Energy storage type MMC submodule circuit having direct current fault clearing capability, and working method

Abstract

Disclosed in the present invention are an energy storage type MMC submodule circuit having a direct-current fault clearing capability, and a working method, belonging to the technical field of voltage source converters. By adding an energy storage converter unit and a fault handling unit to a typical half-bridge submodule circuit topology, the present invention provides an energy storage type MMC submodule circuit having a direct-current fault clearing capability, and a control method. An improved submodule has both a direct-current fault clearing capability and an energy storage capability without losing original functions, thus helping to enhance the technical and economic performance of MMC power stations. Therefore, the MMC power stations have broad applicability.

Description

Energy storage type MMC submodule circuit and working method with DC fault clearing capability Technical Field

[0001] The present invention belongs to the technical field of voltage source converters, and relates to a submodule circuit topology and a working method of a modular multilevel converter, and in particular to an energy storage type MMC submodule circuit topology with DC fault clearing capability and a working method thereof. Background Art

[0002] With the advancement of power electronics technology, modular multilevel converter (MMC)-based high-voltage direct current (HVDC) systems have attracted widespread attention from both industry and academia. MMCs offer advantages such as decoupling of active and reactive power control, elimination of commutation faults, and low harmonics, outperforming line-commutated converters (LCCs). They are widely used in wind farm integration, multi-terminal operation, and passive network power supply scenarios. To date, multi-terminal overhead line HVDC projects have been completed in China, including those in Kunliulong and Zhangbei. However, overhead line transmission systems expose exposed conductors, making them susceptible to transient faults such as short circuits and flashovers. Typical MMCs using half-bridge submodules lack the ability to self-clear DC-side faults, hindering the development of HVDC systems. MMCs using full-bridge submodules offer self-clear DC-side faults due to their ability to dissipate current into the module capacitors when locked, but they come with high investment costs.

[0003] Due to environmental factors, renewable energy output is subject to certain fluctuations and intermittent characteristics. The integration of large-scale renewable energy into the grid will affect the stability of the power system. The high proportion of power electronic devices connected to the power system also reduces the grid's ability to resist disturbances and regulate due to their low inertia, weak stability, and poor interference immunity, thus affecting system stability. Adding energy storage systems to power systems can effectively offset these shortcomings by smoothing power fluctuations, improving system stability and economic efficiency. Modular multilevel converters, with their advantages of modularity, low output voltage harmonics, and reduced switching device voltage stress, are ideal power conversion topologies for energy storage systems.

[0004] Therefore, a half-bridge and full-bridge adaptive MMC sub-module is designed to temporarily reuse the power electronic devices of the distributed energy storage system under DC fault conditions, so that the MMC-based converter station has both DC fault clearing and energy storage functions, which has great application value.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to solve the above-mentioned defects in the prior art and to provide an energy storage type MMC submodule circuit and a working method with DC fault clearing capability.

[0007] To achieve the above objectives, in a first aspect, the present invention provides an energy storage MMC submodule circuit with DC fault clearing capability, which adopts the following technical solutions:

[0008] The present invention discloses an energy storage MMC submodule circuit with DC fault clearing capability, referred to as a submodule for ease of reference. The submodule includes the following components: one reactor, designated L for ease of reference; one capacitor, designated C; six power diodes, designated D1, D2, D3, D3, D4, D5, and D6; two power thyristors, designated V1 and V2; five insulated gate bipolar transistors, designated T1, T2, T3, T4, and T5; and one energy storage element, selected from a battery or supercapacitor, designated ES. Compared to the typical non-isolated energy storage MMC submodule circuit topology, the submodule circuit topology disclosed in the present invention, by adding only one insulated gate bipolar transistor, two power thyristors, and two power diodes, enables the MMC converter station to possess the overhead line DC fault handling capability that is in short supply in flexible DC transmission, while also functioning as an energy storage power station under normal operating conditions.

[0009] The submodule circuit topology disclosed in this invention comprises a conventional half-bridge submodule unit, an energy storage converter unit, and a fault handling unit, each of which includes three terminals. The conventional half-bridge submodule unit provides a relatively stable voltage for the MMC bridge arm, the energy storage converter unit enables energy exchange between the energy storage element and the capacitor, and the fault handling unit is responsible for temporary state transitions between the half-bridge and full-bridge submodules.

[0010] Furthermore, the connection relationship between the half-bridge conventional sub-module unit, the energy storage converter unit and the fault processing unit is as follows: the first terminal of the half-bridge conventional sub-module unit is the positive output terminal of the sub-module disclosed in the present invention, the second terminal of the half-bridge conventional sub-module unit is connected to the first terminal of the energy storage converter unit, and the third terminal of the half-bridge conventional sub-module unit is connected to the third terminal of the energy storage converter unit; the first terminal of the fault processing unit is the negative output terminal of the sub-module disclosed in the present invention, the second terminal of the fault processing unit is connected to the second terminal of the energy storage converter unit, and the third terminal of the fault processing unit is connected to the third terminal of the energy storage converter unit.

[0011] Furthermore, the conventional half-bridge submodule unit includes a capacitor C, power diodes D1 and D2, and insulated gate bipolar transistors T1 and T2. The connection relationship between these five components is as follows: the anode of D1, the cathode of D2, the emitter of T1, and the collector of T2 are connected and serve as the first terminal of the conventional half-bridge submodule unit; one end of capacitor C is connected to the collector of T1 and the cathode of D1 and serve as the second terminal of the conventional half-bridge submodule unit; the other end of capacitor C is connected to the emitter of T2 and the anode of D2 and serve as the third terminal of the conventional half-bridge submodule unit. It can be seen that the topology of the conventional half-bridge submodule unit is consistent with the typical half-bridge submodule topology.

[0012] Furthermore, the energy storage converter unit includes a reactor L, power diodes D3 and D4, power thyristors V1 and V2, insulated gate bipolar transistors T3 and T4, and an energy storage element ES. The connection relationship of the eight elements is as follows: the cathode of D3 and the collector of T3 are both connected and serve as the first terminal of the energy storage converter unit; the anode of D3, the cathode of D4, the anode of V1, the cathode of V2, the emitter of T3, and the collector of T4 are all connected and serve as the second terminal of the energy storage converter unit; one end of L is connected to the positive electrode of ES, the cathode of V1, the anode of V2, and the other end of L are all connected; and the emitter of T4, the anode of D4, and the negative electrode of ES are all connected and serve as the third terminal of the energy storage converter unit. It can be seen that compared with the typical Buck-Boost bidirectional converter topology, the energy storage converter unit topology adds a pair of anti-parallel thyristors. These thyristors can cut off the path of fault current feeding into the energy storage element, achieving the purpose of mutual isolation.

[0013] Furthermore, the fault handling unit includes power diodes D5 and D6, and an insulated gate bipolar transistor T5. The three components are connected as follows: the anode of D5, the cathode of D6, and the collector of T5 are connected and serve as the first terminal of the fault handling unit; the cathode of D5 serves solely as the second terminal of the fault handling unit; and the anode of D6 and the emitter of T5 are connected and serve as the third terminal of the fault handling unit. This fault handling unit topology, independently discovered by the inventors and applied to the energy storage MMC submodule circuit with DC fault clearing capability, enables the submodule to switch between normal half-bridge mode and fault-state full-bridge mode, playing an indispensable role in clearing DC faults.

[0014] In order to achieve the above-mentioned object, in a second aspect, the present invention further provides a control method for the above-mentioned energy storage MMC submodule circuit with DC fault clearing capability, which adopts the following technical solution:

[0015] The control method for the submodule disclosed in the present invention includes six control modes, which are as follows:

[0016] The first control mode involves a fault clearing operation in an MMC employing the submodule disclosed herein. When a bipolar short circuit occurs on the MMC's DC side and the energy storage element is in a charged state, the following steps are performed: First, a shutdown signal is simultaneously applied to the gates of T1, T2, T3, and T5; a conduction signal is applied to the gate of T4, and the conduction signals to the gates of V1 and V2 are disconnected. A 2-millisecond wait is then performed, followed by a shutdown signal applied to the gate of T4. This waiting period ensures the reliable shutdown of the anti-parallel thyristor. As can be seen, after all seven controllable and partially controlled elements are shut down, the fault current in the submodule disclosed herein flows through the negative output terminal, then sequentially through D5, D3, C, and D2, and finally out through the positive output terminal. This control strategy ensures that the voltage between the positive and negative output terminals of the submodule disclosed herein is equal to the reverse voltage of capacitor C, facilitating the absorption of the fault current into the capacitor and thereby blocking the DC fault current.

[0017] The second control mode is to clear the fault by performing the following operations when the bipolar short circuit occurs on the DC side of the MMC and the energy storage element is in a discharged state: first, a shutdown signal is applied to the gates of T1, T2, T4, and T5 simultaneously, a conduction signal is applied to the gate of T3, and the conduction signals of the gates of V1 and V2 are disconnected. Then, a 2-millisecond wait is performed, and finally, a shutdown signal is applied to the gate of T3. The purpose of the waiting period is to ensure that the anti-parallel thyristor is reliably turned off. It can be seen that after all seven controllable and semi-controlled elements are turned off, the fault current flows through the submodule disclosed in the present invention as follows: it flows in from the negative output terminal, then flows through D5, D3, C, D2 in sequence, and finally flows out through the positive output terminal. Through this control, the voltage between the positive and negative output terminals of the submodule disclosed in the present invention is the reverse voltage of capacitor C, which is conducive to absorbing the fault current into the capacitor and thereby blocking the DC fault current.

[0018] The third control mode is to simultaneously perform the following operations to ensure that the submodule disclosed in the present invention outputs a positive voltage to the MMC bridge arm and charges the energy storage element: a turn-on signal is applied to the gates of T1, T3, and T5; a turn-off signal is applied to the gates of T2 and T4; and a turn-on signal is applied to the gates of V1 and V2. This indicates that the voltage between the positive and negative output terminals of the submodule disclosed in the present invention now equals the forward voltage of capacitor C, and the current flowing through the energy storage element flows from the positive electrode of the energy storage element, achieving the predetermined goal of the submodule disclosed in the present invention outputting a positive voltage to the MMC bridge arm and charging the energy storage element.

[0019] The fourth control mode is to simultaneously apply the following operations to the gates of T1, T4, and T5, apply turn-off signals to the gates of T2 and T3, and apply turn-on signals to the gates of V1 and V2, in order to ensure that the submodule disclosed in the present invention outputs a positive voltage to the MMC bridge arm and discharges the energy storage element: At this point, the voltage between the positive and negative output terminals of the submodule disclosed in the present invention equals the forward voltage of capacitor C, and the current flowing through the energy storage element flows out of the positive electrode of the energy storage element, achieving the predetermined goal of the submodule disclosed in the present invention outputting a positive voltage to the MMC bridge arm and discharging the energy storage element.

[0020] The fifth control mode is to simultaneously perform the following operations to ensure that the submodule disclosed in the present invention outputs zero voltage to the MMC bridge arm and charges the energy storage element: a turn-on signal is applied to the gates of T2, T3, and T5; a turn-off signal is applied to the gates of T1 and T4; and a turn-on signal is applied to the gates of V1 and V2. This indicates that the voltage between the positive and negative output terminals of the submodule disclosed in the present invention is less than the conduction voltage drop of the two insulated gate bipolar transistors and is close to zero. The current flowing through the energy storage element flows from the positive terminal of the energy storage element, achieving the predetermined goal of the submodule disclosed in the present invention outputting zero voltage to the MMC bridge arm and charging the energy storage element.

[0021] The sixth control mode is to simultaneously perform the following operations to ensure that the submodule disclosed in the present invention outputs zero voltage to the MMC bridge arm and discharges the energy storage element: a turn-on signal is applied to the gates of T2, T4, and T5; a turn-off signal is applied to the gates of T1 and T3; and a turn-on signal is applied to the gates of V1 and V2. This indicates that the voltage between the positive and negative output terminals of the submodule disclosed in the present invention is less than the conduction voltage drop of the two insulated gate bipolar transistors and is close to zero. The current flowing through the energy storage element flows out of the positive electrode of the energy storage element, achieving the predetermined goal of the submodule disclosed in the present invention outputting zero voltage to the MMC bridge arm and discharging the energy storage element.

[0022] The present invention has the following advantages and effects compared to the prior art:

[0023] In this invention, based on the proposed typical non-isolated energy storage MMC submodule topology, a fault handling unit comprising one insulated gate bipolar transistor, two power diodes, and a pair of anti-parallel thyristors for energy storage element isolation is added, resulting in an energy storage MMC submodule circuit topology with DC fault clearing capabilities. For MMC-based flexible DC converter stations, the proposed submodule circuit topology not only enables them to handle DC faults in overhead lines, a capability that is in short supply in flexible DC transmission, but also enables the converter station to combine the peak and frequency regulation, renewable energy output fluctuation smoothing, and uninterruptible power supply functions of energy storage power stations under normal operating conditions, significantly improving the technical and economic efficiency of the station. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of this application. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

[0025] FIG1 is a circuit topology diagram of an energy storage MMC submodule with DC fault clearing capability disclosed in the present invention;

[0026] FIG2 is a schematic diagram of a current flow path acting on a DC fault condition in accordance with a control method for a circuit topology of an energy storage MMC submodule with DC fault clearing capability disclosed in the present invention;

[0027] FIG3 is a schematic diagram of a current flow path under normal operating conditions, illustrating a control method for a circuit topology of an energy storage MMC submodule with DC fault clearing capability disclosed in the present invention;

[0028] FIG4 is a circuit topology diagram of a modular multilevel converter with a submodule circuit topology disclosed in Embodiment 1 of the present invention;

[0029] FIG5 is a circuit topology diagram of a three-terminal power transmission system in Example 3 of the present invention;

[0030] 6 is a schematic diagram of simulated waveforms of the grid-side voltage and DC voltage of the MMC disclosed in Example 2 of the present invention;

[0031] 7 is a schematic diagram of simulated waveforms of AC, DC and energy storage side power of the MMC disclosed in Example 2 of the present invention;

[0032] FIG8 is a schematic diagram comparing the simulated waveforms of DC fault currents of three different MMCs using a half-bridge submodule, a non-isolated energy storage submodule, and a submodule disclosed in the present invention, under the same MMC control strategy;

[0033] FIG9 is a schematic diagram showing a comparison of simulated waveforms of the A-phase fault current at the AC grid connection point of three different MMCs using a half-bridge submodule, a non-isolated energy storage submodule, and a submodule disclosed in the present invention, under the same MMC control strategy;

[0034] FIG10 is a schematic diagram comparing the simulated waveforms of the fault current of the first energy storage element of the upper bridge arm of phase A of two different MMCs using a non-isolated energy storage submodule and a submodule disclosed in the present invention, under the same MMC control strategy;

[0035] 11 is a schematic diagram of simulated waveforms of the grid-side voltage and DC voltage of the converter station MMC2 in the three-terminal power transmission network in Example 3 of the present invention;

[0036] 12 is a schematic diagram of simulated waveforms of AC, DC and energy storage side power of converter station MMC2 in a three-terminal power transmission network in Example 3 of the present invention;

[0037] FIG13 is a schematic diagram showing a comparison of simulated DC fault current waveforms of three different MMCs using a half-bridge submodule, a non-isolated energy storage submodule, and a submodule disclosed in the present invention, under the same MMC control strategy, in the three-terminal power transmission network of Example 3;

[0038] FIG14 is a schematic diagram comparing the simulated waveforms of the A-phase fault current at the AC grid connection point of converter station MMC2 in the three-terminal power transmission network of Example 3, using three different MMCs, namely, a half-bridge submodule, a non-isolated energy storage submodule, and a submodule disclosed in the present invention, under the same MMC control strategy;

[0039] Figure 15 is a schematic diagram comparing the simulated waveforms of the fault current in the first energy storage element of the A-phase upper bridge arm of the converter station MMC2 of two different MMCs using a non-isolated energy storage submodule and the submodule disclosed in the present invention, under the same MMC control strategy, in the three-terminal transmission network of Example 3. DETAILED DESCRIPTION

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0041] Example 1

[0042] As shown in Figure 1, this embodiment provides a circuit topology for an energy storage MMC submodule with DC fault clearing capabilities. This topology includes the following components: one reactor, named L for ease of reference; one capacitor, named C; six power diodes, named D1, D2, D3, D3, D4, D5, and D6; two power thyristors, named V1 and V2; five insulated gate bipolar transistors, named T1, T2, T3, T4, and T5; and one energy storage element, selected from a battery or supercapacitor, named ES.

[0043] This circuit topology includes three parts: a half-bridge conventional sub-module unit, an energy storage converter unit, and a fault processing unit. Each of the three units includes three terminals and the connection relationship is as follows: the first terminal of the half-bridge conventional sub-module unit is the positive output terminal of the sub-module topology, the second terminal of the half-bridge conventional sub-module unit is connected to the first terminal of the energy storage converter unit, and the third terminal of the half-bridge conventional sub-module unit is connected to the third terminal of the energy storage converter unit; the first terminal of the fault processing unit is the negative output terminal of the sub-module topology, the second terminal of the fault processing unit is connected to the second terminal of the energy storage converter unit, and the third terminal of the fault processing unit is connected to the third terminal of the energy storage converter unit.

[0044] In this embodiment, the half-bridge conventional submodule unit includes components C, D1, D2, T1, and T2, and the connection relationship of the five components is as follows: the anode of D1, the cathode of D2, the emitter of T1, and the collector of T2 are connected and serve as the first terminal of this unit; one end of C is connected to the collector of T1 and the cathode of D1 and serve as the second terminal of this unit, and the other end of C is connected to the emitter of T2 and the anode of D2 and serve as the third terminal of this unit.

[0045] In this embodiment, the energy storage converter unit includes elements L, D3, D4, V1, V2, T3, T4, and ES, and the connection relationship of the eight elements is as follows: the cathode of D3 and the collector of T3 are connected and serve as the first terminal of this unit; the anode of D3, the cathode of D4, the anode of V1, the cathode of V2, the emitter of T3, and the collector of T4 are connected and serve as the second terminal of this unit; one end of L is connected to the positive electrode of ES, and the cathode of V1, the anode of V2, and the other end of L are connected; the emitter of T4, the anode of D4, and the negative electrode of ES are connected and serve as the third terminal of this unit.

[0046] In this embodiment, the fault handling unit includes components D5, D6, and T5, and the connection relationship of the three components is as follows: the anode of D5, the cathode of D6, and the collector of T5 are connected and serve as the first terminal of the unit; the cathode of D5 alone serves as the second terminal of the unit; the anode of D6 and the emitter of T5 are connected and serve as the third terminal of the unit.

[0047] Example 2

[0048] As shown in FIG2 , this embodiment provides an MMC using the submodule disclosed in the present invention, and applies the control method of the circuit topology of the energy storage MMC submodule with DC fault clearing capability described in the present invention to the MMC.

[0049] When the submodule disclosed in the present invention is applied to an MMC, its MMC topology includes a three-phase circuit of A, B, and C, each phase including two upper and lower bridge arms, each bridge arm consisting of four submodules disclosed in the present invention, and upper and lower bridge arm inductors and a resistor for equivalent bridge arm losses are arranged in series between the three phase units A, B, and C. The positive output terminal of the first submodule of the upper bridge arm of phase A is connected to the positive DC bus, the negative output terminal is connected to the positive output terminal of the second submodule, the negative output terminal of the second submodule is connected to the positive output terminal of the third submodule, the negative output terminal of the third submodule is connected to the positive output terminal of the fourth submodule, the negative output terminal of the fourth submodule is connected to the upper end of the upper bridge arm equivalent resistor, the lower end of the upper bridge arm equivalent resistor is connected to the upper end of the upper bridge arm reactor, the lower end of the upper bridge arm reactor is connected to the upper end of the lower bridge arm reactor. Phases B and C are connected to the A-phase AC power supply at the connection point. The lower end of the lower-arm reactor is connected to the upper end of the lower-arm equivalent resistor. The positive output terminal of the first submodule of the lower arm of phase A is connected to the lower end of the lower-arm equivalent resistor, the negative output terminal is connected to the positive output terminal of the second submodule, the negative output terminal of the second submodule is connected to the positive output terminal of the third submodule, the negative output terminal of the third submodule is connected to the positive output terminal of the fourth submodule, and the negative output terminal of the fourth submodule is connected to the negative terminal of the DC bus. The connection method for phases B and C is the same as that for phase A.

[0050] The control method for the energy storage MMC submodule circuit with DC fault clearing capability disclosed in the present invention includes six control modes, as follows:

[0051] The first control mode is to first apply a shutdown signal to the gates of T1, T2, T3, and T5 at the same time, apply a conduction signal to the gate of T4, and disconnect the conduction signals of the gates of V1 and V2, then wait for 2 milliseconds, and finally apply a shutdown signal to the gate of T4.

[0052] The second control mode is to first apply a shutdown signal to the gates of T1, T2, T4, and T5 at the same time, apply a conduction signal to the gate of T3, and disconnect the conduction signals of the gates of V1 and V2, then wait for 2 milliseconds, and finally apply a shutdown signal to the gate of T3.

[0053] The third control mode is to apply a turn-on signal to the gates of T1, T3, and T5, apply a turn-off signal to the gates of T2 and T4, and apply a turn-on signal to the gates of V1 and V2.

[0054] The fourth control mode is to apply a turn-on signal to the gates of T1, T4, and T5, apply a turn-off signal to the gates of T2 and T3, and apply a turn-on signal to the gates of V1 and V2.

[0055] The fifth control mode is to apply a turn-on signal to the gates of T2, T3, and T5, apply a turn-off signal to the gates of T1 and T4, and apply a turn-on signal to the gates of V1 and V2.

[0056] The sixth control mode is to apply a turn-on signal to the gates of T2, T4 and T5, apply a turn-off signal to the gates of T1 and T3, and apply a turn-on signal to the gates of V1 and V2.

[0057] The first and second control modes are DC fault handling modes. In these modes, the fault current flows through the submodule as shown in Figure 3: it flows in from the negative output terminal, then flows through D5, D3, C, and D2 in sequence, and finally flows out through the positive output terminal. The third, fourth, fifth, and sixth control modes are normal operating modes. The current flows through the submodule disclosed in the present invention as shown in Figure 4. In these modes, the submodule disclosed in the present invention provides forward voltage and energy storage for the MMC.

[0058] To demonstrate the capabilities and advantages of this invention, a single-ended MMC-HVDC simulation model with a ±5kV / 1.5MW DC side and a 360kW / 360kWh energy storage side was constructed on the PSCAD / EMTDC simulation platform, as shown in Figure 2. This model simulated the submodules and control methods described in this invention under normal operating conditions and DC fault conditions. Specific parameters of the simulation model are shown in Table 1.

[0059] Table 1 Simulation model parameter table provided by Example 2

[0060] For the MMC and submodule control method provided in this embodiment, under normal working conditions, an AC active power instruction increment ΔP is given at 0.5s. s =0.4pu and give the energy storage active power instruction increment ΔP es =1.4pu, the response of the system is shown in FIG6 . On the one hand, the AC grid-connected point voltage and the DC side voltage of the MMC can remain stable, indicating that the submodule circuit topology proposed in the present invention has the function of constituting a voltage source converter that the submodule should have. On the other hand, as shown in FIG7 , the AC, DC and energy storage powers of the MMC change with the change of the command value and quickly return to stability. Moreover, the power exchange among the AC side, DC side and energy storage side complies with the law of conservation of energy. This indicates that the submodule circuit topology proposed in the present invention has the function of exchanging energy with the AC and DC grids that the energy storage submodule should have.

[0061] For the MMC and submodule control method provided in this embodiment, after a short-circuit fault occurs at the DC side outlet 1.0s later, the system response is shown in Figures 8 and 9. The DC side short-circuit fault current can reliably drop to zero in a short period of time, and the AC side fault current can also reliably drop to zero. Compared with MMCs using half-bridge submodules and non-isolated energy storage submodules, the submodule circuit topology disclosed in this invention has significantly superior DC fault clearing capabilities. Furthermore, as shown in Figure 10, the shutdown of the anti-parallel thyristors reliably isolates the energy storage element from the MMC in the fault state, achieving the same effect as an MMC using non-isolated energy storage submodules.

[0062] Example 3

[0063] As shown in Figure 5, this embodiment provides a three-terminal DC transmission network utilizing the submodules disclosed herein, and a control method for the submodules disclosed herein within this three-terminal DC transmission network. Each terminal of the three-terminal DC transmission network utilizes an MMC based on the submodules disclosed herein, and the topology of the MMC at each terminal is similar to that of Example 2 and Figure 2.

[0064] To demonstrate the capabilities and advantages of the present invention, a 400 kV three-terminal DC transmission system simulation model (Figure 5) was constructed on the PSCAD / EMTDC simulation platform to simulate the submodules and control methods described in the present invention under normal operating conditions and DC fault conditions. Specific parameters of the simulation model are shown in Table 2.

[0065] Table 2 Simulation model parameters provided by Example 3

[0066] For the three-terminal power transmission system and control method provided in this embodiment, under normal working conditions, the converter station MMC2 is given an energy storage active power instruction increment ΔP at 0.2s. es =1.4pu, the response of the system is shown in FIG11 . On the one hand, the AC grid-connected point voltage and the DC side voltage of MMC2 can remain stable, indicating that the submodule circuit topology proposed in the present invention has the function of constituting a voltage source converter that the submodule should have. On the other hand, as shown in FIG12 , the AC, DC and energy storage powers of MMC2 change with the change of the command value and quickly return to stability. Moreover, the power exchange among the AC side, DC side and energy storage side complies with the law of conservation of energy. This indicates that the submodule circuit topology proposed in the present invention has the function of exchanging energy with the AC and DC grids that the energy storage submodule should have.

[0067] For the three-terminal power transmission system and control method provided in this embodiment, after a short-circuit fault occurs at the midpoint of the DC line between MMC1 and MMC2 for 0.5 seconds, the system responds as shown in Figures 13 and 14. The DC-side short-circuit fault current reliably drops to zero in a short period of time, and the AC-side fault current also reliably drops to zero. Compared to three-terminal power transmission systems using half-bridge submodules and non-isolated energy storage submodules, the submodule circuit topology disclosed in this invention significantly improves DC fault clearing capabilities. Furthermore, as shown in Figure 15, the shutdown of the anti-parallel thyristors reliably isolates the energy storage element from the faulty MMC, achieving the same effect as an MMC using non-isolated energy storage submodules.

[0068] In summary, the circuit topology disclosed in the present invention enables the MMC converter station to have the overhead line DC fault handling capability that is urgently needed for flexible DC transmission without losing its original functions, and also has the function of an energy storage power station under normal working conditions, greatly improving the technical and economic performance of the power station and having wide applicability.

[0069] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. An energy storage type MMC submodule circuit with DC fault clearing capability, characterized in that: It comprises a half-bridge conventional submodule unit, an energy storage converter unit and a fault processing unit, wherein the above three units all include three terminals and are connected to each other as follows: the first terminal of the half-bridge conventional submodule unit is the positive output terminal of the energy storage type MMC submodule circuit with DC fault clearing capability, the second terminal of the half-bridge conventional submodule unit is connected to the first terminal of the energy storage converter unit, and the third terminal of the half-bridge conventional submodule unit is connected to the third terminal of the energy storage converter unit; the first terminal of the fault processing unit is the negative output terminal of the energy storage type MMC submodule circuit with DC fault clearing capability, the second terminal of the fault processing unit is connected to the second terminal of the energy storage converter unit, and the third terminal of the fault processing unit is connected to the third terminal of the energy storage converter unit; Wherein, the half-bridge conventional submodule unit includes a capacitor C, power diodes D1, D2, insulated gate bipolar transistors T1, T2, and the connection relationship of the five elements is as follows: the anode of D1, the cathode of D2, the emitter of T1 and the collector of T2 are connected and serve as the first terminal of the half-bridge conventional submodule unit; one end of the capacitor C is connected to the collector of T1 and the cathode of D1 and serves as the second terminal of the half-bridge conventional submodule unit, and the other end of the capacitor C is connected to the emitter of T2 and the anode of D2 and serves as the third terminal of the half-bridge conventional submodule unit; The energy storage converter unit includes a reactor L, power diodes D3 and D4, power thyristors V1 and V2, insulated gate bipolar transistors T3 and T4, and an energy storage element ES, and the connection relationship of the eight elements is as follows: the cathode of D3 and the collector of T3 are connected and serve as the first terminal of the energy storage converter unit; the anode of D3, the cathode of D4, the anode of V1, the cathode of V2, the emitter of T3, and the collector of T4 are connected and serve as the second terminal of the energy storage converter unit; one end of the reactor L is connected to the positive electrode of ES, and the other end of L is connected to the cathode of V1 and the anode of V2; the emitter of T4, the anode of D4 and the negative electrode of ES are connected and serve as the third terminal of the energy storage converter unit; The fault processing unit includes power diodes D5, D6, and an insulated gate bipolar transistor T5, and the connection relationship of the three elements is as follows: the anode of D5, the cathode of D6, and the collector of T5 are connected and serve as the first terminal of the fault processing unit; the cathode of D5 alone serves as the second terminal of the fault processing unit; the anode of D6 and the emitter of T5 are connected and serve as the third terminal of the fault processing unit.

2. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: When the bipolar short circuit occurs on the MMC DC side and the energy storage element is in a charging state, the energy storage MMC submodule circuit performs the following operations to clear the fault: first, a shutdown signal is applied to the gates of T1, T2, T3, and T5 at the same time, a conduction signal is applied to the gate of T4, and the conduction signals of the gates of V1 and V2 are disconnected, then wait for 2 milliseconds, and finally a shutdown signal is applied to the gate of T4.

3. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: When the bipolar short circuit occurs on the MMC DC side and the energy storage element is in a discharge state, the energy storage MMC submodule circuit performs the following operations to clear the fault: first, a shutdown signal is applied to the gates of T1, T2, T4, and T5 at the same time, a conduction signal is applied to the gate of T3, and the conduction signals of the gates of V1 and V2 are disconnected, then wait for 2 milliseconds, and finally a shutdown signal is applied to the gate of T3.

4. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: In order to make the energy storage MMC sub-module circuit output a positive voltage to the MMC bridge arm and charge the energy storage element, the following operations are performed simultaneously: a turn-on signal is applied to the gates of T1, T3, and T5, a turn-off signal is applied to the gates of T2 and T4, and a turn-on signal is applied to the gates of V1 and V2.

5. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: In order to make the energy storage type MMC submodule circuit output a positive voltage to the MMC bridge arm and discharge the energy storage element, the following operations are performed simultaneously: a turn-on signal is applied to the gates of T1, T4 and T5, a turn-off signal is applied to the gates of T2 and T3, and a turn-on signal is applied to the gates of V1 and V2.

6. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: In order to make the energy storage MMC submodule circuit output zero voltage to the MMC bridge arm and charge the energy storage element, the following operations are performed simultaneously: applying a turn-on signal to the gates of T2, T3, and T5, applying a turn-off signal to the gates of T1 and T4, and applying a turn-on signal to the gates of V1 and V2.

7. The energy storage type MMC submodule circuit with DC fault clearing capability according to claim 1, characterized in that: In order to make the energy storage MMC submodule circuit output zero voltage to the MMC bridge arm and discharge the energy storage element, the following operations are performed simultaneously: applying a turn-on signal to the gates of T2, T4, and T5, applying a turn-off signal to the gates of T1 and T3, and applying a turn-on signal to the gates of V1 and V2.

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