Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-04-14
2002-05-21
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S509000, C324S551000, C361S102000, C361S094000
Reexamination Certificate
active
06392422
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and a device for monitoring insulation and fault current in an electrical alternating current (AC) network, in which the differential current, formed by vectorial addition, between at least two network conductors is ascertained, and a load shutoff is performed whenever the differential current exceeds a certain response value.
BACKGROUND OF THE INVENTION
In electrical networks, because of defective insulation, fault currents can flow out via ground or via a protective conductor. The voltage drop generated by the fault current can be dangerous to human beings at parts that are touchable but in normal operation are voltage-free (indirect touch). If open, voltage-carrying parts of a current circuit are touched directly, a fault current can flow via human beings and is limited only by the resistance of the human body. Aside from harmed human beings, fault currents can also cause property damage, by influencing electrical systems or by the development of heat energy at the point of the fault. To protect against danger to human beings and property damage from fault currents, in addition to other protective provisions, fault current protection switches (FI protection switches or RCDs) are used. These devices, via a summation current converter, form the vectorial sum of the currents of the network conductors, and, from the outcome of the total differential current, its amount. The total differential current can include AC components and, when DC consumers are connected, such as drives with frequency rectifiers and a DC intermediate circuit, they can also contain DC components. If the total differential current exceeds a certain limit or response value, then the defective current circuit is turned off.
Fault current protective switches, as is generally known to professionals in the field, can be used only with networks of a certain size, because otherwise the natural capacitive leakage currents become greater than the necessary fault current limit value for protecting human beings. The consequences are defective or unintended or unnecessary load shutoffs.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to embody a method of the type defined at the outset such that by distinguishing resistive fault currents from normal capacitive network leakage currents, it can be employed without problems even in relatively large AC networks, and that even in smaller networks, it allows more-accurate monitoring of the fault currents.
This and other objects are attained in accordance with the present invention, wherein the AC component of the differential current is detected as a first network variable; the AC network voltage between at least two network conductors, or between one network conductor and an equipotential bonding conductor or a neutral conductor, is detected as a second network variable; the product of the amplitude of the AC component of the differential current and the cosine of the phase angle &PHgr; between the two network variables detected is ascertained as a measure of the resistive fault current of the network; and the load shutoff is performed whenever the ascertained product exceeds a certain response value.
Such a method is extremely versatile in use and allows both reliable and accurate network monitoring in a way that is not vulnerable to malfunction, in single or multiphase AC networks. This is also particularly true for relatively large AC networks with correspondingly larger natural (capacitive) network leakage currents, in which professionals in the field had until now assumed that reliable fault current or insulation monitoring by means of fault current protection switches and summation current converters was impossible.
In electrical AC networks, consumers that are capable of generating direct leakage and fault currents are used more and more often and actually uncontrollably. These direct currents are caused by electronic elements, such as rectifiers, thyristors, TRIACs or transistors, which are used to convert the alternating voltage into a direct voltage or into an alternating voltage of a different frequency. If insulation faults occur behind these elements, then the fault current includes major DC components. Examples of such devices are primary-clocked switched-mode power supplies in electrical equipment, rectifiers and interrupt-free power supplies or frequency inverters for variable-rpm motor drives, which are all being used increasingly.
It is thus especially advantageous for safety reasons to be able to take into account not only resistive AC components of the network, but also, in accordance with claim
8
, to provide differential current detection that is sensitive to universal current, that is, one in which even DC components, which must always be assessed as being resistive, can be taken into account. This makes substantially more versatile and safe use possible, even in the presently typical AC networks that have considerable DC components.
It is advantageous for the differential current, detected with universal current sensitivity, to be split into an AC component and a DC component, the latter always to be addressed as a fault current, as well as to ascertain the resistive AC fault current signal from the AC component and then to add the resistive AC and DC fault currents quadratically. Preferably, the AC component after being filtered out is immediately subjected to frequency weighting for the sake of protecting human beings, an example being low-pass filtration that simulates the frequency dependency of the human body.
According to another aspect of the present invention, two limit or response values of different magnitude are provided, namely a lesser one, of 30 mA, for instance, for a comparison with the ascertained resistive fault current, and a greater one, for instance of 300 mA, for a comparison with the total differential current that has been detected with universal current sensitivity. If the limit value is exceeded, a load or network shutoff is effected. The limit values can be adjustable, and they can also be adapted flexibly to prevailing network conditions.
In extensive networks, until now, it was impossible to use fault current devices with limit values for protecting human beings, if for no other reason because the natural leakage current, which was present because of how the installation is constructed and in many cases was not reducible, was above the limit value of the protection device. In many applications, such as on construction sites with cables that can be damaged during the work, this is highly problematic and even today often leads to accidents. Since in the present method the resistive fault current is ascertained in a targeted way as part of the total differential current and evaluated, for the first time appropriate distinctions and separate monitoring for protecting human beings and protecting equipment are possible. No ways of attaining this in a similar way were known, even though the problem of high leakage currents, which initially made it seem impossible to protect human beings via an FI or fault current protective switch, had already existed ever since there had been electrical distribution networks.
Two different limit or response values are provided, which are also differently frequency-weighted, to make it possible simultaneously to protect both human beings and property. Until now, either only a fault current protection provision with a low limit valve (such as 30 mA) could be used for protecting human beings, or a property protection provision with a higher limit value (such as 300 mA) could be used. In the known passive devices until now, there was also no possibility of frequency weighting using a low-pass filter for protecting human beings and a universal pass filter for protecting property, because these passive devices had to be optimized to the network frequency because of the necessary sensitivity and therefore have only a very narrow bandwidth (approximately 30 Hz to a maximum of one kHz) even in experiments with activ
Hackl Dieter
Kammer Michael
Kaul Karl-Hans
Brown Glenn W.
Dip.-Ing. Walther Bender GmbH & Co. KG
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Hamdan Wasseem H.
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