CN102375108B - Reliability analysis method for transformer neutral current suppression device - Google Patents

Abstract

The invention discloses a reliability analysis method for a transformer neutral current suppression device. The reliability analysis method is implemented through the following steps: acquiring line running voltage/current signals by using a current sensor and a voltage sensor; preprocessing the acquired signals by using a line monitoring terminal; transmitting the preprocessed signals into a PC (personal computer) by using a communication unit; and determining a compensation amount according to an acquisition amount by the PC through using a preset compensation method. By using the method disclosed by the invention, a purpose of suppressing neutral current can be ensured to be achieved after the device is incorporated through judging whether a preset compensation amount is reliable based on a transformer neutral current suppression model, thereby avoiding the occurrence of a situation that an expected compensation can not be achieved because of under-compensation.

Description

Reliability analysis method for transformer neutral current suppression device

Technical Field

The invention relates to a reliability analysis method for a neutral current suppression device of a transformer.

Background

At present, most of 10/0.4kV low-voltage distribution transformers in China are Yyn0 connection method. In this type of transformer, when the secondary side load is unbalanced, the transformer winding loss increases and the neutral current increases, resulting in a great waste of electric energy. The low-voltage transformer compensation device in China mostly adopts an SVC (static var compensator), and can effectively reduce the three-phase imbalance of the transformer and restrain the neutral current by judging and acting through collected three-phase line voltage and current signals through a control module. The neutral current suppression device adopts a reactive compensation mode to suppress neutral current, so that three-phase balance is ensured during full compensation, the neutral current tends to be 0, and the neutral current is reduced during under compensation. The setting of the compensation amount and the determination of the method influence the suppression effect of the line current in the transformer, judge whether the suppression device can reach the target or not, and avoid the problem that the suppression effect is not obvious due to insufficient compensation amount. In addition, for the case of full capacitance compensation, it is necessary to determine whether the transformer can achieve the target through full capacitance compensation.

Disclosure of Invention

The invention aims to provide a reliability analysis method for a neutral current suppression device of a transformer. The method is based on the influence of interphase and relatively ground reactive compensation on the outgoing line current of the three-phase four-wire system transformer, the influence of the setting of the suppression method and the compensation range on the compensation result, the line operation voltage and current signals are collected through a current sensor and a voltage sensor, the line operation voltage and current signals are preprocessed through a line monitoring terminal and are transmitted into a PC through a communication unit, the PC determines the compensation amount according to the collection amount through the set compensation method, the neutral current and the power factor of the compensated transformer are calculated, and the reliability of the neutral current suppression device of the transformer is determined through statistical analysis.

The purpose of the invention is realized as follows: a reliability analysis method for a transformer neutral current suppression device is characterized in that a line running voltage and current signal is collected through a current sensor and a voltage sensor, is preprocessed by a line monitoring terminal and a communication unit and then is transmitted into a PC (personal computer) and a GPRS (general packet radio service) terminal, the PC determines a compensation amount according to the collection amount by using a set compensation method, calculates the neutral current of a compensated transformer, and determines the reliability of the transformer neutral current suppression device through statistical analysis; the method comprises the following implementation steps:

a. a voltage sensor and a current sensor are arranged on the outgoing line side of the transformer, the sensors acquire voltage and current signals in the line and transmit the voltage and current signals to a central processing unit through a discrimination and conversion circuit, and the central processing unit judges whether the transformer normally operates according to the acquired signals, namely whether overvoltage and overcurrent phenomena exist or not, and determines three-phase active current I 'at the output end of the transformer during normal operation'_pa、I'_pb、I'_pcOf reactive current I'_qa、I'_qb、I'_qc(ii) a The total data acquisition times are set to be N, the acquisition interval is t, active current and reactive current of the output end of the three-phase transformer acquired at each time are stored in a plurality of groups Dn ', and the total data D acquired for N times are sent to an upper PC for processing, wherein the data formats of Dn' and D are as follows:

D_n'={I'_paI'_pbI'_pcI'_qaI'_qbI'_qc}^T

D={D₁...D_n...D_N}

wherein N ranges from [0N ];

b. according to the amount of reactive compensation

And actual current voltage I 'in line'_pa、I'_pb、I'_pc、I'_qa、I'_qb、I'_qcU' to determine the active current I in the reactive-load compensated network in the line_pa、I_pb、I_pcReactive current I_qa、I_qb、I_qcThe formula is calculated:

$I_{\mathrm{pa}}={I_{\mathrm{pa}}}^{\′}+\frac{1}{2\sqrt{3}{U}^{\′}}(Q_{\mathrm{ca}}^{\mathrm{\Δ}}-Q_{\mathrm{ab}}^{\mathrm{\Δ}})$

$I_{\mathrm{pb}}={I_{\mathrm{pb}}}^{\′}+\frac{1}{2\sqrt{3}{U}^{\′}}(Q_{\mathrm{ab}}^{\mathrm{\Δ}}-Q_{\mathrm{bc}}^{\mathrm{\Δ}})$

$I_{\mathrm{pc}}={I_{\mathrm{pc}}}^{\′}+\frac{1}{2\sqrt{3}{U}^{\′}}(Q_{\mathrm{bc}}^{\mathrm{\Δ}}-Q_{\mathrm{ca}}^{\mathrm{\Δ}})$

$I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{ca}}^{\mathrm{\Δ}}+Q_{\mathrm{ab}}^{\mathrm{\Δ}})+\frac{Q_a^Y}{U^{\′}}$

$I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{ab}}^{\mathrm{\Δ}}+Q_{\mathrm{bc}}^{\mathrm{\Δ}})+\frac{Q_b^Y}{U^{\′}}$

$I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{bc}}^{\mathrm{\Δ}}+Q_{\mathrm{ca}}^{\mathrm{\Δ}})+\frac{Q_c^Y}{U^{\′}}$

c. setting a compensation range of the neutral current suppression device, and determining an actual compensation amount; the calculation formula for determining the reactive compensation amount of the neutral current suppression is as follows:

an objective function:

minf(X)=ω|I₀|²+|I₂|²+|Im(I₁)|²

wherein, I₀For the compensated zero sequence current, Im (I)₁) To compensate for the imaginary part of the post-positive-sequence component, I₂For the compensated negative sequence component, ω is a weight function, and the weight function is set to a number greater than 0; determining the priority of the neutral current suppression by adjusting the omega weight function; i is₀、I₁、I₂The calculation formula is as follows:

$I_0=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+{\mathrm{\α}}^2(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+\mathrm{\α}(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_1=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_2=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+\mathrm{\α}(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+{\mathrm{\α}}^2(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

wherein, $\mathrm{\α}=-\frac{1}{2}+j\frac{\sqrt{3}}{2};$

determination of the constraint:

1) the set compensation range is as follows:

$-{\mathrm{\α}}_1\≤Q_{\mathrm{ab}}^{\mathrm{\Δ}}\≤{\mathrm{\β}}_1;$ $-{\mathrm{\α}}_1\≤Q_{\mathrm{bc}}^{\mathrm{\Δ}}\≤{\mathrm{\β}}_1;$ $-{\mathrm{\α}}_1\≤Q_{\mathrm{ca}}^{\mathrm{\Δ}}\≤{\mathrm{\β}}_1;$

$-{\mathrm{\α}}_2\≤Q_a^Y\≤{\mathrm{\β}}_2;$ $-{\mathrm{\α}}_2\≤Q_b^Y\≤{\mathrm{\β}}_2;$ $-{\mathrm{\α}}_2\≤Q_c^Y\≤{\mathrm{\β}}_2;$

wherein,

is the reactive compensation quantity between the AB phases,

is the reactive compensation quantity between BC phases,

is the reactive compensation quantity among the CA phases,

is the reactive compensation quantity between the phase A and the ground,

is the reactive compensation quantity between the phase B and the ground,

is the reactive compensation between C phase and ground, alpha₁The value is negative for the maximum interphase inductive reactive compensation; alpha is alpha₂The maximum relative ground inductive reactive compensation is achieved, and the value is negative; beta is a₁The value is positive for maximum interphase capacitive reactive compensation; beta is a₂For maximum relative capacitive reactive compensation, its value is positive;

2) reactive compensation constraint conditions, which may be specifically required according to the actual operation condition of the transformer, usually take output of inductive reactive power as the main:

${I_{\mathrm{qa}}}^{\′}\≤I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{ca}}^{\mathrm{\Δ}}+Q_{\mathrm{ab}}^{\mathrm{\Δ}})+\frac{Q_a^Y}{U^{\′}}\≤0$

${I_{\mathrm{qb}}}^{\′}\≤I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{ab}}^{\mathrm{\Δ}}+Q_{\mathrm{bc}}^{\mathrm{\Δ}})+\frac{Q_b^Y}{U^{\′}}\≤0$

${I_{\mathrm{qc}}}^{\′}\≤I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\′}+\frac{1}{{2U}^{\′}}(Q_{\mathrm{bc}}^{\mathrm{\Δ}}+Q_{\mathrm{ca}}^{\mathrm{\Δ}})+\frac{Q_c^Y}{U^{\′}}\≤0$

wherein the voltage amplitude U' is 220V;

3) active power transfer constraint conditions:

$\left\{\begin{array}{c}0\&le;(I_p-I_p^{*})\&le;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I_p^{*}\&GreaterEqual;0\\ 0\&GreaterEqual;(I_p-I_p^{*})\&GreaterEqual;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I_p^{*}<0\end{array}\right.$

wherein,

is the average value of the active current; i is_p' ABC three-phase active current, I, in the pre-compensation grid_pThe method comprises the steps of compensating ABC three-phase active current in a power grid;

4) neutral current constraint

Because the main objective of compensation is to implement neutral current management, in order to ensure that the neutral current is not increased after compensation, the zero sequence current is set to satisfy the constraint condition as shown in the following formula:

|I₀|²≤|I'₀|²

wherein, | I'₀I is the modulus of the zero sequence current before compensation, | I₀I is the module of the zero sequence current after compensation;

d. determining three-phase current after compensation according to actual compensation quantity of the compensation device and load side current so as to obtain neutral current and power factor of secondary side of the transformer after compensation of nth acquisition quantity, wherein the calculation formula is as follows:

I₍₀₎=(I_pa+jI_qa)+α²(I_pb+jI_qb)+α(I_pc+jI_qc)

${\cos \mathrm{\&theta;}}_n=\frac{I_q}{\sqrt{I_p^2+I_q^2}}$

the judgment basis of reliability is as follows:

1) because the objective function is not 0, zero sequence current after the compensation of the transformer is described, the imaginary part of the positive sequence component after the compensation does not tend to 0; the larger the objective function is, the more serious the compensation device is under-compensated; setting a target function control quantity K1, and when the value of the target function is smaller than K1, the imaginary parts of the zero sequence component, the negative sequence component and the positive sequence component of the current tend to 0, and at the moment, the compensation device does not under-compensate; counting the occurrence times of the objective function when the objective function is larger than K1, and determining the proportion M of the occurrence times in the total sampling times N;

2) in practice, neutral current of 0 and reactive compensation of 0 can not be completely achieved through the static reactive compensation device, neutral current and reactive current can only be reduced, the controlled quantity is K2 and the power factor controlled quantity is K3 after neutral current control, when the objective function exceeds K1, whether neutral current and power factor of the transformer exceed the controlled quantities K2 and K3 or not is judged, and if the objective function exceeds K1, the compensation device is unreliable;

and (3) reliability analysis:

setting a reliability requirement value Km, and when M is less than or equal to Km and the target function exceeds a threshold value, keeping the neutral current within a threshold value K2 and keeping the power factor above K3, which shows that the compensation device can effectively restrain the neutral current of the transformer, has high working stability and certain reliability; and judging whether the compensation can be carried out completely through setting the compensation range from 0 to infinity or not, and judging whether the compensation is reliable or not under the condition and whether the compensation requirement is met by adopting the completely capacitive compensation or not.

The line monitoring terminal and the communication unit comprise a current sensor, a voltage sensor, a signal filtering, amplifying and converting circuit, a central processing unit, a field terminal GPRS communication module and a power supply of the acquisition and communication unit; the current sensor and the voltage sensor collect line voltage and current signals, the output end of the current sensor and the voltage sensor is connected with the input end of the signal filtering, amplifying and converting circuit, the output end of the signal filtering, amplifying and converting circuit is connected with the input end of the central processing unit, the output end of the central processing unit is connected with the field end GPRS communication module, and the power supply of the collecting and communicating unit is connected with the power supply ends of the signal filtering, amplifying and converting circuit, the central processing unit and the field end GPRS communication module;

the PC and the GPRS terminal are composed of a GPRS terminal and a PC, and the input end of the GPRS terminal is connected with the serial port of the PC.

The invention has the beneficial effects that:

the method ensures whether the neutral current suppression device of the transformer is guaranteed to reach the target after being merged into the power grid, avoids the problem that the neutral current suppression effect after compensation is not obvious due to insufficient compensation amount, and ensures that the compensation device can effectively reduce the loss of the neutral current of the distribution transformer.

Drawings

FIG. 1 is a system block diagram of the present invention.

FIG. 2 is a process flow diagram of the reliability analysis of the present invention.

Detailed Description

Referring to fig. 1, the system is composed of a line monitoring terminal, a communication unit A, PC, and a GPRS terminal B:

1) the line monitoring terminal and communication unit A comprises a current sensor 11, a voltage sensor 12, a signal filtering, amplifying and converting circuit 13, a central processing unit 14, a field terminal GPRS communication module 15 and a power supply 16 of the acquisition and communication unit; the current sensor 11 and the voltage sensor 12 collect line voltage and current signals, the output ends of the current sensor and the voltage sensor are connected with the input end of the signal filtering, amplifying and converting circuit 13, the output end of the signal filtering, amplifying and converting circuit 13 is connected with the input end of the central processing unit 14, the output end of the central processing unit 14 is connected with the field end GPRS communication module 15, and the power supply 16 of the collecting and communicating unit is connected with the power supply ends of the signal filtering, amplifying and converting circuit 13, the central processing unit 14 and the field end GPRS communication module 15;

2) the PC and the GPRS terminal B are composed of a GPRS terminal 17 (namely a GPRS wireless receiving module) and a PC 18, and the input end of the GPRS terminal is connected with the serial port of the PC;

3) connecting input ends of six signal acquisition modules of a line monitoring terminal and a communication unit to a secondary end of a three-phase of a distribution transformer to be tested; information is transmitted between the GPRS communication module of each line monitoring terminal and communication unit and the GPRS communication modules of the PC and GPRS terminals in a wireless communication mode

In this embodiment, the line monitoring terminal and the communication unit are composed of a current sensor 11, a voltage sensor 12 and a signal filtering, amplifying and converting circuit 13. The sensor and the signal filtering, amplifying and converting electricity thereof are all commercial products. In the line monitoring terminal and the communication unit, the power supply 16 consists of a switching power supply 21, a lithium battery 22, a charging control circuit 23 and a direct-current voltage boosting conversion circuit 24; the charging control circuit 23 is connected to the secondary end of the distribution transformer through the switching power supply 21, and the output end of the charging control circuit 23 is connected in series with the lithium battery 22 and then connected with the direct-current voltage boost conversion circuit 24. The power supply 16 is a known circuit composed of a switching power supply 21, a lithium battery 22, a charge control circuit 23, and a dc voltage boost converter circuit 24.

The PC and the GPRS terminal are installed in the monitoring center, and the PC comprises related software for analyzing the reliability of the transformer unbalance compensation device.

The compensation amount determination of the present invention is based on an algorithm:

an objective function:

minf(X)=ω|I₀|²+|I₂|²+|Im(I₁)|²

wherein, I₀For the compensated zero sequence current, Im (I)₁) To compensate for the imaginary part of the post-positive-sequence component, I₂To compensate for the pre-negative sequence component, ω is a weight function, and the weight function is set to a number greater than 0; determining the priority of the neutral current suppression by adjusting the omega weight function; i is₀、I₁、I₂The calculation formula is as follows:

$I_0=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+{\mathrm{\&alpha;}}^2(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+\mathrm{\&alpha;}(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_1=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_2=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+\mathrm{\&alpha;}(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+{\mathrm{\&alpha;}}^2(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

wherein, $\mathrm{\&alpha;}=-\frac{1}{2}+j\frac{\sqrt{3}}{2}.$

determination of the constraint:

1) the set compensation range is as follows:

$-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$ $-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$ $-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$

$-{\mathrm{\&alpha;}}_2\&le;Q_a^Y\&le;{\mathrm{\&beta;}}_2;$ $-{\mathrm{\&alpha;}}_2\&le;Q_b^Y\&le;{\mathrm{\&beta;}}_2;$ $-{\mathrm{\&alpha;}}_2\&le;Q_c^Y\&le;{\mathrm{\&beta;}}_2;$

wherein,

is the reactive compensation quantity between the AB phases,is the reactive compensation quantity between BC phases,

is the reactive compensation quantity among the CA phases,

is the reactive compensation quantity between the phase A and the ground,

is the reactive compensation quantity between the phase B and the ground,

is the reactive compensation between C phase and ground, alpha₁The value is negative for the maximum interphase inductive reactive compensation; alpha is alpha₂The maximum relative ground inductive reactive compensation is achieved, and the value is negative; beta is a₁The value is positive for maximum interphase capacitive reactive compensation; beta is a₂For maximum relative capacitive reactive compensation, its value is positive;

2) reactive compensation constraint conditions, which may be specifically required according to the actual operation condition of the transformer, usually take output of inductive reactive power as the main:

${I_{\mathrm{qa}}}^{\&prime;}\&le;I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})+\frac{Q_a^Y}{U^{\&prime;}}\&le;0$

${I_{\mathrm{qb}}}^{\&prime;}\&le;I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}+Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})+\frac{Q_b^Y}{U^{\&prime;}}\&le;0$

${I_{\mathrm{qc}}}^{\&prime;}\&le;I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})+\frac{Q_c^Y}{U^{\&prime;}}\&le;0$

wherein the voltage amplitude U' is 220V;

3) active power transfer constraint conditions:

$\left\{\begin{array}{c}0\&le;(I_p-I_p^{*})\&le;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I^{*}\&GreaterEqual;0\\ 0\&GreaterEqual;(I_p-I_p^{*})\&GreaterEqual;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I^{*}<0\end{array}\right.$

wherein,

is the average value of the active current; i is_p' ABC three-phase active current, I, in the pre-compensation grid_pThe method comprises the steps of compensating ABC three-phase active current in a power grid;

4) neutral current constraint

Because the main objective of compensation is to implement neutral current management, in order to ensure that the neutral current is not increased after compensation, the zero sequence current is set to satisfy the constraint condition as shown in the following formula:

|I₀|²≤|I'₀|²

wherein, | I'₀I is the modulus of the zero sequence current before compensation, | I₀And | is a module of the zero sequence current after compensation.

Determining the neutral current of the compensated transformer:

the upper computer performs reactive compensation according to the amount of reactive compensation

And actual current voltage I 'in line'_pa、I'_pb、I'_pc、I'_qa、I'_qb、I'_qcU' to determine the active current I in the reactive-load compensated network in the line_pa、I_pb、I_pcReactive current I_qa、I_qb、I_qcThe formula is calculated:

$I_{\mathrm{pa}}={I_{\mathrm{pa}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}-Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{pb}}={I_{\mathrm{pb}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}-Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{pc}}={I_{\mathrm{pc}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}-Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})+\frac{Q_a^Y}{U^{\&prime;}}$

$I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}+Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})+\frac{Q_b^Y}{U^{\&prime;}}$

$I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})+\frac{Q_c^Y}{U^{\&prime;}}$

determining the neutral line current and the power factor of the transformer after the compensation of the nth acquisition amount, wherein the calculation formula is as follows:

I₍₀₎=(I_pa+jI_qa)+α²(I_pb+jI_qb)+α(I_pc+jI_qc)

${\cos \mathrm{\&theta;}}_n=\frac{I_q}{\sqrt{I_p^2+I_q^2}}$

wherein, I_pAnd I_qShown as three-phase active and reactive current.

The judgment basis of reliability is as follows:

1) the objective function is not 0, which shows that zero sequence current after the compensation of the transformer, the imaginary part of the positive sequence component and the negative sequence component do not tend to 0. The larger the objective function, the more severe the compensation means are under-compensated. And setting the target function control quantity K1, and when the value of the target function is smaller than K1, the imaginary parts of the zero sequence component, the negative sequence component and the positive sequence component of the current tend to 0, and the compensating device does not under compensate at the moment. And (4) counting the occurrence times when the objective function is greater than K1, and determining the proportion M of the occurrence times in the total sampling times N.

2) In practice, neutral current of 0 and reactive compensation of 0 cannot be completely achieved through the static reactive compensation device, only neutral current and reactive current can be reduced, the controlled quantity after neutral current control is set to be K2 and the power factor controlled quantity is set to be K3, when the objective function exceeds K1, whether the neutral current and the power factor of the transformer exceed the controlled quantities K2 and K3 or not is judged, and if the objective function exceeds K1, the compensation device is unreliable.

And (3) reliability analysis:

1. take full capacitance compensation as an example. The compensation range is set to 0 to infinity. Threshold K1=1, K2=10A, K3=0.85, Km =0.2, ω =1000 in the objective function.

2. And collecting the active current and the reactive current of the outgoing line side of the transformer for 5 times, as shown in table 1.

TABLE 1 active and reactive current on the outgoing line side of a transformer

3. The compensation amount is calculated according to the actual condition of the line, as shown in table 2.

TABLE 2 unbalance Compensation

And 4, determining the power quality of the outlet side of the compensated transformer according to the compensation quantity, as shown in the table 3.

TABLE 3 Compensation results after unbalance Compensation

5. Counting the compensation result, and determining the reliability of the compensation device: the compensation result shows that the M value is 0.2, the threshold condition is met, and the power factor and the neutral current of the compensation result meet the compensation requirement. It is proven that full capacitance compensation can be used.

Claims (2)

1. A reliability analysis method for a transformer neutral current suppression device is characterized in that a line running voltage and current signal is collected through a current sensor and a voltage sensor, is preprocessed by a line monitoring terminal and a communication unit and then is transmitted into a PC (personal computer) and a GPRS (general packet radio service) terminal, the PC determines a compensation amount according to the collection amount by using a set compensation method, calculates the neutral current of a compensated transformer, and determines the reliability of the transformer neutral current suppression device through statistical analysis; the method comprises the following implementation steps:

a. the voltage and current sensors are arranged on the outgoing line side of the transformerVoltage and current signals in a sensor acquisition line are transmitted to a central processing unit through a discrimination and conversion circuit, the central processing unit judges whether the transformer normally operates according to the acquired signals, namely whether overvoltage and overcurrent phenomena exist, and determines three-phase active current I 'at the output end of the transformer during normal operation'_pa、I'_pb、I'_pcOf reactive current I'_qa、I'_qb、I'_qc(ii) a The total data acquisition times are set to be N, the acquisition interval is t, active current and reactive current of the output end of the three-phase transformer acquired at each time are stored in a plurality of groups Dn ', and the total data D acquired for N times are sent to an upper PC for processing, wherein the data formats of Dn' and D are as follows:

D_n'={I'_paI'_pbI'_pcI'_qaI'_qbI'_qc}^T

D={D₁...D_n...D_N}

wherein N ranges from [0N ];

b. according to the amount of reactive compensation

And actual current voltage I 'in line'_pa、I'_pb、I'_pc、I'_qa、I'_qb、I'_qcU' to determine the active current I in the reactive-load compensated network in the line_pa、I_pb、I_pcReactive current I_qa、I_qb、I_qcThe formula is calculated:

$I_{\mathrm{pa}}={I_{\mathrm{pa}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}-Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{pb}}={I_{\mathrm{pb}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}-Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{pc}}={I_{\mathrm{pc}}}^{\&prime;}+\frac{1}{2\sqrt{3}{U}^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}-Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})$

$I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})+\frac{Q_a^Y}{U^{\&prime;}}$

$I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}+Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})+\frac{Q_b^Y}{U^{\&prime;}}$

$I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\&prime;}+\frac{1}{2U^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})+\frac{Q_c^Y}{U^{\&prime;}}$

c. setting a compensation range of the neutral current suppression device, and determining an actual compensation amount; the calculation formula for determining the reactive compensation amount of the neutral current suppression is as follows:

an objective function:

minf(X)=ω|I₀|²+|I₂|²+|Im(I₁)|²

wherein, I₀In order to compensate the zero-sequence current,Im(I₁) To compensate for the imaginary part of the post-positive-sequence component, I₂For the compensated negative sequence component, ω is a weight function, and the weight function is set to a number greater than 0; determining the priority of the neutral current suppression through adjusting the weight function; i is₀、I₁、I₂The calculation formula is as follows:

$I_0=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+{\mathrm{\&alpha;}}^2(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+\mathrm{\&alpha;}(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_1=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

$I_2=\frac{(I_{\mathrm{pa}}+{\mathrm{jI}}_{\mathrm{qa}})+\mathrm{\&alpha;}(I_{\mathrm{pb}}+{\mathrm{jI}}_{\mathrm{qb}})+{\mathrm{\&alpha;}}^2(I_{\mathrm{pc}}+{\mathrm{jI}}_{\mathrm{qc}})}{3}$

wherein, $\mathrm{\&alpha;}=-\frac{1}{2}+j\frac{\sqrt{3}}{2};$

determination of the constraint:

1) the set compensation range is as follows:

$-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$ $-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$ $-{\mathrm{\&alpha;}}_1\&le;Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}\&le;{\mathrm{\&beta;}}_1;$

$-{\mathrm{\&alpha;}}_2\&le;Q_a^Y\&le;{\mathrm{\&beta;}}_2;$ $-{\mathrm{\&alpha;}}_2\&le;Q_b^Y\&le;{\mathrm{\&beta;}}_2;$ $-{\mathrm{\&alpha;}}_2\&le;Q_c^Y\&le;{\mathrm{\&beta;}}_2;$

wherein,

is the reactive compensation quantity between the AB phases,is the reactive compensation quantity between BC phases,

is the reactive compensation quantity among the CA phases,

is the reactive compensation quantity between the phase A and the ground,

is the reactive compensation quantity between the phase B and the ground,

is the reactive compensation between C phase and ground, alpha₁The value is negative for the maximum interphase inductive reactive compensation; alpha is alpha₂The maximum relative ground inductive reactive compensation is achieved, and the value is negative; beta is a₁The value is positive for maximum interphase capacitive reactive compensation; beta is a₂For maximum relative capacitive reactive compensation, the value is positive；

2) Reactive compensation constraint conditions, which may be specifically required according to the actual operation condition of the transformer, usually take output of inductive reactive power as the main:

${I_{\mathrm{qa}}}^{\&prime;}\&le;I_{\mathrm{qa}}={I_{\mathrm{qa}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{ca}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ab}}^{\mathrm{\&Delta;}})+\frac{Q_a^Y}{U^{\&prime;}}\&le;0$

${I_{\mathrm{qb}}}^{\&prime;}\&le;I_{\mathrm{qb}}={I_{\mathrm{qb}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{ab}}^{\mathrm{\&Delta;}}+Q_{\mathrm{bc}}^{\mathrm{\&Delta;}})+\frac{Q_b^Y}{U^{\&prime;}}\&le;0$

${I_{\mathrm{qc}}}^{\&prime;}\&le;I_{\mathrm{qc}}={I_{\mathrm{qc}}}^{\&prime;}+\frac{1}{{2U}^{\&prime;}}(Q_{\mathrm{bc}}^{\mathrm{\&Delta;}}+Q_{\mathrm{ca}}^{\mathrm{\&Delta;}})+\frac{Q_c^Y}{U^{\&prime;}}\&le;0$

wherein the voltage amplitude U' is 220V;

3) active power transfer constraint conditions:

$\left\{\begin{array}{c}0\&le;(I_p-I_p^{*})\&le;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I_p^{*}\&GreaterEqual;0\\ 0\&GreaterEqual;(I_p-I_p^{*})\&GreaterEqual;{I_p}^{\&prime;}-I_p^{*},{I_p}^{\&prime;}-I_p^{*}<0\end{array}\right.$

wherein,

is the average value of the active current; i is_p' ABC three-phase active current, I, in the pre-compensation grid_pThe method comprises the steps of compensating ABC three-phase active current in a power grid;

4) neutral current constraint

Because the main objective of compensation is to implement neutral current management, in order to ensure that the neutral current is not increased after compensation, the zero sequence current is set to satisfy the constraint condition as shown in the following formula:

|I₀|²≤|I'₀|²

wherein, | I'₀I is the modulus of the zero sequence current before compensation, | I₀I is the module of the zero sequence current after compensation;

d. determining three-phase current after compensation according to actual compensation quantity of the compensation device and load side current so as to obtain neutral current of the secondary side of the transformer after compensation of the nth acquisition quantity, wherein the calculation formula is as follows:

I₍₀₎=(I_pa+jI_qa)+α²(I_pb+jI_qb)+α(I_pc+jI_qc)

the judgment basis of reliability is as follows:

1) because the objective function is not 0, zero sequence current after the compensation of the transformer is described, the imaginary part of the positive sequence component after the compensation does not tend to 0; the larger the objective function is, the more serious the compensation device is under-compensated; setting a target function control quantity K1, and when the value of the target function is smaller than K1, the imaginary parts of the zero sequence component, the negative sequence component and the positive sequence component of the current tend to 0, and at the moment, the compensation device does not under-compensate; counting the occurrence times of the objective function when the objective function is larger than K1, and determining the proportion M of the occurrence times in the total sampling times N;

2) in practice, neutral current of 0 and reactive compensation of 0 can not be completely achieved through the static reactive compensation device, neutral current and reactive current can only be reduced, the controlled quantity is K2 and the power factor controlled quantity is K3 after neutral current control, when the objective function exceeds K1, whether neutral current and power factor of the transformer exceed the controlled quantities K2 and K3 or not is judged, and if the objective function exceeds K1, the compensation device is unreliable;

and (3) reliability analysis:

setting a reliability requirement value Km, and when M is less than or equal to Km and the target function exceeds the threshold, keeping the neutral current within a threshold K2 and the power factor above K3, which shows that the compensation device can effectively restrain the neutral current of the transformer, has high working stability and certain reliability; and judging whether the compensation can be carried out completely through setting the compensation range from 0 to infinity or not, and judging whether the compensation is reliable or not under the condition and whether the compensation requirement is met by adopting the completely capacitive compensation or not.

2. The reliability analysis method for the transformer neutral current suppression device according to claim 1, wherein the line monitoring terminal and the communication unit comprise a signal filtering, amplifying and converting circuit (13), a central processing unit (14), a field terminal GPRS communication module (15) and a power supply (16) of the acquisition and communication unit; the current sensor (11) and the voltage sensor (12) collect line voltage and current signals, the output ends of the current sensor and the voltage sensor are connected with the input end of the signal filtering, amplifying and converting circuit (13), the output end of the signal filtering, amplifying and converting circuit (13) is connected with the input end of the central processing unit (14), the output end of the central processing unit (14) is connected with the field end GPRS communication module (15), and the power supply (16) of the collecting and communicating unit is connected with the power supply ends of the signal filtering, amplifying and converting circuit (13), the central processing unit (14) and the field end GPRS communication module (15);

the PC and the GPRS terminal are composed of a GPRS terminal (17) and a PC (18), and the input end of the GPRS terminal (17) is connected with the serial port of the PC (18).

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