CN108933445A - DC grid short-circuit current calculation method based on the loop method of analysis - Google Patents
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
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Abstract
The present invention relates to a kind of DC grid short-circuit current calculation method based on the loop method of analysis, belongs to flexible DC transmission technology field.Single-ended converter station equivalent method before converter station is latched under two kinds of fault conditions of flexible direct current power grid monopolar grounding fault and bipolar short trouble, it proposes the choosing method of current loop, complete the modeling of loop current and the calculating of expression formula, and the expression formula of any one DC line short circuit current is finally established, complete the calculating process of entire DC grid short circuit current.It can be applied to the calculation of short-circuit current of N-terminal flexible direct current power grid; for disclosing the changing rule of fault current; help to better solve error protection problem, all still either there is great practical value and meaning for the research of the blocking ability of failure self-cleaning ability submodule for the verification of dc circuit breaker technical indicator.
Description
Technical Field
The invention relates to the technical field of flexible direct current transmission, in particular to a method for calculating short-circuit current of a flexible direct current power grid, and particularly relates to a method for calculating short-circuit current of the direct current power grid based on a loop current method.
Background
In 2001, german scholars r.marquardt and a.leinicar proposed Modular Multilevel Converters (MMC), which promoted the development of high voltage direct current transmission (HVDC) technology. To date, the domestic MMC-HVDC engineering put into operation is as follows: shanghai Hui demonstration engineering, Nanao engineering, Zhoushan engineering, mansion engineering and the like. The projects adopt cables for power transmission, but compared with overhead lines, the cables are high in cost, most of faults are permanent, and the maintenance and the overhaul are inconvenient. Therefore, the expansion of the flexible direct current transmission technology to the overhead line transmission occasion is a trend of future development of the power grid. In addition, the demand of new energy power is continuously increased, and the MMC-HVDC system with a true bipolar structure has great advantages in high-voltage and large-capacity power transmission occasions. At present, the north-opened 500kV flexible-straight engineering under construction adopts overhead line power transmission, and the topology of a converter station is a true bipolar structure.
The overhead line has a higher probability of short-circuit failure than the cable, and therefore, the problem of fault clearing and fault protection is particularly important. In the existing fault clearing mode, the direct current circuit breaker can cut off fault current in a short time, but in practical situations, the fault current is often very large, and due to technical limitations, the capacity of the direct current circuit breaker for cutting off the current is limited. Furthermore, the dc circuit breaker is costly due to the large number of power electronics employed. Another way is to use sub-modules with fail-over capability, such as full-bridge sub-modules, clamped dual sub-modules, etc. Such sub-modules are able to interrupt the fault current for a short time by generating a reverse voltage by themselves. However, compared with the half-bridge type sub-modules, the number and the loss of the power electronic devices of the sub-modules are increased, and the economical efficiency greatly limits the application of the power electronic devices in practical engineering. Up to now, the half-bridge type submodule is mainly matched with the alternating current circuit breaker. In addition, the current short-circuit fault research mainly aims at a flexible direct-current power transmission system with two ends, and short-circuit current calculation is not carried out on a flexible direct-current power grid consisting of a multi-end flexible direct-current system. The method has the advantages that the fault characteristics of the direct-current power grid are researched, the change rule of the fault current can be revealed, the fault protection problem can be better solved, the method has great significance for checking the technical indexes of the direct-current circuit breaker and researching the blocking capability of the fault self-clearing capability submodule, and a complete theoretical method is not formed before.
Disclosure of Invention
The invention aims to provide a direct-current power grid short-circuit current calculation method based on a loop current method, which solves the problems in the prior art and fills the industrial blank. The invention provides an equivalent method of a single-ended converter station under two fault conditions of a single-pole grounding fault and a double-pole short-circuit fault of a flexible direct-current power grid before the converter station is locked, and introduces a calculation method of short-circuit current of the direct-current power grid by taking a four-end flexible direct-current power grid as an example. In addition, the calculation method is expanded to an N-end converter station, and finally a set of completed short-circuit current calculation method is provided.
The above object of the present invention is achieved by the following technical solutions:
the direct-current power grid short-circuit current calculation method based on the loop current method comprises the following steps of:
(1) equivalence of single-ended converter stations;
(2) forming an equivalent circuit of the flexible direct current power grid;
(3) selecting a current loop;
(4) modeling and calculating loop current;
(5) and calculating the short-circuit current of the direct current line.
The equivalence of the single-ended converter station in the step (1) comprises the following two equivalent methods of the single-ended converter station of the flexible direct-current power grid with the true bipolar structure under the short-circuit fault:
(1.1) the equivalent method of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct current power grid with the true bipolar structure is to equivalently form the single-end converter station into a mode that a resistor, an inductor and a capacitor are mutually connected in series, wherein the equivalent resistor is 2R0/3 equivalent inductance of 2L0/3, equivalent capacitance of 6C0/N；
(1.2) the equivalent method of the single-end converter station under the condition of the bipolar short-circuit fault of the flexible direct-current power grid with the true bipolar structure is also in the mode that the single-end converter station is equivalent to a resistor, an inductor and a capacitor which are mutually connected in series, and the equivalent resistor is 4R0/3, equivalent inductance of 4L0/3, equivalent capacitance of 3C0/N；
Wherein, 2R0[ 3 ] is the equivalent resistance of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct-current power grid with the true bipolar structure, R0、L0Resistance value, inductance value, C corresponding to bridge arm reactor of single bridge arm of converter station0And N is the number of the sub-modules of a single bridge arm of the converter station under the condition of not considering redundancy.
Forming the equivalent circuit of the flexible direct current power grid in the step (2), wherein the steps are as follows:
(2.1) respectively establishing equivalent circuits of all converter stations according to the main circuit parameters of the converter stations;
(2.2) respectively establishing a line equivalent circuit between each converter station according to the direct current network system topological graph and the line parameters;
and (2.3) connecting the equivalent circuits and the line equivalent circuits of the converter stations with each other according to the topological diagram of the direct current network system to form a direct current network equivalent circuit.
Selecting the current loop in the step (3), namely selecting the following two loops:
(3.1) a loop flowing from each converter station to a short-circuit point in the direct-current power grid equivalent circuit;
(3.2) a loop running through all lines but not through the converter station.
The modeling and calculation of the loop current in the step (4) adopts differential equation modeling, and comprises the following steps:
(4.1) establishing a second-order zero-input state circuit model according to the selected current loop;
(4.2) establishing a second-order differential equation according to the second-order zero-input state circuit model;
and (4.3) solving the established second order differential equation to obtain an expression of the loop current.
The calculation of the short-circuit current of the direct-current line in the step (5) comprises the following steps:
(5.1) aiming at a direct-current line short-circuit current to be solved, referring to selection of a direct-current power grid equivalent circuit and a current loop, taking a positive value for a loop current in the same direction as the direct-current line current reference direction, and taking a negative value for a loop current in the opposite direction to the direct-current line current reference direction;
(5.2) the direct-current line short-circuit current is superposition of loop currents, and the loop currents are added to obtain a relational expression of the direct-current line short-circuit current and the loop currents;
and (5.3) determining the expression of the line short-circuit current according to the expression of the loop current and the relation between the direct-current line current and the loop current.
The invention has the beneficial effects that: the invention provides a direct-current power grid short-circuit current calculation method for the first time. In recent years, the development trend of direct current transmission is to develop a point-to-point direct current transmission line to a direct current power grid, so that in the past, only a direct current line short-circuit current calculation method under a single-end converter station and a double-end converter station exists, multi-end converter station expansion is not performed, and a real direct current power grid is not formed. The direct-current power grid short-circuit current calculation method provided by the invention is beneficial to solving the problem of direct-current power grid fault protection, and has great practical value and significance for direct-current circuit breaker technical index verification and blocking capability research of the fault self-clearing capability submodule.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the invention without limiting the invention.
FIG. 1 is a flow chart of a method of the present invention;
FIG. 2 is a true bipolar block diagram of the converter station of the present invention;
FIG. 3 is a DC side equivalent circuit diagram of the single-phase earth fault single-ended converter station of the present invention;
FIG. 4 is a DC side equivalent circuit diagram of the inter-pole short circuit fault single-ended converter station of the present invention;
FIG. 5 is a schematic diagram of a short circuit fault of the four-terminal true bipolar flexible DC system of the present invention;
FIG. 6 is an equivalent circuit diagram of the DC side short-circuit fault network of the present invention;
FIG. 7 is a schematic diagram of an N-terminal polygonal true bipolar MMC system of the present invention;
fig. 8 is an equivalent circuit diagram of the N-terminal dc power grid dc side short-circuit fault network of the present invention.
Detailed Description
The details of the present invention and its embodiments are further described below with reference to the accompanying drawings.
Referring to fig. 1 to 8, the method for calculating the short-circuit current of the direct-current power grid based on the loop current method of the present invention firstly provides a single-ended converter station equivalent method under two fault conditions of a unipolar ground fault and a bipolar short-circuit fault of a flexible direct-current power grid before the converter station is locked, and establishes a complete flexible direct-current power grid short-circuit fault equivalent circuit by taking a four-terminal flexible direct-current power grid as an example, and provides a selection method of a current loop, completes modeling and calculation of an expression of the loop current, and finally establishes an expression of the short-circuit current of any one direct-current line, and completes the calculation process of the short-circuit current of the whole direct-current power grid. In addition, the calculation method is expanded from a four-end flexible direct-current power grid to an N-end flexible direct-current power grid, and finally a set of completed short-circuit current calculation method is provided. The method comprises the following steps:
(1) equivalence of single-ended converter stations;
(2) forming an equivalent circuit of the flexible direct current power grid;
(3) selecting a current loop;
(4) modeling and calculating loop current;
(5) and calculating the short-circuit current of the direct current line.
The equivalence of the single-ended converter station in the step (1) comprises the following two equivalent methods of the single-ended converter station of the flexible direct-current power grid with the true bipolar structure under the short-circuit fault:
(1.1) the equivalent method of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct current power grid with the true bipolar structure is to equivalently form the single-end converter station into a mode that a resistor, an inductor and a capacitor are mutually connected in series, wherein the equivalent resistor is 2R0/3 equivalent inductance of 2L0/3, equivalent capacitance of 6C0/N；
(1.2) the equivalent method of the single-end converter station under the condition of the bipolar short-circuit fault of the flexible direct-current power grid with the true bipolar structure is also in the mode that the single-end converter station is equivalent to a resistor, an inductor and a capacitor which are mutually connected in series, and the equivalent resistor is 4R0/3, equivalent inductance of 4L0/3, equivalent capacitance of 3C0/N；
Wherein, 2R0[ 3 ] is the equivalent resistance of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct-current power grid with the true bipolar structure, R0、L0Resistance value, inductance value, C corresponding to bridge arm reactor of single bridge arm of converter station0And N is the number of the sub-modules of a single bridge arm of the converter station under the condition of not considering redundancy.
Forming the equivalent circuit of the flexible direct current power grid in the step (2), wherein the steps are as follows:
(2.1) respectively establishing equivalent circuits of all converter stations according to the main circuit parameters of the converter stations;
(2.2) respectively establishing a line equivalent circuit between each converter station according to the direct current network system topological graph and related line parameters;
and (2.3) connecting the equivalent circuits and the line equivalent circuits of the converter stations with each other according to the topological diagram of the direct current network system to form a direct current network equivalent circuit.
Selecting the current loop in the step (3), namely selecting the following two loops:
(3.1) a loop flowing from each converter station to a short-circuit point in the direct-current power grid equivalent circuit;
(3.2) a loop running through all lines but not through the converter station.
The modeling and calculation of the loop current in the step (4) adopts differential equation modeling, and comprises the following steps:
(4.1) establishing a second-order zero-input state circuit model according to the selected current loop;
(4.2) establishing a second-order differential equation according to the second-order zero-input state circuit model;
and (4.3) solving the established second order differential equation to obtain an expression of the loop current.
The calculation of the short-circuit current of the direct-current line in the step (5) comprises the following steps:
(5.1) aiming at a direct-current line short-circuit current to be solved, referring to selection of a direct-current power grid equivalent circuit and a current loop, taking a positive value for a loop current in the same direction as the direct-current line current reference direction, and taking a negative value for a loop current in the opposite direction to the direct-current line current reference direction;
(5.2) the direct-current line short-circuit current is the superposition of loop currents, and the direct-current line short-circuit current and the loop currents can be obtained by adding the loop currents;
and (5.3) determining the expression of the line short-circuit current according to the expression of the loop current and the relation between the direct-current line current and the loop current.
Example (b):
the invention provides a flexible direct-current power grid short-circuit current calculation method, and a converter station equivalent method, a four-end flexible direct-current power grid short-circuit current calculation method and an N-end flexible direct-current power grid short-circuit current calculation method for the flexible direct-current power grid short-circuit calculation are introduced below respectively.
1. Equivalent method of converter station for flexible direct current power grid short circuit calculation
The flexible direct current power grid direct current side short-circuit fault mainly includes a single-pole grounding fault and a double-pole short-circuit fault, and aims at the converter station with a true double-pole structure, wherein the MMC topology with the true double-pole structure is shown in figure 1, and the equivalent circuits of the converter station under the condition of two short-circuit faults before locking are respectively as follows:
(1) single pole ground fault
The positive pole structure and the negative pole structure of the true bipolar MMC are relatively independent, and the other pole is not affected when the single-pole ground fault occurs, so that only the fault pole needs to be analyzed when the single-pole ground fault occurs.
The equivalent circuit of a single converter station in case of a monopolar earth fault at a true bipolar MMC is shown in fig. 2.
(2) Bipolar short circuit fault
The occurrence of a bipolar earth fault in a true bipolar MMC an equivalent circuit of a single converter station is obtained from the short-circuited current circulation loop as shown in fig. 3.
2. Four-terminal flexible direct-current power grid short circuit calculation method
Taking a four-terminal flexible direct-current Dianwang structure as an example to establish and solve an equivalent model, wherein the short-circuit position is shown in FIG. 4, although the short-circuit current circulation loops of the unipolar grounding short-circuit fault and the interelectrode short-circuit fault are different, the equivalent network of the direct-current side short-circuit fault shown in FIG. 5 can be used for calculation, and the short-circuit currents of the two faults are basically the same.
For the current icir1In the loop, a capacitor C is provided1Has an initial voltage value ofInductor L1Initial current ofThe following second order differential equations can be listed:
wherein,therefore, the oscillation discharge process of the loop is solved as follows:
in the formula,
similarly, current icir2The expressions of capacitance voltage and current of the loop are as follows:
in the formula,
current icir3The expressions of capacitance voltage and current of the loop are as follows:
in the formula,
current icir4The expressions of capacitance voltage and current of the loop are as follows:
in the formula,
for the current icir34In the loop, an inductor L is arranged34At an initial value of current I34(0)The following first order differential equations may be listed:
obtaining by solution:
in the formula,
thus, an expression for the short circuit current on the dc line can be obtained:
i10＝icir1+icir3-icir34(2-29)
i20＝icir2+icir4+icir34(2-30)
i31＝icir3-icir34(2-31)
i42＝icir4+icir34(2-32)3. N-end flexible direct-current power grid short circuit calculation method
On the basis of the four-terminal flexible direct current system, the method is expanded to the N terminal, an N-terminal flexible direct current system similar to the Zhang-North four-terminal flexible direct current structure is established, the N-terminal flexible direct current system is numbered from 1 to N in sequence, as shown in FIG. 6, and an equivalent circuit diagram is shown in FIG. 7.
First, the following notations are made: the equivalent inductance of the k-th end converter station is LkEquivalent resistance of RkEquivalent capacitance of CkThe initial value of the voltage of the equivalent capacitor isThe initial value of the equivalent inductance isThe current flowing through the k-th end converter station (i.e. the direct current between the bus bar and the MMC) is ikThe loop current flowing from the k-th end converter station to the short-circuit point is icirkThe loop current flowing through the DC line but not the MMC is icirline. In addition, the following symbol is also requiredDescription of the No.: the line current flowing from the ith end converter station to the jth end converter station is ii,jLine inductance between the ith end converter station stream and the jth end converter station is Li,jAnd the line resistance between the ith end converter station stream and the jth end converter station is Ri,jIn addition, if j is 0, it represents the physical parameter from the i-th end to the short-circuit point.
If a short-circuit fault occurs in a line between the m-th end converter station stream and the m + 1-th end converter station, and m is more than or equal to 1 and less than N, the loop current i is aimed atcirkThe loop is divided into three conditions:
(1) if k is 1. ltoreq. k < m, the following second order differential equations can be listed:
wherein,therefore, the oscillation discharge process of the loop is solved as follows:
in the formula,
(2) if m +1 < k.ltoreq.N, the following second order differential equations can be listed:
wherein,therefore, the oscillation discharge process of the loop is solved as follows:
in the formula,
(3) if k ═ m or m +1, the following second order differential equations can be listed:
wherein,therefore, the oscillation discharge process of the loop is solved as follows:
in the formula,
for the current icirlineIn the loop, an inductor L is arrangedN1 the initial value of the current is IN,1(0) The following first order differential equations may be listed:
obtaining by solution:
in the formula,
thus, an expression for the short-circuit current on different dc lines can be obtained:
(1) if k is more than or equal to 1 and less than m, the short-circuit current ik,k+1The expression of (a) is:
(2) if m +1 is more than k and less than or equal to N, then the short-circuit current ik,k-1The expression of (a) is:
(3) if k is m, short-circuit current ik,0The expression of (a) is:
(4) if k is m +1, the short-circuit current ik,0The expression of (a) is:
the above description is only a preferred example of the present invention and is not intended to limit the present invention, and various modifications and changes may be made by those skilled in the art. Any modification, equivalent replacement, improvement and the like of the present invention shall be included in the protection scope of the present invention.
Claims (6)
1. A direct current power grid short-circuit current calculation method based on a loop current method is characterized by comprising the following steps: the method comprises the following steps:
(1) equivalence of single-ended converter stations;
(2) forming an equivalent circuit of the flexible direct current power grid;
(3) selecting a current loop;
(4) modeling and calculating loop current;
(5) and calculating the short-circuit current of the direct current line.
2. The direct-current power grid short-circuit current calculation method based on the loop current method as claimed in claim 1, wherein: the equivalence of the single-ended converter station in the step (1) comprises the following two equivalent methods of the single-ended converter station of the flexible direct-current power grid with the true bipolar structure under the short-circuit fault:
(1.1) the equivalent method of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct current power grid with the true bipolar structure is to equivalently form the single-end converter station into a mode that a resistor, an inductor and a capacitor are mutually connected in series, wherein the equivalent resistor is 2R0/3 equivalent inductance of 2L0/3, equivalent capacitance of 6C0/N；
(1.2) the equivalent method of the single-end converter station under the condition of the bipolar short-circuit fault of the flexible direct-current power grid with the true bipolar structure is also in the mode that the single-end converter station is equivalent to a resistor, an inductor and a capacitor which are mutually connected in series, and the equivalent resistor is 4R0/3, equivalent inductance of 4L0/3, equivalent capacitance of 3C0/N；
Wherein, 2R0[ 3 ] is the equivalent resistance of the single-end converter station under the condition of the monopole grounding short circuit fault of the flexible direct-current power grid with the true bipolar structure, R0、L0Resistance value, inductance value, C corresponding to bridge arm reactor of single bridge arm of converter station0And N is the number of the sub-modules of a single bridge arm of the converter station under the condition of not considering redundancy.
3. The direct-current power grid short-circuit current calculation method based on the loop current method as claimed in claim 1, wherein: forming the equivalent circuit of the flexible direct current power grid in the step (2), wherein the steps are as follows:
(2.1) respectively establishing equivalent circuits of all converter stations according to the main circuit parameters of the converter stations;
(2.2) respectively establishing a line equivalent circuit between each converter station according to the direct current network system topological graph and the line parameters;
and (2.3) connecting the equivalent circuits and the line equivalent circuits of the converter stations with each other according to the topological diagram of the direct current network system to form a direct current network equivalent circuit.
4. The direct-current power grid short-circuit current calculation method based on the loop current method as claimed in claim 1, wherein: selecting the current loop in the step (3), namely selecting the following two loops:
(3.1) a loop flowing from each converter station to a short-circuit point in the direct-current power grid equivalent circuit;
(3.2) a loop running through all lines but not through the converter station.
5. The direct-current power grid short-circuit current calculation method based on the loop current method as claimed in claim 1, wherein: the modeling and calculation of the loop current in the step (4) adopts differential equation modeling, and comprises the following steps:
(4.1) establishing a second-order zero-input state circuit model according to the selected current loop;
(4.2) establishing a second-order differential equation according to the second-order zero-input state circuit model;
and (4.3) solving the established second order differential equation to obtain an expression of the loop current.
6. The direct-current power grid short-circuit current calculation method based on the loop current method as claimed in claim 1, wherein: the calculation of the short-circuit current of the direct-current line in the step (5) comprises the following steps:
(5.1) aiming at a direct-current line short-circuit current to be solved, referring to selection of a direct-current power grid equivalent circuit and a current loop, taking a positive value for a loop current in the same direction as the direct-current line current reference direction, and taking a negative value for a loop current in the opposite direction to the direct-current line current reference direction;
(5.2) the direct-current line short-circuit current is superposition of loop currents, and the loop currents are added to obtain a relational expression of the direct-current line short-circuit current and the loop currents;
and (5.3) determining the expression of the line short-circuit current according to the expression of the loop current and the relation between the direct-current line current and the loop current.
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| CN111830431A (en) * | 2020-06-23 | 2020-10-27 | 中国电力科学研究院有限公司 | Method and system for judging fault of true bipolar flexible direct current transmission system |
| CN112505473A (en) * | 2020-10-21 | 2021-03-16 | 北京交通大学 | Analytic calculation method for bipolar short-circuit fault transient current of flexible direct-current power grid |
| CN112684290A (en) * | 2020-12-16 | 2021-04-20 | 华北电力大学 | Efficient calculation method for interelectrode short-circuit fault current of flexible direct-current power grid |
| CN113361886A (en) * | 2021-05-28 | 2021-09-07 | 西安交通大学 | Quantitative evaluation method for interelectrode short-circuit current on direct current side of flexible direct-current power grid |
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| CN113361886A (en) * | 2021-05-28 | 2021-09-07 | 西安交通大学 | Quantitative evaluation method for interelectrode short-circuit current on direct current side of flexible direct-current power grid |
| CN113361886B (en) * | 2021-05-28 | 2023-12-19 | 西安交通大学 | Quantitative evaluation method for interelectrode short-circuit current of direct-current side of flexible direct-current power grid |
| CN113394758A (en) * | 2021-08-18 | 2021-09-14 | 湖北东湖实验室 | Short-circuit current engineering calculation method for ship power system based on DC/DC converter power supply |
| CN114417241A (en) * | 2021-12-15 | 2022-04-29 | 北京四方继保工程技术有限公司 | Single-pole grounding short circuit calculation method and system for offshore wind power flexible-straight system |
| CN114417241B (en) * | 2021-12-15 | 2024-05-10 | 北京四方继保工程技术有限公司 | Monopole grounding short circuit calculation method and system of offshore wind power flexible-direct system |
| CN114336548A (en) * | 2021-12-31 | 2022-04-12 | 苏州汇川控制技术有限公司 | Short-circuit parameter determination method, short-circuit parameter determination equipment, storage medium and short-circuit protection method |
| CN114336548B (en) * | 2021-12-31 | 2024-05-24 | 苏州汇川控制技术有限公司 | Short-circuit parameter determining method, short-circuit parameter determining device, storage medium and short-circuit protection method |
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