CN104391198B - A kind of low pressure power network safety monitoring method - Google Patents

Abstract

The present invention relates to power grid security, and in particular to a kind of low pressure power network safety monitoring method, comprise the following steps：Gather the three-phase transient current signal i of low pressure power network line head end_A(t)、i_B(t)、i_C(t) and three-phase transient voltage signal u_A(t)、u_B(t)、u_CT the data sampling frequency of (), current signal and voltage signal is 10kHz, time window is a half cycles after disturbance；Using any two-phase an as loop, differential equation of higher order is formed；Ask for differential equation of higher order coefficient in step 2；Equivalent parameters are solved using the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters；The features such as fundamental frequency component included in it are extracted from the three-phase transient current signal and three-phase transient voltage signal that collect.Whether the present invention can be combined safe with power grid security to judge with the actual degree of unbalancedness of neutral conductor impedance magnitude and load.

Description

A kind of low pressure power network safety monitoring method

Technical field

The present invention relates to power grid security, and in particular to a kind of low pressure power network safety monitoring method.

Background technology

Low pressure power utilization network is joined directly together as last ring of whole power network with user, and its security is closed extensively Note.National regulations need to ensure that impedance is zero on three-phase load mean allocation, the neutral conductor in the case of three phase supply.But actual fortune The lower three-phase load of row is often unbalanced, in addition the presence of neutral line impedence so that load side dummy neutral voltage occurs Skew, an at least phase load terminal voltage are raised, that is, produce neutral point overvoltage.For a user, electrical equipment is under over-voltage condition Operation easily burns electrical equipment；Secondly, the zero-sequence current that uncompensated load is produced is conducted back to system side by the neutral conductor, once the neutral conductor Due to circuit aging, neutral conductor junction contacts are bad etc. reason causes impedance value to increase extremely when, resistance increase on the neutral conductor Just there is the danger of overheat at position, so as to turn into disaster hidden-trouble.Therefore degree of unbalancedness and voltage protection are carried out with power network to low pressure Safety monitoring is most important.

The three of data predominantly points of common connection (PCC) place that current low pressure power network safety monitoring is used are relatively electric Pressure, current information.It is three-phase but PCC point voltage-to-grounds are restricted by superior voltage and ground potential, under normal operating condition Symmetrical voltage.In the presence of load side dummy neutral voltage, the voltage-to-ground of PCC points cannot represent the true of user side Real pressure drop and the degree of unbalancedness of load.Therefore under the non-faulting state that dummy neutral voltage is present, mainly by three-phase electricity Stream realizes the safety monitoring problem of low pressure power network.But, the pressure drop on load only cannot be equally characterized using current information, And when being monitored for three-phase load unbalance degree, because three-phase current is influenceed by impedance on the neutral conductor, same load shape Neutral line impedence changes under state, and the load unbalanced degree of gained changes therewith, therefore it is inadequate that current information convincingness is used alone.Separately Outward, it is impossible to centering line impedence and its change be monitored be also existing method deficiency.

The content of the invention

It is an object of the invention to provide a kind of low pressure power network safety monitoring method, can threephase load it is asymmetric with In the case of situation, the constraint of stable state port information that neutral line impedence is present, transient information identification Equivalent Model parameter is collected.

To solve above-mentioned technical problem, the present invention uses following technical scheme：

A kind of low pressure power network safety monitoring method, comprises the following steps：

Step one, gathers the three-phase transient current signal i of low pressure power network line head end_A(t)、i_B(t)、i_C(t) and three-phase Transient voltage signal u_A(t)、u_B(t)、u_CT the data sampling frequency of (), current signal and voltage signal is 10kHz, time window is A half cycles (30ms) after disturbance；

Step 2, using any two-phase an as loop, forms differential equation of higher order

Wherein parameter a₀,a₁,…,a_K、b₀,b₁,…,b_KAnd c₀,c₁,…,c_KIt is the coefficient to be identified of the differential equation, equation Middle U_AB(t)^(k)It is voltage microvariations signal U_ABThe k order derivatives of (t), i_A(t)^(k)、i_B(t)^(k)It is electric current microvariations signal i_A(t)、 i_BT the k order derivatives of (), K is the equivalent exponent number of load model；

Step 3, asks for differential equation of higher order coefficient in step 2；

Step 4, equivalent parameters are solved using the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters, The transient state differential equation of higher order coefficient and stable state equivalent parameters, i.e. transient state differential equation of higher order coefficient a₀,a₁,…,a_K、b₀, b₁,…,b_K、c₀,c₁,…,c_KWith stable state equivalent parameters R_Aeq、R_Beq、R_CeqBetween relation it is as follows

When K is even number, orderCalculate

When K is odd number, orderCalculate

Thus, the A phase stable state equivalent parameters R of load model is drawn_AeqAnd L_AeqAnd B phase stable state equivalent parameters R_BeqWith L_Beq：

Step 5, extracts included in it from the three-phase transient current signal and three-phase transient voltage signal that collect Fundamental frequency

Composition, and the expression formula of sin cos functionses is written as, it is as follows

U_Awen=a₁cosωt+a₂sinωt

U_Bwen=a₃cosωt+a₄sinωt

I_Awen=b₁cosωt+b₂sinωt

I_Bwen=b₃cosωt+b₄sinωt

U_Nwen=c₁cosωt+c₂sinωt；

Step 6, the equivalent impedance parameter obtained in step 4 is represented with the coefficient of the sine and cosine expression formula for obtaining, such as Under

c₁=a₁+R_Aeqb₁-ωb₂L_Aeq

c₂=a₂-R_Aeqb₂-ωb₁L_Aeq

c₁=a₃+R_Beqb₃-ωb₄L_Beq

c₂=a₄+R_Beqb₄+ωb₃L_Beq

Step 7, the expression formula obtained by the use of in step 6 is made as constraints, the parameter being calculated in step 4 It is initial value, optimizes, obtains each equal value impedance exact value；

Step 8, passes through

The drift voltage of calculated load side dummy neutral；

Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, table Levy the security of low pressure power network, voltage unbalance factor ε_u, current unbalance factor ε_iComputing formula it is as follows：

Wherein：U₁The positive-sequence component amplitude root mean square of-three-phase voltage；

U₂The negative sequence component amplitude root mean square of-three-phase voltage；

Wherein：I₁The positive-sequence component amplitude root mean square of-three-phase current；

I₂The negative sequence component amplitude root mean square of-three-phase current；

The positive and negative order components of the actual pressure drop on three-phase load are asked for first：

Wherein：A=e^j120°；

Positive and negative, the zero-sequence component of-three-phase voltage,

It is the voltage phasor obtained using the fitting of stable state sampled point,For the void that the correspondence time is obtained Intend neutral point voltage phasor,

Then positive-negative sequence amplitude is takenSeek degree of unbalancedness,

ε_uN, ε_u、ε_iValue it is bigger, the degree of unbalancedness of load is bigger, and the threat to the safe operation with power network is bigger, electric current Degree of unbalancedness ε_iCalculate in the case of identical, improve voltage unbalance factor ε_uCalculating, using the actual negative obtained in preceding step Degree of unbalancedness ε is asked in lotus pressure drop_uN, it is with a high credibility.

The method that low pressure network load equivalent impedance is calculated with transient state component, its feature be due to threephase load not The presence of balance and neutral line impedence, dummy neutral voltage offsets with transformer neutral point voltage, referred to as drift voltage.This When equivalent circuit can not be split as single-phase circuit, drift voltage is influenceed by each phase load, it is impossible to directly eliminate.

This patent feature is separate between biphase current directly to be established an equation using any two-phase in three-phase, in equation Parameter without other phases, without decoupling.

Compared with prior art, the beneficial effects of the invention are as follows：With neutral conductor impedance magnitude and the actual imbalance of load Whether safely degree is combined to judge with power grid security, using dummy neutral voltage is calculated, to obtain the reality in power load Voltage difference (voltage difference between PCC points and load side dummy neutral), asks for voltage unbalance factor, with neutrality by national standard method On line the size of impedance value together, the security of sign power network；In addition, this patent directly obtains the equivalent impedance of three-phase load Value, it can be seen directly that whether three-phase balances.

Brief description of the drawings

Fig. 1 is basic three-phase four-wire system network equivalence circuit diagram.

Fig. 2 is a kind of schematic flow sheet of low pressure power network safety monitoring method of the invention.

Fig. 3 is simple structure analogous diagram.

Fig. 4 is simple structure simulation result.

Fig. 5 is neutral conductor change in the instantaneous impedance analogous diagram.

Fig. 6 is neutral conductor change in the instantaneous impedance simulation result figure.

Fig. 7 is neutral conductor impedance transition mechanism analogous diagram.

Fig. 8 is neutral conductor impedance transition mechanism simulation result figure.

Specific embodiment

In order to make the purpose , technical scheme and advantage of the present invention be clearer, it is right below in conjunction with drawings and Examples The present invention is further elaborated.It should be appreciated that the specific embodiments described herein are merely illustrative of the present invention, and It is not used in the restriction present invention.

Figures 1 and 2 show that a kind of one embodiment of low pressure of the invention with power network safety monitoring method：A kind of low pressure is used Power grid security monitoring method, comprises the following steps：

Step one, gathers the three-phase transient current signal i of low pressure power network line head end_A(t)、i_B(t)、i_C(t) and three-phase Transient voltage signal u_A(t)、u_B(t)、u_CT the data sampling frequency of (), current signal and voltage signal is 10kHz, time window is A half cycles (30ms) after disturbance；

Step 2, using any two-phase an as loop, forms differential equation of higher order

Wherein parameter a₀,a₁,…,a_KAnd b₀,b₁,…,b_K, a and b is the coefficient to be identified of the differential equation, and K is referred to as equivalent rank Number, U in equation_AB(t)^(k)It is voltage microvariations signal U_ABThe k order derivatives of (t), i_A(t)^(k)、i_B(t)^(k)It is electric current microvariations letter Number i_A(t)、i_BT the k order derivatives of (), K is the equivalent exponent number of load model；

Step 3, asks for differential equation of higher order coefficient in step 2；

Step 4, equivalent parameters are solved using the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters, The transient state differential equation of higher order coefficient and stable state equivalent parameters, i.e. transient state differential equation of higher order coefficient a₀,a₁,…,a_K、b₀, b₁,…,b_K、c₀,c₁,…,c_KWith stable state equivalent parameters R_Aeq、R_Beq、R_CeqBetween relation it is as follows：

When K is even number, order

When K is odd number, orderCalculate

Thus, the A phase stable state equivalent parameters R of load model is drawn_AeqAnd L_AeqAnd B phase stable state equivalent parameters R_BeqWith L_Beq：

Step 5, extracts included in it from the three-phase transient current signal and three-phase transient voltage signal that collect Fundamental frequency

Composition, and the expression formula of sin cos functionses is written as, it is as follows

U_Awen=a₁cosωt+a₂sinωt

U_Bwen=a₃cosωt+a₄sinωt

I_Awen=b₁cosωt+b₂sinωt

I_Bwen=b₃cosωt+b₄sinωt

U_Nwen=c₁cosωt+c₂sinωt；

Step 6, the equivalent impedance parameter obtained in step 4 is represented with the coefficient of the sine and cosine expression formula for obtaining, such as Under

c₁=a₁+R_Ab₁-ωb₂L_A

c₂=a₂-R_Ab₂-ωb₁L_A

c₁=a₃+R_Bb₃-ωb₄L_B

c₂=a₄+R_Bb₄+ωb₃L_B

Step 7, the expression formula obtained by the use of in step 6 is made as constraints, the parameter being calculated in step 4 It is initial value, optimizes, obtains each equal value impedance exact value；

Step 8, passes through

The drift voltage of calculated load side dummy neutral；

Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, table Levy the security of low pressure power network, voltage unbalance factor ε_u, voltage unbalance factor ε_iComputing formula it is as follows：

Wherein：U₁The positive-sequence component amplitude root mean square of-three-phase voltage；

U₂The negative sequence component amplitude root mean square of-three-phase voltage；

Wherein：I₁The positive-sequence component amplitude root mean square of-three-phase current；

I₂The negative sequence component amplitude root mean square of-three-phase current；

I₂The negative sequence component amplitude root mean square of-three-phase current；

The positive and negative order components of the actual pressure drop on three-phase load are asked for first：

Wherein：A=e^j120°；

Positive and negative, the zero-sequence component of-three-phase voltage,

It is the voltage phasor obtained using the fitting of stable state sampled point,For the void that the correspondence time is obtained Intend neutral point voltage phasor,

Then positive-negative sequence amplitude is takenSeek degree of unbalancedness ε_uN, ε_u、ε_iValue it is bigger, the degree of unbalancedness of load is got over Greatly, the threat to the safe operation with power network is bigger, current unbalance factor ε_iCalculate in the case of identical, improve Voltage unbalance Degree ε_uCalculating, ask for degree of unbalancedness ε using the actual load pressure drop obtained in preceding step_uN, it is with a high credibility.

The method that low pressure network load equivalent impedance is calculated with transient state component, its feature be due to threephase load not The presence of balance and neutral line impedence, dummy neutral voltage offsets with transformer neutral point voltage, referred to as drift voltage, this When equivalent circuit can not be split as single-phase circuit, drift voltage is influenceed by each phase load, it is impossible to directly eliminate.

This patent feature is separate between biphase current directly to be established an equation using any two-phase in three-phase, in equation Parameter without other phases, without decoupling.

The degree of unbalancedness of true pressure drop on three-phase load is calculated：

Ask for the positive and negative order components of the actual pressure drop on three-phase load：

Wherein：A=e^j120°；

Positive and negative, the zero-sequence component of-three-phase voltage.

It is the voltage phasor obtained using the fitting of stable state sampled point,For the void that the correspondence time is obtained Intend neutral point voltage phasor.

Then positive-negative sequence amplitude U is taken_1N、U_2NSeek degree of unbalancedness ε_uN：

Its result such as table 2：

The load unbalanced degree of table 2

		Conventional method	0/125.24%
Patented method	14.3%/125.24%

In conjunction with emulation, the invention will be further described：

1st, the low pressure power system simulation model set up in the case of three-phase four-wire system using MATLAB simulation softwares herein, will be temporary Network during state represents with three rank circuits as shown in Figure 3, R1=100 Ω, L1=0.003H, L2=0.002H, C1=in Fig. 3 1uF, RB=400 Ω, LB=0.25H, R2=500 Ω, L3=0.001H, R3=400 Ω, R0=4 Ω, L0=0.03H.

Step 1：List differential equation of higher order：

It is collated to obtain：

u_AB+R₁C₂u'_AB+L₁C₁u″_AB=R₁i_A+(L₁+L₂)i'_A+R₁C₁L₂i'_A'+L₁C₁L₂i″'_A+R_bi_B+(R₁C₁R_b+L_b) i_b'+(R₁C₁L_b+L₁R_bC₁)i'_B'+L₁L_bC₁i″'_BK=3 is can be seen that from the above-mentioned differential equation, identification model is：

Needing the parameter of identification has 3* (3+1) -1=11.

Make a₀=1, equation is written as：The matrix of Y=AX forms.

Identification above formula is carried out using least square method herein, 11 coefficients can be obtained：a₀-a₃、b₀-b₃、c₀-c₃。

Have again：

Afterwards according to formula (2) (3), stable state equivalent impedance is translated into：

C phase parameters can similarly be obtained.

Then have：

Step 2：Take sampled value i when stable state_O、i_A、u_AAnd calculated value di/dt is according to formula (5), obtain accurate LO, R_O。

Now by simulation result tabulated below 1：

The rank circuit identification result of table 1

It is Fig. 4 that the voltage waveform of load side dummy neutral is calculated with the sample waveform contrast in emulation.

Step 3：The degree of unbalancedness of true pressure drop on three-phase load is calculated：

Ask for the positive and negative order components of the actual pressure drop on three-phase load：

Wherein：A=e^j120°；

Positive and negative, the zero-sequence component of-three-phase voltage.

It is the voltage phasor obtained using the fitting of stable state sampled point,For the void that the correspondence time is obtained Intend neutral point voltage phasor.

Then positive-negative sequence amplitude U is taken_1N、U_2NSeek degree of unbalancedness ε_uN：

Its result such as table 2：

The load unbalanced degree of table 2

2nd, neutral conductor change in the instantaneous impedance emulation：

As shown in figure 5, the unit of resistance is Ω in Fig. 5, the unit of inductance is H, and emulation breaker in middle breaks suddenly in t=0.1s Open, impedance is undergone mutation on the neutral conductor, and microvariations are caused in monitoring port, the neutral line impedence for accessing suddenly is 4+j0.5, will Identification result typing table 3 below：

The neutral conductor change in the instantaneous impedance of table 3 is emulated

Dummy neutral voltage is as shown in Figure 6 with sampled voltage comparison diagram after change.Emulation explanation, in neutral conductor mutation In the case of, this paper institute's extracting methods still have preferable identification result to impedance on virtual earth, the neutral conductor and three-phase equivalent parameters.

3rd, neutral conductor impedance transition mechanism emulation：

As shown in fig. 7, the unit of resistance is Ω in figure, the unit of inductance is H.R to simulation model₀Constant interval be [0- 5] Ω, rate of change is 0.002 Europe/second, R₀Constant interval be [0-5] Europe, rate of change be 0.002 Europe/second.Due to rate of change compared with Low, the change for relying only on neutral line impedence cannot detect disturbing signal in port, but in low pressure power network, it is internal and Outside microvariations often occur, and program can still be run.Information after double disturbance is contrasted, it is readily obtained Rate of change.

Now by simulation result tabulated below 4：

The neutral conductor impedance transition mechanism of table 4 is emulated

Dummy neutral voltage is as shown in Figure 8 with sampled voltage comparison diagram.

Although reference be made herein to invention has been described for multiple explanatory embodiments of the invention, however, it is to be understood that Those skilled in the art can be designed that a lot of other modification and implementation methods, and these modifications and implementation method will fall in this Shen Please be within disclosed spirit and spirit.More specifically, in the range of disclosure, drawings and claims, can Various variations and modifications are carried out with the building block to theme composite configuration and/or layout.Except to building block and/or layout The deformation and improvement for carrying out are outer, and to those skilled in the art, other purposes also will be apparent.

Claims (1)

1. a kind of low pressure power network safety monitoring method, it is characterised in that comprise the following steps：

Step one, gathers the three-phase transient current signal i of low pressure power network line head end_A(t)、i_B(t)、i_C(t) and three-phase transient state Voltage signal u_A(t)、u_B(t)、u_CT the data sampling frequency of (), current signal and voltage signal is 10kHz, time window is disturbance A half cycles afterwards；

Step 2, using any two-phase an as loop, forms differential equation of higher order

$\underset{k=0}{\overset{K}{\Σ}}{a}_k{U}_{AB}{\left(t\right)}^{\left(k\right)}=\underset{k=0}{\overset{K}{\Σ}}{b}_k{i}_A{\left(t\right)}^{\left(k\right)}+\underset{k=0}{\overset{K}{\Σ}}{c}_k{i}_B{\left(t\right)}^{\left(k\right)}$

Wherein parameter a₀,a₁,…,a_K、b₀,b₁,…,b_KAnd c₀,c₁,…,c_KIt is the coefficient to be identified of the differential equation, U in equation_AB (t)^(k)It is voltage microvariations signal U_ABThe k order derivatives of (t), i_A(t)^(k)、i_B(t)^(k)It is electric current microvariations signal i_A(t)、i_B(t) K order derivatives, K is the equivalent exponent number of load model；

Step 3, asks for differential equation of higher order coefficient in step 2；

Step 4, equivalent parameters are solved using the relation between transient state differential equation of higher order coefficient and stable state equivalent parameters, described Relation is as follows between transient state differential equation of higher order coefficient and stable state equivalent parameters

When K is even number, orderCalculate

$A_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{a}_{2n}{\mathrm{\ω}}^{2n}$

$A_1=\underset{n=0}{\overset{N-1}{\Σ}}{(-1)}^n{a}_{2n+1}{\mathrm{\ω}}^{2n+1}$

$B_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{b}_{2n}{\mathrm{\ω}}^{2n}$

$B_1=\underset{n=0}{\overset{N-1}{\Σ}}{(-1)}^n{b}_{2n+1}{\mathrm{\ω}}^{2n+1}$

$C_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{c}_{2n}{\mathrm{\ω}}^{2n}$

$C_1=\underset{n=0}{\overset{N-1}{\Σ}}{(-1)}^n{c}_{2n+1}{\mathrm{\ω}}^{2n+1}$

When K is odd number, orderCalculate

$A_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{a}_{2n}{\mathrm{\ω}}^{2n}$

$A_1=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{a}_{2n+1}{\mathrm{\ω}}^{2n+1}$

$B_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{b}_{2n}{\mathrm{\ω}}^{2n}$

$B_1=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{b}_{2n+1}{\mathrm{\ω}}^{2n+1}$

$C_0=\underset{n=0}{\overset{N}{\Σ}}{(-1)}^n{c}_{2n}{\mathrm{\ω}}^{2n}$

$C_1=\underset{n=0}{\overset{N-1}{\Σ}}{(-1)}^n{c}_{2n+1}{\mathrm{\ω}}^{2n+1}$

Thus, the A phase stable state equivalent parameters R of load model is drawn_AeqAnd L_AeqAnd B phase stable state equivalent parameters R_BeqAnd L_Beq：

$R_{Aeq}=\frac{A_1{B}_1+A_0{B}_0}{A_0^2+A_1^2}$

$L_{Aeq}=\frac{A_1{B}_0-A_0{B}_1}{\mathrm{\ω}(A_0^2+A_1^2)}$

$R_{Beq}=\frac{A_1{C}_1+{\mathrm{LC}}_0}{A_0^2+A_1^2}$

$L_{Beq}=\frac{A_1{C}_0-A_0{C}_1}{\mathrm{\ω}(A_0^2+A_1^2)};$

Step 5, extracts the base included in it from the three-phase transient current signal and three-phase transient voltage signal that collect Frequency composition, and the expression formula of sin cos functionses is written as, it is as follows

U_Awen=a₁cosωt+a₂sinωt

U_Bwen=a₃cosωt+a₄sinωt

I_Awen=b₁cosωt+b₂sinωt

I_Bwen=b₃cosωt+b₄sinωt

U_Nwen=c₁cosωt+c₂sinωt；

Step 6, the equivalent impedance parameter obtained in step 4 is represented with the coefficient of the sine and cosine expression formula for obtaining, as follows

c₁=a₁+R_Aeqb₁-ωb₂L_Aeq

c₂=a₂-R_Aeqb₂-ωb₁L_Aeq

c₁=a₃+R_Beqb₃-ωb₄L_Beq

c₂=a₄+R_Beqb₄+ωb₃L_Beq

$L_{Aeq}=\frac{a_2{b}_1+b_2{c}_1-b_2{a}_1-b_1{c}_2}{\mathrm{\ω}(b_1^2+b_2^2)}$

$R_{Aeq}=\frac{a_2{b}_2-b_2{c}_2+b_1{a}_1-b_1{c}_1}{b_1^2+b_2^2}$

$\begin{array}{c}{L}_{Beq}=\frac{a_3{b}_4-b_4{c}_1-b_3{a}_4+b_3{c}_2}{\mathrm{\ω}(b_3^2+b_4^2)}\\ R_{Beq}=\frac{b_4{c}_2-a_4{b}_4-b_3{a}_3+b_3{c}_1}{b_1^2+b_2^2}\end{array};$

Step 7, used as constraints, the parameter being calculated in step (4) is used as first for the expression formula obtained by the use of in step 6 Value, optimizes, and obtains each equal value impedance exact value；

Step 8, passes through

$\left\{\begin{array}{c}{U}_N=i_O{R}_O+L_O\frac{{\mathrm{di}}_o}{dt}\\ U_N=U_A-I_A*Z_A\end{array}\right.$

The drift voltage of calculated load side dummy neutral；

Step 9, calculates three-phase load unbalance degree, together with three-phase load size and dummy neutral drift voltage, characterizes low The security of pressure power network, voltage unbalance factor ε_u, current unbalance factor ε_iComputing formula it is as follows：

${\mathrm{\ϵ}}_u=\frac{U_2}{U_1}\×100\%$

Wherein：U₁The positive-sequence component amplitude root mean square of-three-phase voltage；

U₂The negative sequence component amplitude root mean square of-three-phase voltage；

${\mathrm{\ϵ}}_i=\frac{I_2}{I_1}\×100\%$

Wherein：I₁The positive-sequence component amplitude root mean square of-three-phase current；

I₂The negative sequence component amplitude root mean square of-three-phase current；

The positive and negative order components of the actual pressure drop on three-phase load are asked for first：

$\left[\begin{array}{c}\stackrel{\·}{U_{1N}}\\ \stackrel{\·}{U_{2N}}\\ \stackrel{\·}{U_{0N}}\end{array}\right]=\frac{1}{3}\left[\begin{array}{ccc}1& a& a^2\\ 1& a^2& a\\ 1& 1& 1\end{array}\right]*\left[\begin{array}{c}\stackrel{\·}{U_A}-\stackrel{\·}{U_N}\\ \stackrel{\·}{U_B}-\stackrel{\·}{U_N}\\ \stackrel{\·}{U_C}-\stackrel{\·}{U_N}\end{array}\right]$

Wherein：A=e^j120°；

Positive and negative, the zero-sequence component of-three-phase voltage,

It is the voltage phasor obtained using the fitting of stable state sampled point,For correspondence the time obtain it is virtual in Property point voltage phasor,

Then positive-negative sequence amplitude is takenSeek degree of unbalancedness ε_uN, ε_u、ε_iValue it is bigger, the degree of unbalancedness of load is bigger, right Threat with the safe operation of power network is bigger, current unbalance factor ε_iCalculate in the case of identical, improve voltage unbalance factor ε_u's Calculate, degree of unbalancedness ε is asked for using the actual load pressure drop obtained in preceding step_uN, it is with a high credibility.