Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-01-21
2003-01-28
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S714000, C324S704000, C324S705000, C324S720000, C324S543000, C324S650000
Reexamination Certificate
active
06512383
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a system and a method for measuring the alternating current equivalent series resistance of a conductor, in particular when transporting a large current, i.e. of the order of a few thousand Amperes (around 3000 A).
When carrying an alternating electric current, at a frequency of 50 Hz for example, a conductor will exhibit an impedance having a real or active component and an imaginary or reactive component. Measurement of the alternating-current resistance refers to the value, per unit length (&OHgr;/m), of the real component of the impedance of the conductor.
Today, as a result of the rapid increase in the power required by electrical systems, cables are made for high voltage with conductors of greater than 1000 mm
2
cross-section. In order to be able to assess the performance of a cable of this kind and quantify the magnitude of the power losses it is important to know the value of the alternating current equivalent, series resistance of a conductor.
With conductors of such dimensions the nonuniform distribution of current within the cross-section causes a considerable rise in the alternating current equivalent series resistance. As is known, this phenomenon is due principally to two effects referred to as the skin effect and the proximity effect.
The skin effect corresponds to the tendency of the alternating current to flow close to the surface of a conductor, thereby reducing the useful cross-section for passage of the current and increasing the resistance thereof.
The proximity effect entails a redistribution of the current in the conductor, due to the closeness of another conductor.
Considering the difficulty of applying traditional methods of calculating resistance, such as those discussed in the articles listed below and from the CEI (Commission Electronique International) 287 standard, to the conductors used in practice, made up of a very large number of wires more or less insulated from one another, the only means of assessing the alternating current equivalent series resistance is through experimental methods.
The articles relating to the methods of calculation are: “Eddy current losses in single-conductor paper insulated lead covered unarmoured cables of single-phase system”, A. H. Arnold, Vol. 89, Part II, J. IEE, p. 636, 1942; and “Proximity effect in solid and hollow round conductors”, A. H. Arnold, Vol. 88, Part. II, J. IEE, p. 349-359, 1941.
Measurement of the alternating-current resistance is of considerable interest both in the course of research, where it is used to improve the design of the conductor, and in industry, for testing the finished product.
In particular, the method used must guarantee the typical repeatability and accuracy of the methods employed in the course of research, but must be sufficiently simple to be industrially applicable.
Measurement of the alternating-current resistance must take into account the temperature of the cable, the frequency of the flowing current, and the closeness of other conductors.
The alternating-current resistance of cables of 1000 mm
2
cross-section is of the order of 10
−4
−10
−5
&OHgr;/m and the accuracy of the measurement should be at least 0.1%.
One technique for measuring the alternating-current resistance makes use of networks of the bridge type on account of their simplicity and the absence of initial calibrations.
A bridge network consists of a quadrilateral of impedances, one of which is unknown. A null indicator (normally consisting of a galvanometer) is inserted into one of the diagonals, and the power supply into the other. By modifying the value of one or more arms, of known value, so as to zero the null indicator, the value of the unknown impedance is derived from the value of the other impedances. The accuracy of a bridge system depends directly on the accuracy of the known impedances.
For example, an accuracy of measurement of around 0.2% is achievable with impedances having an accuracy of 0.1%. Better accuracies can be obtained only with special preliminary calibrations.
Furthermore, if harmonic contributions at frequencies higher than the working frequency are present in the current flowing in the conductor, as normally happens, measurement with the bridge could overestimate the value of the resistance. The article by F. Castelli, L. Maciotta-Rolandin, P. Riner entitled “A new method for measuring the AC resistance of large cable conductors”, published in March-April 1977 in IEEE Transactions on Power Apparatus and Systems, vol. PAS-96, No. 2 pp. 414-422, describes a bridge for measuring alternating-current resistance, based on the so-called Maxwell bridge which uses a transformer in one arm in such a way that the measurement bridge is not traversed by the high current of the conductor.
The measurement of the alternating-current resistance can be derived from the ratio between the real component of the voltage withdrawn over a predetermined length of the conductor and the current flowing in this conductor. With the current flowing in the conductor known, the measurement of the voltage can be effected with an instrument capable of discriminating and measuring the real component from the imaginary one. An instrument of this type is the so-called lock-in amplifier, such as for example that sold by Stanford Research Systems, 1290-D Reamwood Ave., Sunnyvale, Calif., model SR-830.
This amplifier has a measurement accuracy (or gain accuracy) equal to 1%, deemed insufficient for measuring alternating-current resistance.
German patent DE-1,067,924 discloses a network test device for determining the short circuit current intensity in a network of electrical conductors. In that device a load resistor is syncronously connected and disconnected with a frequency depending on the network frequency. The load resistor temporarily lowers the network voltage. An indicator shows the voltage difference between the connected and the disconnected status. The voltage of the periodically loaded network is sent to two channels. A first channel comprises a variable delay line, a second channel comprises a variable attenuator. The average of the sum (or difference) voltage between the two channels is measured by a rectifier instrument. The two channels are then equalized so that the instrument gives a zero reading in case of unloaded network. The frequency of connection and disconnection of the load resistor can be different from the network frequency, e.g., one half or one third or even double the network frequency.
German patent DE-1,073,621 disclose a method for measuring the internal network resistance (impedance and phase angle) at the network frequency. The method employs a measuring voltage at a frequency higher (harmonic) than the network frequency and a dummy load that is switched to the network terminals with the network frequency. The load current is flown in a compensation device comprising a variometer with a switchable conversion ratio and an ohmic resistance, from which a sum voltage is derived. The sum voltage has a component in phase with the load current and a component advancing in phase by 900 the load current and is adjustable in intensity by the variometer. The sum voltage is switched against a voltage derived from the network voltage. The signal resulting from two voltages is bandpass filtered at the frequency of the measuring voltage and read in an instrument. The variometer and a potentiometer are adjusted until a null reading is achieved on the instrument. The phase angle measurement is then carried out by reading the variometer setting. The instrument is then switched to measure the sum voltage, while at the same time the variometer conversion ratio is switched to a second value. A measurement of the internal impedance of the network is so derived.
The Applicant has found that the measurement accuracy can be greatly increased, beyond the accuracy limit of the available instrument, by measuring with the latter not the value of the quantity to be measured, but rather the difference between the said quantity and a kn
Coletta Giacomo
Gherardi Laura
Gömöry Fedor
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Hamdan Wasseem H.
Le N.
Pirelli Cavi e Sistemi S.p.A.
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