Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
2000-10-18
2003-09-02
Le, N. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
C324S126000, C324S127000
Reexamination Certificate
active
06614218
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a current measuring device for measuring electrical current waveforms using a combination of a Rogowski coil and electronic processing equipment. In particular, this invention relates to improvements in such a device whereby the high frequency bandwidth of the measurement is increased whilst still retaining the capability of measuring low frequency currents.
2. Description of Related Art
A Rogowski coil is so named following publication in 1912 of an article titled “Die Messung der Magnitischen Spannung” (Arch Elektrotech1, pp141-150) by Rogowski W. and Steinhaus W. Its principle of operation is well known and is based on the fact that if a coil of uniformly spaced turns, wound is on a former of constant cross-sectional area, is arranged to form a closed loop, then the voltage induced in the coil at any instant is directly proportional to the rate of change of the total current passing through the loop at that instant. If means can be found of integrating with respect to time the voltage produced by the coil then the voltage obtained is proportional to the current passing through the loop. The combination of a Rogowski coil and means for integrating a voltage with respect to time thereby constitutes a current measuring system commonly referred to as a Rogowski transducer.
In practice there will be some small variation of the turns density of the coil and of the cross sectional area of the former. As a result the voltage produced by the coil will be slightly dependent on the position of the current in relation to the Rogowski coil loop. It is understood that reference in this specification to a Rogowski coil includes these practical tolerances.
The former around which the coil is wound is normally non-magnetic but it may be magnetic provided that the relative permeability of the magnetic material used is sufficiently small such that the material does not magnetically saturate when used to carry a Rogowski coil.
A Rogowski transducer has the advantages that the coil may be looped around a conductor, without the necessity of disconnecting the conductor; to provide a contactless and isolated current measurement and that large currents can be measured without magnetically saturating the transducer. A further advantage is that due to the use of non-magnetic material (which does not suffer from energy losses that increase with frequency), a Rogowski transducer potentially has a very high bandwidth, significantly in excess of 1 MHz and is therefore able to measure very rapidly changing currents.
Examples of known Rogowski transducers are described in the publications by Ray W F and Davis R M: “Wideband Rogowski current transducers: Part 1—The Rogowski Coil”, EPE Journal, Vol 3, No.1, March 1993, pp 51-59, and Ray W F: “Wideband Rogowski current transducers: Part 2 “The Integrator”, EPE Journal, Vol 3, No.2, June 1993, pp116-122.
The means for integrating the Rogowski coil voltage with respect to time may take various forms, some called “passive” means in as much as the means only utilises passive electrical components such as capacitors and resistors. Others are called “active” means in that the means also utilises active electronic components, such as semi-conducting devices and integrated circuits.
FIG. 1
shows a Rogowski current transducer that was proposed by J. A. J. Pettinga and J. Siersema in their paper “A polyphase 500 kA current measuring system with Rogowski coils”, Proc IEE, Vol 130, Pt B, No. 5, September 83, pp 360-363. This measuring system incorporates two types of passive integration, which is relevant at high frequencies, and integration using a conventional non-inverting operational amplifier, which is called “active” integration, for low frequencies.
In the circuit of
FIG. 1
, A represents the coil with distributed inductance L and capacitance C. The coil is connected to the remainder of the circuit by a co-axial cable terminated by a resistor R
C
of 50&OHgr;, which is the characteristic impedance of the cable such that the terminating resistance seen by the coil is R
C
. The components R
3
, R
4
and C
2
comprise a passive integration network for which R
3
>>R
C
and R
3
>>R
4
. A non-inverting operational amplifier circuit D acts as an integrator at low frequencies and as a unity gain amplifier at high frequencies.
FIG. 2
shows the overall frequency characteristic for the integration which falls into three bands—active integration for frequencies f in the range f
0
<f<f
1
, passive CR integration for f
1
<f<f
2
and passive L/R integration for f>f
2
.
The resistors and capacitors of
FIG. 1
are chosen such that for each frequency band the following behaviour occurs:
(a) The resistance R
1
is relatively large and its presence is ineffective for frequencies f>f
0
.
(b) For f
0
<<f<<f
1
the impedance of L and the admittance of C
2
are negligible and the voltage V
+
at the non-inverting input of the operational amplifier is substantially the same as the voltage E induced in the coil. For integrator gains greater than 1 the behaviour of integrator D is represented by the well known relationship:
V
out
=
1
C
1
R
2
∫
E
ⅆ
t
(
1
)
(c) For f
1
<<f<f
2
the impedance of L and C
1
are negligible and the circuit D acts as a unity gain amplifier. Since the impedance of R
4
is also negligible, the network R
3
−C
2
behaves as a passive integrator.
(d) For f>>f
2
circuit D continues to act as a unity gain amplifier and the impedance of C
2
is negligible compared with that of R
4
. The network L-R
C
behaves as a passive L/R integrator. This is the type of integration that has been favoured in other known Rogowski transducers.
It will be appreciated that there are significant design constraints on the relative values for the resistors and capacitors used in order to provide the required transition of behaviour from one mode of integration to the next Furthermore, in order to provide a straight-line gain-frequency relationship for the integration as shown in
FIG. 2
it is important that two pairs of time constants are accurately matched, namely
R
2
C
1
=
R
3
C
2
and
L
R
c
=
R
4
C
2
(
2
)
The matching requirement presents difficulties both in the design and in the practical setting-up and calibration of a transducer. This is disadvantageous.
The circuit of
FIG. 1
suffers from several additional disadvantages that have not until now been appreciated.
Firstly the circuit of
FIG. 1
uses L/R integration. It is commonly thought that L/R integration does not result in unwanted signal oscillations. However, this is because previously published analysis of Rogowski transducers that utilise L/R integration is based on a symmetrical arrangement in which the coil loop is circular, the current to be measured lies along the axis of this circular loop and there are no other currents close to the coil. In this special case each element of the coil generates the same elemental voltage and the transit times from each element to the coil termination can be averaged to produce a smooth output voltage. The coil is therefore not susceptible to oscillations. As a result, L/R integration has been utilised in prior art transducers with satisfactory results provided care is taken to ensure a symmetrical geometry as described above. However, to utilise a symmetrical arrangement it is generally necessary to use a coil with a rigid former that cannot be opened and is therefore less convenient. Furthermore, when measuring currents in closely spaced equipment it is difficult and often impossible to arrange for the current to be central and co-axial with the coil loop. There are also often currents other than the current being measured which are close to the coil. In practice, therefore, the arrangement is not symmetrical. When this is the case, the coil is susceptible to oscillations. One solution that has been proposed is to design the coil so that its natural frequenc
Dicke Billig & Czaja, PLLC
Lair Donald M
Le N.
Power Electronic Measurements Limited
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