Thermal measuring and testing – Temperature measurement – Combined with diverse art device
Reexamination Certificate
1999-05-11
2001-05-01
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Temperature measurement
Combined with diverse art device
C374S183000, C374S170000, C374S172000, C340S595000, C340S870170, C702S130000
Reexamination Certificate
active
06224255
ABSTRACT:
FIELD OF THE INVENTION
Our present invention relates to a temperature pickup for measuring the temperature of an oil-filled vessel, especially for measurement of the oil temperature of a power transformer or tap changer. The invention also relates to a method of measuring a temperature and to a method of using such a temperature pickup.
BACKGROUND OF THE INVENTION
For the measurement of the oil temperature of a transformer and a tap changer, for example, the use of a resistance thermometer is common. Such resistance thermometers are available from a number of manufacturers and have generally standard dimensions and construction. In Germany, for example, the resistance thermometer must conform to the German Industrial Standard DIN 42554 and in other countries similar standards are applicable. In the United States and Canada, for instance, standard screw threads such as ⅜ inch NPT or ⅞-14 UNF are customary. The resistance thermometers which can be used are those made by Messko Hauser, Thermometerwerk Geraberg and Sika GmbH, all of Germany, and a Qualitrol Corp., of New York in the United States. Qualitrol supplies, for oil-filled power transformers and tap changers, thermometers of the Series 165/167, 104, 104-400. These units are described in the Qualitrol Corp. brochures QT-150, QT-165, QT-104-2, QT 104-321 and QT-104-400. Such units can have a housing or body provided with the electronic circuitry and a stem in which the resistor is housed, a screw thread being provided adjacent a prismatic formation which can be engaged by a wrench. The housing may be provided with connectors allowing cabling to electronic circuitry.
The thermometer is normally threaded into a thermometer pocket of the apparatus and can have a lower portion as a perforated protected tube in which a replaceable sensor insert is received. A vent can be provided on the housing as well.
Resistance thermometers of the aforementioned type have a number of drawbacks in the form known from the aforementioned brochures and the housing can have three or four conductor connectors as may be required or desirable.
Because of the contact resistances, the use of electromagnetic interference circuitry and the like, the measurement results of such systems can be falsified, especially as a result of time constants of various components of the electromagnetic interference circuitry. Such circuitry is frequently required to prevent interference from electromagnetic environments.
The output of the resistance thermometer frequently requires additional electronic processing to produce an output which can be provided to a display device or which is suitable as an input to a computer. This is a consequence of the fact that the output signals of some of the known resistance thermometers mentioned above are not compatible for computers or such display devices. Conventional resistance thermometers have a resistance variation &Dgr;R which is dependent upon the temperature e so that &Dgr;R=f (&thgr;). Electronic devices generally require standard well-defined signal levels, for example in the range of 0 to 10V or 0 to 20 mA or 0 . . . 1 mA. As a result, signal processing is practically always required to provide signals in this range for the downstream electronic units.
Finally, monitoring of the functioning of a resistance thermometer and thus the reliability and precision of the temperature measurement in use is not possible. In the past resistance thermometers have only been capable of being monitored by a separate reference measurement off line.
OBJECTS OF THE INVENTION
It is the principal object of the present invention to provide a temperature measurement pickup which avoids these drawbacks and can provide a precise, reliable temperature measurement, remote transmission of the measured signal and further processing of the measurement influence in a convenient manner.
It is also an object of the invention to provide an improved method of measuring temperature in an oil-filled vessel, especially a power transformer or a tap changer whereby drawbacks of earlier methods are avoided.
SUMMARY OF THE INVENTION
These objects are attained, in accordance with the present invention, with a temperature measurement pickup for an oil-filled vessel, especially a power transformer or a tap changer which comprises:
a standard resistance-thermometer housing;
a temperature sensor in the housing having a resistance temperature detector in contact with oil in the oil-filled vessel and generating a duty-factor-modulated output signal;
an interface circuit in the housing connected to the temperature sensor and receiving the output signal therefrom, the interface circuit subjecting the output signal to polarity reversal to produce a ground-potential-symmetrical output for detection of zero passages thereof; and
electromagnetic interference circuitry in the housing connected to the temperature sensor and to the interface circuit.
The method of the invention can comprise the steps of:
(a) initially generating a temperature-dependent duty-factor-modulated output signal representing a temperature in the oil-filled vessel;
(b) subjecting the temperature-dependent duty-factor-modulated output signal to polarity reversal at a beginning of each flank thereof to produce a ground-potential-symmetrical output;
(c) detecting successive zero passages of the ground- potential-symmetrical output;
(d) determining time intervals between successive zero passages; and
(e) calculating a duty factor from a ratio of the time intervals and a temperature in the oil-filled vessel from the duty factor.
The term duty-factor-modulated refers to a modulation based upon the ratio between the “high” and “low” flanks within a pulse so that g(T)=t
d
/t
p
and the temperature is given by T=212.766 (t
d
/t
p
−0.32) in degrees Celsius. It will be apparent that duty factor modulation is a special case of pulse width modulation where the full cycle has a period t
p
and the pulse has a width t
d
. Such a signal has been shown in FIG.
4
.
Duty-factor-modulated temperature sensors are manufactured, for example, by the UK firm, Ginsbury, although, as far as we are aware, such duty factor modulation with resistance thermometers has not been used for thermometer measurements of oil-filled vessels like power transformers and tap changers heretofore in which the sensor is set into the thermometer well of the unit.
According to a feature of the invention, all of the components of the pickup are incorporated in the standard conventional housing of such a resistance thermometer. This has insured full compatibility of the processing circuitry with the resistance thermometer and allows the resistance thermometer to have full capacity while enabling it to be fitted into the thermometer pocket of the transformer. It also allows retrofitting of a resistance thermometer with the processing circuitry of the invention. The pickup of the invention has a number of advantages. For example, it enables self-monitoring in a simple manner since a defect in the generated carrier frequency in a temperature proportional duty-factor-modulated output signal is readily ascertained in the case of an intact electric current supply.
Converters such as analog/digital converters are not required and the path lengths to a remote display or to further processing units for the digital measurement signals which are produced can be substantially greater than is customary. The measurement signal is available in digital form and can be directly processed by a microprocessor.
The method of the invention supplies a temperature-dependent duty-factor-modulated output. In other words pulses are generated whose keying ratios are direct measurements for the actual temperature. This signal can be provided with electromagnetic interference protection utilizing resistance-capacitance (RC) or inductive capacitance (LC) networks. The result of the use of such time constant networks is a flank slurring of the duty-factor-modulated signals without the present invention, such time constant networks can give
Stadelmayer Manfred
Viereck Karsten
Dubno Herbert
Gutierrez Diego
Maschinenfabrik Reinhausen GmbH
Pruchnic Jr. Stanley J.
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