US 6,392,422 B1Granted

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

AssigneeDip.-Ing. Walther Bender GmbH & Co. KG

InventorsMichael Kammer, Karl-Hans Kaul, Dieter Hackl

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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.

Claims24

Dependent claims

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.

6 more elements

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 (IFDC) 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 (IΔAC) 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 (IfAC) 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 (IfAC, IfDC) 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 (IfAC) with a lesser response value (Ifg) suitable for protecting human beings and triggers the load switch (2) to trip it if this response value is exceeded.

3 more elements

Dependencies are inferred from the claim text.

Facts

Application: 09/446,350Filed 14 Apr 2000
Priority: DE 197 25 53217 Jun 1997
Granted: 21 May 2002
Term ends: Not estimatedThe source records don’t support an estimate.

Classification6 codes

H02H 1/06
Details of emergency protective circuit arrangements: arrangements for supplying operative power
H02H 3/337
Using summation current transformers: avoiding disconnection due to reactive fault currents
H02H 5/105
Responsive to mechanical injury, e.g. rupture of line, breakage of earth connection: responsive to deterioration or interruption of earth connection
H02H 3/332
Using summation current transformers: with means responsive to DC component in the fault current

Family

No related US applications are recorded in the indexed filing history.

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