WO2012072528A1

WO2012072528A1 - Method and apparatus for detecting broken wire in current transformer and equipment for relay protection

Info

Publication number: WO2012072528A1
Application number: PCT/EP2011/071100
Authority: WO
Inventors: Shu Yao Zhao; Long Tian Wang
Original assignee: Siemens Aktiengesellschaft
Priority date: 2010-11-30
Filing date: 2011-11-28
Publication date: 2012-06-07
Also published as: CN102478614B; CN102478614A

Abstract

The present invention discloses a method and an apparatus for detecting a broken wire in a current transformer (CT) and equipment for relay protection. The method for detecting CT wires comprises the steps of: sampling a current signal outputted by said current transformer; calculating a forecast current value i _(k-forecast) at the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point; comparing the sampled current value i _(k-actual) at the k^th sampling point with the forecast current value i _{(k- forecast)} thereof; and judging that said current transformer has a broken wire when the comparison result complies with a criterion. The method has the features of being high in detection accuracy, fast in detection speed, etc.

Description

Method and apparatus for detecting broken wire in current transformer and equipment for relay protection

Technical field

The present invention relates to the field of relay protection and, in particular, to a method for detecting broken wires in current transformers (CT) , an apparatus for detecting a broken wire in CTs and equipment for relay protection comprising the same.

Background art

CTs are important components in electrical power systems When a wire in a CT is broken, it will produce a very high induction voltage and may cause damage to both equipment and people . Therefore, it is often required to find promptly a broken wire in a CT so that the equipment for relay

protection can take protective measures.

A currently known method based on zero-sequence current for detecting broken wires in CTs uses the following

criterion :

i^f

< ^{Q 751}Ocaicu ,,*! > ^{then the} broken wire detection is triggered after delaying 200 ms .

if I ocaic_titaud > a minimum current threshold value and

^ocala_ihted - the minimum voltage threshold value, then the CT broken wire is judged after delaying 10 s.

In this case, loa_ttmhted ^{1S a} calculated zero-sequence current, I_Qmeasurtd is a measured zero-sequence current, and

U<icaia at«d i^{s a} measured zero-sequence voltage.

For example, in a three-phase power system, this method only measures a zero-sequence current and does not detect separately the current of each phase, and under the

circumstance that the wire of one certain phase of the CT i broken and the currents of the other phases are normal, it cannot be j udged actually in which phase of the CT the wi e is broken. Furthermore, this method is inevitably affected by the load of the power system (especially in the case of unbalanced loads) . A method based on the rated current for detecting a broken wire in a CT has overcome the defect of failing to judge in which phase of the CT the wire is broken.

Specifically speaking, in this method the currents of three phases are measured respectively and compared with the rated current to judge in which phase of the CT the wire is broken, for example, the judgment can be made on the basis of the following criterion:

if all the three phase currents are greater than 0.2/_n and l_diff > 0.1/„ , then start the detection of a broken wire in a CT, and then, if any one phase current is less than 0.04/_B and at least one phase current remains unchanged and the maximum phase current is less than 1.21_n , then after delaying 10 s it is judged that the CT has a broken wire. Alternatively, if any phase current is less than 0.06I_n and I_dt >0.l5I_n, then after delaying 6 s it is judged that the

CT has a broken wire.

In this case, I_n is the rated current at the secondary side, which generally is 1 A or 5 A, and I_dig is the

differential current value in the same phase, for example, the differential current at the two ends of the wire of phase A. Although this method can detect each phase current separately, since the actual magnitude of the current is often affected by the load of the power system (especially in the case of unbalanced load) , it tends to make faulty

judgments when comparing with the rated current.

Contents of the invention

In view of the various problems existing in the currently available detection methods, the object of the present invention is to provide a method and an apparatus for detecting broken wires in CTs accurately and quickly.

The method for detecting a broken wire in a CT provided by the embodiments of the present invention comprises the following steps of:

1} sampling a current signal outputted by said current transformer;

2 ) calculating a forecast current value i^- forecast) ^a ^ the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point;

3 ) comparing the sampled current value the k^th sampling point with the forecast current val

thereof; and

4} judging that said current transformer has a broken wire when the comparison result complies with a criterion,

wherein, k>l . In step 1 ) , the sampling frequency is an integral multiple of the frequency of said current signal .

In step 2 ) , in the case that said current signal is a sinusoidal signal , the forecast current value at the k^th sampling point can be calculated according to one of the following equations :

(k- N)

4

l{k-forecast) ^{= l}(k-N) '

wherein, N is the number of the sampling points durinc each cycle of said current signal, and is the maximum instantaneous value of said current signal during one cyc^

In step 3) , the magnitudes of k-forec i) x Factor! and '(k-actual)] can be compared, and said criterion comprises

k-foncast) x Factor! > '(k-actual) , Factor! E [θ.1,0.99]

In this case, the method can further comprise step 3A) of : comparing the sampled current value

the k^th sampling point with the sampled current value 2_{(j )} at the (k-

1 ) ^th sampling point, and in the case that the result of this comparison does not comply with a predetermined condition further comparing the sampled current value z^' _(i__artua/) at the k^th sampling point with the sampled current value at the

( k+1 ) ^th sampling point.

In step 3A) , the magnitudes of

^ar>d |*₍*-_acai_a/₎| ^x Factor! can be compared, and in the case that the comparison result does not comply with said predetermined condition

(i-l) (k- ct al) x Factor! , the magnitudes of (k- ctual) and (k+1) x Factor! are further compared, and said criterion can further

comprise : > |i_(i__flCftjai)| x Factor! or |¾__flCftJfl/)| > \ x Factor! ,

Factor! e[1.01,+oo) .

In this case, the method can further comprise step 3B) of: comparing the sampled current values i(k_+m) at M sampling points after the k^th sampling point in turn with the minimum current threshold value ZERO , wherein m=l,2,...M, M>2, and ZERO is determined according to the rated value of said current signal, and said criterion further comprises:

In step 3B) , when the comparison result of the ( k+m) ^th sampling point complies with ≤ ZERO , the comparison for the ( k+m+1 ) ^th sampling point is further carried out until the comparison for the k+M sampling point is completed or the comparison result of one certain sampling point therein does not comply with < ZERO . In step 4} , when the comparison result complies with the criterion, it can be judged that said current transformer has a broken wire after delaying for a period of time.

The apparatus for detecting a broken wire in a current transformer provided by the embodiments of the present invention comprises:

a sampling module for sampling a current signal outputted by said current transformer;

a forecast module for calculating a forecast current value z_(i_^_om._∞i) at the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point;

a comparison module for comparing a sampled current value a^<L the k^th sampling point with the forecast current value thereof; and

a judging module for outputting the judgment that said current transformer has a broken wire when the comparison result complies with a criterion. In the case that said current signal is a sinusoidal signal, said forecast module can calculate the forecast current value at the k^th sampling point according to one of the following equations:

(k-±N)

4

(k-forecost) ¹ ^_

(k-forecast) ^~ '(*-.V)

(k-forec st) and

'(k- forecast) ] I¹ + i

4

Said comparison module compares the magnitudes of l(k-forecast) x Factor! with (k- ctu !) , and said criterion comprises

| ")| ^{x Factorl} > > ^{Factorl E} [0-1,0^.99]

This apparatus can further comprise: a second comparison module for comparing the sampled current value ½__aciHa) at the k sampling point with a sampled current value i₍ (_j*-D₎ at the

( k-1 ) sampling point, and when the comparison result does not comply with a predetermined condition

(*-D (k-actua!) x Factorl , further comparing the sampled current value ¾__QCiHaj) at the k sampling point with a sampled current value i_(k+l) at the (k+1) sampling point, and said criterion further comprises (k-l) (k-actt l) x Factorl or \

x Factorl , Factor 2 e[1.01,- c) .

This apparatus can further comprise: a third comparison module for comparing the sampled current values at M sampling points after the k^th sampling point with the minimum current threshold value ZERO respectively, wherein m=l,2,,.,M, M>2, and ZERO is determined according to the rated value of said current signal, and said criterion further comprises:

This apparatus for detecting a broken wire in a current transformer can further comprise: delay means for delaying for a period of time when the compa ison result complies with the criterion. At this moment, said judging module outputs the judgment that said current transformer has a broken wire after the delay time expires.

The equipment for relay protection provided by the embodiments of the present invention comprises an apparatus for detecting a broken wire in a current transformer as described above. The method and the apparatus for detecting a broken wire in a current transformer and the equipment for relay

protection provided by the embodiments of the present

invention detect the broken wires on the basis of the sampled currents, which overcomes the defect that when detecting on the basis of the rated current (or rather basic quantity values or effective values) without taking the actual loads (especially unbalanced loads) of a power system into

consideration, it tends to make faulty judgments, so that detection accuracy is significantly improved, and a broken wire in a CT can be detected after 3 to 4 sampling points and therefore, rapid detection can be achieved.

Brief description of the accompanying drawings

Fig. 1 is a flow chart of a method for detecting a broken wire in a CT according to a first embodiment of the present invention .

Fig. 2 is a waveform graph of sampling a sinusoidal current signal under normal conditions according to the embodiment of the present invention.

Fig- 3 is a waveform graph of sampling a sinusoidal current signal when a wire in the CT is broken according to the embodiment of the present invention .

Fig. 4 is a flow chart of a method for detecting a broken wire in a CT according to a second embodiment of the present invention.

Fig. 5 is a flow chart of a method for detecting a broken wire in a CT according to a third embodiment of the present invention .

Fig. 6 is a structural diagram of an apparatus for detecting a broken wire in a CT according to a first

embodiment of the present invention.

Fig. 7 is a structural diagram of an apparatus for detecting a broken wire in a CT according to a second embodiment of the present invention.

Fig. 8 is a structural diagram of an apparatus for detecting a broken wire in a CT according to a third

embodiment of the present invention.

Exemplary embodiments

Hereinbelow, a sinusoidal current signal is taken as an example to describe the method for detecting a broken wire in a CT according to a first embodiment of the present

invention. It should be mentioned that the type of current signals is not limited to this.

As shown in Fig. 1, in step 101, sampling is carried out on a current signal outputted by a current transformer in a power system. The sampling frequency should be adapted to the frequency change of the power system. In practice, the sampling frequency is generally set as an integral multiple of the frequency of the power system, and preferably, this multiple is 8, 12 , 16, 20 .... For example, when the frequency of the current signal is 50 Hz, the sampling frequency can be set as 1 kHz ; in this way, N=1000/5Q=20 sampling points can be obtained during each cycle of the current signal . Howeve , when the f equency of the power system is 52 Hz, in order to obtain 20 sampling points during each cycle of the current signal in the same way, the sampling frequency can be set as 1.04 kHz . Fig . 2 shows a waveform graph under the conditions that the frequency of the current signal is 50 Hz and the sampling frequency is 1 kHz . In step 102, a forecast current value i(k-foncast) ^a^ the k^th sampling point is calculated according to the sampled current values at the sampling points before the k^th sampling point . As to a periodic signal , since the signal wavefo m in each cycle is the same or similar, the signal value at one certain sampling point can be forecasted according to the sampling signal value at this sampling point in a previous pe iod .

First of all , four sampling points of * - .v , * - -I.v , k- N and k-^N are selected from the sampling points before the I sampling point, assuming that the sample value at the k^th sampling point is , and the phase angle is θ , wherein there are N samples during each cycle, then:

the phase angle of the sample i^__Nj is (Θ - 1π) , the phase angle of the sample π) , the phase angle of the

sample i , is {θ - π) , and the phase angle of the sample i is ⁽θ--π) .

2

As to a standard sinusoidal current signal, these four samples can be represented by the maximum sample value of this sinusoid and the phase angle Θ at the k^th sampling point :

<₍*-.v₎ ~^I _mix ^x sin(0 - 2π) =I_m x sinθ ,

3

, = /„ x sin(6>

(t-.v) ™ —π) =I_m, x cosθ ,

2

• 2j = I_mma„x ^x sin(6»- πJ) = I_mma„x x(V- sinθ)7 ,'

4

i = I_m„ x sin(0——Λ" = /„„ x(-cos#) , and

4

(*) / xsm6>,

next, a forecast current value ^-forecast) ^a^ the k^th sampling point can be calculated using one of the following equations :

Equation 1 :

x (-sinfl)./^ x (-cosfl) _ ¾ ^{X i(}*

/„„ cos# • 3

-V)

4

Equation 2 :

~) = -( = ^x(-sm6*)) = -i

Equation 3

'it-forecast) ^~

ation 4 :

Equation 5 forecast) ] (* - )

4

Equation 6:

I «₎I=(^/ _m»^xsm _^)² =^L x(l- cos² ^) = L + LC-cos fl x cos fl) = +i _! x i 3

^■ {*-- ■N)

In theory, a correct forecast current value at the k^th sampling point can be obtained using each of the above

equations. However, in practical applications, some special samples may cause deviations, for example, as to Equation 1, when the samples i , , i , , and / , are approximate to the

(k-—N) (k~N) (k~N)

4 4 4

value of zero, the forecast current value will have a

relatively large deviation. Therefore, in order to avoid the deviation caused by special samples, a suitable calculation equation can be selected therefrom.

In step 103, the sampled current value

the k^th sampling point is compared with the forecast current value.

. Under normal circumstances, both of them should be equal or approximately equal to each othe . As shown in Fig .

3, if the wire is broken between the (k-1 ) ^th sampling point and the k^th sampling point of the CT, in theoretical cases, since the circuit is broken at this moment, should be

zero, but in fact, due to the hysteresis effect of the

circuit in the protection equipment and the influence of the sampling circuit, the actual i^_k) does not immediately change to zero but its absolute value drops sharply, thus it has to be smaller than the forecast value.

In practice, the criterion of a broken wire in a CT can be set as by comparing the magnit

this case,

Factor! is a constant, which can be between 0.1 and 0.99, and it is generally set as 0.95 in order to start the detection for a broken wire with a high sensitivity .

In step 104, when the comparison result complies with the criterion, for example when |¾__orec£Ki)Ix Factor! > \i judged that the CT has a broken wire.

In contrast, when the comparison result does not comply with the criterion, it indicates that the current power system operates normally. At this moment, k+1 is assigned to k, i.e. the (k+1 ) ^th sampling point is changed into a sampling point currently to be detected, then returning to step 102 to carry out a new round of detection of a broken wire in the CT for the current sampling point , and so on, thus carrying out in turn the detection of a broken wire in the CT at the

<k+l)^th, (k+2)^th, and (k+3) ^th sampling point.

It can be seen from the first embodiment described above that the method for detecting a broken wire in a CT of the present invention forecasts a present current value on the basis of previously sampled current values and compares the forecast current value with the currently sampled current value, so as to make a judgment regarding whether or not a wire in the CT is broken . Compared with the currently available detection method based on the zero sequence

current, the detection method of this embodiment detects each path of current separately, which can be applied in single- phase or multi-phase power systems. Compared with the

currently available detection method based on the rated current, the detection method of this embodiment neither relies on the differential current value at two ends of a wire nor is affected by the load.

Fig. 4 is a flow chart of a method for detecting a broken wire in a CT according to a second embodiment of the present invention . Here, only the steps which are different from those of the first embodiment will be described, and the sampling step 201 and the forecast step 202 which are the same as the corresponding steps in the fi st embodiment will not be described redundantly. the first comparing step 203, the sampled current i(k-actuai) ^a^ the k^th sampling point is compared with the forecast current value thereof, which is the same as the first embodiment. Under normal circumstances, both of them should be equal or approximately equal to each other. As shown in Fig. 3, if the wire is broken between the (k-1 ) ^th sampling point and the k^th sampling point of the CT, in theoretical cases, since the circuit is broken at this moment, should be zero, however in fact, due to the hysteresis effect of the circuit in the protection equipment and the influence of the sampling circuit, the actual

does not immediately change to zero but its absolute value drops sharply, thus it has to be smaller than the forecast value .

In practice, the first criterion of a broken wire in a CT can be set as |*^' ₍*-/_οτκ-∞_/) x FactorX > [i^^a*,₎ , in other words, by comparing the magnitudes of _forecest x FactorX with ^l{k-actuaT) In this case, Factor! can be between 0.1 and 0.99, and it is generally set as 0.95 so as to start the broken wire

detection with a high sensitivity .

Furthermore, if the wire is broken between the ( k-1 ) ^th sampling point and the k^th sampling point of the CT, due to the hysteresis effect of the circuit and the influence of the sampling circuit, would have the following two

possibilities:

a) the absolute value of z^' _(Jt) drops abruptly to smaller than that of i_(k__x , and the absolute value of i_{(j 1)} is smaller than that of z^' _(i) ; and

b) the absolute value of 2_{(( 1)} drops abruptly to be smaller than that of z_(i)

therefore, when the comparison result of the above first comparing step 203 complies with the fi st criterion, for example, |ϊ₍*_/_ΟΓβ<:α5,₎| ^x

> the second comparing step 204 is performed to compa e the sampled current value i_(t__aclUQj) at the k^th sampling point with the sampled current value i^_k_^ of the ( k-1 ) ^th sampling point , and when the compa ison result does not comply with a predete mined condition, the sampled current value

the k^th sampling point is further compared with the sampled current value z_{(j 1)} of the (k+1 ) ^th sampling point . In practice, the second criterion of a broken wire in a CT can be set as | > |i_(i:__flCftJ£lij| x i¾ctor2 or

|'(*-acf»i/)I > |*(*₊i)I^x Factor! , in other words, by fi st comparing with |i₍i-_{0<:ft<0 )}| ^x Factor , if

Factor! , i.e. the result of this comparison complies with the predetermined condition, then the second comparing step 204 is completed, and the result of this comparison is taken as the comparison result of the second compa ing step 204; in contrast , if

i.e. the result of this compa ison does

not comply with the predete mined condition, then

|*(*-a««_j | ^x Factor! if fu ther compared with |i_(A+1)| x oc/or2 , and comparison result of

and |i_((t+1)| x actor 2 is taken as the compa ison result of the second comparing step 204. In this case, Factor! is a constant which is greater than or equal to 1.01, the specific value of Factor! is related to the circuit hysteresis and the influence of the sampling ci cuit, and affected by the hardware design and, therefo e, its value should be adj usted according to the particular hardware platform. Under normal circumstances, its value can be set as 1.2-2.0.

When the comparison result of the above second comparing step 204 complies with the second crite ion, for example, (i-l) > or |/

(k-aetiiai) ^> (k+i) \ ^x Factor 2 (specifically speaking, the criterion co responding to that for comparing the magnitudes of and (k-actual) x Factor! is

k-i) (k-actial) x Factor! in the second criterion, while the criterion co responding to that for comparing the magnitudes of '(k-actual) and Factor! is

t*0t₊l) | x Factor! in the second criterion) , step 205 is pe formed to j udge the broken wire in the CT.

Compared with the first embodiment, the second compa step 204 is further added in the method for detecting a broken wire in a CT according to the second embodiment, which further improves detection accuracy.

Fig. 5 is a flow chart of a method for detecting a broken wire in a CT according to a third embodiment of the present invention. Here, only the steps which are different from the methods for detecting a broken wire in a CT in the first and second embodiments are described, and the sampling step 301, the forecast step 302 and the first comparing step 303 which are the same as the corresponding steps in the first

embodiment or the second embodiment will not be described redundantly .

In the second comparing step 304, the sampled current value i (k -actual) at the k sampling point is compared with the sampled current value i^_k_^ of the (k-1) sampling point, and when the compa ison result does not comply with the

predetermined condition the sampled current value

the k^th sampling point is further compared with the sampled current value of the (k+1 ) ^th sampling point. In practice, the second criterion of CT broken wire can be set as

(*-D (k-actual) x Factor! or | {k-acMal) \ *" Uk+l) Factor! , in other words first comparing '(k-iy with x Factor! ,

(*-D (k-act al) Factor! , i.e. the result of this compa

complies with the predetermined condition, then the second comparing step 304 is completed, and the result of this comparison is taken as the comparison result of the second comparing step 304; in contrast, if i^^ ^i^^^ x Factor! , i.e. the result of this comparison does not comply with the predetermined condition, then l^_.^^| is further compared with k*₊i)| ^x factor! , and the result of this comparison of |¾__ac<Hflj₎| and

'(i+!) Factor! is taken as the comparison result of the second comparing step 304. In this case, Factor! is a constant which is greater than or equal to 1.01, and the specific value of Factor! is related to the circuit hysteresis and the influence of the sampling circuit, and affected by the hardware design; therefore, its value should be adjusted according to the particular hardware platform. Under normal circumstances, its value can be set as 1.2-2.0.

Furthermore, if the wire is broken between the (k-l)^th sampling point and the k^th sampling point of the CT, the absolute value of i^_k+M is inevitably approximate to zero, m=l , 2 , 3 ....

Therefore, when the comparison result of the above second comparing step 304 complies with the second criterion, for example, Factor!

(specifically speaking, the criterion corresponding to that of comparing the magnitudes of and x Factor!

is hk-l) ¹(k-actal) x Factor! in the second criterion, while the

criterion corresponding to that of comparing the magnitudes of |i₍*__acme/₎ I and |r_{(j )}|x Fflctor2 is |¾.__αί._ήί3η| > |%_t+i₎|^x Factor2 in the second criterion) , the third comparing step 305 is carried out, and the sampled current values i _k+m) at M sampling points after the k^th sampling point are compared in turn with the minimum current threshold value ZERO , m=l,2,..,M; and M>2. In this case, ZERO is the minimum current threshold, the value of which is generally determined by the rated value of the current signal, and it is related to the accuracy of an analog-digital converter and affected by the hardware design. Under the condition that the rated current is between 1 A and 5 A, ZERO is generally between 0.01 A and 0.5 A, and

specifically speaking, as to the current signal with the rated current being 1 A, ZERO is generally set to be 0.04 A, and as to the current signal with the rated current being 5 A, ZERO is generally set to 0.2 A.

In practice, the absolute values of the sampled current value ί_(¾+1) at the k+l^th sampling point and of the sampled current value i (,k_k+₊2₂)₎ at the (k+2 ) ^th sampling point are first compared with the minimum current threshold value ZERO respectively, and when the comparison result complies with I≤ ZERO and |i (*₊2) ^≤ ZERO , an indication that a wire is suspected to be broken is sent out, then the absolute values of subsequent sampling points, i.e. the (k+3) ^th, the (K+4)^th sampling point, etc., are further compared in turn with

ZERO , and the number of the further sampled points can be set as required. Generally within 3 to 4 samples, a broken wire can be judged accurately. During the comparison, if the absolute value of some sampling point is greater than ZERO , then the comparison is stopped, and the indication that a wire is suspected to be broken is reset so as to remove the suspicion. When the comparison result of the above third comparing step 305 complies with the third criterion, for example,

|*₍*₊m₎| - ZERO , step 306 is performed, and it delays a period of time, in which the delay time Timer is between 1 ms and 100 s . Generally the delay time Timer can be set to be 5 ms-20 ms .

After the delay time Timer expires , step 307 is performed to judge a broken wire in the CT. In addition, it should be mentioned that although in the third embodiment , the comparison operation is pe formed according to the order of the first compa ing step 303, the second comparing step 304, and then the third comparing step 305 , the present invention is not limited to this. On the basis of the above description of the third embodiment , those skilled in the art can understand that these comparison steps can be performed in an order different from the order herein and these compa ison steps can even be pe formed in parallel , and then these comparison results can be summarized to make a j udgment .

Hereinbelow, Fig . 3 is used as an example to illustrate the steps for detection of a broken wire in a CT in the third embodiment .

As shown in Fig. 3, a wire of the CT is broken at the sample ip_9j :

- ° - ^{3 2} · _k-i₎ = -1-14 , = -0.031 , and i_(t+2) = -0.02 according to Equation 2 , forecast) ⁼

-0.831 can be obtained by calculation;

as to the first criterion '(k- forecast) x Factorl > |? (k-arual) wherein the value of Factor! is 0.95:

0.831 x 0.95 = 0.789 > 0.32 :

as to the second criterion hk-l) > or

|⁷(*-<ΧΏΜ/)I^> I^x Factorl , wherein the value of Factorl is 2 :

1.144 > 0.32 x 2

0.32 > 0.031 x 2 ;

as to the third criterion \i_{k+\) — ZERO and \i^₊₂₎ — ZERO , wherein the value of ZERO is 0.04:

0.031 < 0.04

0.026 < 0.04

It can be seen from the above that the comparison results comply with all the criteria, the indication that a wire is suspected to be broken is sent out after the sample z₍₄₃₎ , and an indication that a wire is broken is sent out after delaying 10 ms .

The method for the detection of a broken wire in the CT as described in the third embodiment can immediately detect a broken wire in the CT after 3 to 4 samples, for example, for a sampling frequency of 1 kHz , it can be detected 3-4 ms after the CT broken wire occurs, and as compared with the method for detection of a broken wire in a CT in the first and second embodiments , more comparison operations are performed to prevent misj udgment , thus detection accuracy is further improved .

Fig . 6 is a structural diagram of an apparatus for detecting a broken wire in a CT according to a first

embodiment of the present invention. This apparatus for detecting a broken wire in a CT comprises a sampling module 501, a forecast module 502, a comparison module 503, and a judging module 504.

The sampling module 501 samples a current signal

outputted by said current transformer. The sampling frequency should be adapted to the frequency change of a power system, and as shown in Fig. 2 , when the f equency change of the current signal is 50 Hz, the sampling frequency can be set to be 1 kHz; thus, N=1000/50=20 sampling points can be obtained during each cycle of the current signal . After having

obtained the current sampling value at the k^th sampling point, the sampling module 501 outputs it to the forecast module 502 and the comparison module 503.

The forecast module 502 calculates the forecast current value ^at the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point. As to a periodic signal, since the signal waveform of each cycle is the same o similar, the signal value of one certain sampling point can be forecasted

according to the sampling signal value of a previous period of this sampling point. As to a sinusoidal current signal, the forecast module 502 can calculate the forecast current value i(k-forecast) ^at the k^th sampling point according to any one of the above equations 1 to 6.

The comparison module 503 compares the current sampled current value

the k^th sampling point with the forecast current value ia-f_orm:ast) thereof, for example,

comparing the magnitudes of ^•(k-farecmt) x Factorl with Xk-actual) In this case, Factorl can be between 0.1 and 0.99, and it is generally set as 0.95 so as to start the broken wire

detection with a high sensitivity . The judging module 504 outputs the judgment that a wire of said current transformer is broken when the comparison result complies with a criterion, for example,

Fig. 7 is a structural diagram of an apparatus for detecting a broken wire in a CT according to a second

embodiment of the present invention. Here, only those

components which are different from the apparatus for

detecting a broken wi e in a CT of the first embodiment will be described, and the sampling module 601 and the forecast module 602 which are the same as the corresponding components in the first embodiment will not be described redundantly.

In this embodiment, the sampling module 601 outputs the sampled current value at the k^th sampling point to the forecast module 602 and the fi st comparison module 603, and outputs the sampled current values at the k^h, ( k-1 ) ^th, and k+1 ) sampling points to the second compa ison module 604

The first comparison module 603 compares the sampled current value ½__aiA,_oi) at the k^th sampling point with the forecast current value ½__forecajrt thereof . For example, it compares the magnitudes of (k- forecast) x Factor! with ^k-actual) In this case, Factor! can be between 0.1 and 0.99, and it is generally set as 0.95 so as to start the broken wi e

detection with a high sensitivity .

When the comparison result of the first comparison module 603 complies with the first criterion, for example,

,

the second comparison module 604 compares the sampled current value

the k^th sampling point with the sampled current value i_(iM) at the (k-1 ) ^th sampling point , and when the comparison result does not comply with the predete mined condition, it further compares the sampled current value

the k^th sampling point with the sampled current value i_{fj 1)} at the (k+1 ) ^th sampling point. For example, the second comparison module 604 can first compare the magnitudes of

x Factor2 , and if (i-i)I>I' t-_oi-ru_a/₎I^x Factor! , i.e. the result of this comparison complies with the predetermined condition, then the second comparison module 604 outputs the result of this comparison in contrast, if

, i.e. the result of this comparison does not comply with the predetermined condition then the second comparison module 604 further compares the magnitudes of '(k-actual) with <{k₊l) x Factor! , and outputs the result of this comparison. In this case, Factor! is a constant which is greater than or equal to 1.01.

When the comparison result of the second comparison module 604 complies with the second criterion, for example,

^'<*-!) {k-atuci) x Factor! or | {k-actual) \ -* +l) x Factor! , the judging module

605 outputs the judgment that a wire of the CT is broken.

embodiment of the present invention . Here, only those components which are different from the apparatus for detecting a broken wire in a CT of the first and second embodiment will be described, and the sampling module 701, the forecast module 702, and the first comparison module 703 which are the same as the corresponding components in the first embodiment or the second embodiment will not be

described redundantly.

In this embodiment, the sampling module 701 outputs the sampled current value to the forecast module 702 and the first comparison module 703, outputs the sampled current values at the k^th, { k-1 ) ^th, and ( k+1 ) ^th sampling point to the second comparison module 704, and outputs the sampled current value of the (k+m)^th sampling point to the third comparison module 705.

When the comparison result of the first comparison module 703 complies with the first criterion, for example,

Ix Factorl >

_I the second comparison module 704 compares the sampled current value *_(t__aciHa/) at the k^th sampling point with the sampled current value of the ( k-1 ) ^th sampling point , and in the case that the result of this comparison does not comply with the predetermined condition, it further compares the sampled current value i_(it__acftiaj) at the k sampling point with the sampled current value i,_k+r> of the

{ k+1 } sampling point . For example, the second comparison module 704 can first compare the magnitudes of and

'(k-actual) x Factor! , if > (k-actual) x Factor! , i.e. the result of this comparison complies with the predetermined condition, then the second comparison module 704 outputs the result of this comparison; in contrast , if * Factor! ,

i.e. the result of this comparison does not comply with the

predetermined condition, then the second comparison module 704 further compares the magnitudes of with

|'₍*₊i₎| ^x Factor! , and outputs the result of this comparison . In this case, Factor! is a constant which is greater than or equal to 1.01.

When the comparison result of the second comparison module 704 complies with the second criterion, for example,

, the third

comparison module 705 compares the sampled current values (_i+BIj at M sampling points after the k^th sampling point in turn with the minimum current threshold value ZERO , wherein m=l , 2 , ...M, M>2. In this case, ZERO is the minimum current threshold, the value of which is determined by the rated value of the current signal . The absolute value of the sampled current value of the (k+1 ) ^th sampling point and the absolute value of the sampled current value z^' _(i+2) of the

( k+2 ) ^th sampling point can be first compared with the minimum current threshold value ZERO respectively . When the

comparison result complies with ?^' _(t+1) < ZERO and |/_(jt+2) < ZERO , the third comparison module 705 sends an indication that a wire is suspected to be broken. Afterwards, the third

comparison module 705 further compares the absolute values at the subsequent sampling points, i.e. the ( k+ 3 ) ^th, the ( K+ 4 ) ^th sampling point, etc. in turn with ZERO, and the number of the sampled points can be set as required. Generally, a suspected broken wire can be accurately confirmed within 3 to 4 samples. During the further comparison, if the absolute value of any sampling point is greater than ZERO, then the third comparison module 705 resets the indication that a wire is suspected to be broken so as to remove the suspicion.

When the third comparison module 705 finishes its

comparison tasks and all the comparison results comply with the third criterion, for example, |z^' _{(( m)}| < ZERO , the delaying module 706 delays a period of time, and the delay time Timer is between 1 ms and 100 s, and generally set to be 5 ms to 20 ms .

Said judging module 707 outputs the judgment of a broken wire in the CT after the delay time expires.

Corresponding to the method for detecting a broken wire in a CT according to the embodiments of the present

invention, the apparatus for detecting a broken wire in a CT according to the embodiments of the present invention also has the features of being high in detection accuracy, fast in detection speed, etc.

What are mentioned above are merely the preferred

embodiments of the present invention, and they are not intended to limit the protection scope of the present

invention. Any modifications, equivalent substitutions and improvements within the spirit and principle of the present invention are to be covered in the protection scope of the present invention.

Claims

1. A method for detecting a broken wire in a current transformer, comprising:

1} sampling (101) a current signal outputted by said current transformer;

2) calculating (102) a forecast current value i(k-fsrecast) ^a^ the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point;

3) comparing (103} the sampled current value i ..^^ at the k^th sampling point with the forecast current value

thereof; and

4) judging (104} that said current transformer has a broken wire when the comparison result complies with a criterion,

wherein, k>l .

2. The method for detecting a broken wire in a current transformer as claimed in claim 1, characterized that in step 1) the sampling frequency is an integral multiple of the frequency of said current signal.

3. The method for detecting a broken wire in a current transformer as claimed in claim 1, characterized in that said current signal is a sinusoidal signal, and the forecast current value at the k^th sampling point is

calculated according to one of the following equations :

i z ^xi 1

(*--") (*--.v)

4

^I(k-forecest) ^{= l}(k-N) f

and

'(k-forecast)] I¹ + i XI

4

4. The method for detecting a broken wire in a current transformer as claimed in claim 1 , characterized in that in step 3) the magnitude %k- forecast) x Factor! is compared with , and

said criterion comprises: |¾_/_oreccBf)| x Factor! > |¾t__aclHai₎ | ,

Factor! e [0.10.99] .

5. The method for detecting a broken wire in a current transformer as claimed in claim 4 , characterized in that the method further comprises:

3A) comparing the sampled current value ί^-_χηαΐ at the k^th sampling point with the sampled current value i^_k_^ at the

( k-1 ) ^th sampling point, and in the case that the comparison result does not comply with a predetermined condition, further comparing the sampled current value

the k^th sampling point with a sampled current value _(t+1j at the

( k+1 } ^th sampling point.

6. The method for detecting a broken wire in a current transformer as claimed in claim 5, characterized in that in step 3A) the magnitudes of and ^■(k-actual) x Factor! are compared, and in the case that the result of this compa does not comply with said predetermined condition

r^' ₍*-i₎| > ^x Factor! , the magnitudes of |i_(t__{oc )} and x Factor! are further compared; and

said criterion further comprises (k-l) ^:(k-actual) x Factor! or

|*^' ₍*-_1Irtuo/₎ I > I'o _i) I x Factor! , Factor! e [1.01,+00) .

7. The method for detecting a broken wire in a current transformer as claimed in claim 6, characterized in that the method further comprises:

3B) comparing the sampled current values i(_k+m^ at M sampling points after the k^th sampling point in turn with the minimum current threshold value ZERO , wherein m=l , 2 , ...M, M≥2 , and ZERO is determined according to the rated value of said current signal; and

said criterion further comprises < ZERO

8. The method for detecting a broken wire in a current transformer as claimed in claim 7, characterized in that in step 3B) , when the comparison result of the ( k+m) ^h sampling point complies with |/_(i+OT)| < ZERO , the comparison for the { k+m+1 ) ^th sampling point is further carried out until the comparison for the ( k+M) ^th sampling point is completed or the comparison result at a certain sampling point does not comply

9. The method for detecting a broken wire in a current transformer as claimed in claim 1, characterized in that in step 4) , when the comparison result complies with the criterion, it is first delayed for a period of time and then it is judged that said current transformer has a broken wire.

10. An apparatus for detecting a broken wire in a current transformer, comprising:

a sampling module (501, 601, 701) for sampling a current signal outputted by said current transformer;

a forecast module (502, 602, 702) for calculating a forecast current value ¹i^k-f°^r'^cas>> at the k^th sampling point according to the sampled current values at the sampling points before the k^th sampling point;

a comparison module (503} for comparing the sampled current value ^Ι^-^<κΛω at the k^th sampling point with the forecast current value ¾-™^αϊί) thereof; and

a judging module (504, 605, 707) for outputting the judgment that said current transformer has a broken wire when the comparison result complies with a criterion .

11. The apparatus for detecting a broken wire in a current transformer as claimed in claim 10, characterized in that said current signal is a sinusoidal signal, and said forecast module calculates the forecast current value at the k^th sampling point according to one of the following

equations

¹ 2.„ ^{X i},. i

(*—-v) (*—Λ')

j - i t

%k- forecast)

¹ 3

<i— N)

4

ilk-forecast) ^{= l}(k-N) f

IHk- forecast) | = J⁷L " _»n ' ^

(k- forecast)

wherein, N is the number of the sampling points during each cycle of said current signal, and is the maximum instantaneous value of said current signal during one cycle .

12. The apparatus for detecting a broken wire in a current transformer as claimed in claim 10, characterized in that said comparison module (503) compares the magnitudes of the ^■{k-forecast) x FactoA with

said criterion comp

Factorl E [θ.1,0.99] .

13. The apparatus for detecting a broken wire in a current transformer as claimed in claim 12, characterized in that the apparatus further comprises: a second comparison module (604, 704) for comparing the sampled current value i(k-actuai) at the k^th sampling point with a sampled current value (_t_y at the (k-1 ) ^th sampling point, and in the case that the result of this comparison does not comply with a predetermined condition further

comparing the sampled current value at the k sampling point with a sampled current value ι_(Μ) of the (k+l)^th

sampling point; and

said criterion further comprises: |½_D| > |¾-acft_ia?J ^x Factor! or

Factor! e [1.01,+∞)

14. The apparatus for detecting a broken wire in a current transformer as claimed in claim 13, characterized in that the apparatus further comprises: a third comparison module (705) for comparing the sampled current values i_(k+m) at

M sampling points after the k^th sampling point in turn with the minimum current threshold value ZERO , wherein m=l , 2 , ...M, M≥2, and ZERO is determined according to the rated value of said current signal; and

said criterion further comprises: ≤ ZERO .

15. The apparatus for detecting a broken wire in a current transformer as claimed in claim 10, characte ized in that the apparatus for detecting a broken wire in a current transformer further comprises: delay means (706) for delaying for a period of time when the compa ison result complies with the criterion; and

said judging module (707) outputs the judgment that said current transformer has a broken wire after the delay time has expired.

16. Equipment for relay protection, comprising an apparatus for detecting a broken wire in a current

transformer as claimed in any one of claims 10 to 15.