High-voltage direct-current transformer with symmetrical bipolar structure

Abstract

The invention discloses a high-voltage direct-current transformer with a symmetrical bipolar structure, which comprises a low-voltage side current conversion module, an alternating-current bus, a high-voltage side anode current conversion unit and a high-voltage side cathode current conversion unit, and is characterized in that the input end of the low-voltage side current conversion module is electrically connected with a low-voltage side direct-current voltage, and the output end of the low-voltage side current conversion module is electrically connected with the alternating-current bus; the input end of the high-voltage side positive electrode current conversion unit and the input end of the high-voltage side negative electrode current conversion unit are electrically connected with an alternating current bus, the output end of the high-voltage side positive electrode current conversion unit and the output end of the high-voltage side negative electrode current conversion unit serve as the positive electrode output end and the negative electrode output end of the high-voltage direct current transformer, and the low-voltage side current conversion module is of a symmetrical single-pole structure. The high-voltage side current conversion module is of a symmetrical bipolar structure. The high-voltage direct-current transformer is of a bisymmetric structure, the bisymmetric structure can provide higher transmission capacity, transmission of high-voltage-class voltage is achieved, and meanwhile the cost of the high-voltage direct-current transformer is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of high-voltage direct current transformers, and in particular relates to a high-voltage direct current transformer with a symmetrical bipolar structure. Background Art

[0002] With the transformation of the global energy structure and the rapid development of smart grids, the DC grid technology based on flexible DC transmission has increasingly shown its broad application prospects in the consumption of new energy and the construction of smart grids due to its significant controllability, strong adaptability and flexible operation mode. In the current DC grid technology system, high-voltage DC transformers have become key equipment in DC grids because they can achieve conversion and matching between different voltage levels.

[0003] At present, high-voltage DC transformer technology usually adopts a symmetrical unipolar (pseudo-bipolar) structure, and both the high-voltage side and the low-voltage side use a modular multilevel converter topology. When the voltage level is too high, a large number of modular multilevel converters are required. Since modular level converters are relatively expensive, not only are the costs relatively high, but very high requirements are also placed on the hardware system (mainly the control and protection system), making it difficult to truly achieve a higher level of DC voltage.

[0004] Therefore, a high-voltage DC transformer with relatively low cost and high voltage level is needed. Summary of the invention

[0005] In view of this, the present invention provides a high-voltage DC transformer with a symmetrical bipolar structure, the main purpose of which is to solve the problem that the existing high-voltage DC transformer has a relatively high cost and a low voltage level.

[0006] In order to solve the above problems, the present application provides a high-voltage DC transformer with a symmetrical bipolar structure, including a low-voltage side commutation module, an AC bus and a high-voltage side commutation module, wherein the high-voltage side commutation module includes a high-voltage side positive commutation unit and a high-voltage side negative commutation unit, wherein:

[0007] The input end of the low-voltage side converter module is electrically connected to the low-voltage side DC voltage, the output end of the low-voltage side converter module is electrically connected to the AC bus, the input end of the high-voltage side positive converter unit is electrically connected to the AC bus, the output end of the high-voltage side positive converter unit serves as the positive output end of the high-voltage DC transformer, the input end of the high-voltage side negative converter unit is electrically connected to the AC bus, and the output end of the high-voltage side negative converter unit serves as the negative output end of the high-voltage DC transformer, wherein the low-voltage side converter module is a symmetrical unipolar structure, and the high-voltage side converter module is a symmetrical bipolar structure;

[0008] The low-voltage side conversion module is used to convert the low-voltage side DC voltage into an AC voltage, the high-voltage side positive conversion unit is used to convert the AC voltage into a positive DC voltage with a preset amplitude as the positive voltage of the high-voltage DC transformer, and the high-voltage side negative conversion unit is used to convert the AC voltage into a negative DC voltage with a preset amplitude as the negative voltage of the high-voltage DC transformer.

[0009] In one embodiment of the present invention, optionally, the high-voltage side positive current conversion unit includes a first converter transformer, a second converter transformer, a first grid-commutated converter and a second grid-commutated converter, wherein:

[0010] The input end of the first converter transformer is electrically connected to the AC bus, the output end of the first converter transformer is electrically connected to the input end of the first grid-commutated converter, the negative output end of the first grid-commutated converter is electrically connected to the ground, and the positive output end of the first grid-commutated converter is electrically connected to the negative output end of the second grid-commutated converter;

[0011] The input end of the second converter transformer is electrically connected to the AC bus, the output end of the second converter transformer is electrically connected to the input end of the second grid-commutated converter, and the positive output end of the second grid-commutated converter serves as the positive output end of the high-voltage DC transformer.

[0012] In one embodiment of the present invention, optionally, the high-voltage side negative pole commutation unit includes a third converter transformer, a fourth converter transformer, a first modular multilevel converter and a second modular multilevel converter, wherein:

[0013] The input end of the third converter transformer is electrically connected to the AC bus, the output end of the third converter transformer is electrically connected to the input end of the first modular multilevel converter, the positive output end of the first modular multilevel converter is electrically connected to the ground end, and the negative output end of the first modular multilevel converter is electrically connected to the positive output end of the second modular multilevel converter;

[0014] The input end of the fourth converter transformer is electrically connected to the AC bus, the output end of the fourth converter transformer is electrically connected to the input end of the second modular multilevel converter, and the negative output end of the second modular multilevel converter serves as the negative output end of the high-voltage DC transformer.

[0015] In one embodiment of the present invention, optionally, the high-voltage side positive pole commutation unit further includes a plurality of single-tuned filters, the plurality of single-tuned filters are electrically connected to the AC bus, and the rated fundamental reactive power of the single-tuned filters fully compensates for the reactive power absorbed by the first grid-commutated converter and the second grid-commutated converter under rated operating conditions.

[0016] In one embodiment of the present invention, optionally, the winding at the input end of the converter transformer adopts a Y-type connection mode, and the neutral point of the Y-type connection mode is grounded through a series-connected inductor and a resistor.

[0017] In one embodiment of the present invention, optionally, the low-voltage side conversion module is a third modular multilevel converter, the positive and negative input terminals of the third modular multilevel converter are electrically connected to the positive and negative poles of the low-voltage side DC voltage respectively, and the output terminal of the third modular multilevel converter is electrically connected to the AC bus.

[0018] In one embodiment of the present invention, optionally, the symmetrical bipolar high-voltage DC transformer further includes a low-voltage side DC energy consumption module, and the low-voltage side DC energy consumption module includes a hysteresis comparator, a power electronic switch and an energy consumption resistor, wherein:

[0019] The positive phase input terminal of the hysteresis comparator is electrically connected to the threshold voltage, the negative phase input terminal of the hysteresis comparator is electrically connected to the instantaneous effective value voltage of the three-phase voltage of the AC bus, the output terminal of the hysteresis comparator is electrically connected to the control terminal of the power electronic switch, the first end of the power electronic switch is electrically connected to the positive input terminal of the third modular multilevel converter, the second end of the power electronic switch is electrically connected to the first end of the energy dissipation resistor, and the second end of the energy dissipation resistor is electrically connected to the negative input terminal of the third modular multilevel converter.

[0020] In one embodiment of the present invention, optionally, the operating mode of the high-voltage DC transformer includes a normal operating mode and a fault operating mode. In the normal operating mode, the high-voltage side positive pole commutation unit operates based on a constant DC current control mode, the high-voltage side negative pole commutation unit operates based on a V / f control mode, and the low-voltage side commutation module operates based on a constant DC voltage control mode.

[0021] In one embodiment of the present invention, optionally, the fault operation module of the high-voltage DC transformer includes a high-voltage side positive commutation unit exiting operation mode, a high-voltage side negative commutation unit exiting operation mode and a high-voltage side negative line exiting operation mode.

[0022] The present invention provides a high-voltage direct current transformer with a symmetrical bipolar structure. The low-voltage side direct current voltage is converted into an alternating current voltage by a low-voltage side converter module and transmitted to the alternating current bus of the high-voltage direct current transformer. The positive converter unit on the high-voltage side converts the alternating current voltage on the alternating current bus into a positive direct current voltage as the positive voltage of the high-voltage direct current transformer. The negative converter unit on the high-voltage side converts the alternating current voltage on the alternating current bus into a negative direct current voltage as the negative voltage of the high-voltage direct current transformer. The high-voltage direct current transformer presents a symmetrical bipolar structure. The symmetrical bipolar structure can provide a higher transmission capacity and realize the transmission of a high-level voltage of the high-voltage direct current transformer. Since it is difficult to construct a symmetrical bipolar structure using multiple modular multilevel converters, the symmetrical bipolar structure in the present application is not constructed using a large number of modular multilevel converters, thereby reducing the cost of the high-voltage direct current transformer.

[0023] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention, it can be implemented according to the contents of the specification. In order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0025] Figure 1 The structure block diagram of a high-voltage DC transformer with a symmetrical bipolar structure according to an exemplary embodiment of the present invention.

[0026] Figure 2 This is a circuit schematic diagram of a high-voltage DC transformer with a symmetrical bipolar structure according to an exemplary embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram of a constant DC current control strategy of a high-voltage side positive commutation unit in a normal operating mode of a high-voltage DC transformer with a symmetrical bipolar structure according to an exemplary embodiment of the present invention.

[0028] Figure 4 It is a structural schematic diagram of an exemplary embodiment of the present invention in which a positive current commutation unit on the high-voltage side of a symmetrical bipolar high-voltage DC transformer exits an operating mode.

[0029] Figure 5 It is a structural schematic diagram of an exemplary embodiment of the present invention in which a negative pole commutation unit on the high-voltage side of a symmetrical bipolar structure high-voltage DC transformer exits an operating mode.

[0030] Figure 6 This is a schematic diagram of a constant DC current control strategy for a high-voltage side positive commutation unit of a symmetrical bipolar high-voltage DC transformer in an exit operation mode according to an exemplary embodiment of the present invention.

[0031] Figure 7 It is a structural schematic diagram of a high-voltage side negative pole line exiting operation mode of a high-voltage DC transformer with a symmetrical bipolar structure according to an exemplary embodiment of the present invention.

[0032] Figure 8 This is a schematic diagram of a constant DC current control strategy for a high-voltage side positive commutation unit of a high-voltage DC transformer with a symmetrical bipolar structure in an exit operation mode of a high-voltage side negative line according to an exemplary embodiment of the present invention.

[0033] in,

[0034] The numbers are as follows: 11-low-voltage side converter module; 12-AC bus; 13-high-voltage side positive converter unit; 14-high-voltage side negative converter unit; 131-first grid phase-commutating converter; 132-second grid phase-commutating converter; 133-first converter transformer; 134-second converter transformer; 135-single-tuned filter; 141-first modular multilevel converter; 142-second modular multilevel converter; 143-third converter transformer; 144-fourth converter transformer. DETAILED DESCRIPTION

[0035] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0036] In order to further explain the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the specific implementation methods, structures, features and effects of the present invention application are described in detail below in conjunction with the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "embodiment" does not necessarily refer to the same embodiment. In addition, specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form.

[0037] Combine the following Figures 1 to 8 A high-voltage DC transformer with a symmetrical bipolar structure according to some embodiments of the present invention is described.

[0038] In one embodiment, Figure 1 As shown, a high-voltage DC transformer with a symmetrical bipolar structure includes a low-voltage side commutation module 11, an AC bus 12 and a high-voltage side commutation module, wherein the high-voltage side commutation module includes a high-voltage side positive commutation unit 13 and a high-voltage side negative commutation unit 14, wherein:

[0039] The input end of the low-voltage side converter module 11 is electrically connected to the low-voltage side DC voltage 20, the output end of the low-voltage side converter module 11 is electrically connected to the AC bus 12, the input end of the high-voltage side positive converter unit 13 is electrically connected to the AC bus 12, the output end of the high-voltage side positive converter unit 13 serves as the positive output end of the high-voltage DC transformer, the input end of the high-voltage side negative converter unit 14 is electrically connected to the AC bus 12, and the output end of the high-voltage side negative converter unit 14 serves as the negative output end of the high-voltage DC transformer, wherein the low-voltage side converter module 11 is a symmetrical unipolar structure, and the high-voltage side converter module is a symmetrical bipolar structure;

[0040] The low-voltage side conversion module 11 is used to convert the low-voltage side DC voltage into an AC voltage, the high-voltage side positive conversion unit 13 is used to convert the AC voltage into a positive DC voltage with a preset amplitude as the positive voltage of the high-voltage DC transformer, and the high-voltage side negative conversion unit 14 is used to convert the AC voltage into a negative DC voltage with a preset amplitude as the negative voltage of the high-voltage DC transformer.

[0041] Specifically, the low-voltage side converter module is mainly responsible for the conversion and transmission of electric energy on the low-voltage side. In the present application, the low-voltage side converter module converts the DC voltage on the low-voltage side into an AC voltage and transmits it to the AC bus of the high-voltage transformer. The DC positive and negative ports of the low-voltage side converter module are connected to the low-voltage side DC line through a smoothing reactor, and the AC port of the low-voltage side converter module is directly connected to the AC bus of the high-voltage DC transformer. The low-voltage side converter module can be composed of a modular multi-level converter, and the low-voltage side converter module adopts a symmetrical monopole structure, where the positive pole is used for power transmission, and the symmetrical monopole structure can reduce line costs.

[0042] The high-voltage side converter adopts a symmetrical bipolar structure, that is, it has two symmetrical electrodes (positive and negative), the voltage and current of the two electrodes are equal but in opposite directions. The positive commutation unit on the high-voltage side is the positive pole in the symmetrical bipolar structure, and the negative commutation unit on the high-voltage side is the negative pole in the symmetrical bipolar structure. The positive commutation unit on the high-voltage side can adopt a 12-pulse or dual 12-pulse grid-commutated converter (LCC), and the negative commutation unit on the high-voltage side can adopt a modular multilevel converter; the AC port of the positive commutation unit on the high-voltage side is connected to the AC bus, the low-voltage output end of the positive commutation unit on the high-voltage side is connected to the grounding electrode, and the high-voltage end of the positive commutation unit on the high-voltage side serves as the positive DC port of the high-voltage DC transformer, and is connected to the positive DC line on the high-voltage side through a smoothing reactor; the AC port of the negative commutation unit on the high-voltage side is connected to the AC bus, the high-voltage end of the negative commutation unit on the high-voltage side is connected to the grounding electrode, and the low-voltage end of the negative commutation unit on the high-voltage side serves as the negative DC port of the high-voltage DC transformer, and is connected to the negative DC line on the high-voltage side through a smoothing reactor.

[0043] Compared with the prior art, the present application provides a high-voltage DC transformer with a symmetrical bipolar structure, which converts the low-voltage side DC voltage into an AC voltage through a low-voltage side converter module and transmits it to the AC bus of the high-voltage DC transformer. The high-voltage side positive converter unit converts the AC voltage on the AC bus into a positive DC voltage, which serves as the positive voltage of the high-voltage DC transformer. The high-voltage side negative converter unit converts the AC voltage on the AC bus into a negative DC voltage, which serves as the negative voltage of the high-voltage DC transformer. The high-voltage DC transformer presents a symmetrical bipolar structure, which can provide a higher transmission capacity and realize the transmission of high-level voltage of the high-voltage DC transformer. Since it is difficult to construct a symmetrical bipolar structure with multiple modular level converters, the symmetrical bipolar structure in the present application does not use a large number of modular level converters to build it, thereby reducing the cost of the high-voltage DC transformer.

[0044] In one embodiment, Figure 2 As shown, the high-voltage side positive current conversion unit 13 includes a first converter transformer 133, a second converter transformer 134, a first grid-commutated converter 131 and a second grid-commutated converter 132, wherein:

[0045] The input end of the first converter transformer 133 is electrically connected to the AC bus 12, the output end of the first converter transformer 133 is electrically connected to the input end of the first grid-commutated converter 131, the negative output end of the first grid-commutated converter 131 is electrically connected to the ground, and the positive output end of the first grid-commutated converter 131 is electrically connected to the negative output end of the second grid-commutated converter 132;

[0046] The input end of the second converter transformer 134 is electrically connected to the AC bus 12, the output end of the second converter transformer 134 is electrically connected to the input end of the second grid-commutating converter 132, and the positive output end of the second grid-commutating converter 132 serves as the positive output end of the high-voltage DC transformer.

[0047] In this embodiment, the converter transformer has the function of converting voltage. It can convert the AC voltage with a first preset amplitude on the AC bus into an AC voltage with a second preset amplitude required by the converter, thereby ensuring that the converter can work normally. At the same time, it also isolates the converter from the AC bus to protect the converter. Therefore, the first grid phase-commutation converter is connected to the AC bus through the first converter transformer, and the second grid phase-commutation converter is connected to the AC bus through the second converter transformer.

[0048] The low voltage output end of the first grid-commutated converter is connected to the ground electrode, the high voltage end of the first grid-commutated converter is connected to the low voltage output end of the second grid-commutated converter, and the high voltage end of the second grid-commutated converter serves as the positive DC port of the high voltage DC transformer. As a preferred implementation of this embodiment, the first grid-commutated converter and the second grid-commutated converter use 12-pulse or double 12-pulse grid-commutated converters.

[0049] The grid-commutated converter is a power electronic device that uses thyristors and other devices to achieve current commutation through the commutation voltage provided by the AC system. In the rectification state, it can convert AC power into DC power; in the inverter state, it can convert DC power into AC power. The thyristors in the grid-commutated converter can only be turned on and not off, so their bias depends on the power used for commutation on the AC side of the grid. By controlling the trigger angle of the thyristor through the controller, the phase angle control of the AC wave can be achieved, thereby adjusting the direction and magnitude of the current.

[0050] In one embodiment, Figure 2 As shown, the high-voltage side negative pole converter unit 14 includes a third converter transformer 143, a fourth converter transformer 144, a first modular multilevel converter 141 and a second modular multilevel converter 142, wherein:

[0051] The input end of the third converter transformer 143 is electrically connected to the AC bus 12, the output end of the third converter transformer 143 is electrically connected to the input end of the first modular multilevel converter 141, the positive output end of the first modular multilevel converter 141 is electrically connected to the ground end, and the negative output end of the first modular multilevel converter 141 is electrically connected to the positive output end of the second modular multilevel converter 142;

[0052] The input end of the fourth converter transformer 144 is electrically connected to the AC bus 12, the output end of the fourth converter transformer 144 is electrically connected to the input end of the second modular multilevel converter 142, and the negative output end of the second modular multilevel converter 142 serves as the negative output end of the high-voltage DC transformer.

[0053] Specifically, the first modular multilevel converter is connected to the AC bus through the third converter transformer, and the first modular multilevel converter is connected to the AC bus through the fourth converter transformer.

[0054] The high voltage output terminal of the first modular multilevel converter is connected to the ground electrode, the low voltage terminal of the first modular multilevel converter is connected to the high voltage output terminal of the second modular multilevel converter, and the low voltage terminal of the second modular multilevel converter serves as the negative DC port of the high voltage DC transformer.

[0055] The modular multilevel converter is composed of a plurality of cascaded sub-modules (SM) with the same structure. These sub-modules can be half H-bridge type, full H-bridge type or double clamp sub-module type. Each sub-module contains components such as high-voltage DC capacitors, semiconductor switching devices (such as IGBTs) and diodes. The multi-level output of voltage and current is achieved by controlling the switching state of the semiconductor switching devices of the sub-module. Specifically, each sub-module can output several fixed levels, and the specific output level is determined by the controller. In the high-voltage DC transmission project, there are usually hundreds of sub-modules in the bridge arm of the modular multi-level converter. Therefore, under the control of the controller, the sub-module can make the bridge arm output a step wave voltage (i.e., a multi-level voltage), so that the bridge arm voltage approaches its reference value in a nearly continuous manner.

[0056] In one embodiment, Figure 2 As shown, the high-voltage side positive current conversion unit 13 also includes a plurality of single-tuned filters 135, which are electrically connected to the AC bus 12, and the rated fundamental wave reactive power of the single-tuned filters 135 fully compensates for the reactive power absorbed by the first grid-commutated converter 131 and the second grid-commutated converter 132 under rated operating conditions.

[0057] Specifically, a single tuned filter needs to be installed on the AC bus. For example, a 12th order single tuned filter and a 24th order single tuned filter are connected to the AC bus. The rated fundamental wave reactive power of the single tuned filter should be able to fully compensate for the reactive power absorbed by the first grid commutation converter and the second grid commutation converter under the rated operating conditions (denoted as Q n ), the rated operating condition means that the AC bus voltage is the rated value and the active power of the grid-commutated converter is equal to its rated power.

[0058] The single tuned filter is a filter specially used to absorb single harmonics. It is mainly composed of filter capacitors, reactors and resistors in series, and is tuned to the corresponding harmonic frequency through precise calculation and combination. In this way, when a specific harmonic appears in the power grid, the single tuned filter can effectively absorb it and ensure the cleanliness and stability of the power grid.

[0059] As a preferred embodiment, the present application uses a 12th-order single-tuned filter and a 24th-order single-tuned filter, and the rated fundamental reactive power of the 12th-order single-tuned filter and the rated fundamental reactive power of the 24th-order single-tuned filter are allocated in a ratio of 2:1.

[0060] The fundamental rated reactive power of each filter is calculated according to the following steps. First, the commutation angle of the grid-commutated converter under rated conditions is calculated as shown in formula (1):

[0061]

[0062] Then, the reactive power absorbed by the grid-commutated converter under rated conditions is calculated according to formula (2):

[0063]

[0064] Therefore, the rated fundamental reactive power of the 12th and 24th order single tuned filters are 275.72 Mvar and 137.85 Mvar respectively.

[0065] In one embodiment, the winding at the input end of the converter transformer adopts a Y-type connection mode, and the neutral point of the Y-type connection mode is grounded through a series reactor and a resistor.

[0066] Specifically, the windings of the high-voltage side converter transformer connected to the AC bus side are connected in a Y-type manner, and are grounded at the neutral point through a large reactor in series with a large resistor. When a fault occurs at the neutral point of the converter transformer, a grounding fault will be formed. If the grounding is not connected in series with a large reactor, the short-circuit current is extremely large, which may endanger personal safety. However, grounding through a large reactor in series with a large resistor can reduce the short-circuit current to a safe range, thereby reducing the hazard to personal safety. For example, a 200H large reactor is connected in series with a 1000Ω large resistor. The two converter transformers connected to the grid phase-commutating converter have windings connected to the converter side in a Y-type and a △-type connection respectively, and both are ungrounded; the two converter transformers connected to the multilevel converter have windings connected to the converter side in a Y-type connection, and both are ungrounded.

[0067] In one embodiment, Figure 2 As shown, the low-voltage side converter module 11 is a third modular multilevel converter, the positive and negative input terminals of the third modular multilevel converter are electrically connected to the positive and negative poles of the low-voltage side DC voltage, and the output terminal of the third modular multilevel converter is electrically connected to the AC bus 12.

[0068] Specifically, the low-voltage side converter module is a third modular multilevel converter, the DC positive and negative ports of the third modular multilevel converter are connected to the low-voltage side DC line through a smoothing reactor, and the AC port of the third modular multilevel converter is directly connected to the AC bus of the high-voltage DC transformer. The third modular multilevel converter adopts a symmetrical monopole structure to reduce costs, and the positive pole is used for power transmission.

[0069] In one embodiment, the high-voltage DC transformer with a symmetrical bipolar structure further includes a low-voltage DC energy consumption module, which includes a hysteresis comparator, a power electronic switch and an energy consumption resistor, wherein:

[0070] The positive phase input terminal of the hysteresis comparator is electrically connected to the threshold voltage, the negative phase input terminal of the hysteresis comparator is electrically connected to the instantaneous effective value voltage of the three-phase voltage of the AC bus 12, the output terminal of the hysteresis comparator is electrically connected to the control terminal of the power electronic switch, the first end of the power electronic switch is electrically connected to the positive input terminal of the third modular multilevel converter, the second end of the power electronic switch is electrically connected to the first end of the energy dissipation resistor, and the second end of the energy dissipation resistor is electrically connected to the negative input terminal of the third modular multilevel converter.

[0071] Specifically, the overall on and off control of the low-voltage side DC energy consumption module is determined by a hysteresis comparator; the positive input terminal of the hysteresis comparator is connected to the threshold voltage, and the negative input terminal of the hysteresis comparator is connected to the instantaneous effective value voltage (the square root of the sum of the instantaneous values ​​of the three-phase voltage) u of the AC bus. acrms The upper threshold of the hysteresis comparator is 1.15 (per unit), and the lower threshold is 1.05 (per unit). acrms When the upper threshold of the hysteresis comparator is exceeded, the hysteresis comparator outputs a signal to turn on the DC energy consumption device, that is, the power electronic switch is closed, and the energy consumption resistor forms a loop with the low-voltage DC voltage. acrms When the voltage is lower than the lower threshold of the hysteresis comparator, the hysteresis comparator outputs a shutdown signal of the DC energy consumption device, that is, the power electronic switch is disconnected, and the energy consumption resistor and the low-voltage DC voltage do not form a loop. When the instantaneous effective value voltage of the three-phase voltage of the AC bus exceeds the upper threshold, the low-voltage DC energy consumption module is turned on as a whole. When a system failure occurs, the surplus power is discharged through the DC energy consumption device to avoid the increase of the DC transmission line voltage, thereby protecting the safety of the system and equipment.

[0072] In one embodiment, the operating mode of the high-voltage DC transformer includes a normal operating mode and a fault operating mode. In the normal operating mode, the high-voltage side positive pole commutation unit operates based on a constant DC current control mode, the high-voltage side negative pole commutation unit operates based on a V / f control mode, and the low-voltage side commutation module operates based on a constant DC voltage control mode.

[0073] Specifically, Figure 2The connection structure shown is the connection structure of the normal operating mode of the high-voltage DC transformer. In the normal operating mode, the high-voltage side positive converter unit adopts a constant DC current control method, the high-voltage side negative converter unit adopts V / f control for AC side network control, and the low-voltage side converter module adopts a constant DC voltage control. Among them, the voltage command value of the high-voltage side negative converter unit is the rated value of the AC bus voltage (the per-unit value is equal to 1.0pu), and the frequency command value is 50Hz; the control target of the constant DC voltage control of the low-voltage side converter module is the low-voltage side DC voltage, and its command value is equal to 1.0pu; the current command of the constant DC current control of the high-voltage side positive converter unit needs to be given by the controller, and the control target is that the active power of the high-voltage side positive converter is equal to the active power of the high-voltage side negative converter.

[0074] In normal operation mode, the positive commutation unit on the high voltage side works based on a constant DC current control method, such as Figure 3 As shown, its input is the active power measurement value P of the negative pole commutation unit on the high voltage side. dcHn and the active power measurement value P of the positive commutation unit on the high voltage side dcHp After the two are subtracted, the PI controller outputs the current command value I of the positive commutation unit on the high-voltage side. dcp_ref ;I dcp_ref and the positive DC voltage measurement value I dcp After subtraction, the PI controller outputs the trigger angle command value α of the positive commutation unit on the high-voltage side. ord , the positive commutation unit on the high-voltage side is based on α ord Execute the trigger angle.

[0075] In one embodiment, the fault operation mode of the high-voltage DC transformer includes a high-voltage side positive commutation unit exiting operation mode, a high-voltage side negative commutation unit exiting operation mode, and a high-voltage side negative line exiting operation mode.

[0076] Specifically, the structural connection diagram of the high-voltage DC transformer when the high-voltage side positive commutation unit exits the operation mode is as follows: Figure 4 In this mode, the AC side grid control of the negative commutation unit on the high-voltage side adopts V / f control, and its voltage command value is the rated value of the AC bus voltage (the per-unit value is equal to 1.0pu), and the frequency command value is 50Hz; the DC voltage command value of the constant DC voltage control of the low-voltage side commutation module is equal to its rated value, the positive DC line on the high-voltage side needs to be connected to the grounding electrode as a metal return line, and the 12th single-tuned filter and the 24th single-tuned filter need to be disconnected from the AC bus.

[0077] Specifically, the structural connection diagram of the high-voltage DC transformer when the high-voltage side negative pole commutation unit exits the operation mode is as follows: Figure 5In this mode, the AC side grid control of the negative pole commutation unit on the high voltage side adopts V / f control, with the voltage command value being the rated value of the AC bus voltage (the per unit value is equal to 1.0 pu), and the frequency command value being 50 Hz; Figure 6 As shown, the input of the constant DC current control of the positive commutation unit on the high voltage side is the reference value u of the DC voltage on the low voltage side. dcL_ref (taken as 1.0pu) and the measured value u dcL After the two are subtracted, the PI controller outputs the current command value I of the positive commutation unit on the high-voltage side. dcp_ref ;I dcp_ref and the positive DC current measurement value I dcp After subtraction, the PI controller outputs the trigger angle command value α of the positive commutation unit on the high-voltage side. ord , the positive commutation unit on the high-voltage side is based on α ord Execute the trigger angle. In addition, the negative DC line on the high voltage side needs to be connected to the ground electrode as a metal return line.

[0078] The structural connection diagram of the high-voltage DC transformer when the high-voltage side negative line is out of operation mode is as follows: Figure 7 In this mode, the AC side grid control of the negative pole commutation unit on the high voltage side adopts V / f control, with the voltage command value being the rated value of the AC bus voltage (the per unit value is equal to 1.0 pu), and the frequency command value being 50 Hz; Figure 8 As shown, the input of the constant DC current control of the positive commutation unit on the high-voltage side is the reference value u of the negative DC voltage on the high-voltage side. dcHn_ref (taken as 1.0pu) and the measured value u dcHn After the two are subtracted, the PI controller outputs the current command value I of the positive commutation unit on the high-voltage side. dcp_ref ;I dcp_ref and the positive DC voltage measurement value I dcp After subtraction, the PI controller outputs the trigger angle command value α of the positive commutation unit on the high-voltage side. ord , the positive commutation unit on the high-voltage side is based on α ord Execute the trigger angle; the control target of the fixed DC voltage control of the low-voltage side converter module is the low-voltage side DC voltage, and its command value is equal to 1.0pu.

[0079] In one embodiment, taking a certain high-voltage DC transformer as an example, its basic parameters are shown in Table 1.

[0080] Table 1

[0081]

[0082] It should be understood that various modifications may be made to the embodiments of the present application. Therefore, the above description should not be considered as limiting, but only as an example of an embodiment. Other modifications within the scope and spirit of the present application will occur to those skilled in the art.

[0083] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present application and, together with the general description of the present application given above and the detailed description of the embodiments given below, serve to explain the principles of the present application.

[0084] These and other characteristics of the present application will become apparent from the following description of a preferred form of embodiment given as a non-limiting example with reference to the accompanying drawings.

[0085] It should also be understood that although the present application has been described with reference to some specific examples, those skilled in the art will be able to readily implement many other equivalent forms of the present application.

[0086] The above and other aspects, features and advantages of the present application will become more apparent in view of the following detailed description when taken in conjunction with the accompanying drawings.

[0087] Specific embodiments of the present application are described hereinafter with reference to the accompanying drawings; however, it should be understood that the embodiments applied for are merely examples of the present application, which may be implemented in a variety of ways. Well-known and / or repeated functions and structures are not described in detail to avoid unnecessary or redundant details that obscure the present application. Therefore, the specific structural and functional details applied for herein are not intended to be limiting, but merely serve as a basis and representative basis for the claims to teach those skilled in the art to use the present application in a variety of ways with substantially any suitable detailed structure.

[0088] This specification may use the phrases "in one embodiment," "in another embodiment," "in yet another embodiment," or "in other embodiments," all of which may refer to one or more of the same or different embodiments according to the present application.

[0089] The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application. The protection scope of the present application is defined by the claims. Those skilled in the art may make various modifications or equivalent substitutions to the present application within the essence and protection scope of the present application, and such modifications or equivalent substitutions shall also be deemed to fall within the protection scope of the present application.

Claims

1. A high voltage DC transformer with a symmetrical bipolar structure, characterized in that: include: A low-voltage side commutation module, an AC busbar and a high-voltage side commutation module, wherein the high-voltage side commutation module includes a high-voltage side positive commutation unit and a high-voltage side negative commutation unit, wherein: The input end of the low-voltage side converter module is electrically connected to the low-voltage side DC voltage, the output end of the low-voltage side converter module is electrically connected to the AC bus, the input end of the high-voltage side positive converter unit is electrically connected to the AC bus, the output end of the high-voltage side positive converter unit serves as the positive output end of the high-voltage DC transformer, the input end of the high-voltage side negative converter unit is electrically connected to the AC bus, and the output end of the high-voltage side negative converter unit serves as the negative output end of the high-voltage DC transformer, wherein the low-voltage side converter module is a symmetrical unipolar structure, and the high-voltage side converter module is a symmetrical bipolar structure; The low-voltage side conversion module is used to convert the low-voltage side DC voltage into an AC voltage, the high-voltage side positive conversion unit is used to convert the AC voltage into a positive DC voltage with a preset amplitude as the positive voltage of the high-voltage DC transformer, and the high-voltage side negative conversion unit is used to convert the AC voltage into a negative DC voltage with a preset amplitude as the negative voltage of the high-voltage DC transformer.

2. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 1, characterized in that: The high-voltage side positive current conversion unit includes a first converter transformer, a second converter transformer, a first grid-commutated converter and a second grid-commutated converter, wherein: The input end of the first converter transformer is electrically connected to the AC bus, the output end of the first converter transformer is electrically connected to the input end of the first grid-commutated converter, the negative output end of the first grid-commutated converter is electrically connected to the ground, and the positive output end of the first grid-commutated converter is electrically connected to the negative output end of the second grid-commutated converter; The input end of the second converter transformer is electrically connected to the AC bus, the output end of the second converter transformer is electrically connected to the input end of the second grid-commutated converter, and the positive output end of the second grid-commutated converter serves as the positive output end of the high-voltage DC transformer.

3. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 1, characterized in that: The high-voltage side negative pole commutation unit comprises a third converter transformer, a fourth converter transformer, a first modular multilevel converter and a second modular multilevel converter, wherein: The input end of the third converter transformer is electrically connected to the AC bus, the output end of the third converter transformer is electrically connected to the input end of the first modular multilevel converter, the positive output end of the first modular multilevel converter is electrically connected to the ground end, and the negative output end of the first modular multilevel converter is electrically connected to the positive output end of the second modular multilevel converter; The input end of the fourth converter transformer is electrically connected to the AC bus, the output end of the fourth converter transformer is electrically connected to the input end of the second modular multilevel converter, and the negative output end of the second modular multilevel converter serves as the negative output end of the high-voltage DC transformer.

4. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 2, characterized in that: The high-voltage side positive pole commutation unit also includes a plurality of single-tuned filters, which are electrically connected to the AC bus, and the rated fundamental wave reactive power of the single-tuned filters fully compensates for the reactive power absorbed by the first grid-commutated converter and the second grid-commutated converter under rated operating conditions.

5. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 2 or 3, characterized in that: The winding at the input end of the converter transformer adopts a Y-type connection mode, and the neutral point of the Y-type connection mode is grounded through a series-connected reactor and a resistor.

6. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 1, characterized in that: The low-voltage side conversion module is a third modular multilevel converter, the positive and negative input terminals of the third modular multilevel converter are electrically connected to the positive and negative poles of the low-voltage side DC voltage respectively, and the output terminal of the third modular multilevel converter is electrically connected to the AC bus.

7. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 6, characterized in that: The high-voltage DC transformer of the symmetrical bipolar structure further includes a low-voltage side DC energy consumption module, and the low-voltage side DC energy consumption module includes a hysteresis comparator, a power electronic switch and an energy consumption resistor, wherein: The positive phase input terminal of the hysteresis comparator is electrically connected to the threshold voltage, the negative phase input terminal of the hysteresis comparator is electrically connected to the instantaneous effective value voltage of the three-phase voltage of the AC bus, the output terminal of the hysteresis comparator is electrically connected to the control terminal of the power electronic switch, the first end of the power electronic switch is electrically connected to the positive input terminal of the third modular multilevel converter, the second end of the power electronic switch is electrically connected to the first end of the energy dissipation resistor, and the second end of the energy dissipation resistor is electrically connected to the negative input terminal of the third modular multilevel converter.

8. The high-voltage DC transformer with a symmetrical bipolar structure according to any one of claims 1 to 7, characterized in that: The operating mode of the high-voltage DC transformer includes a normal operating mode and a fault operating mode. In the normal operating mode, the high-voltage side positive pole commutation unit operates based on a constant DC current control mode, the high-voltage side negative pole commutation unit operates based on a V / f control mode, and the low-voltage side commutation module operates based on a constant DC voltage control mode.

9. The high-voltage DC transformer with a symmetrical bipolar structure according to claim 8, characterized in that: The fault operation mode of the high-voltage DC transformer includes a high-voltage side positive commutation unit exit operation mode, a high-voltage side negative commutation unit exit operation mode and a high-voltage side negative line exit operation mode.

Images