Multi-port ac-dc hybrid converter and multi-port ac-dc hybrid system

Abstract

This invention discloses a multi-port AC / DC hybrid converter and a multi-terminal AC / DC hybrid system. The device includes n converter units with independent adjustment and control capabilities and several multi-winding transformers. The DC sides of the n converter units are connected in series and n+1 DC terminals are led out. The n+1 DC terminals can be combined into a maximum of n independently controllable DC ports. The multi-winding transformers include n windings connected to the AC side of the converter units and m windings connected to the AC power grid, where n≥2 and m≥0. When m=0, the device can realize DC-DC conversion functions of two or more DC ports. When m≥1, the device can simultaneously realize AC-DC conversion and DC-DC conversion functions. The device achieves direct DC-DC conversion by connecting multiple DC ports and adopts device multiplexing technology, requiring fewer power electronic devices, thus improving cost-effectiveness and efficiency. The system can achieve flexible AC-DC conversion, control, and transmission.

Description

Technical Field

[0001] This invention relates to the field of power system transmission and distribution technology, specifically to a multi-port AC / DC hybrid converter and a multi-port AC / DC hybrid system. Background Technology

[0002] With the development of DC power grids and DC transmission and distribution technologies, diversified converter technologies based on power electronic devices are attracting increasing attention. Researchers are constantly seeking low-cost, high-efficiency, and high-power-density DC transformers and AC / DC converters. On the other hand, with the increasingly widespread application of distributed renewable energy and the proliferation of various DC loads, power distribution networks are also showing a trend towards power electronics. Hybrid AC / DC flexible distribution has become a recent hot topic, as evidenced by the construction of flexible DC distribution demonstration projects in Zhuhai, Guangdong, and Hangzhou, Zhejiang. Currently, hybrid AC / DC transmission and distribution systems and projects generally use voltage source converters or current source converters for AC / DC conversion and DC transformers for DC conversion. Further improvements are needed in the technical and economic performance of the equipment and the flexibility and versatility of the system's functions. In particular, the high cost of main equipment and systems in hybrid AC / DC flexible distribution hinders its application and development. Summary of the Invention

[0003] In view of the shortcomings of the prior art, the present invention provides a multi-port AC / DC hybrid converter device and a converter method for the device, as well as a multi-port AC / DC hybrid system. Through the device and method, AC / DC conversion and DC / DC conversion can be realized simultaneously with lower cost and better technical performance. Through the system, flexible AC / DC conversion, control and transmission between local and remote locations can be realized.

[0004] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0005] The present invention relates to a multi-port AC / DC hybrid converter.

[0006] It includes n converter units with independent control and regulation capabilities and several multi-winding transformers. The several multi-winding transformers are combined and lead out n first AC ports connected to the AC side of the converter unit and m second AC ports connected to the AC power grid, where m≥0. Under the DC transmission and distribution unipolar structure, the number of converter units n is not less than 2, and under the DC transmission and distribution true bipolar structure and pseudo bipolar structure, n is not less than 3.

[0007] The n converter units are used for AC / DC power conversion, AC / DC current conversion, and AC / DC voltage conversion; the multi-winding transformer is used for electrical isolation, AC voltage transformation, and power exchange between the n first AC ports and the m second AC ports.

[0008] The DC sides of n converter units are connected in series, and at least n+1 DC terminals are led out, so that a maximum of n independently adjustable and controllable DC ports can be formed, or a maximum of (n+1)*n / 2 DC ports including independently controllable or non-independently controllable ones can be formed. The number of independently adjustable DC ports is not less than 2 under the DC transmission and distribution unipolar structure and pseudo-bipolar structure, and not less than 3 under the DC transmission and distribution true bipolar structure.

[0009] When m = 0, this multi-port AC / DC hybrid converter realizes the DC-DC conversion function of more than two DC ports. When m ≥ 1, this multi-port AC / DC hybrid converter realizes both AC-DC conversion and DC-DC conversion functions.

[0010] Furthermore, the multi-winding transformer includes one multi-winding transformer or autotransformer with n first AC ports and m second AC ports, or it is composed of several transformers connected in series and parallel to form n first AC ports and m second AC ports, and realizes electrical isolation, AC transformation and power exchange between the n first AC ports and m second AC ports.

[0011] The n converter units adopt the same or different converter topologies, including two-level voltage source converters, multi-level voltage source converters with more than two levels, modular multi-level voltage source converters, and other AC / DC converter structures. The n converter units can select circuit topologies, voltage and current ratings as needed. Moreover, each converter unit can also adopt a multi-level structure composed of multiple converter circuits connected in series and parallel.

[0012] The multi-winding transformer and the converter unit are both single-phase, three-phase, or a hybrid structure of single-phase and three-phase, and the AC port of the converter unit and the first AC port of the multi-winding transformer are respectively single-phase, three-phase, or a hybrid structure of single-phase and three-phase.

[0013] When the number of external AC ports in a multi-winding transformer is m=0, the multi-port AC-DC hybrid converter forms a topology with two or more ports for DC-DC conversion, and becomes a multi-port DC-DC converter.

[0014] Furthermore, among the DC ports on the DC side of the n converter units, the voltage of the DC port with a higher voltage level is formed by superimposing the voltage of the DC port with a lower voltage level and the DC voltage of the converter unit between them. This allows the converter units and power electronic devices in each DC port to be reused in the DC port with a higher voltage level, significantly reducing the number of power electronic devices required. At the same time, it enables direct DC-DC conversion between different DC ports, and reduces the number of devices through which current flows during direct DC-DC conversion, thereby improving the efficiency of DC-DC conversion.

[0015] Neglecting converter losses, the converter power satisfies the following relationship:

[0016]

[0017] Where I du，i V du，i P du，i and Q du，i Let I represent the input DC current, DC voltage, input active power, and reactive power input to the multi-winding transformer of the i-th converter unit, respectively. dt，i V dt，i and P dt，i P represents the input DC current, DC voltage, and input active power of the i-th DC port, respectively. ac and Q ac V represents the active and reactive power input to the AC port of the converter. d,i Let be the potential of the i-th DC terminal.

[0018] When a converter device containing n converter units is used in a unipolar DC system, it can draw out a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 2; the DC ports are arranged from high to low voltage, and the drawn DC ports are d1-d... n+1 d2-d n+1 , ..., d n -d n+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: V dt,i =V d,i -V d,n+1 Let i = 1, 2, ..., n, then the DC current I of the converter unit is... du,i DC voltage V du,i DC current I at the DC port dt,i DC voltage Vdt,i, The relationship between them satisfies the following formula;

[0019]

[0020]

[0021] When a converter device containing n converter units is applied in a true bipolar DC system, it can produce a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 3; for a symmetrical structure, n is even and not less than 4; at this time, d at the DC midpoint potential... n / 2+1 The terminals are connected to a neutral line and grounded. The upper and lower ends of the neutral line are the positive and negative terminals, respectively, and the positive and negative terminals can be controlled independently. They are arranged from high to low DC port voltage, and the DC ports are d1-d. n / 2+1 d2-d n / 2 +1, ..., d n / 2 -d n / 2+1 d n / 2+2 -d n / 2+1 , ..., d n+1 -d n / 2+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n Furthermore, the voltage at the aforementioned ports is half positive and half negative. The relationship between the DC port voltage and the DC terminal potential is: V dti =V di -V dn / 2+1 , when i=1, 2,...,n / 2; V dt,i =V d,i+1 -V d,n / 2+1 When i = n / 2 + 1, n / 2 + 2, ..., n, the DC current I of the converter unit is... du,i DC voltage V du,i DC current I at the DC port dt,i DC voltage V dt,i, The relationship between them satisfies the following formula;

[0022]

[0023]

[0024] For asymmetric structures, n is an odd number and not less than 3. In this case, the positive and negative poles of the bipolar mode are asymmetrical and are configured according to the unipolar mode respectively.

[0025] When a converter device containing n converter units is applied in a pseudo-bipolar DC system, where n is an odd number and not less than 3, it can produce a maximum of (n+1) / 2 independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. Arranged from highest to lowest DC port voltage, the produced DC ports are d1-d... n+1 d2-d n , ..., d (n+1) / 2 -d (n+3) / 2 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,(n+1) / 2 The relationship between the DC port voltage and the DC terminal potential is: V dti =V di -V dn+2-i , i = 1, 2, ..., (n+1) / 2, at this time the DC current I of the converter unit du,i DC voltage V du,i DC current I at the DC port dt,i DC voltage V dt,i, The relationship satisfies the following formula:

[0026]

[0027]

[0028] A converter device containing n converter units also includes a hybrid application of unipolar, true bipolar, and pseudo bipolar DC. In this case, n is not less than 3, and the number of DC ports that can be independently controlled is not less than 3. At the same time, some DC ports are connected in a unipolar structure, some DC ports are led out using the positive and negative DC terminals and neutral line terminals of the corresponding voltage level to form a true bipolar DC connection, and some DC ports are led out using the positive and negative DC terminals of the corresponding voltage level to form a pseudo bipolar DC connection. In this case, the neutral line of this device is not connected. Two or all of the above three structures of unipolar, true bipolar, and pseudo bipolar are used to form a hybrid structure.

[0029] Furthermore, each of the converter units is connected to a control unit, which is used to control and regulate the voltage, current, and power of the DC side and / or AC side of the converter unit, thereby directly or indirectly controlling and regulating the voltage, current, and power of the DC port and / or AC port of this multi-port AC / DC hybrid converter. Moreover, since the control of lower voltage level converter units must consider the influence of the DC current flowing through higher voltage level converter units, the control unit is set according to all the above formulas, and on this basis, completes the required voltage, current, or power control.

[0030] Alternatively, the n converter units are connected to a device-level total control unit, which coordinates and controls the input, output and total power of all converter units, thereby enabling flexible power conversion, control and stable operation of the AC and DC ports of this multi-port AC / DC hybrid converter.

[0031] When controlling the converter unit, the control unit or device-level main control unit should satisfy the following formula:

[0032]

[0033] In the formula, nt indicates that there are nt DC ports in the converter, na indicates that the voltage control of na DC ports is completed by other equipment in the connected DC circuit, nb indicates that the DC voltage control of nb DC ports is completed by this multi-port AC-DC hybrid converter, nc indicates that there are nc converter units without directly leading DC ports that use DC voltage control, and nd indicates that there are nd converter units that use non-DC voltage control.

[0034] Furthermore, the multi-port AC / DC hybrid converter can enable the access and power interaction of DC loads and distributed DC power sources of different voltage levels. Through the selection and coordination of control methods, it can also realize power conversion and interaction between different DC ports, and power conversion and interaction between DC ports and AC ports, specifically including:

[0035] When the input power of any one or more of the DC ports that form the first part DC port A is greater than zero or less than zero, and the sum of the input power is P A The input power of the remaining DC ports of the second part B, which are combined, is either less than zero or greater than zero, and the sum of the input powers is P. B The input active power of the AC port is P. AC Ignoring the losses of this multi-port AC / DC hybrid converter, we have: when m = 0, P A +P B =0; when m≥1, P A +P B +P AC =0, and:

[0036] When m≥0, if P A +P B =0, P A If P > 0, then it represents the power conversion and transmission from DC port A in the first part to DC port B in the second part; conversely, if P < 0, then it represents the power conversion and transmission from DC port A in the first part to DC port B in the second part. A +P B =0, P A If <0, then it represents the power conversion and transmission from the second DC port B to the first DC port A;

[0037] When m≥1, if P A +P B >0, P A >0, P B If P < 0, then it represents the power conversion and transmission from the first DC port A to the second DC port B and AC port; conversely, if P < 0, then it represents the power conversion and transmission from the first DC port A to the second DC port B and AC port. A +P B <0, P A <0, P B If the value is >0, then it represents the power conversion and power transmission from the second part DC port B and AC port to the first part DC port A.

[0038] When m≥1, if P A +P B >0, P A >0, P B If P > 0, then it represents the power conversion and transmission from the first DC port A and the second DC port B to the AC port; conversely, if P < 0, then it represents the power conversion and transmission from the first DC port A and the second DC port B to the AC port. A +P B <0, P A <0, P B When <0, it represents the power conversion and transmission from the AC port to the first DC port A and the second DC port B.

[0039] The present invention also provides a multi-terminal AC / DC hybrid power transmission and distribution system, comprising two or more of the aforementioned multi-port AC / DC hybrid converters. One or more DC ports of the two or more aforementioned multi-port AC / DC hybrid converters are respectively connected to DC circuits of corresponding voltage levels, and one or more AC ports of the two or more aforementioned multi-port AC / DC hybrid converters are respectively connected to AC circuits of corresponding voltage levels, thereby forming a multi-terminal AC / DC hybrid power transmission and distribution system, realizing the access, interconnection and interaction of DC loads and DC power supplies of multiple locations and different voltage levels.

[0040] Furthermore, the multi-terminal AC / DC hybrid transmission and distribution system also includes a system-level control unit, which is used to control the coordinated operation of each multi-terminal converter, realize flexible AC / DC power conversion and mutual assistance across different regions, and achieve stable operation of the AC / DC transmission and distribution network; wherein, the flexible AC / DC power conversion and mutual assistance across different regions includes:

[0041] Flexible power exchange and mutual assistance between AC circuits of the same or different voltage levels in different locations are achieved through DC circuits of different voltage levels.

[0042] Flexible power exchange and mutual assistance between DC circuits of the same or different voltage levels in different locations;

[0043] Flexible power conversion and mutual assistance between AC and DC in different locations.

[0044] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0045] The multi-port AC / DC converter of the present invention can simultaneously realize AC-DC conversion and DC-DC conversion functions, and has a low overall cost and requires fewer power electronic devices.

[0046] The multi-terminal AC / DC hybrid system of the present invention can simultaneously achieve

[0047] 1. Power exchange and mutual assistance between AC power grids of the same or different voltage levels in different locations can be achieved through DC circuits of different voltage levels.

[0048] 2. Power conversion and distribution of DC power at different locations with the same or different voltage levels.

[0049] 3. Flexible power conversion between AC and DC in different locations, thereby enabling more diverse AC and DC hybrid operation modes and achieving more flexible AC and DC conversion, control and transmission.

[0050] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the structure of the multi-port AC / DC converter of the present invention.

[0052] Figure 2 This is a schematic diagram of the structure of the single AC port multi-port AC / DC converter of the present invention;

[0053] Figure 3 This is a schematic diagram of the multi-terminal AC / DC hybrid power transmission and distribution system of the present invention;

[0054] Figure 4 This is the main circuit for the Zhuhai Tangjiawan flexible AC / DC power distribution system.

[0055] Figure 5 This is the main circuit diagram for the Zhuhai Tangjiawan project based on a multi-port AC / DC converter.

[0056] Figure 6 Topology diagram of the converter used in the Zhuhai Tangjiawan project;

[0057] Figure 7 Topology diagram of DC transformer used in the Zhuhai Tangjiawan project;

[0058] Figure 8 This is a schematic diagram of a single-stage multi-port AC / DC converter that can be used in the Zhuhai Tangjiawan project.

[0059] Figure 9 This is a schematic diagram of a pseudo-bipolar multi-port AC / DC converter that can be used in the Zhuhai Tangjiawan project. Detailed Implementation

[0060] The multi-port AC / DC hybrid converter of the present invention includes:

[0061] It includes n converter units with independent control and regulation capabilities and several multi-winding transformers. The several multi-winding transformers are combined and lead out n first AC ports connected to the AC side of the converter unit and m second AC ports connected to the AC power grid, where m≥0. Under the DC transmission and distribution unipolar structure, the number of converter units n is not less than 2, and under the DC transmission and distribution true bipolar structure and pseudo bipolar structure, n is not less than 3.

[0062] The n converter units are used for AC / DC power conversion, AC / DC current conversion, and AC / DC voltage conversion; the multi-winding transformer is used for electrical isolation, AC voltage transformation, and power exchange between the n first AC ports and the m second AC ports.

[0063] The DC sides of n converter units are connected in series, and at least n+1 DC terminals are led out, so that a maximum of n independently adjustable and controllable DC ports can be formed, or a maximum of (n+1)*n / 2 DC ports including independently controllable or non-independently controllable ones can be formed. The number of independently adjustable DC ports is not less than 2 under the DC transmission and distribution unipolar structure and pseudo-bipolar structure, and not less than 3 under the DC transmission and distribution true bipolar structure.

[0064] When m = 0, this multi-port AC / DC hybrid converter realizes the DC-DC conversion function of more than two DC ports. When m ≥ 1, this multi-port AC / DC hybrid converter realizes both AC-DC conversion and DC-DC conversion functions.

[0065] In one possible implementation, the multi-winding transformer comprises a multi-winding transformer or autotransformer with n first AC ports and m second AC ports, or is composed of several transformers connected in series and parallel to form n first AC ports and m second AC ports, thereby achieving electrical isolation, AC transformation and power exchange between the n first AC ports and m second AC ports.

[0066] The n converter units adopt the same or different converter topologies, including two-level voltage source converters, multi-level voltage source converters with more than two levels, modular multi-level voltage source converters, and other AC / DC converter structures. The n converter units can select circuit topologies, voltage and current ratings as needed. Moreover, each converter unit can also adopt a multi-level structure composed of multiple converter circuits connected in series and parallel.

[0067] The multi-winding transformer and the converter unit are both single-phase, three-phase, or a hybrid structure of single-phase and three-phase, and the AC port of the converter unit and the first AC port of the multi-winding transformer are respectively single-phase, three-phase, or a hybrid structure of single-phase and three-phase.

[0068] When m=0 in a multi-winding transformer, this multi-port AC / DC hybrid converter forms a topology with two or more ports for DC-DC conversion.

[0069] Furthermore, among the DC ports on the DC side of the n converter units, the voltage of the DC port with a higher voltage level is formed by superimposing the voltage of the DC port with a lower voltage level and the DC voltage of the converter unit between them. This allows the converter units and power electronic devices in each DC port to be reused in the DC port with a higher voltage level, significantly reducing the number of power electronic devices required. At the same time, it significantly reduces the number of devices through which current flows when exchanging power between different DC ports, thereby improving the DC-DC converter efficiency.

[0070] When a converter device containing n converter units is used in a unipolar DC system, it can draw out a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 2; the DC ports are arranged from high to low voltage, and the drawn DC ports are d1-d... n+1 d2-d n+1 , ..., d n -d n+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: V dt,i =V d,i -V d,n+1 For i = 1, 2, ..., n, the basic current and voltage relationships between the n converter units and the DC port satisfy the following formulas respectively;

[0071]

[0072]

[0073] When a converter device containing n converter units is applied in a true bipolar DC system, it can produce a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 3; for a symmetrical structure, n is even and not less than 4; at this time, d at the DC midpoint potential... n / 2+1The terminals are connected to a neutral line and grounded. The upper and lower ends of the neutral line are the positive and negative terminals, respectively, and the positive and negative terminals can be controlled independently. They are arranged from high to low DC port voltage, and the DC ports are d1-d. n / 2+1 d2-d n / 2+1 , ..., d n / 2 -d n / 2+1 d n / 2+2 -d n / 2+1 , ..., d n+1 -d n / 2+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n Furthermore, the voltage at the aforementioned ports is half positive and half negative. The relationship between the DC port voltage and the DC terminal potential is: V dt,i =V d,i -V d,n / 2+1 , when i=1, 2,...,n / 2; V dt,i =V d,i+1 -V d,n / 2+1 When i = n / 2 + 1, n / 2 + 2, ..., n, the basic current and voltage relationships between the n converter units and the DC port satisfy the following formulas respectively;

[0074]

[0075]

[0076] For asymmetric structures, n is an odd number and not less than 3. In this case, the positive and negative poles of the bipolar mode are asymmetrical and are configured according to the unipolar mode respectively.

[0077] When a converter device containing n converter units is applied in a pseudo-bipolar DC system, where n is an odd number and not less than 3, it can produce a maximum of (n+1) / 2 independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. Arranged from highest to lowest DC port voltage, the produced DC ports are d1-d... n+1 d2-d n , ..., d (n+1) / 2 -d (n+3) / 2 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,(n+1) / 2 The relationship between the DC port voltage and the DC terminal potential is: V dt,i =V d,i -V d,n+2-i , i = 1, 2, ..., (n+1) / 2, at this time the basic current and voltage relationships between each converter unit and the DC port satisfy the following formulas respectively;

[0078]

[0079]

[0080] A converter device containing n converter units also includes a hybrid application of unipolar, true bipolar, and pseudo bipolar DC. In this case, n is not less than 3, and the number of DC ports that can be independently controlled is not less than 3. At the same time, some DC ports are connected in a unipolar structure, some DC ports are led out using the positive and negative DC terminals and neutral line terminals of the corresponding voltage level to form a true bipolar DC connection, and some DC ports are led out using the positive and negative DC terminals of the corresponding voltage level to form a pseudo bipolar DC connection. In this case, the neutral line of this device is not connected. Two or all of the above three structures of unipolar, true bipolar, and pseudo bipolar are used to form a hybrid structure.

[0081] In one possible implementation, each converter unit is connected to a control unit, which controls and regulates the voltage, current, and power on the DC and / or AC sides of the converter unit, thereby directly or indirectly controlling and regulating the voltage, current, and power at the DC and / or AC ports of this multi-port AC / DC hybrid converter. Furthermore, since the control of lower voltage level converter units must consider the influence of the DC current flowing through higher voltage level converter units, the control unit is set according to all the above formulas and, based on this, performs the required voltage, current, or power control.

[0082] Alternatively, the n converter units are connected to a device-level total control unit, which coordinates and controls the input, output and total power of all converter units, thereby enabling flexible power conversion, control and stable operation of the AC and DC ports of this multi-port AC / DC hybrid converter.

[0083] When controlling the converter unit, the control unit or device-level main control unit should satisfy the following formula:

[0084]

[0085] In the formula, nt indicates that there are nt DC ports in the converter, na indicates that the voltage control of na DC ports is completed by other equipment in the connected DC circuit, nb indicates that the DC voltage control of nb DC ports is completed by this multi-port AC-DC hybrid converter, nc indicates that there are nc converter units without directly leading DC ports that use DC voltage control, and nd indicates that there are nd converter units that use non-DC voltage control.

[0086] In one possible implementation, the multi-port AC / DC hybrid converter can enable the access and power interaction of DC loads of different voltage levels and distributed DC power sources. Through the selection and coordination of control methods, it can also achieve power conversion and interaction between different DC ports, and between DC ports and AC ports, specifically including:

[0087] When the input power of any one or more of the DC ports that form the first part DC port A is greater than zero or less than zero, and the sum of the input power is P A The input power of the remaining DC ports of the second part B, which are combined, is either less than zero or greater than zero, and the sum of the input powers is P. B The input active power of the AC port is P. AC Ignoring the losses of this multi-port AC / DC hybrid converter, we have: when m = 0, P A +P B =0; when m≥1, P A +P B +P AC =0, and:

[0088] When m≥0, if P A +P B =0, P A If P > 0, then it represents the power conversion and transmission from DC port A in the first part to DC port B in the second part; conversely, if P < 0, then it represents the power conversion and transmission from DC port A in the first part to DC port B in the second part. A +P B =0, P A If <0, then it represents the power conversion and transmission from the second DC port B to the first DC port A;

[0089] When m≥1, if P A +P B >0, P A >0, P B If P < 0, then it represents the power conversion and transmission from the first DC port A to the second DC port B and AC port; conversely, if P < 0, then it represents the power conversion and transmission from the first DC port A to the second DC port B and AC port. A +P B <0, P A <0, P B If the value is >0, then it represents the power conversion and power transmission from the second part DC port B and AC port to the first part DC port A.

[0090] When m≥1, if P A +P B >0, P A >0, P B If P > 0, then it represents the power conversion and transmission from the first DC port A and the second DC port B to the AC port; conversely, if P < 0, then it represents the power conversion and transmission from the first DC port A and the second DC port B to the AC port.A +P B <0, P A <0, P B When <0, it represents the power conversion and transmission from the AC port to the first DC port A and the second DC port B.

[0091] like Figure 1 As shown, Figure 1 The example is a single-stage system of a three-phase multi-port AC / DC converter. Taking this system as an example, the n converter units of the device are connected in series on the DC side and n+1 DC terminals are led out. The AC side of each converter unit is connected to different windings of a multi-winding transformer. The other m windings of the transformer lead out 3m AC terminals, thus forming a multi-port AC / DC converter with n+1 (n≥2) DC terminals and 3m (m≥1) AC terminals.

[0092] For a multi-port three-phase AC / DC converter, a set of three-phase AC terminals (a, b, c) constitutes one AC port, and 3m AC terminals constitute m AC ports. Furthermore, any two DC terminals can form a DC port, thus allowing for a maximum of (n+1)*n / 2 DC ports to be drawn out, forming a multi-port AC / DC converter. This facilitates simultaneous connection to AC and DC circuits of different voltage levels, enabling flexible AC / DC power conversion and distribution. Additionally, according to the design rules of the above system, it can also be easily modified and applied to a single-phase AC system to form a multi-port single-phase AC / DC converter. The corresponding single-phase multi-port AC / DC converter would contain n+1 (n+1≥3) DC terminals and m (m≥1) single-phase AC terminals.

[0093] Each of the n converter units is connected to a control unit, which is used to control and regulate the voltage, current and power output of the DC side and / or AC side of the converter unit, thereby directly or indirectly controlling and regulating the voltage, current and power of the DC and / or AC ports of the device.

[0094] Each of the n converter units is equipped with a relatively independent control unit, allowing for flexible control of the AC-DC conversion of the n converter units and ultimately enabling regulation and control of the n DC ports and / or AC ports, such as adjusting the voltage, current, or power of these DC ports. This device has a maximum of (n+1)*(n) / 2 DC ports on its DC side, but only a maximum of n DC ports can be independently and flexibly controlled. The other (n+1)*(n) / 2-n DC port outputs are combinations of the aforementioned n controllable DC port outputs and do not possess controllability or adjustability. Therefore, the number of converter units and the number of DC ports can be set as needed.

[0095] Each converter unit can regulate the voltage, current, or power on the DC side through its own control unit, as well as the voltage, current, or power on the AC side. Regardless of the control method used, the active power on the AC and DC sides of the same converter unit is consistent (ignoring converter losses), while the reactive power output on the AC side can be adjusted by the amplitude and phase relationship of the AC voltage and current of the converter unit.

[0096] Multi-winding transformers connect the converter output and the AC network to achieve electrical isolation between windings, AC voltage transformation, and power exchange. However, transformers do not have flexible current and power regulation capabilities.

[0097] The device of this invention combines a converter unit and a multi-winding transformer to achieve flexible power conversion, control, and transmission between DC and AC ports. For better illustration, several embodiments using the device of this invention are listed below.

[0098] 1) DC loads of different voltage levels and distributed DC power sources are connected and interact with the power supply.

[0099] For example Figure 1 As shown, a converter with n independent adjustable DC ports can lead out n DC transmission and distribution lines of different voltage levels, which can provide access for DC loads of different voltage levels (such as electric vehicle charging, DC buildings, etc.) and distributed DC power sources (such as photovoltaics, energy storage, etc.), and realize power interaction through DC transmission and distribution lines.

[0100] 2) Power conversion, control, and distribution between two or more DC ports. Flexible power exchange, control, and distribution between DC ports of different or the same voltage levels can be achieved through converter units and multi-winding transformers. As shown in the figure, n+1 DC terminals can be combined into n independently controllable DC ports of different (or the same) voltage levels, thereby achieving conversion, control, and ultimately power conversion, control, and distribution between DC circuits of different or the same voltage levels through the following path.

[0101] The relationship between power conversion and transmission / distribution paths is as follows:

[0102] DC ports A, B, etc. → converter units A, B, etc. → multi-winding transformer → converter units C, D, etc. → DC ports C, D, etc.

[0103] 3) Power conversion, control, and transmission from one or more DC ports to AC ports. As shown in the figure, by controlling n converter units through a converter unit controller, power aggregation from multiple DC ports and its transmission to AC ports can be achieved. The power distribution at each AC port of this converter device can be switched via an AC switch; therefore, the power of multiple AC ports cannot be adjusted simultaneously. The power conversion and transmission path from DC ports to AC ports is as follows:

[0104] DC ports A, B, etc. → converter units A, B, etc. → multi-winding transformer realizes the aggregation and exchange of electrical energy of multiple converter units → AC ports A or B, etc.

[0105] 4) Power conversion and distribution from AC port to one or more DC ports. The AC port is connected to a multi-winding transformer, and through the transformer and n converter units, power conversion, control, and distribution of AC power from the AC port to n DC ports of different (or the same) voltage levels can be achieved. Similarly, this device can individually regulate and control the power distribution of the n DC ports, but cannot simultaneously regulate the power transmitted from each AC port; this can only be done by switching the connection at different times using a switch. The power conversion and distribution path from AC port to DC port is as follows:

[0106] AC port A or B, etc. → Multi-winding transformer realizes AC port power aggregation and power exchange - Converter unit A, B, etc. → DC port A, B, etc.

[0107] Alternatively, it can also be used Figure 2 The multi-DC-port single-AC-port converter topology shown enables power conversion and power interaction between one or more DC ports and AC ports. Figure 1 A three-phase multi-winding transformer can also be composed of three single-phase multi-winding transformers, etc. Figure 1 , 2 If the three-phase converter unit and the three-phase multi-winding transformer in the original text are replaced by a single-phase converter unit and a single-phase multi-winding transformer, a single-phase multi-port AC / DC converter device can be constructed.

[0108] in addition, Figure 1 , Figure 2 The three-phase windings of the transformer can adopt different connection methods such as star and delta and different connection groups.

[0109] The n converter units can employ the same or different converter topologies, including two-level voltage source converters, multi-level voltage source converters with more than two levels, modular multi-level voltage source converters, and other types of current source converters. A hybrid application of the above-mentioned different converter topologies is also possible.

[0110] Each converter unit can be configured with different voltage and current ratings as needed. A converter unit can be composed of multiple valve modules connected in series. Different converter units can also use different valve module topologies, different valve module parameters, and different power electronic device ratings. However, the valve modules and their parameters within the same converter unit are generally consistent.

[0111] A multi-port device consisting of n converter units can independently control n DC ports through the n converter units. The independent control ports can be the DC output terminals of each converter unit, such as d1-d2, d2-d3, d3-d4, etc., or different combinations of the output terminals of different converter units, such as d1-d4. n+1 d2-d n The multi-port converter, with components d1-d3, enables flexible power conversion and transmission control. A typical single-pole multi-port converter, containing n converter units and a multi-winding transformer, provides n positive (or negative) DC ports. A true bipolar symmetrical multi-port converter, containing n (where n is even) converter units and a multi-winding transformer, provides n / 2 positive ports and n / 2 negative ports. A pseudo-bipolar multi-port converter, containing n (where n is odd) converter units and a multi-winding transformer, provides (n+1) / 2 DC ports. Generally, the positive and negative topologies of a bipolar multi-port converter are symmetrical.

[0112] In addition, the number of converter units in a multi-port AC / DC converter is determined by the number of DC ports that need to be controlled independently. The voltage withstand capability and the number of power electronic devices connected in series in each converter unit are mainly determined by the rated voltage and overvoltage multiple of its DC port and AC side. The total DC voltage withstand capability and the total number of power electronic devices connected in series in a multi-port AC / DC converter are determined by the rated voltage of the highest voltage DC port and its corresponding AC side voltage and overvoltage multiple.

[0113] For a unipolar DC structure containing n converter units, if the absolute values ​​of the rated DC voltages of the required n independently controlled DC ports are respectively V dt,1 >V dt,2 >V dt,3 >…>V dt,n The corresponding DC port drawn out is d1-d n+1 d2-d n+1 , ..., d n -d n+1 ,like Figure 1 , Figure 2 As shown. And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: V dt,i =V d,i -V d,n+1 , i = 1, 2, ..., n.

[0114] For a true bipolar DC symmetrical structure containing n converter units (where n is an even number), if the absolute values ​​of the rated DC voltages of the required n independent control DC ports are respectively V dt,1 >V dt,2 >V dt,3 >…>Vdt,n The corresponding DC port drawn out is d1-d n / 2+1 d2-d n / 2+1 , ..., d n / 2 -d n / 2+1 d n / 2+2 -d n / 2+1 , ..., d n+1 -d n / 2+1 ,like Figure 1 , Figure 2 As shown. And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: when i = 1, 2, ..., n / 2, V dt,i =V d,i -V d,n / 2+1 ;When i=n / 2+1,n / 2+2,…,n,V dt,i =V d,i+1 -V d,n / 2+1 .

[0115] For a pseudo-bipolar DC structure containing n converter units (where n is an odd number), if the absolute values ​​of the rated DC voltages of the required (n+1) / 2 independent control DC ports are respectively V dt,1 >V dt,2 >V dt,3 >…>V dt,(n+1) / 2 The corresponding DC port drawn out is d1-d n+1 d2-d n , ..., d (n+1) / 2 -d (n+3) / 2 ,like Figure 1 , Figure 2 As shown. And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: V dt,i =V d,i -V d,n+2-i ,i=1,2,…,(n+1) / 2.

[0116] The number of converter unit devices connected in series is determined by their corresponding AC / DC voltages and their overvoltage multiples. The rated current of each converter unit is determined by the total rated current flowing through it.

[0117] The control methods for converter units can vary depending on their topology. In principle, the control methods used in each converter topology are applicable to the corresponding control units of this multiport device; however, the characteristics of this device's topology and their impact must still be considered, mainly including...

[0118] 1) The impact of potential differences between converter units. The converter units are connected in series on the DC side, resulting in significant differences in potential between each converter unit and a single converter. For example, for the first converter unit 1, its high-voltage DC bus potential is V...d,1 The low-voltage DC bus potential is V. d,2 The high and low voltage DC bus voltages of the second converter unit 2 are V and V, respectively. d,2 and V d,3 The magnitude of the AC bus voltage to ground in the converter unit also depends on its position in the series-connected converter unit and the design of the grounding location.

[0119] 2) The impact of current differences in the converter unit. The AC and DC currents required for converter unit control also need to consider the impact of the series structure used in the device. For example... Figure 2 As shown, the DC current of the converter unit is not the same as the DC current of the connected DC port. The DC current used in the control should be selected appropriately according to the controlled object.

[0120] The control of AC / DC conversion power, conversion current, and DC voltage in a multi-port converter is mainly achieved through the converter units. For a multi-port converter with n converter units, each converter unit can choose one of three control modes: DC voltage, AC (or DC) current, or AC (or DC) power. Other control modes based on these three variables can be considered as one of these three. For example, AC voltage stabilization control can be categorized as AC current control, etc.

[0121] The n converter units are connected to a device-level total control unit, which is used to coordinate and control the total power of all converter units.

[0122] Specifically, the methods for power converter control of the aforementioned device, including the control units and the device-level main control unit, include:

[0123] When controlling each converter unit, the control method settings must meet the following conditions:

[0124]

[0125] nt indicates that the converter has nt DC ports, na indicates that the voltage control of na DC ports is completed by other equipment in the connected DC circuit, nb indicates that the DC voltage control of nb DC ports is completed by this multi-port converter, nc indicates that there are nc converter units without directly led-out DC ports that use DC voltage control, and nd indicates that there are nd converter units that use non-DC voltage control.

[0126] The method enables the connection and power interaction of DC loads and distributed DC power sources at different voltage levels. Through the selection and coordination of the aforementioned control methods, it can also achieve power conversion and interaction between different DC ports, and between DC ports and AC ports, specifically including:

[0127] When the input power of the first part DC port A, which is formed by any combination of one or more DC ports, is either greater than zero or less than zero, and the sum of the input powers is P A The remaining part consists of one or more DC ports combined to form port B, where the input power of each port is either less than zero or greater than zero, and the sum of the input power is P. B The active power input at the AC port is P. AC Ignoring the losses of the multi-port converter, we have P A +P B +P AC =0, and:

[0128] 1) When P A +P B =0, P A When P > 0, the power conversion and transmission from the first DC port A to the second DC port B are performed; conversely, if P < 0, the power conversion and transmission are performed from the first DC port A to the second DC port B. A +P B =0, P A If <0, then the power conversion and power transmission from the second DC port B to the first DC port A are performed.

[0129] 2) When P A +P B >0, P A >0, P B When P < 0, the power conversion and transmission from the first part DC port A to the second part DC port B and AC port are performed; conversely, if P A +P B <0, P A <0, P B If the value is >0, then the power conversion and power transmission from the second part DC port B and AC port to the first part DC port A are performed.

[0130] 3) When P A +P B >0, P A >0, P B When P > 0, the power conversion and transmission from the first DC port A and the second DC port B to the AC port are performed; conversely, if P A +P B <0, P A <0, P B When <0, the AC port converts and transmits power to the first DC port A and the second DC port B.

[0131] When a converter device containing n converter units is used in a unipolar DC system (where n is not less than 2), such as Figure 1As shown, it can draw out a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. The DC ports are arranged from high to low voltage, and the drawn DC ports are d1-d2. n+1 d2-d n+1 , ..., d n -d n+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n And the DC port voltage V dt,i With DC terminal potential V d,i The relationship is: V dt,i =V d,i -V d,n+1 , i = 1, 2, ..., n.

[0132] The relationship between the DC current of each converter unit and the DC port is as follows:

[0133]

[0134] The DC voltage relationship between each converter unit and the DC port is as follows:

[0135]

[0136] The power relationship satisfied by the converter is:

[0137]

[0138] Where I du，i V du，i P du，i and Q du，i Let I represent the input DC current, DC voltage, input active power, and reactive power input to the transformer of the i-th converter unit, respectively. dt，i V dt，i and P dt，i P represents the input DC current, DC voltage, and input active power of the i-th DC port, respectively. ac and Q ac V represents the active and reactive power input to the AC port of the converter. d,i Let be the potential of the i-th DC terminal.

[0139] It is evident that the closer the converter unit is to the low-voltage DC end, the higher the DC current, thus requiring a larger rated current to be designed. The current and power ratings of each converter unit can be determined based on the required device port voltage, maximum port power, and other parameters according to formulas (2) and (3), and the voltage, current, and power control of each converter unit also need to refer to the above relationships.

[0140] When a converter device containing n converter units is applied in a true bipolar DC system, it can produce a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 3; for a symmetrical structure, n is even and not less than 4; at this time, d at the DC midpoint potential... n / 2+1 The terminals have a neutral wire leading out and are generally grounded. The neutral wire has positive and negative poles above and below it, and each pole can be controlled independently. The DC ports are arranged from highest to lowest voltage, and the DC ports are designated d1-d. n / 2+1 d2-d n / 2 +1, ..., d n / 2 -d n / 2+1 d n / 2+2 -d n / 2+1 , ..., d n+1 -d n / 2+1 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,n Furthermore, the voltage at the aforementioned ports is half positive and half negative. The relationship between the DC port voltage and the DC terminal potential is as follows:

[0141] V dt,i =V d,i -V d,n / 2+1 , when i=1, 2,...,n / 2;

[0142] V dt,i =V d,i+1 -V d,n / 2+1 , when i=n / 2+1, n / 2+2,...,n.

[0143] The relationship between the DC current of each converter unit and the DC port is as follows:

[0144]

[0145] The DC voltage relationship between each converter unit and the DC port is as follows:

[0146]

[0147] The power of the converter satisfies the above relationship (4);

[0148] The rated DC current and rated DC voltage of each converter unit can also be obtained from the rated power and rated DC voltage of each DC port according to the relationship described in (5) and (6). For asymmetrical structures, n is an odd number and not less than 3. At this time, its positive and negative poles are asymmetrical and configured according to the above single-pole mode. The rated DC current, DC voltage and other parameters of its converter unit can also be calculated accordingly.

[0149] When a converter device containing n converter units is applied in a pseudo-bipolar DC system, where n is an odd number and not less than 3, it can produce a maximum of (n+1) / 2 independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. Arranged from highest to lowest DC port voltage, the produced DC ports are d1-d... n+1 d2-d n , ..., d (n+1) / 2 -d (n+3) / 2 The corresponding port voltage is V dt,1 >V dt,2 >V dt,3 >…>V dt,(n+1) / 2 The relationship between the DC port voltage and the DC terminal potential is: V dt,i =V d,i -V d,n+2-i ,i=1,2,…,(n+1) / 2;

[0150] The relationship between the DC current of each converter unit and the DC port is as follows:

[0151]

[0152] The DC voltage relationship between each converter unit and the DC port is as follows:

[0153]

[0154] The power of the converter satisfies the above relationship (4);

[0155] The rated DC current and rated DC voltage of each converter unit can also be obtained from the rated power and rated DC voltage of each DC port according to the relationship described in (7) and (8).

[0156] A converter device containing n converter units also includes a hybrid application of unipolar, true bipolar, and pseudo bipolar DC, where n is not less than 3 and the number of independently controllable DC ports is not less than 3. In this case, some DC ports are wired in a unipolar structure, some DC ports are led out using the positive and negative DC terminals and neutral line terminals of the corresponding voltage level to form a true bipolar DC connection, and some DC ports are led out using the positive and negative DC terminals of the corresponding voltage level to form a pseudo bipolar DC connection. In this case, the neutral line of this device is not connected. Two or all of the above unipolar, true bipolar, and pseudo bipolar structures are used to form a hybrid structure.

[0157] In addition, the mutual influence caused by dynamic operation must also be considered. For example, if the converter unit is adjusted from constant DC voltage control to constant power control, its DC output voltage may change dynamically, further affecting the voltage of other DC ports.

[0158] Multi-port AC / DC converters typically include DC ports with different voltage levels, such as low, medium, and high. For low-voltage DC, if MMC technology is used, a combination of pulse amplitude and pulse width modulation can be employed to reduce harmonics and improve power quality, thereby reducing the number of power electronic modules required in series. This can be achieved through hybrid modulation techniques based on the principles of nearest-nearest-level modulation and pulse width modulation.

[0159] A multi-port AC / DC hybrid power transmission and distribution system includes two or more multi-port AC / DC hybrid converters as described in this invention. One or more DC ports of the two or more multi-port AC / DC hybrid converters are respectively connected to DC circuits of corresponding voltage levels, and one or more AC ports of the two or more multi-port AC / DC hybrid converters are respectively connected to AC circuits of corresponding voltage levels, forming a multi-port AC / DC hybrid power transmission and distribution system to realize the access, interconnection and interaction of DC loads and DC power supplies of multiple locations and different voltage levels.

[0160] The multi-terminal AC / DC hybrid transmission and distribution system of this invention also includes a system-level control unit, which is used to control the coordinated operation of each multi-terminal converter, realize flexible AC / DC power conversion and mutual assistance across different regions, and achieve stable operation of the AC / DC transmission and distribution network; wherein, the flexible operation mode includes:

[0161] Flexible power exchange and mutual assistance between different locations with the same or different voltage levels;

[0162] Flexible power exchange and mutual assistance between different DC power sources with the same or different voltage levels;

[0163] Flexible power conversion and mutual assistance between AC and DC in different locations.

[0164] In addition, AC / DC converters at different ports of the same multi-terminal AC / DC hybrid system also need to be coordinated through device-level control, which in this embodiment refers to the system-level control unit described below.

[0165] Multi-port AC / DC converters can flexibly realize local AC / DC conversion, regulation and distribution. Multiple (two or more) multi-port AC / DC converters can form a multi-port AC / DC hybrid system to realize AC / DC conversion, regulation and distribution between different locations, and can also realize power regulation and distribution between multiple AC circuits in different locations.

[0166] The DC ports of each multi-port AC / DC converter can be selected to connect to the number and voltage level of DC circuits as needed, while their AC ports can be switched and controlled by AC switches to connect to local AC power grids of different voltage levels, or to a single local AC power grid of a specific voltage level. Specifically, the achievable remote AC / DC transmission and distribution can include:

[0167] 1) Flexible AC power exchange and mutual assistance between different locations, such as power exchange between AC networks in location A and those in locations B and C, not only provides an alternative power exchange method besides AC network interconnection, but also does not affect short-circuit current and allows for flexible control and adjustment of transmitted power, possessing unique advantages not available in AC interconnection. The flexible AC power conversion and transmission paths between different locations are as follows:

[0168] AC power grid in location A → AC port of multi-port converter in location A → DC port of multi-port converter in location A → DC port of multi-port converter in locations B and C → DC port of multi-port converter in locations B and C → AC port of multi-port converter in locations B and C → AC power grid in locations B and C.

[0169] 2) Flexible DC power exchange between different locations, such as power exchange between a DC circuit of a certain voltage level in location A and DC circuits of the same or different voltage levels in locations B and C, thereby achieving high-reliability operation of the DC transmission and distribution network. Flexible DC power conversion and transmission between DC circuits of different voltage levels in different locations is slightly more complex, and its power conversion and transmission path is as follows:

[0170] DC circuits in locations A and B → DC ports of multi-port converters in locations A and B → DC ports of multi-port converters in locations C and D → Multi-port converters in locations C and D → DC ports of other different voltage levels of multi-port converters in locations C and D → DC circuits corresponding to other different voltage levels of DC ports.

[0171] 3) Flexible AC / DC power exchange between different locations, such as AC power exchange at a certain voltage level in location A with DC power exchange at one or more voltage levels in locations B and C, thereby achieving power mutual assistance between AC and DC in different locations, facilitating energy allocation, and improving the reliability of AC and DC transmission and distribution networks. The power conversion and transmission paths in this case are as follows:

[0172] AC power grid in location A → AC port of multi-port converter in location A → multi-port converter in location A → DC port (one or more) of multi-port converter in location A → DC circuit (one or more) → DC ports (one or more) in locations B, C, etc.

[0173] Furthermore, multi-terminal AC / DC hybrid transmission and distribution systems require coordinated control of converters in various locations, resulting in more diverse AC / DC power exchange and mutual assistance methods, and more complex control. However, generally speaking, the system-level control unit can employ multi-level control, similar to multi-terminal DC systems, including system control, converter (or converter station) control, and converter unit control. System-level control primarily manages the coordinated operation of converters in different locations, ensuring stable operation of AC / DC transmission and distribution networks across different regions and facilitating AC / DC power exchange and mutual assistance between different regions. Converter (or converter station) control primarily ensures stable operation of locally controlled AC / DC circuits and local power exchange, as well as the coordinated operation of various converter units. Converter unit control controls the AC / DC conversion power exchange of the converter units.

[0174] For a hybrid AC / DC transmission and distribution system with n DC ports and L terminals (i.e., L AC / DC converters connected), it is necessary to regulate and control each DC port of each converter at the system level. If all converters have a total of NS DC ports connected to the DC transmission and distribution system, of which NU DC ports are controlled by DC voltage and NV DC ports are controlled by other methods besides DC voltage regulation, then the control of each port can be flexibly allocated according to the following formula.

[0175]

[0176] An AC / DC converter that uses DC voltage control at a specific DC port can be referred to as the grid-connecting equipment for the connected DC circuit. Generally, this equipment should have a large rated power and a wide power regulation range at the corresponding port to ensure the system operates within a safe and stable range. Other converters connected to this DC circuit can use current control, power control, or other controls at their respective ports. In this case, the DC current or DC power at that port is generally relatively fixed, and its variation range is relatively small. Furthermore, the grid-connecting equipment for each DC circuit and the control of each port of each converter should be rationally selected based on the system's operational and reliability requirements, the breaking capacity of switching devices, the capacity of multi-winding transformers, and system and equipment costs.

[0177] The following is a specific example of the application of a multi-port converter in a flexible AC / DC distribution network:

[0178] Taking the Zhuhai Tangjiawan Three-Terminal Flexible DC Distribution Project as an example, this project, which began operation on December 25, 2018, was the world's largest multi-terminal flexible DC distribution network project in terms of capacity and voltage levels at the time. The Tangjiawan Three-Terminal Flexible DC Distribution Network Project consists of the Jishan 1 Converter Station (10MW), Jishan 2 Converter Station (10MW), and Tangjia Converter Station (20MW) connected by underground cables. It connects to the wind, solar, energy storage, charging, and various DC loads in Tangjiawan Science and Technology Park, forming a multi-terminal, multi-level, network-reconfigurable ±10kV / 40MW flexible DC distribution network. This realizes the flexible DC interconnection of multiple AC substations and backup power support, improving the reliability of the system power supply.

[0179] The main equipment of the Zhuhai Tangjiawan three-terminal flexible DC distribution network mainly includes three 10kV MMC converters, one ±10kV / ±375V DC transformer, three converter transformers, and DC circuit breakers, etc. Figure 4 As shown. The functions implemented by this system include:

[0180] 1) Power exchange and mutual assistance between the three AC circuits are achieved through converters and ±10kV medium-voltage DC circuits;

[0181] 2) Power exchange between the three-location AC circuit and the ±10kV medium-voltage DC circuit is achieved through a converter;

[0182] 3) Power exchange between the ±10kV medium-voltage DC circuit and the ±375V low-voltage DC circuit is achieved through a DC transformer.

[0183] If a multi-port AC / DC hybrid system based on the multi-port converter unit of this embodiment is adopted, the system topology diagram can be obtained as follows: Figure 5 As shown. The multi-port converter mainly includes two converter units and a three-winding converter transformer. The number of power electronic devices connected in series in each converter unit depends primarily on the highest DC port voltage ±10kV, the highest AC side voltage 10kV, and the overvoltage multiple; therefore, its number is comparable to that of current engineering applications. Compared to existing solutions, the system structure using this embodiment of the invention has the following characteristics:

[0184] 1. It eliminates the need for a 2MW ±10kV / ±375V DC transformer and a DC circuit breaker on the ±10kV DC side of the DC transformer, significantly reducing system costs;

[0185] 2. The cost of the equipment required for the system is comparable to that of other equipment used in the Tangjiawan DC project (excluding DC transformers and their DC side circuit breakers).

[0186] The system functions that can be implemented include:

[0187] 1. Power exchange and mutual assistance between the three AC circuits are realized through a multi-port converter and a ±10kV medium-voltage DC circuit.

[0188] 2. Power exchange between the three-location AC circuit and the ±10kV medium-voltage DC circuit is achieved through a multi-port converter.

[0189] 3. Power exchange between the ±10kV medium-voltage DC circuit and the ±375V low-voltage DC circuit is achieved through a multi-port converter.

[0190] 4. A certain degree of power exchange and mutual assistance between the three AC circuits is achieved through a multi-port converter and a ±375V low-voltage DC circuit.

[0191] 5. Power exchange between the three-location AC circuit and the ±375V low-voltage DC circuit is achieved through a multi-port converter.

[0192] As can be seen from the above analysis, the AC / DC hybrid flexible power distribution system based on the embodiments of the present invention requires fewer devices, has lower costs, and can achieve more functions and operate in a more flexible and diverse manner.

[0193] The converter used will be described in more detail below.

[0194] The main equipment for the Tangjiawan AC / DC hybrid distribution network includes three AC / DC converters and one DC transformer, with their topologies as follows: Figure 6 and Figure 7 As shown in the diagram. The AC / DC converter adopts a modular multilevel converter topology, and the number of power electronic devices required for series connection is determined by the 10kV AC, 20kV DC voltages and their overvoltage multiples. The DC transformer adopts a series-input parallel-output topology composed of 25 isolated dual active bridges. That is, the 25 input active bridges are connected in series on the medium-voltage side, and each bridge withstands a DC voltage of approximately 800V; while the 25 output active bridges are connected in parallel on the low-voltage side, with a rated voltage of 750V and a low-voltage DC rated value of nearly 2700A, and each bridge outputs a DC of 107A. The DC transformer has a relatively complex series-parallel multi-level structure, requiring a total of 200 fully controlled power electronic devices, in addition to 16 isolation transformers.

[0195] If the converter unit adopts an MMC structure, a single-pole multi-port converter device consisting of two converter units and a three-winding transformer is obtained, as shown below. Figure 8 As shown, this multi-port converter includes two DC ports and one AC port, thus consisting of two converter units connected in series, plus a three-winding transformer. The control method for each converter unit can be flexibly selected according to the following relationship.

[0196]

[0197] At this time, the currents of each converter unit have the following relationship.

[0198]

[0199] The DC voltage of each converter unit is:

[0200]

[0201] And there are

[0202]

[0203] By flexibly controlling the current, voltage, and power of each converter unit through the above relationships, flexible AC and DC power conversion, control, and transmission can be achieved. The number of single-phase series power electronic devices in this converter device is mainly determined by the 10kV AC, 20kV DC voltages and their overvoltage multiples. The number of power electronic devices used is comparable to that used in the current Tangjiawan project converter. The difference lies in the fact that the power electronic devices used in the 750V low-voltage converter unit require a larger current rating to achieve a 2MW low-voltage DC capacity. The capacity and number of converter transformers are also comparable to those in the current Tangjiawan project. For ±375V low voltage, if MMC technology is adopted, a hybrid modulation technology based on the nearest-level modulation and pulse width modulation principles can be used to reduce harmonics and improve power quality.

[0204] If a pseudo-bipolar DC system structure is adopted, the topology of this dual-DC-port single-AC-port converter is as follows: Figure 9 As shown, it includes three converter units. The current relationships of each converter unit are as follows:

[0205]

[0206] The DC voltage of each converter unit is:

[0207]

[0208] And there are

[0209]

[0210] At this time, the rated DC voltages of the three converter units of the multi-terminal converter are 9.625kV, 750V, and 9.625kV, respectively. The number of power electronic devices connected in series per phase is... Figure 8 The single-pole multi-port converter is similar, but requires an additional control unit to control the three converter units separately. A multi-winding transformer, on the other hand, requires an additional AC winding; the upper and lower converter units have the same number of turns in their windings, and the number of turns is... Figure 8The number of turns in the winding connected to the first converter unit shown is half. By flexibly controlling the current, voltage, and power of each converter unit through the above relationships, flexible AC / DC power conversion, control, and transmission can be achieved. If a true bipolar structure is adopted, then... Figure 8 The single-pole converter structure can be replicated on the other pole, but the number of devices connected in series on each pole is halved. At this time, the converter unit structure is symmetrical about the neutral line, and the transformer adopts a four-winding structure.

[0211] As can be seen from the above analysis, the device using the embodiments of the present invention does not require a DC transformer in its system topology. The number of power electronic devices used in the multi-port converter is comparable to that used in the current Tangjiawan project converter, and the capacity of the multi-winding transformer used in the multi-port converter is also comparable to that of the converter transformers currently used in engineering projects. In summary, it can reduce system costs, provide more AC / DC conversion functions, and enable more flexible operation.

[0212] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A multi-port AC / DC hybrid converter, characterized in that: It includes n converter units with independent control and regulation capabilities and several multi-winding transformers. The several multi-winding transformers are combined and lead out to n first AC ports connected to the AC side of the converter unit in the AC-DC hybrid converter device and m second AC ports connected to the AC power grid, where m≥0. Under the DC transmission and distribution unipolar structure, the number of converter units n is not less than 2, and under the DC transmission and distribution true bipolar structure and pseudo bipolar structure, n is not less than 3. The n converter units are used for AC / DC power conversion, AC / DC current conversion, and AC / DC voltage conversion, and are connected to have direct DC-DC conversion function; the multi-winding transformer is used for electrical isolation, AC voltage transformation, and power exchange between the n first AC ports and the m second AC ports; The DC sides of n converter units are connected in series, and at least n+1 DC terminals are led out, so that a maximum of n independently adjustable and controllable DC ports can be formed, or a maximum of (n+1)*n / 2 DC ports including independently controllable and non-independently controllable ones can be formed. The number of independently adjustable DC ports is not less than 2 under the DC transmission and distribution unipolar structure and pseudo-bipolar structure, and not less than 3 under the DC transmission and distribution true bipolar structure. When m=0, the multi-port AC-DC hybrid converter realizes the DC-DC conversion function of more than two DC ports. When m≥1, the multi-port AC-DC hybrid converter realizes both AC-DC conversion and DC-DC conversion functions.

2. The multi-port AC / DC hybrid converter as described in claim 1, characterized in that: The multi-winding transformer includes one multi-winding transformer or autotransformer with n first AC ports and m second AC ports, or it is composed of several transformers connected in series and parallel to form n first AC ports and m second AC ports, and realizes electrical isolation, AC transformation and power exchange between the n first AC ports and m second AC ports; the m second AC ports are led out through AC terminals to form m external AC ports of the device; The n converter units adopt the same or different converter topologies, including multi-level voltage source converters with two or more levels, modular multi-level voltage source converters, and bridge AC / DC converter structures. The n converter units can select circuit topologies, voltage and current ratings as needed. Moreover, each converter unit can also adopt a multi-level structure composed of multiple converter circuits connected in series and parallel. The multi-winding transformer and the converter unit are both single-phase, three-phase, or a hybrid structure of single-phase and three-phase, and the AC port of the converter unit and the first AC port of the multi-winding transformer are respectively single-phase, three-phase, or a hybrid structure of single-phase and three-phase. When the number of external AC ports in a multi-winding transformer is m=0, the multi-port AC-DC hybrid converter forms a topology with two or more ports for DC-DC conversion, thus becoming a multi-port DC-DC converter.

3. The multi-port AC / DC hybrid converter as described in claim 1, characterized in that: Among the DC ports on the DC side of the n converter units, the voltage of the DC port with a higher voltage level is formed by the superposition of the voltage of the DC port with a lower voltage level and the DC voltage of the converter unit between them. This allows the converter units and power electronic devices in each DC port to be reused in the DC port with a higher voltage level, significantly reducing the number of power electronic devices required. At the same time, it enables direct DC-DC conversion between different DC ports, and the number of devices through which current flows during direct DC-DC conversion is reduced, thereby improving the DC-DC conversion efficiency. When a converter device containing n converter units is applied in a unipolar DC system, it can draw out a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. The DC ports are arranged from high to low voltage, and the drawn DC ports are d1-dn+1, d2-dn+1, ..., dn-dn+1, with corresponding port voltages of... V dt,1> V dt,2> V dt,3>…> V dt,n, and DC port voltage V dt, i DC terminal potential V d, i The relationship is: V dt, i = V d, i - V d,n+1, i =1, 2, ..., n, where the DC current of the converter unit is... I du, i DC voltage V du, i DC current at DC port I dt, i DC voltage V dt, i The relationship between and satisfies the following formula; P dt,i This represents the input active power of the i-th DC port; When a converter device containing n converter units is applied in a true bipolar DC system, it can draw out a maximum of n independently controllable DC ports, and the number of independently controllable DC ports is not less than 3; for a symmetrical structure, n is an even number and not less than 4; at this time, a neutral line is drawn out from the dn / 2+1 terminal at the DC midpoint potential and grounded. The upper and lower terminals of the neutral line are positive and negative, respectively, and the positive and negative terminals can be independently controlled. Arranged from high to low DC port voltage, the drawn DC ports are d1-dn / 2+1, d2-dn / 2+1, ..., dn / 2-dn / 2+1, dn / 2+2-dn / 2+1, ..., dn+1-dn / 2+1, with corresponding port voltages of... V dt,1> V dt,2> V dt,3>…> V Given dt,n, and the aforementioned port voltages are half positive and half negative, the relationship between the DC port voltage and the DC terminal potential is: V dt, i = V d, i - V d,n / 2+1, when i =1, 2, ..., n / 2; V dt, i = V d, i+ 1- V d,n / 2+1, when i =n / 2+1, n / 2+2, ...,n, where the DC current of the converter unit is... I du, i DC voltage V du, i DC current at DC port I dt, i DC voltage V dt, i The relationship between and satisfies the following formula; For asymmetric structures, n is an odd number and not less than 3. In this case, the positive and negative poles of the bipolar mode are asymmetrical and are configured according to the unipolar mode described above. When a converter device containing n converter units is applied in a pseudo-bipolar DC system, where n is an odd number and not less than 3, it can produce a maximum of (n+1) / 2 independently controllable DC ports, and the number of independently controllable DC ports is not less than 2. Arranged from highest to lowest DC port voltage, the produced DC ports are d1-dn+1, d2-dn, ..., d(n+1) / 2-d(n+3) / 2, with corresponding port voltages of... V dt,1> V dt,2> V dt,3>…> V The relationship between the DC port voltage and the DC terminal potential is as follows: dt,(n+1) / 2 V dt, i = V d, i - V d,n+2-i, i =1, 2, ..., (n+1) / 2, where the DC current of the converter unit is... I du, i DC voltage V du, i DC current at DC port I dt, i DC voltage V dt, i The relationship between and satisfies the following formula; A converter device containing n converter units also includes a hybrid application of unipolar, true bipolar, and pseudo bipolar DC. In this case, n is not less than 3, and the number of DC ports that can be independently controlled is not less than 3. At the same time, some DC ports are connected in a unipolar structure, some DC ports are led out using the positive and negative DC terminals and neutral line terminals of the corresponding voltage level to form a true bipolar DC connection, and some DC ports are led out using the positive and negative DC terminals of the corresponding voltage level to form a pseudo bipolar DC connection. In this case, the neutral line of the device is not connected. Two or all of the above three structures of unipolar, true bipolar, and pseudo bipolar are used to form a hybrid structure.

4. The multi-port AC / DC hybrid converter as described in claim 3, characterized in that: Each of the converter units is connected to a control unit, which controls and regulates the voltage, current, and power on the DC and / or AC sides of the converter unit, thereby directly or indirectly controlling and regulating the voltage, current, and power at the DC and / or AC ports of the multi-port AC / DC hybrid converter. Furthermore, since the control of lower voltage level converter units must consider the influence of the DC current flowing through higher voltage level converter units, the control unit is set according to all the above formulas and, based on this, performs the required voltage, current, or power control. Alternatively, the n converter units are connected to a device-level total control unit, which coordinates and controls the input, output and total power of all converter units, thereby enabling flexible power conversion, control and stable operation of the AC and DC ports of the multi-port AC / DC hybrid converter. When controlling the converter unit, the control unit or device-level main control unit should satisfy the following formula: In the formula, nt indicates that there are nt DC ports in the converter device, na indicates that the voltage control of na DC ports is completed by other equipment in the connected DC circuit, nb indicates that the DC voltage control of nb DC ports is completed by the multi-port AC-DC hybrid converter device, nc indicates that there are nc converter units without directly leading DC ports that use DC voltage control, and nd indicates that there are nd converter units that use non-DC voltage control.

5. The multi-port AC / DC hybrid converter as described in claim 4, characterized in that: The multi-port AC / DC hybrid converter can realize the access and power interaction of DC loads and distributed DC power sources of different voltage levels. By selecting and coordinating control methods, it can also realize power conversion and interaction between different DC ports, and power conversion and interaction between DC ports and AC ports, specifically including: When the input power of any one or more of the DC ports that form the first part DC port A is greater than zero or less than zero, and the sum of the input power is PA The input power of the remaining DC ports of the second part B, which are combined, is either less than zero or greater than zero, and the sum of the input powers is... PB The input active power of the AC port is PAC Ignoring the losses of the multi-port AC / DC hybrid converter, we have: when m=0, PA + PB =0; when m≥1, PA + PB + PAC =0, and: When m≥0, if PA + PB =0, PA If the value is greater than 0, then it represents the power conversion and transmission from the first DC port A to the second DC port B; conversely, if... PA + PB =0, PA If <0, then it represents the power conversion and transmission from the second DC port B to the first DC port A; When m≥1, if PA + PB >0, PA >0, PB If <0, then it represents the power conversion and transmission from the first part (DC port A) to the second part (DC port B and AC port); conversely, if PA + PB <0, PA <0, PB If >0, then it represents the power conversion and power transmission from the second part DC port B and AC port to the first part DC port A. When m≥1, if PA + PB >0, PA >0, PB If the value is greater than 0, then it represents the power conversion and transmission from the first DC port A and the second DC port B to the AC port; conversely, if... PA + PB <0, PA <0, PB When <0, it represents the power conversion and transmission from the AC port to the first DC port A and the second DC port B.

6. A multi-terminal AC / DC hybrid transmission and distribution system, characterized in that: The system includes two or more multi-port AC / DC hybrid converters as described in any one of claims 1-5, wherein one or more DC ports of the two or more multi-port AC / DC hybrid converters are respectively connected to DC circuits of corresponding voltage levels, and one or more second AC ports of the two or more multi-port AC / DC hybrid converters are respectively connected to AC circuits of corresponding voltage levels, thereby forming a multi-port AC / DC hybrid transmission and distribution system to realize the access, interconnection and interaction of DC loads and DC power supplies of multiple locations and different voltage levels.

7. The multi-terminal AC / DC hybrid transmission and distribution system as described in claim 6, characterized in that: It also includes a system-level control unit, which is used to control the coordinated operation of various multi-port AC / DC hybrid converters to achieve flexible AC / DC power conversion and mutual assistance across different regions, as well as to achieve stable operation of AC / DC transmission and distribution networks; wherein, the flexible AC / DC power conversion and mutual assistance across different regions includes: flexible power exchange and mutual assistance between AC circuits of the same or different voltage levels in different locations through DC circuits of different voltage levels; Flexible power exchange and mutual assistance between DC circuits of the same or different voltage levels in different locations; Flexible power conversion and mutual assistance between AC and DC in different locations.

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