An isolated terra bipolar DC system

Abstract

An isolated terra bipolar DC system is provided comprising a positive conductor, a negative conductor, and a midpoint conductor, wherein the DC system is designed to be symmetric around protective earth at the midpoint conductor, wherein the DC system comprises a predetermined impedance (120) arranged between the midpoint conductor and protective earth, a current measurement circuit (130-134) configured to determine a magnitude of a current flowing through the predetermined impedance, and a fault detection circuit (140) configured to compare the magnitude of the current against a threshold, to compensate for the current using a current source (160), and to trigger a fault action if the magnitude of the current exceeds the threshold By measuring the current, and thereby fault current transients, instead of DC voltage transients, faster detection of faults is obtained. By way of current compensation and the predetermined impedance, post-fault currents may be limited to acceptable levels and thereby avert destructive consequences.

Description

[0001]AN ISOLATED TERRA BIPOLAR DC SYSTEMTECHNICAL FIELD The invention relates to a DC system and to a method for use with the DCsystem, wherein the DC system is an isolated terra bipolar DC system comprising a positive conductor, a negative conductor, and a midpoint conductor, wherein the DCsystem is designed to be symmetric around protective earth at the midpoint conductor.BACKGROUND In the realms of sustainable and durable energy, power conversion and transportation circuits are increasingly delivering higher power levels. Consequently,the complexity of these systems and the number of interconnected ports is also on therise. These increases may necessitate the implementation of robust protectionmechanisms to ensure safe circuit operation. To address the challenge of higher power levels, system voltages areincreased to keep current levels manageable. This practice is prevalent in variousapplications, including electric vehicle (EV) DC systems, EV supply equipment (EVSE or chargers), stationary durable energy resources (DERs) like batteries, solar panels,and electrolysers, and in local micro-grid systems that interconnect AC grids, EVSEsand DERs together. A critical requirement across these applications is the necessity for the DC section to be an Isolated Terra (IT) circuit. This configuration is essential for human protection and permits the occurrence of a single fault without destructive outcomes. Despite the DC system facilitating easier interconnection and power transfer amongassets (e.g., batteries, solar panels, motor drives, DC grids), DC systems presentsignificant protection challenges. One potential solution for increasing the DC system'svoltage is the adoption of a symmetric, bipolar configuration. There exist current industry standards for Insulation Monitoring Devices(IMDs) in IT circuits, such as IEC 61557-8 and, in the field of conductive EV charging,IEC 61851 or UL equivalent UL2202 and UL2231. These standards reveal the limitations of IMDs in Megawatt Charging Standard (MCS) applications. Furtherelaboration on this is provided in, for example, the 'CharIN Whitepaper on MegawattCharging System' (https: / / www.charin.global / media / pages / technology / knowledge-base / c708ba3361-1670238823 / whitepaper_megawatt_charging_system_1.0.pdf), fromwhich the following conclusions may be drawn:- An IMD can only detect the presence of a fault in the IT-circuit butcannot pinpoint its location. -The normally continued operation of an IT-circuit after occurrence of thefirst fault is not allowed, nor used in conductive charging nor in PV installations. The combined requirement in the relevant standards of an IT-circuit (galvanic isolation mandatory) of stopping it after a first fault (IMD) makes the system expensive and more complex without the advantages of continued operation. -The periodic measure-process-act principle of IMD detection introducesa significant delay between fault occurrence and system safety. -A symmetrically forced DC system minimizes potential discharge currentfrom the Y-capacitance between the DC+ or DC- pole and PE, thereby reducing thesystem's hazard level. -An IMD's capability to measure total Y-capacitance in an IT-circuit islimited, and this measurement does not provide any evaluation parameters for isolation properties of the system. -The intended touch-safe protection is not sustainable for increased DCsystem voltages or power levels, or their combinations beyond the MCS operationalrange. This also applies to symmetric DC circuits.The complexity of high-power systems is partly due to their physical size,affecting components such as EV batteries, (traction) machines, charger converter isolation, and cabling in DER or infrastructure systems. These components inadvertently introduce stray capacitance. Furthermore, each product and the system collectively must adhere to EMC standard limits to ensure interoperable use, necessitating additional Y-capacitance that exacerbates the issue. Examples of relevant IT-circuit-based systems include bipolar and unipolar DC systems (such as HVDC infrastructure or LVDC equivalent and chargers or PV installations, respectively), and split-phase electric power AC systems (like the 240 V AC system with two 120 V AC phases in anti-phase, commonly used in the USA). It would be desirable to obtain a protection system which can facilitate safeoperation of an IT circuit with a symmetric bipolar DC section, for example by being able to monitor the safe operation and to trigger a fault action upon detecting a fault. SUMMARY In accordance with a first aspect of the invention, an isolated terra bipolarDC system is provided comprising:- a positive conductor, a negative conductor, and a midpoint conductor,wherein the DC system has a first system impedance between the positive conductor and protective earth and a second system impedance between the negative conductorand the protective earth, wherein the DC system is designed to be symmetric aroundprotective earth at the midpoint conductor;- a predetermined impedance arranged between the midpoint conductorand the protective earth; -a current measurement circuit configured to determine a magnitude of acurrent flowing through at least a resistive part of the predetermined impedance; -a current source arranged in a current path of the predeterminedimpedance; -a fault detection circuit configured to control the current source to reducethe current flowing through the predetermined impedance, to compare the magnitude of the current against a threshold, and to trigger a fault action if the magnitude of the current exceeds the threshold. In accordance with a further aspect of the invention, a protection method isprovided for a DC system, wherein the DC system is an isolated terra bipolar DCsystem comprising a positive conductor, a negative conductor, and a midpoint conductor, wherein the DC system is designed to be symmetric around protective earth at the midpoint conductor, wherein the protection method comprises: -arranging a predetermined impedance between the midpoint conductorand protective earth; -arranging a current source in a current path of the predeterminedimpedance; and during operation of the DC system: -determining a current flowing through at least a resistive part of thepredetermined impedance; -determining a magnitude of the current;- comparing the magnitude of the current against a threshold;- controlling the current source to reduce the current flowing through thepredetermined impedance, and triggering a fault action if the magnitude of the current exceeds the threshold. The above measures involve providing a predetermined impedance, i.e., animpedance with a predetermined value, between the DC system’s midpoint andProtective Earth (PE) and monitoring the magnitude, and optionally also the direction,of the current flowing through the predetermined impedance. Thereby, the IT circuitmay be effectively converted a ZM-S earthing system analogous to IEC terminology(IEC 60364) as also explained elsewhere in this specification with reference to Fig. 5.The DC system may be provided with components which together mayrepresent a protection system. Exemplary components of the protection system mayinclude, but are not limited, one or more of: a predetermined impedance, a faultdetection circuit, a current source, etc., as described elsewhere in this specification. References to the protection system, including its properties and advantages,represent references to the relevant components within the protection system.The DC system may be designed to be symmetric around protective earth by regulating the potential of the midpoint conductor to match the potential of protective earth. This regulation or control may ensure that the midpoint conductor serves as a stable reference point to maintain equal and opposite voltages for the positive and negative conductors relative to protective earth and thereby maintain the symmetry ofthe DC system. The operation of the protection system may be explained as follows.When a balanced DC section (this DC section also being referred to as DC systemelsewhere) of a circuit experiences a fault condition to protective earth (PE), thistypically causes an imbalance in the operation of the DC section, which in turn causesa current to flow through the impedance. This current flow may alter the referencepotential of the midpoint conductor relative to PE, disrupting the symmetry of the system. Specifically, the voltages of the positive and negative conductors relative to PE may no longer be equal in magnitude and opposite in polarity. By measuring the current, such imbalances may be detected. In particular, by measuring the magnitudeof the current and comparing it against a threshold, the severity of the fault may bedetermined. This may enable the protection system to trigger fault actions, such as anexternal alert or triggering a circuit breaker. In particular, the current through a resistivepart of the impedance, e.g., a resistance, may be measured. Advantageously, bymeasuring the current, and thereby fault current transients instead of DC voltagetransients, the protection system may be capable of faster detection than prior artIMDs, as small fault currents, that may be accurately measured by an appropriatecurrent measurement circuit, such as a current transducer, typically only cause a slowrate of voltage change over the impedance’s capacitance and thus require significantlymore time to reliably measure. Furthermore, the protection system may allow the DCsystem to continue operating after a single fault. Namely, by being able to quickly andaccurately measure small fault currents, the protection system may initially only triggeran alert, e.g., when the fault current exceeds a first threshold, and only proceed totrigger a circuit breaker or in another manner stop operations if the fault current exceeds a second, higher, threshold. This level of discrimination is typically not offeredby the aforementioned prior art IMDs as these are incapable of determining where inthe IT circuit the fault is caused. Therefore, a single fault must already lead to a full stop of the system. Additionally, slow reaction time of an IMD may necessitate stopping operation out of precaution. Moreover, the chosen value of the predetermined impedance may limit post-fault current to acceptable levels and thereby avert destructive consequences. In accordance with the above measures, the protection system may further comprise a current source arranged in a current path of the predetermined impedance, wherein the fault detection circuit is configured to control the current source to reduce the current flowing through the predetermined impedance. This way, current flowingthrough the predetermined impedance, for example due to unintended common mode(CM) stray current during normal operation or a fault current in case of a failure (e.g.,full short or reduced IT-PE impedance below a defined threshold), may be compensated. In some examples, the fault detection circuit may be configured to control the current source to regularize the current to remain below a threshold. The compensation of unintended CM stray current may be especially relevant for, e.g., marine applications, where such currents can lead to corrosion in metal parts of ships or underwater structures. In these environments, stray currents can accelerate galvanic corrosion. This can compromise the structural integrity of hulls, propellers, and other metal components. Additionally, in sensitive marine equipment like sonar, navigation systems, and communication devices, CM stray currents can cause electromagnetic interference, leading to malfunctions or inaccurate readings. This interference is particularly problematic in precision instruments, where even slight deviations can lead to significant errors. By being able to compensate for such CM stray currents, at least to a certain degree, these negative effects may be mitigated. In an embodiment, the current measurement circuit is further configured todetermine a direction of the current, and the fault detection circuit is configured to, on the basis of the direction, determine a fault location of an isolation fault with respect tothe protective earth. For example, for DC faults, the fault location may be indicative ofwhether the isolation fault is between the positive conductor and the protective earth orbetween the negative conductor and the protective earth. Namely, by determining thedirection of the fault current, the type of imbalance, and thereby the location of the fault,being either in the VDC+ or VDC- section of the DC system, may be determined. It isnoted that AC faults may similarly cause an imbalance in the operation of the DCsystem, and based the direction of the current, it may be concluded where such an ACfault is located. It is further noted that in order to distinguish between AC and DC faults,the protection system may detect AC currents flowing through the impedance, and inparticular flowing through a capacitive part of the impedance, as such AC currents maybe indicative of a fault being an AC fault. Thereby, the protection system enables fault detection, which may allow a fault to be faster detected and resolved. In an embodiment, the fault action comprises one or more of: -triggering a circuit breaker;- sending an alert; and- activating an auxiliary protection system.In an embodiment, the impedance comprises or consists of a parallelarrangement of a predetermined resistance and a predetermined capacitance. Thecurrent measurement circuit may measure at least the current through thepredetermined resistance to detect DC and AC faults and the current level (which mayalso be referred to as an ‘absolute limiting current level’ due to the resistance imposingan absolute limit on this current). The capacitance may steer high frequency currentscreated by electromagnetic interference away from the resistance and may act as afilter to avoid or reduce the chance of the fault detection circuit erroneously triggering.In some embodiments, also the current through the predetermined capacitance may be measured, or otherwise computed, to detect AC currents that exhibit transient behaviour and which may be the result of stray effects and / or current mode effects. The fault detection circuit may detect and analyse AC currents through the capacitanceto determine that a detected fault is an AC fault at an AC side of the IT circuit.In an embodiment the current measurement circuit comprise at least two of:a current transducer arranged in a current path of the predetermined resistance, a current transducer arranged in a current path of the predetermined capacitance, and acurrent transducer arranged in a common current path of the predetermined resistanceand the predetermined capacitance. Current transducers have been found to be wellsuited to quickly and accurately measure fault currents. Since the current in thecommon current path corresponds to the sum of the current through the predeterminedresistance and the current through the predetermined capacitance, it is possible todetermine, using at least two of said current transducers, the third current by eitheraddition or subtraction of the other two currents. Advantageously, two currenttransducers may suffice to be able to determine all three currents. In an embodiment, the fault detection circuit is configured to determine a resistance Rfaultof an isolation fault between on the one hand the positive conductor or the negative conductor and on the other hand protective earth.In an embodiment, the fault detection circuit is configured to determine aresistance Rfault of an isolation fault between on the one hand the positive conductor orthe negative conductor and on the other hand protective earth by solving for Rfault in theformula iR(M-PE) = VDC / (Rfault + RM-PE), where iR(M-PE) is a value of a measured currentthrough the predetermined resistance, R(M-PE) is a value of the predeterminedresistance, and VDC is a DC voltage at the respective positive or negative conductor.In an embodiment, the DC system further comprises a control circuitconfigured to, in response to the fault action being triggered, adjust a ratio between the positive DC voltage and the negative DC voltage to reduce the current flowing through the predetermined impedance. In an embodiment, the one or more power converters comprise an odd-levelmulti-level DC converter configured to independently control the positive DC voltage and the negative DC voltage with respect to the midpoint conductor, and the control circuit is configured to adjust the ratio between the positive DC voltage and the negative DC voltage by controlling the odd-level multi-level DC converter. In an embodiment, the DC system further comprises an external ground current compensating source, wherein the control circuit is further configured to control the external ground current compensating source to reduce the current flowing through the predetermined impedance. In an embodiment, the predetermined resistance is chosen from a range of 10-1000 Ω. In an embodiment, the predetermined capacitance is chosen from a range of 5-50 μF. In an embodiment, the DC system is designed to comprise a system resistance between i) each one of the positive conductor and the negative conductor and ii) protective earth, wherein the system resistance is within a range of 0.1-20 MΩ.The DC system may be configured with a specific system resistance between each ofthe positive and negative conductors and the protective earth, with this system resistance being within the range of 0.1 to 20 MΩ. These resistances, also referred to as the first and second system impedances, may correspond to the impedances between the positive and negative conductors, e.g., the DC lines, and protective earth.These system impedances may not be solely incidental but may be in part intentionallyprovided, for example to meet electromagnetic compatibility (EMC) requirements. Additionally, the DC lines may exhibit parasitic resistances and capacitances relative to the protective earth, contributing further to these system impedances. In an embodiment, the DC system is designed to comprise a system capacitance between i) each one of the positive conductor and the negative conductor and ii) protective earth, wherein the system capacitance is within a range of 30-2000 μF, preferably 100-2000 μF. In an embodiment, the DC system is or comprises a DC charger, for example an EV DC charger. In an embodiment, the DC system is configured to operate as a symmetricalDC system at a maximum DC voltage at or above 920 V.In an embodiment, the DC system comprises one or more power converters configured to convert AC power to DC power, wherein the one or more power converters establish a positive DC voltage between the positive conductor and the midpoint conductor and a negative DC voltage between the midpoint conductor and the negative conductor. In an embodiment, the DC system comprises multiple instances of theprotection system. The DC system may thus comprise several protection systemswhich may together provide the protection functionality in a decentralized manner. It will be appreciated by those skilled in the art that two or more of the above-mentioned embodiments, implementations, and / or aspects of the invention may be combined in any way deemed useful. Modifications and variations of any one of the systems or methods, which correspond to the described modifications and variations of another one of these systems or methods, may be carried out by a person skilled in the art on the basis of the present description. BRIEF DESCRIPTION OF THE DRAWINGS These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings, Fig.1 shows an IT circuit as a composition of AC and / or DC components;Fig.2 shows a basic example of an IT circuit in form of EVSE-EVcombination with a symmetric DC section with VDC+ = VDC-;Fig. 3A shows a basic ZM circuit diagram indicating the equivalent overallimpedance with respect to PE of an IT circuit, ZIT-PE, and the added impedance, ZM-PE,by which the circuit arrangement is changed to an ZM-S earthing system; Fig. 3B shows AC-DC conversion via a multilevel power converter topology,where a symmetrical midpoint (M) is controlled to correct voltage imbalances; Fig. 3C shows a midpoint created by two power converters in series;Fig. 4A shows sensors and components details of the ZM-S earthingsystem; Fig. 4B is similar to Fig. 4A, additionally showing a control circuit configuredto control the symmetrical midpoint to correct voltage imbalances;Fig. 5 shows the ZM-S earthing system with a compensation current source;Fig. 6 shows a ZM equivalent to the IT circuit in Fig.2;Fig. 7 shows examples of fault locations that may be detected by the faultdetection circuit, wherein Rfault1 to Rfault3 are the values of the resistance at therespective fault locations; Fig. 8 show an example of an insulation fault between a positive pole andprotective earth, showing a main fault current path and a detection fault current path;Fig.9 illustrates the circuit used for the calculation of the fault resistance; Fig.10 shows additional fault locations which may be detected by the faultdetection circuit; Fig. 11 shows a protection method for a DC system; andFig. 12 shows a non-transitory computer-readable medium comprising data.Reference signs list The following list of references and abbreviations is provided for facilitatingthe interpretation of the drawings and shall not be construed as limiting the claims orclauses. 100, 106, 109 isolated terra (IT) circuit101 - 105 DC section / system107, 108 DC section / system110, 112 protection system115 control circuit120 predetermined impedance122 predetermined resistance124 predetermined capacitance130-134 current transducer140 fault detection circuit150 detection main current path152 detection fault current path160 current source181 first system impedance182 second system impedance200 protection method for a symmetric, isolated terra bipolar DCsystem 210 providing predetermined impedance220 operation of the DC system230 determining current flowing through predetermined impedance240 determining magnitude of current250 comparing magnitude of current against threshold260 triggering fault action300 non-transitory computer-readable medium310 dataDESCRIPTION OF EMBODIMENTS Fig. 1 shows a generic isolated terra (IT) circuit 100. The IT circuit maycomprise converters that have a defined minimum resultant impedance ZIT-PE with respect to protective earth (PE) and converters that may or may not be isolated andtherefore their respective outputs may be included (non-isolated) or excluded (isolated)from the IT circuit. The overall IT circuit has a defined minimum resultant impedance(ZIT-PE) with respect to PE. This circuit, which may be considered to ‘float’, mayrepresent anything from an AA size battery connected to an LED up to a DC grid with numerous resources and loads connected to multiple AC grid points using galvanically isolated AC / / DC converters. By way of example, Fig.1 shows the power conversionfrom AC to DC as a two-stage power conversion, namely as a first stage from AC toDC and as a second stage from DC to DC. However, the AC to DC power conversion may also be a one-stage power conversion. The AC network(s) connected to the ITcircuit may be single phase or multi-phase. Typically, the electricity grid is a threephase network. The overall impedance between an IT circuit and PE, which may bereferred to as ZIT-PE, is normally of a high value, (ideally infinite) and may be consideredas the Thevenin equivalent impedance value of all intended and unintended RLCcomponents in the IT circuit with respect to PE, or as the result of measuring theimpedance while shortening all AC phases together and the DC phases together. Thisoverall impedance may elsewhere also be referred to as ‘system impedance’ and isshown to be arranged between the positive conductor and PE and between the negative conductor and PE (cf.181, 182 in Fig.3A). In practice, ZIT-PE may be approximated by neglecting the differential-mode (DM) phase-to-phase impedances,since the current-mode (CM) impedance may be dominant. The insulation level of theIT circuit, and thereby the impedance value of ZIT-PE, must typically be kept above aminimum value, which may be referred to as Zmin. It is desirable to provide a protection system for such an IT circuit. A primary function of such a protection system is totrigger a fault in case ZIT-PE<Zmin. Such triggering of a fault may involve triggering a faultaction, such as triggering a circuit breaker, sending an alert, such as an external alertto alert an operator or an external system, and / or activating an auxiliary protectionsystem. Fig. 2 shows a basic example of an IT circuit used for EV charging. The ITcircuit comprises, from left to right, the EVSE (charger) which is represented by animpedance ZIT-PE(EVSE), wiring in the field which is represented by an impedance ZS, andthe EV (electric vehicle or electric vessel with battery to be charged) which isrepresented as an impedance ZIT-PE(EV) and a battery with Vbat. The IT circuit comprisesa DC section 101 (which is an example of a DC system as referenced elsewhere)which includes the DC part of the EVSE, the wiring in the field and the EV. The DCsection may establish DC symmetry if VDC+ = VDC- = Vbat / 2. Such symmetry is in manyapplications mandatory or highly favoured for the DC section of an IT circuit. Thecombined impedances ZIT-PE(EVSE) / / ZS / / ZIT-PE(EV) = ZIT-PE, as defined in Fig.1(neglecting ZDM). The impedance ZIT-PErepresents the connection between the IT circuit and PE, and may be distributed along multiple points in the circuit. It is typically arequirement for an IT circuit that the impedance ZIT-PE is high, typically > 1 MΩ.It would be desirable to be able to monitor the symmetry of the DC section,insulation of the DC section with respect to PE, and to detect and limit fault currentstowards PE. For that purpose, a protection system may be provided which may beconfigured for use with a symmetric DC section of an IT circuit as shown in Fig.2.Fig. 3A illustrates one aspect of the protection system. Namely, to detectand limit faults towards PE, a defined impedance, ZM-PE may be added in between themiddle-point (M) of the DC section 102 of the IT circuit, or in general the middle-point ofan isolated DC system, and the PE. This way, the DC section may effectively beconverted into a ZM-S circuit, as also explained elsewhere in this specification. As aresult of an insulation fault towards PE, a current iZ(M-PE) may flow through theimpedance ZM-PE. This fault current iZ(M-PE) may be limited to a defined safe level,determined by the value of impedance ZM-PE. The fault current iZ(M-PE) may bemeasured, and the measured value of the fault current iZ(M-PE) may be analysed by afault detection circuit (not shown explicitly in Fig.3) and used for protection purposes, e.g., to trigger fault actions. Inequalities between the positive-half voltage of theconverter, VDC+, and the negative half voltage of the converter, VDC-, may be detectedas this condition also causes a current circulation through ZM-PE. The polarity of thecurrent iZ(M-PE) may determine whether the fault is in the positive or negative pole.Fig. 3B is similar to Fig.3A but shows the AC-DC conversion to beachieved via a multilevel power converter topology and the symmetrical midpoint (M) tobe actively controlled to correct voltage imbalances. In general, the compensation of such imbalances may be realized through a current source in the current path of thedefined impedance ZM-PE, as elucidated with reference in Fig. 5. Additionally, or as analternative to using a current source, the power converters may be controlled by acontrol circuit (not explicitly shown in Fig.3B) to adjust the ratio between the positiveDC voltage and the negative DC voltage in order to reduce the voltage imbalances andthereby reduce the current flowing through the predetermined impedance. Forexample, the control circuit may adjust control signals Ctl+ and Ctl- to independentlymanage the positive DC voltage (VDC+) and the negative DC voltage (VDC-) relative tothe midpoint M to actively compensate for leakage currents to address voltageimbalance. Such active compensation of leakage current may be especially applicablein systems using power converters, including configurations with series-connected power converters or multilevel topologies. In these arrangements, a symmetrical midpoint (M) may be established and maintained through converter configurations, which allows management of common mode voltages and grounding currents. Forexample, as shown in Fig.3C, the midpoint may be created by two series-connectedpower converters, or by a single power converter using a multilevel topology, such as athree-level NPC converter, which allows control over the voltage ratio between Vdc+and Vdc-. Unlike battery-based DC systems, which may lack this control capability, theconfiguration shown in Fig. 3B, 3C and others may facilitate precise voltage balancingbetween the positive and negative conductors with respect to the midpoint, significantlyimproving the control of leakage currents. This control of the power converters may forexample be effected in response to a fault action triggered by the fault detection circuit.The DC system may regulate the overall differential voltage between VDC+and VDC-, and the ratio between VDC+ and VDC-, and may manage leakage current usingone of several control methods, each offering varying degrees of control: 1. Odd-Level Multi-Level DC Converter Control: An odd-level multi-level DC converter, such as a three-level neutral-point-clamped (NPC) converter, may independently control the positive and negative conductor voltages relative to the midpoint. This configuration provides a high control bandwidth, enabling precise voltage adjustments at the midpoint and targeted reduction of leakage current. 2. Even-Level Series-Connected Power Converter with Midpoint Converter: The DC system may utilize a series-connected even-level power converter (e.g., a two- level or battery string balancing converter) to control the total voltage across the positive and negative conductors. A midpoint converter with a higher bandwidth may be combined with this setup, allowing fine-tuning of the midpoint, minimizing current to protective earth (PE), and thereby reducing leakage. 3. Odd-Level Multi-Level DC Converter with External Ground Current Compensation Source: In configurations utilizing an odd-level multi-level DC converter, an external ground current compensating source may be added, with a bandwidth exceeding that of the primary power converters, to quickly mitigate ground leakage currents. This auxiliary control approach further reduces current through the predetermined impedance. Using these methods, the DC system’s midpoint M may be actively managed by power converters that compensate for ground leakage or fault currents, keeping them within safety-defined limits. Fig. 4A shows the protection system 110 (which may also be referred to asa protection circuit) for a DC system 103 in more detail. The protection system 110 maybe composed of a combination of limiting and sensing devices that together may perform the task of monitoring a balanced symmetrical circuit, detecting imbalances,and / or limiting the fault current in case of a failure (e.g., full short or reduced IT-PEimpedance below a defined threshold). In addition, the protection system 110 may acton fault current transients instead of DC voltage transients and therefore may becapable of much faster detection. The protection system is shown to comprise theaforementioned impedance ZM-PE in form of a parallel arrangement of a predeterminedresistance RM-PE 122 and a predetermined capacitance CM-PE 124. To perform themonitoring task, the fault current iR(M-PE) may be measured by a current transducer 132 which may be arranged in a current path of the resistance RM-PE 122. The fault current iR(M-PE) is one of the currents flowing through the impedance ZM-PE. By measuring thefault current iR(M-PE) with the current transducer 132, DC faults and an absolute limitingcurrent level may be measured through the resistance RM-PE 122. In someembodiments, additionally or alternatively, a current transducer 134 may be provided inthe current path of the predetermined capacitance CM-PE 124 to measure the faultcurrent iC(M-PE). By measuring the fault current iC(M-PE) with the current transducer 134,AC currents that exhibit transient behaviour and which may be the result of stray effectsand / or current mode effects may be measured through the capacitor. In someembodiments, for example as shown in Fig.5, additionally or alternatively to themeasurement of the fault current iR(M-PE) and the fault current iC(M-PE), a fault current iZ(M-PE)may be measured using a current transducer 130 in the common current path of thepredetermined resistance RM-PE 122 and the predetermined capacitance CM-PE 124. Bymeasuring the fault current iC(M-PE) with the current transducer 130, it may suffice toadditionally either provide the current transducer 132 or the current transducer 134 to be able to determine all three currents iR(M-PE), iC(M-PE), and iZ(M-PE). It further is noted thatthe fault current iC(M-PE) itself may also be computed by summing iR(M-PE) and iC(M-PE),thereby avoiding the need for the current transducer 130 if the current transducers 132,134 are provided. The determination of the fault current iC(M-PE) may be relevant for thecompensation by a current source as also explained with reference to Fig.5. A fault detection circuit 140 may be provided which may have access to theone or more measured fault currents. The fault detection circuit 140 may be configuredto detect an imbalance by detecting a transient or steady-state excess with respect to a defined level of current, e.g., a threshold. The value of the capacitance across RM-PE,being CM-PE, may determine a range of transient detection for a given application.Fig.4A further shows the voltage sources providing VDC+and VDC-beingcontrolled by input reference values Ctl+ and Ctl-. As elucidated with reference to Fig.3B and 4B, such control of the ratio and overall differential voltage between VDC+ andVDC- may allow voltage imbalances to be corrected to reduce the fault current iZ(M-PE).Fig. 4B shows the output voltages VDC+ and VDC- to be individuallycontrolled in accordance with input reference values Ctl+ and Ctl-, previouslyintroduced in Fig. 4A. Fig.4B shows a control circuit 115 to receive a total DC voltage(e.g., VDC+ + VDC-) as a reference input and to adjust each power converter’s output voltage to maintain the specified total DC voltage while reducing leakage currents. Thiscontrol may be achieved by adjusting the ratio between VDC+ and VDC-, by incorporatinga compensation current source, as further detailed in Fig.5, which provides additional fine-tuning to offset leakage currents beyond the power converters' control, or both.The leakage current compensation may activate if the current iZ(M-PE) between themidpoint M and PE exceeds a set threshold or if the voltage asymmetry between thepositive and negative sides (VT+ and VT-) surpasses an allowable limit, ensuring safeoperational parameters are maintained. With continued reference to Fig. 4B, it is notedthat the control circuit may integrate the fault detection circuit 140, allowing the control circuit to receive current and voltage measurements and act upon such measurementsto address any deviations in voltage symmetry and / or excessive leakage current iZ(M-PE)Fig. 5 shows a DC system 104 which comprises a ZM protection circuit 112with an added compensation current source 160. This current source 160 may becontrolled, e.g., by the fault detection circuit 140 or by the control circuit 115 (notshown), to compensate for unintended current mode (CM) stray current during normaloperation and may compensate and limit the fault current in case of a failure (e.g., fullshort or reduced IT-PE impedance below a defined threshold). As such, the active ZMprotection circuit 112 may be composed of a combination of controlling, limiting andsensing devices that together perform the task of monitoring a balanced symmetricalcircuit, detecting imbalances, compensating imbalances, and / or limiting the faultcurrent. Since the protection circuit 112 acts on fault current transients as well as DCvoltage transients, it may be capable of faster compensation and detection. In thiscontext, the capacitance CM-PE 124 may represent a low impedance for high-frequencysignals, which allows for rapid circulation of high-frequency components, contributing tothe protection system's ability be responsive to transient imbalances. The compensation current source 160 may act based on iZ(M-PE) as measured with current transformer 130 and may regulate iZ(M-PE) to an intended value i*Z(M-PE) by superimposinga current such that iZ(M-PE) == i*Z(M-PE). For example, the regularization may be such thatiZ(M-PE)is equal or below a predetermined threshold. This threshold may, but does not need to be, equal to the fault threshold as referenced elsewhere. With continued reference to the protection system, analogue to the IEC terminology (IEC 60364), the protection system may be considered as a ZM-S earthingsystem. The following definitions may be used to characterize the protection system:o Z: defined impedance (Z) between symmetrical middle-point (M) andearth (PE). oM: symmetrical middle-point (M) with respect to the electrical supplynetwork. In this respect, it is noted that since there is impedance in between M and PE,the term ‘neutral’ cannot be used.o S: the middle-point conductor and PE conductor are separated (S)because there is impedance in between M and PE. Unlike a neutral conductor, the middle-point conductor does not need to be provided throughout the ZM system, asshown in Fig.6 for a DC system 105. Therefore, the S conductor may only exist insideof the product generating M. All converters in the ZM circuit may be connected to the symmetric DCsection in a symmetrical configuration. The symmetrical middle-point (M) may beactively enforced with converter configurations or passively obtained with an impedance divider.Typical value ranges of circuit components as shown in Fig.4 and 5 may beas follows (here, differential mode (DM) impedances and voltage sources VDC may beconsidered ideal): 1. RIT-PE (== RY+IT-PE / / RY-IT-PE): 0.1-20 MΩ (lower may be alloweddepending on application requirements)2. CIT-PE (== CY+IT-PE / / CY-IT-PE): 100-2000 µF (higher may be alloweddepending on application requirements) 3. RM-PE: 10-1000 Ω (lower may be allowed may be limited as determinedby safety standard IEC 60364-4-41) 4. CM-PE: 5-50 µFIt will be appreciated that the above values may be for the overall IT circuit,or at least for the overall DC section of the IT circuit. ZIT-PE may be distributed over thewhole IT circuit and all components within (e.g., deliberate components ZYand stray components ZS). If the protections are distributed over multiple converters, thecombination of the impedances may determine ZIT-PE as defined elsewhere in thisspecification. In the above, the converter sources may be assumed to be ideal and themidpoint (M) to lie exactly in between DC+ and DC-. The protection system may be generally applicable to detection and limitation of fault currents and with that the protection of circuit and users. The following nevertheless provides guidance for applicability in certain domains and applications. Since the protection system may monitor the level of symmetry between 2 DC voltages, it is particularly suitable for systems in which a balanced 3-level DC voltage is used. An exemplary application of the protection system may be one where one ormore of the following is exceeded: 1. Voltage in a balanced 3-level DC voltage systemsa. In DC charging applications, this may correspond to a voltageabove HPC (> 920 V DC, IEC 61851 part 23)i. Within MCS, this may be the case for forced symmetrycircuits (mandatory for VDC> 920 V DC and allowed below that value) ii. Above the voltages listed in Table 1 below alwaysmandatory. 2. High system level Y-capacitance valuea. In DC charging applications, the Y-capacitance value may belimited per power level as listed in Table 1.3. Fault detection time with other protection means exceeds that definedper standard a. In DC charging applications, detection times exceeding thevalues in Table 1. 4. In the application of charging, this is typically the case for systems of 1MW and above. Table 1 IEC 61851 – limit values per part (time and CY per respective sideEVSE / EV) * According to ISO 6469, @VDCmax. Other EV side safety standards may be leading; however principle remains applicable.Part Max power [kW] tlim [s] VDC max [V] CY-EVSE [µF] CY-EV [µF]23 560 10 920 1 0,47*23-1 1000 10 1000 4 0,4*23-3 3750 60 1000 15 151200 8 81250 5 5Fig. 7 illustrates three examples of fault situations that may be detected,limited and discriminated by the ZM protection circuit. These fault situations may bereferred to as fault 1, 2, and 3, and their fault resistances as Rfault1, Rfault2, and Rfault3:Fault point 1: Rfault1 is the fault resistance value in between DC+ and PE Fault point 2: Rfault2 is the fault resistance value in between DC- and PEFault point 3: Rfault3is the fault resistance value in between any of the AC terminals of non-insulated equipment in the ZM circuit and PE. In general, a possible fault (Rfault1 / 2 / 3) may be limited through RM-PE since thelatter may provide a limiting effect for the fault current caused by the discharge of:Rfault1: In Fig.8, an example is shown of fault 1 with main fault currentpath 150 and detection fault current path 152 through ZM-PE. It can be seen that CY+(IT-PE)may be discharged via Rfault1 and RM-PE based on the ratio between both resistances.VDC+ may be pulled down toward PE level by closing S1 and conduction through Rfault1.As a result, the approximated voltage across RM-PE becomes VDC+ resulting in a currentthrough RM-PEin indicated direction leading to a negative iZ(M-PE). Rfault2: CY-(IT-PE)discharge. Situation equal to fault 1 but with currents inreversed direction and resulting in a positive iZ(M-PE). See also Fig.9 which shows theswitch S2 and CY-(IT-PE) between the negative pole DC- and PE. Rfault3: coupling of the fault to the DC lines via the converter and with that CY+(IT-PE) and / or CY-(IT-PE), respectively. In general, from the polarity of iZ(M-PE), e.g., the sign thereof, it may bedetermined whether fault 1 (negative sign) or fault 2 (positive sign) occurs. Based oniR(M-PE), the ratio between Rfault and RM-PE may be determined, and with that Rfault may bederived (neglecting RY since that will typically be of much higher value). In a faultcondition (e.g., after a transient time of around 3 times the time constant defined by RM-PE times CM-PE), the measured current iR(M-PE) may be given by (here, x may refer eitherto fault 1, 2, or 3) iR(M-PE) = VDCx / (Rfaultx + RM-PE). As the value of RM-PE may bepredetermined and the voltages VDC+ and VDC- (commonly termed VDCx) may also beknown or measured, the fault resistance value may be determined by Rfaultx = (VDCx / iR(M-PE)) - RM-PE. Or in other words, one may solve for Rfault in the formula iR(M-PE) = VDC / (Rfault+ RM-PE), where VDCis a DC voltage at the respective positive or negative conductor. Fig.10 shows additional fault locations and may be explained as follows.When a fault condition to PE is experienced at the AC side (represented by the Rfault3 inFig. 10 but which may occur with respect to any of the AC phases), this typicallycauses also an imbalance in the operation of the DC section, as the AC / DC powerconverter has no isolation and its internal active or passive switches (semiconductorspower components like IGBTs or diodes), due to their conduction action, connectalternately the DC side (DC+ or DC- terminals) to the AC side, which in turn causes acurrent to flow through the impedance ZM-PE 120. By measuring the current, such AC fault currents through Rfault3 may be detected in a similar way as explained for the DC side. Namely, by measuring the magnitude of the current iZ(M-PE) and comparing itagainst a threshold, the severity of the fault may be determined. In particular, in case ofan PE fault at the AC side, the fault current iZ(M-PE) may contain AC components thatmay be detected by the fault protection circuit of the protection system. If such ACcomponents are detected, the fault protection circuit may conclude that the current iZ(M-PE) is at least in part due to a fault condition to PE at the AC side. By way of example, Fig.10 shows the power conversion from AC to DC as a two-stage power conversion, namely as a first stage from AC to DC and as a second stage from DC to DC. In such a two-stage power conversion, there may be fault condition to PE at the DC input side of the second stage, represented by Rfault4 and Rfault5 in Fig.10. Such fault conditions also typically cause an imbalance in the DC output side which again may cause a current to flow through the impedance ZM-PE 120. By measuring the current, such fault currents through Rfault4and Rfault5may be detected in a similar way as previously explained for Rfault1 and Rfault2, namely by measuring the magnitude and direction of the current iZ(M-PE). In general, the above-described protection system may be used as an IT circuit grounding method for DC systems with (high) capacitances to protective earth. In particular, the protection system may be used for monitoring, detecting and limiting of fault current in a symmetric IT circuit. The protection system may further help involtage balancing and may limit ground fault currents to safe values with fast detectionfor protection against safety ground faults on DC and AC sides. The detection may befast, for example typically 100 ms, in comparison to currently used IMD devices (10-60seconds) since iZ(M-PE) is used to detect a fault instead of an indirect RC estimation. Forthe protection, there is no limitation in the allowed maximum IT circuit Y-capacitancevalue. Similarly to an IMD, the protection system may detect a too low RIT-PE valuebased on the magnitude of the fault current. The protection system may further detectand limit permanent values of short-circuit of DC+ or DC- to PE, detect voltageimbalance in DC sections, and limit value of short-circuit current to predetermined values. The protection system may represent a passive method, free of injecting signal, that does not cause any interference or disturbance in the monitored DC system. The protection system may be deployed centrally to detect faults everywhere in the IT circuit, not only in the (symmetric) DC part of the system. The protection system may comprise one or more (micro)processors whichexecute appropriate software and which may implement certain functionality of theprotection system, specifically the functionality of the fault detection circuit. Thesoftware may have been downloaded and / or stored in a corresponding memory ormemories, e.g., in volatile memory such as RAM or in non-volatile memory such asFlash. Alternatively, the fault detection circuit may be implemented in the form ofprogrammable logic, e.g., as a Field-Programmable Gate Array (FPGA). Fig. 11 shows a protection method 200 for a symmetric, isolated terrabipolar DC system. The method 200 may comprise providing 210 a predeterminedimpedance between the midpoint conductor and protective earth, and during operation 220 of the DC system, determining 230 a current flowing through at least a resistive part of the predetermined impedance, determining 240 a magnitude of the current, comparing 250 the magnitude of the current against a threshold, and triggering 260 afault action if the magnitude of the current exceeds the threshold. For example, thethreshold may be chosen for human protection, for example as defined in IEC 60479-1Part 1 with reference to the effects of current on human beings and livestock. Another example is that the threshold may be chosen in accordance with automotive industrystandards, for example as 10 mA as defined by SAE J1766 for continuous monitoring.Select steps of methods described in this specification, for example in anyof the claims or clauses, may be implemented on a computer as a computerimplemented method, as dedicated hardware, or as a combination of both. Instructions for the computer, e.g., executable code, may be stored on a computer-readable medium 300 as for example shown in Fig.12, e.g., in the form of a series 310 of machine-readable physical marks and / or as a series of elements having different electrical, e.g., magnetic, or optical properties or values. The executable code may be stored in a transitory or non-transitory manner. Examples of computer-readable mediums include memory devices, optical storage devices, integrated circuits, etc. Fig.12 shows a memory card 300.The following clauses outline independently claimable aspects of the invention, each specifying a distinct feature or combination of features that may serve as a standalone element of patent protection: Clause 1. A protection system for a DC system, wherein the DC system isan isolated terra bipolar DC system comprising a positive conductor, a negative conductor, and a midpoint conductor, wherein the DC system is designed to be symmetric around a midpoint at the midpoint conductor, wherein the protection system comprises: -a predetermined impedance arranged between the midpoint conductorand protective earth; -a current measurement circuit configured to determine a magnitude of acurrent flowing through at least a resistive part of the predetermined impedance; -a fault detection circuit configured to compare the magnitude of thecurrent against a threshold, and to trigger a fault action if the magnitude of the current exceeds the threshold. Clause 2. The protection system according to clause 1, wherein the currentmeasurement circuit is further configured to determine a direction of the current, and the fault detection circuit is configured to, on the basis of the direction, determine a fault location of an isolation fault with respect to the protective earth. Clause 3. The protection system according to clause 1 or 2, wherein thefault action comprises one or more of: -triggering a circuit breaker;- sending an alert to an external entity; and- activating an auxiliary protection system.Clause 4. The protection system according to any one of clauses 1 to 3,wherein the impedance comprises or consists of a parallel arrangement of a predetermined resistance and a predetermined capacitance. Clause 5. The protection system according to clause 4, wherein the currentmeasurement circuit comprise at least two of: a current transducer arranged in a current path of the predetermined resistance, a current transducer arranged in a current path of the predetermined capacitance, and a current transducer arranged in a common current path of the predetermined resistance and the predetermined capacitance. Clause 6. The protection system according to clause 5, wherein the faultdetection circuit is configured to determine a resistance Rfault of an isolation fault between on the one hand the positive conductor or the negative conductor and on the other hand the protective earth by solving for Rfault in the formula iR(M-PE) = VDC / (Rfault + RM-PE), where iR(M-PE) is a value of a measured current through the predetermined resistance, R(M-PE) is a value of the predetermined resistance, and VDC is a DC voltage at the respective positive or negative conductor. Clause 7. The protection system according to any one of clauses 4 to 6,wherein the predetermined resistance is chosen from a range of 10-1000 Ω. Clause 8. The protection system according to any one of clauses 4 to 7,wherein the predetermined capacitance is chosen from a range of 5-50 μF. Clause 9. The protection system according to any one of clauses 1 to 8,further comprising a current source arranged in a current path of the predetermined impedance, wherein the fault detection circuit is configured to control the current source to reduce the current flowing through the predetermined impedance. Clause 10.A DC system in form of an isolated terra bipolar DC system,wherein the DC system comprises a positive conductor, a negative conductor, and a midpoint conductor, wherein the DC system is designed to be symmetric around a midpoint at the midpoint conductor, wherein the DC system comprises the protection system according to any one of clauses 1 to 9. Clause 11.The DC system according to clause 10, wherein the DC systemis designed to comprise a system resistance between i) each one of the positive conductor and the negative conductor and ii) protective earth, wherein the system resistance is within a range of 0.1-20 MΩ. Clause 12.The DC system according to clause 10 or 11, wherein the DCsystem is designed to comprise a system capacitance between i) each one of the positive conductor and the negative conductor and ii) protective earth, wherein the system resist capacitance is within a range of 100-2000 μF. Clause 13.The DC system according to any one of clauses 10 to 12,wherein the DC system is or comprises a DC charger, for example an EV DC charger. Clause 14.The DC system according to any one of clauses 10 to 13,wherein the DC system is configured to operate as a symmetrical DC system at a maximum DC voltage at or above 920 V. Clause 15.A protection method for a DC system, wherein the DC system isan isolated terra bipolar DC system comprising a positive conductor, a negative conductor, and a midpoint conductor, wherein the DC system is designed to be symmetric around a midpoint at the midpoint conductor, wherein the protection method comprises: -arranging a predetermined impedance between the midpoint conductorand protective earth; and during operation of the DC system: -determining a current flowing through at least a resistive part of thepredetermined impedance; -determining a magnitude of the current;- comparing the magnitude of the current against a threshold; and- triggering a fault action if the magnitude of the current exceeds thethreshold. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design manyalternative embodiments without departing from the scope of the appended claims orclauses. In the claims or clauses, any reference signs placed between parenthesesshall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or stages other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Expressions such as “at least one of” when preceding a list or group of elements represent a selection of all or of any subset of elements from the list or group. For example, the expression, “at least one of A, B, and C” should be understood as including only A, only B, only C, both A and B, both A and C, both B and C, or all of A, B, and C. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMSClaim 1. An isolated terra bipolar DC system (101-104, 106, 107) comprising:- a positive conductor, a negative conductor, and a midpoint conductor,wherein the DC system has a first system impedance (181) between the positive conductor and protective earth and a second system impedance (182) between the negative conductor and the protective earth, wherein the DC system is designed to besymmetric around protective earth at the midpoint conductor;- a predetermined impedance (120) arranged between the midpointconductor and the protective earth;- a current measurement circuit (130-134) configured to determine amagnitude of a current flowing through at least a resistive part of the predetermined impedance;- a current source (160) arranged in a current path of the predeterminedimpedance (120);- a fault detection circuit (140) configured to control the current source toreduce the current flowing through the predetermined impedance, to compare themagnitude of the current against a threshold, and to trigger a fault action if themagnitude of the current exceeds the threshold.Claim 2. The DC system (101-104, 106, 107) according to claim 1, wherein thecurrent measurement circuit is further configured to determine a direction of the current,and the fault detection circuit is configured to, on the basis of the direction, determine afault location of an isolation fault with respect to the protective earth.Claim 3. The DC system (101-104, 106, 107) according to claim 1 or 2, whereinthe fault action comprises one or more of:- triggering a circuit breaker;- sending an alert to an external entity; and- activating an auxiliary protection system.Claim 4. The protection system (110) according to any one of claims 1 to 3,wherein the impedance (120) comprises or consists of a parallel arrangement of apredetermined resistance (122) and a predetermined capacitance (124).Claim 5. The DC system (101-104, 106, 107) according to claim 4, wherein thecurrent measurement circuit (130-134) comprise at least two of: a current transducer(132) arranged in a current path of the predetermined resistance (122), a currenttransducer (134) arranged in a current path of the predetermined capacitance (124),and a current transducer (130) arranged in a common current path of thepredetermined resistance (122) and the predetermined capacitance (124).Claim 6. The DC system (101-104, 106, 107) according to claim 5, wherein thefault detection circuit (140) is configured to determine a resistance Rfault of an isolationfault between on the one hand the positive conductor or the negative conductor and onthe other hand the protective earth, for example by solving for Rfault in the formula iR(M-PE) = VDC / (Rfault + RM-PE), where iR(M-PE) is a value of a measured current through thepredetermined resistance (122), R(M-PE) is a value of the predetermined resistance, andVDC is a DC voltage at the respective positive or negative conductor.Claim 7. The DC system (101-104, 106, 107) according to any one of claims 1 to6, further comprising a control circuit configured to, in response to the fault action beingtriggered, adjust a ratio between the positive DC voltage and the negative DC voltageto reduce the current flowing through the predetermined impedance.Claim 8. The DC system (101-104, 106, 107) according to claim 7, wherein:- the one or more power converters comprise an odd-level multi-level DCconverter configured to independently control the positive DC voltage and the negative DC voltage with respect to the midpoint conductor; and- the control circuit is configured to adjust the ratio between the positiveDC voltage and the negative DC voltage by controlling the odd-level multi-level DC converter.Claim 9. The DC system (101-104, 106, 107) according to claim 8, furthercomprising an external ground current compensating source, wherein the control circuit is further configured to control the external ground current compensating source to reduce the current flowing through the predetermined impedance.Claim 10. The DC system (101-104, 106, 107) according to claim 7, wherein:- the one or more power converters comprise an even-level series-connected power converter configured to control a DC voltage between the positiveconductor and the negative conductor;- the DC power system further comprises a midpoint converter; and- the control circuit is configured to control the even-level series-connected power converter to adjust the DC voltage and the midpoint converter to adjust the ratio between the positive DC voltage and the negative DC voltage.Claim 11. The DC system (101-104, 106, 107) according to any one of claims 1 to10, wherein the first system impedance (181) and / or the first system impedance (182)comprises a resistance within a range of 0.1-20 MΩ.Claim 12. The DC system (101-104, 106, 107) according to any one of claims 1 to11, wherein the first system impedance (181) and / or the first system impedance (182)comprises a capacitance within a range of 30-2000 μF, preferably 100-2000 μF.Claim 13. The DC system (101-104, 106, 107) according to any one of claims 1 to12, wherein the DC system:- is or comprises a DC charger, for example an EV DC charger; and / or- is configured to operate as a symmetrical DC system at a maximum DCvoltage at or above 920 V.Claim 14. The DC system (101-104, 106, 107) according to any one of claims 1 to13, wherein the DC system comprises one or more power converters configured to convert AC power to DC power, wherein the one or more power converters establish a positive DC voltage between the positive conductor and the midpoint conductor and a negative DC voltage between the midpoint conductor and the negative conductor.Claim 15. A method (200) for use with a DC system, wherein the DC system is anisolated terra bipolar DC system comprising a positive conductor, a negative conductor,and a midpoint conductor, wherein the DC system is designed to be symmetric around protective earth at the midpoint conductor, wherein the method comprises:- arranging (210) a predetermined impedance between the midpointconductor and protective earth;- arranging a current source in a current path of the predeterminedimpedance;and during operation of the DC system:- determining (230) a current flowing through at least a resistive part of thepredetermined impedance;- determining (240) a magnitude of the current;- comparing (250) the magnitude of the current against a threshold;- controlling the current source to reduce the current flowing through thepredetermined impedance, and triggering (260) a fault action if the magnitude of the current exceeds the threshold.

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