Method and insulation monitoring arrangement for a functionally grounded electric installation operated using a supply direct voltage

Abstract

A method and insulation monitoring arrangement for insulation monitoring of an electric installation operated using a supply direct voltage and has a first insulation resistance between the positive active conductor and ground and a second insulation resistance between the negative active conductor and ground as well as a functional grounding between the negative active conductor and ground by a ground resistance. The method involves measuring a ground current, which flows in the path of the functional grounding, by means of a DC measuring device; measuring the supply direct voltage by means of a voltage measuring device; computing the first insulation resistance from the supply direct voltage divided by the ground current by means of a computing unit; the condition is valid during operation of the electric installation that the second insulation resistance being at least 100 times greater than the ground resistance.

Claims

1. A method for insulation monitoring of an electric installation (2) which is operated using a supply direct voltage (U.sub.DC) and has a first insulation resistance (R.sub.iso1) between the positive active conductor (L+) and ground (PE) and a second insulation resistance (R.sub.iso2) 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 (R.sub.E), the method comprising the following steps: measuring a ground current (I.sub.E), which flows in the path of the functional grounding, by means of a DC measuring device (14), measuring the supply direct voltage (U.sub.DC) by means of a voltage measuring device (12), computing the first insulation resistance (R.sub.iso1) from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E) by means of a computing unit (20), the condition being valid during operation of the electric installation (2) that the second insulation resistance (R.sub.iso2) is at least 100 times greater than the ground resistance (R.sub.E).

2. The method according to claim 1, wherein 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 (R.sub.E) 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 (U.sub.D) being measured by means of another voltage measuring device (12), the first insulation resistance (R.sub.iso1) being computed in the closed state from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E), and the second insulation resistance (R.sub.iso2) being computed by dividing a diode voltage change (U.sub.D) between the two states and a ground current change (I.sub.E) between the two states by means of the computing unit (20).

3. The method according to claim 1, wherein an antiparallel diode circuit (16) without a bypass switch (18) switched parallel thereto is disposed in series to the ground resistance (R.sub.E) in the path of the functional grounding, the first insulation resistance (R.sub.iso1) being computed from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E) by means of the computing unit (20).

4. The method according to claim 1, wherein the ground current (I.sub.E) measured by means of the DC measuring device (14) is detected in a range less than 100 mA.

5. The method according to claim 1, wherein a DC residual current (I.sub.F) 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, wherein the DC residual current (I.sub.F) 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 claim 1, wherein the electric installation (2) operated using the supply direct voltage (U.sub.DC) is a hydrogen electrolysis installation.

8. An insulation monitoring arrangement (10) for an electric installation which is operated using a supply direct voltage (U.sub.DC) and has a first insulation resistance (R.sub.iso1) between the positive active conductor (L+) and ground (PE) and a second insulation resistance (R.sub.iso2) 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 (R.sub.E), the insulation monitoring arrangement (10) having a DC measuring device (14) for measuring a ground current (I.sub.E) flowing in the path of the functional grounding, a voltage measuring device (12) for measuring the supply direct voltage (U.sub.DC), a computing unit (20), which is configured for computing the first insulation resistance (R.sub.iso1) from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E), the condition being valid during operation of the electric installation (2) that the second insulation resistance (R.sub.iso2) is at least 100 times greater than the ground resistance (R.sub.E).

9. The insulation monitoring arrangement (10) according to claim 8, wherein 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 (R.sub.E) 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 (R.sub.iso1) from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E) in the closed state and for computing the second insulation resistance (R.sub.iso2) by dividing the voltage change (U.sub.D) between the two states and the current change (I.sub.D) between the two states.

10. The insulation monitoring arrangement (10) according to claim 8, wherein an antiparallel diode circuit (16) is disposed in series to the ground resistance (R.sub.E) in the path of the functional grounding, the computing unit (20) being configured for computing the first insulation resistance (R.sub.iso1) from the supply direct voltage (U.sub.DC) divided by the ground current (I.sub.E).

11. The insulation monitoring arrangement (10) according to claim 8, wherein the DC measuring device (14) is designed to be highly sensitive for detecting the ground current (I.sub.E) in the range of less than 100 mA.

12. The insulation monitoring arrangement (10) according to claim 8, further including a DC residual-current measuring device (30) installed in the path of the functional grounding and configured for detecting a DC residual current (I.sub.F).

13. The insulation monitoring arrangement (10) according to claim 12, wherein 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 claim 8, wherein the electric installation (2) is a hydrogen electrolysis installation.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantageous embodiment features are derived from the following description and drawings, which describe a preferred embodiment of the invention by means of examples.

(2) FIG. 1 shows the development of electric corrosion via a DC stray current in a hydrogen electrolysis installation.

(3) FIG. 2 shows the hydrogen electrolysis installation with a functional grounding.

(4) FIG. 3 shows an insulation monitoring arrangement according to the invention for the functionally grounded hydrogen electrolysis installation.

(5) FIG. 4 shows simulation results of the computed first insulation resistance as a function of the second insulation resistance.

(6) FIG. 5 shows an insulation monitoring arrangement according to the invention and having an antiparallel diode circuit and bypass switch.

(7) FIG. 6 shows an insulation monitoring arrangement according to the invention and having an antiparallel diode circuit without a bypass switch.

(8) FIG. 7 shows the insulation monitoring arrangement according to FIG. 6 and having a DC residual-current measuring device.

DETAILED DESCRIPTION

(9) FIG. 1 shows the development of electric corrosion via a DC stray current I.sub.S in an electric installation 2 using the example of a hydrogen electrolysis installation 2 having a PEM (proton exchange membrane) electrolyzer.

(10) The PEM electrolyzer is fed by a three-phase alternating current network 3 and is connected to the positive active conductor L.sub.+ and the negative active conductor L.sub. via an AC/DC rectifier. Between positive active conductor L.sub.+ and ground PE, the insulation state of hydrogen electrolysis installation 2 is characterized by a first insulation resistance R.sub.iso1; accordingly, a second insulation resistance R.sub.iso2 is shown between negative active conductor L.sub. and ground PE. Insulation resistances R.sub.iso1 and R.sub.iso2 can thus form an electrically conductive current path via the process-water connection of the PEM electrolyzer and a parasitic current path of metallically conductive parts and an electrolically conductive foundation (for example a concrete foundation).

(11) A DC stray current I.sub.S flows in this current circuit, DC stray current I.sub.S being able to cause electric corrosion in the electrolytically conductive (concrete) foundation.

(12) FIG. 2 shows hydrogen electrolysis installation 2 having a functional grounding via a ground resistance R.sub.E.

(13) Negative active conductor L.sub. is connected to ground PE via ground resistance R.sub.E. Since this ground resistance R.sub.E is configured to be many times lower in impedance than insulation resistance R.sub.iso2 switched parallel, the majority of stray current I.sub.S flows as a ground current I.sub.E via the path of the functional grounding. As a result, the risk of corrosion is significantly lower there via second insulation resistance R.sub.iso2 because of the now reduced stray current portion.

(14) FIG. 3 shows an insulation monitoring arrangement 10 according to the invention for functionally grounded hydrogen electrolysis installation 2.

(15) For this purpose, a DC measuring device 14 is provided in the path of the functional grounding, DC measuring device 14 detecting ground current I.sub.E flowing via ground resistance R.sub.E. To measure supply direct voltage U.sub.DC, a voltage measuring device 12 is located between positive active conductor L.sub.+ and negative active conductor L.sub.. First insulation resistance R.sub.iso1 is computed in a computing unit 20. According to Ohm's Law, the division of supply direct voltage U.sub.DC by ground current I.sub.E equals the first insulation resistance

(16) $R_{\mathrm{iso}1}=\frac{U_{\mathrm{DC}}}{I_E}$

(17) Insulation monitoring arrangement 10 according to the invention consequently comprises voltage measuring device 12, DC measuring device 14 and computing unit 20 as the main components in order to evaluate the measured variables and for computing first (and second) insulation resistance R.sub.iso1 (R.sub.iso2).

(18) During operation of hydrogen electrolysis installation 2, the condition is to be maintained that second insulation resistance R.sub.iso2 is at least 100 times greater than ground resistance R.sub.E so that the possibly corrosive stray current portion via second insulation resistance R.sub.iso2 is low.

(19) FIG. 4 shows a simulation result of computed first insulation resistance R.sub.iso1 as a function of second insulation resistance R.sub.iso2.

(20) In practice, many manufacturers of functionally grounded hydrogen electrolysis installations 2 find it difficult to assess whether the condition that second insulation resistance R.sub.iso2 is many times greater than ground resistance R.sub.E is fulfilled and thus whether the realized functional grounding actually protects from electric corrosion caused by a DC stray current I.sub.S.

(21) To verify this condition, second insulation resistance R.sub.iso2 ranging from 10 m to 1 k is changed step by step in the simulation using a constant first insulation resistance R.sub.iso1 of 100 k and a constant ground resistance R.sub.E of 10 m and the respective value of first insulation resistance R.sub.iso1 is computed from DC nominal voltage U.sub.DC and ground current I.sub.E as computed above.

(22) FIG. 4 shows that the desired value of 100 k of first insulation resistance R.sub.iso1 is reached at a second insulation resistance value of R.sub.iso2=1. The condition can thus be that second insulation resistance R.sub.iso2 (=1) should be 100 times greater than ground resistance R.sub.E (=10 m).

(23) FIG. 5 shows an insulation monitoring arrangement 10 according to the invention and having an antiparallel diode circuit 16 and a bypass circuit 18.

(24) In this exemplary embodiment, insulation monitoring arrangement 10 according to the invention additionally comprises antiparallel diode circuit 16 having another voltage measuring device 12 and bypass circuit 18. In this context, antiparallel diode circuit 16 switched in series to ground resistance R.sub.E in the path of the functional grounding and bypass switch 18 is disposed parallel to antiparallel diode circuit 16. Bypass switch 18 is cyclically switched between a high-impedance open state and a low-impedance closed state via control signals originating from computing unit 20.

(25) In the closed state of bypass switch 20, diode circuit 16 is short-circuited, meaning first insulation resistance R.sub.iso1 is yielded from supply direct voltage U.sub.DC is divided by ground current I.sub.E at a low-impedance ground resistance R.sub.E, as described above.

(26) Second insulation resistance R.sub.iso2 is computed by dividing voltage change U.sub.D between the two states and the voltage change I.sub.E between the two states

(27) $R_{\mathrm{iso}2}=\frac{U_D}{I_E}$

(28) FIG. 6 shows another embodiment of an insulation monitoring 10 according to the invention and having an antiparallel diode circuit 16 without a bypass switch 18.

(29) Simulation results show that even without determining second insulation resistance R.sub.iso2, it can be assumed that the condition that second insulation resistance R.sub.iso2 should be 100 times greater than ground resistance R.sub.E is fulfilled when DC voltage drop Up via antiparallel diode circuit 16 is significantly above one percent of the diode let-through voltage. In this case, bypass switch 18 can be foregone in this optimized embodiment and DC voltage drop Up via antiparallel diode circuit 16 is monitored to discover that this voltage value is still significantly above one percent of the diode let-through voltage.

(30) In this case, it is presumed that first insulation resistance R.sub.iso1 is computed sufficiently precisely by dividing DC nominal voltage U.sub.DC by ground voltage I.sub.E.

(31) FIG. 7 shows an insulation monitoring according to the invention and FIG. 6 and having a DC residual-current measuring device 30.

(32) In order to effect a quick shutdown of electric installation 2 when a DC residual current I.sub.F flowing in the path of the functional grounding occurs, an additional DC residual-current measuring device 30 is installed in the functional-grounding path.

(33) While DC measuring device 14 is tasked with detecting ground voltage I.sub.E as sensitively as possible using measuring technology during regular operation, additional DC residual-current measuring device 30 is configured for a significantly greater residual current I.sub.F occurring in the event of a fault (ground fault).

(34) Preferably, DC residual-current measuring device 30 is designed as a DC-sensitive modular residual-current device (MRCD).