Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Plural inputs
Reexamination Certificate
1999-09-28
2001-08-14
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Plural inputs
C324S141000, C324S120000, C702S061000
Reexamination Certificate
active
06275021
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the field of electrical engineering. More specifically, the invention relates to an electricity meter having signal channels which are passed to a digital signal processing device and each of which has an analog/digital converter. Voltage signals of an electrical consumer, which are detected by means of transformers, can thereby be connected to a first input of the analog/digital converter of a first signal channel, and current signals of an electrical consumer, which are detected by means of transformers, can be connected to a first input of the analog/digital converter of the second signal channel. In this context, the term current signals also refers to voltage signals that are proportional to the respective currents. Such an electricity meter is described, for example, in my earlier German patent DE 195 26 723 C1. In that electricity meter, which is designed for a high level of self-monitoring of its measurement or signal channels, the signals which are associated with the respective different signal channels are used to form comparison values. This allows a plausibility check to be carried out on the detected measured values or signals. In this way, it is possible, for example, to find out whether an internal or external fault is present, in which case logic operations in the digital signal processing device may allow the fault to be traced in more detail, or localized.
German patent DE 740 487 C discloses a rotary current three-phase meter which, by using an Aron (two-wattmeter) circuit, can be used for three-conductor and four-conductor connection.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a electricity meter, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be used both as a four-conductor meter and as a three-conductor meter while having a simple circuit design at the same time. It is a further object of the invention to disclose a particularly suitable method of operating the electricity meter.
With the foregoing and other objects in view there is provided, in accordance with the invention, an electricity meter, comprising:
a digital signal processing device;
first and second signal channels each connected to the digital signal processing device;
a first analog/digital converter connected in the first signal channel and having a first input receiving voltage signals of an electrical consumer, which are detected by means of transformers, and a second input forming a difference input with the first input;
a second analog/digital converter connected in the second signal channel and having a first input receiving current signals of the electrical consumer, which are detected by means of transformers; and
a changeover switch connected upstream of the second input of the first analog/digital converter for selectively passing one of the voltage signals or a neutral conductor signal to the second input.
The invention is based on the idea that the electricity meter, i.e. the circuit on which the meter is based, can be used not only for operation as a three-conductor meter but also for operation as a four-conductor meter, provided the circuitry for the so-called Aron circuit (two-watt meter circuit) is incorporated. This can in turn be achieved in a simple manner if it is possible to detect directly difference voltages in the signal channel which detects the voltage signals. By suitable configuration, it is then possible to find out whether operation as a four-conductor meter with a neutral conductor connected or operation as a three-conductor meter without a neutral conductor being connected is envisaged. A virtual star point is then formed for three-conductor operation.
When operated as a four-conductor meter with a neutral conductor connected, the voltage signals of all three phases (R, S, T) are detected while, when operated as a three-conductor meter, the difference voltages between the phases R and S as well as T and S are detected directly. For this purpose, the analog/digital converter in the signal channel for the voltage values (voltage channel) has a second input, to which the voltage signal from, expediently, phase S can be connected for operation as a three-conductor meter. For operation as a four-conductor meter, a neutral conductor potential is connected to this input as the input signal.
In accordance with an added feature of the invention, the changeover switch is a multiplexer.
In accordance with an additional feature of the invention, a multiplexer is connected upstream of each first input of the analog/digital converter in each channel, the multiplexer detecting three-phase measured values on an input side thereof.
In other words, multiplexers are preferred as the changeover switches for the individual voltage and current signals. By means of these multiplexers, it is expediently possible to supply a neutral conductor signal not only to both inputs of the analog/digital converter in the voltage channel, but also to the first input of the analog/digital converter in the second signal channel (current channel).
The changeover switch or multiplexer associated with the second input of the analog/digital converter in the voltage channel, is designed to detect one phase and the neutral conductor signal. In contrast, the number of inputs of the multiplexer which is associated with the respective first input of the analog/digital converter in the respective signal channel is designed to detect three-phase measured values. In this case, the multiplexers which are connected upstream of the analog/digital converters are expediently synchronized to one another. It is thus possible to compare simultaneous measured values for signals. This applies in particular to the formation of power values for consumption measurement.
In accordance with a further feature of the invention, when the voltage signal is connected to the second input of the first analog/digital converter, the first input is connected to another one of the voltage signals, and the first analog/digital converter outputs a corresponding difference voltage on its output side.
In accordance with again an added feature of the invention, a decimation filter is connected in each of the first and second signal channels between the analog/digital converter and the digital signal processing device.
The analog/digital converters are expediently sigma-delta modulators, preferably of the second order. In that case it is an advantageous refinement to have the decimation filter connected downstream of each analog/digital converter. The decimation filter converts a bit stream at a high sampling rate emitted by the analog/digital converter into a digital signal which is available for further signal processing and has a comparatively low sampling rate, but high resolution.
In accordance with again an additional feature of the invention, a high-pass filter is connected in the second signal channel. While the voltage channel is high-pass filtered with a variable time constant, expediently within the digital signal processing device, the separate high-pass filter in the second channel filters out direct-current elements. The separate high-pass is connected upstream of the signal processing device, within the current channel.
In accordance with again another feature of the invention, a common signal clock apparatus is connected to the first and second analog/digital converters and to the digital signal processing device. The clock apparatus provides a system clock for both the signal processing device and for the analog/digital converters. The clock apparatus is expediently a divider, which divides an external clock source downward by a specific ratio.
With the above and other objects in view there is also provided, in accordance with the invention, a method of operating the above-described electricity meter. The method comprises the following steps:
selectively operating the electricity meter in a four-conductor meter mode or in a three-c
Deb Anjan K
Greenberg Laurence A.
Lerner Herbert L.
Metjahic Safet
Siemens Aktiengesellschaft
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