CN100336275C - Transformer differential protection method based on virtual magnetic flux-differential current characteristics - Google Patents

Abstract

The present invention relates to a transformer differential protection method based on virtual magnetic flux-differential current characteristics, which belongs to the technical field of transformer relay protection. The present invention is characterized in that the present invention provides a concept of the virtual magnetic flux, namely that leakage magnetic flux is converted to a primary side to become main magnetic flux with a fixed winding turn number, and simultaneously, an asymmetry degree is defined to reflect the central symmetry of graphs according to the characteristic that the shape of a virtual magnetic flux-differential current curve of an internal fault is approximate to an ellipse and has good central symmetry. When differential current exceeds an action threshold value under the control of a computer, an average of the asymmetry degrees of a plurality of continuous sampling points in the first two industrial frequency periods is compared with a predetermined braking threshold value. So long as the average of the asymmetry degrees of any one phase is more than or equal to the braking threshold value, the present invention judges that the differential current is an excitation surge. Otherwise, the present invention judges that the differential current is the internal fault. The computer transmits corresponding control commands according to the judgment, and the present invention has the advantages of simplicity, convenience, high flexibility, etc.

Description

Transformer differential protection method based on virtual flux-differential current characteristic

Technical field

The present invention relates to a kind of transformer differential protection method, this method is differentiated magnetizing inrush current and internal fault according to the centre symmetry quality of the virtual flux-differential current curve of transformer, belongs to the transformer relay protecting technical field.

Background technology

In the transformer relay protecting field; differential protection is adopted in main protection usually; differential protection is based on the amplitude and the phase place of each side electric current of comparison transformer; the principle of application Kirchhoff's current law (KCL) realizes; and secondary side current poor of transformer both sides differential current after referring to primary side current and amounting to; the transformer differential electric current of normal operation is zero, and magnetizing inrush current and internal fault current all corresponding bigger differential current.This distinguishes magnetizing inrush current and internal fault rapidly and exactly with regard to needs, the sensitive action of protection when internal fault make to take place, and protection can reliably be braked when magnetizing inrush current occurring.

Related content according to document " electric main equipment Principles of Relay Protection and application " and " characteristics of several transformer excitation flow method of discrimination and the analysis of inner link thereof "; in the method for identification magnetizing inrush current and internal fault; most schemes are discerned according to waveform (voltage or electric current) feature; as secondary harmonic brake principle, interval angle principle, waveform correlation analysis, waveform fitting method or the like, wherein the practical application of secondary harmonic brake principle is the most extensive.The other schemes synthesis is analyzed transformer parameter and shape information, mainly comprises magnetic flux characteristic recognition principle, equivalent circuit principle or the like.

It is to be solved to still have a lot of problems to have from the various schemes of practical experience, with regard to the magnetic flux characteristic principle, though can fundamentally distinguish magnetizing inrush current and internal fault from the feature of transformer main flux, but tend on key parameters such as accurate acquisition transformer leakage inductance, meet difficulty, particularly Magnetic Leakage Field of Transformer still needs to further investigate under the internal fault situation, so practical application is subjected to certain limitation.

Summary of the invention

The objective of the invention is to propose a kind of based on virtual flux---the protection scheme of the differential protection of differential current characteristic, the method only need be monitored the electric current of transformer primary side voltage and both sides, compare with the magnetic flux characteristic scheme and need not the transformer inner parameter, less demanding to the hardware sample frequency, be swift in response, and the braking threshold has bigger nargin;

Need accurate transformer inner parameter because actual main flux calculates, thereby be difficult to realize; The present invention proposes the notion of virtual flux, be about to leakage flux and be converted into the magnetic flux amount of calculation that the fixing main flux of umber of turn obtains afterwards in the lump, the branch voltage equation of primary side that comprises virtual flux is as follows:

$u=\mathrm{Ri}+W\frac{d{\mathrm{\Φ}}_v}{\mathrm{dt}}$

U wherein, i is respectively the terminal voltage and the electric current of this side; R is an all-in resistance; W is the number of turn of this side winding; Φ _vFor virtual

${\mathrm{\Φ}}_v=\frac{1}{W}\∫(u-\mathrm{Ri})\mathrm{dt}\≈\frac{1}{W}\∫\mathrm{udt}\≈\frac{1}{W}{\∫U}_{1m}\sin (\mathrm{\ωt}+\mathrm{\α})\mathrm{dt}=-\frac{U_{1m}}{\mathrm{W\ω}}\cos (\mathrm{\ωt}+\mathrm{\α})$

U wherein _1mBe the amplitude of phase voltage, ω is a power frequency angular speed, and α is the phase voltage initial phase angle;

Therefore virtual flux is similar to the integration of terminal voltage, and need not to carry out power transformer interior fault modeling accurately, relatively is fit to the actual demand of relaying protection; Distinguishing the criterion of magnetizing inrush current and internal fault then extracts from the centre symmetry of virtual flux-differential current curve, under the internal fault situation,, can be similar to sinusoidal form and represent the short circuit differential current because short circuit differential current transient state component is very little, and in conjunction with preceding surface analysis,

$\left\{\begin{array}{c}{\mathrm{\Φ}}_v\≈-\frac{U_{1m}}{\mathrm{W\ω}}\cos (\mathrm{\ωt}+\mathrm{\α})\\ i_d\≈I_{\mathrm{dm}}\sin (\mathrm{\ωt}+\mathrm{\β})\end{array}\right.$

Wherein β is for thinking the pairing initial phase angle of sinusoid with the short circuit differential current is approximate;

Corresponding Φ _v～i _dThe curve shape sub-elliptical, centre symmetry is good; And the magnetizing inrush current wave distortion is serious, and therefore corresponding center of curve symmetry is relatively poor.According to can adjust out the criterion of braking of the centre symmetry of virtual flux-differential current curve, definition degree of asymmetry E _SymThe quality of the centre symmetry of reflection figure also is used for the differentiation of magnetizing inrush current and internal fault;

Protection scheme of the present invention is characterised in that it contains following steps successively:

Step 1, set following parameter and formula and deposit computer in:

The action threshold value of differential current, differential current are meant the transformer primary side current and secondary side current are folded to the difference of the later electric current of primary side between the two that action threshold value is meant the differential current value when described protection braking maneuver starts;

Degree of asymmetry is E _SymThe braking threshold value be k _e, k _eBe meant that spending 1.5th to 2.0 cycle of action after the threshold value in differential current is the E of any phase in the transformer three-phase in the 1st cycle that records in the decision-making time _SymThe threshold value of mean value is as the E of certain phase _SymMean value is greater than k _eThe time, computer just is judged to be magnetizing inrush current, the control signal of output protection braking; Otherwise just be judged to be internal fault, the control signal of output action:

$E_{\mathrm{sym}}=k_1{\left(\frac{x_0^{\&prime;}-x_0}{D_x}\right)}^2+k_2{\left(\frac{{\mathrm{<figure-callout\; id="0"\; label="y"\; filenames="C20041000908800091.png,C20041000908800101.png"\; state="\{\{state\}\}">y</figure-callout>}}_0^{\&prime;}-y_0}{D_y}\right)}^2$

Wherein, x is the instantaneous value of differential current, and y is the instantaneous value of virtual flux;

x ₀=(x _t+ x _T+T/2)/2, x ₀Be meant according to the curve of differential current time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t and two time difference streaming currents of t+T/2;

y ₀=(y _t+ y _T+T/2)/2, y ₀Be meant according to the curve of virtual flux time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t and t+T/2 two moment virtual flux;

x ₀'=(x _T+T/4+ x _T+3T/4)/2, x ₀' be meant according to the curve of differential current time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t+T/4 and two time difference streaming currents of t+3T/4;

y ₀'=(y _T+T/4+ y _T+3T/4)/2, y ₀' be meant according to the curve of virtual flux time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t+T/4 and t+3T/4 two moment virtual flux;

D _x＝max(|x _t-x _t+T/2|，|x _t+T/4-x _t+3T/4|)；

D _y＝max(|y _t-y _t+T/2|，|y _t+T/4-y _t+3T/4|)；

k ₁, k ₂Be rule of thumb to E _SymThe weight coefficient used when regulating of braking threshold value, k in the application ₁, k ₂Each value should satisfy k in interval [0.5,1.5] ₁+ k ₂=2.0;

Above-mentioned virtual flux Φ _vExpression, its approximate expression is

${\mathrm{\&Phi;}}_v\&ap;-\frac{U_{1m}}{\mathrm{W\&omega;}}\cos (\mathrm{\&omega;t}+\mathrm{\&alpha;})$

U wherein _1mBe the amplitude of phase voltage, W is the number of turn of first side winding, and ω is a power frequency angular speed, and α is the phase voltage initial phase angle;

Above-mentioned differential current i _dExpression, the differential current of magnetizing inrush current is non-linear very strong, and its approximate expression is when internal fault takes place

i _d≈I _dm?sin(ωt+β)；

I wherein _DmBe the differential current amplitude, β is an initial phase angle, and sample frequency is f _s

Method described in the step 2, step 1 is carried out under computer control successively according to the following steps:

Step 2.1, computer continues to monitor the three-phase voltage of transformer primary side respectively by three voltage transformers and the A/D converter that is connected in series with it by sample frequency, simultaneously respectively by three current transformers with continue to monitor the three-phase current of transformer primary side with the A/D converter that it is gone here and there mutually, also simultaneously respectively by other three current transformers with continue to monitor the three-phase current of Circuit Fault on Secondary Transformer with the A/D converter that it is gone here and there mutually, when differential current reaches the action threshold value, record is t constantly, and carries out following steps;

Step 2.2 is in the 1.5th times of power frequency period after differential current is crossed the action threshold value, by sample frequency f _sPointwise is calculated this and is put pairing Φ _vAnd i _d, and deposit internal memory in;

Step 2.3 is promptly pressed sample frequency f in 1.5～2.0 times of power frequency periods at ensuing half power frequency period _sΦ is calculated in pointwise _vAnd i _dAnd deposit internal memory in, and calculate this and put interior E of previous cycle _SymValue, during this period of time after at each mutually respectively to the E of all sampled points _SymValue is averaged;

Step 2.4 is above-mentioned three each phase average E _SymThe value respectively with k _eMake comparisons, as long as a value is arranged more than or equal to k _e, computer-made decision is a magnetizing inrush current; Otherwise, be judged to be internal fault, send control signal corresponding according to the result who judges subsequently.Machine is judged to be magnetizing inrush current; Otherwise, be judged to be internal fault, send control signal corresponding according to the result who judges subsequently;

Braking scheme of the present invention is compared the magnetic flux characteristic scheme owing to do not need parameter such as accurate leakage inductance, realize easily, the characteristic quantity of differentiation that scheme is used is degree of asymmetry Esym mean value, and at present the most general employed characteristic quantity of secondary harmonic brake scheme is the ratio I of second harmonic component and fundametal compoment in the differential current _D2/ I _D1Experimental results demonstrate that the differentiation effect of this programme can reach the level of the braking scheme of second harmonic, as to a certain magnetizing inrush current data, the three-phase Esym mean value that this programme calculates is respectively 0.44,1.10,0.49, the three-phase I that afterwards adopts secondary harmonic brake scheme principle to calculate _D2/ I _D1Be 0.57,0.66,0.50; And for a certain A phase turn-to-turn short circuit data, the three-phase Esym mean value that this programme calculates is respectively 1.9e-4,1.1e-3,2.7e-3, the three-phase I that afterwards adopts secondary harmonic brake scheme principle to calculate _D2/ I _D1Be 1.7e-3,5.8e-3,6.0e-3.As seen the effect of this programme is to a certain extent even be better than the secondary harmonic brake scheme in internal fault characteristic identification.

Description of drawings

Fig. 1 is the schematic diagram of a kind of define method of degree of asymmetry.

Fig. 2 is the program flow diagram of protection scheme of the present invention.

Fig. 3 is a protection scheme hardware block diagram of the present invention.

The three-phase differential current waveform that Fig. 4 shoves for typical no-load three phase excitation.

Fig. 5 is shove three-phase virtual flux waveform under the situation of no-load for this reason.

Fig. 6 no-load situation allowance below nominal size that shoves for this reason flows through the virtual flux-differential current curve in two power frequency periods after the action threshold value.

Fig. 7 is the typical three-phase differential current waveform of transformer generation internal fault in service.

Fig. 8 is the three-phase virtual flux waveform under the internal fault situation for this reason.

Fig. 9 internal fault situation allowance below nominal size for this reason flows through the virtual flux-differential current curve in two power frequency periods after the action threshold value.

Embodiment

If Fig. 4 for the monitoring gained the differential current waveform (the three-phase no-load is shoved, Y ₀/ Y ₀The three-phase three-limb experimental transformer of connection), protection braking implementation step is as follows:

1. continue to monitor differential current with sample frequency fs, after no-load was shoved and taken place, differential current had surpassed the action threshold value, and protection scheme starts;

2. after flowing through threshold value be (data time t in 1.5 times of power frequency periods in difference ₀), calculate this by frequency f s pointwise and put pairing three-phase differential current and virtual flux (as Fig. 5), store standby;

3. (Esym t computing time in half power frequency period after step 2 is finished ₁In), calculate this by frequency f s continuation pointwise and put pairing three-phase differential current and virtual flux, and calculate Esym value in the power frequency period before this point, again with Esym in computing time each mutually the Esym value of all sampled point correspondences average, each Esym mean value of calculating gained three-phase is respectively 0.51,0.71 and 0.47; Difference flow through after the action threshold value in two power frequency periods virtual flux-the differential current curve as shown in Figure 6;

With above three values respectively with selected braking threshold value 0.2 relatively, all greater than the braking threshold value, so the three-phase result of determination is " shoving ", get " or " after, the control signal of final output protection braking;

5. return step 1, this algorithm circulates;

And for differential current waveform shown in Figure 7 (A in service 0～10% turn-to-turn short circuit mutually, Y ₀/ Y ₀The three-phase three-limb experimental transformer of connection), the implementation step of protection action is as follows:

1. continue to monitor differential current with sample frequency fs, take place after the internal fault generation, differential current has surpassed the action threshold value, and protection scheme starts;

2. after flowing through threshold value be (data time t in 1.5 times of power frequency periods in difference ₀), calculate this by frequency f s pointwise and put pairing three-phase differential current and virtual flux (as Fig. 8), store standby;

3. (Esym t computing time in half power frequency period after step 2 is finished ₁), calculate this by frequency f s continuation pointwise and put pairing three-phase differential current and virtual flux, and calculate Esym value in the power frequency period before this point, again with Esym in computing time each mutually the Esym value of all sampled point correspondences average, each Esym mean value of calculating gained three-phase is respectively 2.9e-05,1.4e-4 and 2.4e-4; Differential current cross after the action threshold value in two power frequency periods virtual flux-the differential current curve as shown in Figure 9;

With above three values respectively with selected braking threshold value 0.2 relatively, all less than the braking threshold value, so the three-phase result of determination is " fault ", get " or " after, the control signal of final output protection action;

5. return step 1, this algorithm circulates;

Each Esym mean value of the three-phase that then obtains all is far smaller than the braking threshold value, and the three-phase result of determination all is " fault ", the control signal of therefore final output protection action.

Claims (1)

1. based on the transformer differential protection braking method of virtual flux-differential current characteristic, it is characterized in that it contains following steps successively:

Step 1, set following parameter and formula and deposit computer in:

The action threshold value of differential current, differential current are meant the transformer primary side current and secondary side current are folded to the difference of the later electric current of primary side between the two that action threshold value is meant the differential current value when described protection braking maneuver starts;

Degree of asymmetry is E _SymThe braking threshold value be k _e, k _eBe meant that spending 1.5th to 2.0 cycle of action after the threshold value in differential current is the E of any phase in the transformer three-phase in the 1st cycle that records in the decision-making time _SymThe threshold value of mean value is as the E of certain phase _SymMean value is greater than k _eThe time, computer just is judged to be magnetizing inrush current, the control signal of output protection braking; Otherwise just be judged to be internal fault, the control signal of output action:

$E_{\mathrm{sym}}=k_1{\left(\frac{x_0^{\&prime;}-x_0}{D_x}\right)}^2+k_2{\left(\frac{y_0^{\&prime;}-y_0}{D_y}\right)}^2$

Wherein, x is the instantaneous value of differential current, and y is the instantaneous value of virtual flux;

x ₀=(x _t+ x _T+T/2)/2, x ₀Be meant according to the curve of differential current time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t and two time difference streaming currents of t+T/2;

y ₀=(y _t+ y _T+T/2)/2, y ₀Be meant according to the curve of virtual flux time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t and t+T/2 two moment virtual flux;

x ₀'=(x _T+T/4+ x _T+3t/4)/2, x ₀' be meant according to the curve of differential current time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t+T/4 and two time difference streaming currents of t+3T/4;

y ₀'=(y _T+T/4+ y _T+3T/4)/2, y ₀' be meant according to the curve of virtual flux time t, in first power frequency period T of differential current after t moves threshold value constantly excessively, the mean value of t+T/4 and t+3T/4 two moment virtual flux;

D _x＝max(|x _t-x _t+T/2|，|x _t+T/4-x _t+3T/4|)；

D _y＝max(|y _t-y _t+T/2|，|y _t+T/4-y _t+3T/4|)；

k ₁, k ₂Be rule of thumb to E _SymThe weight coefficient used when regulating of braking threshold value, k in the application ₁, k ₂Each value should satisfy k in interval [0.5,1.5] ₁+ k ₂=2.0;

Above-mentioned virtual flux Φ _vExpression, its approximate expression is

${\mathrm{\&Phi;}}_v\&ap;-\frac{U_{1m}}{W_{\mathrm{\&omega;}}}\cos (\mathrm{\&omega;t}+\mathrm{\&alpha;})$

U wherein _1mBe the amplitude of phase voltage, W is the number of turn of first side winding, and ω is a power frequency angular speed, and α is the phase voltage initial phase angle;

Above-mentioned differential current i _dExpression, the differential current of magnetizing inrush current is non-linear very strong, and its approximate expression is when internal fault takes place

i _d≈I _dmsin(ωt+β)；

I wherein _DmBe the differential current amplitude, β is an initial phase angle, and sample frequency is f _s

Method described in the step 2, step 1 is carried out under computer control successively according to the following steps:

Step 2.1, computer continues to monitor the three-phase voltage of transformer primary side respectively by three voltage transformers and the A/D converter that is connected in series with it by sample frequency, simultaneously respectively by three current transformers with continue to monitor the three-phase current of transformer primary side with the A/D converter that it is gone here and there mutually, also simultaneously respectively by other three current transformers with continue to monitor the three-phase current of Circuit Fault on Secondary Transformer with the A/D converter that it is gone here and there mutually, when differential current reaches the action threshold value, record is t constantly, and carries out following steps;

Step 2.2 is in the 1.5th times of power frequency period after differential current is crossed the action threshold value, by sample frequency f _sPointwise is calculated this and is put pairing Φ _vAnd i _d, and deposit internal memory in;

Step 2.3 is promptly pressed sample frequency f in 1.5～2.0 times of power frequency periods at ensuing half power frequency period _sΦ is calculated in pointwise _vAnd i _dAnd deposit internal memory in, and calculate this and put interior E of previous cycle _SymValue, during this period of time after at each mutually respectively to the E of all sampled points _SymValue is averaged;

Step 2.4 is above-mentioned three each phase average E _SymThe value respectively with k _eMake comparisons, as long as a value is arranged more than or equal to k _e, computer-made decision is a magnetizing inrush current; Otherwise, be judged to be internal fault, send control signal corresponding according to the result who judges subsequently.

Images