U.S. patent application number 09/092657 was filed with the patent office on 2002-01-17 for method for detecting short-circuit conditions and device which uses this method.
Invention is credited to LORITO, FABRIZIO.
Application Number | 20020005721 09/092657 |
Document ID | / |
Family ID | 26331451 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005721 |
Kind Code |
A1 |
LORITO, FABRIZIO |
January 17, 2002 |
METHOD FOR DETECTING SHORT-CIRCUIT CONDITIONS AND DEVICE WHICH USES
THIS METHOD
Abstract
Method and device for the rapid detection of short-circuits in
an electrical network. The method is based on estimating the
electrical characteristics of the short-circuit load and
calculating the peak value of the current on the basis of N
successive samplings of the instantaneous current (i(t)). If the
calculated value, typically available before the current actually
attains this value, is greater than a pre-established threshold
(Icc), a signal (C) is emitted indicative of a short-circuit fault.
The calculation of the peak value may occur automatically for
example upon switching on, or be subject to the exceeding of a
guard threshold (Is) by the instantaneous current (i(t)). Moreover,
the method makes it possible concurrently to determine the value of
the power factor of the short-circuit (cos .phi.) and the phase of
the voltage at the moment of the short-circuit.
Inventors: |
LORITO, FABRIZIO; (MILAN,
IT) |
Correspondence
Address: |
LAWRENCE G KURLAND
BRYAN CAVE
245 PARK AVENUE
NEW YORK
NY
101670034
|
Family ID: |
26331451 |
Appl. No.: |
09/092657 |
Filed: |
June 5, 1998 |
Current U.S.
Class: |
324/424 |
Current CPC
Class: |
G01R 31/52 20200101;
H02H 3/44 20130101 |
Class at
Publication: |
324/424 |
International
Class: |
G01R 031/02; G01R
031/327 |
Claims
What is claimed is:
1. method for detecting short-circuit conditions in an electrical
network comprising at least one voltage source connected to at
least one load, comprising the following operational phases a)
measuring the values of the instantaneous current flowing in the
load and of its first derivative, b) performing n successive
samplings of the values of the instantaneous current, c) estimating
the effective parameters of the circuit viewed by the said source,
d) generating a short-circuit detection signal when the value of
the estimated load is such as to cause the steady peak current to
exceed an assigned threshold level
2. method according to claim 1, wherein the said phase (b) can be
subject to the attaining, by the instantaneous value of the
instantaneous current, of a further threshold.
3. Method according to claim 1, wherein the number N of successive
samples of the values of the instantaneous current is fixed and
defined as N=Tr/Dt, where Tr is the time required for detection and
Dt is the duration of the sampling interval.
4. Method according to claim 3, wherein the estimate of the said
effective parameters of the circuit is obtained by the least
squares method
5. Method according to claim 1, wherein the said source is a
sinusoidal voltage generator.
6. Device for detecting short-circuit conditions in an electrical
network comprising at least one voltage source connected to at
least one load, the said device comprising an analogue
differentiator which receives as input a signal proportional to the
instantaneous current and delivers as output a signal which
represents the first derivative thereof, two analogue/digital
converters whose inputs are connected to the said signal and to the
output of the said differentiator; and processing and detection
means connected to the outputs of the said converters and able to
output a signal representative of the short-circuit conditions.
7. Device for detecting short-circuit conditions in an electrical
network comprising at least one voltage source connected to at
least one load, the said device comprising a digital/analogue
converter which receives as input a signal proportional to the
instantaneous current, a digital differentiator connected to the
output of said converter, and processing and detection means,
connected to the outputs of said converters and digital
differentiator, said means being able to output a signal
representative of the short-circuit conditions.
8. Device according to claim 6, wherein the said processing and
detection means are able additionally to output signals
representative of the value of the power factor of the
short-circuit and the phase of the voltage at the moment of the
short-circuit.
9. Device according to claim 7, wherein the said processing and
detection means are able additionally to output signals
representative of the value of the power factor of the
short-circuit and the phase of the voltage at the moment of the
short-circuit.
10. Method according to claim 1, wherein the said distribution
network is a low or medium-voltage network.
11. Method according to claim 1, wherein the said network is a
three-phase network, and the instantaneous current is the current
in each phase
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a method able rapidly to
detect the initiating of short-circuit conditions in an electrical
network, as well as to a device which uses this method.
[0002] The invention will be illustrated with particular reference
to a preferred application in a low-voltage electrical power
distribution network, although this is not to be understood as
limiting, and in any event the invention applies in general to both
medium and low-voltage networks.
[0003] In modern automatic circuit breakers for low and medium
voltage, devices for detecting faults are integrated in the circuit
breakers, and such detection of faults is often performed by means
of devices of electronic type.
[0004] However, the detection of short-circuits, characterized by
very high current and by the need for very rapid actuation so as to
eliminate the fault, is still carried out at the present time by
electromechanical devices based on the electrodynamic effect.
[0005] The calibration of these short-circuit detection devices is
very difficult and is carried out on an empirical basis. This leads
to low accuracy in the determination of the actuation times of the
circuit breaker.
[0006] Moreover, with these electromechanical devices it is not
possible to gather, in the case of a short-circuit fault, detailed
information about the fault such as, for example, the peak current
characterizing the fault, the complex impedance of the circuit or
the angle of the voltage at the moment of extinction of the fault.
This information would be useful among other things for diagnostic
and statistical purposes.
[0007] Finally, in an automatic circuit breaker based on the known
short-circuit detection mechanism, it is not possible to prevent
the opening of the circuit breaker once the fault has been detectd.
However, in many situations, such as for example when the circuit
breaker is part of a power distribution network which comprises
other circuit breakers, it is desirable for just one of them to
open the circuit, even if the fault was detected by several circuit
breakers simultaneously.
[0008] There is therefore a technical need to make available a
circuit breaker device which is capable of detecting a
short-circuit fault without necessarily involving the opening of
said circuit breaker, since the opening operation may be subject to
other parameters and/or assigned to other circuit breakers present
in the network.
SUMMARY OF THE INVENTION
[0009] The objective of the present invention is to solve the
technical problem illustrated above, by overcoming the limitations
of the prior art, and in particular to carry out the rapid
detection of short-circuit conditions in a network, with
simultaneous calculation of the value of the power factor of the
short-circuit (cos .phi.) and of the phase of the voltage at the
moment of the short-circuit.
[0010] These objectives are achieved by means of the invention
which relates to a method for detecting short-circuit conditions in
an electrical network comprising at least one voltage source
connected to at least one load, characterized in that it comprises
the following operational phases:
[0011] a) measuring the values of the instantaneous current i(t)
flowing in the load and of its first derivative di(t)/dt;
[0012] b) performing N successive samplings of the values of the
instantaneous current i(t);
[0013] c) estimating the effective value of the load R, L viewed by
the said source;
[0014] d) generating a short-circuit detection signal C when the
value of the estimated load is such as to cause the steady peak
current to exceed an assigned threshold level Icc.
[0015] The invention relates, moreover, to a device for detecting
short-circuit conditions in an electrical network comprising at
least one voltage source connected to at least one load, the said
device being characterized in that it comprises:
[0016] an analogue differentiator which receives as input a signal
proportional to the instantaneous current i(t) and delivers as
output a signal which represents the first derivative di(t)/dt
thereof;
[0017] two analogue/digital converters whose inputs are connected
to the said signal i(t) and to the output of the said
differentiator; and
[0018] processing and detection means connected to the outputs of
the said converters and able to output a signal representative of
the short-circuit conditions C.
[0019] The invention relates, moreover, to a device for detecting
short-circuit conditions in an electrical network comprising at
least one voltage source connected to at least one load, the said
device being characterized in that it comprises a digital/analogue
converter which receives as input a signal proportional to the
instantaneous current i(t), a digital differentiator (or filter)
connected to the output of said converter, and processing and
detection means, connected to the outputs of said converters and
digital differentiator, said means being able to output a signal
representative of the short-circuit conditions C.
[0020] According to the invention, the detection of short-circuit
conditions is based on estimating the electrical characteristics of
the load under short-circuit conditions, and more particularly its
impedance. The peak value of the current is typically calculated
from this, before the current actually attains the said peak value.
If this value is greater than a desired threshold Icc, then the
fault is detectd as a short-circuit.
[0021] Alternatively, more complex fault conditions can be defined,
in which the treshold value Icc depends upon the value of cos
.phi..
[0022] The method according to the invention makes it possible to
achieve detection very rapidly, typically in a few
milliseconds.
[0023] Advantageously, the method according to the invention makes
it possible concurrently to determine other characteristics of the
circuit at the moment of the fault, such as the value of the power
factor of the short-circuit (cos .phi.) and the phase of the
voltage at the moment of the short-circuit.
BRIEF DESCRIPTION OF THE DRAWINGS:
[0024] The invention will now be described with reference to the
appended drawings relating to its preferred but non-limiting
embodiments, in which:
[0025] FIG. 1 schematically illustrates a structure of the
detection device according to the invention;
[0026] FIG. 2 illustrates an other embodiment of the device;
[0027] FIG. 3 is a schematic representation of a network for
illustrating the principles of the invention; and
[0028] FIG. 4 is a graph which illustrates the pattern of behaviour
of short-circuit conditions.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] The basic principles of the method according to the
invention will firstly be illustrated with reference to FIG. 3.
[0030] In this figure, the electrical network whereto the invention
is applied is represented by means of a voltage source 10, and a
load having a reactive component L (of inductive type) and a
resistive component R. The source 10 is for example a sinusoidal
generator (V) of known frequency (for example 50 Hz) and known peak
value (for example 220.times.{square root}2 volt). It should be
stressed that the source 10, illustrated in FIG. 3 as a voltage
generator, is in reality a schematization of different sources and
generators present in the network. The network can be a
single-phase, three-phase or in general a polyphase network.
[0031] The device according to the invention is represented by
means of a block 11 provided with a circuit breaker 12 inserted in
series between the generator 10 and load (i.e. the rest of the
network). The opening of the circuit breaker is controlled by
circuitry (not shown) which comprises suitable sensors and which
uses the method of the invention to detect a possible short-circuit
condition.
[0032] With reference also to FIG. 4, according to an alternative
embodiment, the method of the invention provides for the definition
of a first "guard" threshold level indicated Is. In other words,
the instantaneous current i=i(t) flowing in the load is monitored
and it is determined whether it exceeds a first threshold level.
According to another more general form, the method does not provide
for such a constraint, and passes directly to the phases described
below continuatively and/or periodically, starting from the instant
at which the network is powered, or following some other
preestablished event(s).
[0033] If there is provision for the guard threshold, upon
exceeding this threshold, which may constitute the beginning of a
short-circuit or form part of a transient, a sampling is begun of
successive values taken by the current, at pre-established
intervals, for example those indicated by the times t1, t2, t3 etc.
in FIG. 4. The behaviour pattern of the current i(t) is
reconstructed or predicted on the basis of these values, and the
parameters of the load are derived from it. These parameters are
then compared with reference values so as to establish whether a
short-circuit is occurring. If there is no provision for the guard
threshold, the sampling can be carried out continuosly.
[0034] It is important to note that, according to the invention,
the detection of the short-circuit conditions (i.e. when the
threshold peak current is >Icc) is not normally obtained from a
measurement, since the proposed method is capable of estimating or
forecasting in advance--based on previously stored values--the
behaviour pattern of the current, and hence the imminent exceeding
of the threshold Icc. Even when, in the case of a very fast
short-circuit transient, detection is carried out with a very short
or practically zero advance, the method of the invention
nevertheless makes it possible to obtain information about this
short-circuit.
[0035] According to a preferred embodiment, illustrated in detail
below, the estimate of the behaviour pattern of the current from
the samples is obtained by the least squares method and using
normalized values of the current for simplicity of processing. The
method for detecting a short-circuit according to the invention
therefore provides for the following steps:
[0036] when the instantaneous value of the normalized current I(t)
exceeds a normalized threshold value Is, i.e. Is=3, the behaviour
of the normalized current I(t) is calculated. In the circuit of
FIG. 3, the normalized current I(t) can be expressed by the
following relation:
I(t)=+.alpha.I'(t)+.beta. cos(wt)+.gamma. sin(wt) (1)
[0037] where:
1 .alpha. = -tan.phi.; (2) .beta. = ki cos.theta./cos.phi.; (3)
.gamma. = ki sin.theta./cos.phi.; (4) .phi. = arctan(L/R) Io =
normalization constant (typically the nominal current which
characterizes the circuit breaker); Vo = nominal peak voltage (for
example VO = 220{square root}2 V) w = nominal angular frequency of
the electrical network (e.g. 50 Hz), with w = 2.pi.f I(t) = i(t)/Io
I'(t) = [di(t)/dt] .multidot. (1/wIo) .theta. = phase of the
voltage.
[0038] The parameter ki is the ratio between the peak value which
the current i(t) attains when steady and the value Io, and kilo
represents the peak value which the current will attain when
steady. Ki can be exspressed as:
ki=[(Vo)/R.sup.2+W.sup.2L.sup.2).sup.1/2]/Io
[0039] or considering the above-given definitions:
ki=(.beta..sup.2+.gamma..sup.2)/(1+.alpha..sup.2)].sup.1/2
[0040] It is clear from the above that in order to estimate the
effective parameters of the circuit (R, L), it is enough to
estimate the value of the parameter Id.
[0041] The phase of the voltage .theta. is a parameter such that,
having fixed the time origin, the time profile of the voltage v(t)
of the sinusoidal generator 10 of FIG. 3 can be defined as:
v(t)=Vo cos(wt+.theta.)
[0042] As already indicated, FIG. 3 is an equivalent representation
("Thevenin equivalent") of the electrical network considered, so
that the components 10, R, L are not necessarily physical
components, but the "equivalent generator" and the "equivalent
load" viewed by the device 11.
[0043] The parameter N can be determined as follows: if Dt is the
duration of the sampling interval (for example 100 microseconds),
and Tr is the time required for detection (for example 3 ms), then
N can be expressed as:
N=Tr/Dt
[0044] The relation 1 can be expressed in the following vectorial
form:
[I(t)]=[.epsilon.40 ][.PHI.(t)]
[0045] where, the vector .PHI.(t) is defined as 1 ( t ) = ' ( t )
sin wt cos wt
[0046] and let .PHI.(t)' be the transposed vector of .PHI.(t).
[0047] The vector .epsilon.' is the transposed of the vector 2
=
[0048] The vector .epsilon. is calculated as 3 * = [ t = 1 N ( t )
( ( t ) ) ' ] - 1 [ t = 1 N ( t ) I ( t ) ]
[0049] and the elements of .epsilon.* are defined as: 4 * = * *
*
[0050] so that ki* is expressed by:
ki*=(.beta.*.sup.2+.gamma.*.sup.2)/(1+.alpha.*.sup.2).sup.1/2
[0051] The value ki* is compared with the threshold k0. Preferably,
a short-circuit situation is defined as a condition in which the
value ki exceeds a preset threshold ko, typically, but not
exclusively ko=12.
[0052] If ki*>k0 then a short-circuit condition does exist;
otherwise the method is reiterated from the step of monitoring the
current. The sign * indicates that these are values obtained by
estimation.
[0053] From the vector .epsilon. indicated above it is also
possible to calculate also the estimate cos .phi.* of the parameter
cos .phi. and the estimate .theta.* of the parameter .theta. using
the following formulae:
cos .phi.*=1/(1+.alpha..sup.2*).sup.1/2
.theta.*=arc cos(cos .phi.*/ki*)
[0054] As indicated previously, other alternative methods, such as
that of the mean quadratic value (known in the art by the initials
LMS), etc., may be employed to estimate the short-circuit
current.
[0055] FIG. 1 illustrates an embodiment of the device according to
the invention which comprises an appropriate analogue
differentiator filter 1 which calculates the instantaneous
derivative of the current.
[0056] The analogue differentiator 1 receives as input a signal
proportional to the instantaneous current i(t), for example via the
sensor 5, and delivers as output a signal which represents the
first derivative thereof, i.e. "di(t)/dt". The device moreover
comprises two analogue/digital converters, 2 and 3 respectively,
which convert into digital form the signal i(t) and its derivative
output by the differentiator 1. The outputs from the converters are
fed to the processing and detection block 4 which, using the method
of the invention, detects the short-circuit and outputs a signal C,
as well as the signals cos .phi. and 0 which are representative of
the angular phase and of the angle of extinction.
[0057] Preferably, the processing in the block 4 is executed by
means of a programmed microprocessor system, although it is
possible to use analogous discrete devices, either analogue or
digital.
[0058] In the variant embodiment of FIG. 2, on the other hand, in
which the same references have been used for corresponding parts,
there is provided an A/D converter 2 which receives as input the
signal i(t), and a digital differentiator (or filter) 6 connected
to the output of this converter. The outputs of the blocks 2 and 6
are then connected to the block 4, in a manner analogous to the
above.
[0059] The above description relates to single-phase systems,
however the invention can be applied also to the detection of
short-circuits in two-phase or three-phase systems. In the case of
systems on several phases, the method is applied independently on
the different phases.
[0060] The invention has been described with reference to
particular and preferred embodiments, however it is not limited to
what was described, but embraces the variants and modifications
which will be evident to a person skilled in the art.
* * * * *