Method For Overcoming Influence Of Out-Flowing Current On Bus Differential Protection

Abstract

The invention provides a method for overcoming the influence of out-flowing current on bus differential protection. The method comprises the following steps: acquiring and processing branch current signals; selecting a fault bus, and determining the branch current with maximum amplitude from branches connected with the fault bus; calculating differential current and restraint current of a large differential element, and determining whether the large differential element acts. The method for overcoming the influence of out-flowing current on bus differential protection does not need to reduce the braking coefficient during splitting operation in a two-bus connecting mode, can adaptively improve the sensitivity of bus differential protection under an internal fault in the presence of out-flowing current, and simultaneously ensures the reliability under an external fault.

Claims

1. A method for overcoming the influence of out-flowing current on bus differential protection, comprising the following steps: step 1, acquiring and processing branch current signals; step 2, selecting a fault bus, and determining the branch current with maximum amplitude from the branches connected with the fault bus: step 3, calculating differential current and restraint current of a large differential element, and judging whether the large differential element acts.

2. The method for overcoming the influence of out-flowing current on bus differential protection of claim 1 comprising the following steps: step 1-1, acquiring current sampling values of all branches connected with a bus, and performing low-pass filtration to obtain a k.sup.th current sampling value i.sub.j(k) of the j.sup.th branch, wherein j=1, 2, . . . , n, and n is the total number of branches connected with the bus; step 1-2, performing Fourier transformation on the i.sub.j(k) to obtain a real part X.sub.j and an imaginary part Y.sub.j of the current phasor i.sub.j of the j.sup.th branch, $X_j=\frac{1}{N}\left[2.Math.\underset{k=1}{\overset{N-1}{.Math.}}.Math..Math.i_j\left(k\right).Math.\sin \left(k.Math.\frac{2.Math.\pi }{N}\right)\right]$ $Y_j=\frac{1}{N}\left[2.Math.\underset{k=1}{\overset{N-1}{.Math.}}.Math..Math.i_j\left(k\right).Math.\cos \left(k.Math.\frac{2.Math.\pi }{N}\right)\right]$ wherein N is the number of sampling points of fundamental wave within one cycle; and obtaining amplitude I.sub.jM and phase angle θ.sub.j of İ.sub.j via the real part X.sub.j and the imaginary part Y.sub.j: $I_{\mathrm{jM}}=\sqrt{\frac{X_j^2+Y_j^2}{2}}$ ${\theta }_j=\mathrm{arc}.Math..Math.\mathrm{tg}.Math.\frac{Y_j}{X_j}.$

3. The method for overcoming the influence of out-flowing current on bus differential protection of claim 2 comprising the following steps: step 2-1, calculating differential current and restraint current of a small differential element, the differential current and the restraint current of the small differential element being respectively expressed by and , $=.Math.\underset{j=1}{\overset{m}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.$ $=\underset{j=1}{\overset{m}{.Math.}}.Math..Math..Math.{\stackrel{.}{I}}_j.Math.$ wherein, m is the number of all branches connected with a single-sectional bus; step 2-2, if the differential current and the restraint current of the small differential element corresponding to a certain bus satisfy >k.sub.res1, determining the bus as a fault bus, wherein k.sub.res1 is a percentage restraint coefficient of the small differential element, and is generally 0.6; and step 2-3, selecting the branch current İ.sub.max with maximum amplitude from the branches connected with the determined fault bus.

4. The method for overcoming the influence of out-flowing current on bus differential protection of claim 3 comprising the following steps: step 3-1, calculating the differential current of the large differential element, $I_{\mathrm{cd}}=.Math.\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.$ wherein I.sub.cd is the differential current of the large differential element; step 3-2, calculating the restraint current of the large differential element,
I.sub.zd=|(İ.sub.cd−İ.sub.max)−İ.sub.max| wherein I.sub.zd is the restraint current of the large differential element, İ.sub.cd is the differential current phasor of the large differential element, and ${\stackrel{.}{I}}_{\mathrm{cd}}=\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j;$ judging whether the large differential element acts, wherein if the ratio braking criterion I.sub.cd>k.sub.res1I.sub.zd is satisfied, $.Math.\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.>k_{\mathrm{res}}.Math..Math.\left({\stackrel{.}{I}}_{\mathrm{cd}}-{\stackrel{.}{I}}_{\max }\right)-{\stackrel{.}{I}}_{\max }.Math.$ it indicates the large differential element acts, otherwise, it indicates the large differential element does not act, and k.sub.res is the percentage restraint coefficient of the large differential element and is 0.8.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a schematic diagram of out-flowing current of two-bus connecting internal fault in the prior art;

[0031] FIG. 2 is a flow diagram of a method for overcoming the influence of out-flowing current on bus differential protection in an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] As shown in FIG. 2, the present invention provides a method for overcoming the influence of out-flowing current on bus differential protection, which does not need to reduce the braking coefficient during splitting operation in a two-bus connecting mode, can adaptively improve the sensitivity of bus differential protection for an internal fault in the presence of out-flowing current, and simultaneously ensures the reliability under an external fault.

[0033] Overcoming the influence of out-flowing current on bus differential protection, comprising the following steps:

[0034] step 1, acquiring and processing branch current signals;

[0035] step 2, selecting a fault bus, and determining the branch current with maximum amplitude from the branches connected with the fault bus:

[0036] step 3, calculating differential current and restraint current of a large differential element, and judging whether the large differential element acts.

[0037] step 1 comprising the following steps:

[0038] step 1-1, acquiring current sampling values of all branches connected with a bus, and performing low-pass filtration to obtain a k.sup.th current sampling value i.sub.j(k) of the j.sup.th branch, wherein j=1, 2, . . . , n, and n is the total number of branches connected with the bus;

[0039] step 1-2, performing Fourier transformation on the i.sub.j(k) to obtain a real part X.sub.j and an imaginary part Y.sub.j of the current phasor i.sub.j of the j.sup.th branch,

[00011]$X_j=\frac{1}{N}\left[2.Math.\underset{k=1}{\overset{N-1}{.Math.}}.Math..Math.i_j\left(k\right).Math.\sin \left(k.Math.\frac{2.Math.\pi }{N}\right)\right]$$Y_j=\frac{1}{N}\left[2.Math.\underset{k=1}{\overset{N-1}{.Math.}}.Math..Math.i_j\left(k\right).Math.\cos \left(k.Math.\frac{2.Math.\pi }{N}\right)\right]$

[0040] wherein N is the number of sampling points of fundamental wave within one cycle; and

[0041] obtaining amplitude I.sub.jM and phase angle θ.sub.j of İ.sub.j via the real part X.sub.j and the imaginary part Y.sub.j:

[00012]$I_{\mathrm{jM}}=\sqrt{\frac{X_j^2+Y_j^2}{2}}$${\theta }_j=\mathrm{arc}.Math..Math.\mathrm{tg}.Math.\frac{Y_j}{X_j}.$

[0042] step 2 comprising the following steps:

[0043] step 2-1, calculating differential current and restraint current of a small differential element,

[0044] the differential current and the restraint current of the small differential element being respectively expressed by and ,

[00013]$=.Math.\underset{j=1}{\overset{m}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.$$=\underset{j=1}{\overset{m}{.Math.}}.Math..Math..Math.{\stackrel{.}{I}}_j.Math.$

[0045] wherein, m is the number of all branches connected with a single-sectional bus;

[0046] step 2-2, if the differential current and the restraint current of the small differential element corresponding to a certain bus satisfy >k.sub.res1, determining the bus as a fault bus, wherein k.sub.res1 is a percentage restraint coefficient of the small differential element, and is generally 0.6; and

[0047] step 2-3, selecting the branch current İ.sub.max with maximum amplitude from the branches connected with the determined fault bus.

[0048] step 3 comprising the following steps:

[0049] step 3-1, calculating the differential current of the large differential element,

[00014]$I_{\mathrm{cd}}=.Math.\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.$

[0050] wherein I.sub.cd is the differential current of the large differential element;

[0051] step 3-2, calculating the restraint current of the large differential element,

I.sub.zd=|(İ.sub.cd−İ.sub.max)−İ.sub.max|

[0052] wherein I.sub.zd is the restraint current of the large differential element, İ.sub.cd is the differential current phasor of the large differential element, and

[00015]${\stackrel{.}{I}}_{\mathrm{cd}}=\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j;$

[0053] judging whether the large differential element acts, wherein if the ratio braking criterion I.sub.cd>k.sub.resI.sub.zd is satisfied,

[00016]$.Math.\underset{j=1}{\overset{n}{.Math.}}.Math..Math.{\stackrel{.}{I}}_j.Math.>k_{\mathrm{res}}.Math..Math.\left({\stackrel{.}{I}}_{\mathrm{cd}}-{\stackrel{.}{I}}_{\max }\right)-{\stackrel{.}{I}}_{\max }.Math.$

[0054] it indicates the large differential element acts, otherwise, it indicates the large differential element does not act, and k.sub.res is the percentage restraint coefficient of the large differential element and is 0.8.

[0055] Finally it should be noted that the described embodiments are merely a part, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all of other embodiments obtained by those of ordinary skill without any creative effort are within the protection scope of the present application.