METHOD AND ISOLATION MONITORING ARRANGEMENT FOR A FUNCTIONALLY GROUNDED ELECTRICAL SYSTEM OPERATED WITH A SUPPLY DC VOLTAGE

Description

[0001] The invention relates to a method for insulation monitoring of an electrical system operated with a DC supply voltage, which has a first insulation resistance between the positive active conductor and earth and a second insulation resistance between the negative active conductor and earth, as well as a functional earthing between the negative active conductor and earth by means of an earthing resistor (path of functional earthing).

[0002] Furthermore, the invention relates to an insulation monitoring arrangement implementing the inventive method.

[0003] When increased demands are placed on the operational, fire, and contact safety of electrical installations, an ungrounded power supply system is used, also known as an isolated network (French: "Isolé Terre" - IT) or IT power supply system. In this type of power supply system, the live parts are separated from earth potential. The advantage of these networks is that in the event of an insulation fault, the function of the connected electrical loads is not affected, since, due to the ideally infinite insulation resistance between a live conductor of the network and earth, a closed circuit cannot form. Therefore, the network can continue to operate even in the event of a single insulation fault.

[0004] The resistance of the ungrounded power supply system to earth (insulation resistance - in case of a fault also insulation fault resistance or fault resistance) must therefore be constantly monitored, since a possible further fault on another active conductor would create a fault loop and the fault current flowing in conjunction with an overcurrent protection device would result in a shutdown of the system with operational standstill.

[0005] Particularly in electrical installations powered by an ungrounded AC power supply system and operated with a DC supply voltage via an AC / DC rectifier, there is a risk of DC stray currents flowing through conductive metal components and insulation resistance paths into electrolytically conductive foundations. Unprotected metallic components located in an electrolytic environment, such as steel reinforcement in a concrete foundation or buried metallic fixtures, are thus exposed to corrosion from DC stray currents originating from power supply systems. In reinforced concrete structures, the resulting material loss from the reinforcing steel can weaken the structural integrity, or, in the case of buried pipelines, cause leaks.

[0006] This particularly applies to hydrogen electrolysis plants, where stray currents can arise in the electrolytic background due to electrolytically conductive current paths via a process water connection of the electrolyzer or a parasitic electrolytically conductive current path, which, depending on the DC nominal voltage at the electrolyzer and depending on the magnitude of the (electrolytic) insulation resistance paths, shorten the service life of the electrical system.

[0007] With regard to limiting DC stray currents and the associated dangers of electrocorrosion, insulation monitoring is of particular importance.

[0008] In hydrogen electrolysis plants, some manufacturers have succeeded in designing the electrical system in such a way that the parasitic electrolytic current paths have only very high resistance in contact with ground. Insulation resistance values ​​well over 100 Ω / V DC nominal voltage are achievable.

[0009] The DC stray currents occurring at this level are mostly uncritical and do not lead to a shortened service life of the system.

[0010] Where this is not achievable due to design constraints, system manufacturers are resorting to connecting the negative live conductor (negative terminal) to earth potential on the DC side of the electrical system via a low-resistance earthing resistor. This functional earthing does not alter the potential (electrolytic) current flow through the first insulation resistance between the positive live conductor and earth; however, a portion of this current now flows not through the second insulation resistance between the negative live conductor and earth, but through the functional earthing resistor connected in parallel to the second insulation resistance.

[0011] The aim is to design the metallic current path via the earth resistance to be so low-impedance that the remaining potential DC stray current component via the second insulation resistance does not lead to a reduced service life of the electrical system due to electrocorrosion.

[0012] Determining the insulation resistance in conjunction with functional grounding presents a problem for the plant operator, as this functional grounding creates a low-resistance connection between the negative live conductor of the electrical system and earth. It is known that attempts are made to estimate the insulation status of the electrical system by measuring the DC current or DC differential current on the AC side of the rectifier. However, since load currents in the kA range sometimes occur, particularly in hydrogen electrolyzers, this DC current or DC differential current measurement on the AC side cannot be implemented with the required accuracy, according to the current state of the art, to detect unwanted fault currents below the range of a few hundred mA.

[0013] According to current technology, functional grounding can also be achieved via a so-called GFDI (Ground Fault Detector Interrupter), which briefly disconnects the functional ground at specific intervals, for example, daily before commissioning the electrical system, to enable insulation monitoring. A disadvantage of this solution is the requirement for a sufficiently high-impedance disconnect switch for the GFDI, which must be rated for the full DC nominal voltage. Furthermore, it is a disadvantage that virtually uninterrupted insulation monitoring is not possible throughout the entire operating cycle of the electrical system.

[0014] Another common approach is to implement functional grounding via a monitored resistance ground of the negative active conductor to earth. For example, the NGR (Neutral Ground Resistor) monitoring solution, widely used in the US market, is employed here. However, a disadvantage of this approach is that, depending on the expected power consumption of the electrical system, a separate control cabinet is required for the NGR, and that, depending on the magnitude of the stray current and the resistance value, the voltage drop across the NGR can be in a range (>1V DC) where stray current corrosion cannot be prevented. Document CN 116298522A discloses a method for correcting the impedance of an insulation detection resistor of a photovoltaic inverter and includes determining a first insulation impedance of the positive electrode and a second insulation impedance of the negative electrode to earth.The insulation resistances are calculated using the known insulation detection resistance or via leakage currents and voltages across the respective insulation resistances.

[0015] The present invention is therefore based on the objective of developing a method and an arrangement implementing this method which enables reliable insulation monitoring for an electrical system operated with a DC supply voltage and having a functional earth.

[0016] This task is solved, with reference to a method, by measuring an earthing current flowing in the functional earthing path using a DC current measuring device, measuring the supply DC voltage using a voltage measuring device, calculating the first insulation resistance from the supply DC voltage divided by the earthing current using a computing unit, whereby during the operation of the electrical system the condition applies that the second insulation resistance is at least one hundred times greater than the earthing resistance.

[0017] The basic idea of ​​the present invention is to install a (highly sensitive - see below) DC current measuring device in the functional earthing path, which detects the earthing current flowing through the earthing resistance, and simultaneously measures the supply DC voltage by means of a voltage measuring device.

[0018] The first insulation resistance is then calculated by dividing the supply DC voltage by the earthing current in a computing unit.

[0019] Therefore, determining the first insulation resistance as accurately as possible is of primary interest, since the vast majority of stray current flows through the low-resistance earthing conductor—and not through the parallel second insulation resistance—due to the functional earthing conductor running in parallel with the second insulation resistance. The second insulation resistance is only subject to constraints regarding its relative magnitude to the earthing resistance.

[0020] To prevent electrostatic corrosion, it is crucial that the DC voltage between the negative live conductor and earth, i.e., the DC voltage drop across the earth resistance, is so small that no corrosion processes are initiated at the potential site of electrostatic corrosion, namely the path across the second insulation resistance – in the case of a hydrogen electrolysis plant, an electrolytically conductive current path via a domestic water connection of the electrolyzer. Experience has shown that this is ensured when this DC voltage drop is significantly below 1 V (DC).

[0021] Simulation results show that insulation monitoring with determination of the first insulation resistance is sufficiently accurate if, during the operation of the electrical system, the condition is that the second insulation resistance is at least one hundred times greater than the earth resistance.

[0022] In a further embodiment, an antiparallel diode circuit is arranged in series with the earth resistance in the functional earthing path, with a bypass switch connected in parallel to the antiparallel diode circuit, which cyclically switches between a high-resistance open state and a low-resistance closed state, wherein (across the antiparallel diode circuit) a diode voltage is measured by means of a further voltage measuring device, in the closed state the first insulation resistance is calculated from the supply DC voltage divided by the earth current and the second insulation resistance is calculated by dividing a diode voltage change between the two states and a ground current change between the two states by means of the arithmetic unit.

[0023] Since manufacturers of functionally grounded electrical systems often cannot assess whether the condition for sufficiently accurate insulation monitoring – namely, that the second insulation resistance is at least one hundred times the earth resistance – can be met in operation and thus whether the implemented functional earthing actually protects against electrocorrosion caused by DC stray currents, the second insulation resistance is calculated to verify this condition.

[0024] For this purpose, on the DC side of the functionally grounded electrical system, a combination of antiparallel diodes with a parallel bypass switch is inserted into the functional ground path in series with the grounding resistor. The DC decoupling via the antiparallel diode circuit prevents the occurrence of high reverse voltages, which could damage parallel components if only a single diode were used.

[0025] Both the diodes and the bypass switch must be designed for the expected maximum grounding current. The DC voltage drop across the antiparallel diode circuit (diode voltage) is measured.

[0026] The bypass switch is preferably implemented using a very low-resistance semiconductor switch. The resistance of the bypass switch when it is switched on should be no higher than the resistance of the grounding circuit.

[0027] The bypass switch is now controlled to cyclically switch between a high-resistance open state and a low-resistance closed state. The dwell times for the open / closed states are selected to ensure that the measured variables—ground current, ground current change, diode voltage, diode voltage change, and supply DC voltage—can be recorded with sufficient accuracy.

[0028] During the closed state of the bypass switch, in which the diode circuit is short-circuited, the first insulation resistance - as already stated in claim 1 - is calculated from the supply DC voltage divided by the earth current.

[0029] Additionally, after each period with the bypass switch in the open and closed state, the diode voltage change between the two states and the earth current change between the two states are cyclically recorded and used to calculate the second insulation resistance from the quotient of the diode voltage change and the earth current change.

[0030] As an alternative to the previously described design with antiparallel diode circuit and bypass switch, in another design an antiparallel diode circuit without a parallel bypass switch is inserted into the functional earthing path in series with the earthing resistance, whereby the first insulation resistance is calculated from the supply DC voltage divided by the earthing current using the computing unit.

[0031] Simulation results show that even without determining the second insulation resistance, the condition that the second insulation resistance is one hundred times greater than the earth resistance is sufficiently fulfilled even if the DC voltage drop across the antiparallel diode circuit is significantly above one percent of the diode forward voltage.

[0032] From this, it can be concluded that the bridging switch can be dispensed with and the diode voltage dropping across the antiparallel diode circuit can be used in such a way that this diode voltage is monitored to ensure that the measured voltage value is significantly above one percent of the diode forward voltage.

[0033] In this case, it is assumed that the calculation of the first insulation resistance from the supply DC voltage divided by the earthing current is sufficiently accurate.

[0034] It is advantageous to record the earth current measured by the DC current measuring device in a range of less than 100mA.

[0035] The DC current measuring device must be designed so that the earthing current can be measured with high sensitivity in the mA range below 100mA during normal operation of the electrical system.

[0036] Furthermore, a DC fault current is detected by means of a DC fault current measuring device installed in the functional earthing path.

[0037] In addition, supplementary fault protection can be implemented in all versions to ensure a rapid shutdown of the electrical system in the event of a DC fault current in the functional path that could endanger people or the system. For this purpose, a DC fault current measuring device is installed in the functional earthing path.

[0038] In contrast to the highly sensitive DC current measuring device for measuring the earth current, the DC fault current measuring device for detecting the DC fault current must be designed for the significantly higher fault current that occurs in the event of a fault (earth fault).

[0039] Without this additional fault protection, the functional earthing path would have to have an extremely low loop impedance in order to quickly and reliably trip an existing overcurrent protection device. This would also mean that all components in the functional earthing path would have to be designed for such high currents – on the order of several hundred amps up to the kiloampere range.

[0040] With this additional fault protection, however, a quick and reliable shutdown of the electrical system can be achieved even with significantly smaller fault currents in the single-digit ampere range.

[0041] The components in the functional earthing path can be smaller and are therefore easier and more cost-effective to obtain.

[0042] Preferably, the DC fault current is detected using a DC fault current measuring device designed as a modular fault current device.

[0043] The design of the DC residual current measuring device as a (DC-sensitive) modular residual current device (MRCD) makes it possible to detect incipient fault currents at an early stage through the DC-sensitive differential current monitoring integrated in the MRCD and, in conjunction with an external switching element, to bring about a shutdown of the electrical system within a normatively prescribed time.

[0044] The inventive method for insulation monitoring proves to be particularly advantageous if the electrical system operated with the supply DC voltage is a hydrogen electrolysis system.

[0045] In the course of the shift towards renewable energies as the basis for hydrogen production, the inventive method can be used very efficiently in conjunction with the operation of hydrogen electrolysis plants, since the electrolytically conductive process water piping system poses particular challenges for monitoring the insulation resistance.

[0046] The claimed structural features of the insulation monitoring arrangement according to the invention carry out the corresponding process steps of the insulation monitoring method according to the invention. Thus, the technical effects and resulting advantages achieved with the method also apply equally to the insulation monitoring arrangement.

[0047] Further advantageous design features will become apparent from the following description and the drawings, which illustrate a preferred embodiment of the invention by means of examples.

[0048] They show: Fig. 1 the formation of electrocorrosion due to a DC stray current in a hydrogen electrolysis plant; Fig. 2 the hydrogen electrolysis plant with a functional grounding; Fig. 3 an insulation monitoring arrangement according to the invention for the functionally grounded hydrogen electrolysis plant; Fig. 4 Simulation result of the calculated first insulation resistance as a function of the second insulation resistance; Fig. 5 an insulation monitoring arrangement according to the invention with antiparallel diode circuit and bypass switch; Fig. 6 an insulation monitoring arrangement according to the invention with antiparallel diode circuit without bypass switch and Fig. 7 the insulation monitoring arrangement according to Fig. 6 with a DC fault current measuring device.

[0049] Fig. 1 The figure shows the formation of electrocorrosion by a DC stray current IS in an electrical system 2 using the example of a hydrogen electrolysis system 2 with a PEM (proton exchange membrane) electrolyzer.

[0050] The PEM electrolyzer is powered by a three-phase AC network 3 and is connected via an AC / DC rectifier to the positive active conductor L+ and the negative active conductor L-. Between the positive active conductor L+ and ground PE, the insulation state of the hydrogen electrolysis system 2 is characterized by a first insulation resistance Risol, and correspondingly, between the negative active conductor L- and ground PE, a second insulation resistance Riso2 is shown. The insulation resistances Risol and Riso2 can thus form an electrolytically conductive current path via the service water connection of the PEM electrolyzer, as well as a parasitic current path through metallically conductive parts and an electrolytically conductive substrate (e.g., the concrete foundation).

[0051] In this circuit, a DC stray current IS flows, which can lead to electrocorrosion in the electrolytically conductive (concrete) soil.

[0052] Fig. 2 The hydrogen electrolysis plant 2 is shown with a functional grounding via a grounding resistor RE.

[0053] The negative active conductor L- is connected to earth PE via the earthing resistor RE. Since this earthing resistor RE has a significantly lower resistance than the parallel insulation resistance R iso2, the majority of the stray current IS flows as earthing current IE through the functional earthing path. Consequently, the risk of corrosion is considerably reduced due to the now minimal stray current flowing through the second insulation resistance R iso2.

[0054] Fig. 3 shows an insulation monitoring arrangement 10 according to the invention for the functionally grounded hydrogen electrolysis plant 2.

[0055] For this purpose, a DC current measuring device 14 is provided in the functional grounding path, which detects the grounding current IE flowing through the grounding resistance RE. A voltage measuring device 12 is located between the positive active conductor L+ and the negative active conductor L- to measure the supply DC voltage UDC. The first insulation resistance Risol is calculated in a processing unit 20. According to Ohm's law, the first insulation resistance is obtained by dividing the supply DC voltage UDC by the grounding current IE. R iso 1 = U DC I E .

[0056] The insulation monitoring arrangement 10 according to the invention thus comprises as main components the voltage measuring device 12, the DC current measuring device 14 and the computing unit 20 for evaluating the measured quantities and for calculating the first (and the second) insulation resistance R iso1 (R iso2 ).

[0057] During the operation of the hydrogen electrolysis plant 2, the condition must be met that the second insulation resistance R iso2 is at least one hundred times greater than the earthing resistance RE, so that the potentially corrosion-causing stray current component via the second insulation resistance R iso2 is low.

[0058] Fig. 4 shows a simulation result of the calculated first insulation resistance R iso1 as a function of the second insulation resistance R iso2 .

[0059] In practice, it is difficult for many manufacturers of functionally grounded hydrogen electrolysis plants 2 to assess whether the condition is met that the second insulation resistance R iso2 is many times greater than the earthing resistance RE and thus whether the implemented functional earthing actually protects against electrocorrosion caused by a DC stray current IS.

[0060] To verify this condition, with a constant first insulation resistance R isol of 100kΩ and a constant earth resistance RE of 10mΩ, the second insulation resistance R iso2 is gradually changed in the simulation in a range from 10mΩ to 1kΩ, and the respective value of the first insulation resistance R iso1 is calculated from the DC nominal voltage U DC and the earth current IE as above.

[0061] From the Fig. 4 It is evident that the desired target value of 100 kΩ for the first insulation resistance Risol is achieved with a second insulation resistance value of Riso2 = 1 Ω. Therefore, the condition can be that the second insulation resistance Riso2 (= 1 Ω) should be one hundred times greater than the earth resistance RE (= 10 mΩ).

[0062] Fig. 5 shows an insulation monitoring arrangement 10 according to the invention with antiparallel diode circuit 16 and bridging switch 18.

[0063] In this embodiment, the insulation monitoring arrangement 10 according to the invention additionally comprises the antiparallel diode circuit 16 with a further voltage measuring device 12 and the bypass switch 18. The antiparallel diode circuit 16 is connected in series with the grounding resistor RE in the functional grounding path, and the bypass switch 18 is arranged in parallel with the antiparallel diode circuit 16. The bypass switch 18 is cyclically switched between a high-resistance open state and a low-resistance closed state via control signals from the processing unit 20.

[0064] In the closed state of the bridging switch 20, the diode circuit 16 is short-circuited, so that - as described above - with low earth resistance RE, the first insulation resistance R iso1 results from the supply DC voltage U DC divided by the earth current IE.

[0065] The second insulation resistance R iso2 is calculated by dividing the voltage change ΔU D between the two states and the current change ΔI E between the two states. R iso 2 = ΔU D ΔI E .

[0066] Fig. 6 In a further embodiment, an insulation monitoring device 10 according to the invention with an antiparallel diode circuit 16 without a bypass switch 18 is shown.

[0067] Simulation results indicate that, even without determining the second insulation resistance Riso2, it can be assumed that the condition that the second insulation resistance Riso2 should be one hundred times greater than the grounding resistance RE is met if the DC voltage drop UD across the antiparallel diode circuit 16 is significantly above one percent of the diode forward voltage. In this case, the bypass switch 18 can be omitted in this optimized embodiment, and the DC voltage drop UD across the antiparallel diode circuit 16 is monitored to ensure that this voltage value remains significantly above one percent of the diode forward voltage.

[0068] In this case, it is assumed that the calculation of the first insulation resistance R iso1 follows sufficiently accurately from the division of the DC nominal voltage U DC by the earthing current IE.

[0069] Fig. 7 shows the insulation monitoring according to the invention. Fig. 6 with a DC fault current measuring device 30.

[0070] To ensure a rapid shutdown of the electrical system 2 in the event of a DC fault current IF flowing in the functional earthing path, an additional DC fault current measuring device 30 is installed in the functional earthing path.

[0071] While the DC current measuring device 14 has the task of measuring the earth current IE as sensitively as possible during normal operation, the additional DC fault current measuring device 30 is designed for a significantly higher fault current IF that occurs in the event of a fault (earth fault).

[0072] Preferably, the DC residual current measuring device 30 is designed as a DC-sensitive modular residual current device (MRCD).

Claims

1. A method for insulation monitoring of an electric installation (2) which is operated using a supply direct voltage (UDC) and has a first insulation resistance (Riso1) between the positive active conductor (L+) and ground (PE) and a second insulation resistance (Riso2) between the negative active conductor (L-) and ground (PE) as well as a functional grounding between the negative active conductor (L-) and ground (PE) by means of a ground resistance (RE), the method comprising the following steps: measuring a ground current (IE), which flows in the path of the functional grounding, by means of a DC measuring device (14), measuring the supply direct voltage (UDC) by means of a voltage measuring device (12), computing the first insulation resistance (Riso1) from the supply direct voltage (UDC) divided by the ground current (IE) by means of a computing unit (20), the condition being valid during operation of the electric installation (2) that the second insulation resistance (Riso2) is at least 100 times greater than the ground resistance (RE).

2. The method according to claim 1, characterized in that an antiparallel diode circuit (16) having a bypass switch (18) switched parallel to the antiparallel diode circuit (16) is disposed in series to the ground resistance (RE) in the path of the functional grounding, the bypass switch (18) cyclically alternating between a high-impedance open state and a low-impedance closed state, a diode voltage (UD) being measured by means of another voltage measuring device (12), the first insulation resistance (Riso1) being computed in the closed state from the supply direct voltage (UDC) divided by the ground current (IE), and the second insulation resistance (Riso2) being computed by dividing a diode voltage change (ΔUD) between the two states and a ground current change (ΔIE) between the two states by means of the computing unit (20).

3. The method according to claim 1, characterized in that an antiparallel diode circuit (16) without a bypass switch (18) switched parallel thereto is disposed in series to the ground resistance (RE) in the path of the functional grounding, the first insulation resistance (Riso1) being computed from the supply direct voltage (UDC) divided by the ground current (IE) by means of the computing unit (20).

4. The method according to any one of the claims 1 to 3, characterized in that the ground current (IE) measured by means of the DC measuring device (14) is detected in a range less than 100 mA.

5. The method according to any one of the claims 1 to 4, characterized in that a DC residual current (IF) is detected by means of DC residual-current measuring device (30) installed in the path of the functional grounding.

6. The method according to claim 5, characterized in that the DC residual current (IF) is detected by means of a DC residual-current measuring device (30) configured as a modular residual current device.

7. An application of the method for insulation monitoring according to any one of the claims 1 to 6, characterized in that the electric installation (2) operated using the supply direct voltage (UDC) is a hydrogen electrolysis installation.

8. An insulation monitoring arrangement (10) for an electric installation which is operated using a supply direct voltage (UDC) and has a first insulation resistance (Riso1) between the positive active conductor (L+) and ground (PE) and a second insulation resistance (Riso2) between the negative active conductor (L-) and ground (PE) and a functional grounding between the negative active conductor (L-) and ground (PE) by means of a ground resistance (RE), the insulation monitoring arrangement (10) having a DC measuring device (14) for measuring a ground current (IE) flowing in the path of the functional grounding, a voltage measuring device (12) for measuring the supply direct voltage (UDC), a computing unit (20), which is configured for computing the first insulation resistance (Riso1) from the supply direct voltage (UDC) divided by the ground current (IE), the condition being valid during operation of the electric installation (2) that the second insulation resistance (Riso2) is at least 100 times greater than the ground resistance (RE).

9. The insulation monitoring arrangement (10) according to claim 8, characterized in that an antiparallel diode circuit (16) having a bypass switch (18) switched parallel to the antiparallel diode circuit (16) is disposed in series to the ground resistance (RE) in the path of the functional grounding, the bypass switch (18) cyclically alternating between a high-impedance open state and a low-impedance closed state, the computing unit (20) being configured for computing the first insulation resistance (Riso1) from the supply direct voltage (UDC) divided by the ground current (IE) in the closed state and for computing the second insulation resistance (Riso2) by dividing the voltage change (ΔUD) between the two states and the current change (ΔID) between the two states.

10. The insulation monitoring arrangement (10) according to claim 8, characterized in that an antiparallel diode circuit (16) is disposed in series to the ground resistance (RE) in the path of the functional grounding, the computing unit (20) being configured for computing the first insulation resistance (Riso1) from the supply direct voltage (UDC) divided by the ground current (IE).

11. The insulation monitoring arrangement (10) according to any one of the claims 8 to 10, characterized in that the DC measuring device (14) is designed to be highly sensitive for detecting the ground current (IE) in the range of less than 100 mA.

12. The insulation monitoring arrangement (10) according to any one of the claims 8 to 11, characterized by a DC residual-current measuring device (30) installed in the path of the functional grounding and configured for detecting a DC residual current (IF).

13. The insulation monitoring arrangement (10) according to claim 12, characterized in that the DC residual-current measuring device is configured as a modular residual current device.

14. A usage of the insulation monitoring arrangement (10) according to any one of the claims 8 to 13, characterized in that the electric installation (2) is a hydrogen electrolysis installation.