CN103149478A - Method for detecting working point of current transformer under direct current magnetic biasing condition - Google Patents

Abstract

The invention discloses a method for detecting a working point of a current transformer under a direct current magnetic biasing condition. The method comprises the following steps of determining alternating current magnetic flux of the current transformer by a numerical integration algorithm; superposing one direct current magnetic flux on the alternating current magnetic flux, calculating a corresponding exciting current instantaneous value and a direct current exciting current average value according to a magnetizing curve of an iron core, and calculating the direct current magnetic flux of the iron core of the current transformer by utilizing an iterative approximation method to determine the characteristic quantity associated with the working point of the current transformer under the direct current magnetic biasing condition. The method disclosed by the invention can be used for calculating and determining the characteristic quantities of the sizes of the alternating current magnetic flux and the direct current magnetic flux of the iron core of the current transformer under the direct current magnetic biasing condition, the value of exciting current, the values of alternating current exciting current and direct current exciting current, the current transformer secondary current and the like associated with the working point of the current transformer.

Description

Operation of current transformers point detecting method under a kind of Under Direct Current Bias

Technical field

The present invention relates to determine under Under Direct Current Bias the numerical computation method of operation of current transformers point, comprise computational algorithm, two aspects of operation of current transformers point detection system platform of current transformer core magnetic flux, belong to the power automation technical field.

Background technology

The operation of HVDC (High Voltage Direct Current) transmission system monopolar ground circuit operation mode or magnetic storm etc. may form the distribution of the earth DC potential; DC current is entered AC network from the directly grounded transformer of neutral point; cause the current transformer of AC system to produce DC magnetic biasing; DC magnetic biasing may be accelerated the protection of current transformer, and bias current more great current mutual inductor is saturated faster.

DC magnetic biasing mainly concentrates under Under Direct Current Bias the current transformer protection to the impact of differential protection and the aspects such as relation between DC magnetic biasing and current transformer saturation characteristic to the impact of current transformer research.Research is thought, under normal circumstances, less exciting current can produce the nominal operation magnetic flux, if have a certain amount of direct current to enter current transformer this moment, the direct current flux that produces is much larger than specified magnetic flux, if this two parts magnetic flux directly superposes, main flux is offset greatly to a direction, cause that the iron core height is saturated, exciting current seriously distorts.For the current transformer DC magnetic bias phenomena, a kind of misunderstanding that is easy to produce is: under DC magnetic biasing, the magnetic flux of current transformer equals to enter the direct current flux that the DC current of current transformer produces and directly directly superposes with original exchange flux, produces the DC magnetic biasing effect.And true really not so, analysis and research show makes non-linear due to the current transformer core magnetization characteristic when current transformer once-through AC load electric current, if produce simultaneously DC magnetic bias current, this DC current finally can be all for the iron core excitation; Iron core magnetic flux during DC magnetic biasing is not equal to iron core exchange flux and the during DC current independent excitation direct stack of direct current flux of current transformer when an AC load electric current independent role; During an identical load current, different magnetic bias direct current electric currents can make the exchange flux in iron core different; And during a different load current, identical DC current can produce different direct current fluxes in iron.Therefore, utilize that data algorithm carries out that Under Direct Current Bias operation of current transformers point automatically detects, iron core magnetic flux and exciting current are determined etc. become problem demanding prompt solution.

Summary of the invention

Problem to be solved by this invention is to provide under a kind of Under Direct Current Bias determines operation of current transformers point value computing method, is used for the characteristic quantities relevant with the current transformer operating point such as size, Current Transformer Secondary electric current of size, AC excitation electric current and DC excitation electric current of size, the exciting current of current transformer core exchange flux and direct current flux under the calculative determination Under Direct Current Bias.

For solving the problems of the technologies described above, the invention provides operation of current transformers point detection side under a kind of Under Direct Current Bias

Method is characterized in that, comprises the following steps:

1) establish variable k=1, ξ is threshold value, establishes the direct current flux initial value

DC excitation electric current initial value i _{μ dc, 0}=0.0;

2) at known exchange flux Any one direct current flux of upper stack

T is time variable, direct current flux Utilize current transformer iron exciting characteristic curve ψ=f (i _μ) determine i _μBe exciting current, cut-off stream magnetic flux

3) obtain total magnetic flux

4) utilize current transformer core exciting characteristic curve ψ=f (i _μ), calculate total magnetic flux

Corresponding exciting current instantaneous value i _{μ, k}(t), calculate exciting current instantaneous value i _{μ, k}(t) average DC excitation current i in a cycle _{μ dc, k},

$i_{\mathrm{\μdc},k}=\frac{1}{T}\underset{T}{\∫}{i}_{\mathrm{\μ},k}\left(\mathrm{\τ}\right)\mathrm{d\τ}$

Wherein, T is power frequency period (20ms), and τ is integration variable;

5) average DC excitation current i _{μ dc, k}With stack DC current i _DCIf, | i _{μ dc, k}-i _DC|＜ξ turns step

6); Otherwise turn step 7);

6) utilize secant method to calculate direct current flux

Wherein, sign () is sign function, that is, x＜0 o'clock sign (x)=-1, sign during x=0 (x)=0, x＞0 o'clock sign (x)=1, x are variable, Be the core sataration magnetic flux, K 〉=1.0 are coefficient; And make k=k+1, turn step 2);

7) stop iteration, in the DC magnetic biasing situation

Direct current flux

Exciting current i _μ: i _μ=i _{μ, k}

The DC excitation current i _{μ dc}: i _{μ dc}=i _{μ dc, k}=i _DC

The AC excitation current i _{μ ac}: i _{μ ac}=i _μ-i _{μ dc}

The Current Transformer Secondary current i ₂: i ₂=i ₁-i _μ=i _ac-i _{μ ac}, i ₁Be the primary current of reduction to secondary side, i ₂Be secondary current.

Description of drawings

Fig. 1 is current transformer DC magnetic biasing analysis circuit;

Fig. 2 is the current transformer equivalent circuit of Fig. 1;

Fig. 3 is the process flow diagram of the inventive method;

Fig. 4 is operation of current transformers point detection system platform in the present invention.

Embodiment

Determine the numerical computation method of operation of current transformers point under Under Direct Current Bias: with the bias phenomenon that the current transformer primary side current adopts the first-harmonic ac current source mode analog AC electric current in parallel with the direct current current source and DC current acting in conjunction to be, press circuit shown in Figure 1 magnetization characteristic under the current transformer DC magnetic biasing and exciting current are carried out analytical calculation.

According to the equivalent electrical circuit of current transformer, a winding is converted secondary side to simplify the analysis, ignore core loss, field excitation branch line is the nonlinear inductance loop, uses L _μExpression, the leakage inductance of a winding and resistance can be ignored concerning primary current, and resistance and the leakage inductance of Secondary Winding is integrated into together with secondary load equivalent resistance and inductance, use L ₂, R ₂Expression.i ₁Be the primary current of reduction to secondary side, i ₂Be secondary current, i _μBe reduction to the exciting current of secondary side, equivalent circuit as shown in Figure 2,

Wherein

i ₁＝i _ac+i _dc

$i_{\mathrm{ac}}=\sqrt{2}{I}_1\sin \mathrm{\ωt}$

Because the excitation property of current transformer core has nonlinear characteristic, the calculating of DC magnetic biasing size can not utilize superposition principle to think that simply the skew of working point analyzes, as for a nonlinear excitation system:

When add a DC current i in iron core _DC, obtain a direct current flux

When doing separately the test of AC excitation characteristic simultaneously: add an ac-excited current i _{μ ac}, obtain exchange flux and be output as:

That is:

But can not basis

With

Directly i is determined in stack _DCAnd i _{μ ac}Make simultaneously the magnetic flux of used time, namely

Determine that according to DC magnetic bias current the operation of current transformers characteristic can calculate by the method for iterative approximation in reality, utilize the exciting characteristic curve of Current Mutual Inductance

By add a direct current flux on exchange flux is folded, determine corresponding exciting current, and calculate the mean value of exciting current; The direct current flux of revising stack makes the exciting current mean value of calculating approach DC current, thus the final direct current flux of determining correspondence.Two aspects of calculating of determining to reach direct current flux that mainly comprise exchange flux.

At first, determine the current transformer exchange flux.According to Faraday's electromagnetic induction law, the transient voltage data at winding two ends are carried out integral and calculating about the time, obtain the waveform of this current transformer core transient state interlinkage flux.In fact according to the law of electromagnetic induction, induction electromotive force is magnetic flux rate over time, namely has

Exchange flux is as follows,

Wherein,

For measuring the magnetic flux of initial time.

Suppose that exchange flux is the stack of fundamental frequency sine wave and higher hamonic wave thereof, exchange flux

Integration in a power frequency period is zero

Can determine initial magnetic flux

As seen, measure the magnetic flux of initial time Relevant with measuring voltage u (t), if u (t) is sinusoidal quantity, Relevant with the initial phase angle of measurement of u (t).

It is to be noted: above definite magnetic flux

Be that the alternating current impression magnetic flux is not considered direct current flux, because direct current flux is 0 to the rate of change of time, so induction electromotive force is also 0, can not obtain direct current flux in induction voltage waveform, so the following formula just alternating current impression magnetic flux of calculating.

Can utilize numerical algorithm to realize the integral and calculating of voltage to the time in above principle, establishing each cycle (20ms) has M instantaneous voltage sampled point, i.e. sampling period

For simplicity, numerical integration adopts trapezoidal method to calculate, and gets t ₀=k ₀DT

n＝1,2,L

Exchange flux calculates by following formula

n＝1,2,L

Further, utilize the method for iterative approximation to carry out direct current flux calculating, direct current of stack is straight-through on exchange flux, magnetization curve according to the correspondence of magnetic linkage, calculate the exciting current instantaneous value that has, and calculate average DC excitation electric current in a cycle, if this average DC excitation electric current and DC current i _DCEquating, is exactly the working point of magnetic linkage with regard to the working point that be described this moment so, if unequal, so according to search strategy, proceeds to iterate search, until dc point like approaching recently.

Adopt secant method to carry out iterative, specific algorithm is as follows:

1) establish k=1, ξ is threshold value, establishes initial value

i _{μ dc, k}=0.0;

2) at known exchange flux

Any one direct current flux of upper stack

Direct current flux

Initial value utilizes current transformer iron magnetization curve ψ=f (i _μ) determine, get

i _μBe the exciting current of reduction to secondary side;

3) obtain total magnetic flux

4) utilize current transformer core exciting characteristic curve ψ=f (i _μ), calculate total magnetic flux

Corresponding exciting current instantaneous value i _{μ, k}(t), calculate exciting current instantaneous value i _{μ, k}(t) average DC excitation current i in a cycle _{μ dc, k}

$i_{\mathrm{\μdc},k}=\frac{1}{T}\underset{T}{\∫}{i}_{\mathrm{\μ},k}\left(\mathrm{\τ}\right)\mathrm{d\τ}$

5) average DC excitation current i _{μ dc, k}With stack DC current i _DCIf, | i _{μ dc, k}-i _DC|＜ξ turns step

6); Otherwise turn step 7);

6) utilize secant method to calculate direct current flux

Wherein, sign () is sign function, Be the core sataration magnetic flux, K 〉=1.0 are coefficient; And make k=k+1, turn step 2);

7) stop iteration, in the DC magnetic biasing situation

Direct current flux

Exciting current i _μ: i _μ=i _{μ, k}

The DC excitation current i _{μ dc}: i _{μ dc}=i _{μ dc, k}=i _DC

The AC excitation current i _{μ ac}: i _{μ ac}=i _μ-i _{μ dc}

The Current Transformer Secondary current i ₂: i ₂=i ₁-i _μ=i _ac-i _{μ ac}

Other application: carry out harmonic content analysis of current transformer exciting current and secondary current in the DC magnetic biasing situation etc. by fast fourier transformation algorithm.Algorithm flow chart is illustrated in fig. 3 shown below.

Operation of current transformers point detection system platform under Under Direct Current Bias:

Under Under Direct Current Bias in the utilization of operation of current transformers point detection system platform the method for a joint carry out the work of current transformer when DC magnetic biasing and determine.System platform is pressed modularized equipment, comprising: voltage measurement and data acquisition module, detection data processing module and input/output module three parts:

1) voltage measurement and data acquisition link utilize the high-accuracy data acquisition integrated circuit board to realize the measurement of Current Transformer Secondary voltage, are converted into digital quantity through the high-speed a/d transducer and flow to main control computer and carry out the detection method data and process;

2) operation of current transformers point detection data processing module is the core of this system, utilize the software programming exploitation to realize operation of current transformers point detecting method under upper one Under Direct Current Bias that saves, utilize the current transformer magnetization curve data point of input, by difference arithmetic, fundamental magnetization curve is carried out match; Utilize the software of the algorithm flow of Fig. 3 to realize carrying out calculating and other application of the operation of current transformers point correlated characteristic amounts such as current transformer alternating current-direct current magnetic flux, exciting current.

3) input/output module utilizes software programming to realize user interface, input and the setting of convenient tested current mutual-inductor parameter, and the form of result of calculation with figure and data shown.

Under above Under Direct Current Bias, operation of current transformers point detection system platform structure is illustrated in fig. 4 shown below.This system platform can be completed the detection of operation of current transformers point automatically, and test data is exported and stored.

Under the Under Direct Current Bias that proposes in the present invention, operation of current transformers point detecting method and system platform have been carried out verification experimental verification in the laboratory.Utilize current lifting device to pass into alternating current one time to the current transformer of reality in the laboratory, and pass into the DC excitation electric current with direct supply, make operation of current transformers under different magnetic bias conditions, assess validity and the system platform practicality of mutual inductor working point detection algorithm in this invention.Testing result shows, utilizes this system platform can detect fast and accurately the working point correlated characteristic amount such as magnetic flux, exciting current of current transformer under the magnetic bias condition.

In addition to the implementation, all employings are equal to replaces or technical scheme that the form of equivalent transformation obtains, within all dropping on protection scope of the present invention.

Claims (1)

1. operation of current transformers point detecting method under a Under Direct Current Bias, is characterized in that, comprises the following steps:

1) establish variable k=1, ξ is threshold value, establishes the direct current flux initial value

DC excitation electric current initial value i _{μ dc, 0}=0.0;

2) at known exchange flux Any one direct current flux of upper stack

T is time variable, direct current flux

Utilize current transformer iron exciting characteristic curve ψ=f (i _μ) determine i _μBe exciting current, cut-off stream magnetic flux

3) obtain total magnetic flux

4) utilize current transformer core exciting characteristic curve ψ=f (i _μ), calculate total magnetic flux

Corresponding exciting current instantaneous value i _{μ, k}(t), calculate exciting current instantaneous value i _{μ, k}(t) average DC excitation current i in a cycle _{μ dc, k},

$i_{\mathrm{\μdc},k}=\frac{1}{T}\underset{T}{\∫}{i}_{\mathrm{\μ},k}\left(\mathrm{\τ}\right)\mathrm{d\τ}$

Wherein, T is power frequency period, and τ is integration variable;

5) average DC excitation current i _{μ dc, k}With stack DC current i _DCIf, | i _{μ dc, k}-i _DC|＜ξ turns step

6); Otherwise turn step 7);

6) utilize secant method to calculate direct current flux

Wherein, sign () is sign function, that is, x＜0 o'clock sign (x)=-1, sign during x=0 (x)=0, x＞0 o'clock sign (x)=1, x are variable, Be the core sataration magnetic flux, K 〉=1.0 are coefficient; And make k=k+1, turn step 2);

7) stop iteration, in the DC magnetic biasing situation

Direct current flux

Exciting current i _μ: i _μ=i _{μ, k}

The DC excitation current i _{μ dc}: i _{μ dc}=i _{μ dc, k}=i _DC

The AC excitation current i _{μ ac}: i _{μ ac}=i _μ-i _{μ dc}

The Current Transformer Secondary current i ₂: i ₂=i ₁-i _μ=i _ac-i _{μ ac}, i ₁Be the primary current of reduction to secondary side, i ₂Be secondary current.

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