Background technology
Luo-coil be a kind of employing with lead equably on the uniform nonmagnetic substance framework in cross section; Let the lead that is connected with big electric current vertically pass the center of coil; Through the variation of the magnetic flux that tested current produced, induce the measurement mechanism that can react the voltage signal of tested size of current.The voltage signal that secondary side induces is the micro component of the big electric current of primary side, so Luo-coil has derivative characteristic.
In tranformer protection of today, because differential protection performance is good, and some other advanced resist technology cost is high, and it is difficult to realize, so differential protection is the main salvo of present transformer.
Transformer can produce excitation surge current when idle-loaded switching-on, the distortion current when this shoves and breaks down with transformer has false similarity, causes the relay equipment malfunction easily.Therefore need be to shoving and fault current is differentiated.Mature methods has at present: the method for quadratic harmonics; The balancing waveform method; Interval angle detection method etc.
Because the work magnetic of transformer core properties of materials, iron core is close, the switching angle when saturation magnetic induction, remanent magnetism, air-drop or the like (wherein remanent magnetism and switching angle are at random), all can influence transformer magnetizing current in various degree and distort.Therefore, the distortion degree of shoving is not unique, and the characteristic quantity that needs to detect also is uncertain.Though other harmonic wave proportions are bigger relatively for the shared first-harmonic ratio of second harmonic in the excitation surge current, have uncertainty, can change within the specific limits.Under the lower situation of the degree of shoving, the very low situation of second harmonic content might occur, thereby be prone to cause the protective device malfunction.
The definite value of interval angle generally is taken between 60 degree-70 degree, and this can only guarantee that saturation magnetic induction and the close ratio of work magnetic are 1.1-1.3, and remanent magnetism is 0.5 left and right sides Shi Keyong.Interval angle is also very big with the variation of excitation factors vary in fact, possibly almost not have the appearance of interval angle sometimes.Waveform symmetry can only be applied in the even-order harmonic braking that occurs under the symmetrical situation of shoving; Under some specific situation, just can occur; And when overexcitation appears in transformer,, also can't use the waveform symmetry method to differentiate excitation surge current though its excitation current waveform is symmetrical.In a word, the distortion that present the whole bag of tricks all only shoves from some aspects reflection of current distortion degree, therefore the distortion situation that does not have reflection comprehensively to shove all has tangible limitation, and redundance is lower, occurs malfunction or action delay easily.
Summary of the invention
The objective of the invention is: to the limitation of above-mentioned existing inrush current of transformer distortion detection method existence; Propose a kind of inrush current of transformer distortion detection method based on Luo-coil that can reflect more comprehensively that the distortion degree of shoving, redundance are high, thereby significantly improve the differential protection performance factor.
In order to achieve the above object; The applicant is carrying out finding behind the off-line analysis to the transformer excitation flow recorder data; When adopting Fourier transform (FFT) to ask the effective value of each harmonic; Carry out Fourier transform behind the first differential again if will shove, then find
subharmonic effective value has been enlarged
doubly.Associate thus.If the differentiation of shoving when utilizing the harmonic wave effective value enlarged to carry out transient state; Then the redundance of criterion will significantly improve; The derivative characteristic that combines Luo-coil more promptly utilizes Luo-coil that the big electric current of transformer primary side is measured, and the harmonic wave of the differentiated voltage signal that obtains is carried out multianalysis; New criterion as inrush current of transformer is differentiated then can reach the foregoing invention purpose in theory.
On this understanding basis, the inrush current of transformer distortion detection method based on Luo-coil that the applicant proposes may further comprise the steps:
The first step, by Luo-coil measuring transformer primary side current
, obtain differential voltage simulating signal
;
Second the step, differential voltage simulating signal
is carried out analog to digital conversion and Filtering Processing, obtain corresponding differential voltage digital signal
(
; Requirement according to existing sampling precision; The value of
generally is taken as 20, i.e. 20 points of a cycle sampling);
The 3rd step, according to the criterion of in the differential protection abnormal current being judged (usually when difference of phase currents greater than 0.5
---
is differential definite value; Perhaps be on duty stream greater than 0.8
; Judge that then abnormal current appears in transformer); Judge whether to occur abnormal current; Then carry out next step in this way, as otherwise return the first step;
The 4th step, from first cycle behind the abnormal current occurring; Differential voltage digital signal
is handled through the Fourier transform that adds Cosine Window, obtained each harmonic effective value
(
;
), with harmonic wave effective value
the substitution computes degree of distortion
:
Where:
(
) for the transient differential voltage signal when the first
a cycle
first
harmonics RMS;
is the sampling number of a cycle;
for the first
one cycle of the fundamental rms;
exists during for transient state the cycle number of harmonic wave (because the decay of transformer transient state obviously; So
for there being preceding
individual cycle of obvious transient state situation, the value of
generally is taken as 3);
The 5th goes on foot, judges whether to occur shoving according to the degree of distortion
; When
then is judged to be excitation surge current greater than threshold values; Then be judged to be internal fault (threshold values differentiated of the degree of distortion of shoving can pass through experiment and obtain, experimental verification to try to achieve be 12.5) repeatedly less than threshold values.
The present invention adopts fast Fourier (FFT) algorithm that adds the Cosine Window interpolation accurately to obtain the effective value of differentiated voltage signal flow control
individual cycle
subharmonic; Ask the ratio of harmonic wave total amount
and first-harmonic effective value
again; This ratio is the degree of distortion of shoving
(
=
) of
individual cycle; (
) asks root-mean-square value
to
then, the degree of distortion value of shoving when the value of promptly using
defines transient state.The high real-time of considering transformer differential protection requires and the excitation surge current characteristics of decay fast, and when choosing unusual cycle data window (the 4th go on foot choose unusual cycle data window), the data of choosing first three cycle are comparatively suitable.
Compared with prior art; Remarkable substantive distinguishing features one of the present invention is by the big current signal of Luo-coil measuring transformer primary side; Obtain reacting the differentiated voltage signal of big current signal characteristic; This voltage signal is carried out frequency analysis, to the numerical value of harmonic content effective value enlarge with the redundance that enlarges criterion with strengthen its validity; The 2nd, adopt Fast Fourier Transform (FFT) (FFT) algorithm that adds Cosine Window accurately to calculate the effective value of each harmonic, this algorithm is a kind of improvement algorithm that proposes to the not high shortcoming of conventional fast fourier transform (FFT) algorithm accuracy; The 3rd, finally confirm the degree of distortion value of shoving with the root mean square method, can reduce the error of calculation effectively.
When abnormal current is fault current; Because harmonic content is minimum; Ratio
can be very little; And it is can not become multiple to change, so constant basically through the each harmonic effective value behind the Luo-coil differential for harmonic wave effective value before and after the differential; When being excitation surge current; Contain a large amount of integral frequency harmonizing waves;
value will be very big; For the micro component behind the process Luo-coil differential;
subharmonic effective value before than differential enlarges
to its
subharmonic effective value doubly, and the degree of distortion value of shoving that then obtains will enlarge several times (value through evidence
will enlarge 3-4 times); Therefore being easy to distinguish transformer current according to the size of the degree of distortion
is fault electric current or excitation surge current when unusual.And the degree of distortion
has reflected the distortion degree of inrush current of transformer all sidedly; Therefore can guarantee differential protection not malfunction when no-load transformer closes a floodgate reliably; And strict exciting current and the fault current distinguished; Criterion value has very high redundance; Judge accurately reliably, so the present invention has outstanding progress.
Description of drawings
Below in conjunction with accompanying drawing the present invention is further described.
Fig. 1 is the system flowchart of one embodiment of the invention.
Fig. 2 is three-phase and door mode of braking structural representation.
Fig. 3 is one of sampled data oscillogram before the differential.
Fig. 4 is one of harmonic wave effective value chart of first three unusual cycle before the differential.
Fig. 5 is for adding one of Luo-coil differential post-sampling data waveform figure.
Fig. 6 is one of harmonic wave effective value chart of first three unusual cycle behind the differential.
Fig. 7 is two of the preceding sampled data oscillogram of differential.
Fig. 8 be first, second and third cycle of unusual back harmonic wave effective value chart two.
Fig. 9 is for adding two of Luo-coil differential post-sampling data waveform figure.
Figure 10 be first three unusual cycle behind the differential harmonic wave effective value chart two.
Embodiment
Embodiment one
The main flow process based on the inrush current of transformer of Luo-coil distortion detection method of present embodiment is as shown in Figure 1, calculates output element by Luo-coil measurement links, analog to digital conversion and filtering link, the degree of distortion and constitutes.The concrete steps of each link are following:
1, adopts Luo-coil at measuring section; The big electric current of measuring transformer primary side
obtains differentiated voltage signal
;
2,
sent into analog to digital conversion and filtering (with prior art roughly the same; The method of existing many maturations is not given unnecessary details in addition) link obtains digital signal
(
after handling; The sampling number of
cycle);
3, judge whether to occur abnormal current according to the criterion of in the differential protection abnormal current being judged; If abnormal current then gets into the calculating link in following the 4th step; When difference of phase currents greater than 0.5
(
is differential definite value); Perhaps be on duty stream greater than 0.8
, judge that then abnormal current appears in transformer;
4,
through the Fourier transform that adding Cosine Window, obtain each harmonic effective value
(
;
), with calculating the degree of distortion
in harmonic wave effective value
the substitution degree of distortion solution formula.
The high real-time of considering transformer differential protection requires the quick decay property with transformer excitation flow; First three cycle that occurs according to shoving is abundant and comprehensive to the comparison of harmonic information reaction; In the practice of differential protection, an intercepting occurs the sampled data of first three cycle of back unusually and calculates; SF is 1000HZ in transformer differential protection, and promptly sampling number is 20 points, and this is enough to satisfy the calculation requirement that the transformer difference is protected.For the cycle is the excitation surge current of 20 sampled points; The value of its higher hamonic wave is very little in fact; Obtain the effective value of preceding 10 subharmonic according to Fast Fourier Transform (FFT); Only get final product so participate in the harmonic wave effective value of computing, then the value of
with getting 2-10 time:
Try to achieve through lots of emulation and test:
The threshold limit value value of <img file=" 229236DEST_PATH_IMAGE012.GIF " he=" 18 " img-content=" drawing " img-format=" jpg " inline=" no " orientation=" portrait " wi=" 17 " /> can get 12.5; Promptly as <img file=" 728350DEST_PATH_IMAGE012.GIF " he=" 18 " img-content=" drawing " img-format=" jpg " inline=" no " orientation=" portrait " wi=" 17 " >be judged as excitation surge current 12.5 the time, <be judged as fault current 12.5 the time as <img file=" 824482DEST_PATH_IMAGE012.GIF " he=" 18 " img-content=" drawing " img-format=" jpg " inline=" no " orientation=" portrait " wi=" 17 " />.
As shown in Figure 2, can adopt three-phase AND gate mode of braking to carry out the differential protection of transformer three-phase, can guarantee that like this transformer main protection can malfunction when air-drop.Ra, rb, rc are respectively A, B, the C three phase excitation judged result of shoving among the figure, r be ra, rb, rc with result's (rc is a true or false according to the judged result value of A, B, C three-phase for ra, rb).
Be that example describes with two groups of real recorder datas below.
Fig. 3 to Fig. 6 is that the recorder data behind the flat IB3 phase excitation surge current of stubble unusual is analyzed.Wherein Fig. 3, Fig. 4 calculate the degree of distortion: the R=25.94 that shoves by degree of distortion computing formula of the present invention; Fig. 5, Fig. 6 calculate the degree of distortion: the R=81.08 that shoves by degree of distortion computing formula of the present invention
Fig. 7 to Figure 10 is the recorder data analysis of Yinchuan new city fault current after unusual.Wherein Fig. 7, Fig. 8 draw degree of distortion value: the R=1.92 that shoves after calculating through the present invention; Fig. 9, Figure 10 draw degree of distortion value: the R=1.68 that shoves through after this algorithm computation.
Can prove that thus the method for present embodiment is practical, have enough validity and reliability.
Except that the foregoing description, the present invention can also have other embodiments.All employings are equal to the technical scheme of replacement or equivalent transformation formation, all drop on the protection domain of requirement of the present invention.