CN105137270B - A kind of excitation surge current method for quickly identifying based on difference current discrete cosine transform - Google Patents
A kind of excitation surge current method for quickly identifying based on difference current discrete cosine transform Download PDFInfo
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- CN105137270B CN105137270B CN201510633156.8A CN201510633156A CN105137270B CN 105137270 B CN105137270 B CN 105137270B CN 201510633156 A CN201510633156 A CN 201510633156A CN 105137270 B CN105137270 B CN 105137270B
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Abstract
The present invention relates to a kind of transformer excitation flow method for quickly identifying based on difference current discrete cosine transform, belong to Relay Protection Technology in Power System field.When the present invention is that transformer occurs internal short-circuit or generates excitation surge current, measurement end detects and records to obtain segregated phase differential current data.Each difference flow data is converted respectively using discrete cosine transform；Ten before gained sequence of real numbers standard deviations are sought after each phase discrete cosine transform respectively, and take its maximum value σmaxLogarithm as criterion characteristic quantity；By comparing criterion characteristic quantity and the size for pre-setting threshold value Th, inside transformer short circuit and excitation surge current are judged：If log (σmax) ＞ Th, then it is judged to inside transformer short circuit；If log (σmax)≤Th, then be judged to excitation surge current.Theory analysis and a large amount of emulation show of the invention correct effective.
Description
Technical field
The present invention relates to a kind of excitation surge current method for quickly identifying based on difference current discrete cosine transform, belong to electric power
System relay protection technical field.
Background technology
Transformer is as a kind of a kind of essential primary equipment being connected between different voltages grade, safe operation
It has great significance to the stabilization of whole system.The main protection of transformer at present based on harmonic braking formula differential protection,
Protection to transformer is completed to be constituted with second harmonic, differential quick-action, percentage differential.It is to utilize one secondary side of transformer
Between electric current make the difference after carry out a series of processing, to obtain the criterion of excitation surge current.But since static exciter is gushed
Stream is often as high as rated current several times, and waveform is in peaked wave and is biased to time shaft side, and differential protection is built upon transformer
On the basis of stable state magnetic circuit balance, the medium transient state of voltage recovery process after transformer during no-load closing, overexcitation or failure removal
Under process, electric current can compare with short circuit current, to which this equilibrium relation can be because magnetic circuit saturation generates transformer excitation flow
And be not being met, so that make differential protection misoperation.Thus, the difficult point of transformer differential protection is that shoving and internal
Reliable, the rapid identification of short circuit, to ensure to protect correct action.
Around the differentiation of power transformer excitation surge current, domestic and foreign scholars have carried out many researchs, successively emerge many
Method, and the transformer differential for being generally based on secondary harmonic brake principle and interrupted angle principle to come into operation at present.It is secondary
Harmonic restraint method is the characteristics of being much larger than internal short-circuit situation according to the second harmonic that excitation surge current generates, to calculate two in difference stream
Subharmonic content is judged to shoving if its value is larger.But secondary harmonic brake method has the disadvantage that：Because currently used for
Make transformer core is typically high magnetic conduction cold rolled grain silicon steel material, and easily saturation is so that certain in excitation surge current
The second harmonic component ratio of a little phases substantially reduces, and leads to false protection.Interrupted angle principle is utilized inrush and discontinuously can
The feature of larger interval angle is generated to discriminate whether to generate excitation surge current, but when due to the CT progress of disease, CT saturations often occur
Situation, this will caused by interruption corner characteristics unobvious even disappear the problem of to judge to fail.Waveform symmetry principle is
Serious distortion is will produce to extract feature by the methods of least square method using difference stream waveform when excitation surge current occurs
Variable distinguishes excitation surge current and internal short-circuit.The principle is considered from waveform, can regard the popularization of interrupted angle principle as.
But inrush is related with many factors, has uncertain, diversity；In wink electric current complex shape at the beginning of failure, carry merely
It takes local feature amount often to cause to judge by accident, the diversity of fault condition and the complexity of fault waveform is must take into consideration in real system
Property.
Invention content
It is existing to inside transformer short circuit and excitation surge current discrimination method the purpose of the present invention is overcoming the shortcomings of, it proposes
One kind being based on difference current discrete cosine transform transformer excitation flow quick discrimination method.
The technical scheme is that：Consecutive points distance in a kind of plane constituted based on the adjacent order difference of difference current
Excitation surge current method for quickly identifying, when transformer occur internal short-circuit or generate excitation surge current when, measurement end detects and remembers
Record to obtain segregated phase differential current data.Each difference flow data is converted respectively using discrete cosine transform；It seeks respectively each mutually discrete
Ten standard deviations before sequence of real numbers obtained by after cosine transform, and take its maximum value σmaxAs criterion characteristic quantity；Sentence by comparing
According to characteristic quantity and the size for pre-setting threshold value Th, inside transformer short circuit and excitation surge current are judged：If log (σmax)
＞ Th are then judged to inside transformer short circuit；If log (σmax)≤Th, then be judged to excitation surge current.Theory analysis and a large amount of emulation tables
The bright present invention is correct effective.
It is as follows：
(1) when internal short-circuit or excitation surge current occur for transformer, measurement end detects and records segregated phase differential current, predetermined threshold value
Th=4；
(2) the segregated phase differential current data i after the short circuit of extraction inside transformer or excitation surge current occur when 10ms in windowA(n)、iB
(n)、iC(n), A, B, C are the number of Three-Phase Transformer, and n is n-th of sampled point of discrete signal；
(3) discrete cosine transform is carried out to the difference stream in each phase 10ms forms respectively:
In formula, n is n-th of sampled point of discrete signal；N-1 is total number of samples；For Three-Phase Transformer
Number；For the sequence of real numbers after discrete cosine transform；
It enables
Then sequenceDiscrete cosine transform be represented by
WhereinForGained column vector after difference stream sampled data discrete cosine transform, CNFor with Ck,nFor matrix element group
At N × N rank matrixes,To flow sampled data as the column vector of element using each difference；
(4) each phase is sought respectively by following formulaTen average value before sequence
It utilizesSeek each phaseTen standard deviations before sequence
Seek each phaseMaximum value and be denoted as σmax；
(5) differentiation of inside transformer short circuit and excitation surge current is carried out according to following criterion：
If log (σmax) ＞ Th, then it is judged to inside transformer short circuit；
If log (σmax)≤Th, then be judged to excitation surge current.
The principle of the present invention is：
One, the principle of discrete cosine transform
Length is the sequence x (n) of N, and discrete cosine transform is
It enables
Wherein
The then discrete cosine transform of sequence x (n) is
Due to transformation matrix CNFor normalized orthogonal matrix, so, discrete cosine transform is orthogonally transformed, such sequence x
(n) it is to say sequence to be, then element is also real number after its discrete cosine transform.
Two, average value and standard deviation are sought
By the definition of average value, each phaseThe average value of sequence
In formulaFor the number of the three-phase of transformer；
Since standard deviation refers to the statistical result amplitude that error fluctuates up and down in a certain amount of time, utilizeSeek each correspondence
PhaseThe standard deviation of sequence
Three, the differentiation of inside transformer short circuit and excitation surge current
Discrete cosine transform can characterize the totality of signal with seldom several transformation coefficients well very much, can enhance letter
Number dispersion；And the sequence low frequency part amplitude meeting in one timing window of internal short-circuit after discrete cosine transform is occurring
It is better than excitation surge current：The σ of dispersion degree will be characterizedmaxLogarithm be compared with threshold value Th：
If log (σmax) ＞ Th, then it is judged to inside transformer short circuit；
If log (σmax)≤Th, then be judged to excitation surge current.
The beneficial effects of the invention are as follows：
1, this method reduces influence of the distortion to judgement in the reinforcement for carrying out characteristic quantity using discrete cosine transform；
2, decision analysis is carried out using 10ms short time-windows, taken window is shorter；
3, this method has the function of decorrelation, reduces redundancy.
Description of the drawings
Fig. 1 is transformation of embodiment of the present invention internal short-circuit and Inrush Simulation system model；
Fig. 2 is that 28.5% winding earth failure of step down side generation A phases respectively differs flow graph；
Respectively difference flows the sequence chart after discrete cosine transform when Fig. 3 short circuit groundings；
Fig. 4 respectively differs flow graph when being transformation excitation surge current；
Respectively difference flows the sequence chart after discrete cosine transform when Fig. 5 excitation surge currents.
Specific implementation mode
Below in conjunction with the drawings and specific embodiments, the invention will be further described.
When transformer occurs internal short-circuit or generates excitation surge current, measurement end measures the segregated phase differential current of transformer.It carries
Segregated phase differential current data after taking inside transformer short circuit or excitation surge current to occur when 10ms in window, are distinguished using discrete cosine transform
Each difference flow data is converted；Ten before gained sequence of real numbers standard deviations are sought after each phase discrete cosine transform respectively,
And take its maximum value σmaxLogarithm as criterion characteristic quantity；By comparing criterion characteristic quantity and pre-set the big of threshold value Th
It is small, inside transformer short circuit and excitation surge current are judged：If σmax＞ Th are then judged to inside transformer short circuit；If σmax≤
Th is then judged to excitation surge current.
Embodiment 1：Transformer fault as shown in Figure 1 and Inrush Simulation system model are established, wherein transformer is
Three single-phase three-winding transformers are transformer primary side its high-voltage winding access 110kV systems using Yd11 connections, in
Winding is pressed to constitute transformer secondary with low pressure winding cascade, the parameter of the equivalent two-winding transformer constituted is as follows：Specified appearance
Amount is 250MVA, nominal transformation ratio 110kV/10.5kV, equivalent resistance 0.002pu, equivalent reactance 0.08pu.It magnetizes ginseng
Number is as shown in table 1:
Table 1
It is now assumed that single-phase earthing at 28.5% winding of A phases occurs for transformer secondary winding, ground resistance is 1 ohm, sampling
Frequency is 20kHz, and under the model, the sampled result of each difference stream is as shown in Fig. 2, can be in the hope of using following formula to sampled result
The value of each element in sequence after its discrete cosine transform：
Each phaseSequence is as shown in figure 3, seek each phaseTen standard deviation sigmas before sequenceA=5731, σB=
29944、σC=6615, and take its maximum value log (σmax)=4.4763>4；
According to criterion, because of σmax＞ Th thus be judged to inside transformer short circuit, and assume consistent, correct judgment.
Embodiment 2：Transformer fault as shown in Figure 1 and Inrush Simulation system model are established, parameter is being implemented
It elaborates, is described again here in example 1.It is now assumed that transformer generates excitation surge current in system, sample frequency is
20kHz, under the model, each difference stream sampled data are as shown in Figure 4.
Corresponding each phase can be similarly acquired by the data of sampled pointSequence, as shown in Figure 5.Because of σA=4840,
σB=9341, σC=9792, so log (σmax)=3.9909<4, excitation surge current is determined as according to criterion.It is consistent with hypothesis, sentence
It is disconnected correct.
Claims (1)
1. a kind of transformer excitation flow method for quickly identifying based on difference current discrete cosine transform, it is characterised in that：Become
When depressor occurs internal short-circuit or generates excitation surge current, measurement end detects and records to obtain segregated phase differential current data, using discrete remaining
String transformation respectively converts each difference flow data；It seeks respectively after each phase discrete cosine transform ten before gained sequence of real numbers
Standard deviation, and take its maximum value σmaxLogarithm as criterion characteristic quantity；By comparing criterion characteristic quantity and pre-set threshold
The size of value Th judges inside transformer short circuit and excitation surge current：If log (σmax) ＞ Th, then it is judged to inside transformer
Short circuit；If log (σmax)≤Th, then be judged to excitation surge current；
It is as follows：
(1) when internal short-circuit or excitation surge current occur for transformer, measurement end detects and records segregated phase differential current, predetermined threshold value Th=
4；
(2) the segregated phase differential current data i after the short circuit of extraction inside transformer or excitation surge current occur when 10ms in windowA(n)、iB(n)、
iC(n), A, B, C are the number of Three-Phase Transformer, and n is n-th of sampled point of discrete signal；
(3) discrete cosine transform is carried out to the difference stream in each phase 10ms forms respectively：
In formula, n is n-th of sampled point of discrete signal；N-1 is total number of samples；For the volume of Three-Phase Transformer
Number；For the sequence of real numbers after discrete cosine transform；
It enables
Wherein
Then sequenceDiscrete cosine transform be represented by：
WhereinForGained column vector after difference stream sampled data discrete cosine transform, CNFor with Ck,nFor matrix element composition
N × N rank matrixes,To flow sampled data as the column vector of element using each difference；
(4) each phase is sought respectively by following formulaTen average value before sequence
It utilizesSeek each phaseTen standard deviations before sequence
Seek each phaseMaximum value and be denoted as σmax；
(5) differentiation of inside transformer short circuit and excitation surge current is carried out according to following criterion：
If log (σmax) ＞ Th, then it is judged to inside transformer short circuit；
If log (σmax)≤Th, then be judged to excitation surge current.
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