A method and system for calculating bipolar short-circuit fault current of a multi-terminal direct current power distribution network

Abstract

The application provides a multi-terminal direct-current power distribution network bipolar short-circuit fault current calculation method and system, comprising obtaining a multi-terminal direct-current power distribution network topology, converter station parameters, line parameters and a short-circuit fault position; determining each short-circuit current component at the short-circuit fault position before and after multi-converter station locking respectively, and calculating the short-circuit fault current before and after multi-converter station locking by using the superposition principle based on the multi-terminal direct-current power distribution network topology, converter station parameters and line parameters. The application considers the fault development process before and after multi-converter locking, accurately calculates the multi-terminal direct-current power distribution network bipolar short-circuit current by using the superposition principle, and realizes complete short-circuit fault current calculation.

Description

Technical Field

[0001] This invention belongs to the field of DC distribution network technology, specifically relating to a method and system for calculating bipolar short-circuit fault current in multi-terminal DC distribution networks. Background Technology

[0002] Compared to AC distribution networks, DC distribution networks enable efficient and flexible integration of distributed renewable energy sources, DC loads, and frequency converter loads, significantly improving the flexibility of distribution-side operation and control. Compared to AC grid faults, DC distribution networks experience lower damping and faster fault current rise. Furthermore, the absence of a natural zero-crossing point in DC networks places higher demands on protection technologies.

[0003] When a bipolar short-circuit fault occurs in a DC distribution network, the fault current is affected by various factors, including the type of DC distribution network equipment, the topology of the converter equipment, the location of the fault, and the time stage. To clarify the parameter requirements of the main equipment in the DC distribution network and provide crucial support for fault current suppression, it is essential to determine the spatiotemporal distribution of the fault current. Currently, fault current analysis in DC distribution networks is mostly limited to individual devices, failing to fully consider the impact of different converter equipment in multi-terminal DC distribution networks on the fault current. Summary of the Invention

[0004] In view of this, the present invention aims to solve the problem that the fault current analysis of existing DC distribution networks is mostly limited to a single device and does not fully consider the impact of different converter devices on fault current in multi-terminal DC distribution networks.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] This invention provides a method for calculating bipolar short-circuit fault current in multi-terminal DC distribution networks, applicable to any multi-terminal DC distribution network including full-bridge MMC converter stations, half-bridge MMC converter stations, and DC transformers, comprising the following steps:

[0007] Obtain multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault locations;

[0008] Before the blocking of multiple converter stations, the components of each short-circuit current at the short-circuit fault location are determined. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the superposition principle is used to calculate the short-circuit fault current before the blocking of multiple converter stations.

[0009] Before the multiple converter stations are blocked, the components of the short-circuit current at the short-circuit fault location are determined. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the superposition principle is used to calculate the short-circuit fault current after the multiple converter stations are blocked.

[0010] Furthermore, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the short-circuit fault current i before the multiple converter stations are blocked is... fault1_DC The calculation formula is as follows:

[0011]

[0012] In the formula, These are the submodule capacitor discharge current of the i-th MMC converter station or DC transformer, the submodule capacitor discharge current on the high-voltage side of the DC transformer, and the AC feed-in current through the MMC converter station, respectively.

[0013] Furthermore, before the multiple converter stations are locked out, the calculation formulas for each short-circuit current component are as follows:

[0014] Where τ1 is the decay time constant of the MMC capacitor discharge current, U dc The voltage before the fault is represented by C0, where C0 is the capacitance value of the submodule, n is the number of bridge arm submodules, and L is the DC voltage before the fault. bridge ω is the inductance value of the bridge arm reactor, I0 is the angular frequency of the discharge current oscillation, and I0 is the DC output current of the MMC instant before the fault.

[0015] Where τ2 is the decay time constant of the DC transformer capacitor discharge current, U dc The DC voltage before the fault, ω d L is the oscillation angular frequency of the capacitor discharge current of the DC transformer. s This is the equivalent inductance value of the discharge circuit;

[0016] Where L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the transformer is connected, i0 is the DC current output by the DC transformer just before the fault, and t0 is the time when the fault occurs.

[0017] Furthermore, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the short-circuit fault current i after multiple converter stations are blocked is... fault2_DC The calculation formula is as follows:

[0018]

[0019] In the formula, m′1 represents the number of half-bridge submodules in m1 MMC converter stations connected to the upstream AC distribution network, and m′2 represents the number of half-bridge submodules in m2 MMC converter stations connected to AC loads. These are the feed-in current through the half-bridge MMC converter station, the MMC arm freewheeling current, and the DC transformer freewheeling current, respectively, for the i-th half-bridge MMC converter station or the AC side of the DC transformer.

[0020] Furthermore, after multiple converter stations are locked out, the calculation formulas for each short-circuit current component are as follows:

[0021] Where L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the connection transformer is given, i0 is the DC current output by the DC transformer instant before the fault, and t0 is the time when the fault occurs.

[0022] Where I 0_MM For the bridge arm current at the moment of converter station blocking, τ3 = L bridge / R L L bridge For the bridge arm inductance, R L This represents the resistance value of the discharge circuit.

[0023] Where I 0_DCT The short-circuit current of the DC transformer at the moment of converter station blocking is τ4=L T / R L L T R is the inductance value of the high-frequency transformer. L This represents the resistance value of the discharge circuit.

[0024] Secondly, the present invention provides a bipolar short-circuit fault current calculation system for multi-terminal DC distribution networks, applicable to any multi-terminal DC distribution network including full-bridge MMC converter stations, half-bridge MMC converter stations, and DC transformers, comprising:

[0025] The parameter acquisition unit is used to acquire the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault locations.

[0026] The first calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the unit uses the superposition principle to calculate the short-circuit fault current before the multiple converter stations are blocked.

[0027] The second calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, it calculates the short-circuit fault current after the multiple converter stations are blocked using the superposition principle.

[0028] Furthermore, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the first calculation unit calculates the short-circuit fault current i before the multi-converter station is blocked according to the following formula. fault1_DC :

[0029]

[0030] In the formula, These are the submodule capacitor discharge current of the i-th MMC converter station or DC transformer, the submodule capacitor discharge current on the high-voltage side of the DC transformer, and the AC feed-in current through the MMC converter station, respectively.

[0031] Furthermore, the first calculation unit specifically includes: a first component calculation unit, a second component calculation unit, and a third component calculation unit;

[0032] The first component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, τ1 is the time constant of the discharge current decay of the MMC capacitor, U dc The voltage before the fault is represented by C0, where C0 is the capacitance value of the submodule, n is the number of bridge arm submodules, and L is the DC voltage before the fault. bridge ω is the inductance value of the bridge arm reactor, I0 is the angular frequency of the discharge current oscillation, and I0 is the DC output current of the MMC instant before the fault.

[0033] The second component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, τ2 is the decay time constant of the DC transformer capacitor discharge current, U dc The DC voltage before the fault, ω d L is the oscillation angular frequency of the capacitor discharge current of the DC transformer. s This is the equivalent inductance value of the discharge circuit;

[0034] The third component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the transformer is connected, i0 is the DC current output by the DC transformer just before the fault, and t0 is the time when the fault occurs.

[0035] Furthermore, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the second calculation unit calculates the short-circuit fault current i after the multi-converter station is blocked according to the following formula. fault2_DC :

[0036]

[0037] In the formula, m′1 represents the number of half-bridge submodules in m1 MMC converter stations connected to the upstream AC distribution network, and m′2 represents the number of half-bridge submodules in m2 MMC converter stations connected to AC loads. These are the feed-in current through the half-bridge MMC converter station, the MMC arm freewheeling current, and the DC transformer freewheeling current, respectively, for the i-th half-bridge MMC converter station or the AC side of the DC transformer.

[0038] Furthermore, the second calculation unit specifically includes: a fourth component calculation unit, a fifth component calculation unit, and a sixth component calculation unit;

[0039] The fourth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula, L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the connection transformer is given, i0 is the DC current output by the DC transformer instant before the fault, and t0 is the time when the fault occurs.

[0040] The fifth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula I 0_MMC For the bridge arm current at the moment of converter station blocking, τ3 = L bridge / R L L bridge For the bridge arm inductance, R L This represents the resistance value of the discharge circuit.

[0041] The sixth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula I 0_DCT The short-circuit current of the DC transformer at the moment of converter station blocking is τ4=L T / R L L T R is the inductance value of the high-frequency transformer. L This represents the resistance value of the discharge circuit.

[0042] In summary, this invention provides a method and system for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network. The method includes acquiring the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault location; determining the various short-circuit current components at the short-circuit fault location before and after multiple converter station blocking; and calculating the short-circuit fault current before and after multiple converter station blocking based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters using the superposition principle. This invention considers the fault development process before and after multiple converter blocking, accurately calculates the bipolar short-circuit current in the multi-terminal DC distribution network using the superposition principle, and achieves complete short-circuit fault current calculation. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 A flowchart illustrating a method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network, provided by an embodiment of the present invention;

[0045] Figure 2 This is a schematic diagram of a typical multi-terminal DC distribution network provided in an embodiment of the present invention;

[0046] Figure 3 A flowchart for calculating short-circuit fault current provided in an embodiment of the present invention;

[0047] Figure 4 This is a structural block diagram of a bipolar short-circuit fault current calculation system for a multi-terminal DC distribution network provided in an embodiment of the present invention. Detailed Implementation

[0048] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0049] Compared to AC distribution networks, DC distribution networks enable efficient and flexible integration of distributed renewable energy sources, DC loads, and frequency converter loads, significantly improving the flexibility of distribution-side operation and control. Compared to AC grid faults, DC distribution networks experience lower damping and faster fault current rise. Furthermore, the absence of a natural zero-crossing point in DC networks places higher demands on protection technologies.

[0050] When a bipolar short-circuit fault occurs in a DC distribution network, the fault current is affected by various factors, including the type of DC distribution network equipment, the topology of the converter equipment, the location of the fault, and the time stage. To clarify the parameter requirements of the main equipment in the DC distribution network and provide crucial support for fault current suppression, it is essential to determine the spatiotemporal distribution of the fault current. Currently, fault current analysis in DC distribution networks is mostly limited to individual devices, failing to fully consider the impact of different converter equipment in multi-terminal DC distribution networks on the fault current.

[0051] Existing methods primarily study the transient processes of a single converter station during DC faults, neglecting the coupling effects between different converter stations, making it difficult to accurately calculate fault currents in multi-terminal DC distribution networks. Therefore, this invention proposes a method and system for calculating bipolar short-circuit fault currents in multi-terminal DC distribution networks, capable of calculating fault currents considering multiple development stages and the influence of multiple converter devices.

[0052] The following is a detailed description of an embodiment of the method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network according to the present invention.

[0053] Please see Figure 1 This embodiment provides a method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network, applicable to any multi-terminal DC distribution network including full-bridge MMC converter stations, half-bridge MMC converter stations, and DC transformers, and includes the following steps:

[0054] S100: Obtain multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault locations.

[0055] To perform short-circuit fault current calculations before and after the blocking of the converter station in a DC distribution network, it is necessary to obtain the basic parameters required for the calculation, including the DC distribution network topology, converter station parameters (full-bridge MMC / half-bridge MMC), line parameters, and short-circuit fault location. The parameters to be determined include the DC voltage before the fault, the number of bridge arm sub-modules, the capacitance value of the MMC sub-modules, the inductance value of the bridge arm reactor, and the inductance value of the connecting transformer.

[0056] The DC distribution network topology parameters represent the number of half-bridge MMCs, full-bridge MMCs, and DC transformer stations and their interconnection relationships. According to the Thevenin equivalent circuit principle, its circuit parameters are related to the circuit topology and the location of the short-circuit fault point, and directly affect the short-circuit current value at the fault point.

[0057] S200: Determine the short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters, calculate the short-circuit fault current before the multiple converter stations are blocked using the superposition principle.

[0058] Before the converter station is locked out, short-circuit current is fed into the short-circuit point from each node. The short-circuit current includes the discharge current i of the MMC submodule capacitor. c_MMC DC transformer high-voltage side submodule capacitor discharge current i c_DCT AC side feed-in current i via MMC converter station ac_feed .

[0059] For a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the following calculation formula is proposed to calculate the fault current before the multi-converter station is blocked:

[0060]

[0061] in These are the submodule capacitor discharge current of the i-th MMC converter station (or DC transformer), the submodule capacitor discharge current on the high-voltage side of the DC transformer, and the AC feed-in current through the MMC converter station, respectively.

[0062] The above calculation formula reflects the contribution of each converter station or DC transformer in any multi-terminal DC distribution network to the short-circuit fault current. The short-circuit current before the blocking of multiple converters can be calculated by using the superposition principle.

[0063] The following uses Figure 2 The calculation of each component in the above fault current calculation formula is explained using a typical multi-terminal DC distribution network structure.

[0064] For example Figure 2 The diagram shows a typical multi-terminal DC distribution network. Node 1 is a full-bridge MMC (Modular Multilevel Converter) converter station node, Node 2 is a DC transformer node, and Node 3 is a half-bridge MMC converter station node. The short-circuit fault in the diagram occurs on the DC line between Node 1 and Node 2, and the short-circuit fault type is an inter-pole fault on the DC line. The calculation formulas for each current component are as follows:

[0065] Where τ1 is the decay time constant of the MMC capacitor discharge current, U dc The voltage before the fault is represented by C0, where C0 is the capacitance value of the submodule, n is the number of bridge arm submodules, and L is the DC voltage before the fault. bridge ω is the inductance value of the bridge arm reactor, I0 is the angular frequency of the discharge current oscillation, and I0 is the DC output current of the MMC at the instant before the fault.

[0066] Where τ2 is the decay time constant of the DC transformer capacitor discharge current, U dc The DC voltage before the fault, ω d L is the oscillation angular frequency of the capacitor discharge current of the DC transformer. s This is the equivalent inductance value of the discharge circuit.

[0067] Where L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the transformer is connected, i0 is the DC current output by the DC transformer just before the fault, and t0 is the time when the fault occurs.

[0068] Among them, targeting Figure 2 The typical multi-terminal DC distribution network structure shown has a short-circuit fault current i before the blocking of multiple converter stations. fault1_DC =i c_MMC +i c_DCT +i ac_feed .

[0069] By combining the above formulas for calculating the current components with the formulas for calculating the current components using the superposition principle, the short-circuit fault current before the blocking of multiple converter stations can be calculated.

[0070] S300: Determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters, calculate the short-circuit fault current after the multiple converter stations are blocked using the superposition principle.

[0071] After the converter station is locked out, the submodule capacitor discharge stops. The full-bridge MMC can isolate DC faults after being locked out. However, the half-bridge MMC and DC transformer cannot achieve DC fault isolation through the locked-out converter station. Therefore, the short-circuit current includes the AC side current fed into the converter station via the half-bridge MMC. ac_HF MMC bridge arm freewheeling current i MMC_bridge and DC transformer freewheeling current i DCT .

[0072] For a multi-terminal DC distribution network with m1 MMC converter stations connected to the upstream AC distribution network (where the number of half-bridge submodules is m′1), m2 MMC converter stations connected to AC loads (where the number of half-bridge submodules is m′2), and n DC transformers, the fault current before the multi-converter station blocking is:

[0073]

[0074] in These are the AC side feed-in current of the i-th half-bridge MMC converter station (or DC transformer) via the half-bridge MMC converter station, the MMC arm freewheeling current, and the DC transformer freewheeling current, respectively.

[0075] Also based on Figure 2 The calculation of various current components of the short-circuit fault current after multiple converter stations are blocked in the typical multi-terminal DC distribution network structure shown is explained below:

[0076] Where L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the transformer is connected, i0 is the DC current output by the DC transformer just before the fault, and t0 is the time when the fault occurs.

[0077] Where I 0_MMC For the bridge arm current at the moment of converter station blocking, τ3 = L bridge / R L L bridge For the bridge arm inductance, R L This represents the resistance value of the discharge circuit.

[0078] Where I 0_DCT The short-circuit current of the DC transformer at the moment of converter station blocking is τ4=L T / R L L T R is the inductance value of the high-frequency transformer. L This represents the resistance value of the discharge circuit.

[0079] Among them, targeting Figure 2 The typical multi-terminal DC distribution network structure shown has a short-circuit fault current of i after multiple converter stations are blocked. fault2_DC =i ac_HF +i MMC_bridge +i DCT .

[0080] It is understandable that the calculation of each short-circuit current component before and after converter station blocking in the above typical multi-terminal DC distribution network structure is applicable to the calculation of short-circuit fault current in any multi-terminal DC distribution network. Based on the topology of the multi-terminal DC distribution network, combined with the calculation formulas for each current component and the general calculation formula using the superposition principle, the short-circuit fault current of any multi-terminal DC distribution network can be accurately calculated.

[0081] Figure 3 The flowchart for calculating the short-circuit fault current obtained according to the above superposition principle is as follows: the calculation of the short-circuit fault current before blocking includes calculating the discharge current of the MMC submodule capacitor, calculating the discharge current of the DC transformer high-voltage side submodule capacitor, and calculating the AC side feed-in current through the MMC converter station; the calculation of the short-circuit fault current after blocking includes calculating the AC side feed-in current through the MMC converter station, calculating the MMC bridge arm reactor freewheeling current, and calculating the DC transformer freewheeling current.

[0082] This embodiment provides a method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network. The method includes acquiring the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault location; determining the various short-circuit current components at the short-circuit fault location before and after multiple converter station blocking; and calculating the short-circuit fault current before and after multiple converter station blocking based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters using the superposition principle. This invention considers the fault development process before and after multiple converter blocking, accurately calculates the bipolar short-circuit current in the multi-terminal DC distribution network using the superposition principle, and achieves complete short-circuit fault current calculation.

[0083] The above is a detailed description of an embodiment of the bipolar short-circuit fault current calculation method for a multi-terminal DC distribution network according to the present invention. The following will provide a detailed description of an embodiment of the bipolar short-circuit fault current calculation system for a multi-terminal DC distribution network according to the present invention.

[0084] Please see Figure 4 This embodiment provides a bipolar short-circuit fault current calculation system for a multi-terminal DC distribution network, including a parameter acquisition unit, a first calculation unit, and a second calculation unit.

[0085] In this embodiment, the parameter acquisition unit is used to acquire the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault location.

[0086] In this embodiment, the first calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the unit calculates the short-circuit fault current before the multiple converter stations are blocked using the superposition principle.

[0087] Specifically, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the first calculation unit calculates the short-circuit fault current i before the multiple converter stations are blocked according to the following formula. fault1_DC :

[0088]

[0089] In the formula, These are the submodule capacitor discharge current of the i-th MMC converter station or DC transformer, the submodule capacitor discharge current on the high-voltage side of the DC transformer, and the AC feed-in current through the MMC converter station, respectively.

[0090] In addition, the first calculation unit specifically includes a first component calculation unit, a second component calculation unit, and a third component calculation unit.

[0091] The first component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, τ1 is the time constant of the discharge current decay of the MMC capacitor, U dc The voltage before the fault is represented by C0, where C0 is the capacitance value of the submodule, n is the number of bridge arm submodules, and L is the DC voltage before the fault. bridge ω is the inductance value of the bridge arm reactor, I0 is the angular frequency of the discharge current oscillation, and I0 is the DC output current of the MMC instant before the fault.

[0092] The second component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, τ2 is the decay time constant of the DC transformer capacitor discharge current, U dc The DC voltage before the fault, ω d L is the oscillation angular frequency of the capacitor discharge current of the DC transformer. s This is the equivalent inductance value of the discharge circuit;

[0093] The third component calculation unit is used to calculate the short-circuit current component before blocking. in In the formula, L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the transformer is connected, i0 is the DC current output by the DC transformer just before the fault, and t0 is the time when the fault occurs.

[0094] In this embodiment, the second calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the unit calculates the short-circuit fault current after the multiple converter stations are blocked using the superposition principle.

[0095] Specifically, for a multi-terminal DC distribution network containing m1 MMC converter stations connected to the upstream AC distribution network, m2 MMC converter stations connected to AC loads, and n DC transformers, the second calculation unit calculates the short-circuit fault current i after the multi-converter station is blocked according to the following calculation formula. fault2_DC :

[0096]

[0097] In the formula, m′1 represents the number of half-bridge submodules in m1 MMC converter stations connected to the upstream AC distribution network, and m′2 represents the number of half-bridge submodules in m2 MMC converter stations connected to AC loads. These are the feed-in current through the half-bridge MMC converter station, the MMC arm freewheeling current, and the DC transformer freewheeling current, respectively, for the i-th half-bridge MMC converter station or the AC side of the DC transformer.

[0098] Furthermore, the second calculation unit specifically includes: a fourth component calculation unit, a fifth component calculation unit, and a sixth component calculation unit;

[0099] The fourth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula, L0 is the inductance value of the connecting transformer, u φ To connect the transformer grid-side voltage, u c The voltage on the valve side of the connection transformer is given, i0 is the DC current output by the DC transformer instant before the fault, and t0 is the time when the fault occurs.

[0100] The fifth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula I 0_MMC For the bridge arm current at the moment of converter station blocking, τ3 = L bridge / R L L bridge For the bridge arm inductance, R L This represents the resistance value of the discharge circuit.

[0101] The sixth component calculation unit is used to calculate the short-circuit current component after blocking. in In the formula I 0_DCT The short-circuit current of the DC transformer at the moment of converter station blocking is τ4=L T / R L L T R is the inductance value of the high-frequency transformer. L This represents the resistance value of the discharge circuit.

[0102] It should be noted that the multi-terminal DC distribution network bipolar short-circuit fault current calculation system provided in this embodiment is used to implement the multi-terminal DC distribution network bipolar short-circuit fault current calculation method provided in the aforementioned embodiment. The specific settings of each unit are based on the complete implementation of the method, and will not be repeated here.

[0103] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network, characterized in that, This method is applicable to any multi-terminal DC distribution network that includes full-bridge MMC converter stations, half-bridge MMC converter stations, and DC transformers, and includes the following steps: Obtain the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault location; Before the multiple converter stations are blocked, the short-circuit current components at the short-circuit fault location are determined. Based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters, the short-circuit fault current before the multiple converter stations are blocked is calculated using the superposition principle. Before the multiple converter stations are blocked, the short-circuit current components at the short-circuit fault location are determined. Based on the multi-terminal DC distribution network topology, converter station parameters, and line parameters, the short-circuit fault current after the multiple converter stations are blocked is calculated using the superposition principle. For containing An MMC converter station connected to the upstream AC distribution network. An MMC converter station connected to an AC load. The short-circuit fault current of a multi-terminal DC distribution network with a DC transformer before the blocking of the multiple converter stations. The calculation formula is as follows: ； In the formula, , , The first The discharge current of the submodule capacitors of an MMC converter station or DC transformer, the discharge current of the submodule capacitors on the high-voltage side of the DC transformer, and the AC feed current through the MMC converter station are measured.

2. The method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network according to claim 1, characterized in that, Before the multiple converter stations are locked out, the calculation formulas for each short-circuit current component are as follows: ,in The discharge current decay time constant of the MMC capacitor is... This is the DC voltage before the fault. This refers to the capacitance value of the submodule. The number of bridge arm sub-modules, The inductance value of the bridge arm reactor. The angular frequency of the discharge current oscillation. The DC current output by the MMC at the instant before the fault; ,in This is the decay time constant of the capacitor discharge current of the DC transformer. This is the DC voltage before the fault. This refers to the oscillation frequency of the discharge current of the DC transformer capacitor. This is the equivalent inductance value of the discharge circuit; ,in To connect the transformer inductance value, To connect the transformer grid-side voltage, To connect the transformer valve side voltage, The DC current output by the DC transformer at the instant before the fault. The time when the fault occurred.

3. The method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network according to claim 1, characterized in that, For containing An MMC converter station connected to the upstream AC distribution network. An MMC converter station connected to an AC load. A multi-terminal DC distribution network with one DC transformer, short-circuit fault current after the blocking of the multiple converter stations. The calculation formula is as follows: ； In the formula, for The number of half-bridge sub-modules in an MMC converter station connected to the upstream AC distribution network for The number of half-bridge submodules in an MMC converter station connected to an AC load , , The first The AC side of the half-bridge MMC converter station or DC transformer receives the feed current from the half-bridge MMC converter station, the freewheeling current of the MMC bridge arm, and the freewheeling current of the DC transformer.

4. The method for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network according to claim 3, characterized in that, After multiple converter stations are locked out, the calculation formulas for each short-circuit current component are as follows: ,in To connect the transformer inductance value, To connect the transformer grid-side voltage, To connect the transformer valve side voltage, The DC current output by the DC transformer at the instant before the fault. The time when the fault occurred; ,in The bridge arm current at the moment of converter station blocking. , For bridge arm inductance, This represents the resistance value of the discharge circuit. ,in This refers to the short-circuit current of the DC transformer at the moment the converter station is locked out. , This is the inductance value of the high-frequency transformer. This represents the resistance value of the discharge circuit.

5. A system for calculating bipolar short-circuit fault current in a multi-terminal DC distribution network, characterized in that, Applicable to any multi-terminal DC distribution network including full-bridge MMC converter stations, half-bridge MMC converter stations, and DC transformers, including: The parameter acquisition unit is used to acquire the multi-terminal DC distribution network topology, converter station parameters, line parameters, and short-circuit fault location. The first calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked. Based on the multi-terminal DC distribution network topology, converter station parameters and line parameters, the unit calculates the short-circuit fault current before the multiple converter stations are blocked using the superposition principle. The second calculation unit is used to determine the various short-circuit current components at the short-circuit fault location before the multiple converter stations are blocked, and to calculate the short-circuit fault current after the multiple converter stations are blocked based on the multi-terminal DC distribution network topology, converter station parameters and line parameters using the superposition principle. For containing An MMC converter station connected to the upstream AC distribution network. An MMC converter station connected to an AC load. In a multi-terminal DC distribution network with multiple DC transformers, the first calculation unit calculates the short-circuit fault current before the blocking of multiple converter stations according to the following formula. : ； In the formula, , , The first The discharge current of the submodule capacitors of an MMC converter station or DC transformer, the discharge current of the submodule capacitors on the high-voltage side of the DC transformer, and the AC feed current through the MMC converter station are measured.

6. The multi-terminal DC distribution network bipolar short-circuit fault current calculation system according to claim 5, characterized in that, The first calculation unit specifically includes: a first component calculation unit, a second component calculation unit, and a third component calculation unit; The first component calculation unit is used to calculate the short-circuit current component before blocking. ,in In the formula The discharge current decay time constant of the MMC capacitor is... This is the DC voltage before the fault. This refers to the capacitance value of the submodule. The number of bridge arm sub-modules, The inductance value of the bridge arm reactor. The angular frequency of the discharge current oscillation. The DC current output by the MMC at the instant before the fault; The second component calculation unit is used to calculate the short-circuit current component before blocking. ,in In the formula This is the decay time constant of the capacitor discharge current of the DC transformer. This is the DC voltage before the fault. This refers to the oscillation frequency of the discharge current of the DC transformer capacitor. This is the equivalent inductance value of the discharge circuit; The third component calculation unit is used to calculate the short-circuit current component before blocking. ,in In the formula To connect the transformer inductance value, To connect the transformer grid-side voltage, To connect the transformer valve side voltage, The DC current output by the DC transformer at the instant before the fault. The time when the fault occurred.

7. The multi-terminal DC distribution network bipolar short-circuit fault current calculation system according to claim 5, characterized in that, For containing An MMC converter station connected to the upstream AC distribution network. An MMC converter station connected to an AC load. In a multi-terminal DC distribution network with multiple DC transformers, the second calculation unit calculates the short-circuit fault current after multiple converter stations are blocked according to the following formula. : ； In the formula, for The number of half-bridge sub-modules in an MMC converter station connected to the upstream AC distribution network for The number of half-bridge submodules in an MMC converter station connected to an AC load , , The first The AC side of the half-bridge MMC converter station or DC transformer receives the feed current from the half-bridge MMC converter station, the freewheeling current of the MMC bridge arm, and the freewheeling current of the DC transformer.

8. The multi-terminal DC distribution network bipolar short-circuit fault current calculation system according to claim 7, characterized in that, The second calculation unit specifically includes: a fourth component calculation unit, a fifth component calculation unit, and a sixth component calculation unit; The fourth component calculation unit is used to calculate the short-circuit current component after blocking. ,in In the formula To connect the transformer inductance value, To connect the transformer grid-side voltage, To connect the transformer valve side voltage, The DC current output by the DC transformer at the instant before the fault. The time when the fault occurred; The fifth component calculation unit is used to calculate the short-circuit current component after blocking. ,in In the formula The bridge arm current at the moment of converter station blocking. , For bridge arm inductance, This represents the resistance value of the discharge circuit. The sixth component calculation unit is used to calculate the short-circuit current component after blocking. ,in In the formula This refers to the short-circuit current of the DC transformer at the moment the converter station is locked out. , This is the inductance value of the high-frequency transformer. This represents the resistance value of the discharge circuit.

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