CN108120878B - Direct current resistance selection method and system of complex power distribution network comprehensive optimization device - Google Patents
Direct current resistance selection method and system of complex power distribution network comprehensive optimization device Download PDFInfo
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Abstract
The invention discloses a direct current resistance selection method and a direct current resistance selection system of a complex power distribution network comprehensive optimization device. The direct current resistance selection method comprises the following steps: 1) constructing a positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and an equivalent circuit under sampling frequency of the complex power distribution network comprehensive optimization device; 2) constructing positive sequence, negative sequence and zero sequence equivalent circuits of the power distribution system at the fault point under the fundamental frequency and the sampling frequency; 3) calculating to obtain a relational expression between fault current and direct current resistance; 4) determining a lower limit value of the direct current resistance; 5) calculating to obtain a relational expression between the current of the sampling frequency of the fault point and the direct current resistance; 6) determining an upper limit value of the direct current resistance; 7) and obtaining the direct current resistance according to the lower limit value, the upper limit value and the relationship between the lower limit value and the upper limit value. The selected direct current resistance value can give consideration to effective arc extinction and accurate line selection of a power distribution system, the control performance of the comprehensive optimization device can be effectively improved, and the substitution effect of the comprehensive optimization device on an arc extinction coil is realized.
Description
Technical Field
The invention belongs to the technical field of flexible power distribution networks, and particularly relates to a direct current resistance selection method and a direct current resistance selection system of a complex power distribution network comprehensive optimization device.
Background
In recent years, many new electrical devices based on power electronic technology appear in power supply and distribution systems, and these nonlinear loads have nonlinear and impulsive characteristics, which not only cause the distortion of bus voltage and current waveforms in the system, but also cause the phenomena of flicker, fluctuation and three-phase imbalance of the bus voltage of the power grid, cause serious pollution to the power supply quality, and affect the economic and safe operation of the electrical devices in the power grid. The voltage source type converter has excellent control operation performance, and is widely used as an active filter, a reactive compensator and the like at present to optimize the quality of electric energy and alleviate the problems.
With the enlargement of the distribution network scale and the increase of cable lines, the capacitance current of the distribution network seriously exceeds the standard, and the requirement of the operation of the power grid can not be met by a neutral point ungrounded mode. At present, most of 10kV power distribution networks in China adopt an arc suppression coil grounding mode to suppress fault current. However, due to the compensation effect of the arc suppression coil, the characteristics of zero-sequence currents of a fault line and a non-fault line in a steady state may be the same, which greatly increases the difficulty of fault line selection, and leads to inaccurate line selection and false alarm phenomena. Meanwhile, the traditional arc suppression coil grounding mode cannot automatically and continuously follow the parameter change of a power grid to carry out optimal adjustment, and cannot enable the residual current of a fault point to be in a minimum state at any time.
The voltage source type converter under the direct current side grounding mode has zero sequence voltage and current regulation capacity, fully utilizes equipment such as an active filter and the like, adopts time-sharing regulation means (for example, when the converter is in normal operation, the converter is used for filtering and optimizing the electric energy quality, and when the converter is in fault, control means such as zero sequence current and the like are used for arc suppression and line selection), can enable a power distribution network to remove the use of an arc suppression coil, and only one converter is used for realizing the functions of the active filter and the arc suppression coil. The voltage source type converter with multiple functions can be called a complex power distribution network comprehensive optimization device. The method can reduce the equipment investment cost, can also compensate the fault current by utilizing the quick adjustment capacity of the zero-sequence current of the converter to maintain the minimum state of the fault current, and can effectively improve the line selection accuracy by injecting a voltage current signal with certain frequency by utilizing a means similar to an S injection method.
The resistance value of the direct current resistor influences the arc extinction effect of the converter and the line selection accuracy effect under the condition of certain frequency signal injection. In order to effectively suppress the instantaneous ground current and overvoltage of a single-phase ground fault and simultaneously consider the line selection accuracy of a system under the condition of injecting a certain frequency signal, parameter design needs to be carried out on the resistance value of a direct current resistor.
Disclosure of Invention
Aiming at the problems of the direct current resistance of the converter, the invention provides the direct current resistance selection method of the complex power distribution network comprehensive optimization device, which can realize effective arc extinction, inhibit the single-phase grounding short-circuit current within a specified range, meet the high-accuracy line selection requirement of the system and provide conditions for the good control performance of the comprehensive optimization device.
Therefore, the invention adopts the following technical scheme: a direct current resistance selection method of a complex power distribution network comprehensive optimization device is characterized in that the complex power distribution network comprehensive optimization device is a transformer-free hybrid MMC, the alternating current side of the hybrid MMC is directly connected with a substation bus through an alternating current breaker, and a connection transformer is not arranged in the middle of the hybrid MMC; positive pole of DC side and capacitor C1Connected with the negative electrode of the DC side and a capacitor C2Connected to a capacitor C1Another terminal of (1) and a capacitor C2Is connected to the other end of the resistor and is simultaneously connected to a direct current resistor R0Connected, DC resistance R0The other end of the first and second electrodes is directly grounded;
the direct current resistance selection method comprises the following steps:
1) positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and sampling frequency f for constructing complex power distribution network comprehensive optimization devicesThe equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series;
2) selecting n (n is an integer) typical single-phase earth fault points, and constructing the fundamental frequency and the sampling frequency f of the power distribution system at the fault pointssA lower positive sequence, negative sequence and zero sequence equivalent circuit;
3) setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between;
4) based on maximum fault current not exceeding maximum short-circuit current If0Determining a lower limit value R of the direct current resistancemin；
5) According to sampling frequency f at fault pointsCalculating a positive sequence, a negative sequence and a zero sequence equivalent circuit to obtain a fault point sampling frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between;
6) based on fault grounding resistance being smaller than set resistance Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax；
7) R obtained according to step 4)minR obtained in step 6)maxAnd the relation between the two to obtain the direct current resistance R0。
As a complement to the above-mentioned method of selecting the dc resistance, in step 1),
the positive sequence fundamental frequency equivalent voltage source is e+Positive sequence fundamental frequency equivalent impedance of Z+(ii) a The equivalent voltage source of the negative sequence fundamental frequency is 0, and the equivalent impedance of the negative sequence fundamental frequency is Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is Z0(ii) a Wherein the content of the first and second substances,
ω0＝2πf0，f050 Hz; j represents the imaginary unit; r and L are equivalent resistance and inductance of a bridge arm respectively; r0Is a direct current resistance; cdIs a capacitor C1And C2The capacitance value of (a); e.g. of the type+The positive-sequence fundamental frequency voltage output by the MMC is obtained by the MMC according to the control of a power distribution system;
positive sequence fsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
Wherein the content of the first and second substances,
ωs＝2πfs，fs＝220Hz；
frequency of output for MMCsThe zero sequence voltage of (2) is obtained by controlling the MMC according to the power distribution system.
As a complement to the above-mentioned dc resistance selection method, in step 2),
the positive sequence fundamental frequency equivalent voltage source at the fault point is k11Es+k12e+Positive sequence fundamental frequency equivalent impedance is k13+k14Z+(ii) a The negative sequence fundamental frequency equivalent voltage source is 0, and the positive sequence fundamental frequency equivalent impedance is k13+k14Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is k15+k16Z0(ii) a Wherein the content of the first and second substances,es is the power supply voltage of the power distribution network, k11、k12、k13、k14、k15、k16The fault location is determined by the structure and parameters of the power distribution network and is constant for the specified fault location;
positive sequence f at fault pointsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
ks1、ks2、ks3、ks4、ks5It is a constant for a given fault location, determined by the distribution network structure and parameters.
As a supplement to the above-mentioned method of selecting the dc resistance, in step 3), the fault current IfAnd a direct current resistance R0The relationship between them is as follows:
wherein the content of the first and second substances,
k17and are constants related to the power distribution network structure and parameters.
As a complement to the above-mentioned dc resistance selection method, in step 4),
for the ith fault point, I is more than or equal to 1 and less than or equal to n, and the maximum short-circuit current I isf0Under the limitation of (3), obtaining a direct current resistance R0Lower limit value of
Wherein If0Taking out the mixture of 30A and 30B,
selecting
The medium maximum value is the direct current resistance R0Lower limit value R ofmin，
As a supplement to the above-mentioned method of selecting the direct current resistance, in step 5), the current IfsAnd a direct current resistance R0The relationship between them is as follows:
wherein the content of the first and second substances,
ks6and are constants related to the power distribution network structure and parameters.
As a complement to the above-mentioned dc resistance selection method, in step 6),
ith fault point, R0Upper limit value of
Comprises the following steps:
wherein the content of the first and second substances,
set to less than 10% of the rated voltage of the fundamental frequency; rf0The selection relates to the effective grounding problem of the direct current side of the device, and 100 omega, I is takenfs0Taking 1A; selecting
The minimum value is the DC resistance R0Upper limit value R ofmax，
As a complement to the above-mentioned dc resistance selection method, in step 7),
if R ismax≥RminThen the direct current resistance R0In { Rmin，RmaxSelecting within the range; the selection bias is R based on the further requirements of fault current control and line selection accuracymaxOr Rmin；
If R ismax<RminThen in step 6) by decreasing Rf0Calculation of RmaxParameter until R is satisfiedmax≥RminUnder the condition of (1) R0Is set as RminOr RmaxAnd recording the corresponding Rf0. In this case, the following meanings are included: in the R0If the grounding resistance is less than the corresponding Rf0Otherwise, other existing line selection methods are required to participate in line selection calculation together so as to ensure the line selection accuracy.
As a supplement to the direct current resistance selection method, each bridge arm of the mixed MMC is formed by cascading a plurality of submodules, the bridge arm reactors are connected in series, the types of the submodules in the bridge arms are HBSM and FBSM, the number ratio of the submodules in the two types is 1:1, and a driving circuit board of each submodule is powered by capacitance voltage of the corresponding submodule.
The invention also provides a direct current resistance selection system of the complex power distribution network comprehensive optimization device, wherein the complex power distribution network comprehensive optimization device is a transformer-free hybrid MMC; the direct current resistance selection system comprises:
equivalent circuit first construction unit: constructing complex power distribution network comprehensive optimizationPositive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and sampling frequency f of chemical conversion devicesThe equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series;
the equivalent circuit second construction unit: selecting n typical single-phase earth fault points, and constructing the fundamental frequency and the sampling frequency f of the power distribution system at the fault pointssA lower positive sequence, negative sequence and zero sequence equivalent circuit;
a first relational expression calculation unit: setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between;
a direct current resistance lower limit value determination unit: based on maximum fault current not exceeding maximum short-circuit current If0Determining a lower limit value R of the direct current resistancemin；
A second relational expression calculation unit: according to sampling frequency f at fault pointsCalculating a positive sequence, a negative sequence and a zero sequence equivalent circuit to obtain a fault point sampling frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between;
a direct current resistance upper limit value determination unit: based on fault grounding resistance being smaller than set resistance Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax；
A direct current resistance obtaining unit: according to Rmin、RmaxAnd the relation between the two to obtain the direct current resistance R0。
The invention has the following beneficial technical effects:
(1) the direct-current resistance value selected by the method can give consideration to effective arc extinction and accurate line selection of a power distribution system, can effectively improve the control performance of the comprehensive optimization device, and realizes the substitution effect of the comprehensive optimization device on the arc extinction coil.
(2) The invention has strong universality and is suitable for power distribution networks with different network topology structures.
Drawings
FIG. 1 is a schematic diagram of a power distribution system including an integrated optimization device in accordance with an embodiment of the present invention;
FIG. 2 is a flow chart of a method for selecting a ground resistor according to an embodiment of the present invention;
fig. 3 is an equivalent model diagram of a hybrid MMC in an embodiment of the invention.
Detailed Description
To describe the present invention more specifically, the following detailed description of the technical solution and the related principles of the present invention is made with reference to the drawings and the detailed description of the present invention.
Example 1
The embodiment provides a direct current resistance selection method of a complex power distribution network comprehensive optimization device.
As shown in fig. 1, the integrated optimization device adopts a transformer-free hybrid MMC (MMC belongs to one of voltage source converters), the alternating current side of the hybrid MMC is directly connected with a bus of a transformer substation through an alternating current breaker Q, and a connection transformer is not arranged in the middle; positive pole of DC side and capacitor C1Connected with the negative electrode of the DC side and a capacitor C2Connected to a capacitor C1Another terminal of (1) and a capacitor C2Is connected to the other end of the resistor and is simultaneously connected to a direct current resistor R0Connected, DC resistance R0And the other end of the same is directly grounded.
The hybrid MMC adopts a three-phase six-bridge arm structure, each bridge arm is formed by cascading n submodules and is connected with a bridge arm reactor in series, wherein the number n of the submodules is determined by direct current voltage, alternating current voltage, submodule voltage and bridge arm redundancy. Each bridge arm comprises a half-bridge sub-module (HBSM) and a full-bridge sub-module (FBSM), the number ratio of the two sub-modules is 1:1, and a driving circuit board of each sub-module is powered by capacitor voltage of the sub-module. The HBSM comprises two IGBTs (T)1And T2) Two antiparallel diodes (D)1And D2) And a capacitor C; the FBSM includes four IGBTs (T)1-T4) Four anti-parallel diodes (D)1-D4) And a capacitor C.
In the invention, the selection of the direct current resistor needs to simultaneously consider the ground current suppression and f at the moment of single-phase ground faults(fsTypically chosen to be a value intermediate N times and N +1 times the fundamental frequency, such as 220Hz, where N is an integer) frequency signal injection to the line selection accuracy, i.e., arc suppression and option capability, of the lower system. Therefore, the resistance of the direct current resistor needs to be restricted from both arc extinction and line selection, so as to obtain a proper direct current resistor. Fig. 2 shows the specific steps of the ground resistance selection method, which are respectively:
the first step is as follows: positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and frequency f for constructing comprehensive optimization device of power distribution networksAnd the equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series.
The second step is that: selecting a plurality of typical single-phase earth fault points, and constructing the fundamental frequency and the frequency f of the system at the fault pointssAnd the lower positive sequence, negative sequence and zero sequence equivalent circuits.
The third step: setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between them.
The fourth step: based on maximum fault current not exceeding limit value If0Determining a lower limit value R of the direct current resistancemin。
The fifth step: according to frequency f at fault pointsCalculating the positive sequence, negative sequence and zero sequence equivalent circuit to obtain a fault point frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between them.
And a sixth step: based on fault grounding resistance being smaller than set resistance Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax。
The seventh step: r obtained according to the fourth stepminR obtained in the sixth stepmaxAnd the relation between the two to obtain the direct current resistance R0。
In the first step, fig. 3 shows an equivalent model of the hybrid MMC, and the following equations can be listed
Udp-Udn＝Udc(3)
isj＝ijn-ijp(4)
Wherein u isjAnd isj(j ═ a, b, c) represents the alternating voltage and alternating current on the alternating current outlet side of the MMC, ijpAnd ijnIs j-phase upper and lower bridge arm current, ujpAnd ujnIs the output voltage of the sub-module group connected with the upper bridge arm and the lower bridge arm in series of j phase, UdpAnd UdnIs a positive DC voltage and a negative DC voltage, UdcIs a DC interpolar voltage uoThe voltage of a neutral point on the direct current side is R and L are equivalent resistance and inductance of a bridge arm respectively.
By substituting the equations (1) to (4), the following expression can be obtained:
wherein the content of the first and second substances,
substituting j ═ a, b, c into the above formula, the abc triphase equation can be obtained, and the equation can be used
The mathematical models that can be obtained for positive, negative and zero sequences are:
wherein the superscripts "+", "-" and "0" represent positive, negative and zero sequence components, respectively, as can be seen from the structure of figure 3,
uo＝(idcp+idcn)R0(10)
wherein, CdIs a capacitor C1And C2The capacitance value of (2). According to the kirchhoff's current theorem, the total current flowing in is equal to the total current flowing out, there
The positive sequence, negative sequence and zero sequence fundamental frequency equivalent models can be obtained by substituting equations (10) - (13) into equations (7) - (9) and adopting Fourier transform.
Wherein the content of the first and second substances,
ω0＝2πf0，f0the equivalent impedance of the positive and negative sequences is the same at 50 Hz.
Under the condition of normal operation, the comprehensive optimization device of the power distribution network only outputs a positive sequence voltage component, and the negative sequence and the zero sequence control are put into use after the system fails. Then it can be found that: the positive sequence fundamental frequency equivalent voltage source is e+Positive sequence fundamental frequency equivalent impedance of Z+(ii) a The equivalent voltage source of the negative sequence fundamental frequency is 0, and the equivalent impedance of the negative sequence fundamental frequency is Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is Z0。
In the same way, the injection frequency of the comprehensive optimization device to the power distribution network is fsThe frequency f can be obtained similarly when the zero sequence current signal is obtainedsThe following equivalent model.
Wherein the content of the first and second substances,
ωs＝2πfs，fs220 Hz. Also, the positive and negative sequence have the same equivalent impedance.
Due to fsThe current signal is zero sequence component, and there is no positive sequence power supply and negative sequence power supply under this frequency, so can obtain: positive sequence fsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
In the second step, typically n fault points are selected, where n is an integer. Because the distribution network is a radial structure, typical positions such as the head end, the tail end and the contact point of the feeder line are generally selected. Then, for each fault point, constructing a system fundamental frequency equivalent circuit and f at the fault pointsAnd a frequency equivalent circuit.
In the construction process, the equivalent model of the comprehensive optimization device obtained in the first step is analogized to electric power elements such as transformers and distribution lines, the sequence component decomposition calculation is carried out on the whole power distribution system except the fault point by using a symmetric component method according to the existing traditional electric power analysis means, and finally, the positive sequence fundamental frequency equivalent voltage source k at the fault point can be obtained11Es+k12e+Positive sequence fundamental frequency equivalent impedance is k13+k14Z+(ii) a The negative sequence fundamental frequency equivalent voltage source is 0, and the positive sequence fundamental frequency equivalent impedance is k13+k14Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is k15+k16Z0. Wherein Es is the power supply voltage of the power distribution network, k11、k12、k13、k14、k15、k16Depending on the distribution network configuration and parameters, it may be considered constant for a given fault location.
Similarly, the frequency f can be obtainedsNext, the equivalent model of each sequence component at the fault point, wherein the positive sequence f at the fault pointsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
ks1、ks2、ks3、ks4、ks5Depending on the distribution network configuration and parameters, it may be considered constant for a given fault location.
In the third step, the single-phase earth fault resistor RfMay correspond to different resistance values such as 0 Ω,1 Ω, 10 Ω, 100 Ω, etc. The method comprises the following steps of (1) utilizing a composite sequence network equation of the single-phase grounding short circuit provided in the existing asymmetric short circuit calculation method:
the fault current I can be calculated by combining the fundamental frequency equivalent model provided in the second stepfAnd a direct current resistance R0The relation between them.
Wherein the content of the first and second substances,
it can be seen that k17And are constants related to the power distribution network structure and parameters.
In the fourth step, the slave formula (22) mayIt is seen that except e+，RfAnd R0The three variables and the rest parameters are determined by the grid structure and the parameters of the power distribution system and are constants. Under the condition of normal operation, the comprehensive optimization device only outputs the positive sequence voltage e+And e is a+Is close to the ac rated voltage of the distribution network, so that at the instant of a ground fault, e+Can be considered approximately as a constant. From IfIt can be seen that in determining R0Under the premise of (1), when the fault is detected, the ground resistance RfWhen equal to 0, IfA maximum value will be obtained. Fault current IfConstrained by factors such as system overcurrent and the like, the maximum short-circuit current is set to be If0。
Thus, for the ith (1 ≦ I ≦ n) fault point, at maximum short circuit current If0Under the limitation of (3), R can be obtained0Lower limit value of
Selecting
The highest value being the direct resistance R0Lower limit value R ofmin。
In the fifth step, similar to the calculation process in the third step, the complex sequence network equation according to the formulas (20) and (21) is combined with the f provided in the step 2sEquivalent model under frequency, can be calculated to obtain fsFrequency of flow through fault point at frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between them.
Wherein the content of the first and second substances,
it can be seen that ks6And are constants related to the power distribution network structure and parameters.
In the sixth step, as can be seen from formula (25), except
RfAnd R0The three variables and the rest parameters are determined by the grid structure and the parameters of the power distribution system and are constants. After a fault, the comprehensive optimization device has the injection frequency fsThe voltage and current signals of (2) cause most of the signals to flow through fault points, thereby being matched with the traditional line selection method to improve the line selection accuracy. The injected voltage signal is used for ensuring that the power distribution system does not generate resonance, overvoltage and the like
It should not be too large, and the amplitude is generally set to be less than 10% of the rated voltage of the fundamental frequency.
To enable a faulty line to be monitored effectivelysFrequency signal of alternating current, IfsShould be greater than the minimum current I that can be considered as a valid signalfs0. Determination of good IfsMinimum sum of
Can obtain (3R)f+3ks5R0) Is measured. I isfsIs easily influenced by the shunt resistance of the feeder line to the ground, and if the parameter of the ground capacitance of a certain cable feeder line is 0.27 mu F/km and the length of the cable is 10km, the frequency F issThe lower capacitive reactance to ground is 268 Ω. To ensure the accuracy of line selection, a fault ground resistor RfMust not be larger than 268 omega, otherwise, the line selection error is caused. Thus, RfShould be less than a certain resistance Rf0Such as 100 omega. When the grounding resistance exceeds Rf0In time, the invention should cooperate with other existing line selection methods to select lines, so as to ensure the accuracy of line selection.After R is determinedfThen, the ith fault point, R, can be obtained0Upper limit value of
Selecting
The smallest value being the direct resistance R0Upper limit value R ofmax。
In the seventh step, R calculated in the fourth stepminAnd R calculated in the sixth stepmaxR may exist due to different parameters related to the twomin>RmaxThe case (1). If R ismax≥RminThen the direct current resistance R0May be in { Rmin，RmaxSelecting within the range; depending on further requirements of fault current control and line selection accuracy, a bias towards R may be selectedmaxOr Rmin。
If R ismax<RminThen in the sixth step by decreasing Rf0Calculation of RmaxParameter until R is satisfiedmax≥RminUnder the condition of (1) R0Is set as Rmin(Rmax) And record the corresponding Rf0. In this case, the meaning of: in the R0If the grounding resistance is less than the corresponding Rf0Otherwise, other existing line selection methods are required to participate in line selection calculation together so as to ensure the line selection accuracy.
Example 2
The embodiment provides a direct current resistance selection system of a complex power distribution network comprehensive optimization device. The comprehensive optimization device for the complex power distribution network is transformer-freeThe alternating current side of the hybrid MMC is directly connected with a bus of a transformer substation through an alternating current breaker, and a connecting transformer is not arranged in the middle of the hybrid MMC; positive pole of DC side and capacitor C1Connected with the negative electrode of the DC side and a capacitor C2Connected to a capacitor C1Another terminal of (1) and a capacitor C2Is connected to the other end of the resistor and is simultaneously connected to a direct current resistor R0Connected, DC resistance R0And the other end of the same is directly grounded. Each bridge arm of the hybrid MMC is formed by cascading a plurality of submodules, the bridge arm reactors are connected in series, the number ratio of the submodules in the bridge arms is 1:1, and the driving circuit board of each submodule is powered by the capacitor voltage of the corresponding submodule.
The direct current resistance selection system comprises:
equivalent circuit first construction unit: positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and sampling frequency f for constructing complex power distribution network comprehensive optimization devicesThe equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series;
the equivalent circuit second construction unit: selecting n typical single-phase earth fault points, and constructing the fundamental frequency and the sampling frequency f of the power distribution system at the fault pointssA lower positive sequence, negative sequence and zero sequence equivalent circuit;
a first relational expression calculation unit: setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between;
a direct current resistance lower limit value determination unit: based on maximum fault current not exceeding maximum short-circuit current If0Determining a lower limit value R of the direct current resistancemin；
A second relational expression calculation unit: according to sampling frequency f at fault pointsCalculating a positive sequence, a negative sequence and a zero sequence equivalent circuit to obtain a fault point sampling frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between;
a direct current resistance upper limit value determination unit: base ofWhen the fault grounding resistance is smaller than the set resistance value Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax；
A direct current resistance obtaining unit: according to Rmin、RmaxAnd the relation between the two to obtain the direct current resistance R0。
It should be noted that, for those skilled in the art, it is possible to make various improvements and modifications to the present invention without departing from the principle of the present invention, and those improvements and modifications also fall within the scope of the claims of the present invention.
Claims (9)
1. A direct current resistance selection method of a complex power distribution network comprehensive optimization device is characterized in that the complex power distribution network comprehensive optimization device is a transformer-free hybrid MMC, the alternating current side of the hybrid MMC is directly connected with a substation bus through an alternating current breaker, and a connection transformer is not arranged in the middle of the hybrid MMC; positive pole of DC side and capacitor C1Connected with the negative electrode of the DC side and a capacitor C2Connected to a capacitor C1Another terminal of (1) and a capacitor C2Is connected to the other end of the resistor and is simultaneously connected to a direct current resistor R0Connected, DC resistance R0The other end of the first and second electrodes is directly grounded; it is characterized in that the preparation method is characterized in that,
the direct current resistance selection method comprises the following steps:
1) positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and sampling frequency f for constructing complex power distribution network comprehensive optimization devicesThe equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series;
2) selecting n typical single-phase earth fault points, and constructing the fundamental frequency and the sampling frequency f of the power distribution system at the fault pointssA lower positive sequence, negative sequence and zero sequence equivalent circuit;
3) setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between；
4) Based on maximum fault current not exceeding maximum short-circuit current If0Determining a lower limit value R of the direct current resistancemin；
5) According to sampling frequency f at fault pointsCalculating a positive sequence, a negative sequence and a zero sequence equivalent circuit to obtain a fault point sampling frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between;
6) based on fault grounding resistance being smaller than set resistance Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax；
7) R obtained according to step 4)minR obtained in step 6)maxAnd the relation between the two to obtain the direct current resistance R0(ii) a If R ismax≥RminThen the direct current resistance R0In { Rmin，RmaxSelecting within the range; the selection bias is R based on the further requirements of fault current control and line selection accuracymaxOr Rmin；
If R ismax<RminThen in step 6) by decreasing Rf0Calculation of RmaxParameter until R is satisfiedmax≥RminUnder the condition of (1) R0Is set as RminOr RmaxAnd recording the corresponding Rf0。
2. The method for selecting direct current resistance according to claim 1, wherein, in the step 1),
the positive sequence fundamental frequency equivalent voltage source is e+Positive sequence fundamental frequency equivalent impedance of Z+(ii) a The equivalent voltage source of the negative sequence fundamental frequency is 0, and the equivalent impedance of the negative sequence fundamental frequency is Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is Z0(ii) a Wherein the content of the first and second substances,
ω0＝2πf0，f0＝50Hz；j represents the imaginary unit; r and L are equivalent resistance and inductance of a bridge arm respectively; r0Is a direct current resistance; cdIs a capacitor C1And C2The capacitance value of (a); e.g. of the type+The positive-sequence fundamental frequency voltage output by the MMC is obtained by the MMC according to the control of a power distribution system;
positive sequence fsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
Wherein the content of the first and second substances,
ωs＝2πfs，fs＝220Hz；
frequency of output for MMCsThe zero sequence voltage of (2) is obtained by controlling the MMC according to the power distribution system.
3. The method for selecting direct current resistance according to claim 2, wherein, in the step 2),
the positive sequence fundamental frequency equivalent voltage source at the fault point is k11Es+k12e+Positive sequence fundamental frequency equivalent impedance is k13+k14Z+(ii) a The negative sequence fundamental frequency equivalent voltage source is 0, and the positive sequence fundamental frequency equivalent impedance is k13+k14Z-(ii) a The zero-sequence fundamental frequency equivalent voltage source is 0, and the zero-sequence fundamental frequency equivalent impedance is k15+k16Z0(ii) a Wherein Es is the power supply voltage of the power distribution network, k11、k12、k13、k14、k15、k16The fault location is determined by the structure and parameters of the power distribution network and is constant for the specified fault location;
positive sequence f at fault pointsFrequency equivalent voltage source of 0, positive sequence fsFrequency equivalent impedance of
Negative sequence fsFrequency equivalent voltage source of 0, negative sequence fsFrequency equivalent impedance of
Zero sequence fsA frequency equivalent voltage source of
Zero sequence fsFrequency equivalent impedance of
ks1、ks2、ks3、ks4、ks5It is a constant for a given fault location, determined by the distribution network structure and parameters.
4. The method of claim 3, wherein in step 3), the fault current I is selectedfAnd a direct current resistance R0The relationship between them is as follows:
wherein the content of the first and second substances,
k17also related to distribution network structure and parametersA constant value.
5. The method for selecting DC resistance according to claim 4, wherein, in the step 4),
for the ith fault point, I is more than or equal to 1 and less than or equal to n, and the maximum short-circuit current I isf0Under the limitation of (3), obtaining a direct current resistance R0Lower limit value of
Wherein If0Taking out the mixture of 30A and 30B,
selecting
The medium maximum value is the direct current resistance R0Lower limit value R ofmin，
6. The method for selecting DC resistance according to claim 5, wherein in step 5), the current IfsAnd a direct current resistance R0The relationship between them is as follows:
wherein the content of the first and second substances,
ks6and are constants related to the power distribution network structure and parameters.
7. The method for selecting direct current resistance according to claim 6, wherein, in step 6),
the (i) th point of failure,R0upper limit value of
Comprises the following steps:
wherein the content of the first and second substances,
set to less than 10% of the rated voltage of the fundamental frequency; rf0Take 100 Ω, Ifs0Taking 1A; selecting
The minimum value is the DC resistance R0Upper limit value R ofmax，
8. The method according to claim 1, wherein the DC resistance is selected from the group consisting of,
each bridge arm of the hybrid MMC is formed by cascading a plurality of submodules, the bridge arm reactors are connected in series, the number ratio of the submodules in the bridge arms is 1:1, and the driving circuit board of each submodule is powered by the capacitor voltage of the corresponding submodule.
9. A direct current resistance selection system of a complex power distribution network comprehensive optimization device is disclosed, wherein the complex power distribution network comprehensive optimization device is a mixed MMC without a transformer; the direct current resistance selection system is characterized by comprising:
equivalent circuit first construction unit: positive sequence, negative sequence and zero sequence three-sequence base frequency equivalent circuit and sampling frequency f for constructing complex power distribution network comprehensive optimization devicesThe equivalent circuit is formed by connecting an equivalent voltage source and an equivalent impedance in series;
the equivalent circuit second construction unit: selecting n typical single-phase earth fault points, and constructing the fundamental frequency and the sampling frequency f of the power distribution system at the fault pointssA lower positive sequence, negative sequence and zero sequence equivalent circuit;
a first relational expression calculation unit: setting different fault ground resistances RfCalculating to obtain fault current I according to the positive sequence, negative sequence and zero sequence base frequency equivalent circuit at the fault pointfAnd a direct current resistance R0The relation between;
a direct current resistance lower limit value determination unit: based on maximum fault current not exceeding maximum short-circuit current If0Determining a lower limit value R of the direct current resistancemin；
A second relational expression calculation unit: according to sampling frequency f at fault pointsCalculating a positive sequence, a negative sequence and a zero sequence equivalent circuit to obtain a fault point sampling frequency fsCurrent of (I)fsAnd a direct current resistance R0The relation between;
a direct current resistance upper limit value determination unit: based on fault grounding resistance being smaller than set resistance Rf0And the frequency of the inflow fault point is fsIs not less than the minimum current I of the effective signalfs0Determining an upper limit value R of the direct current resistancemax；
A direct current resistance obtaining unit: according to Rmin、RmaxAnd the relation between the two to obtain the direct current resistance R0；
If R ismax≥RminThen the direct current resistance R0In { Rmin，RmaxSelecting within the range; the selection bias is R based on the further requirements of fault current control and line selection accuracymaxOr Rmin；
If R ismax<RminThen in step 6) by decreasing Rf0Calculation of RmaxParameter until R is satisfiedmax≥RminUnder the condition of (1) R0Is set as RminOr RmaxAnd recording the corresponding Rf0。
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