DC power transmission receiving end and DC power transmission system

Abstract

The invention discloses a direct-current power transmission receiving end and a direct-current power transmission system. The direct-current power transmission receiving end comprises an inverter circuit and a receiving end transformer; the inverter circuit comprises a hybrid power grid commutation converter and a modular multi-level converter; a direct current first end of the hybrid power grid commutation converter is used for connecting a direct current power transmission line, a direct current second end of the hybrid power grid commutation converter is connected with a direct current first end of the modular multilevel converter, and an alternating current end of the hybrid power grid commutation converter is connected with a receiving end alternating current bus through a corresponding receiving end transformer; the direct current second end of the modular multilevel converter is grounded, and the alternating current end of the modular multilevel converter is connected with the receiving end alternating current bus through the corresponding receiving end transformer. In the embodiment of the invention, the hybrid power grid commutation converter and the modular multi-level converter which are connected in series are used at the direct-current transmission receiving end, so that the direct-current transmission receiving end has direct-current side fault ride-through capability and alternating-current side fault ride-through capability.

Description

Technical Field

[0001] The present application relates to the technical field of DC power transmission, and particularly to a receiving end of DC power transmission and a DC power transmission system. Background Art

[0002] The core equipment of DC power transmission technology is the converter. Currently, the mainstream converters are divided into current source converters (CSC) and voltage source converters (VSC).

[0003] Among them, the reactive power regulation ability of CSC has certain limitations. When a fault occurs on the AC side, it is necessary to rely on the AC power grid to balance the reactive power, which is difficult to meet the fast and active support requirements of high-voltage DC power transmission; due to its voltage source characteristics, VSC cannot effectively block the fault current loop when a fault occurs on the DC side. Summary of the Invention

[0004] The present application relates to the technical field of DC power transmission, and particularly to a receiving end of DC power transmission and a DC power transmission system, which have the ability to ride through DC side faults and AC side faults.

[0005] In a first aspect, an embodiment of the present application provides a receiving end of DC power transmission, including: an inverter circuit and a receiving end transformer; The inverter circuit includes a hybrid line-commutated converter and a modular multilevel converter; The first DC end of the hybrid line-commutated converter is used to connect to the DC transmission line. The second DC end of the hybrid line-commutated converter is connected to the first DC end of the modular multilevel converter. The AC end of the hybrid line-commutated converter is connected to the receiving end AC bus through the corresponding receiving end transformer; The second DC end of the modular multilevel converter is grounded. The AC end of the modular multilevel converter is connected to the receiving end AC bus through the corresponding receiving end transformer.

[0006] In a possible implementation, the receiving end of DC power transmission is a monopolar receiving end, and the receiving end of DC power transmission includes one inverter circuit; the DC transmission line is a DC positive bus.

[0007] In a possible implementation, the receiving end of DC power transmission is a bipolar receiving end, and the receiving end of DC power transmission includes two inverter circuits, namely a first inverter circuit and a second inverter circuit; the DC transmission line includes a DC positive bus and a DC negative bus; The first DC end of the hybrid line-commutated converter in the first inverter circuit is used to connect to the DC positive bus; The first DC end of the hybrid line-commutated converter in the second inverter circuit is used to connect to the DC negative bus.

[0008] In a possible implementation, the receiving end of the DC power transmission further includes: a controller; The controller is configured to, when the AC voltage drops, obtain the reactive power demand of the AC system; obtain a first reactive power command value according to the reactive power demand and the reactive power distribution coefficient of the hybrid line-commutated converter, and control the delay trigger angle of the hybrid line-commutated converter according to the first reactive power command value to reduce the reactive power consumed by the hybrid line-commutated converter or make the hybrid line-commutated converter generate reactive power; Obtain a second reactive power command value according to the reactive power demand and the reactive power distribution coefficient of the modular multilevel converter; control the three-phase AC voltage of the modular multilevel converter according to the second reactive power command value to make the modular multilevel converter generate reactive power.

[0009] In a second aspect, an embodiment of the present application provides a DC power transmission system, including: a DC power transmission sending end and the DC power transmission receiving end described in the first aspect embodiment; wherein, the DC power transmission sending end includes a rectification circuit and a sending end transformer; The DC first end of the rectification circuit is used to connect to the DC transmission line, the DC second end of the rectification circuit is grounded, and the AC end of the rectification circuit is connected to the sending end AC bus through the corresponding sending end transformer.

[0010] In a possible implementation, the DC power transmission sending end is a monopolar sending end, the DC power transmission sending end includes a rectification circuit, and the rectification circuit is a first rectification circuit; the DC transmission line is a DC positive bus.

[0011] In a possible implementation, the DC power transmission sending end is a bipolar sending end, the DC power transmission sending end includes two rectification circuits, which are a first rectification circuit and a second rectification circuit respectively; the DC transmission line includes a DC positive bus and a DC negative bus; The DC first end of the first rectification circuit is used to connect to the DC positive bus; The DC first end of the second rectification circuit is used to connect to the DC negative bus.

[0012] In a possible implementation, the first rectification circuit includes a hybrid line-commutated converter and a modular multilevel converter; The DC first end of the hybrid line-commutated converter is used to connect to the DC transmission line, the DC second end of the hybrid line-commutated converter is connected to the DC first end of the modular multilevel converter, and the AC end of the hybrid line-commutated converter is connected to the sending end AC bus through the corresponding sending end transformer; The DC second end of the modular multilevel converter is grounded, and the AC end of the modular multilevel converter is connected to the sending end AC bus through the corresponding sending end transformer.

[0013] In a possible implementation, the first rectification circuit includes a first modular multilevel converter and a second modular multilevel converter; The first DC terminal of the first modular multilevel converter is used to connect to the DC transmission line. The second DC terminal of the first modular multilevel converter is connected to the first DC terminal of the second modular multilevel converter. The AC terminal of the first modular multilevel converter is connected to the sending-end AC bus through the corresponding sending-end transformer. The second DC terminal of the second modular multilevel converter is grounded. The AC terminal of the second modular multilevel converter is connected to the sending-end AC bus through the corresponding sending-end transformer.

[0014] In a possible implementation, the first rectifier circuit includes a first line-commutated converter and a second line-commutated converter. The first DC terminal of the first line-commutated converter is used to connect to the DC transmission line. The second DC terminal of the first line-commutated converter is connected to the first DC terminal of the second line-commutated converter. The AC terminal of the first line-commutated converter is connected to the sending-end AC bus through the corresponding sending-end transformer. The second DC terminal of the second line-commutated converter is grounded. The AC terminal of the second line-commutated converter is connected to the sending-end AC bus through the corresponding sending-end transformer.

[0015] In the DC power receiving end provided by the embodiments of the present application, the inverter circuit includes two different types of inverter modules, namely a hybrid line-commutated converter and a modular multilevel converter. Since the hybrid line-commutated converter has unidirectional conductivity, in the case of a DC side fault, the fault current loop can be quickly blocked. Moreover, for the modular multilevel converter, in the case of an AC side fault, it can provide dynamic reactive power support to support the rapid recovery of the AC side fault. Therefore, the DC power receiving end provided by the embodiments of the present application has the ability to ride through DC side faults and AC faults. In addition, since neither the hybrid line-commutated converter nor the modular multilevel converter has the problem of commutation failure, the DC power receiving end provided by the embodiments of the present application does not have the problem of commutation failure. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of a DC power receiving end provided by an embodiment of the present application; Figure 2 It is an equivalent circuit diagram of a DC side fault provided by an embodiment of the present application; Figure 3 It is a schematic diagram of coordinating the reactive power of the HCC and the reactive power of the MMC provided by an embodiment of the present application; Figure 4 It is a schematic diagram of the control loop of the HCC provided by an embodiment of the present application; Figure 5 It is a schematic diagram of the control loop of the MMC provided by an embodiment of the present application; Figure 6 Schematic diagram of the receiving end of bipolar DC power transmission provided by an embodiment of the present application; Figure 7 Schematic diagram of an HCC provided by an embodiment of the present application; Figure 8 Schematic diagram of an MMC provided by an embodiment of the present application; Figure 9 Schematic diagram of a DC power transmission system provided by an embodiment of the present application; Figure 10 Schematic diagram of an LCC provided by an embodiment of the present application; Figure 11 Schematic diagram of another DC power transmission system provided by an embodiment of the present application; Figure 12 Schematic diagram of yet another DC power transmission system provided by an embodiment of the present application; Figure 13 Schematic diagram of still another DC power transmission system provided by an embodiment of the present application; Figure 14 Schematic diagram of a DC power transmission system provided by an embodiment of the present application; Figure 15 Schematic diagram of another DC power transmission system provided by an embodiment of the present application; Figure 16 Schematic diagram of a control device provided by an embodiment of the present application. Detailed implementation manners

[0017] In order to enable those skilled in the art to better understand the solution of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0018] The terms "first", "second", etc. in the specification and claims of the present application are used to distinguish different objects, rather than to describe a specific order of the objects. For example, the first sub-module and the second sub-module are used to distinguish different sub-modules, rather than to describe the specific order of the sub-modules.

[0019] In the embodiments of the present application, words such as "exemplary" or "for example" are used to indicate examples, illustrations or explanations. Any embodiment or design solution described as "exemplary" or "for example" in the embodiments of the present application should not be construed as being more preferred or having more advantages than other embodiments or design solutions. Specifically, the use of words such as "exemplary" or "for example" is intended to present relevant concepts in a specific manner.

[0020] In the description of the embodiments of the present application, unless otherwise specified, the meaning of "a plurality" refers to two or more. For example, a plurality of processing units means two or more processing units, etc.; a plurality of elements means two or more elements, etc.

[0021] To make the above objects, features, and advantages of the present application more apparent and understandable, the embodiments of the present application will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] See Figure 1 , which is a schematic diagram of a receiving end of a DC power transmission provided by an embodiment of the present application.

[0023] The receiving end of the DC power transmission provided by the embodiment of the present application includes: an inverter circuit 1000 and a receiving-end transformer (a first transformer T1 and a second transformer T2); the inverter circuit includes a hybrid line commutated converter (HCC) and a modular multilevel converter (MMC); the first DC end of the hybrid line commutated converter HCC is used to connect to the DC transmission line DC Line, the second DC end of the hybrid line commutated converter HCC is connected to the first DC end of the modular multilevel converter MMC, and the AC end of the hybrid line commutated converter HCC is connected to the receiving-end AC bus through the corresponding first transformer T1; the second DC end of the modular multilevel converter MMC is grounded, and the AC end of the modular multilevel converter MMC is connected to the receiving-end AC bus through the corresponding second transformer T2, and the receiving-end AC bus is connected to the power grid AC1.

[0024] In the embodiment of the present application, the receiving end of the DC power transmission includes two different types of inverter modules, namely HCC and MMC, and utilizes the respective advantages of HCC and MMC. HCC includes an integrated gate-commutated thyristor (IGCT). Since IGCT has unidirectional conductivity, the receiving end of the DC power transmission can block the fault current loop and achieve DC-side fault ride-through. In the case of an AC-side fault, MMC can provide dynamic reactive power support for the AC side, thereby supporting the rapid recovery of the AC-side fault. Therefore, the receiving end of the DC power transmission provided by the embodiment of the present application has the capabilities of DC-side fault ride-through and AC-side fault ride-through.

[0025] In the embodiments of the present application, the HCC includes IGCTs, where the IGCTs have the ability of self - turn - off and can quickly turn off the current when needed to avoid commutation failure; the sub - modules in the MMC include controllable switching tubes, such as IGBTs / IGCTs. Since the IGBTs / IGCTs have the ability of self - turn - off and can quickly turn off the current when needed to avoid commutation failure. Therefore, there is no problem of commutation failure in the receiving end of the DC power transmission provided in the embodiments of the present application.

[0026] In the embodiments of the present application, the MMC can achieve grid - forming control and provide active power and reactive power support. Therefore, when the HCC adopts the phase - controlled mode, the MMC can be used to realize reactive power support and then effectively regulate the receiving - end AC bus voltage. Since the HCC and the MMC can simultaneously perform rapid adjustment of active power to achieve frequency support, the receiving end of the DC bus provided in the embodiments of the present application has the ability of grid - forming control, thereby improving the stability of the DC power transmission system, enhancing the adaptability of the DC power transmission system, and promoting the consumption of new energy.

[0027] It should be understood that the phase - controlled mode of the HCC mentioned in the embodiments of the present application is to realize the regulation of the output voltage and power of the HCC by controlling the trigger angle of the IGCT. Among them, the phase - controlled mode of the HCC can be applied to large - scale energy base power transmission projects, such as long - distance power transmission from large - scale hydropower stations and thermal power stations.

[0028] Next, the ability of the receiving end of the DC power transmission to cross DC - side faults and AC - side faults will be introduced.

[0029] In the embodiments of the present application, when a fault occurs on the DC side of the receiving end of the DC power transmission, since the MMC adopts a fixed DC voltage control strategy, that is, constant - voltage control, the DC voltage U of the MMC dc_MMC can be stabilized within a certain range, and the MMC can be approximately equivalent to a voltage source. The DC voltage U of the HCC dc_HCC will change, and the HCC can be approximately equivalent to a series of IGCTs and a variable voltage source.

[0030] Exemplarily, the equivalent circuit diagram of the sending end of the DC power transmission in the embodiments of the present application during a DC - side fault is as Figure 2 shown; among them, Figure 2 the DC voltage of the HCC in (1) can be obtained through the following formula: a i is the delay trigger angle, T i is the transformation ratio of the commutation transformer, U aci is the line voltage of the commutation bus, and N is the number of 6 - pulse converters.

[0031] Further, according to Kirchhoff's voltage law, the voltage u across the IGCT valve can be obtained through the following formula (2) IGCT : (2) According to the above formula (2), in the case of a fault occurring on the DC side of the receiving end of DC transmission, the voltage u across the IGCT valve IGCT reverses.

[0032] In the embodiment of the present application, due to the unidirectional conductivity of the IGCT in the HCC, when the voltage u of the IGCT IGCT reverses, the IGCT can prevent the reverse flow of current, thereby quickly blocking the fault current.

[0033] When the voltage drops due to a fault on the AC side, since the HCC uses the fully controlled device IGCT as the commutation device, there is no need to increase the turn-off margin after the fault to avoid commutation failure problems. At the same time, as shown in the following formula (3), the HCC can reduce the reactive power consumed by the HCC or even make it generate reactive power by increasing the delay trigger angle a i in this way. Therefore, the HCC can provide a certain amount of reactive power support under transient conditions.

[0034] (3) where μ is the commutation time, P is the active power, and Q coni is the reactive power consumed by the HCC.

[0035] In the embodiment of the present application, for the HCC, the reactive power consumed by the HCC can be reduced or reactive power can be generated by the delay trigger angle a i to provide a certain amount of reactive power support for the AC system.

[0036] Further, since the MMC control is fast and flexible, in the case of an AC side fault, when the AC voltage drops, the MMC can quickly adjust the reactive power output to provide dynamic reactive power compensation, making up for the problem of insufficient reactive power regulation ability of the HCC during AC faults, thereby improving the voltage support ability of the AC power grid. The embodiment of the present application can support the rapid recovery of the AC side fault at the receiving end of DC transmission by coordinating the reactive power of the HCC and the reactive power of the MMC.

[0037] For the convenience of understanding, the embodiment of the present application provides a schematic diagram of coordinating the reactive power of the HCC and the reactive power of the MMC. Refer to Figure 3 .

[0038] As Figure 3As shown in the figure, the first DC terminal of the hybrid line-commutated converter (HCC) is used to connect to the DC transmission line. The second DC terminal of the HCC is connected to the first DC terminal of the modular multilevel converter (MMC). The AC terminal of the HCC is connected to the receiving-end AC bus through the corresponding first transformer T1. The second DC terminal of the MMC is grounded. The AC terminal of the MMC is connected to the receiving-end AC bus through the corresponding second transformer T2. The receiving-end AC bus is connected to the power grid AC1.

[0039] First, by making full use of the reactive power regulation capabilities of each converter and comprehensively considering multi-dimensional constraints such as devices, equipment, and system operation modes, the maximum reactive power regulation amount ΔQm_HCC of the HCC and the maximum reactive power regulation amount ΔQm_MMC of the MMC are obtained. Among them, the maximum reactive power regulation amount ΔQm_HCC of the HCC and the maximum reactive power regulation amount ΔQm_MMC of the MMC are respectively the maximum ranges of the reactive power regulated by the HCC and the MMC.

[0040] Secondly, based on the maximum reactive power regulation amount ΔQm_HCC of the HCC and the maximum reactive power regulation amount ΔQm_MMC of the MMC, the reactive power distribution coefficient K of the HCC HCC and the reactive power distribution coefficient K of the MMC MMC are obtained. Among them, the reactive power distribution coefficients of the HCC are respectively K HCC and the reactive power distribution coefficient K of the MMC MMC The expressions of are shown in the following formulas (4) and (5): (4) (5) Then, in the case of an AC-side fault, the AC voltage drops from U to U f , and the reactive power demand of the AC system is obtained as ΔQ f . By fully considering the reactive power coordinated control capabilities of the HCC and the MMC, ΔQ f is distributed. Through the following formula (6), the reactive power distribution amount ΔQ of the HCC under the AC-side fault can be obtained HCC_ref , and through the following formula (7), the reactive power distribution amounts of the MMC are respectively ΔQ MMC_ref .

[0041] (6) (7) Finally, the reactive power distribution amount ΔQ of the HCC HCC_ref and the reactive power distribution amount ΔQ of the MMC HCC_refThey are respectively input into the reactive power control links of HCC and MMC as command values, so as to realize the coordinated regulation of the reactive power of HCC and the reactive power of MMC, and support the rapid recovery of the AC side fault at the receiving end of the DC transmission.

[0042] Exemplarily, an embodiment of the present application provides a schematic diagram of the control loop of HCC. Refer to Figure 4 .

[0043] As Figure 4 shown, according to the reactive power distribution ΔQ HCC_ref of HCC, the initial reactive power command Q HCC_ref and the system reactive power Q HCC , the reactive power difference ΔQ is obtained; the reactive power difference ΔQ is used as the input of the HCC control link, and the HCC control link outputs the delayed trigger angle a i ; by controlling the delayed trigger angle a i , the reactive power consumed by HCC is reduced or even it emits part of the reactive power, providing a certain reactive power support effect. It should be understood that the HCC control link in the embodiment of the present application is a mature technical means in the art and will not be elaborated here.

[0044] Exemplarily, an embodiment of the present application provides a schematic diagram of the control loop of MMC. Refer to Figure 5 .

[0045] As Figure 5 shown, according to the initial active power command P MMC_ref and the system active power P HCC , the active power difference ΔP is obtained; the active power difference ΔP is used as the input of the active power control link, and the active power link outputs the phase θ * of the AC voltage; according to the reactive power distribution ΔQ MMC_ref of MMC, the initial reactive power command Q MMC_ref and the system reactive power Q MMC , the reactive power difference ΔQ is obtained; the reactive power difference ΔQ is used as the input of the reactive power control link, and the reactive power control link outputs the amplitude V * of the AC voltage; the phase θ * of the AC voltage and the amplitude V * of the AC voltage are used as the inputs of the MMC inner loop control, and the MMC inner loop control outputs the three-phase AC voltage reference values u ca_ref , u cb_ref and u cc_ref ; by controlling the three-phase AC voltage, MMC emits part of the reactive power to provide reactive power support. It should be understood that the active power control link, the reactive power control link and the MMC inner loop control in the embodiment of the present application are all mature technical means in the art and will not be elaborated here.

[0046] In the embodiments of the present application, by jointly controlling the reactive power of the HCC and the reactive power of the MMC, the rapid recovery of the AC-side fault can be supported.

[0047] In the above embodiments, the receiving end of the unipolar DC power transmission is introduced. In order to improve the transmission efficiency of the DC power transmission, the embodiments of the present application also provide a receiving end of the bipolar DC power transmission, which will be described in detail below with reference to the accompanying drawings.

[0048] See Figure 6 , which is a schematic diagram of a receiving end of a bipolar DC power transmission provided by the embodiments of the present application.

[0049] As Figure 6 shown, the receiving end of the bipolar DC power transmission includes two inverter circuits, namely the first inverter circuit 1000 and the second inverter circuit 2000.

[0050] Among them, the first inverter circuit 1000 includes HCC1 and MMC1. The first DC end of HCC1 is connected to the DC positive bus, the second DC end of HCC1 is connected to the first DC end of MMC1, and the AC end of HCC1 is connected to the receiving-end AC bus DC Line1 through the receiving-end transformer T1; the second DC end of MMC1 is grounded, and the AC end of MMC1 is connected to the receiving-end AC bus through the receiving-end transformer T2. The second inverter circuit 2000 includes HCC2 and MMC2. The first DC end of HCC2 is connected to the DC negative bus DC Line2, the second DC end of HCC2 is connected to the first DC end of MMC2, and the AC end of HCC2 is connected to the receiving-end AC bus through the receiving-end transformer T3; the second DC end of MMC2 is grounded, and the AC end of MMC2 is connected to the receiving-end AC bus through the receiving-end transformer T4.

[0051] Exemplarily, the current of the DC positive bus DC Line1 is +800 kV, and the current of the DC negative bus DC Line2 is -800 kV.

[0052] Exemplarily, the embodiments of the present application provide a schematic diagram of an HCC. See Figure 7 .

[0053] As Figure 7As shown, the HCC includes a three-phase full-bridge circuit. Each phase includes an upper half-bridge arm and a lower half-bridge arm, and the midpoints of the upper half-bridge arm and the lower half-bridge arm are connected to alternating current. Among them, the first end of the first switching tube Q1 is connected to the DC positive pole, the second end of the first switching tube Q1 is connected to the first end of the fourth switching tube Q4, the second end of the fourth switching tube Q4 is connected to the DC negative pole, and the second end of the first switching tube Q1 is connected to ua; the first end of the third switching tube Q3 is connected to the DC positive pole, the second end of the third switching tube Q3 is connected to the first end of the sixth switching tube Q6, the second end of the sixth switching tube Q6 is connected to the DC negative pole, and the second end of the third switching tube Q3 is connected to ub; the first end of the fifth switching tube Q5 is connected to the DC positive pole, the second end of the fifth switching tube Q5 is connected to the first end of the second switching tube Q2, the second end of the second switching tube Q2 is connected to the DC negative pole, and the second end of the fifth switching tube Q5 is connected to uc.

[0054] It should be noted that the first switching tube Q1, the second switching tube Q2, the third switching tube Q3, the fourth switching tube Q4, the fifth switching tube Q5, and the sixth switching tube Q6 in the embodiments of the present application are all IGCTs.

[0055] Exemplarily, the embodiments of the present application provide a schematic diagram of an MMC. Refer to Figure 8 .

[0056] As Figure 8 shown, the MMC includes a three-phase power conversion circuit. Each phase includes an upper half-bridge arm and a lower half-bridge arm, and the midpoints of the upper half-bridge arm and the lower half-bridge arm are connected to alternating current. Among them, the upper half-bridge arm includes n sub-modules connected in series, the lower half-bridge arm includes n sub-modules connected in series, and n is an integer greater than or equal to 2.

[0057] It should be understood that the types of the first sub-module SM1, the second sub-module SM2... and the Nth sub-module are not specifically limited in the embodiments of the present application. For example, the first sub-module SM1, the second sub-module SM2... and the Nth sub-module can be half-bridge sub-modules or full-bridge sub-modules.

[0058] In addition, the types of the controllable switching tubes in the first sub-module SM1, the second sub-module SM2... and the Nth sub-module are not specifically limited in the embodiments of the present application. For example, the controllable switching tubes in the first sub-module SM1, the second sub-module SM2... and the Nth sub-module can be insulated gate bipolar transistors (IGBTs) or IGCTs.

[0059] Based on the DC power transmission receiving end introduced in the foregoing embodiments, the embodiments of the present application further provide a DC power transmission system, which will be introduced in detail below with reference to the accompanying drawings.

[0060] Refer to Figure 9, this figure is a schematic diagram of a DC power transmission system provided by an embodiment of the present application.

[0061] As Figure 9 shown, the DC power transmission system includes: a DC power transmission sending end and a DC power transmission receiving end. Among them, the DC power transmission sending end includes a rectifier circuit 3000 and a sending-end transformer (the third transformer T3 and the fourth transformer T4), and the DC power transmission receiving end includes an inverter circuit 1000 and a receiving-end transformer (the first transformer T1 and the second transformer T2).

[0062] The rectifier circuit 3000 includes a first line-commutated converter (LCC) and LCC2; among them, the first DC end of the LCC1 is connected to the DC transmission line DC Line, the second DC end of the LCC1 is connected to the first DC end of the LCC2, the AC end of the LCC1 is connected to the sending-end AC bus through the third transformer T3, and the sending-end AC bus is connected to the second power grid AC2; the second DC end of the LCC2 is grounded, and the AC end of the LCC2 is connected to the sending-end AC bus through the fourth transformer T4, and the sending-end AC bus is connected to the second power grid AC2.

[0063] The inverter circuit 1000 includes an HCC and an MMC. The first DC end of the HCC is connected to the DC transmission line DC Line, the second DC end of the HCC is connected to the first DC end of the MMC, the AC end of the HCC is connected to the receiving-end AC bus through the first transformer T1, and the receiving-end AC bus is connected to the first power grid AC1; the second DC end of the MMC is grounded, and the AC end of the MMC is connected to the receiving-end AC bus through the second transformer T2, and the receiving-end AC bus is connected to the first power grid AC1.

[0064] In the embodiment of the present application, the LCC is used as the rectifier circuit at the DC power transmission sending end, and it has the following advantages: First, the LCC mainly uses thyristors as power switching devices, and thyristors have the characteristics of mature manufacturing process and low production cost; Second, the LCC can withstand high voltages and currents and has strong power transmission capabilities. Therefore, in a high-voltage DC power transmission system, it can achieve large-capacity power transmission and meet the power transmission requirements of long distances and high powers; Third, the LCC depends on the AC system to provide commutation voltage and has good operating performance when connected to a strong AC system.

[0065] Exemplarily, the schematic diagram of the LCC is as Figure 10 shown.

[0066] As Figure 10As shown in the figure, the LCC includes a three-phase full-bridge circuit. Each phase includes an upper half-bridge arm and a lower half-bridge arm. The midpoints of the upper half-bridge arm and the lower half-bridge arm are connected to alternating current. Among them, the first end of the first switch Q1 is connected to the positive direct current, the second end of the first switch Q1 is connected to the first end of the fourth switch Q4, the second end of the fourth switch Q4 is connected to the negative direct current, and the second end of the first switch Q1 is connected to ua; the first end of the third switch Q3 is connected to the positive direct current, the second end of the third switch Q3 is connected to the first end of the sixth switch Q6, the second end of the sixth switch Q6 is connected to the negative direct current, and the second end of the third switch Q3 is connected to ub; the first end of the fifth switch Q5 is connected to the positive direct current, the second end of the fifth switch Q5 is connected to the first end of the second switch Q2, the second end of the second switch Q2 is connected to the negative direct current, and the second end of the fifth switch Q5 is connected to uc.

[0067] It should be noted that the first switch Q1, the second switch Q2, the third switch Q3, the fourth switch Q4, the fifth switch Q5, and the sixth switch Q6 in the embodiments of the present application are all thyristors.

[0068] Furthermore, in order to improve the transmission efficiency of the DC power transmission system, another DC power transmission system provided by the embodiments of the present application, for the schematic diagram of this DC power transmission system, see Figure 11 .

[0069] As Figure 11 shown in the figure, the DC power transmission system includes a DC power transmission sending end and a DC power transmission receiving end. Among them, the DC power transmission sending end includes a first rectifier circuit 3000, a second rectifier circuit 4000, and sending-end transformers (the fifth transformer T5, the sixth transformer T6, the seventh transformer T7, and the eighth transformer T8); the DC power transmission receiving end includes a first inverter circuit 1000, a second inverter circuit 2000, and receiving-end transformers (the first transformer T1, the second transformer T2, the third transformer T3, and the fourth transformer T4).

[0070] The first rectifier circuit 3000 includes LCC1 and LCC2. The first DC end of LCC1 is connected to the positive DC bus DCLine1. The second DC end of LCC1 is connected to the first DC end of LCC2. The AC end of LCC1 is connected to the sending-end AC bus through the fifth transformer T5. The sending-end AC bus is connected to the second power grid AC2; the second DC end of LCC2 is grounded. The AC end of LCC2 is connected to the sending-end AC bus through the sixth transformer T6. The sending-end AC bus is connected to the second power grid AC2.

[0071] The second rectifier circuit 4000 includes LCC3 and LCC4. The DC first end of LCC4 is connected to the DC positive bus DCLine2. The DC second end of LCC4 is connected to the DC first end of LCC3. The AC end of LCC4 is connected to the sending-end AC bus through the seventh transformer T7, and the sending-end AC bus is connected to the second power grid AC2. The DC second end of LCC3 is grounded, and the AC end of LCC3 is connected to the sending-end AC bus through the eighth transformer T8, and the sending-end AC bus is connected to the second power grid AC2.

[0072] It should be understood that the connection of the receiving end of DC power transmission has been introduced accordingly in the foregoing embodiments, and will not be elaborated here.

[0073] By adopting a bipolar DC power transmission sending end and a bipolar DC power transmission receiving end in the embodiments of the present application, the transmission efficiency of the DC power transmission system can be improved.

[0074] See Figure 12 , which is a schematic diagram of another DC power transmission system provided by the embodiments of the present application.

[0075] As Figure 12 shown, the DC power transmission system includes a DC power transmission sending end and a DC power transmission receiving end. Among them, the DC power transmission sending end includes a rectifier circuit 3000 and a sending-end transformer (the third transformer T3 and the fourth transformer T4); the DC power transmission receiving end includes an inverter circuit 1000 and a receiving-end transformer (the first transformer T1 and the second transformer T2).

[0076] The rectifier circuit includes MMC2 and MMC3. The DC first end of MMC2 is connected to the DC positive bus DC Line. The DC second end of MMC2 is connected to the DC first end of MMC3. The AC end of MMC is connected to the sending-end AC bus through the third transformer T3, and the sending-end AC bus is connected to the second power grid AC2. The DC second end of MMC3 is grounded, and the AC end of MMC3 is connected to the sending-end AC bus through the fourth transformer T4, and the sending-end AC bus is connected to the second power grid AC2.

[0077] It should be understood that the connection of the receiving end of DC power transmission has been introduced accordingly in the foregoing embodiments, and will not be elaborated here.

[0078] In the embodiments of the present application, using MMC as the rectifier circuit at the sending end of DC power transmission has the following advantages: First, MMC can quickly adjust the active power transmitted to the DC line according to the actual situation, and at the same time, accurately control the reactive power output according to the requirements of the sending-end AC side bus voltage to maintain voltage stability and improve the system operation efficiency; Second, when a fault occurs in the sending-end AC system (such as a short-circuit fault), MMC has a certain fault ride-through ability and maintains a certain operating state during the fault to avoid rapid blocking of the converter due to the fault; Third, compared with LCC, MMC does not need to rely on the AC system to provide commutation voltage. Even if the short-circuit capacity of the sending-end AC system is small and the voltage support ability is weak, MMC can still operate stably, expanding the application range of the DC power transmission system in different power grid environments.

[0079] Further, in order to improve the transmission efficiency of DC power transmission, another DC power transmission system provided by the embodiments of the present application, the schematic diagram of the DC power transmission system, see Figure 13 .

[0080] As Figure 13 shown, the DC power transmission system includes: a DC power transmission sending end and a DC power transmission receiving end. Among them, the DC power transmission sending end includes a first rectifier circuit 3000, a second rectifier circuit 4000, and sending-end transformers (the fifth transformer T5, the sixth transformer T6, the seventh transformer T7, and the eighth transformer T8); the DC power transmission receiving end includes a first inverter circuit 1000, a second inverter circuit 2000, and receiving-end transformers (the first transformer T1, the second transformer T2, the third transformer T3, and the fourth transformer T4).

[0081] The first rectifier circuit 3000 includes MMC3 and MMC4. The first DC end of MMC3 is connected to the DC positive bus DCLine1. The second DC end of MMC3 is connected to the first DC end of MMC4. The AC end of MMC3 is connected to the sending-end AC bus through the fifth transformer T5. The sending-end AC bus is connected to the second power grid AC2; the second DC end of MMC4 is grounded, and the AC end of MMC4 is connected to the sending-end AC bus through the sixth transformer T6. The sending-end AC bus is connected to the second power grid AC2.

[0082] The second rectifier circuit 4000 includes MMC5 and MMC6. The first DC end of MMC6 is connected to the DC positive bus DCLine2. The second DC end of MMC6 is connected to the first DC end of MMC5. The AC end of MMC6 is connected to the sending-end AC bus through the seventh transformer T7. The sending-end AC bus is connected to the second power grid AC2; the second DC end of MMC5 is grounded, and the AC end of MMC5 is connected to the sending-end AC bus through the eighth transformer T8. The sending-end AC bus is connected to the second power grid AC2.

[0083] It should be understood that the connection of the receiving end of DC power transmission has been introduced accordingly in the foregoing embodiments, and will not be elaborated herein.

[0084] By adopting a bipolar DC power transmission sending end and a bipolar DC power transmission receiving end in the embodiments of the present application, the transmission efficiency of the DC power transmission system can be improved.

[0085] See Figure 14 , which is a schematic diagram of a DC power transmission system provided by an embodiment of the present application.

[0086] As Figure 14 shown, the DC power transmission system includes: a DC power transmission sending end and a DC power transmission receiving end. Among them, the DC power transmission sending end includes a rectifier circuit 3000 and a sending end transformer (the third transformer T3 and the fourth transformer T4); the DC power transmission receiving end includes an inverter circuit 1000 and a receiving end transformer (the first transformer T1 and the second transformer T2).

[0087] The rectifier circuit includes HCC3 and MMC3. The first DC end of HCC3 is connected to the DC positive bus DC Line, the second DC end of HCC3 is connected to the first DC end of MMC4, the AC end of HCC3 is connected to the sending end AC bus through the third transformer T3, and the sending end AC bus is connected to the second power grid AC2; the second DC end of MMC4 is grounded, and the AC end of MMC4 is connected to the sending end AC bus through the fourth transformer T4, and the sending end AC bus is connected to the second power grid AC2.

[0088] It should be understood that the connection of the receiving end of DC power transmission has been introduced accordingly in the foregoing embodiments, and will not be elaborated herein.

[0089] According to the introduction of the DC power transmission receiving end in the foregoing embodiments, by using HCC and MMC connected in series in the DC power transmission receiving end, the DC power transmission receiving end has the ability to cross AC faults and the ability to cross DC faults. Similarly, by using HCC and MMC connected in series in the DC power transmission sending end, the DC power transmission sending end can also have the ability to cross AC faults and the ability to cross DC faults.

[0090] Furthermore, in order to improve the transmission efficiency of DC power transmission, another DC power transmission system provided by the embodiments of the present application, a schematic diagram of this DC power transmission system, see Figure 15 .

[0091] As Figure 15As shown in the figure, the HVDC transmission system includes: an HVDC transmission sending end and an HVDC transmission receiving end. Among them, the HVDC transmission sending end includes a first rectifier circuit 3000, a second rectifier circuit 4000, and sending-end transformers (the fifth transformer T5, the sixth transformer T6, the seventh transformer T7, and the eighth transformer T8); the HVDC transmission receiving end includes a first inverter circuit 1000, a second inverter circuit 2000, and receiving-end transformers (the first transformer T1, the second transformer T2, the third transformer T3, and the fourth transformer T4).

[0092] The first rectifier circuit 3000 includes HCC3 and MMC3. The first DC terminal of HCC3 is connected to the DC positive bus DCLine1. The second DC terminal of HCC3 is connected to the first DC terminal of MMC3. The AC terminal of HCC3 is connected to the sending-end AC bus through the fifth transformer T5. The sending-end AC bus is connected to the second power grid AC2; the second DC terminal of MMC3 is grounded. The AC terminal of MMC3 is connected to the sending-end AC bus through the sixth transformer T6. The sending-end AC bus is connected to the second power grid AC2.

[0093] The second rectifier circuit 4000 includes HCC4 and MMC4. The first DC terminal of HCC4 is connected to the DC positive bus DCLine2. The second DC terminal of HCC4 is connected to the first DC terminal of MMC4. The AC terminal of HCC4 is connected to the sending-end AC bus through the seventh transformer T7. The sending-end AC bus is connected to the second power grid AC2; the second DC terminal of MMC4 is grounded. The AC terminal of MMC4 is connected to the sending-end AC bus through the eighth transformer T8. The sending-end AC bus is connected to the second power grid AC2.

[0094] It should be understood that the connection of the HVDC transmission receiving end has been introduced accordingly in the foregoing embodiments, and will not be elaborated here.

[0095] By adopting a bipolar HVDC transmission sending end and a bipolar HVDC transmission receiving end in the embodiments of the present application, the transmission efficiency of the HVDC transmission system can be improved.

[0096] In a possible implementation manner, refer to Figure 16 , which is a schematic diagram of a control device provided by the embodiments of the present application.

[0097] The control device may include a memory 1011 and a processor 1012. The processor 1012 may be connected to the converter to control the switching operation of the controllable switch tubes in the converter. As Figure 16As shown, the memory can be a random access memory (RAM), flash memory, read only memory (ROM), EPROM memory, non-volatile read only memory (Electronic Programmable ROM, EPROM), register, hard disk, removable disk, etc.

[0098] The memory 1011 can store computer instructions. When the computer instructions stored in the memory 1011 are executed by the processor 1012, the processor 1012 can be used to execute the detection method. The memory 1011 can also store data, such as the preset range, preset threshold and other information involved in the above embodiments.

[0099] In the above embodiments, it can be implemented in whole or in part by software, hardware, firmware or any combination thereof. When implemented using software, it can be implemented in whole or in part in the form of a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, the processes or functions according to the embodiments of the present application are generated in whole or in part. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable devices. The computer instructions can be stored in a computer-readable storage medium, or transmitted from one computer-readable storage medium to another computer-readable storage medium. For example, the computer instructions can be transmitted from one website, computer, server or data center to another website, computer, server or data center by wire (such as coaxial cable, optical fiber, digital subscriber line (DSL)) or wirelessly (such as infrared, wireless, microwave, etc.). The computer-readable storage medium can be any available medium that can be accessed by a computer or a data storage device such as a server, data center, etc. that includes one or more available media integrated. The available medium can be a magnetic medium (such as a floppy disk, hard disk, magnetic tape), or a semiconductor medium (such as a solid state disk (SSD)), etc.

[0100] The embodiments of the present application also provide a readable storage medium for storing the method provided in the above embodiments. For example, a random access memory (RAM), flash memory, read only memory (ROM), EPROM memory, non-volatile read only memory (Electronic Programmable ROM, EPROM), register, hard disk, removable disk or any other form of storage medium in the art.

[0101] It should be noted that the embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. For the same or similar parts among the embodiments, reference can be made to each other. For the methods disclosed in the embodiments, since they correspond to the product embodiments disclosed in the embodiments, the description is relatively simple. For the relevant parts, reference can be made to the description in the product embodiment section.

[0102] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A receiving end of a DC power transmission, characterized in that, Comprising: An inverter circuit and a receiving-end transformer; The inverter circuit includes a hybrid line-commutated converter and a modular multilevel converter; The first DC terminal of the hybrid line-commutated converter is used to connect to a DC transmission line, the second DC terminal of the hybrid line-commutated converter is connected to the first DC terminal of the modular multilevel converter, and the AC terminal of the hybrid line-commutated converter is connected to a receiving-end AC bus through the corresponding receiving-end transformer; The second DC terminal of the modular multilevel converter is grounded, and the AC terminal of the modular multilevel converter is connected to the receiving-end AC bus through the corresponding receiving-end transformer.

2. The receiving end of the DC power transmission according to claim 1, characterized in that The receiving end of the DC power transmission is a monopolar receiving end, and the receiving end of the DC power transmission includes one such inverter circuit; the DC transmission line is a DC positive bus.

3. The receiving end of the DC power transmission according to claim 1, characterized in that The receiving end of the DC power transmission is a bipolar receiving end, and the receiving end of the DC power transmission includes two such inverter circuits, namely a first inverter circuit and a second inverter circuit; the DC transmission line includes a DC positive bus and a DC negative bus; The first DC terminal of the hybrid line-commutated converter in the first inverter circuit is used to connect to the DC positive bus; The first DC terminal of the hybrid line-commutated converter in the second inverter circuit is used to connect to the DC negative bus.

4. The receiving end of DC power transmission according to any one of claims 1-3, characterized in that, Further comprising: A controller; The controller is used to obtain the reactive power demand of the AC system in the case of an AC voltage dip; According to the reactive power demand and the reactive power distribution coefficient of the hybrid line-commutated converter, obtain a first reactive power command value, and control the delay trigger angle of the hybrid line-commutated converter according to the first reactive power command value to reduce the reactive power consumed by the hybrid line-commutated converter or make the hybrid line-commutated converter emit reactive power; According to the reactive power demand and the reactive power distribution coefficient of the modular multilevel converter, obtain a second reactive power command value; control the three-phase AC voltage of the modular multilevel converter according to the second reactive power command value to make the modular multilevel converter emit reactive power.

5. A DC power transmission system, characterized in that, Comprising: A sending end of DC power transmission and the receiving end of DC power transmission according to any one of claims 1-4; wherein, the sending end of DC power transmission includes a rectifier circuit and a sending-end transformer; The first DC terminal of the rectifier circuit is used to connect to the DC transmission line, the second DC terminal of the rectifier circuit is grounded, and the AC terminal of the rectifier circuit is connected to a sending-end AC bus through the corresponding sending-end transformer.

6. The DC power transmission system according to claim 5, characterized in that, The sending end of the DC power transmission is a monopolar sending end, the sending end of the DC power transmission includes one such rectifier circuit, and the rectifier circuit is a first rectifier circuit; the DC transmission line is a DC positive bus.

7. The DC power transmission system according to claim 5, characterized in that, The sending end of the DC power transmission is a bipolar sending end, the sending end of the DC power transmission includes two such rectifier circuits, namely a first rectifier circuit and a second rectifier circuit; the DC transmission line includes a DC positive bus and a DC negative bus; The first DC terminal of the first rectifier circuit is used to connect to the DC positive bus; The first DC terminal of the second rectifier circuit is used to connect to the DC negative bus.

8. The DC power transmission system according to claim 6 or 7, characterized in that, The first rectifier circuit includes a hybrid line-commutated converter and a modular multilevel converter; The first DC terminal of the hybrid line-commutated converter is used to connect to the DC transmission line. The second DC terminal of the hybrid line-commutated converter is connected to the first DC terminal of the modular multilevel converter. The AC terminal of the hybrid line-commutated converter is connected to the sending-end AC bus through the corresponding sending-end transformer. The second DC terminal of the modular multilevel converter is grounded. The AC terminal of the modular multilevel converter is connected to the sending-end AC bus through the corresponding sending-end transformer.

9. The DC power transmission system according to claim 6 or 7, characterized in that The first rectifier circuit includes a first modular multilevel converter and a second modular multilevel converter. The first DC terminal of the first modular multilevel converter is used to connect to the DC transmission line. The second DC terminal of the first modular multilevel converter is connected to the first DC terminal of the second modular multilevel converter. The AC terminal of the first modular multilevel converter is connected to the sending-end AC bus through the corresponding sending-end transformer. The second DC terminal of the second modular multilevel converter is grounded. The AC terminal of the second modular multilevel converter is connected to the sending-end AC bus through the corresponding sending-end transformer.

10. The HVDC transmission system according to claim 6 or 7, characterized in that, The first rectifier circuit includes a first line-commutated converter and a second line-commutated converter. The first DC terminal of the first line-commutated converter is used to connect to the DC transmission line. The second DC terminal of the first line-commutated converter is connected to the first DC terminal of the second line-commutated converter. The AC terminal of the first line-commutated converter is connected to the sending-end AC bus through the corresponding sending-end transformer. The second DC terminal of the second line-commutated converter is grounded. The AC terminal of the second line-commutated converter is connected to the sending-end AC bus through the corresponding sending-end transformer.

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