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

Abstract

The invention relates to power grid safety, in particular to a low-voltage power grid safety monitoring method, comprising the following steps: collecting three-phase transient current signals i _A (t), i _B (t), and i _C at the head end of a low-voltage power grid line (t) and three-phase transient voltage signals u _A (t), u _B (t), u _C (t), the data sampling frequency of the current signal and the voltage signal is 10kHz, and the time window is a half cycle after the disturbance; Take any two phases as a loop to form a high-order differential equation; obtain the coefficients of the high-order differential equation in step 2; use the relationship between the coefficients of the transient high-order differential equation and the steady-state equivalent parameters to solve the equivalent parameters; from the acquisition to The fundamental frequency components contained in the three-phase transient current signal and the three-phase transient voltage signal are extracted. The invention can judge whether the power grid is safe or not by combining the impedance of the neutral line and the actual unbalanced degree of the load.

Description

A method for monitoring the safety of a low-voltage power grid

technical field

The invention relates to the safety of a power grid, in particular to a method for monitoring the safety of a low-voltage power grid.

Background technique

As the last link of the entire power grid, the low-voltage power network is directly connected to users, and its security has received widespread attention. The national standard stipulates that in the case of three-phase power supply, it is necessary to ensure that the three-phase load is evenly distributed and the neutral line impedance is zero. However, in actual operation, the three-phase load is often unbalanced, and the existence of the neutral line impedance makes the virtual neutral point voltage on the load side shift, and at least one phase of the load terminal voltage rises, that is, the neutral point overvoltage occurs. For the user, it is easy to burn out the electrical appliance when it is running under the condition of overvoltage; secondly, the zero-sequence current generated by the unbalanced load is transmitted back to the system side by the neutral line. The reason is that when the impedance value increases abnormally, the part with increased resistance on the neutral line will be in danger of overheating, which will become a fire hazard. Therefore, it is very important to monitor the unbalance degree and voltage protection of the low-voltage power grid.

At present, the data used for safety monitoring of low-voltage power grids are mainly the three-phase-to-ground voltage and current information at the point of common connection (PCC). However, the PCC point-to-ground voltage is restricted by the upper-level voltage and the ground potential, and it is a three-phase symmetrical voltage under normal operating conditions. In the presence of a virtual neutral point voltage on the load side, the ground voltage at the PCC point cannot represent the real voltage drop on the user side and the unbalanced degree of the load. Therefore, in the non-fault state where the virtual neutral point voltage exists, the safety monitoring of the low-voltage power grid is mainly realized through the three-phase current. However, it is also impossible to characterize the voltage drop on the load by only using current information, and when it is used for three-phase load unbalance monitoring, since the three-phase current is affected by the impedance of the neutral line, the impedance of the neutral line changes under the same load state , the resulting load imbalance changes accordingly, so the current information alone is not convincing enough. In addition, the inability to monitor the neutral line impedance and its changes is also a deficiency of the existing methods.

Contents of the invention

The purpose of the present invention is to provide a low-voltage power grid safety monitoring method, which can collect transient information to identify equivalent model parameters in the presence of three-phase load asymmetry and neutral line impedance, and under the constraints of steady-state port information .

In order to solve the above-mentioned technical problems, the present invention adopts the following technical solutions:

A low-voltage power grid safety monitoring method, comprising the following steps:

Step 1, collect the three-phase transient current signals i _A (t), i _B (t), i _C (t) and the three-phase transient voltage signals u _A (t), u _B ( t), u _C (t), the data sampling frequency of the current signal and the voltage signal is 10kHz, and the time window is a half cycle (30ms) after the disturbance;

Step 2, taking any two phases as a loop to form a high-order differential equation

Among them, the parameters a ₀ , a ₁ ,…,a _K , b ₀ ,b ₁ ,…,b _K and c ₀ ,c ₁ ,…,c _K are the coefficients to be identified of the differential equation. In the equation, U _AB (t) ^{( k)} is the k-order derivative of the voltage small disturbance signal U _AB (t), i _A (t) ^(k) and i _B (t) ^(k) are the current small disturbance signals i _A (t), i _B (t) The k-order derivative of , K is the equivalent order of the load model;

Step 3, obtaining the coefficients of the high-order differential equation in step 2;

Step 4, using the relationship between the transient high-order differential equation coefficients and the steady-state equivalent parameters to solve the equivalent parameters, the transient high-order differential equation coefficients and the steady-state equivalent parameters, that is, the transient high-order differential equation coefficients The relationship between a ₀ , a ₁ ,…,a _K , b ₀ ,b ₁ ,…,b _K , c ₀ ,c ₁ ,…,c _K and the steady-state equivalent parameters R _Aeq , R _Beq , R _Ceq as follows

When K is an even number, let calculate

When K is an odd number, let calculate

Thus, the A-phase steady-state equivalent parameters R _Aeq and L _Aeq and B-phase steady-state equivalent parameters R _Beq and L _Beq of the load model are obtained:

Step five, extract the fundamental frequency contained in the collected three-phase transient current signal and three-phase transient voltage signal

components, and written as the expression of the sine and cosine functions, as follows

U _Awen ＝a ₁ cosωt+a ₂ sinωt

U _Bwen ＝a ₃ cosωt+a ₄ sinωt

I _Awen ＝b ₁ cosωt+b ₂ sinωt

I _Bwen ＝b ₃ cosωt+b ₄ sinωt

U _Nwen = c ₁ cos ωt + c ₂ sin ωt;

Step 6, use the coefficients of the obtained sine-cosine expression to represent the equivalent impedance parameters obtained in step 4, as follows

c ₁ ＝a ₁ +R _Aeq b ₁ -ωb ₂ L _Aeq

c ₂ ＝a ₂ -R _Aeq b ₂ -ωb ₁ L _Aeq

c ₁ ＝a ₃ +R _Beq b ₃ -ωb ₄ L _Beq

c ₂ ＝a ₄ +R _Beq b ₄ +ωb ₃ L _Beq

Step 7, using the expression obtained in step 6 as the constraint condition, and the parameters calculated in step 4 as the initial value, optimize to obtain the exact value of each equal value impedance;

Step eight, pass

Calculate the drift voltage of the virtual neutral point on the load side;

Step nine, calculate the three-phase load unbalance degree, together with the three-phase load size and the virtual neutral point drift voltage, characterize the safety of the low-voltage power grid, the calculation formula of the voltage unbalance degree ε _u and the current unbalance degree ε _i as follows:

Among them: U ₁ — the root mean square of the positive sequence component amplitude of the three-phase voltage;

U ₂ —the negative sequence component amplitude root mean square of the three-phase voltage;

Among them: I ₁ —the root mean square of the amplitude of the positive sequence component of the three-phase current;

I ₂ —the root mean square of the amplitude of the negative sequence component of the three-phase current;

First calculate the positive and negative sequence components of the actual voltage drop on the three-phase load:

Wherein: a=e ^j120° ;

- the positive, negative and zero sequence components of the three-phase voltage,

In order to use the steady-state sampling points to fit the obtained voltage phasor, is the virtual neutral point voltage phasor calculated corresponding to time,

Then take the positive and negative sequence amplitude Find the degree of imbalance,

The greater the value of _εuN , εu, and _{εi ,} the greater the unbalanced load, and the greater the threat to the safe operation of the power grid. Under the same calculation of the current unbalanced degree _εi , the voltage unbalanced degree can be improved For the calculation of ε _u , the unbalance degree ε _uN is obtained by using the actual load voltage drop obtained in the previous steps, which has high reliability.

The method of using the transient component to calculate the equivalent impedance of the low-voltage power grid load is characterized in that due to the unbalanced three-phase load and the existence of the neutral line impedance, the virtual neutral point voltage and the transformer neutral point voltage have an offset, called is the drift voltage. At this time, the equivalent circuit cannot be divided into single-phase circuits, and the drift voltage is affected by the load of each phase and cannot be eliminated directly.

The feature of this patent is that any two-phase sequence equation of the three-phase is directly used, and the two-phase currents are independent of each other. There are no parameters of other phases in the equation, and no decoupling is required.

Compared with the prior art, the beneficial effect of the present invention is: the combination of the neutral line impedance and the actual unbalanced degree of the load is used to judge whether the safety of the power grid is safe, and the calculation of the virtual neutral point voltage is used to obtain the power consumption The actual voltage difference on the load (the voltage difference between the PCC point and the virtual neutral point on the load side), the voltage unbalance degree is obtained according to the national standard method, together with the impedance value on the neutral line, it represents the safety of the power grid In addition, this patent directly obtains the equivalent impedance value of the three-phase load, and it can be directly seen whether the three-phase is balanced.

Description of drawings

Figure 1 is the equivalent circuit diagram of the basic three-phase four-wire power grid.

Fig. 2 is a schematic flowchart of a method for monitoring the safety of a low-voltage power grid according to the present invention.

Figure 3 is a simulation diagram of a simple structure.

Figure 4 shows the simulation results of a simple structure.

Figure 5 is a simulation diagram of a sudden change in neutral line impedance.

Figure 6 is a graph of the simulation results of the neutral line impedance mutation.

Fig. 7 is a simulation diagram of neutral line impedance gradual change.

Fig. 8 is a simulation result diagram of neutral line impedance gradual change.

detailed description

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention, not to limit the present invention.

Fig. 1 and Fig. 2 have shown an embodiment of a kind of low-voltage power grid security monitoring method of the present invention: a kind of low-voltage power grid safety monitoring method comprises the following steps:

Step 1, collect the three-phase transient current signals i _A (t), i _B (t), i _C (t) and the three-phase transient voltage signals u _A (t), u _B ( t), u _C (t), the data sampling frequency of the current signal and the voltage signal is 10kHz, and the time window is a half cycle (30ms) after the disturbance;

Step 2, taking any two phases as a loop to form a high-order differential equation

Among them, the parameters a ₀ , a ₁ ,…,a _K and b ₀ ,b ₁ ,…,b _K , a and b are the coefficients to be identified of the differential equation, and K is called the equivalent order. In the equation, U _AB (t) ^(k) is the k-order derivative of the voltage small disturbance signal U _AB (t), i _A (t) ^(k) and i _B (t) ^(k) are the current small disturbance signals i _A (t), i _B (t ) k-order derivative, K is the equivalent order of the load model;

Step 3, obtaining the coefficients of the high-order differential equation in step 2;

Step 4, using the relationship between the transient high-order differential equation coefficients and the steady-state equivalent parameters to solve the equivalent parameters, the transient high-order differential equation coefficients and the steady-state equivalent parameters, that is, the transient high-order differential equation coefficients The relationship between a ₀ , a ₁ ,…,a _K , b ₀ ,b ₁ ,…,b _K , c ₀ ,c ₁ ,…,c _K and the steady-state equivalent parameters R _Aeq , R _Beq , R _Ceq as follows:

When K is an even number, let

When K is an odd number, let calculate

Thus, the A-phase steady-state equivalent parameters R _Aeq and L _Aeq and B-phase steady-state equivalent parameters R _Beq and L _Beq of the load model are obtained:

Step five, extract the fundamental frequency contained in the collected three-phase transient current signal and three-phase transient voltage signal

components, and written as the expression of the sine and cosine functions, as follows

U _Awen ＝a ₁ cosωt+a ₂ sinωt

U _Bwen ＝a ₃ cosωt+a ₄ sinωt

I _Awen ＝b ₁ cosωt+b ₂ sinωt

I _Bwen ＝b ₃ cosωt+b ₄ sinωt

U _Nwen = c ₁ cos ωt + c ₂ sin ωt;

Step 6, use the coefficients of the obtained sine-cosine expression to represent the equivalent impedance parameters obtained in step 4, as follows

c ₁ ＝a ₁ +R _A b ₁ -ωb ₂ L _A

c ₂ ＝a ₂ -R _A b ₂ -ωb ₁ L _A

c ₁ ＝a ₃ +R _B b ₃ -ωb ₄ L _B

c ₂ ＝a ₄ +R _B b ₄ +ωb ₃ L _B

Step 7, using the expression obtained in step 6 as the constraint condition, and the parameters calculated in step 4 as the initial value, optimize to obtain the exact value of each equal value impedance;

Step eight, pass

Calculate the drift voltage of the virtual neutral point on the load side;

Step 9, calculate the three-phase load unbalance degree, together with the three-phase load size and the virtual neutral point drift voltage, characterize the safety of the low-voltage power grid, the calculation formula of the voltage unbalance degree ε _u and the voltage unbalance degree ε _i as follows:

Among them: U ₁ — the root mean square of the positive sequence component amplitude of the three-phase voltage;

U ₂ —the negative sequence component amplitude root mean square of the three-phase voltage;

Among them: I ₁ —the root mean square of the amplitude of the positive sequence component of the three-phase current;

I ₂ —the root mean square of the amplitude of the negative sequence component of the three-phase current;

I ₂ —the root mean square of the amplitude of the negative sequence component of the three-phase current;

First calculate the positive and negative sequence components of the actual voltage drop on the three-phase load:

Wherein: a=e ^j120° ;

- the positive, negative and zero sequence components of the three-phase voltage,

In order to use the steady-state sampling points to fit the obtained voltage phasor, is the virtual neutral point voltage phasor calculated corresponding to time,

Then take the positive and negative sequence amplitude Calculate the unbalance degree ε _uN , the larger the value of ε _u and ε _i , the greater the unbalance degree of the load, the greater the threat to the safe operation of the power grid, and the calculation of the current unbalance degree ε _i is the same, the improved The calculation of the voltage unbalance degree ε _u uses the actual load voltage drop obtained in the previous steps to obtain the unbalance degree ε _uN , which has high reliability.

The method of using the transient component to calculate the equivalent impedance of the low-voltage power grid load is characterized in that due to the unbalanced three-phase load and the existence of the neutral line impedance, the virtual neutral point voltage and the transformer neutral point voltage have an offset, called is the drift voltage. At this time, the equivalent circuit cannot be divided into single-phase circuits. The drift voltage is affected by the load of each phase and cannot be eliminated directly.

The feature of this patent is that any two-phase sequence equation of the three-phase is directly used, and the two-phase currents are independent of each other. There are no parameters of other phases in the equation, and no decoupling is required.

Calculation of the unbalance degree of the real voltage drop on the three-phase load:

Find the positive and negative sequence components of the actual voltage drop across a three-phase load:

Wherein: a=e ^j120° ;

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

In order to use the steady-state sampling points to fit the obtained voltage phasor, The virtual neutral point voltage phasor obtained for the corresponding time.

Then take the positive and negative sequence amplitude values U _1N and U _2N to find the unbalance degree ε _uN :

The results are shown in Table 2:

Table 2 load imbalance

traditional method 0/125.24% patented method 14.3%/125.24%

Now in conjunction with simulation the present invention is further described:

1. This paper uses MATLAB simulation software to establish a low-voltage power grid simulation model under the condition of three-phase four-wire system, and the transient network is represented by a third-order circuit as shown in Figure 3. In Figure 3, R1=100Ω, L1= 0.003H, L2=0.002H, C1=1uF, RB=400Ω, LB=0.25H, R2=500Ω, L3=0.001H, R3=400Ω, R0=4Ω, L0=0.03H.

Step 1: List the higher order differential equations:

After sorting out:

u _AB +R ₁ C ₂ u' _AB +L ₁ C ₁ u″ _AB ＝R ₁ i _A +(L ₁ +L ₂ )i' _A +R ₁ C ₁ L ₂ i' _A '+L ₁ C ₁ L ₂ i″' _A +R _b i _B +(R ₁ C ₁ R _b +L _b )i _b '+(R ₁ C ₁ L _b +L ₁ R _b C ₁ )i' _B '+L ₁ L _b C ₁ i″' _B It can be seen from the above differential equation that K=3, and the identification model is:

There are 3*(3+1)-1=11 parameters to be identified.

Let a ₀ =1, and write the equation as: Y=A matrix in the form of AX.

In this paper, the least square method is used to identify the above formula, and 11 coefficients can be obtained: a ₀ -a ₃ , b ₀ -b ₃ , c ₀ -c ₃ .

And again:

Then according to the formula (2) (3), it is transformed into the steady-state equivalent impedance:

Similarly, the phase C parameters can be obtained.

Then there are:

Step 2: Take the sampled values i _O , i _A , u _A and the calculated value di/dt at steady state, and calculate the accurate LO and R _O according to formula (5).

The simulation results are now listed in Table 1 below:

Table 1-order circuit identification results

The comparison between the calculated voltage waveform of the virtual neutral point on the load side and the sampling waveform in the simulation is shown in Figure 4.

Step 3: Calculation of the unbalance degree of the true voltage drop on the three-phase load:

Find the positive and negative sequence components of the actual voltage drop across a three-phase load:

Wherein: a=e ^j120° ;

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

In order to use the steady-state sampling points to fit the obtained voltage phasor, The virtual neutral point voltage phasor obtained for the corresponding time.

Then take the positive and negative sequence amplitude values U _1N and U _2N to find the unbalance degree ε _uN :

The results are shown in Table 2:

Table 2 load imbalance

2. Simulation of neutral line impedance mutation:

As shown in Figure 5, the unit of resistance in Figure 5 is Ω, and the unit of inductance is H. In the simulation, the switch is suddenly disconnected at t=0.1s, and the impedance of the neutral line changes suddenly, causing a small disturbance at the monitoring port, and the sudden connection The input neutral line impedance is 4+j0.5, and the identification results are recorded in Table 3 below:

Table 3 Neutral line impedance mutation simulation

The comparison chart of the virtual neutral point voltage and the sampling voltage after the change is shown in Figure 6. The simulation shows that in the case of a sudden change in the neutral line, the method proposed in this paper still has good identification results for the virtual ground, neutral line impedance and three-phase value parameters.

3. Simulation of neutral line impedance gradient:

The simulation model is shown in Figure 7. The unit of resistance in the figure is Ω, and the unit of inductance is HR. The change interval of ₀ is [0-5]Ω, and the change rate is 0.002 ohms/second. The change interval of R ₀ is [0- 5] ohms, the rate of change is 0.002 ohms/second. Due to the low rate of change, the disturbance signal cannot be detected at the port only by the change of the neutral line impedance. However, in the low-voltage power grid, internal and external small disturbances often occur, and the program can still run. Comparing the information after two consecutive disturbances, it is easy to obtain the rate of change.

The simulation results are now listed in Table 4 below:

Table 4 Neutral line impedance gradient simulation

The comparison chart of the virtual neutral point voltage and the sampling voltage is shown in Figure 8.

Although the invention has been described herein with reference to a number of illustrative embodiments thereof, it is to be understood that numerous other modifications and implementations can be devised by those skilled in the art which will fall within the scope of the disclosure disclosed in this application. within the scope and spirit of the principles. More specifically, within the scope of the disclosure, drawings and claims of the present application, various modifications and improvements can be made to the components and/or layout of the subject combination layout. In addition to variations and modifications to the component parts and/or layout, other uses will be apparent to those skilled in the art.

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.