CN112765920B - Direct-current short-circuit current calculation method and system based on differential-common mode transformation - Google Patents
Direct-current short-circuit current calculation method and system based on differential-common mode transformation 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 differential-common mode transformation, wherein the method comprises the following steps: respectively establishing a converter frequency domain model and a direct current circuit frequency domain model, and respectively carrying out differential-common mode transformation to respectively obtain a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model; establishing an equivalent differential-common-mode network according to boundary conditions of faults based on the converter common-mode model, the converter differential-mode model, the direct-current circuit common-mode model and the direct-current circuit differential-mode model, solving the equivalent differential-common-mode network, and obtaining common-mode short-circuit current and differential-mode short-circuit current at fault points; taking common-mode short-circuit current and differential-mode short-circuit current at fault points as excitation, and respectively solving the current of each network in the common-mode network and the differential-mode network; and carrying out differential-common mode inverse transformation and Laplacian inverse transformation on the current in the common-mode network and the network in the differential-mode network, and obtaining a time domain analysis 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 differential-common mode transformation.
Background
Along with the continuous development of society, the production mode of people is more and more abundant, and the use demand of electric energy is also more and more great. At present, the power distribution network of a part of first-line cities in China faces the problems of lack of power supply corridor and insufficient power supply capacity. Conventional ac distribution networks have power limitations and require high costs to make new power supply corridors. Meanwhile, the traditional alternating current power distribution network has the problems of unbalanced three phases, insufficient reactive support of nodes and the like, and the problems are more remarkable under the trend that the power consumption requirement is greatly increased. In addition, the rise of many high and new industries puts higher demands on power supply reliability and power quality, and high-quality power supply is difficult to realize due to problems of harmonic waves, impact loads and the like caused by converter equipment in a network. This series of problems has driven the step of technological innovation in power distribution networks.
With the development of power electronics technology, the technology of converters is becoming mature, and direct current power distribution technology is gradually coming into the field of view of people. The direct current distribution network has the advantages of large transmission capacity, low line cost, small network loss, high power supply reliability, high electric energy quality and the like, and becomes a feasible way for solving a series of problems of the traditional alternating current 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 distribution network, the fault current rises quickly, and can reach rated current of five to ten times in a few milliseconds, and high requirements are put on the rapidity and the breaking capacity of the relay protection device. The direct current short circuit current calculation is the basis of selection of switching-off equipment and current limiting equipment such as a direct current distribution network relay protection setting, a direct current breaker and the like, and is an important basic work. However, the computer simulation calculation method commonly adopted at present is accurate, but the topology can not be flexibly changed, the calculation time is long, and the requirement of network planning can not be met.
Disclosure of Invention
The invention provides a direct current short-circuit current calculation method and system based on differential-common mode transformation, which are used for solving the problem of how to quickly determine fault component currents of various parts of a network.
In order to solve the above problems, according to an aspect of the present invention, there is provided a direct current short-circuit current calculation method based on differential-common mode transformation, the method including:
Respectively establishing a converter frequency domain model and a direct current circuit frequency domain model, and respectively carrying out differential-common mode transformation on the converter frequency domain model and the direct current circuit frequency domain model to respectively obtain a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model;
Establishing an equivalent differential-common-mode network according to boundary conditions of faults based on the converter common-mode model, the converter differential-mode model, the direct-current circuit common-mode model and the direct-current circuit differential-mode model, solving the equivalent differential-common-mode network, and obtaining common-mode short-circuit current and differential-mode short-circuit current at fault points;
taking common-mode short-circuit current and differential-mode short-circuit current at fault points as excitation, and respectively solving the current of each network in the common-mode network and the differential-mode network;
And carrying out differential-common mode inverse transformation and Laplacian inverse transformation on the current in the common-mode network and the network in the differential-mode network, and obtaining a time domain analysis solution of the short-circuit current fault component.
Preferably, the method performs differential-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 I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
Preferably, wherein the converter common mode model and the converter differential mode model comprise: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit differential mode model.
Preferably, if the short-circuit fault is a negative electrode ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, flowing from the fault point in kA.
Preferably, the method performs differential-common mode inverse transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
According to another aspect of the present invention, there is provided a direct current short-circuit current calculation system based on differential-common mode transformation, the system comprising:
The model determining unit is used for respectively establishing a converter frequency domain model and a direct current circuit frequency domain model, respectively carrying out differential-common mode transformation on the converter frequency domain model and the direct current circuit frequency domain model, and respectively obtaining a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model;
The differential-mode short-circuit current determining unit is used for establishing an equivalent differential-mode network according to boundary conditions of faults based on the converter common-mode model, the converter differential-mode model, the direct-current circuit common-mode model and the direct-current circuit differential-mode model, solving the equivalent differential-mode network and obtaining common-mode short-circuit current and differential-mode short-circuit current at fault points;
The network everywhere current determining unit is used for taking common-mode short-circuit current and differential-mode short-circuit current at fault points as excitation and respectively solving the current everywhere in the common-mode network and the differential-mode network;
and the short-circuit current determining unit is used for carrying out differential-common mode inverse transformation and Laplacian inverse transformation on the current of each part of the common-mode network and the network in the differential-mode network to obtain a time domain analysis solution of a short-circuit current fault component.
Preferably, the model determining unit performs differential-common mode transformation on the converter frequency domain model and/or the direct current line model in the following manner, and includes:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
Preferably, wherein the converter common mode model and the converter differential mode model comprise: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit differential mode model.
Preferably, if the short-circuit fault is a negative electrode ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, flowing from the fault point in kA.
Preferably, the short-circuit current determining unit performs differential-common mode inverse transformation on the converter frequency domain model and/or the direct current line model in the following manner, and includes:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, 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 differential-common mode transformation, which are characterized in that a differential-common mode model of a converter and a direct current circuit is established for calculation, and the direct current power grid is regarded as a linear steady circuit for analysis under the assumption that the states of switching devices of each bridge arm of the converter are unchanged before MMC locking in a short time after a fault occurs; 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 differential-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 used for analyzing and calculating the fault component current, the topology can be flexibly transformed through the input parameter matrix, the calculation speed which is much faster than simulation is ensured, and certain reliability and conservation are maintained.
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Exemplary embodiments of the present invention may be more completely understood in consideration of the following drawings:
Fig. 1 is a flowchart of a method 100 for calculating a dc short-circuit current based on differential-common mode transformation 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 an inverter when the AC side is not grounded while the midpoint of the DC side capacitor is grounded according to an embodiment of the present 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 midpoint of the DC side clamping resistor is grounded in accordance with an embodiment of the present invention;
fig. 5 is a schematic diagram of a common mode model and a differential mode model of an inverter when an ac side is grounded and a midpoint of a dc side capacitor is grounded according to an embodiment of the present invention;
FIG. 6 is a schematic diagram of a common mode model and a differential mode model of the inverter with AC side ground and DC side clamp resistor neutral to ground in accordance with an embodiment of the present invention;
FIG. 7 is a schematic diagram of a differential-common mode model of a DC link 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 system diagram of simulation verification using PSCAD according to an embodiment of the present invention;
FIG. 10 is a schematic diagram of the 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 of the converter station 1 to the circulation station 2 according to an embodiment of the invention;
fig. 12 is a schematic diagram of the variation of the negative current on the line of the converter station 2 to the converter station 1 according to an embodiment of the invention;
fig. 13 is a schematic view showing a change in the negative electrode current at the outlet of the converter station 1 according to the embodiment of the present invention
Fig. 14 is a schematic diagram of a dc short-circuit current calculation system 1400 based on differential-common mode transformation 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 examples described herein, which are provided to fully and completely disclose the present invention and fully convey the scope of the 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, like elements/components are referred to by like reference numerals.
Unless otherwise indicated, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. In addition, it will be understood that terms 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 method 100 for calculating a dc short-circuit current based on differential-to-common mode transformation according to an embodiment of the present invention. As shown in fig. 1, according to the differential-common mode transformation-based direct current short-circuit current calculation method provided by the invention, a differential-common mode model of a converter and a direct current line is established to calculate, and before MMC locking occurs in a short time after a fault occurs, the states of switching devices of each bridge arm of the converter and a bypass can be assumed to be unchanged, and a direct current power grid is regarded as a linear steady circuit to be analyzed; 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 differential-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 used for analyzing and calculating the fault component current, the topology can be flexibly transformed through the input parameter matrix, the calculation speed which is much faster than simulation is ensured, and certain reliability and conservation are maintained. The method 100 for calculating the direct current short circuit current based on the differential-common mode transformation provided by the embodiment of the invention starts from step 101, respectively establishes a converter frequency domain model and a direct current circuit frequency domain model in step 101, respectively carries out differential-common mode transformation on the converter frequency domain model and the direct current circuit frequency domain model, and respectively acquires a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model.
Preferably, the method performs differential-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 I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
Preferably, wherein the converter common mode model and the converter differential mode model comprise: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit differential mode model.
In the embodiment of the invention, frequency domain models of the converter and the direct current circuit are required to be respectively established, and differential-common mode transformation is carried out on the models to form two models of a common mode and a differential mode.
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 is in unipolar symmetrical connection, the frequency domain model of the converter is established as shown in the left graph of FIG. 2. The dashed line in the left diagram of fig. 2 indicates that only when the ac/dc side of the MMC is grounded in a corresponding manner, a circuit connection exists there. The left dashed line indicates ac-side ground, the middle dashed line indicates dc-side ground through the clamp resistor midpoint, and the right dashed line indicates dc-side ground through the capacitor midpoint. L ac in the figure represents 1/3 of the zero sequence inductance of the alternating current side when the alternating current side is grounded; r g represents the resistance value of the grounding point connected with each pole when the direct current side is grounded through the middle point of the clamping resistor; r cg represents the resistance value between the midpoint of the capacitor and the ground when the direct current side is grounded through the midpoint of the capacitor; c g represents the capacitance value of the grounding resistor connected with each pole when the direct current side is grounded through the midpoint of the capacitor; n is the number of sub-modules of each bridge arm of the MMC, and C 0 is the capacitance value of the sub-modules; l dc is the 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 differential-common mode transformation is as follows:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
In the present invention, after the differential-common mode conversion, the common mode and differential mode models of the inverter respectively include: the common mode model and the differential mode model of the converter when the midpoint of the capacitor on the alternating side is not grounded and the midpoint of the capacitor on the direct side is grounded as shown in the left and right diagrams in fig. 3, the common mode model and the differential mode model of the converter when the midpoint of the clamp resistor on the alternating side is not grounded and the midpoint of the capacitor on the direct side is grounded as shown in the left and right diagrams in fig. 4, the common mode model and the differential mode model of the converter when the midpoint of the capacitor on the alternating side is grounded as shown in the left and right diagrams in fig. 5, and the common mode model and the differential mode model of the converter when the midpoint of the clamp resistor on the alternating side is grounded as shown in the left and right diagrams in fig. 6. After the differential-common mode conversion, the differential mode model of the dc line is the same as the common mode model, and the model structure is shown in fig. 7.
In step 102, an equivalent differential-common-mode network is established according to the boundary condition of the fault 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 the equivalent differential-common-mode network is solved to obtain the common-mode short-circuit current and the differential-mode short-circuit current at the fault point.
Preferably, if the short-circuit fault is a negative electrode ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, 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 common-mode and differential-mode short-circuit currents at the fault point. If the short circuit fault is a negative electrode ground fault, the boundary conditions of the fault are as follows:
Wherein U f,n is the negative voltage/kV at the fault point; i f,p、If,n is positive current and negative current/kA flowing to the ground from the fault point respectively; r f is the transition resistance/Ω between the point of failure and ground.
After differential-common mode transformation, the fault boundary condition becomes:
Wherein U f,∑、Uf,Δ is the common mode voltage/kV at the fault point; i f,∑、I f,Δ is the common mode node flow, differential mode current/kA, from the point of failure, respectively.
The resulting equivalent differential-common-mode network is shown in fig. 8, based on fault boundary conditions. Wherein U f,Δ(0) is the normal component/kV of the differential mode voltage at the fault point; z Δ、Z∑ is the dc distribution network equivalent differential mode impedance, common mode impedance/Ω, respectively, as seen from the fault point.
And 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 the currents of the common mode network and the differential mode network are respectively solved.
In step 104, the current in the common mode network and the network is subjected to inverse differential mode transformation and inverse laplace transformation, so as to obtain a time domain analysis solution of the short-circuit current fault component.
Preferably, the method performs differential-common mode inverse transformation on the converter frequency domain model and/or the direct current line model by using the following method, including:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
In the invention, common mode short-circuit current and differential mode short-circuit current at fault points are used as excitation, and the current of each part of the network in the common mode network and the differential mode network is solved respectively; and performing differential-common mode inverse transformation and Laplacian inverse transformation on the differential-common mode current at each part of the network to obtain a time domain analysis solution of the fault component of the short-circuit current.
In the present invention, the calculation of the short-circuit current is performed based on the system shown in fig. 9, and the comparison verification is performed with the result of the PSCAD simulation. Wherein, each parameter in the system is shown in table 1. The power in the table is the injection power of the alternating current side, and the reactive power of the alternating current side of the converter is controlled to be zero.
TABLE 1 System parameter Table
In order to verify converter models with different grounding modes, different grounding modes are set for each MMC in the power distribution network: MMC1 ac side ground (L ac =10 mH), dc side capacitance midpoint ground (C g＝8mF,Rcg =0.5Ω); the alternating current side of the MMC2 is not grounded, and the midpoint of the clamping resistor of the direct current side is grounded (R g = 4MΩ); the alternating current side of the MMC3 is not grounded, and the midpoint of the capacitor at the direct current side is grounded (C g＝8mF,Rcg =0.5Ω); MMC4 ac side ground (L ac =10mh), dc side clamp resistor midpoint ground (R g =4mΩ).
After the circuit is stable, the negative electrode at the midpoint of the direct current line from the converter station 1 to the converter station 2 is grounded and shorted (t=0s at the moment), and R f =0 is taken as the short-circuit current at the fault point, the negative electrode current on the line from the converter station 1 to the converter station 2, the negative electrode current on the line from the converter station 2 to the converter station 1 and the negative electrode current at the outlet of the converter station 1, and the calculated value and the simulation value are compared as shown in fig. 10 to 13 respectively.
As can be seen from comparison of the fault current curves in fig. 10 to 13, there is a small error in the calculated value relative to the simulated value, and the error increases gradually with time. The cause of this error is on the one hand related to the change of the MMC operating state; on the other hand, the influence of the bridge arm reactors is ignored in the process of establishing the model, so that the model is more conservative.
It can be seen that in a short time frame after the fault, the error of the calculated value relative to the simulated value is not large, the calculated value is still quite reliable in the excision time required by the direct current fault, and the calculation shows conservation. Therefore, the method provided by the invention has a considerable reference significance. In addition, the calculation method can flexibly transform the topology of the direct current power grid, the calculation speed is much faster than that of simulation, and the method is an effective direct current short-circuit current calculation method.
Fig. 14 is a schematic diagram of a dc short-circuit current calculation system 1400 based on differential-common mode transformation according to an embodiment of the present invention. As shown in fig. 14, a dc short-circuit current calculation system 1400 according to an embodiment of the present invention includes: a model determination unit 1401, a differential-common-mode short-circuit current determination unit 1402, a network-everywhere current determination unit 1403, 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 direct current line frequency domain model, and respectively perform differential-common mode transformation on the converter frequency domain model and the direct current line frequency domain model, so as to respectively obtain 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.
Preferably, the model determining unit 1401 performs differential-to-common mode transformation on the converter frequency domain model and/or the dc line model in the following manner, including:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
Preferably, wherein the converter common mode model and the converter differential mode model comprise: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit 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 boundary condition of a fault 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 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 electrode ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, flowing from the fault point in kA.
Preferably, the network-wide current determining unit 1403 is configured to solve the current of the network-wide part in the common-mode network and the differential-mode network, respectively, by using the common-mode short-circuit current and the differential-mode short-circuit current at the fault point as excitation.
Preferably, the short-circuit current determining unit 1404 is configured to perform inverse differential-common-mode transformation and inverse laplace transformation on the current in the common-mode network and the network around the differential-mode network, so as to obtain a time-domain analysis solution of the short-circuit current fault component.
Preferably, the short-circuit current determining unit performs differential-common mode inverse transformation on the converter frequency domain model and/or the direct current line model in the following manner, and includes:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
The dc short-circuit current calculation system 1400 based on differential-common mode transformation according to the embodiment of the present invention corresponds to the dc short-circuit current calculation method 100 based on differential-common mode transformation according to another embodiment of the present invention, and is not described herein.
The invention has been described with reference to a few embodiments. However, as is well known to those skilled in the art, other embodiments than the above disclosed invention are equally possible within the scope of the invention, as defined by 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 therein. All references to "a/an/the [ means, component, etc. ]" are to be interpreted openly as referring to at least one instance of said means, 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.
It will be appreciated by those skilled in the art that 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 flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations 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 aspects of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the above embodiments, it should be understood by those of ordinary skill in the art that: modifications and equivalents may be made to the specific embodiments of the invention without departing from the spirit and scope of the invention, which is intended to be covered by the claims.
Claims (8)
1. The direct current short-circuit current calculation method based on differential-common mode transformation is characterized by comprising the following steps of:
Respectively establishing a converter frequency domain model and a direct current circuit frequency domain model, and respectively carrying out differential-common mode transformation on the converter frequency domain model and the direct current circuit frequency domain model to respectively obtain a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model;
Establishing an equivalent differential-common-mode network according to boundary conditions of faults based on the converter common-mode model, the converter differential-mode model, the direct-current circuit common-mode model and the direct-current circuit differential-mode model, solving the equivalent differential-common-mode network, and obtaining common-mode short-circuit current and differential-mode short-circuit current at fault points;
taking common-mode short-circuit current and differential-mode short-circuit current at fault points as excitation, and respectively solving the current of each network in the common-mode network and the differential-mode network;
performing differential-common mode inverse transformation and Laplacian inverse transformation on the current in the common-mode network and the network in the differential-mode network to obtain a time domain analysis solution of a short-circuit current fault component;
Wherein, the converter common mode model and the converter differential mode model include: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit differential mode model.
2. A method according to claim 1, characterized in that it performs a differential-to-common mode transformation of the converter frequency domain model and/or direct current line model by means of:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
3. The method of claim 1, wherein if the short circuit fault is a negative ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, flowing from the fault point in kA.
4. A method according to claim 1, characterized in that it uses the following way to perform a differential-to-common mode inverse transformation of the converter frequency domain model and/or direct current line model, comprising:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
5. A differential-to-common-mode conversion-based direct-current 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 circuit frequency domain model, respectively carrying out differential-common mode transformation on the converter frequency domain model and the direct current circuit frequency domain model, and respectively obtaining a converter common mode model, a converter differential mode model, a direct current circuit common mode model and a direct current circuit differential mode model;
The differential-mode short-circuit current determining unit is used for establishing an equivalent differential-mode network according to boundary conditions of faults based on the converter common-mode model, the converter differential-mode model, the direct-current circuit common-mode model and the direct-current circuit differential-mode model, solving the equivalent differential-mode network and obtaining common-mode short-circuit current and differential-mode short-circuit current at fault points;
The network everywhere current determining unit is used for taking common-mode short-circuit current and differential-mode short-circuit current at fault points as excitation and respectively solving the current everywhere in the common-mode network and the differential-mode network;
The short-circuit current determining unit is used for carrying out differential-common mode inverse transformation and Laplacian inverse transformation on the current of each part of the common-mode network and the network in the differential-mode network to obtain a time domain analysis solution of a short-circuit current fault component;
Wherein, the converter common mode model and the converter differential mode model include: the AC side is not grounded while the midpoint of the DC side capacitor is grounded, the AC side is not grounded while the midpoint of the DC side clamping resistor is grounded, the AC side is grounded while the midpoint of the DC side capacitor is grounded, and the AC side is grounded while the midpoint of the DC side clamping resistor is grounded;
The direct current circuit common mode model is the same as the direct current circuit differential mode model.
6. The system according to claim 5, wherein the model determination unit performs differential-to-common mode transformation on the converter frequency domain model and/or the direct current line model by means of:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters, respectively, corresponding to the voltage.
7. The system of claim 5, wherein if the short circuit fault is a negative ground fault, the boundary condition of the fault is:
after differential-common mode transformation, the boundary condition of the fault becomes:
Wherein U f,n is the negative voltage at the fault point, and the unit is kV; i f,p and I f,n are positive and negative currents flowing to the ground from a fault point respectively, and the unit is kA; r f is the transition resistance between the fault point and the ground, and the unit is omega; u f,∑ and U f,Δ are respectively common mode low voltage and differential mode voltage at a fault point, and the unit is kV; i f,∑ and I f,Δ are the common mode current and the differential mode current, respectively, flowing from the fault point in kA.
8. The system according to claim 5, wherein the short-circuit current determining unit performs differential-to-common mode inverse transformation on the converter frequency domain model and/or the direct current line model by:
Wherein, I ∑ and I Δ are current common mode component and current differential mode component respectively; u ∑ and U Δ are the voltage common mode component and the voltage difference mode component, respectively; i p and I n are positive electrode parameters and negative electrode parameters corresponding to the current respectively; u p and U n are positive and negative parameters corresponding to the voltage respectively.
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