Grid-connected grounding topological structure of flexible converter device without connection transformer
By using a topology consisting of a remote three-winding power transformer, a Z-type grounding transformer, and small resistors, the problems of difficult grounding fault handling and weak zero-sequence current suppression capability of the flexible converter without connection transformer in grid-connected operation are solved. This achieves fast and reliable fault handling and current suppression, improving the safety and stability of the system.
Patent Information
- Application Number
- CN202511917079.9
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-18
- Publication Date
- 2026-05-15
AI Technical Summary
When a flexible converter without a connecting transformer is running in grid-connected mode, the lack of electrical isolation and grounding buffering provided by a traditional power frequency transformer leads to difficulties in handling system grounding faults and weak zero-sequence current suppression capabilities.
The topology consists of a remote three-winding power transformer, a Z-type grounding transformer, and small resistors. By connecting a low-impedance grounding branch in parallel between the AC side of the flexible converter and the ground, and combining it with the precise deployment of three-phase current transformers, a highly reliable zero-sequence current discharge channel is formed, enabling fast and reliable fault handling.
It effectively suppresses the penetration of DC bias current into the AC power grid, ensures the rapid and reliable tripping of AC circuit breakers, limits the peak fault current, and improves system safety and stability.
Smart Images
Figure CN122052497A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of power system technology, and in particular to a grid-connected grounding topology for a flexible converter device without a connecting transformer. Background Technology
[0002] With the rapid development of renewable energy grid connection and DC transmission technology, flexible DC converter devices have become core equipment for building new power systems. Traditional flexible converter devices generally use power frequency transformers to electrically connect the AC system and the converter valves. This technical solution has several significant drawbacks. First, there are size and cost bottlenecks; power frequency transformers occupy approximately 30% of the converter station's floor space, a problem particularly acute in space-constrained scenarios such as offshore wind power platforms. Second, operating losses are substantial. When using high-frequency modulation strategies (such as near-nearest-level (NLM) technology), eddy current losses in the transformer core and skin effect in the windings increase significantly. Experimental data shows that when the switching frequency exceeds 2kHz, transformer losses can account for 15%-18% of the total system losses. Transformerless flexible DC transmission technology is a product driven by advancements in power electronics technology and the demands of new energy systems. Its core lies in achieving system simplification, cost reduction, and efficiency improvement through highly reliable devices, advanced topologies, and control strategies. With the further development of semiconductor technology and DC grid architecture, this technology is expected to play an important role in offshore wind power transmission, urban power grid expansion, DC grid interconnection, and island power supply and renewable energy grid integration. Summary of the Invention
[0003] In view of the aforementioned existing problems, the present invention is proposed.
[0004] Therefore, the present invention provides a grid-connected grounding topology for a flexible converter device without a connecting transformer, which can solve the technical problems of difficult system grounding fault handling and weak zero-sequence current suppression capability caused by the lack of electrical isolation and grounding buffering effect of traditional power frequency transformers when the flexible converter device without a connecting transformer is running in grid connection.
[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: In a first aspect, the present invention provides a grid-connected grounding topology for a flexible converter device without a connecting transformer, comprising: The flexible converter unit body, Z-type grounding transformer, small resistor and remote three-winding power transformer; The flexible converter unit body includes an upper bridge arm and a lower bridge arm. A positive DC bus and a negative DC bus are led out between the upper bridge arm and the lower bridge arm. The midpoints of the upper bridge arm and the lower bridge arm are respectively used as the AC side A phase, B phase and C phase terminals. The Z-type grounding transformer includes three primary windings and one secondary neutral point lead-out terminal. The first ends of the three primary windings are respectively connected to the AC A-phase, B-phase, and C-phase terminals of the flexible converter body. The second ends of the three primary windings are connected to each other to form a zigzag winding structure. The secondary neutral point lead-out terminal is connected to the ground after being connected in series with a small resistor. The remote three-winding power transformer is located on the AC grid side. The three terminals of its third winding are connected to the AC phase A, B, and C terminals of the flexible converter body through AC transmission lines. The third winding adopts a star connection, while the first or second winding adopts a delta connection.
[0006] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, the upper and lower bridge arms of the flexible converter device body are both composed of multiple cascaded half-bridge sub-modules, and each half-bridge sub-module includes two power electronic switching devices connected in series and a DC capacitor connected in parallel.
[0007] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, wherein: a bridge arm reactor is connected in series between the upper and lower bridge arms of the flexible converter device body, one end of the bridge arm reactor is connected to the output terminal of the upper bridge arm, and the other end is connected to the input terminal of the lower bridge arm, and the AC side A-phase, B-phase, and C-phase terminals are respectively led out from the midpoint of the corresponding bridge arm reactor.
[0008] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, the three primary windings of the Z-type grounding transformer are wound on the same three-column iron core, and each phase winding consists of two coil segments wound in opposite directions. The opposite ends of the two coil segments are connected, and the same ends serve as the two ends of the phase winding.
[0009] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, the small resistor is a non-inductive resistive element with a fixed resistance value, one end of which is connected to the neutral point lead-out terminal of the secondary side of the Z-type grounding transformer, and the other end is directly connected to the grounding electrode.
[0010] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, the AC transmission line includes three phase conductors. One end of each phase conductor is connected to the AC side A-phase, B-phase, and C-phase terminals of the flexible converter device body, and the other end is connected to the three star-connected terminals of the third winding of the remote three-winding power transformer.
[0011] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, wherein: the first winding of the remote three-winding power transformer is connected to the high-voltage AC power grid, the second winding is connected to the medium-voltage distribution network, and the third winding is connected to the flexible converter device body; the first winding adopts a delta connection, and the second and third windings both adopt a star connection.
[0012] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, wherein: the Z-type grounding transformer and the grounding branch composed of small resistors are connected in parallel between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body and the ground, without any circuit breaker or disconnecting switch connected in series in between.
[0013] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, the flexible converter device body, the Z-type grounding transformer and the small resistor are installed in the same station, and the remote three-winding power transformer is installed in a substation more than 1 kilometer away from the station.
[0014] As a preferred embodiment of the grid-connected grounding topology of the flexible converter device without a connecting transformer described in this invention, a three-phase current transformer is provided between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter device body and the primary winding of the Z-type grounding transformer, and the secondary output of the three-phase current transformer is connected to a protection and control device.
[0015] Compared with existing technologies, the beneficial effects of this invention are that it proposes a grid-connected grounding topology for a flexible converter device without a connecting transformer. Based on a topology using a remote three-winding transformer plus a Z-type transformer grounded through a small resistor, it solves the problem of the difficulty in quickly and completely shutting down and closing three-phase AC circuit breakers when using a traditional arc suppression coil grounding system without a connecting transformer. When a single-pole ground fault occurs on the DC side of the flexible device system with a connecting transformer, a large amount of DC bias current will flow into the AC grid side due to the lack of a connecting transformer between the AC grid and the flexible device. At this time, this topology can greatly reduce the impact of the DC bias current on the three-phase AC circuit breakers, ensuring that each phase of the three-phase circuit breaker on the AC side shuts down and closes quickly and completely. In addition, this topology can also limit the peak fault current on the AC side when a single-pole ground fault occurs on the DC side of the system. Attached Figure Description
[0016] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. 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.
[0017] Figure 1 This is a topology diagram of a half-bridge flexible converter device with a grid-connected grounding topology, provided as an embodiment of the present invention.
[0018] Figure 2 This is a topology diagram of a flexible converter device without a connecting transformer, provided as an embodiment of the present invention, which is a flexible converter device without a connecting transformer connected to the grid and grounded, consisting of a three-winding transformer and a Z-type transformer grounded through a small resistor.
[0019] Figure 3 This is a system equivalent structure diagram of a DC-side single-pole grounding fault in a grid-connected grounding topology of a flexible converter device without a connecting transformer, provided as an embodiment of the present invention.
[0020] Figure 4 The simulation waveform of a flexible converter device without a coupling transformer and grounded through an arc suppression coil is provided as an embodiment of the present invention.
[0021] Figure 5 The experimental simulation waveforms of a flexible converter device without a connecting transformer, with a grid-grounded topology, provided in one embodiment of the present invention, are shown. The flexible converter device is equipped with a three-winding transformer and a Z-type transformer and grounded through a small resistor. Detailed Implementation
[0022] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.
[0023] Example 1, referring to Figure 1 This is the first embodiment of the present invention, which provides a grid-connected grounding topology for a flexible converter device without a connecting transformer, comprising: Figure 1 A half-bridge flexible converter topology diagram is shown, illustrating a grid-connected and grounded topology for a transformerless flexible converter, including: The flexible converter unit body, Z-type grounding transformer, small resistor and remote three-winding power transformer; The flexible converter unit body includes an upper bridge arm and a lower bridge arm. A positive DC bus and a negative DC bus are led out between the upper bridge arm and the lower bridge arm. The midpoints of the upper bridge arm and the lower bridge arm are respectively used as the AC side A phase, B phase and C phase terminals. The Z-type grounding transformer includes three primary windings and one secondary neutral point lead-out terminal. The first ends of the three primary windings are respectively connected to the AC A-phase, B-phase, and C-phase terminals of the flexible converter body. The second ends of the three primary windings are connected to each other to form a zigzag winding structure. The secondary neutral point lead-out terminal is connected to the ground after being connected in series with a small resistor. The remote three-winding power transformer is located on the AC grid side. The three terminals of its third winding are connected to the AC phase A, B, and C terminals of the flexible converter body through AC transmission lines. The third winding adopts a star connection, while the first or second winding adopts a delta connection.
[0024] In this embodiment of the invention, the upper and lower arms of the flexible converter body are both composed of multiple cascaded half-bridge sub-modules, and each half-bridge sub-module includes two power electronic switching devices connected in series and a DC capacitor connected in parallel.
[0025] A bridge arm reactor is connected in series between the upper and lower bridge arms of the flexible converter body. One end of the bridge arm reactor is connected to the output terminal of the upper bridge arm, and the other end is connected to the input terminal of the lower bridge arm. The AC phase A, phase B, and phase C terminals are respectively led out from the midpoint of the corresponding bridge arm reactor.
[0026] In an embodiment of the present invention, the three primary windings of the Z-type grounding transformer are wound on the same three-column iron core. Each phase winding consists of two coil segments wound in opposite directions. The opposite ends of the two coil segments are connected, and the same ends serve as the two ends of the phase winding.
[0027] The small resistor is a non-inductive resistive element with a fixed resistance value. One end of it is connected to the neutral point lead-out terminal of the secondary side of the Z-type grounding transformer, and the other end is directly connected to the grounding electrode.
[0028] In an embodiment of the present invention, the AC transmission line includes three phase conductors. One end of each phase conductor is connected to the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body, and the other end is connected to the three star terminals of the third winding of the remote three-winding power transformer.
[0029] The first winding of the remote three-winding power transformer is connected to the high-voltage AC power grid, the second winding is connected to the medium-voltage power distribution network, and the third winding is connected to the flexible converter device body. The first winding adopts a delta connection, while the second and third windings both adopt a star connection.
[0030] It should be noted that traditional flexible DC transmission systems generally rely on power frequency connection transformers to achieve electrical isolation and voltage matching between the AC grid and the converter valves. Such solutions have significant limitations in practical engineering applications.
[0031] On the one hand, power frequency transformers are large and heavy, making them difficult to deploy in space-constrained scenarios such as offshore wind power platforms and urban underground substations. They also occupy about 30% of the floor space of the converter station, which seriously restricts the compact design of the system.
[0032] On the other hand, when flexible converters adopt high-frequency modulation strategies such as the nearest level approximation technique, the eddy current loss of the power frequency transformer core and the skin effect of the winding increase sharply. Experimental data show that when the switching frequency exceeds 2 kHz, the transformer loss can account for 15% to 18% of the total system loss, significantly reducing the overall energy efficiency.
[0033] Furthermore, in a transformerless architecture, if the traditional arc suppression coil grounding method is used, once a single-pole metallic grounding fault occurs on the DC side, due to the lack of transformer isolation, a large amount of DC bias current will be directly injected into the AC grid, causing the AC circuit breaker to be unable to reliably trip because the current cannot cross zero, seriously threatening the safe operation of the system.
[0034] Understandably, this scheme constructs a novel transformerless grid-connected grounding topology by setting up a remote three-winding power transformer to work in coordination with a local Z-type grounding transformer.
[0035] Furthermore, the first winding of the remote three-winding power transformer is connected to the high-voltage AC power grid in a delta connection, the second winding is connected in a star connection to serve the medium-voltage distribution network, and the third winding is connected in a star connection to the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body via three phase conductors, forming a clear multi-voltage level coupling path.
[0036] Furthermore, a Z-type grounding transformer is configured within the flexible converter station, with its primary winding connected to the three-phase AC terminals respectively, and the secondary neutral point grounded through a fixed-value non-inductive small resistor, forming a low-impedance zero-sequence path.
[0037] Furthermore, when a single-pole ground fault occurs on the DC side, this structure effectively suppresses the penetration of DC bias current into the AC grid, enabling the current in each phase of the AC circuit breaker to quickly cross zero after the operation command is issued, ensuring that the three-phase circuit breaker completes reliable tripping within 10 milliseconds. At the same time, it limits the peak value of the bridge arm short-circuit fault current to within 500 amperes, which is significantly better than the peak value of 1,300 amperes under the traditional arc suppression coil grounding method.
[0038] In this embodiment of the invention, the Z-type grounding transformer and the grounding branch consisting of a small resistor are connected in parallel between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body and the ground, without any circuit breaker or disconnecting switch connected in series.
[0039] The flexible converter unit, Z-type grounding transformer, and small resistor are installed in the same station, while the remote three-winding power transformer is installed in a substation more than 1 kilometer away from the station.
[0040] It should be noted that traditional flexible converter systems without coupling transformers often use arc suppression coils or high-resistance grounding methods in their grounding design, which have obvious defects in actual operation.
[0041] On the one hand, if a circuit breaker or disconnector is connected in series in the grounding branch, it is convenient for maintenance and isolation. However, when a single-pole grounding fault occurs on the DC side, due to the delay in control logic or the lag in mechanical action, a stable zero-sequence current path cannot be provided in time, resulting in an abnormal rise in the zero-sequence voltage of the system, which in turn induces the risk of overvoltage.
[0042] On the other hand, if the grounding equipment is distributed in a decentralized manner or relies on the neutral point grounding of a remote transformer, the grounding impedance path is too long and the parameters are uncontrollable. During the fault transient process, it is difficult to effectively suppress the DC bias current, which makes the AC circuit breaker unable to reliably trip due to the continuous deviation of the current from zero.
[0043] Furthermore, continuing with the example of offshore wind power grid connection mentioned above, if a circuit breaker is configured on the grounding branch of a compact offshore converter platform, it will not only increase the size of the equipment and the complexity of maintenance, but also introduce the risk of uncontrollable interruption during the critical window period for fault clearing, which will seriously weaken the system's fault ride-through capability.
[0044] Understandably, this solution directly connects the Z-type grounding transformer and the grounding branch consisting of a small resistor in parallel between the three-phase terminals of the AC side of the flexible converter and the ground, and ensures that no switching devices are connected in series along the entire branch, thereby forming a low-impedance, high-reliability zero-sequence current discharge channel.
[0045] Furthermore, this grounding branch is integrated with the flexible converter unit body, Z-type grounding transformer, and small resistor in the same converter station, achieving tight physical coupling and coordinated optimization of electrical parameters.
[0046] Furthermore, the remote three-winding power transformer is deployed at an onshore or near-shore substation more than one kilometer away. Its third winding is connected to the flexible converter in a star configuration, and its first winding is connected to the high-voltage power grid in a delta configuration, effectively blocking the propagation of zero-sequence components to the main grid.
[0047] Furthermore, under this architecture, when a unipolar metallic ground fault occurs on the DC side, the zero-sequence impedance and small resistance provided by the Z-type grounding transformer work together to quickly establish a path to ground, enabling the three-phase current on the AC side to cross zero within 10 milliseconds, ensuring reliable circuit breaker tripping, and avoiding continuous oscillation of fault current and equipment overheating caused by grounding branch interruption or high impedance path.
[0048] Among them, the grounding branch refers to the complete electrical path formed by connecting the primary side of the Z-type grounding transformer to the three-phase AC terminals, the secondary side neutral point connected in series with a small resistor, and then connecting to the ground. This path has no circuit breaker, disconnector or other disconnectable components throughout.
[0049] It should be noted that the flexible converter unit body refers to the upper and lower bridge arm structure composed of multiple cascaded half-bridge sub-modules, which has the functions of positive and negative DC bus output and AC side three-phase terminal lead-out.
[0050] It should be noted that the Z-type grounding transformer has a three-limb iron core structure. The primary winding of each phase consists of two reverse-wound coil segments, with opposite ends connected to form a zigzag connection, which is used to provide high zero-sequence impedance and low positive-sequence impedance characteristics.
[0051] It should be noted that the small resistor is a non-inductive metal alloy resistor with a fixed resistance value. The resistance value is precisely set according to the system short-circuit capacity and ground fault current limit to ensure that the neutral point voltage is kept stable while limiting the fault current.
[0052] It should be noted that "same converter station" refers to the flexible converter unit, Z-type grounding transformer and small resistor being placed in the same building or platform to achieve the shortest electrical connection and the best protection coordination.
[0053] It should be noted that the remote three-winding power transformer refers to a three-winding transformer whose installation location is more than one kilometer away from the flexible converter station. It undertakes the triple functions of high-voltage power transmission, medium-voltage power distribution and flexible DC access. Its physical separation design not only meets the functional requirements of the system, but also avoids the negative impact of power frequency transformers on the space and losses of the converter station.
[0054] In an embodiment of the present invention, a three-phase current transformer is provided between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body and the primary winding of the Z-type grounding transformer, and the secondary side output of the three-phase current transformer is connected to the protection and control device.
[0055] It should be noted that traditional unconnected transformer flexible converter systems have significant shortcomings in fault detection and protection configuration.
[0056] Existing solutions typically place current transformers on the side of a remote three-winding power transformer or at the outlet of an AC transmission line, resulting in a lag in sensing the local AC current status of the flexible converter. This is especially true when a single-pole ground fault occurs on the DC side, making it impossible to accurately capture the zero-sequence current component flowing through the Z-type grounding transformer and the three-phase imbalance characteristics.
[0057] This type of arrangement makes it difficult for protection devices to distinguish between internal faults and external disturbances, which can easily lead to false tripping or failure to trip.
[0058] Example 2 continues to use the offshore wind power grid connection scenario from the above examples. For instance, in a compact offshore converter platform, if the current transformer is far from the flexible converter body, when a metallic grounding fault occurs at the positive terminal of the DC bus, due to the lack of real-time monitoring of the AC side near-end current, the protection and control device cannot promptly identify the rapid rise trend of the bridge arm short-circuit current, resulting in a delay in fault clearing. This may cause overvoltage breakdown of the submodule capacitor or thermal failure of the power electronic switching device, seriously threatening the safe and stable operation of the system.
[0059] Understandably, this solution achieves high-precision, low-delay sampling of the three-phase current flowing into the Z-type grounding transformer by precisely deploying three-phase current transformers on the electrical connection path between the A-phase, B-phase, and C-phase terminals of the AC side of the flexible converter unit and the primary winding of the Z-type grounding transformer.
[0060] Furthermore, the secondary side output signal of the three-phase current transformer is directly connected to the local protection and control device, enabling the protection system to acquire the AC side current amplitude, phase and zero-sequence component information in real time, complete the fault identification within milliseconds after the DC side single-pole ground fault occurs, and trigger the circuit breaker tripping command.
[0061] Furthermore, this arrangement ensures that protection actions are based on the current characteristics closest to the fault source, significantly improving sensitivity and selectivity. It also supports online monitoring of the operating status of the Z-type grounding transformer, preventing grounding failures caused by inter-turn short circuits or abnormal low resistance.
[0062] Furthermore, simulation results show that with this current sampling location, the protection and control device can reliably trip the three-phase circuit breaker within 10 milliseconds, and the peak fault current is effectively limited to within 500 amperes, which is significantly better than the response performance under the traditional arrangement scheme.
[0063] Example 3, referring to Figures 1-5 , Figure 1 This is a topology diagram of a half-bridge flexible converter. In the diagram, i dc DC side current, V dcu For the upper bridge arm voltage, V dcl R is the lower arm voltage, R is the equivalent resistance of the arm, and L is the arm reactance. The flexible DC device itself has a small impedance and a weak ability to suppress DC electrical components.
[0064] Figure 2This is a topology diagram of a flexible converter unit without connection, equipped with a three-winding transformer and a Z-type transformer grounded through a small resistor. The Z-type transformer is connected in parallel to the AC side of the flexible converter unit through the small resistor grounding structure. The remote three-phase transformer is commonly found in AC lines, but it is usually far away from the flexible converter unit. The distribution device winding of the remote transformer usually adopts a delta connection. Therefore, the grounding point of the flexible converter unit is provided by the Z-type transformer grounded through a small resistor structure. F1 is a single-pole metallic grounding fault on the DC side.
[0065] Figure 3 This is the equivalent system structure diagram for a DC-side unipolar ground fault, e a e b e c For the three-phase equivalent three-phase voltage source of the flexible converter device, k a k b k c L is the equivalent three-phase AC circuit breaker on the AC side of the flexible converter unit. a L b L c For a three-phase bridge arm reactor, R a R b R c Lz is the equivalent impedance of the three-phase transformer, which is generally composed of the self-impedance of each phase winding and the mutual impedance between the windings. Rz is the small resistance connected in series with the Z-type transformer. line Z represents the line impedance of the transmission line between the flexible converter and the remote transformer. bianyaqi This is the equivalent impedance of the remote transformer.
[0066] Figure 4 The simulation waveform is for a non-connected flexible converter device grounded through an arc suppression coil. A 2.5s setting enables a DC-side unipolar fault F1, and a 2.6s setting disconnects the AC circuit breaker. Figure 4 (a) Simulated waveforms of active and reactive power transmitted from the flexible converter to the DC side. Figure 4 (b) Simulated waveforms of active and reactive power transmitted from the flexible converter to the AC grid. Figure 4(c) shows the simulated waveform when the flexible converter only transmits reactive power to the AC grid. From the simulation results of the three power flow scenarios above, it can be seen that when the three-phase AC circuit breaker is disconnected at 2.6s, due to the relatively low overall impedance of the system and the absence of a connected transformer, a large amount of DC bias current will enter the AC grid side. Furthermore, the impedance of the DC bias current path is relatively low, resulting in a weak attenuation of the DC bias current. Therefore, the AC current in phase A of the grid-side AC circuit breaker cannot cross zero in a short time (phase A crosses zero at least 4.15s), causing the AC circuit breaker to fail to close or shut down in a short period. Simultaneously, during the fault process, the peak value of the three-phase bridge arm short-circuit fault current is approximately 1300A, while under the same operating conditions, the peak value of the steady-state three-phase current is 250A.
[0067] Figure 5 Simulated waveforms were generated for a seamless flexible converter unit equipped with a three-winding transformer and a Z-type transformer grounded via a small resistor. A 2.5s setting was used to enable a DC-side single-pole fault F1, and a 2.6s setting was used to disconnect the AC circuit breaker. Figure 5 (a) Simulated waveforms of active and reactive power transmitted from the flexible converter to the DC side. Figure 5 (b) Simulated waveforms of active and reactive power transmitted from the flexible converter to the AC grid. Figure 5 (c) shows the simulated waveform of the flexible converter transmitting reactive power only to the AC grid. The simulation results of the three power flow scenarios above show that when the three-phase AC circuit breaker is disconnected at 2.6s, the three-phase AC current on the grid side crosses zero instantaneously (phase C crosses zero at the latest 2.61s). Each phase of the three-phase AC circuit breaker can be quickly switched off and closed. Simultaneously, during the fault, the peak value of the bridge arm short-circuit fault current is limited to approximately 500A, while under the same operating conditions, the peak value of the steady-state three-phase current is 250A.
[0068] The above technical solution has the following effects: Based on the topology of a remote three-winding transformer plus a Z-type transformer grounded through a small resistance, the problem of three-phase AC circuit breakers being unable to quickly and completely turn off and close when paired with a traditional arc suppression coil grounding without a connecting transformer can be solved. When a single-pole ground fault occurs on the DC side of a flexible device paired with an unconnected transformer system, a large amount of DC bias current flows into the AC grid side due to the lack of a connecting transformer between the AC grid and the flexible device. In this case, the topology of using a remote three-phase transformer plus a Z-type transformer grounded through a small resistor can greatly reduce the impact of the DC bias current on the three-phase AC circuit breaker, ensuring that each phase of the three-phase circuit breaker on the AC side can be quickly and completely turned off and closed. The topology based on a remote three-winding transformer plus a Z-type transformer grounded through a small resistor can limit the peak fault current on the AC side when a DC-side single-pole ground fault occurs in the system.
[0069] It should be noted that 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.
[0070] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.
[0071] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.
Claims
1. A grid-connected grounding topology for a flexible converter device without a connecting transformer, characterized in that, include: The flexible converter unit body, Z-type grounding transformer, small resistor and remote three-winding power transformer; The flexible converter unit body includes an upper bridge arm and a lower bridge arm. A positive DC bus and a negative DC bus are led out between the upper bridge arm and the lower bridge arm. The midpoints of the upper bridge arm and the lower bridge arm are respectively used as the AC side A phase, B phase and C phase terminals. The Z-type grounding transformer includes three primary windings and one secondary neutral point lead-out terminal. The first ends of the three primary windings are respectively connected to the AC A-phase, B-phase, and C-phase terminals of the flexible converter body. The second ends of the three primary windings are connected to each other to form a zigzag winding structure. The secondary neutral point lead-out terminal is connected to the ground after being connected in series with a small resistor. The remote three-winding power transformer is located on the AC grid side. The three terminals of its third winding are connected to the AC phase A, B, and C terminals of the flexible converter body through AC transmission lines. The third winding adopts a star connection, while the first or second winding adopts a delta connection.
2. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 1, characterized in that, The upper and lower arms of the flexible converter body are both composed of multiple cascaded half-bridge sub-modules. Each half-bridge sub-module includes two power electronic switching devices connected in series and a DC capacitor connected in parallel.
3. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 2, characterized in that, The upper and lower bridge arms of the flexible converter body are connected in series with bridge arm reactors. One end of the bridge arm reactor is connected to the output terminal of the upper bridge arm, and the other end is connected to the input terminal of the lower bridge arm. The AC phase A, phase B, and phase C terminals are respectively led out from the midpoint of the corresponding bridge arm reactor.
4. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 3, characterized in that, The three primary windings of the Z-type grounding transformer are wound on the same three-column iron core. Each phase winding consists of two coil segments wound in opposite directions. The opposite ends of the two coil segments are connected, and the same ends serve as the two ends of the phase winding.
5. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 4, characterized in that, The small resistor is a non-inductive resistive element with a fixed resistance value. One end of it is connected to the neutral point lead-out terminal of the secondary side of the Z-type grounding transformer, and the other end is directly connected to the grounding electrode.
6. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 5, characterized in that, The AC transmission line includes three phase conductors. One end of each phase conductor is connected to the AC side terminals of phase A, phase B, and phase C of the flexible converter body, and the other end is connected to the three star terminals of the third winding of the remote three-winding power transformer.
7. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 6, characterized in that, The first winding of the remote three-winding power transformer is connected to the high-voltage AC power grid, the second winding is connected to the medium-voltage power distribution network, and the third winding is connected to the flexible converter body. The first winding adopts a delta connection, and the second and third windings both adopt a star connection.
8. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 7, characterized in that, The Z-type grounding transformer and the grounding branch consisting of small resistors are connected in parallel between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body and the ground, without any circuit breaker or disconnecting switch connected in series.
9. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 8, characterized in that, The flexible converter body, Z-type grounding transformer and small resistor are installed in the same station, while the remote three-winding power transformer is installed in a substation more than 1 kilometer away from the station.
10. The grid-connected grounding topology of a flexible converter device without a connecting transformer as described in claim 9, characterized in that, A three-phase current transformer is installed between the AC side A-phase, B-phase, and C-phase terminals of the flexible converter body and the primary winding of the Z-type grounding transformer. The secondary output of the three-phase current transformer is connected to the protection and control device.