CN101162255A - Method for testing CT yarn cutting through estimating out-of-balance current variation - Google Patents

Abstract

The invention relates to a method for detecting CT wire break through estimation of the change of unbalance current, comprising the following steps of: obtaining the angular difference and ratio difference of the CT under any current within the load current scope according to the maximum angular difference and ratio difference performance curves of the CT within the load current scope; calculating the CT secondary current error phasor and the variable quantity of the CT secondary current error phasor of the circuit branch at the reference time and at the current time; accumulating the variable quantity of the CT secondary current error phasor in all circuit branches and estimating the total unbalance current variable quantity of all circuit branches; setting reference times at certain time intervals and comparing the estimated value of the variable quantity of the CT unbalance current and the variable quantity of the actual secondary current error at the current time with those at the reference times to judge the CT wire break. The invention is free from the influence of the change of the operation mode of the system and does not make misjudgment when a big change of current occurs in the system; moreover, the invention can detect the CT wire break delicately regardless the fact that the wire break occurs before the operation of the circuit or the load changes slowly after the circuit operates. The invention has a relatively high sensitivity, low misjudgment rate and strong adaptive capacity to the operation modes of grid.

Description

By estimating that change of unbalance current detects the method for CT broken string

Technical field

The present invention relates to a kind of electric system and detect guard method, specifically relate to a kind of by estimating that change of unbalance current detects the method for CT broken string, belongs to Automation of Electric Systems control technology field.

Background technology

In order to guarantee safe operation of power system, must be equipped with necessary automatic safety device, as differential protection etc.The CT broken string is the major hidden danger of differential protection malfunction, and high sensitivity, low detection time and low False Rate are the most important indexs of CT wire break detection method.Previously used CT wire break detection method is:

1, the asymmetric detection method of three-phase of CT broken string: its criterion is: 3I ₀＞max[k (I _A+ I _B+ I _C), I _{0_th}], 3I in the formula ₀Be zero-sequence current, k is a restraint coefficient, I _A, I _B, I _CBe respectively A, B, the C phase secondary current of CT, I _{0_th}For starting threshold.The asymmetric detection method of three-phase of the CT broken string of this kind reflection zero-sequence current is more reasonable, safer and sensitivity is higher, but must avoid earth fault and open-phase operation in the electric system.

2, the current imbalance detection method of CT broken string: the CT wire break detection method based on current imbalance has the asymmetric advantage that influences, adapts to symmetry or asymmetric broken string of the electric system of not being subjected to.The CT wire break detection method of current imbalance commonly used has two classes, and the one, the fixing current imbalance detection method of threshold, its operating criterion is: spill current＞fixed current imbalance threshold, its sensitivity is lower; The 2nd, the current imbalance detection method of floating threshold, operating criterion is: the uneven threshold of the spill current＞fixed current+uneven threshold of floating current, this detection method sensitivity is higher, but before putting into operation, taken place under the CT broken string situation, may can not detect broken string, variation when system because of the method for operation, during the out-of-balance current flip-flop, erroneous judgement is broken easily.

Summary of the invention

Be to solve the deficiencies in the prior art, the object of the present invention is to provide a kind of sensitivity that can improve the CT burn out detection, reduce the CT broken string False Rate, improve reliability, to system operation mode have strong adaptive faculty pass through estimate that change of unbalance current detects the method that CT breaks.

CT secondary out-of-balance current derives from the difference of each branch road CT progress of disease error.It is limited that the normal operating mode variation causes the variation of out-of-balance current, and after the CT broken string takes place, change of unbalance current will exceed the possible range that normal operating mode changes.Relatively the estimated value and the measured value of change of unbalance current judge whether CT breaks, and this method is applicable to the situation of multiple branch circuit CT.

For addressing the above problem, the present invention is achieved by the following technical solutions:

A kind of by estimating that change of unbalance current detects the method for CT broken string, is characterized in that may further comprise the steps:

(1), according to ratio and angular difference limit value under cold-reduced silicon sheet magnetization characteristic and the protected level CT secondary rated current, obtain maximum angular difference and the ratio family curve of CT in the load current scope, linear interpolation obtains CT any angular difference under the electric current and compare difference in the load current scope;

(2), get with reference to the angle value of delta of secondary current amplitude the greater constantly and in the current time and separately constantly the secondary current correspondence than difference ε, calculate branch road with reference to constantly and the branch road CT secondary current error phasor I of current time _eAnd I _e', count branch road load trend phase change; Calculate CT secondary current error phasor variation delta I _e

(3), according to discriminant $\left\{\begin{array}{c}\mathrm{\Δ}{I}_d>\max (k\underset{m=1}{\overset{N}{\mathrm{\Σ}}}{R}_m\mathrm{\Δ}{I}_{e\_m},\mathrm{\Δ}{I}_{d0})\\ \mathrm{\Δ}{I}_d=I_d^{\′}-I_d\end{array}\right.$ Each branch road CT secondary current error phasor variation delta I adds up _e, estimating the change of unbalance current amount that all branch roads are total, k is a safety factor in the formula, Δ I _D0For starting threshold, R _mBe the CT no-load voltage ratio reduction coefficient of m bar branch road, Δ I _{E_m}Be the Amplitude Estimation value of the secondary current error phasor variable quantity of m bar branch road, I _dBe the actual measurement spill current with reference to the moment, I _d' be the actual measurement spill current of current time;

(4), at regular intervals set up one with reference to constantly, relatively whether the estimated value of the CT change of unbalance current amount between present moment and the reference constantly and the second difference electric current changing value of actual measurement detect CT and break.

Aforesaid by estimating that change of unbalance current detects the method for CT broken string, is characterized in that described branch road CT secondary current error phasor variation delta I _eComputing formula be: $\mathrm{\Δ}{I}_e=\sqrt{I_e^2+I_e^{\′2}-2I_e{I}_e^{\′}\cos ({\mathrm{\θ}}^{\′}-\mathrm{\θ}+\mathrm{\β})};$ I in the formula _eBe the CT secondary current error phasor with reference to the moment, I _e' be the CT secondary current error phasor of current time, θ is with reference to moment CT secondary current error phasor I _eWith CT primary current I ₁Angle, θ ' is current time CT secondary current error phasor I _e' and CT primary current I ₁' angle, β is the phasing degree that primary current changes.

Aforesaid by estimating that change of unbalance current detects the method for CT broken string, it is characterized in that in step (4), being kept in the buffer zone with reference to each branch current phasor constantly, buffer zone recycles, and surpasses one day, and old data with reference to the moment are covered by new data.

The invention has the beneficial effects as follows: the present invention is based on electric system when normally moving CT secondary change of unbalance current limited, the very big feature of change of unbalance current after the CT broken string takes place, relatively whether the estimated value of change of unbalance current amount and actual measurement second difference electric current changing value detect CT and break.When bigger variation takes place for the influence that the present invention is not changed by system operation mode, system load flow, do not judge by accident.Even the CT broken string takes place before circuit puts into operation, when the afterload that puts into operation slowly changes, can sensitivity detect the CT broken string.The present invention has higher sensitivity, has lower False Rate simultaneously, and power system operating mode is had very strong adaptive ability.

Description of drawings

Fig. 1 is a CT secondary current error phasor graph;

Fig. 2 is CT secondary current error phasor I _eThe approximate treatment phasor graph.

Embodiment

In CT load current scope, the ratio of CT and angular difference monotonicity and polarity do not change with primary current, and ratio is negative and monotone increasing, and angular difference is for just and monotone decreasing.According to ratio and the angular difference limit value under cold-reduced silicon sheet magnetization characteristic and the protected level CT secondary rated current; obtain the ratio and the angular difference family curve of CT maximum in the load current scope, linear interpolation can get ratio and the angular difference value under any electric current in the load current scope.CT secondary current error vector is mainly by the factor decision of variable quantity three aspects of angular difference, ratio, primary current phase place.CT secondary current error phasor I _eAs shown in Figure 1, Fig. 1 is a CT secondary current error phasor graph, I among the figure ₁Be reference moment CT primary current, I _eBe reference moment CT secondary current error phasor, θ is with reference to moment CT secondary current error phasor I _eWith CT primary current I ₁Angle, I ₁' be current time CT primary current, I _e' be current time CT secondary current error phasor, θ ' is with reference to moment CT secondary current error phasor I _e' and CT primary current I ₁' angle, I ₂Be reference moment CT secondary current, I ₂' be current time CT secondary current, β is the phasing degree that primary current changes.Reference is secondary current error phasor variation delta I constantly _eThe time reference point that calculates, current time are the time points after after a while.

The derivation of the estimation formulas of single branch road CT secondary current error phasor variable quantity is as follows:

Because angular difference is little, can be similar to and thinks I ₁And I ₂Parallel, I wherein ₁Be CT primary current, I ₂Be the CT secondary current, length of side BC is divided by I ₁Be similar to the radian δ that differs, with current error phasor AB, difference in magnitude AC form right angle triangle, as shown in Figure 2, I among the figure ₁Be reference moment CT primary current, I ₂Be reference moment CT secondary current, I _eBe CT secondary current error phasor, θ is with reference to moment CT secondary current error phasor I _eWith CT primary current I ₁Angle.Can be by figure with reference to secondary current error phasor I constantly _eAmplitude:

I ₁≈I ₂

$I_e=\sqrt{{\left(\mathrm{\ϵ}{I}_1\right)}^2+{\left(\mathrm{\δ}{I}_1\right)}^2}=I_1\sqrt{{\mathrm{\ϵ}}^2+{\mathrm{\δ}}^2}$

$\mathrm{\θ}={\tan }^{-1}\frac{{\mathrm{\δI}}_1}{\mathrm{\ϵ}{I}_1}={\tan }^{-1}\frac{\mathrm{\δ}}{\mathrm{\ϵ}}$

In the following formula, ε is with reference to CT ratio constantly, and δ is with reference to the CT angular difference of primary current amplitude the greater in the moment and the current time, gets the secondary current error phasor variation delta I that this CT angular difference draws _eBe conservative and estimation bigger than normal.

Ignore the influence that angular difference changes, can get the secondary current error phasor I in the current moment _e' amplitude:

I ₁′≈I ₂′

$I_e^{\′}=I_1^{\′}=\sqrt{{\mathrm{\ϵ}}^{\′2}+{\mathrm{\δ}}^2}$

${\mathrm{\θ}}^{\′}={\tan }^{-1}\frac{\mathrm{\δ}}{{\mathrm{\ϵ}}^{\′}}$

In the following formula, ε ' is the CT ratio of current time.

Consider the influence of the phasing degree β that primary current changes, the Amplitude Estimation value that can get secondary current error phasor variable quantity is:

$\mathrm{\Δ}{I}_e=\sqrt{I_e^2+I_e^{\′2}-2I_e{I}_e^{\′}\cos ({\mathrm{\θ}}^{\′}-\mathrm{\θ}+\mathrm{\β})}$

According to single branch road CT secondary current error phasor variable quantity, the total change of unbalance current amount of the Amplitude Estimation that adds up is the most conservative estimation.Can get following discriminant:

$\left\{\begin{array}{c}\mathrm{\Δ}{I}_d>\max (k\underset{m=1}{\overset{N}{\mathrm{\Σ}}}{R}_m\mathrm{\Δ}{I}_{e\_m},\mathrm{\Δ}{I}_{d0})\\ \mathrm{\Δ}{I}_d=I_d^{\′}-I_d\end{array}\right.$

In the following formula, k is a safety factor, can be taken as 2; Δ I _D0Be to start threshold, erroneous judgement when preventing that the estimated value of current error variable quantity is too small can be set at 0.02I _n, I wherein _nBe branch road CT secondary current ratings, R _mIt is the CT no-load voltage ratio reduction coefficient of m bar branch road; Δ I _{E_m}It is the Amplitude Estimation value of the current error phasor variable quantity of m bar branch road.I _dBe the actual measurement spill current with reference to the moment, I _d' be the actual measurement spill current of current time.

With the variation of ruuning situation, set up one at regular intervals as time passes with reference to constantly.Be kept in the buffer zone with reference to each branch current phasor constantly, buffer zone recycles, and surpasses one day, and old data with reference to the moment are covered by new data.Calculate current time with respect to each the change of unbalance current amount with reference to the moment in the buffer zone according to discriminant, it is then alarm or locking of CT broken string that any one result of calculation is differentiated.

The foregoing description does not limit the present invention in any form, and all technical schemes that mode obtained of taking to be equal to replacement or equivalent transformation all drop in protection scope of the present invention.

Claims (3)

1. by estimating that change of unbalance current detects the method for CT broken string, is characterized in that may further comprise the steps:

(1), according to ratio and angular difference limit value under cold-reduced silicon sheet magnetization characteristic and the protected level CT secondary rated current, obtain maximum angular difference and the ratio family curve of CT in the load current scope, linear interpolation obtains CT any angular difference under the electric current and compare difference in the load current scope;

(2), get with reference to the angle value of delta of secondary current amplitude the greater constantly and in the current time and separately constantly the secondary current correspondence than difference ε, calculate branch road with reference to constantly and the branch road CT secondary current error phasor I of current time _eAnd I _e'; Count branch road load trend phase change, calculate CT secondary current error phasor variation delta I _e

(3), according to discriminant $\left\{\begin{array}{c}\mathrm{\Δ}{I}_d>\max (k\underset{m=1}{\overset{N}{\mathrm{\Σ}}}{R}_m\mathrm{\Δ}{I}_{e\_m},\mathrm{\Δ}{I}_{d0})\\ \mathrm{\Δ}{I}_d=I_d^{\′}-I_d\end{array}\right.$ Each branch road CT secondary current error phasor variation delta I adds up _e, estimating the change of unbalance current amount that all branch roads are total, k is a safety factor in the formula, Δ I _D0For starting threshold, R _mIt is the CT no-load voltage ratio reduction coefficient of m bar branch road; Δ I _{E_m}Be the Amplitude Estimation value of the secondary current error phasor variable quantity of m bar branch road, I _dBe the actual measurement spill current with reference to the moment, I _d' be the actual measurement spill current of current time;

(4), at regular intervals set up one with reference to constantly, relatively whether the estimated value of the CT change of unbalance current amount between present moment and the reference constantly and the second difference electric current changing value of actual measurement detect CT and break.

2. according to claim 1 by estimating that change of unbalance current detects the method for CT broken string, is characterized in that described branch road CT secondary current error phasor variation delta I _eComputing formula be: $\mathrm{\Δ}{I}_e=\sqrt{I_e^2+I_e^{\′2}-2I_e{I}_e^{\′}\cos ({\mathrm{\θ}}^{\′}-\mathrm{\θ}+\mathrm{\β})};$ I in the formula _eBe the branch road CT secondary current error phasor with reference to the moment, I _e' be the branch road CT secondary current error phasor of current time, θ is with reference to moment CT secondary current error phasor I _eWith CT primary current I ₁Angle, θ ' is current time CT secondary current error phasor I _e' and CT primary current I ₁' angle, β is the phasing degree that primary current changes.

3. according to claim 1 by estimating that change of unbalance current detects the method for CT broken string, it is characterized in that in step (4), being kept in the buffer zone with reference to each branch current phasor constantly, buffer zone recycles, and surpasses one day, and old data with reference to the moment are covered by new data.

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