Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With coupling means
Reexamination Certificate
1999-04-14
2002-02-12
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With coupling means
Reexamination Certificate
active
06346805
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention concerns a method for current measurement, especially in gas-insulated voltage switchgear, for example, in a high or medium-voltage unit. It also pertains to a current measurement configuration for execution of the method.
A so-called measurement transformer or measurement sensor is ordinarily used as a current transformer to measure currents in such gas-insulated switchgear, in which a gas referred to as SF
6
is generally used as an insulating gas. The measurement transformer essentially includes, as an information sensor, a coil that concentrically surrounds a current conductor of the unit that conducts a primary current. A secondary current flowing in the coil is then referred to as measurement signal, which is processed in a subassembly of an automation system also referred to as process control technique for the switchgear (LSA).
As described in the essay “20 Years of Research in the Field of Nonconventional Transformers”, in ELEKTRIE, Berlin 43 (1989) 6, pages 205 to 207, two different transformer principles can be distinguished. In a so-called conventional transformer, the information sensor and a power supply connected to it are coupled on the primary side to the current conductor of the unit and connected together to the actual load on the secondary side. However, the transformer requires large installation volume. On the other hand, in a so-called non-conventional transformer only the information sensor is coupled to the current conductor of the unit on the primary side. An auxiliary power supply is therefore required in the non-conventional transformer, which is capable of producing a power of up to a few kW, at least for a short time, to maintain the desired nominal output currents of the secondary circuit of 5 or 1. A power amplifier having a corresponding parameter, via which the information sensor is connected to the load, is therefore required. An optical solution is particularly suited for this type of transformer, but can only be implemented at a very high cost.
An intermediate solution is therefore often sought, which underlies the ordinary or conventional measurement principle, but at a low-power level. A low-power measurement transformer based on the intermediate solution, however, is particularly complicated to manufacture, especially if a large saturation-free time is required.
It is also known from Published, European Patent Application 0 510 311 A2, corresponding to U.S. Pat. No. 5,272,460, that a current transformer that operates according to the principle of a Rogowski coil can be used for a metal-encapsulated, gas-insulated high-voltage unit. A current measurement configuration according to the principle of the Rogowski coil is also known from Published, Non-Prosecuted German Patent Application DE 44 24 368 A1. To achieve high imaging trueness in the measurement range while using a Rogowski coil, high expenditure, with correspondingly high costs, is required. The reason for this is that the measurement accuracy of the Rogowski coil, on the one hand, is determined by a number of factors, and that special precautions are required, on the other hand, for high reliability and constancy of parameters, owing to the system-related off-line configuration.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a configuration for current measurement that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for measuring current, which includes: comparing a first signal generated by a first low-power measurement transformer having a magnetic core and a second signal generated by a second nonmagnetic measurement transformer; selecting and further processing one of the first signal and the second signal in dependence on an imaging accuracy of a measured current; and evaluating a deviation between the first signal and the second signal as comparison criterion.
The underlying task of the invention is therefore to provide a particularly suited method and a particularly suited configuration for precise measurement of currents with high dynamic range.
The task is solved with reference to the method by comparing a first signal of a first low-power measurement transformer with a magnetic core and a second signal of a second nonmagnetic, especially optical, measurement transformer. Depending on the accuracy of imaging of the measured current, one of the two signals is then selected and further processed. A deviation of the two signals emitted by the measurement transformers of different type from each other is evaluated as comparison criterion.
The invention starts from the consideration that accurate measurement of currents with high dynamic range can be achieved at low cost and limited dimensions by using a combined current measurement element. Only a sensor or transformer is to be operated in the actual measurement range, whereas another sensor is used, essentially in the overcurrent range. The imaging trueness or accuracy in the measurement range can be guaranteed by a low-powered measurement transformer, whereas a Rogowski coil or an optical measurement transformer, operating according to the Faraday effect, takes over imaging of high, stationary or displaced currents. Rotation of the plane of polarization of linearly polarized light occurs, according to the Faraday effect in an optically inactive material in the presence of a magnetic field parallel to the direction of light propagation.
The signals recorded separately by the two measurement transformers, i.e., their output voltages, are expediently processed together. The output voltage of the low-power measurement transformer drops in the event of its saturation. The gradient, for example, the first or second derivative, of its output voltage is then greater than the gradient of the output voltage of the measurement transformer operating according to the principle of the Rogowski coil or according to the Faraday effect. The derivative of output voltages, according to time, can therefore be referred to as a gauge of the accuracy of the two signals. The gradient of the output voltages or output signals of the two measurement transformers is therefore expediently considered as criterion for switching the output signal of the low-power measurement transformer to the output signal of the Rogowski coil or the optical measurement transformer. As an alternative or in addition, a check and evaluation of characteristic features of the two output signals can occur by a fuzzy controller and/or a neuronal net, to form a switching criterion and thus to determine the degree of accuracy.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for measuring current, including: a first low-power measurement transformer having a magnetic core and recording a primary current conducted in a conductor traversed by current and converting the primary current into a first secondary current; a second nonmagnetic measurement transformer for recording the primary current conducted in the conductor traversed by the current and converting the primary current into a second secondary current; a processing component; an input component connected to and downstream from the first low-power measurement transformer for converting the first secondary current of the first low-power measurement transformer to a first output signal and feeding the first output signal to the processing component; a series circuit connected to and downstream of the second nonmagnetic measurement transformer and including an integrator component and a signal amplifier for converting the second secondary current of the second nonmagnetic measurement transformer into a second output signal and feeding the second output signal to the processing component; and the processing component switching from processing the first output signal to the second output signal
Bauerschmidt Peter
Ermisch Jochen
Stephan Uwe
Thieme Frank
Greenberg Laurence A.
Kerveros James C
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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