CN112765920A - DC short-circuit current calculation method and system based on difference-common mode conversion - Google Patents
DC short-circuit current calculation method and system based on difference-common mode conversion Download PDFInfo
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Abstract
The invention discloses a direct current short circuit current calculation method and a direct current short circuit current calculation system based on difference-common mode conversion, wherein the method comprises the following steps: respectively establishing a current converter frequency domain model and a direct-current line frequency domain model, respectively carrying out difference common-mode transformation, and respectively obtaining a current converter common-mode model, a current converter differential-mode model, a direct-current line common-mode model and a direct-current line differential-mode model; establishing an equivalent differential-common mode network according to fault boundary conditions based on the converter common mode model, the converter differential mode model, the direct-current line common mode model and the direct-current line differential mode model, and solving the equivalent differential-common mode network to obtain common mode short-circuit current and differential mode short-circuit current at a fault point; using the common-mode short-circuit current and the differential-mode short-circuit current at the fault point as excitation, and respectively solving the currents at each position of the network in the common-mode network and the differential-mode network; and performing differential common mode inverse transformation and inverse Laplace transformation on the current at each position of the network in the common mode network and the differential mode network to obtain a time domain analytical solution of the short circuit current fault component.
Description
Technical Field
The invention relates to the technical field of short-circuit current calculation, in particular to a direct-current short-circuit current calculation method and system based on difference-common mode transformation.
Background
With the continuous development of society, people have more and more abundant production modes and have more and more large use requirements on electric energy. At present, the power distribution network of a part of front-line cities in China faces the problems of lack of power supply corridors and insufficient power supply capacity. The traditional ac distribution network has power limit, and the new power supply corridor needs to be carried out with high cost. Meanwhile, the traditional alternating-current power distribution network has the problems of unbalanced three phases, insufficient node reactive power support and the like, and is more and more prominent under the trend that the power consumption demand is greatly increased. In addition, the rise of many high and new industries puts higher requirements on power supply reliability and electric energy quality, and high-quality power supply is difficult to realize due to the problems of harmonic waves, impact loads and the like caused by converter equipment in a network. The series of problems push the step of power distribution network technology innovation.
With the development of power electronic technology, the technology of the current converter is more mature, and the direct current distribution technology gradually comes into the visual field of people. The direct-current power distribution network has the advantages of large transmission capacity, low line cost, small network loss, high power supply reliability, high power quality and the like, and becomes a feasible way for solving a series of difficult problems of the traditional alternating-current power distribution network. However, the fault characteristics of a dc distribution network are very different from those of a conventional ac distribution network.
When a short-circuit fault occurs on the direct-current side of the direct-current power distribution network, the fault current rises quickly and can reach five to ten times of rated current within a few milliseconds, and high requirements are provided for the speed and the breaking capacity of the relay protection device. The direct current short circuit current calculation is the basis for the relay protection setting of a direct current power distribution network, selection of switching-off equipment and current limiting equipment such as a direct current breaker and the like, and is important basic work. However, although the currently generally adopted computer simulation calculation method is accurate, the topology cannot be flexibly changed, the calculation time is long, and the requirement of network planning cannot be met.
Disclosure of Invention
The invention provides a direct current short-circuit current calculation method and a direct current short-circuit current calculation system based on difference-common mode conversion, and aims to solve the problem of how to quickly determine fault component currents of all parts of a network.
In order to solve the above problem, according to an aspect of the present invention, there is provided a method for calculating a dc short-circuit current based on a differential-to-common mode conversion, the method including:
respectively establishing a converter frequency domain model and a direct-current line frequency domain model, respectively carrying out difference common-mode transformation on the converter frequency domain model and the direct-current line frequency domain model, and respectively obtaining a converter common-mode model, a converter differential-mode model, a direct-current line common-mode model and a direct-current line differential-mode model;
establishing an equivalent differential-common mode network according to fault boundary conditions based on the converter common mode model, the converter differential mode model, the direct-current line common mode model and the direct-current line differential mode model, and solving the equivalent differential-common mode network to obtain common mode short-circuit current and differential mode short-circuit current at a fault point;
using the common-mode short-circuit current and the differential-mode short-circuit current at the fault point as excitation, and respectively solving the currents at each position of the network in the common-mode network and the differential-mode network;
and performing differential common mode inverse transformation and inverse Laplace transformation on the current at each position of the network in the common mode network and the differential mode network to obtain a time domain analytical solution of the short circuit current fault component.
Preferably, the method performs difference common-mode transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
Preferably, the converter common mode model and the converter differential mode model include: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
Preferably, if the short-circuit fault is a negative ground fault, the boundary conditions of the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively the common mode current and the differential mode current flowing from the fault point, in kA.
Preferably, the method performs inverse difference-common mode transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
According to another aspect of the present invention, there is provided a dc short-circuit current calculation system based on differential-to-common mode conversion, the system comprising:
the model determining unit is used for respectively establishing a converter frequency domain model and a direct-current line frequency domain model, respectively carrying out difference common-mode transformation on the converter frequency domain model and the direct-current line frequency domain model, and respectively obtaining a converter common-mode model, a converter differential-mode model, a direct-current line common-mode model and a direct-current line differential-mode model;
the differential-mode and common-mode short-circuit current determination unit is used for establishing an equivalent differential-mode and common-mode network according to the boundary condition of the fault based on the current converter common-mode model, the current converter differential-mode model, the direct-current line common-mode model and the direct-current line differential-mode model, solving the equivalent differential-mode and common-mode network and obtaining the common-mode and differential-mode short-circuit currents at the fault point;
the current determining unit at each part of the network is used for respectively solving the currents at each part of the network in the common mode network and the differential mode network by taking the common mode short-circuit current and the differential mode short-circuit current at the fault point as excitation;
and the short-circuit current determining unit is used for performing differential common mode inverse transformation and Laplace inverse transformation on the current at each position of the network in the common mode network and the differential mode network to obtain a time domain analysis solution of the short-circuit current fault component.
Preferably, the model determining unit performs difference common-mode transformation on the converter frequency domain model and/or the dc link model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
Preferably, the converter common mode model and the converter differential mode model include: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
Preferably, if the short-circuit fault is a negative ground fault, the boundary conditions of the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively the common mode current and the differential mode current flowing from the fault point, in kA.
Preferably, the short-circuit current determination unit performs inverse difference-common mode transformation on the converter frequency domain model and/or the dc line model in the following manner, and includes:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
The invention provides a direct current short-circuit current calculation method and a direct current short-circuit current calculation system based on difference-common mode conversion.A difference-common mode model of a converter and a direct current circuit is established for calculation, and after a fault occurs, in a short time and before MMC locking, the states of switching devices input and bypassed by each bridge arm of the converter can be assumed to be unchanged, and a direct current power grid is taken as a linear constant circuit for analysis; in the analysis and calculation process, if the short circuit is an asymmetric fault, the network becomes complex and difficult to solve, so the invention adopts a difference-common mode transformation method to divide the network into two symmetric networks of a common mode and a differential mode, thereby reducing the complexity of solving the high-order asymmetric network; the method is applied to the analytic calculation of the fault component current, the topology can be flexibly changed by inputting the parameter matrix, the calculation speed which is much higher than that of simulation is ensured, and certain reliability and conservatism are kept.
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A more complete understanding of exemplary embodiments of the present invention may be had by reference to the following drawings in which:
fig. 1 is a flowchart of a dc short-circuit current calculation method 100 based on differential-to-common mode conversion according to an embodiment of the present invention;
fig. 2 is a schematic diagram of a converter frequency domain model and a dc link frequency domain model according to an embodiment of the invention;
fig. 3 is a schematic diagram of a common mode model and a differential mode model of the inverter when the ac side is not grounded while the dc side capacitor is midpoint grounded according to an embodiment of the invention;
fig. 4 is a schematic diagram of a common mode model and a differential mode model of the inverter when the ac side is not grounded while the dc side clamp resistor is midpoint grounded according to an embodiment of the invention;
FIG. 5 is a schematic diagram of a common mode model and a differential mode model of the inverter when the AC side is grounded and the DC side capacitor is midpoint grounded according to an embodiment of the invention;
fig. 6 is a schematic diagram of a common mode model and a differential mode model of the inverter when the ac side is grounded while the dc side clamp resistor is midpoint grounded according to an embodiment of the present invention;
FIG. 7 is a schematic diagram of a differential-to-common mode model of a DC line according to an embodiment of the present invention;
FIG. 8 is a schematic diagram of an equivalent differential-common mode network according to an embodiment of the present invention;
FIG. 9 is a diagram of a system utilizing PSCAD simulation verification, according to an embodiment of the present invention;
fig. 10 is a schematic diagram illustrating a change in short-circuit current at a fault point according to an embodiment of the present invention;
fig. 11 is a schematic diagram of the variation of the negative current on the line from the converter station 1 to the circulating current station 2 according to the embodiment of the present invention;
fig. 12 is a schematic diagram of the variation of the negative current on the line from the converter station 2 to the converter station 1 according to the embodiment of the invention;
fig. 13 is a schematic diagram of the variation of the negative current at the outlet of the converter station 1 according to an embodiment of the invention
Fig. 14 is a schematic structural diagram of a dc short-circuit current calculation system 1400 based on differential-to-common mode conversion according to an embodiment of the present invention.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein, which are provided for complete and complete disclosure of the present invention and to fully convey the scope of the present invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, the same units/elements are denoted by the same reference numerals.
Unless otherwise defined, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Further, it will be understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
Fig. 1 is a flowchart of a dc short-circuit current calculation method 100 based on differential-to-common mode conversion according to an embodiment of the present invention. As shown in fig. 1, the method for calculating the dc short-circuit current based on the differential-common mode transformation according to the present invention calculates by establishing a differential-common mode model of the converter and the dc line, and assumes that states of switching devices input and bypassed by each bridge arm of the converter are unchanged in a short time after a fault occurs and before the MMC is locked, and analyzes the dc power grid as a linear constant circuit; in the analysis and calculation process, if the short circuit is an asymmetric fault, the network becomes complex and difficult to solve, so the invention adopts a difference-common mode transformation method to divide the network into two symmetric networks of a common mode and a differential mode, thereby reducing the complexity of solving the high-order asymmetric network; the method is applied to the analytic calculation of the fault component current, the topology can be flexibly changed by inputting the parameter matrix, the calculation speed which is much higher than that of simulation is ensured, and certain reliability and conservatism are kept. In the method 100 for calculating a dc short-circuit current based on differential-common mode transformation according to the embodiment of the present invention, starting from step 101, a converter frequency domain model and a dc line frequency domain model are respectively established in step 101, and differential-common mode transformation is respectively performed on the converter frequency domain model and the dc line frequency domain model to respectively obtain a converter common mode model, a converter differential mode model, a dc line common mode model and a dc line differential mode model.
Preferably, the method performs difference common-mode transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔAre respectively voltage common mode divisionMagnitude and voltage differential mode components; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
Preferably, the converter common mode model and the converter differential mode model include: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
In the embodiment of the invention, frequency domain models of the current converter and the direct current line need to be established respectively, and the models are subjected to difference-common mode conversion to form a common mode model and a differential mode model.
According to the superposition theorem, the fault component current is zero-state response of the direct-current power distribution network under the excitation of the fault component power supply at the fault point, so that the established frequency domain model is a zero-state response model. If the converter is MMC and has unipolar symmetrical connections, the frequency domain model of the converter is established as shown in the left diagram of fig. 2. Wherein, the dashed line in the left diagram of fig. 2 indicates that there is a circuit connection only when the ac-dc side of the MMC is grounded in a corresponding manner. The left dotted line represents the ac side to ground, the middle dotted line represents the dc side to ground through the clamp resistor midpoint, and the right dotted line represents the dc side to ground through the capacitor midpoint. L in the figureac1/3 showing zero sequence inductance on the AC side when the AC side is grounded; rgThe resistance value of the grounding point connected with each pole when the direct current side is grounded through the midpoint of the clamping resistor is shown; rcgThe resistance value between the midpoint of the capacitor and the ground when the direct current side is grounded through the midpoint of the capacitor is shown; cgThe capacitance value of the grounding resistor connected with each pole when the direct current side is grounded through the midpoint of the capacitor is shown; n is the number of submodules of each bridge arm of MMC, C0Is the sub-module capacitance value; l isdcThe inductance value of the smoothing reactor. The frequency domain model of the dc link is shown in the right diagram of fig. 2.
For the frequency domain model, the formula for performing the difference common-mode transformation is as follows:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
In the invention, after the difference-common mode conversion, the common mode and the difference mode models of the converter respectively comprise: the common mode model and the differential mode model of the inverter when the ac side is not grounded while the dc side capacitance is midpoint grounded as shown in the left and right diagrams of fig. 3, the common mode model and the differential mode model of the inverter when the ac side is not grounded while the dc side clamping resistance is midpoint grounded as shown in the left and right diagrams of fig. 4, the common mode model and the differential mode model of the inverter when the ac side is grounded while the dc side capacitance is midpoint grounded as shown in the left and right diagrams of fig. 5, and the common mode model and the differential mode model of the inverter when the ac side is grounded while the dc side clamping resistance is midpoint grounded as shown in the left and right diagrams of fig. 6. After the differential-to-common mode conversion, the differential mode model of the dc line is the same as the common mode model, and the model structure thereof is as shown in fig. 7.
In step 102, based on the converter common mode model, the converter differential mode model, the direct-current line common mode model and the direct-current line differential mode model, an equivalent differential-common mode network is established according to the boundary condition of the fault, the equivalent differential-common mode network is solved, and the common mode short-circuit current and the differential mode short-circuit current at the fault point are obtained.
Preferably, if the short-circuit fault is a negative ground fault, the boundary conditions of the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively the common mode current and the differential mode current flowing from the fault point, in kA.
In the invention, an equivalent differential-common mode network is established according to the boundary condition of the fault, and the equivalent network is solved to obtain the common mode and differential mode short-circuit current at the fault point. If the short-circuit fault is a negative earth fault, the boundary conditions of the fault are as follows:
in the formula of Uf,nThe voltage of the negative electrode at the fault point is/kV; i isf,p、If,nRespectively positive pole current and negative pole current/kA flowing from a fault point to the ground; rfIs the transition resistance/omega between the fault point and ground.
After the differential common-mode transformation, the fault boundary conditions become:
in the formula of Uf,∑、Uf,ΔCommon mode and difference at fault pointMode voltage/kV; i isf,∑、I f,ΔThe common mode node current and the differential mode current/kA respectively flow out of the fault point.
The resulting equivalent differential-common mode network is shown in fig. 8, depending on the fault boundary conditions. Wherein, Uf,Δ(0)Normal component/kV of differential mode voltage at fault point; zΔ、Z∑The equivalent differential mode impedance and the common mode impedance/omega of the direct current distribution network are respectively seen from a fault point.
In step 103, the common-mode short-circuit current and the differential-mode short-circuit current at the fault point are used as excitation, and currents at various parts of the network in the common-mode network and the differential-mode network are respectively solved.
In step 104, the currents at various positions of the network in the common mode network and the differential mode network are subjected to differential common mode inverse transformation and laplacian inverse transformation, and a time domain analysis solution of the short-circuit current fault component is obtained.
Preferably, the method performs inverse difference-common mode transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
In the invention, common-mode and differential-mode short-circuit currents at fault points are used as excitation, and currents at each position of a network in a common-mode network and a differential-mode network are respectively solved; and performing differential-common mode inverse transformation and inverse Laplace transformation on the differential-common mode current at each position of the network to obtain a time domain analytic solution of the short-circuit current fault component.
In the present invention, the short circuit current is calculated based on the system shown in fig. 9, and compared with the result of the PSCAD simulation for verification. Wherein, each parameter in the system is shown in table 1. And the power in the table is the injection power at the alternating current side, and the reactive power at the alternating current side of the converter is controlled to be zero.
TABLE 1 System parameter Table
For the transverter model of verifying different ground connection modes, set up different ground connection modes for each MMC in the distribution network: MMC1 AC side ground (L)ac10mH), dc side capacitor midpoint grounded (C)g＝8mF,Rcg0.5 Ω); MMC2 AC side is not grounded, DC side clamp resistor midpoint is grounded (R)g4M Ω); the AC side of MMC3 is not grounded, and the capacitor midpoint of the DC side is grounded (C)g＝8mF,Rcg0.5 Ω); MMC4 AC side ground (L)ac10mH), dc side clamp resistor midpoint grounded (R)g＝4MΩ)。
After the circuit is stabilized, the negative electrode at the midpoint of the dc line from the converter station 1 to the converter station 2 is short-circuited to ground (at this time, t is 0s), and R is set to be short-circuited to groundfTaking the short-circuit current at the fault point, the negative current on the line from the converter station 1 to the converter station 2, the negative current on the line from the converter station 2 to the converter station 1 and the negative current at the outlet of the converter station 1 as 0, comparing the calculated values with the simulated values as shown in fig. 10 to 13 respectively.
As can be seen from the comparison of the fault current curves in fig. 10 to 13, the calculated value has a small error compared to the simulated value, and the error gradually increases with time. The cause of such errors is related to the change of the MMC operating state on the one hand; on the other hand, the influence of the bridge arm reactors is ignored when the model is established, so that the model is more conservative.
It can be seen that the calculated values have a small error from the simulated values within a short time span after the fault, are still fairly reliable within the required cut-off time of the dc fault, and such calculation is conservative. Therefore, the method provided by the invention has a corresponding reference meaning. In addition, the calculation method can flexibly transform the topology of the direct current power grid, has a calculation speed which is much higher than that of simulation, and is an effective direct current short-circuit current calculation method.
Fig. 14 is a schematic structural diagram of a dc short-circuit current calculation system 1400 based on differential-to-common mode conversion according to an embodiment of the present invention. As shown in fig. 14, the dc short-circuit current calculation system 1400 based on the differential-to-common mode conversion according to the embodiment of the present invention includes: a model determination unit 1401, a difference common mode short circuit current determination unit 1402, a current determination unit 1403 at each place of the network, and a short circuit current determination unit 1404.
Preferably, the model determining unit 1401 is configured to respectively establish a converter frequency domain model and a dc line frequency domain model, and respectively perform difference-common mode transformation on the converter frequency domain model and the dc line frequency domain model to respectively obtain a converter common mode model, a converter differential mode model, a dc line common mode model, and a dc line differential mode model.
Preferably, the model determining unit 1401, performing difference common mode transformation on the converter frequency domain model and/or the dc link model by using the following method, including:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
Preferably, the converter common mode model and the converter differential mode model include: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
Preferably, the differential-common mode short-circuit current determining unit 1402 is configured to establish an equivalent differential-common mode network according to a fault boundary condition based on the converter common mode model, the converter differential mode model, the dc line common mode model, and the dc line differential mode model, and solve the equivalent differential-common mode network to obtain a common mode short-circuit current and a differential mode short-circuit current at a fault point.
Preferably, if the short-circuit fault is a negative ground fault, the boundary conditions of the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively flowing from the point of failureCommon mode current and differential mode current in kA.
Preferably, the current determination unit 1403 in each network part is configured to use the common-mode short-circuit current and the differential-mode short-circuit current at the fault point as excitation to respectively solve the currents in each network part in the common-mode network and the differential-mode network.
Preferably, the short-circuit current determining unit 1404 is configured to perform inverse differential-common mode transformation and inverse laplace transformation on currents at various places in the common-mode network and the differential-mode network to obtain a time-domain analytic solution of the short-circuit current fault component.
Preferably, the short-circuit current determination unit performs inverse difference-common mode transformation on the converter frequency domain model and/or the dc line model in the following manner, and includes:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
The dc short-circuit current calculation system 1400 based on differential-to-common mode transformation according to the embodiment of the present invention corresponds to the dc short-circuit current calculation method 100 based on differential-to-common mode transformation according to another embodiment of the present invention, and is not described herein again.
The invention has been described with reference to a few embodiments. However, other embodiments of the invention than the one disclosed above are equally possible within the scope of the invention, as would be apparent to a person skilled in the art from the appended patent claims.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the [ device, component, etc ]" are to be interpreted openly as referring to at least one instance of said device, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (10)
1. A direct current short circuit current calculation method based on difference-common mode conversion is characterized by comprising the following steps:
respectively establishing a converter frequency domain model and a direct-current line frequency domain model, respectively carrying out difference common-mode transformation on the converter frequency domain model and the direct-current line frequency domain model, and respectively obtaining a converter common-mode model, a converter differential-mode model, a direct-current line common-mode model and a direct-current line differential-mode model;
establishing an equivalent differential-common mode network according to fault boundary conditions based on the converter common mode model, the converter differential mode model, the direct-current line common mode model and the direct-current line differential mode model, and solving the equivalent differential-common mode network to obtain common mode short-circuit current and differential mode short-circuit current at a fault point;
using the common-mode short-circuit current and the differential-mode short-circuit current at the fault point as excitation, and respectively solving the currents at each position of the network in the common-mode network and the differential-mode network;
and performing differential common mode inverse transformation and inverse Laplace transformation on the current at each position of the network in the common mode network and the differential mode network to obtain a time domain analytical solution of the short circuit current fault component.
2. The method according to claim 1, wherein the method performs a differential common mode transformation on the converter frequency domain model and/or the dc link model by:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shapeΣAnd UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
3. The method of claim 1, wherein the converter common mode model and converter differential mode model comprise: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
4. The method of claim 1, wherein if the short circuit fault is a negative ground fault, then the boundary conditions for the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively the common mode current and the differential mode current flowing from the fault point, in kA.
5. The method according to claim 1, wherein the method performs an inverse differential-common mode transformation on the converter frequency domain model and/or the dc link model by using the following method, comprising:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shapeΣAnd UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
6. A differential-to-common mode conversion based dc short circuit current calculation system, the system comprising:
the model determining unit is used for respectively establishing a converter frequency domain model and a direct-current line frequency domain model, respectively carrying out difference common-mode transformation on the converter frequency domain model and the direct-current line frequency domain model, and respectively obtaining a converter common-mode model, a converter differential-mode model, a direct-current line common-mode model and a direct-current line differential-mode model;
the differential-mode and common-mode short-circuit current determination unit is used for establishing an equivalent differential-mode and common-mode network according to the boundary condition of the fault based on the current converter common-mode model, the current converter differential-mode model, the direct-current line common-mode model and the direct-current line differential-mode model, solving the equivalent differential-mode and common-mode network and obtaining the common-mode and differential-mode short-circuit currents at the fault point;
the current determining unit at each part of the network is used for respectively solving the currents at each part of the network in the common mode network and the differential mode network by taking the common mode short-circuit current and the differential mode short-circuit current at the fault point as excitation;
and the short-circuit current determining unit is used for performing differential common mode inverse transformation and Laplace inverse transformation on the current at each position of the network in the common mode network and the differential mode network to obtain a time domain analysis solution of the short-circuit current fault component.
7. The system according to claim 6, wherein the model determining unit performs a differential common mode transformation on the converter frequency domain model and/or the dc link model by:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shape∑And UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InAre respectively asThe positive electrode parameter and the negative electrode parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
8. The system of claim 6, wherein the converter common mode model and converter differential mode model comprise: the alternating current side is not grounded and the midpoint of a direct current side capacitor is grounded, the alternating current side is not grounded and the midpoint of a direct current side clamping resistor is grounded, the alternating current side is grounded and the midpoint of the direct current side capacitor is grounded, and the alternating current side is grounded and the direct current side is clamped;
the DC line common mode model is the same as the DC line differential mode model.
9. The system of claim 6, wherein if the short circuit fault is a negative ground fault, then the boundary conditions for the fault are:
after differential-to-common mode conversion, the boundary conditions for the fault become:
wherein, Uf,nThe unit is the voltage of a negative electrode at a fault point and is kV; i isf,pAnd If,nRespectively the positive and negative currents flowing from the fault point to the earth, and the unit is kA; rfThe unit is omega, and the unit is the transition resistance between the fault point and the ground; u shapef,∑And Uf,ΔCommon mode low voltage and differential mode voltage at fault points are respectively provided, and the unit is kV; i isf,∑And If,ΔRespectively the common mode current and the differential mode current flowing from the fault point, in kA.
10. The system according to claim 6, wherein the short-circuit current determination unit performs inverse differential-common mode transformation on the converter frequency domain model and/or the direct current line model by using the following method, comprising:
wherein, I∑And IΔRespectively a current common mode component and a current differential mode component; u shapeΣAnd UΔRespectively a voltage common mode component and a voltage differential mode component; i ispAnd InRespectively representing a positive parameter and a negative parameter corresponding to the current; u shapepAnd UnRespectively, positive electrode parameters and negative electrode parameters corresponding to the voltage.
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