SVG topological structure and staged starting method

Abstract

The invention discloses an SVG topological structure and a staged starting method. The SVG topological structure comprises a starting loop, an MMC voltage source type converter branch and a super capacitor energy storage branch. The starting loop comprises a first switch and a starting resistor and is used for pre-charging the MMC voltage source type converter branch in a starting stage; the MMC voltage source type converter branch comprises six bridge arms and is used for converting alternating-current electric energy transmitted by the starting loop into direct-current electric energy and controlling voltage, current and reactive power of the alternating-current electric energy; the super capacitor energy storage branch comprises an energy storage unit and a current limiting reactor and is used for providing power support for the system through charging and discharging. According to the SVG topological structure and the staged starting method provided by the invention, the cost can be greatly reduced while the function of a conventional phase modifier is realized, and the starting stability of the SVG is improved.

Description

Technical Field

[0001] The invention relates to the technical field of SVG of power system, and in particular to an SVG topological structure and a phased starting method. Background Art

[0002] With the access of a high proportion of new energy stations, the proportion of power electronic converters has gradually increased, and the problems of insufficient traditional rotating voltage sources in the power system, low inertia, weak damping and insufficient synchronous voltage support have become prominent. Applying power electronic network control technology to power electronic converters can establish voltage through power electronic control, realize the voltage source external characteristics of rotating motors, and realize flexible and dynamic adjustment of the system.

[0003] At present, the existing grid construction technology routes mainly include three different grid construction technology routes and engineering applications based on low-voltage energy storage, SVG, and flexible direct current. Among them, low-voltage energy storage includes centralized grid construction energy storage and low-voltage integrated cabinet grid construction energy storage; SVG includes high-voltage direct-hanging grid construction energy storage, static synchronous phase shifter, and grid construction SVG. The submodules can be configured with lithium batteries, supercapacitors, and conventional capacitors; flexible direct current includes grid construction flexible direct current and energy storage grid construction flexible direct current. The three routes can basically meet the system AC synchronous voltage support requirements in different scenarios of new power.

[0004] In SVG, the DC side is based on the configuration of energy storage batteries to realize the functions of conventional generators, with energy storage and peak filling; the DC side is based on the configuration of supercapacitors to realize the functions of conventional phase regulators, with reactive fast support, inertia, damping and primary frequency modulation, etc.; the DC side is based on the configuration of conventional capacitors to achieve reactive fast response without delay, suppress transient overvoltage, and reduce the risk of system oscillation. Compared with energy storage batteries and conventional capacitors, supercapacitors have the characteristics of long service life and fast charging and discharging speed. Therefore, designing a grid-type SVG based on supercapacitors has broad application prospects.

[0005] The supercapacitor SVG based on the MMC voltage source converter is composed of the MMC voltage source converter and the supercapacitor branch connected in parallel on the DC side. The capacitance of the MMC voltage source converter submodule is at the millifarad level, and the capacitance of a single supercapacitor cell is at the kilofarad level. Therefore, the charging time constant RC of the supercapacitor SVG based on the MMC voltage source converter during the startup process is much greater than that of the flexible DC. If the traditional flexible DC startup method is used, on the one hand, the startup time will be too long, and on the other hand, the long charging time will cause the MMC voltage source converter submodule capacitor voltage to diverge, thereby causing control instability. In addition, the traditional charging method has the problem of current shock at the moment when the supercapacitor branch starts charging. Summary of the invention

[0006] The present invention aims to provide an SVG topology structure to achieve the functions of a traditional phase-shifting converter while reducing costs. On this basis, the present invention proposes a staged supercapacitor branch zero-voltage starting method for a supercapacitor type SVG based on an MMC voltage source converter to solve the problems of long starting time of the supercapacitor type SVG based on the MMC voltage source converter, capacitor voltage divergence of the MMC voltage source converter submodule, and supercapacitor current impact during the starting process, and give full play to the technical advantages of the supercapacitor type SVG based on the MMC voltage source converter.

[0007] In order to solve the above technical problems, an embodiment of the present invention provides an SVG topology structure, including:

[0008] A starting circuit, an MMC voltage source converter branch and a supercapacitor energy storage branch, wherein a DC circuit breaker is provided between the MMC voltage source converter and the supercapacitor, wherein:

[0009] The starting circuit is composed of a first switch and a starting resistor in parallel; the MMC voltage source converter branch is composed of six bridge arms; and the supercapacitor energy storage branch is composed of M energy storage units and two current limiting reactors in series.

[0010] As one of the preferred solutions, the bridge arm includes N sub-modules and 1 bridge arm inductor, and the bridge arm sub-module includes one or more of a half-bridge sub-module and a full-bridge sub-module; the half-bridge sub-module, the full-bridge sub-module and the bridge arm inductor are connected in series.

[0011] As one preferred solution, the half-bridge submodule includes a first DC capacitor, a first switch module and a second switch module; wherein,

[0012] The first switch module and the second switch module are connected in series and then connected in parallel with the first DC capacitor;

[0013] The first switch module includes a first switch tube and a first diode anti-parallel connected to the first switch tube, and the second switch module includes a second switch tube and a second diode anti-parallel connected to the second switch tube.

[0014] As one preferred solution, the full-bridge submodule includes a second DC capacitor, a third switch module, a fourth switch module, a fifth switch module and a sixth switch module; wherein,

[0015] After the third switch module and the fourth switch module are connected in series, and after the fifth switch module and the sixth switch module are connected in series, they are respectively connected in parallel with the second DC capacitor.

[0016] As one of the preferred schemes, the third switch module includes a third switch tube and a third diode anti-parallel to the third switch tube, the fourth switch module includes a fourth switch tube and a fourth diode anti-parallel to the fourth switch tube, the fifth switch module includes a fifth switch tube and a fifth diode anti-parallel to the fifth switch tube, and the sixth switch module includes a sixth switch tube and a sixth diode anti-parallel to the sixth switch tube.

[0017] As one preferred solution, the energy storage unit includes a supercapacitor, a third DC capacitor, a seventh switch module, an eighth switch module, a second switch and a third switch; wherein,

[0018] The seventh switch module and the eighth switch module are connected in series and then connected in parallel with the third DC capacitor; the second switch, the third switch and the super capacitor are connected in series and then connected in parallel with the third DC capacitor.

[0019] Another embodiment of the present invention provides a phased startup method for an SVG topology structure, including an MMC voltage source converter full-half hybrid bridge startup phase, a supercapacitor branch zero voltage startup phase, a supercapacitor uncontrolled charging phase, and a supercapacitor constant current charging phase:

[0020] In the full-half hybrid bridge startup phase of the MMC voltage source converter, the circuit breaker is set to the disconnected state; the MMC voltage source converter branch is uncontrolled charged by an AC power supply until the voltage value of the MMC voltage source converter branch reaches a first preset threshold; the MMC voltage source converter branch is controllably charged according to a first preset strategy until the capacitor voltage of the MMC voltage source converter submodule reaches a second preset threshold;

[0021] In the zero voltage starting stage of the supercapacitor branch, the MMC voltage source converter is set to a fixed DC voltage control, the output voltage of the MMC voltage source converter branch is controlled, the DC side voltage of the MMC voltage source converter branch is controlled to zero, and the circuit breaker is closed;

[0022] In the uncontrolled charging stage of the supercapacitor branch, the power module of the supercapacitor branch is set in a locked state, the DC side voltage of the MMC voltage source converter branch is raised to the rated voltage, and the supercapacitor is uncontrolled charged through each diode until the voltage value of the supercapacitor branch submodule reaches a third preset threshold value;

[0023] In the controllable charging stage of the supercapacitor branch, the power submodules of the supercapacitor branch are unlocked in order, and the supercapacitor is controllably charged according to the second preset strategy until the voltage value of the supercapacitor submodule reaches a fourth preset threshold to complete the startup process.

[0024] As a preferred solution, the second preset threshold is greater than the first preset threshold.

[0025] As one preferred solution, the first preset threshold is expressed as:

[0026]

[0027] Among them, U ac is the equivalent phase potential amplitude of the AC system, and N is the number of sub-modules in a single bridge arm of the MMC voltage source converter.

[0028] As one of the preferred solutions, the voltage value of the supercapacitor branch is expressed as:

[0029]

[0030] Among them, U(t) is the DC side voltage of the MMC converter; T is the charging time of the supercapacitor branch; I(t) is the charging current of the supercapacitor branch; C C is the total capacitance of the supercapacitor branch; R c is the internal resistance of the supercapacitor. Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following:

[0031] (1) The present invention provides a supercapacitor-based meshed SVG topology structure, in which the MMC voltage source converter branch adopts a full half-bridge sub-module hybrid method, which can effectively reduce the cost compared with the traditional topology structure. The DC side adopts supercapacitors, which can not only realize the functions of conventional phase regulators, but also quickly support reactive power, inertia, damping and primary frequency modulation. At the same time, it has the advantage of adjustable voltage. The overall technical solution has broad application prospects.

[0032] (2) The present invention designs a staged supercapacitor branch zero-voltage starting method for a supercapacitor type SVG based on an MMC voltage source converter. This starting method of first charging the MMC voltage source converter and then charging the supercapacitor branch effectively avoids the problem of capacitor voltage divergence of the MMC voltage source converter submodule, while ensuring that the DC voltage of the supercapacitor type SVG based on the MMC voltage source converter quickly rises to a voltage close to that during normal operation.

[0033] (3) The zero voltage starting method proposed in the present invention can effectively solve the overcurrent problem of the supercapacitor branch under the traditional soft DC starting mode, so that the supercapacitor branch does not generate excessive charging current while charging quickly, thereby improving the starting stability of the supercapacitor SVG based on the MMC voltage source converter, giving full play to the technical advantages of the supercapacitor SVG based on the MMC voltage source converter, and having broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of an SVG topological structure in one embodiment of the present invention;

[0035] Figure 2 is a schematic diagram of a half-bridge submodule SM1 in one embodiment of the present invention;

[0036] Figure 3 is a schematic diagram of a full-bridge submodule SM2 in one embodiment of the present invention;

[0037] Figure 4 is a schematic diagram of an energy storage unit CM in one embodiment of the present invention;

[0038] Figure 5 It is a flow chart of a method for zero-voltage starting of a staged supercapacitor branch of an SVG topology structure in one embodiment of the present invention. DETAILED DESCRIPTION

[0039] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions 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. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0040] In the description of this application, the terms "first", "second", "third", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", "third", etc. may explicitly or implicitly include one or more of the feature. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0041] In the description of the present application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be a connection between the two elements. The terms "vertical", "horizontal", "left", "right", "upper", "lower" and similar expressions used herein are only for illustrative purposes, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0042] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by those skilled in the art. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood by specific circumstances.

[0043] An embodiment of the present invention provides an SVG topology structure, specifically, Figure 1 As shown, the supercapacitor SVG topology structure based on the MMC voltage source converter consists of a starting circuit, an MMC voltage source converter branch and a supercapacitor energy storage branch: the starting circuit is composed of a switch K1 and a starting resistor R1 in parallel; the MMC voltage source converter branch is composed of 6 bridge arms, wherein each bridge arm is composed of one or more of N half-bridge sub-modules SM1 and full-bridge sub-modules SM2 and a bridge arm reactor L1 in series; the energy storage branch is composed of M energy storage units CM and 2 current limiting reactors L2 in series.

[0044] The starting circuit is formed by the switch K1 and the starting resistor R1 in parallel, which is used to provide the system with initial current when the equipment is started, and gradually build up the voltage, while limiting the current shock during the starting process.

[0045] The MMC voltage source converter branch is the core part of the system, consisting of 6 bridge arms, each of which is composed of one or more of N half-bridge submodules SM1 and full-bridge submodules SM2 and a bridge arm reactor L1 in series. This structure enables the converter to output multi-level voltage, thereby improving voltage quality and system flexibility.

[0046] Supercapacitor energy storage branch: It is composed of M energy storage units CM and two current limiting reactors L2 in series, used to store and release energy to support the reactive power compensation function of SVG. Supercapacitors have the characteristics of high energy density, long cycle life and fast charge and discharge, and are very suitable for this dynamic reactive power compensation occasion.

[0047] As one preferred solution, the bridge arm includes N submodules and 1 bridge arm reactor, and the submodule includes one or more of a half-bridge submodule and a full-bridge submodule; the half-bridge submodule, the full-bridge submodule and the bridge arm reactor are connected in series;

[0048] Among them, the submodule in the bridge arm can select the half-bridge submodule SM1 or the full-bridge submodule SM2, or a combination of the two. The half-bridge submodule has the advantages of simple structure and low cost, while the full-bridge submodule has higher flexibility and control ability. The half-bridge submodule, the full-bridge submodule and the bridge arm reactor are connected in series to form a complete bridge arm. The bridge arm reactor is used to limit the current change rate and improve the stability and response speed of the system.

[0049] Specifically, when switch K1 is closed, the starting resistor R1 limits the current impact, allowing the system to gradually build up voltage. Once the voltage reaches a stable state, switch K1 is disconnected and the starting circuit is isolated. The MMC voltage source converter can output voltage waveforms of different levels by controlling the switching state of the submodule in the bridge arm. The supercapacitor energy storage branch continuously stores and releases energy during the operation of the SVG. When the grid needs reactive power support, the supercapacitor releases energy; when the grid has excess reactive power, the SVG absorbs reactive power and stores it in the supercapacitor.

[0050] As one preferred solution, the half-bridge submodule includes a first DC capacitor, a first switch module and a second switch module; wherein,

[0051] The first switch module and the second switch module are connected in series and then in parallel with the first DC capacitor;

[0052] The first switch module includes a first switch tube and a first diode anti-parallel connected to the first switch tube, and the second switch module includes a second switch tube and a second diode anti-parallel connected to the second switch tube.

[0053] Specifically, the first switch module and the second switch module each include a switch tube and a diode connected in anti-parallel with the switch tube. This design enables the switch tube to switch more smoothly when turned on and off, and the presence of the diode can also provide a reverse current path when the switch tube is turned off, protecting the switch tube from damage.

[0054] Among them, when the first switch tube is turned on, the positive electrode of the power supply forms a current loop through the first switch tube and the load to achieve forward energy transmission. When the first switch tube is turned off, the inductor element in the load will discharge reversely to the first switch tube. At this time, the second switch tube is turned on, and the negative electrode of the power supply forms a current loop through the second switch tube and the load to achieve reverse energy transmission. This energy conversion process enables the half-bridge submodule to flexibly control the energy flow between the power supply and the load.

[0055] For details, see Figure 2 , Figure 2 The figure shows a schematic diagram of a half-bridge submodule SM1 in one embodiment of the present invention. The half-bridge submodule topology structure of the supercapacitor SVG topology based on the MMC voltage source converter includes a first DC capacitor C1, a first switch module S1, and a second switch module S2; wherein the first switch module S1 and the second switch module S2 are connected in series and then connected in parallel with the first DC capacitor C1; wherein the first switch module S1 includes a switch tube I GBT1 and a diode VD1 anti-parallel to the switch tube, and the second switch module S2 includes a switch tube I GBT2 and a diode VD2 anti-parallel to the switch tube.

[0056] As one preferred solution, the full-bridge submodule includes a second DC capacitor, a third switch module, a fourth switch module, a fifth switch module and a sixth switch module; wherein,

[0057] After the third switch module and the fourth switch module are connected in series, the fifth switch module and the sixth switch module are connected in series and then connected in parallel with the second DC capacitor respectively;

[0058] The third switch module includes a third switch tube and a third diode anti-parallel to the third switch tube, the fourth switch module includes a fourth switch tube and a fourth diode anti-parallel to the fourth switch tube, the fifth switch module includes a fifth switch tube and a fifth diode anti-parallel to the fifth switch tube, and the sixth switch module includes a sixth switch tube and a sixth diode anti-parallel to the sixth switch tube.

[0059] The third switch module and the fourth switch module are connected in series. Specifically, the third switch module includes a third switch tube and a third diode connected in anti-parallel with the switch tube, and the fourth switch module includes a fourth switch tube and a fourth diode connected in anti-parallel with the switch tube. After they are connected in series, they are connected in parallel with the second DC capacitor to form a current path.

[0060] The fifth switch module and the sixth switch module are also connected in series. The fifth switch module includes a fifth switch tube and a fifth diode connected in anti-parallel with the switch tube, and the sixth switch module includes a sixth switch tube and a sixth diode connected in anti-parallel with the switch tube. After they are connected in series, they are connected in parallel with the second DC capacitor to form another current path.

[0061] The working principle of the full-bridge submodule mainly depends on the on and off states of the four switch modules. By accurately controlling the switch states of these switch modules, the voltage output and regulation of the full-bridge submodule in different working modes can be achieved. When the full-bridge submodule is required to output a positive voltage, the third switch module and the sixth switch module can be turned on, while the fourth switch module and the fifth switch module are turned off. At this time, the current starts from the positive pole of the second DC capacitor, passes through the third switch tube, and then flows into the main circuit of the MMC, then flows back from the main circuit, passes through the sixth switch tube, and finally returns to the negative pole of the second DC capacitor. In this way, the full-bridge submodule outputs a positive voltage.

[0062] When the full-bridge submodule is not required to output voltage, the third switch module and the fourth switch module can be turned off by controlling the sixth switch module and the fifth switch module to be turned off (or one of them is turned on to provide a current path, but no voltage output is formed at this time). In this way, the full-bridge submodule is cut off from the main circuit of the MMC and does not participate in the voltage output.

[0063] When the full-bridge submodule needs to output a negative voltage, the fourth switch module and the fifth switch module can be turned on, while the third switch module and the sixth switch module are turned off. At this time, the current starts from the negative electrode of the second DC capacitor, passes through the fourth switch tube, and then flows into the main circuit of the MMC, then flows back from the main circuit, passes through the fifth switch tube, and finally returns to the positive electrode of the second DC capacitor. In this way, the full-bridge submodule outputs a negative voltage.

[0064] For details, see Figure 3 , Figure 3The figure shows a schematic diagram of a full-bridge submodule SM2 in one embodiment of the present invention. The full-bridge submodule of the supercapacitor SVG topology based on the MMC voltage source converter includes a second DC capacitor C2, a third switch module S3, a fourth switch module S4, a fifth switch module S5, and a sixth switch module S6; wherein the third switch module S3 and the fourth switch module S4 are connected in series, and the fifth switch module S5 and the sixth switch module S6 are connected in series, and are respectively connected in parallel with the second DC capacitor C2; wherein the third switch module S3 includes a switch tube IGBT3 and a diode VD3 anti-parallel to the switch tube, the fourth switch module S4 includes a switch tube IGBT4 and a diode VD4 anti-parallel to the switch tube, the fifth switch module S5 includes a switch tube IGBT5 and a diode VD5 anti-parallel to the switch tube, and the sixth switch module S6 includes a switch tube IGBT6 and a diode VD6 anti-parallel to the switch tube.

[0065] As one preferred solution, the energy storage unit includes a supercapacitor, a third DC capacitor, a seventh switch module, an eighth switch module, a second switch and a third switch; wherein,

[0066] The seventh switch module and the eighth switch module are connected in series and then connected in parallel with the third DC capacitor. The second switch, the third switch and the super capacitor are connected in series and then connected in parallel with the third DC capacitor.

[0067] For details, see Figure 4 , Figure 4 It is a schematic diagram of an energy storage unit CM in one embodiment of the present invention. The energy storage unit CM of the supercapacitor SVG topology based on the MMC voltage source converter is composed of a supercapacitor SC, a DC capacitor C3, a seventh switch module S7, an eighth switch module S8, a switch K2 and a switch K3; wherein the seventh switch module S7 and the eighth switch module S8 are connected in series and in parallel with the DC capacitor C3, and then in parallel with the supercapacitor SC through switches K2 and K3.

[0068] Another embodiment of the present invention provides a method for starting an SVG topology structure in stages. Figure 5 , Figure 5 The flowchart of the method for zero-voltage startup of a staged supercapacitor branch of an SVG topology structure in one embodiment of the present invention is shown, which includes S1-S5:

[0069] S1: In the uncontrolled charging stage of the MMC voltage source converter full and semi-hybrid bridge, the circuit breaker is set to the disconnected state; the MMC voltage source converter branch is uncontrolled charged by the AC power supply until the voltage value of the MMC voltage source converter branch reaches a first preset threshold;

[0070] Specifically, the circuit breaker is disconnected, and a soft start mode with a series start resistor is adopted (the start resistor is withdrawn at the end of the start), and the capacitors of the full and half hybrid submodules are uncontrolled charged through the AC system. At this time, there are always 2 bridge arms in short-circuit mode and 4 bridge arms in charging mode. When the charging process is completed, the potential in the bridge arm is equal to the AC line voltage amplitude.

[0071] As one of the preferred solutions, the first preset threshold is expressed as:

[0072]

[0073] Among them, U ac is the equivalent phase potential amplitude of the AC system, and N is the number of sub-modules in a single bridge arm of the MMC voltage source converter.

[0074] S2: Controllably charge the MMC voltage source converter branch according to the first preset strategy until the capacitor voltage of the MMC voltage source converter submodule reaches a second preset threshold;

[0075] Wherein, the second preset threshold is greater than the first preset threshold;

[0076] Specifically, the starting resistor is exited, the converter valve submodule is unlocked in order, and the capacitor voltage of the bridge arm submodule is continuously increased to a second preset threshold value, where the second preset threshold value is the rated voltage of the bridge arm submodule.

[0077] S3: In the zero voltage starting stage of the supercapacitor branch, the MMC voltage source converter is set to a fixed DC voltage control, the output voltage of the MMC voltage source converter branch is controlled, the DC side voltage of the MMC voltage source converter branch is controlled to zero, and the DC circuit breaker is closed;

[0078] Specifically, the MMC voltage source converter is set to a fixed DC voltage control, the DC voltage of the MMC voltage source converter is controlled to be zero, and the DC circuit breaker is closed. At the moment of closing, the voltage difference between the DC side of the MMC voltage source converter and the over-capacity branch is zero, and there is no impact current in the over-capacity branch.

[0079] S4: In the uncontrolled charging stage of the supercapacitor branch, the power module of the supercapacitor branch is set to be in a locked state, the DC side voltage of the MMC voltage source converter branch is raised to the rated voltage, and the supercapacitor is uncontrolled charged through each diode until the voltage value of the supercapacitor branch reaches a third preset threshold;

[0080] Specifically, during this process, the power module of the supercapacitor branch is in a locked state, and the DC voltage is gradually increased until it reaches the rated voltage, so as to charge the supercapacitor with a constant current.

[0081] S5: In the controllable charging stage of the supercapacitor branch, the power submodules of the supercapacitor branch are unlocked in order, and the supercapacitor is controllably charged according to the second preset strategy until the voltage value of the power submodule reaches the fourth preset threshold to complete the startup process.

[0082] Specifically, the supercapacitor submodules are unlocked in order and charged with a constant current until the voltage of the supercapacitor submodule reaches the rated voltage, the charging is completed, and the system is switched to a normal operating state.

[0083] As one of the preferred solutions, the voltage value of the supercapacitor branch is expressed as:

[0084]

[0085] Among them, U(t) is the voltage of the supercapacitor branch; T is the charging time of the supercapacitor branch; I(t) is the charging current of the supercapacitor branch; C C is the total capacitance of the supercapacitor branch; R c is the internal resistance of the supercapacitor.

[0086] The derivation process of the voltage value of the supercapacitor branch is as follows:

[0087] Supercapacitor branch charging energy E C for:

[0088]

[0089] Where: T is the supercapacitor branch charging time; Uc(t) is the supercapacitor branch voltage; I(t) is the supercapacitor branch charging current.

[0090] And because:

[0091]

[0092] Where: C C is the total capacitance of the supercapacitor branch.

[0093] get:

[0094]

[0095] And because:

[0096] U(t)=U C (t)+RcI(t)

[0097] Where: U(t) is the DC side voltage of the MMC voltage source converter.

[0098] get;

[0099]

[0100] After steps S1-S5, the startup is completed, the MMC converter and the supercapacitor branch are unlocked, the MMC converter adopts grid-type control, and the supercapacitor branch adopts constant DC voltage control.

[0101] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0102] The supercapacitor-based meshed SVG topology structure proposed in the present invention can effectively control the target voltage, reactive power and average voltage of sub-modules, control the output to participate in building the system voltage amplitude or frequency, and actively provide inertia or reactive power support during the transient steady state, which has great practical value and broad application prospects.

[0103] (2) The present invention designs a staged supercapacitor branch zero-voltage starting method for a supercapacitor type SVG based on an MMC voltage source converter, which can effectively solve the problem of long starting time under the traditional soft DC starting method, avoid the voltage divergence of the MMC voltage source converter submodule capacitor without generating excessive charging current, and improve the starting stability of the supercapacitor type SVG based on the MMC voltage source converter. It has great practical value and broad application prospects.

[0104] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. An SVG topological structure, characterized in that: include: A starting circuit, an MMC voltage source converter branch and a supercapacitor energy storage branch, wherein a circuit breaker is provided between the MMC voltage source converter and the supercapacitor, wherein: The starting circuit is composed of a first switch and a starting resistor in parallel; the MMC voltage source converter branch is composed of six bridge arms; and the supercapacitor energy storage branch is composed of M energy storage units and two current limiting reactors in series.

2. The SVG topological structure according to claim 1, characterized in that: The bridge arm includes N submodules and 1 bridge arm reactor, and the submodule includes one or more of a half-bridge submodule and a full-bridge submodule; the half-bridge submodule, the full-bridge submodule and the bridge arm reactor are connected in series.

3. The SVG topological structure according to claim 2, characterized in that: The half-bridge submodule includes a first DC capacitor, a first switch module and a second switch module; wherein, The first switch module and the second switch module are connected in series and then connected in parallel with the first DC capacitor; The first switch module includes a first switch tube and a first diode anti-parallel connected to the first switch tube, and the second switch module includes a second switch tube and a second diode anti-parallel connected to the second switch tube.

4. The SVG topological structure according to claim 2, characterized in that: The full-bridge submodule includes a second DC capacitor, a third switch module, a fourth switch module, a fifth switch module and a sixth switch module; wherein, After the third switch module and the fourth switch module are connected in series, and after the fifth switch module and the sixth switch module are connected in series, they are respectively connected in parallel with the second DC capacitor.

5. The SVG topological structure according to claim 4, characterized in that: The third switch module includes a third switch tube and a third diode anti-parallel to the third switch tube, the fourth switch module includes a fourth switch tube and a fourth diode anti-parallel to the fourth switch tube, the fifth switch module includes a fifth switch tube and a fifth diode anti-parallel to the fifth switch tube, and the sixth switch module includes a sixth switch tube and a sixth diode anti-parallel to the sixth switch tube.

6. The SVG topological structure according to claim 1, characterized in that: The energy storage unit includes a super capacitor, a third DC capacitor, a seventh switch module, an eighth switch module, a second switch and a third switch; wherein, The seventh switch module and the eighth switch module are connected in series and then connected in parallel with the third DC capacitor; the second switch, the third switch and the super capacitor are connected in series and then connected in parallel with the third DC capacitor.

7. A phased startup method for an SVG topology structure, comprising an MMC voltage source converter full-half hybrid bridge startup phase, a supercapacitor branch zero voltage startup phase, a supercapacitor uncontrolled charging phase, and a supercapacitor constant current charging phase, applied to the SVG topology structure as claimed in any one of claims 1 to 6, characterized in that: include: During the full-half hybrid bridge startup phase of the MMC voltage source converter, the circuit breaker is set to an open state; The MMC voltage source converter branch is uncontrolledly charged by an AC power supply until the voltage value of the MMC voltage source converter branch reaches a first preset threshold value; the MMC voltage source converter branch is controllably charged according to a first preset strategy until the capacitor voltage of the MMC voltage source converter submodule reaches a second preset threshold value; In the zero voltage starting stage of the supercapacitor branch, the MMC voltage source converter is set to a fixed DC voltage control, the output voltage of the MMC voltage source converter branch is controlled, the DC side voltage of the MMC voltage source converter branch is controlled to zero, and the circuit breaker is closed; In the uncontrolled charging stage of the supercapacitor branch, the power module of the supercapacitor branch is set in a locked state, the DC side voltage of the MMC voltage source converter branch is raised to the rated voltage, and the supercapacitor is uncontrolled charged through each diode until the voltage value of the supercapacitor branch submodule reaches a third preset threshold value; In the controllable charging stage of the supercapacitor branch, the power submodules of the supercapacitor branch are unlocked in order, and the supercapacitor is controllably charged according to the second preset strategy until the voltage value of the supercapacitor submodule reaches a fourth preset threshold to complete the startup process.

8. The phased startup method of the SVG topology structure according to claim 7, characterized in that: The second preset threshold is greater than the first preset threshold.

9. The method for starting up the SVG topology structure in stages according to claim 7, characterized in that: The first preset threshold is expressed as: Among them, U ac is the equivalent phase potential amplitude of the AC system, and N is the number of sub-modules in a single bridge arm of the MMC voltage source converter.

10. The phased startup method of the SVG topology structure according to claim 7, characterized in that: The voltage value of the supercapacitor branch is expressed as: Among them, U(t) is the DC side voltage of the MMC converter; T is the charging time of the supercapacitor branch; I(t) is the charging current of the supercapacitor branch; C C is the total capacitance of the supercapacitor branch; R c is the internal resistance of the supercapacitor.

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