Monitoring insulation and fault current in an A/C current network to provide load shutoff whenever differential current exceeds a certain response value

• US 6,392,422 B1
• Filed: 04/14/2000
• Issued: 05/21/2002
• Est. Priority Date: 06/17/1997
• Status: Expired due to Fees

• Current Assignee: Dipl.-Ing. Walther Bender GmbH & Co. KG.
• Original Assignee: DIP -ING Walther Bender GmbH & Company KG
• Inventors: Kammer, Michael, Hackl, Dieter, Kaul, Karl-Hans
• Primary Examiner(s): Brown, Glenn W.
• Assistant Examiner(s): Hamdan, Wasseem H.
• Application Number:
  US09/446,350
• Time in Patent Office: 767 Days
• Field of Search: 324/650, 324/519, 324/509, 324/510, 324/551, 324/76.12, 324/76.39, 324/76.44, 324/76.52, 324/76.68, 324/76.78, 361/94, 361/77, 361/45, 361/102, 361/42, 361/47
• US Class Current: 324/650
• CPC Class Codes: H02H 1/06 Arrangements for supplying operative power power supply arrangements in general G05F, H02M
• CPC Class Codes: H02H 3/332 with means responsive to dc component in the fault current
• CPC Class Codes: H02H 3/337 avoiding disconnection due to reactive fault currents
• CPC Class Codes: H02H 5/105 responsive to deterioration or interruption of earth connection for preventing switching-on H02H11/001

Abstract

A method and device for monitoring insulation and fault current in an electrical alternating current network. The differential current formed by vectorial addition is detected between at least two network conductors. Load-cut occurs when the differential current exceeds a specific threshold value. In order to increase safety and to protect human beings, the amount of alternating current contained in the differential current is detected as a first network variable, and the network alternating current between at least both network conductors and between at least one network conductor and an equipotential bonding conductor or a neutral conductor is detected as a second network variable. The product between the amplitude of the amount of alternating current contained in the differential current and the cosine of the phase angle φ between both detected network variables is determined as a measure for the resistive fault current of the network and load cut-out occurs when the determined product exceeds a specific threshold value. The product can be calculated directly or, even better, it can be determined indirectly without knowledge of the individual variables. One particular advantage of this technique is that, by including DC components of the differential current, it enables work with all different types of currents in view of the fact that alternating current networks nowadays mainly consist of a large number of alternating current components which, like resistive AC components, involve fault currents endangering the lives of human beings. Thus, it is possible to ensure simultaneous protection of human beings and installations.

Claims

1. A method for monitoring insulation and fault current in an electrical alternating current (AC) network, in which a differential current, formed by a vectorial addition, between at least two network conductors is ascertained, and furthermore a product of an amplitude of the AC component of the differential current and a cosine of a phase angle φ

between the AC component of the differential current and the AC network voltage between at least two conductors of the network is ascertained, and a load shutoff is performed whenever a resistive fault current exceeds a certain response value, characterized in that

the differential current, containing AC and DC components, of the AC network is detected with universal current sensitivity;

that the AC component, including both a capacitive component and a resistive component, of the differential current detected with a universal current sensitively, is obtained by a high-pass filtration with a first limit frequency that is below the network frequency;

that the DC component, to be assessed as a resistive fault current signal, of the differential current detected with the universal current sensitivity is obtained by a low-pass filtration with a second limit frequency located below the first limit frequency;

that after the high-pass filtration, for the sake of protecting human beings the AC component of the differential current is weighted as a function of frequency;

that the product of the amplitude of the AC component of the differential current and the cosine of the phase angle φ

is ascertained indirectly by an effectiveness measurement;

that in said effectiveness measurement, a multiplication of the AC component of the differential current by a multiplication signal and an ensuing arithmetic averaging are performed, and the multiplication signal corresponds to the AC network voltage, which is kept constant with regard to its effective value, or a second network variable;

that the resistive fault current signal on a DC side and the resistive fault current signal on an AC side ascertained from the AC component are subjected to a quadratic addition in order to ascertain a resistive total fault current signal; and

that the load shutoff is performed whenever the resistive total fault current signal exceeds a certain response value.

2. The method of claim 1, characterized in that from the two network variables detected, the phase angle φ

between these network variables is determined, and that the product of the amplitude of the AC component of the differential current and the cosine of the phase angle φ

is calculated.

3. The method of claim 1, characterized in that the product of the amplitude of the AC component of the differential current and the cosine of the phase angle φ

is ascertained indirectly by synchronous rectification (correct-phase rectification).

4. The method of claim 3, characterized in that in the synchronous rectification, a phase-dependent full-travel rectification of the AC component of the differential current and an ensuing arithmetic averaging are performed.

5. The method of claim 3, characterized in that in the synchronous rectification a multiplication of the AC component of the differential current by a multiplication signal and an ensuing arithmetic averaging are performed, and the multiplication signal is a network-synchronous, differential voltage-free square signal or sine wave signal of constant amplitude that is symmetrical to the AC network voltage or to the second network variable.

6. The method of claim 1, characterized in that the resistive fault current ascertained is compared with a lesser response value suitable for protecting human beings;

that the total differential current detected, which includes the AC components and can at the same time include DC components, is compared with a greater response value suitable for protecting property;

and that a load or network shutoff is performed whenever at least one of the response values is exceeded.

7. The method of claim 6, characterized in that the lesser response value and/or the greater response value are adjustable.

8. The method of claim 7, characterized in that the lesser response value for protecting human beings is adapted flexibly to the network conditions prevailing in operation, and the ohmic grounding resistance is measured, and the response value is adjusted such that it corresponds to the quotient of the maximum allowable touch voltage and the ohmic grounding resistance.

9. The method of claim 1, for an AC network with an equipotential bonding conductor, characterized in that for checking the function of the equipotential bonding conductor, the current flowing in the normal situation as a certain component of the total differential current through the equipotential bonding conductor is detected, and that a load or network shutoff is then performed if this current is less than the certain component.

10. The method of claim 1, for an AC network with an equipotential bonding conductor, characterized in that for checking the function of the equipotential bonding conductor, a test current is fed into the equipotential bonding conductor;

that this test current, which in the normal situation flows through the equipotential bonding conductor and the ohmic grounding resistance, is detected; and

that then a load or network shutoff is performed if an elevation of the total grounding resistance above an allowable value is detectable.

11. The method of claim 1, characterized in that for monitoring a three-phase electrical AC network with or without neutral conductors, the method is performed separately for each of the three network conductors, in that the AC component of the differential current of all three network conductors is detected as a first network variable, furthermore the three AC network voltages between each network conductor and a neutral conductor, or in the absence of the latter between each network conductor and an equipotential bonding conductor, are detected, and the corresponding three products of the amplitude of the AC component of the differential current and the cosine values of the three phase angles φ

are ascertained, and the load shutoff is performed whenever one of these products exceeds the certain response value.

12. The method of one claim 1, characterized in that the load or network shutoff is performed with universal polarity.

13. The method of claim 1, characterized in that after a load or network shutoff dictated by the differential current or fault current, or some other network shutoff, the insulation resistance of the shut-off network is measured and compared with a limit value representing the normal situation;

and that the load or network is turned back on again only whenever the measured insulation resistance of the network exceeds the limit value.

14. A device for performing the method of claim 1, having a differential current sensor (3) that includes at least two network conductors of an AC network, and having a differential current relay (1), which via a power or load switch (2) performs a load or network shutoff as soon as the differential current (IΔ

) exceeds a certain response value, characterized in that

the differential current relay (1) has an electronic phase module (13), which from the AC component (IΔ

_AC) of the differential current (IΔ

) which component is ascertained via a high-pass filter (11), and from the AC network voltage or a comparison voltage derived from it, taking into account the phase angle φ

, a fault current signal (If_AC) is ascertained, which represents the AC-dictated resistive fault current of the AC network; and

that the phase module (13) is followed by a comparator module (19), which compares the fault current signal (If_AC) with a lesser response value (If_g) suitable for protecting human beings and triggers the load switch (2) to trip it if this response value is exceeded.

15. A device for monitoring insulation and fault current in an electrical alternating current (AC) network having a differential current sensor (3) that includes at least two conductors of an AC network, and having a differential current relay (1), which via a power or load switch (2) performs a load or network shutoff as soon as a differential current (IΔ

) exceeds a certain response value, characterized in that

the differential current sensor (3) detects the differential current (IΔ

) with a universal current sensitivity, including AC and DC components;

that a first low-pass filter (10) connected to the differential current sensor (3) furnishes a fault current signal (IF_DC) on its output side that represents a DC-dictated resistive fault current of the AC network;

that the differential current relay (1) has an electronic phase module (13), which from the AC component (IΔ

_AC) of the differential current (IΔ

) which component is ascertained via a high-pass filter (11), and from a comparison voltage derived from the AC network voltage, taking into account a phase angle φ

, a fault current signal (If_AC) is ascertained, which represents an AC-dictated resistive fault current of the AC network;

that a frequency-weighted second low-pass filter (15) is connected between the high-pass filter (11) and the phase module (13) and simulates the frequency dependency of the human body, which decreases as the frequency increases;

that the outputs of the first low-pass filter (10) and of the phase module (13) are connected to an adder module (14), which quadratically adds the two fault current signals (If_AC, If_DC) to form a total fault current signal (IF) on the output side that corresponds to a DC signal;

that the output of the adder module (14) is connected to the input of a comparator module (19), which compares the fault current signal (If_AC) with a lesser response value (If_g) suitable for protecting human beings and triggers the load switch (2) to trip it if this response value is exceeded.

16. The device of claim 15, characterized in that the comparator module (19) is connected on the input side, for taking the total differential current (IΔ

) into account, directly with the differential current sensor (3) as well and has two adjusting means (16, 17) for specifying a lesser response value (If_g) for the fault current signal (IF) and a greater response value (IΔ

_g) for the total differential current IΔ

.

17. The device of claim 15, characterized in that for voltage supply, the electronically constructed differential current relay (1) has a power supply (4) which is supplied from the AC network and is embodied with a constantly operating functional self-monitoring means, which even in the event of malfunctions of the differential current relay (1) leads to a load or network shutoff by the differential current relay (1).

18. The device of claim 15, characterized in that the differential current relay (1), in three-phase AC networks, has a power supply with a voltage supply that is redundant on the network side and assures that in the event of a phase failure or a neutral conductor failure, the differential current relay (1) will continue to operate as long as two phases, or one phase and the neutral conductor, are still available.

19. The device of claim 15, characterized in that a storage capacitor (5) charged by the power supply (4) is present, which in the event of failure of the internal voltage supply of the apparatus temporarily furnishes the energy for operating portions (18, 21, 22) of the differential current relay (1) and causes the load switch (2) to be tripped whenever, in a multiphase network, all the network phases except for one have failed, or in a single-phase network if the neutral conductor has failed.

20. The device of claim 15, characterized in that for monitoring the equipotential bonding conductor (PE) of the AC network, a current converter or differential current sensor (25) includes the equipotential bonding conductor (PE) and is connected to the comparator module (19), and a load or network shutoff is effected if the output signal of the current converter or differential current sensor (25) indicates an elevation of the grounding resistance (R_G) above an allowable value.

21. The device of claim 15, characterized in that for monitoring the equipotential bonding conductor (PE) of the AC network, the equipotential bonding conductor is carried through two current converters;

that the first current converter impresses a low test current into the loop formed by the equipotential bonding conductor (PE) and the grounding resistance (R_G);

that the second current converter detects this test current; and

that a load or network shutoff is effected if the output signal of the second current converter becomes less than a certain proportion that indicates an elevation of the resistance (R_PE) of the protective conductor or equipotential bonding conductor above an allowable value.

22. The device of claim 15, characterized in that the comparator module (19) is connected to a digital interface (20, 23), to which at least one additional means can be connected, such as a measurement data display means and/or an insulation monitoring device that monitors the insulation resistance of the network to be monitored in the off state, but including the consumers connected to the network, and/or a means for monitoring the resistance of the equipotential bonding conductor (PE).

23. The device of claim 15, characterized in that the fault current or differential current relay (1), the differential current sensor (3), and the load or power switch (2) are combined into a structural module, or are distributed between two structural modules with separate load switches (2), or are distributed to three structural modules.

24. The device of claim 15, characterized in that the differential current sensor (2) with universal current sensitivity for detecting the AC components and DC components has, in a manner known per se, a soft-magnetic core surrounding at least two network conductors of the AC network to be monitored, the core having two contrary windings (W1, W2) with the same number of turns and also has two electronic switches (S1, S2) that operate in alternation, and these elements are components of an oscillation circuit, and the oscillation signal on the output side, with universal current sensitivity, represents a total differential current flowing through the core.