CN110829455A - Reactive compensation method for capacitor of power distribution network - Google Patents

Reactive compensation method for capacitor of power distribution network Download PDF

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Publication number
CN110829455A
CN110829455A CN201911178746.0A CN201911178746A CN110829455A CN 110829455 A CN110829455 A CN 110829455A CN 201911178746 A CN201911178746 A CN 201911178746A CN 110829455 A CN110829455 A CN 110829455A
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compensation
capacitor
voltage
reactive
harmonic
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沈煜
白敬强
黄文焘
王昊
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Shanghai Jiaotong University
Electric Power Research Institute of State Grid Hubei Electric Power Co Ltd
Electric Power Research Institute of State Grid Beijing Electric Power Co Ltd
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Shanghai Jiaotong University
Electric Power Research Institute of State Grid Hubei Electric Power Co Ltd
Electric Power Research Institute of State Grid Beijing Electric Power Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
    • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
    • H02J3/1871—Methods for planning installation of shunt reactive power compensators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/30—Reactive power compensation
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
    • Y02E40/40—Arrangements for reducing harmonics

Abstract

A reactive compensation method for a capacitor of a power distribution network comprises the steps of firstly, determining an optimal compensation point by adopting a reactive margin mode, monitoring the voltage of the compensation point on line, and controlling a switched capacitor by taking the voltage as a constraint condition so as to dynamically adjust the compensation capacity; and then, capacitor compensation is carried out on voltage drop caused by impact load, harmonic wave influence caused by the delay of the parallel capacitors when the capacitors are connected into a power distribution network is analyzed, and reactors are connected in series on the capacitors to inhibit the amplification effect of the parallel capacitors on the harmonic wave, so that the influence of load impact on a power grid is reduced.

Description

Reactive compensation method for capacitor of power distribution network
Technical Field
The invention relates to a technology in the field of power grid control, in particular to a reactive power compensation method for a capacitor of a power distribution network.
Background
Reactive compensation is an important measure for keeping reactive power balance of a power system, reducing network loss and improving power supply quality, and is widely applied to power networks of various voltage levels. With the increasing requirements of the users on the power quality and the power supply stability of the power grid, the application of reactive compensation is an important subject for improving the economic and social benefits of power enterprises.
At present, various reactive compensation modes exist in a power distribution network in China, and the main technical problems to be solved urgently are as follows: the reactive compensation research on the low-voltage power grid is in a starting stage, and the low-voltage power grid has the characteristics of large impact, complex properties and the like for small users, thereby causing great difficulty in real-time reactive power optimization control.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides the reactive power compensation method for the capacitor of the power distribution network, which can stably improve the voltage of a line and an impact load point and effectively ensure the voltage quality of a power grid. When the reactive power compensation of the power distribution network needs to be connected with a plurality of capacitor banks, the frequency characteristic of the harmonic impedance of the system can be changed accordingly, and an amplification effect is generated on the harmonic current.
The invention is realized by the following technical scheme:
the invention relates to a reactive compensation method for a capacitor of a power distribution network, which comprises the steps of firstly, determining an optimal compensation point by adopting a reactive margin mode, monitoring the voltage of the compensation point on line, and controlling a switched capacitor by taking the voltage as a constraint condition so as to dynamically adjust the compensation capacity; and then, capacitor compensation is carried out on voltage drop caused by impact load, harmonic wave influence caused by the delay of the parallel capacitors when the capacitors are connected into a power distribution network is analyzed, and reactors are connected in series on the capacitors to inhibit the amplification effect of the parallel capacitors on the harmonic waves.
The reactive margin mode is as follows: and under the condition of stable static voltage, the electrical distance between the system operation point and the critical breakdown voltage point is large. When the reactive margin value of a node is large, the reactive compensation capacity required by the node is small, and otherwise, the required reactive compensation capacity is large. Through the size of the reactive margin value, the point where the system needs reactive compensation most can be found out.
The dynamic adjustment compensation capacity refers to: when the voltage is too low, the compensation point is added into the capacitor for compensation; when the voltage is too high, the compensation point is to exit the capacitor.
The harmonic influence refers to that: the harmonic current generated during start-stop of the motor is amplified due to the change in the frequency characteristics of the system harmonic impedance caused by the use of the capacitor.
Technical effects
Compared with the prior art, the invention integrally solves the technical problems that: the low-voltage power grid reactive compensation optimization is difficult, and the compensation point selection and the compensation capacity of the low-voltage power grid reactive compensation scheme are analyzed in detail; and by the method, harmonic wave influence can be generated on the power grid when the low-voltage power grid is subjected to reactive power compensation, and the harmonic wave can be eliminated by a corresponding method.
Drawings
FIG. 1 is a schematic diagram of reactive compensation of a parallel capacitor;
in the figure: (a) parallel compensation wiring diagram (b) parallel compensation vector diagram
FIG. 2 is a simplified current circuit diagram of an embodiment;
FIG. 3 is a schematic diagram of an embodiment of power circle intersection tangency;
in the figure: (a) are crossed; (b) tangency;
FIG. 4 is a schematic diagram of an equivalent impedance of an electrical power system according to an embodiment;
FIG. 5 is a schematic diagram illustrating the harmonic current amplification effect of the parallel capacitor according to the embodiment;
in the figure: (a) a simple system; (b) a schematic current distribution diagram;
FIG. 6 is a schematic diagram of an embodiment high pass filter;
in the figure: (a) primary wiring of the high-pass filter (b) equivalent circuit schematic of the high-pass filter;
FIG. 7 is a schematic diagram of an embodiment asynchronous motor start-stop model with parallel capacitors;
FIG. 8 is a schematic diagram of harmonic analysis (with large capacitors connected in parallel) of the voltage at the outlet of the asynchronous motor according to the embodiment;
FIG. 9 is a schematic diagram of an asynchronous motor start-stop model with parallel large capacitors (series inductors) according to an embodiment;
FIG. 10 is a schematic diagram of voltage frequency analysis (with parallel capacitance and series inductance) at the outlet of an asynchronous motor according to an embodiment;
FIG. 11 is a schematic diagram of a harmonic analysis of the system resonance caused by an excessive parallel capacitance according to the embodiment;
FIG. 12 is a schematic diagram of a main line node location in an embodiment;
fig. 13 is a schematic diagram illustrating the effect of main line voltage distribution before and after compensation.
Detailed Description
As shown in fig. 1, this embodiment relates to a method for reactive compensation of a capacitor of a power distribution network, which includes the following steps:
step 1, determining an optimal compensation point by adopting a reactive margin mode and monitoring the voltage of the compensation point on line, wherein the reactive margin value of a node is large, so that the reactive compensation capacity required by the node is small, otherwise, the required reactive compensation capacity is large, so that the point which is most required to perform reactive compensation of a system can be found out through the magnitude of the reactive margin value, and the reactive margin value of each node is obtained through the following modes:
1.1) carrying out load flow calculation according to the structure of the system and the line parameters to obtain the voltage and the line power value of each node;
1.2) calculating r of each nodeP、rQThe value of D;
1.3) calculating the reactive margin value of each node, and sequencing from small to large;
1.4) obtaining the arrangement sequence of the reactive power shortage of each node of the system, and taking corresponding reactive power compensation measures at the nodes.
Step 2, the compensation point voltage is used as a constraint condition to control the switched capacitor so as to dynamically adjust the compensation capacity, namely after the compensation node is determined, in order to enable the voltage to meet the requirement, the method for determining the compensation capacity by improving the voltage specifically comprises the following steps:
2.1) network Voltage before installing the Compensation capacitor
Figure BDA0002290708960000031
2.2) after the compensation capacitor is installed, the supply voltage U1Bus voltage U of substation is considered unchanged2Go to U'2Then compensating capacitor
Figure BDA0002290708960000032
I.e. the compensation capacitance C is inversely proportional to X.
Step 3, compensating a capacitor for voltage drop caused by impact load, analyzing harmonic influence caused by the delay of the capacitors connected in parallel in the power distribution network, and installing a high-pass filter on the capacitors to inhibit the amplification effect of the capacitors connected in parallel on the harmonic, specifically:
3.1) acquiring/calculating/analyzing the influence of the parallel capacitor on the harmonic impedance of the system;
3.2) changing cut-off frequency by changing resistance and capacitance of high-pass filter circuit
3.3) harmonic frequencies below the cut-off frequency are difficult to pass through, thereby eliminating harmonics of the frequency.
Most of the loads, whether industrial or civil, are inductive loads. All inductive loads need to consume a large amount of reactive power, and there are two main ways to provide these reactive powers: firstly, power transmission system supply; the second is the provision of a compensation capacitor. When provided by a transmission system, the transmission system is designed to take into account both active power and reactive power. The transmission of reactive power by the transmission system will cause the increase of the loss of the transmission line and the transformer, and reduce the economic benefit of the system. And the compensation capacitor provides reactive power on site, so that the transmission of the reactive power by a power transmission system can be avoided, the reactive loss is reduced, and the transmission power of the system is improved. Therefore, the reactive compensation is carried out by adopting a compensation capacitor mode in the project.
As shown in fig. 1, for the basic principle of reactive compensation of parallel capacitors in a power distribution network, in fig. 1(a), resistive load r and inductive load X form branch 1, capacitive load C forms branch 2, and the currents through them are respectively
Figure BDA0002290708960000034
And
Figure BDA0002290708960000035
FIG. 1(b) is a vector diagram of current, where the current
Figure BDA0002290708960000036
Is that
Figure BDA0002290708960000037
And
Figure BDA0002290708960000038
phasor sum. Inductive load current due to inductive impedance of branch 1
Figure BDA0002290708960000039
Lags behind voltage
Figure BDA00022907089600000310
Phase difference of phi1. After the parallel capacitor is connected, the current is due to the capacitance
Figure BDA00022907089600000311
Leading voltage
Figure BDA00022907089600000312
90 deg. so that part of the inductive current can be compensated. At the moment, the synthesized current is from
Figure BDA00022907089600000313
Is reduced to
Figure BDA00022907089600000314
The phase difference is reduced from phi1Is reduced to phi2And thus the power factor is improved. This is the principle of reactive compensation by the parallel capacitors.
As shown in FIG. 2, the simple AC branch circuit with S as the sending end and R as the receiving end in the tidal current power direction takes the line resistance R into accountSRReactance xSR(considering the grounding capacitance branch at the two ends of the branch is merged into the nodes at the two ends of the branch), the voltage at the S, R end is taken as US、URThe branch current is ISRThe active power and the reactive power input by the branch sending end S are respectively PSR、QSRThe active power and the reactive power output by the receiving end R are respectively PRS、QRS。
The branch voltage equation of the simple ac branch in fig. 2 is:
Figure BDA0002290708960000041
wherein: u shapeRx、URyAnd ISRx、ISRyRespectively a voltage at a receiving end URAnd branch current ISRActive and reactive components.
The power output by the receiving end is as follows:
Figure BDA0002290708960000042
after eliminating its current component, the above equation set can be expressed as
Figure BDA0002290708960000043
Wherein: branch admittance of
Figure BDA0002290708960000044
Susceptance b in a common transmission lineSRLess than 0, and taking the conductance g ═ g to highlight the mathematical significance of the positive and negative of the reactive powerSRSusceptance b ═ bSRThe standard form of the circle of the system of quadratic equations with voltage components as variables is then:
Figure BDA0002290708960000045
by URx、URyAs variable, with the centre of the circle as OPRadius rPRound U ofPAnd the center of the circle is OQRadius rQRound U ofQThen the center O of the two circlesP、OQThe coordinates of (a) are:
Figure BDA0002290708960000046
radius is respectively
Figure BDA0002290708960000047
The distance between the centers of the two circles is
Figure BDA0002290708960000048
As shown in fig. 3(a) and 3(b), the two power circles intersect and are tangent to each other, and the operating point P corresponding to the tangency of the two power circles is shown1Referred to as the boundary point between the ac circuit operating domain and the breakdown domain, i.e., the threshold voltage breakdown operating point of the circuit of fig. 2.
The reactive margin can thus be defined as: under the condition of stable static voltage, the electrical distance between the system operation point and the critical breakdown voltage point is QRPM＝rP+rQ-D。
According to the definition, the equivalent reactive margin of the node i can be obtainedWherein: n is a radical ofiThe number of branches connected with the node i is shown; j is the node number connected to node i.
The influence of the parallel capacitor on the harmonic impedance of the system is as follows: when no capacitive device is present and the capacitance to ground of the transmission line is not considered, the harmonic impedance of the power distribution system is
Figure BDA0002290708960000052
The impedance of the practical power system is a complex RLC combined circuit, and if the harmonic impedance is calculated by the above formula, the impedance will be greatly different from the practical value. Now to satisfySystem harmonic impedance Z of the above-mentioned inductive elementsnAnd harmonic capacitive reactance XCnEquivalent harmonic impedance Z of parallel systems′nFor analysis, the circuit is as shown in fig. 4:
when the fundamental impedance of the capacitor is XCN-th harmonic capacitive reactance of XCnThen, then
Figure BDA0002290708960000053
When Z iss′n＝Rs′n+jXs′nThen, then
Figure BDA0002290708960000054
Therefore, the frequency characteristics of the harmonic impedance of the system can be changed by the parallel capacitor, so that the equivalent harmonic impedance of the system is capacitive. For a certain harmonic, the parallel capacitor may have a parallel resonance with the system, when the equivalent harmonic impedance reaches a maximum.
In addition to the effect of harmonics, the parallel capacitors connected to the power supply system also cause greater harmonic distortion in the power system when the parallel capacitors are erroneously inserted. Not only does this pose a hazard to the system and other equipment, but the parallel capacitors themselves will also prematurely break down under large higher harmonic over-currents. This is caused by a harmonic amplification effect and a resonance phenomenon at the k-th harmonic frequency when the capacitor is applied to an inductive power system. As shown in fig. 5, the amplification effect of the parallel capacitor on the harmonic current is shown schematically, where (a) is the simplification of the power system and (b) is the distribution of the current in the system.
The split-flow of the harmonic current is calculated as:
Figure BDA0002290708960000055
Figure BDA0002290708960000056
wherein: harmonic current I into the capacitor circuitCnAnd the harmonic current I flowing into the systemsnAre all greater than harmonic current InI.e. discharge of the capacitor to harmonicsA large phenomenon. Greater ICnOverloading the capacitor. The most serious condition is when
Figure BDA0002290708960000057
Time, system equivalent impedance nXsAnd capacitor bank loop impedance
Figure BDA0002290708960000058
Current resonance occurs when a resonance condition circuit is formed, and at this time:
Figure BDA0002290708960000061
wherein: q. q.snThe quality factor of the circuit is the ratio of the electric field energy and the magnetic field energy to the active power, namely the quality factor of the circuit. Even a small higher harmonic current is artificially amplified, so that operation under low harmonic resonance conditions for a long period of time is rather disadvantageous for the capacitor.
From the harmonic resonance condition, the number of harmonic resonance can be obtained as
Figure BDA0002290708960000062
Wherein:
Figure BDA0002290708960000063
wherein: pkShort circuit capacity for the power supply bus; pCThe capacity of the connected parallel capacitor bank; f. of0Is the natural frequency of the circuit; f is the fundamental frequency of the circuit, ω ═ 2 π f.
In case the harmonic source has been determined, to prevent the injection of harmonic currents into the grid, it is necessary to install a parallel filter, usually a high pass filter, at the harmonic source, as shown in fig. 6.
The primary wiring and equivalent circuit of a conventional high pass filter is shown in fig. 7. Since the inductor L and the resistor R are connected in parallel, the inductance L and the resistor R have a lower valueImpedance frequency range. When the frequency is lower than a certain frequency f0In time, the filter impedance is increased significantly due to the increase in capacitive reactance, and low-order harmonic currents are difficult to pass. When the frequency is higher than f0In this case, the capacitance is not large, and the total impedance is not changed much, thereby forming a passband. n is0＝f0/f1Called cut-off harmonic order, f0I.e. the cut-off frequency, and
Figure BDA0002290708960000065
when parameter
Figure BDA0002290708960000066
FIG. 6(b) can be obtained.
When reactive compensation is carried out on a system, a plurality of capacitors with larger capacity can be put into the system, and when all the capacitors used for reactive compensation are put into the system, partial harmonic increase can be caused according to analysis. In order to show the amplification effect of the parallel capacitor on the harmonic wave, a large capacitor with the capacity of 1000uF is put into operation at the outlet of the motor in the model, and a system is set up in PSCAD/EMTDC for simulation verification, wherein the model is shown in FIG. 8: and (3) carrying out voltage harmonic analysis on the outlet of the simulated asynchronous motor, wherein the abscissa is time/s, the ordinate is voltage/kV, and the amplitude of the fundamental wave is far greater than that of the harmonic wave. As can be seen from a comparison of fig. 8, the operation of the parallel large capacitors results in an increase of the harmonics; moreover, some higher harmonic waveforms, such as 11 th harmonic in fig. 8, have large distortion and may cause great damage to the system; other harmonics are smaller and are not labeled. To avoid the above problem, a reactor may be connected in series with the parallel capacitor, as shown in fig. 9: for convenience, firstly, a 1H inductor is supposed to be connected in series with a parallel capacitor branch, frequency analysis is carried out on the voltage at the outlet of the asynchronous motor, wherein the abscissa is time/s, the ordinate is voltage/kV, the amplitude of fundamental waves is far greater than that of harmonic waves, and the fundamental waves are not marked one by one. As can be seen from the figure, after the series inductance is added, the higher harmonic voltage is restored to the previous state, so that the series inductance is needed together with the parallel capacitance to prevent some harmonic voltages from rising too high during reactive compensation.
In addition, when the capacity of the capacitor is increased to 100mF, the voltage harmonic analysis at the outlet of the asynchronous motor is shown in fig. 10 after the simulation.
As shown in fig. 11, when the parallel capacitor is not properly selected, the system resonance may be caused, which may cause a greater hazard.
In summary, the parallel capacitor has an amplifying effect on the harmonic current, and improper selection of the parallel capacitor may cause the system to resonate. During reactive compensation, the parallel capacitor is connected with a corresponding inductor in series, so that the amplification effect of the parallel capacitor on harmonic waves can be effectively reduced.
As shown in fig. 12, there are 21 nodes on the main line, which are labeled in the figure. And (4) simulating in the PSCAD, and obtaining the voltage, active value and reactive value of each node in the main line. As shown in table 1:
table 1 table of voltage, active and reactive values of the compensation front main line node, where U represents the node voltage, P1Q1 represents the ingress node power, and P2Q2 represents the egress node power.
Number of nodes U P1 Q1 P2 Q2
1 9.926 / / / /
2 9.869 18.64 16.71 18.55 16.63
3 9.824 18.54 16.47 17.98 16.01
4 9.769 17.97 15.82 17.88 15.74
5 9.714 17.87 15.55 17.78 15.48
6 9.66 17.77 15.29 16.66 14.37
7 9.61 16.65 14.2 15.74 13.45
8 9.562 15.74 13.3 15.19 12.85
9 9.517 15.18 12.71 15.15 12.68
10 9.472 15.14 12.54 14.97 12.4
11 9.427 14.97 12.27 14.93 12.24
12 9.383 14.93 12.11 14.79 12
13 9.34 14.79 11.86 14.26 11.44
14 9.298 14.26 11.32 13.99 11.1
15 9.258 13.99 10.98 13.91 10.91
16 9.218 13.9 10.8 13.77 10.69
17 9.178 13.77 10.58 13.68 10.51
18 9.139 13.68 10.4 13.52 10.27
19 9.101 13.52 10.16 13.36 10.03
20 9.063 13.35 9.918 13.2 9.789
21 9.027 / / / /
And then, according to a reactive margin algorithm, the data in the table 1 are substituted into the reactive margin calculation of each node, and because the main line has 21 points, the first and the last nodes do not meet the reactive margin calculation requirement, and only the other 19 points are calculated.
ans＝
Columns 1 through 10
9.6648 9.6072 9.5611 9.5078 9.4502 9.4014 9.3571 9.3147 9.2698 9.2270
Columns 11 through 19
9.1833 9.1388 9.0998 9.0608 9.0213 8.9829 8.9443 8.9066 8.8704
And determining the compensation position according to the arrangement of the reactive margin from small to large. As can be seen from the above data, the reactive margin at the 19 th node is the smallest, and the compensation position is the end of the line. The compensation capacitance is calculated as follows:
Figure BDA0002290708960000081
in practice, the compensated reactive power is partially lost due to loss of the line and the like. After testing, the actual compensation value was 550 μ F. The compensation results are shown in Table 2. It can be seen that the voltage of the whole line is obviously improved, and the worst voltage is over 9.88.
TABLE 2 Voltage, active and reactive value table of compensation rear main line node
Number of nodes U P1 Q1 P2 Q2
1 9.919 / / / /
2 9.911 22.11 1.493 22.01 1.416
3 9.904 22.01 1.292 21.43 0.8222
4 9.898 21.43 0.6721 21.33 0.5953
5 9.892 21.33 0.4466 21.23 0.3699
6 9.888 21.23 0.2225 20.06 -0.7449
7 9.887 20.06 -0.876 19.09 -1.674
8 9.889 19.09 -1.79 18.51 -2.278
9 9.894 18.5 -2.392 18.46 -2.423
10 9.899 18.46 -2.536 18.27 -2.691
11 9.905 18.27 -2.802 18.23 -2.833
12 9.911 18.23 -2.944 18.08 -3.067
13 9.918 18.07 -3.177 17.48 -3.658
14 9.927 17.47 -3.761 17.17 -4.009
15 9.937 17.17 -4.11 17.07 -4.186
16 9.948 17.07 -4.286 16.92 -4.411
17 9.96 16.91 -4.509 16.82 -4.587
18 9.972 16.81 -4.685 16.63 -4.841
19 9.985 16.62 -4.937 16.43 -5.094
20 9.998 16.43 -5.189 16.24 -5.346
21 10.01 / / / /
If the system voltage is still not satisfactory, the selection of the second compensation location in the line is further calculated. Similarly, calculate its reactive compensation margin, have
ans＝
Columns 1 through 10
9.7000 9.6903 9.6867 9.6818 9.6720 9.6722 9.6764 9.6830 9.6875 9.6938
Columns 11 through 19
9.6995 9.7042 9.7142 9.7252 9.7357 9.7473 9.7589 9.7711 9.7834
Therefore, the compensation is continued at the node 5, and a better full-line voltage effect can be achieved.
At this time, the system can be regarded as a double reactive power system, and the compensation capacitor at the node 5 is obtained by calculationHowever, to prevent overvoltage in the end line, the end compensation capacitance is subtracted in proportion to the capacitance inversely proportional to X to obtain an end compensation capacitance value of 550-222.9/19 × 5-491.3 μ F.
After the secondary compensation, the voltage of each node of the main line is as follows:
number of nodes U
1 9.94
2 9.947
3 9.953
4 9.964
5 9.975
6 9.987
7 9.98
8 9.977
9 9.976
10 9.975
11 9.975
12 9.975
13 9.976
14 9.979
15 9.984
16 9.989
17 9.994
18 10
19 10.01
20 10.02
21 10.02
Therefore, the main line voltage reaches over 9.94kv, and the better reactive compensation requirement is met. The main line voltage distribution before and after compensation is as shown in fig. 13. In addition, in the implementation process, after the main line 001 is subjected to centralized compensation, the voltages of the 002 branch circuits and the 003 branch circuits are also well improved to reach over 9.80kv, and the requirements are approximately met.
Compared with the prior art, the method not only solves the selection of the reactive compensation point of the low-voltage power grid and the determination of the compensation capacity, but also solves the harmonic influence on the power grid caused by the installation of the reactive compensator.
The foregoing embodiments may be modified in many different ways by those skilled in the art without departing from the spirit and scope of the invention, which is defined by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims (6)

1. A reactive compensation method for a capacitor of a power distribution network is characterized in that firstly, an optimal compensation point is determined by adopting a reactive margin mode, the voltage of the compensation point is monitored on line, and the voltage is used as a constraint condition to control a switched capacitor so as to dynamically adjust the compensation capacity; and then, capacitor compensation is carried out on voltage drop caused by impact load, harmonic wave influence caused by the delay of the parallel capacitors when the capacitors are connected into a power distribution network is analyzed, and reactors are connected in series on the capacitors to inhibit the amplification effect of the parallel capacitors on the harmonic wave, so that the influence of load impact on a power grid is reduced.
2. The method as claimed in claim 1, wherein the reactive margin manner is: under the condition of stable static voltage, the electrical distance between a system operation point and a critical breakdown voltage point is large, when the reactive margin value of a node is large, the reactive compensation capacity required by the node is small, otherwise, the required reactive compensation capacity is large, and the point, which is most required by the system to perform reactive compensation, can be found out through the size of the reactive margin value.
3. The method of claim 1, wherein the dynamically adjusting the compensation capacity is: when the voltage is too low, the compensation point is added into the capacitor for compensation; when the voltage is too high, the compensation point needs to exit the capacitor, specifically:
1) simulating in a PSCAD to obtain the voltage, active value and reactive value of each node in the main line;
2) calculating the reactive margin of each node of the main line except two ends, arranging the reactive margins from small to large, taking the node with the minimum reactive margin as a compensation position, and installing a compensation capacitor to obtain the power supply voltage U1Bus voltage U of substation is considered unchanged2Go to U'2Then compensating capacitor
Figure FDA0002290708950000011
I.e. the compensation capacitance C is inversely proportional to X.
4. A method as claimed in claim 3, wherein when the compensated system voltage does not meet the requirements, the step 2) is further repeated to recalculate the compensation position in the line and the corresponding compensation capacitance, and to prevent overvoltage in the end line, the end compensation capacitance is subtracted according to a capacitance value proportional to X in inverse proportion to obtain the end compensation capacitance value.
5. The method of claim 1, wherein said harmonic effects are: the method comprises the steps of compensating a capacitor for voltage drop caused by impact load, analyzing harmonic influence caused by delay of the parallel capacitors when the capacitors are connected into a power distribution network, and installing a high-pass filter on the capacitors to inhibit the amplification effect of the parallel capacitors on the harmonic.
6. The method according to claim 1 or 5, characterized in that said harmonic influence is in particular: when the fundamental impedance of the capacitor is XCN-th harmonic capacitive reactance of XCnThen, then
Figure FDA0002290708950000012
Z'sn＝R′sn+jX′snThen, then
Figure FDA0002290708950000021
The split-flow of the harmonic current is calculated as:
Figure FDA0002290708950000022
Figure FDA0002290708950000023
wherein: harmonic current I into the capacitor circuitCnAnd the harmonic current I flowing into the systemsnAre all greater than harmonic current InI.e. amplification of harmonics by capacitors, larger ICnOverload the capacitor, the most serious condition being
Figure FDA0002290708950000024
Time, system equivalent impedance nXsAnd capacitor bank loop impedance
Figure FDA0002290708950000025
Current resonance occurs when a resonance condition circuit is formed, and at this time:
Figure FDA0002290708950000026
Figure FDA0002290708950000027
wherein: q. q.snThe quality factor of the circuit is the ratio of the electric field energy and the magnetic field energy to the active powerI.e. the quality factor of the circuit.
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