. They can then be determined (i.e. extracted) as the first and second in-phase harmonic components at the AC side and the DC side at a later stage, depending on the fundamental voltage and the DC voltage. By comparing the first predetermined in-phase harmonic voltage with the determined first in-phase harmonic component and / or comparing the second predetermined in-phase harmonic voltage with the determined second in-phase harmonic component, a further way of determining the presence of a ground fault is provided. During normal operation, it is expected that the first predetermined in-phase harmonic voltage and the first in-phase harmonic component are equal, and the second predetermined in-phase harmonic voltage and the second in-phase harmonic component are equal. If the predetermined in-phase harmonic voltages and the in-phase harmonic components are not equal (or at least differ from each other by at least a predetermined threshold), this is an indication of a ground fault at the AC side or the DC side, as this ground fault causes a change in the distribution of the generated in-phase harmonic voltages, and thus a change in the distribution of the determined in-phase harmonic components. In this context, the predetermined value or predetermined voltage is a value (or voltage) of an in-phase harmonic voltage of a desired form, as for example set by an operator. The predetermined in-phase harmonic voltages can have a low amplitude to reduce the interaction during normal operation of the converters.

[0024] The method can further comprise comparing the first in-phase harmonic component and the second in-phase component. A ground fault can be detected by comparing the first in-phase harmonic component with the second in-phase harmonic component to detect a change in their relationship. As an example, the method can be designed such that the first in-phase harmonic component and the second in-phase harmonic component are expected to have the same value when determined. In this design, a ground fault is detected when the relationship (e.g. the ratio of the difference) between the first in-phase harmonic component and the second in-phase harmonic component changes. This allows an operator to implement further parameters for detecting whether a ground fault is present.

[0025] The method can further comprise obtaining a control signal based on the first in-phase harmonic component and the second in-phase harmonic component. The comparing can comprise comparing the control signal to a threshold value. The first in-phase harmonic component and the second in-phase harmonic component can be used to obtain the control signal. The control signal is a constant value during normal operation. The control signal can then be compared to a threshold value to detect if a ground fault is present. The possibility of using a threshold value allows an operator to set the threshold value with a safety margin, such that false indications of a ground fault are avoided.

[0026] The control signal can be obtained from a first root mean square, RMS, value of the first in-phase harmonic component and a second RMS value of the second in-phase harmonic component.

[0027] RMS values are advantageous in electrical control systems as they provide a more reliable and more accurate determination of the effective value of an alternating quantity. RMS values facilitate the analysis of signals.

[0028] The control signal can be obtained by subtracting the second in-phase harmonic component from the first in-phase harmonic component. If the method is designed such that the first in-phase harmonic component and the second in-phase harmonic component are equal during normal operation, this will result in the control signal being zero during normal operation. When the control signal deviates from zero, the presence of a ground fault can be detected.

[0029] The first in-phase component can be determined from a residual voltage at the AC side. The residual voltage can be calculated as the sum of all phase-to-ground voltages at the AC side.

[0030] Before obtaining the control signal, the first in-phase harmonic component can be multiplied by a factor 1 / k. The factor 1 / k can be used to scale the first in-phase harmonic component such that it can be easily compared to the second in-phase harmonic component.

[0031] The parameter k can be the ratio of the DC side ground capacitance to the AC side (RC circuit) filter capacitance.

[0032] The second in-phase harmonic component can be determined as the sum of the pole-to-ground voltage measured at the first DC pole and the pole-to-ground voltage measured at the second DC pole.

[0033] The method can further comprise tripping the energy storage system if a ground fault is detected. When a ground fault is detected, it can be advantageous to trip the energy storage system such that it is disconnected from the AC side. This prevents the ground fault from affecting other components at, for example, the AC side.

[0034] The energy storage system can further comprise voltage measurement means for measuring a respective phase-to-ground voltage for each phase at the converter AC side and a respective pole-to-ground voltage for each of the first DC pole and the second DC pole.

[0035] The AC side and the DC side can be ungrounded or high impedance grounded. Typically, the energy storage system connected to the AC side by the first converter and the second converter is preferably an ungrounded or high impedance grounded system as it provides the advantage that if a ground fault occurs on the DC side, this will only result in a low fault current compared to a grounded system. This approach is particularly advantageous in ungrounded and high impedance grounded systems as it is challenging to detect a ground fault at the electrical neutral in such systems.

[0036] As an example, a high impedance grounding can be implemented with an energy storage system comprising: a first capacitor ground for a first DC pole and a second capacitor ground for a second DC pole; and at the AC side a resistance-capacitance, RC, filter for each phase. The values of the first and second capacitor and the capacitance of the RC filter should preferably provide a high impedance to ground at low frequencies.

[0037] The effects and features of the second and third aspects can be largely analogous to those described above in connection with the first aspect. The embodiments mentioned in relation to the first aspect can be at least largely compatible with the second and third aspects. It should further be noted that the present disclosure relates to all possible combinations of features unless otherwise clearly indicated.

[0038] Further areas of applicability of the present disclosure will become apparent from the detailed description given below. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description.

[0039] It will thus be appreciated that the present disclosure is not limited to the specific methodology, control unit and energy storage system described herein. It will also be appreciated that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. BRIEF DESCRIPTION OF DRAWINGS

[0040] The above and other aspects of the present disclosure will now be described in more detail with reference to the appended drawings, in which:

[0041] Figure 1 An energy storage system comprising a control unit is schematically illustrated.

[0042] Figure 2 A method for detecting a ground fault is schematically illustrated.

[0043] Figure 3 A flowchart of the method is schematically illustrated. DETAILED DESCRIPTION

[0044] The present disclosure is described below by means of several illustrative examples. It will be appreciated that these examples are provided merely for illustration and explanation, and are not intended to limit the scope of the present disclosure.

[0045] Figure 1 An exemplifying embodiment of a system 1 is disclosed, the system comprising a first converter 10A, a second converter 10B, an energy storage system 20 and a control unit 30. Each of the first converter 10A and the second converter 10B comprises an AC side 4 (which can be a connection to e.g. an AC grid) and a DC side 2. The first converter is connected to a first DC pole 12 of the DC side 2 and the second converter is connected to a second DC pole 14 of the DC side 2. The energy storage system 20 is connected between the first DC pole 12 and the second DC pole 14. The AC side 4 is provided with three phases A, B, C, each phase being connected to the first converter 10A and the second converter 10B.

[0046] The energy storage system 20 comprises a plurality of strings 21, each string having a plurality of energy storage modules 22 connected in series. Each string 21 is connected between the first DC pole 12 and the second DC pole 14. Depending on system requirements, the energy storage modules 22 can be connected both in series and in parallel. In Figure 1 In an embodiment, the energy storage modules 22 are arranged only in series. Further, each energy storage module can comprise a plurality of energy storage cells (not shown) which can be arranged in parallel and / or in series with each other.

[0047] The energy storage system 20 further comprises a first capacitor ground 42 for the first DC pole 12 and a second capacitor ground 43 for the second DC pole 14. At the AC side 4, a resistance-capacitance, RC, filter 40 is provided for each phase. In this case, the AC side 4 and the DC side 2 are high impedance grounded, thereby providing a high impedance grounded system. However, it is also common in case of ungrounded systems. Typically, the energy storage system 20 is preferably ungrounded or high impedance grounded, as this provides the advantage that if a single ground fault occurs on the DC side 2, this will only result in a low fault current compared to grounded systems. The method 100 will be further discussed in connection with Figure 2 and Figure 3 The method 100 is further advantageous for ungrounded and high impedance grounded systems, as it is challenging to detect a ground fault at the electrical neutral in ungrounded and high impedance systems.

[0048] The energy storage system 20 can further comprise voltage measuring devices (not shown) for each phase at the AC side 4 and for each of the first DC pole 12 and the second DC pole 14. These voltage measuring devices can be utilized to measure the phase-to-ground voltage and the pole-to-ground voltage. These voltage measuring devices are capable of measuring both AC voltage and DC voltage.

[0049] The control unit 30 is provided to control the first converter 10A and the second converter 10B and to perform the method 100 according toFigure 2 and Figure 3 The method 100 is further described to operate.

[0050] In Figure 2 , an overview of the method 100 is illustrated. In Figure 3 , the flowchart illustrates how the method 100 can be implemented.

[0051] Figure 2 A method 100 for detecting a ground fault in an energy storage system 20 as described in connection with Figure 1 is provided, the method having three steps. The method 100 comprises determining 110 a first in-phase harmonic component based on at least one phase-to-ground voltage 112A-112C measured at the AC side 4. This provides a measurement on the AC side 4 where it can be monitored if the first in-phase harmonic component is changing. The method 100 further comprises determining 120 a second in-phase harmonic component based on at least one pole-to-ground voltage 122A, 122B measured at the DC side 2. This provides a measurement on the DC side 2 where it can be monitored if the second in-phase harmonic component is changing. In order to determine if there is a ground fault present, the method 100 further comprises determining 130 the presence of a ground fault if the first in-phase harmonic component is increasing and / or the second in-phase harmonic component is decreasing. By monitoring at least one of the first in-phase harmonic component and the second in-phase harmonic component, it is possible to determine the presence of a ground fault in the energy storage system 20. It is expected that the first in-phase harmonic component at the AC side 4 will experience an increase if there is a ground fault present at the energy storage system 20. Further, it is expected that the second in-phase harmonic component at the DC side 2 will experience a decrease if there is a ground fault present at the energy storage system 20. It is sufficient to observe only one of the first in-phase harmonic component and the second in-phase harmonic component in order to detect the presence of a ground fault in the energy storage system 20. However, by using both the first in-phase harmonic component and the second in-phase harmonic component, a more accurate way of determining the presence of a ground fault is provided. It is expected that if the first in-phase harmonic component is increasing, the second in-phase harmonic component should decrease. If this is not the case, further troubleshooting can be necessary.

[0052] This method 100 of detecting a ground fault in the energy storage system 20 is particularly advantageous for ground faults in the vicinity of the electrical midpoint of the string 21, as it is challenging to detect these ground faults with conventional methods for detecting ground faults. The electrical midpoint can be seen as a neutral point, where the electrical potential with respect to ground or a common reference is zero. This point can exist inside one of the energy storage modules 22 or between two energy storage modules 22, which are at approximately the same electrical distance from each of the first DC pole 12 and the second DC pole 14. With the provided method 100, it is possible to detect ground faults in the vicinity of one of the first DC pole 12 and the second DC pole 14 and also ground faults at or close to the electrical midpoint.

[0053] The first in-phase harmonic component is based on the at least one measured phase-to-ground voltage 112A-112C. The method 100 typically measures the phase-to-ground voltage 112A-112C of all phases A-C of the AC side 4 to determine the first in-phase harmonic component.

[0054] The second in-phase harmonic component is based on the at least one pole-to-ground voltage 122A, 122B measured at the DC side 2. Typically, both the pole-to-ground voltage 122A of the first DC pole 12 and the pole-to-ground voltage 122B of the second DC pole 14 are measured and used to determine the second in-phase harmonic component.

[0055] Further, it is also possible to detect a ground fault at the AC side 4 based on whether a DC component appears in the measured phase-to-ground voltage 112A-112C.

[0056] The method 100 can detect a ground fault based on whether the first in-phase harmonic component has increased and / or the second in-phase harmonic component has decreased for a predetermined operating time 132. When measuring voltages, sudden elevations of the measured values can occur, which can be referred to as spikes. Spikes are a temporary increase in voltage or current that can occur due to, for example, lightning, switching operations, and electromagnetic interference. When the method 100 directly indicates that there is a ground fault based on the first in-phase harmonic component having increased and / or the second in-phase harmonic component having decreased, there can be occasions where a spike triggered a false alarm. By determining the presence of a ground fault based on the first in-phase harmonic component having increased and / or the second in-phase harmonic component having decreased for a predetermined operating time 132, it is possible to avoid or at least reduce false detections such as caused by spikes.

[0057] The previously described steps allow the method 100 to detect a ground fault in the energy storage system 20. To allow the method 100 to also locate the ground fault, the method 100 can further comprise determining 140 a pole imbalance voltage based on the pole-to-ground voltages measured at the first DC pole 12 and the second DC pole 14 for determining whether the ground fault is located at the electrical midpoint of the string 21 or closer to one of the first DC pole 12 and the second DC pole 14. As an example, if the first DC pole 12 is the positive DC pole and the second DC pole 14 is the negative DC pole, a positive pole imbalance voltage indicates that the ground fault is located closer to the second DC pole 14, while a negative pole imbalance voltage indicates that the ground fault is located closer to the first DC pole 12. Based on the magnitude and polarity of the pole imbalance voltage, the approximate location of the ground fault in the string 21 can be determined. However, if the fault is at the electrical midpoint, there will be no or only a small pole imbalance voltage. Therefore, considering only the pole imbalance voltage will not be sufficient to detect and locate a ground fault at the electrical midpoint. With both the first in-phase harmonic component and the second in-phase harmonic component being used for detecting the ground fault and the pole imbalance voltage being used for locating the ground fault, the presence of a ground fault at the electrical midpoint can be determined. The first in-phase harmonic component and / or the second in-phase harmonic component will indicate whether a ground fault is present in the energy storage system. If the first in-phase harmonic component and / or the second in-phase harmonic component indicate that a ground fault is present in the energy storage system and there is no or only a small pole imbalance voltage, the method 100 can determine that the ground fault is located at or close to the electrical midpoint.

[0058] The first converter 10A and the second converter 10B can be used to generate 150 a first predetermined in-phase harmonic voltage at the AC side 4 and a second predetermined in-phase harmonic voltage at the DC side 2. This generated first predetermined in-phase harmonic voltage and second predetermined in-phase harmonic voltage can be used as a reference for comparing with the determined first in-phase harmonic component (at a later stage) and the determined second in-phase harmonic component (at the same later stage). In other words, the method 100 can further comprise generating 150 a first predetermined in-phase harmonic voltage with the first converter and the second converter and / or generating 150 a second predetermined in-phase harmonic voltage with the first converter and the second converter. The method 100 can further comprise comparing the first predetermined in-phase harmonic voltage with the determined first in-phase harmonic component and / or comparing the second predetermined in-phase harmonic voltage with the determined second in-phase harmonic component.

[0059] The first converter 10A and the second converter 10B generate a fundamental voltage and a DC voltage to exchange active and reactive power between the AC side and the energy storage system 1. Together with the fundamental voltage and the DC voltage, the first converter 10A and the second converter 10B are also controlled to generate a first predetermined in-phase harmonic voltage and a second predetermined in-phase harmonic voltage. The generated first and second predetermined in-phase harmonic voltages propagate together with the fundamental voltage and the DC voltage. They can then be determined (i.e. extracted) as first and second in-phase harmonic components at the AC side 4 and the DC side 2 at a later stage according to the converter generated voltages. By comparing the first predetermined in-phase harmonic voltage with the determined first in-phase harmonic component and / or comparing the second predetermined in-phase harmonic voltage with the determined second in-phase harmonic component, a further way of determining the presence of a ground fault is provided. During normal operation, it is expected that the first predetermined in-phase harmonic voltage and the first in-phase harmonic component are equal and the second predetermined in-phase harmonic voltage and the second in-phase harmonic component are equal. If the predetermined in-phase harmonic voltages and the in-phase harmonic components are not equal (or at least differ from each other by at least a predetermined threshold), this is an indication of a ground fault at the AC side or the DC side, as the ground fault causes a change in the distribution of the generated in-phase harmonic voltages and thus a change in the distribution of the determined in-phase harmonic components. The in-phase harmonic voltages are the voltages generated by the first converter 10A and the second converter 10B. The in-phase harmonic components refer to triple-n harmonics of the fundamental frequency (where n is 1, 2, 3, etc.) and it has the same fundamental as the fundamental frequency. As an example, if the fundamental frequency is 50 Hz and the triple harmonic is 150 Hz in a 3-phase system, a 150 Hz component will be generated in all three phases and they will be aligned in phase with each other (i.e. due to the in-phase component).

[0060] The in-phase harmonic voltages are generated at a low amplitude for keeping the active and reactive power at a minimum.

[0061] When a ground fault is detected, it can be advantageous to trip / disconnect the energy storage system 20 so that the energy storage system 20 is disconnected from the AC side 4. In other words, the method 100 can further comprise tripping 190 the energy storage system 20 if a ground fault is detected.

[0062] As from Figure 2 It will be appreciated that the dashed boxes represent optional steps that can be implemented in the method 100 to provide further functionality.

[0063] In Figure 3In detail, a detailed example of how the method 100 can operate is provided. Starting from the top of the figure, a measurement of the phase-to-ground voltages 112A-112C (one for each phase A-C of the AC side 2) is provided. The phase-to-ground voltages 112A-112C are used to determine a residual voltage 114 of the AC side. The residual voltage 114 can be calculated as the sum of all phase-to-ground voltages 112A-112C at the AC side 4. Thereafter, the residual voltage 114 of the phase-to-ground voltages 112A-112C (i.e. the sum of all three phase-to-ground voltages) can be used to determine 110 a first in-phase harmonic component.

[0064] A second in-phase harmonic component can be determined 120 as the sum 124 of a pole-to-ground voltage 122A measured at the first DC pole 12 and a pole-to-ground voltage 122B measured at the second DC pole 14.

[0065] Figure 3 The method 100 in further comprises comparing the first in-phase harmonic component and the second in-phase harmonic component. This comparison can be used to obtain 160 a control signal. In one embodiment, the comparison means that the control signal can be obtained by subtracting the second in-phase harmonic component from the first in-phase harmonic component.

[0066] In order to improve the control signal, the first in-phase harmonic component can be influenced by a factor 1 / k 116 before the control signal is obtained. The factor 1 / k 116 can be used to scale the first in-phase harmonic component so that it can easily be compared with the second in-phase harmonic component. The factor 1 / k 116 can be the DC side ground capacitances 42, 43 divided by the AC side filter capacitance 40. The control signal can be obtained from a first root mean square, RMS, value 118 of the first in-phase harmonic component and a second RMS value 128 of the second in-phase harmonic component.

[0067] With the design described in Figure 3 With the design described in, the first in-phase harmonic component and the second in-phase harmonic component are expected to have the same value when compared, thereby resulting in a control signal that is zero or close to zero during normal operation. In this design, when the relationship between the first in-phase harmonic component and the second in-phase harmonic component changes, a ground fault is detected. This allows the operator to study the change in the control signal to detect a ground fault. The control signal can be compared to a threshold value 132 to detect if a ground fault is present. The possibility of using a threshold value 132 allows the operator to set the threshold value with a safety margin so that false alarms are avoided. Further, the threshold value 132 can also be correlated to the in-phase harmonic components generated by the converter 10A, 10B. As understood from the above description, the method 100 can determine the first in-phase harmonic component and the second in-phase harmonic component and detect a ground fault based on whether the first in-phase harmonic component is increasing and / or the second in-phase harmonic component is decreasing.

[0068] In general, the first converter 10A and the second converter 10B are used to generate a first predetermined in-phase harmonic voltage and a second predetermined in-phase harmonic voltage. In this context, a predetermined value or a predetermined voltage is a value (or voltage) of an in-phase harmonic voltage in a desired form, as for example set by an operator. The determined in-phase harmonic component can be compared to the generated predetermined in-phase harmonic voltage to detect if anything has affected the generated predetermined in-phase harmonic voltage when propagating. If something has affected the generated predetermined in-phase harmonic voltage, the in-phase harmonic component will deviate from the generated predetermined in-phase harmonic voltage, thereby indicating that there is a ground fault. If the generated predetermined first in-phase harmonic voltage and the second in-phase harmonic voltage are equal, the first in-phase harmonic component and the second in-phase harmonic component will also be equal in the absence of a ground fault. During normal operation, this comparison will output a value close to zero, and if a ground fault occurs, the value will increase or decrease. Thereby, a ground fault can be detected.

[0069] While the disclosure is susceptible to various modifications and alternative forms, specific examples are shown by way of example in the drawings and are described in detail herein. However, it should be understood that the detailed description and specific examples are not intended to limit the disclosure to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the claims including all possible combinations of the various elements being described herein.

Claims

1. A method (100) for detecting a ground fault in an energy storage system (20), the energy storage system comprising at least one string (21) of a plurality of energy storage modules (22), wherein, The at least one string is connected between a first direct current, DC, pole (12) and a second DC pole (14) of a DC side (2), and wherein the first DC pole is connected to an AC side (4) by a first converter (10A) and the second DC pole is connected to the AC side by a second converter (10B), the method comprising: determining (110) a first in-phase harmonic component based on at least one phase-to-ground voltage (112A-112C) measured at the AC side, determining (120) a second in-phase harmonic component based on at least one pole-to-ground voltage (122A, 122B) measured at the DC side, and determining (130) a presence of a ground fault if the first in-phase harmonic component is increasing and / or the second in-phase harmonic component is decreasing.

2. The method of claim 1, further comprising: determining (140) a pole imbalance voltage based on voltages measured at the first DC pole and the second DC pole for determining if the ground fault is located at an electrical midpoint of the string or close to a DC pole.

3. The method according to any of the preceding claims, wherein, determining a ground fault if the first in-phase harmonic component is increasing and / or the second in-phase harmonic component is decreasing for a predetermined operating time (132).

4. The method of any of the preceding claims, further comprising: generating (150) a first predetermined in-phase harmonic voltage with the first converter and the second converter and / or generating (150) a second predetermined in-phase harmonic voltage with the first converter and the second converter, and comparing the first predetermined in-phase harmonic voltage to the determined first in-phase harmonic component and / or comparing the second predetermined in-phase harmonic voltage to the determined second in-phase harmonic component.

5. The method of any of the preceding claims, further comprising: comparing the first in-phase harmonic component and the second in-phase harmonic component.

6. The method of claim 5, further comprising: obtaining (160) a control signal based on the first in-phase harmonic component and the second in-phase harmonic component, wherein the comparing comprises comparing the control signal to a threshold value (172).

7. The method of claim 6, wherein, The control signal is obtained from a first root mean square, RMS, value (118) of the first in-phase harmonic component and a second RMS value (128) of the second in-phase harmonic component.

8. The method of claim 6 or 7, wherein, The control signal is obtained by subtracting the second in-phase harmonic component from the first in-phase harmonic component.

9. The method of any of the preceding claims, wherein, The first in-phase harmonic component is determined from a residual voltage (114) of the AC side.

10. The method of any of the preceding claims, wherein, The second in-phase harmonic component is determined as a sum (124) of a pole-to-ground voltage measured at the first DC pole and a pole-to-ground voltage measured at the second DC pole.

11. The method of any one of claims 6 to 10, wherein, The first in-phase harmonic component is multiplied by a factor 1 / k (116) before obtaining the control signal.

12. The method of claim 11, wherein, k is a ratio of a DC side ground capacitance to an AC side filter capacitance.

13. The method of any of the preceding claims, further comprising: tripping (190) the energy storage system if a ground fault is detected.

14. A control unit (30) for an energy storage system (20), the energy storage system comprising at least one string (21) of a plurality of energy storage modules (22), wherein, The at least one string is connected between a first direct current, DC, pole (12) and a second DC pole (14) of a DC side (2), and wherein the first DC pole is connected to an AC side (4) by a first converter (10A) and the second DC pole is connected to the AC side by a second converter (10B), the control unit is configured to: determining a first common-mode harmonic component based on at least one phase-to-ground voltage measured at the AC side, determining a second common-mode harmonic component based on at least one pole-to-ground voltage measured at the DC side, determining the presence of a ground fault if the first common-mode harmonic component increases and the second common-mode harmonic component decreases.

15. An energy storage system (20) comprising: at least one string (21) of a plurality of energy storage modules (22), wherein the at least one string is connected between a first direct current, DC, pole (12) and a second DC pole (14) providing a DC side (2), and wherein the first DC pole is connected to an AC side (4) through a first converter (10A) and the second DC pole is connected to the AC side through a second converter (10B), and a control unit according to claim 14.

16. The energy storage system of claim 15, further comprising: a first capacitor ground (42) for the first DC pole and a second capacitor ground (43) for the second DC pole, and a resistor-capacitor, RC, filter (40) for each phase at the AC side.

17. The energy storage system according to claim 15 or 16, further comprising voltage measuring means measuring a respective phase-to-ground voltage for each phase at the converter AC side and a respective pole-to-ground voltage for each of the first DC pole and the second DC pole.

18. The energy storage system of any one of claims 15-17, wherein, the AC side and the DC side are ungrounded or high impedance grounded.

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