Fault-tolerant four-port active bridge converter for bipolar direct current system interconnection and control method

Abstract

The invention discloses a fault-tolerant four-port active bridge converter for interconnection of a bipolar direct-current system and a control method, and provides a fault-tolerant four-port active bridge converter topology scheme with power forward and reverse flow capability for interconnection of the bipolar direct-current system, and the fault-tolerant four-port active bridge converter is provided with two high-voltage ports and two low-voltage ports. Based on four full-bridge converters and two three-winding transformers, low-power conversion level and high-reliability interconnection of bipolar direct current systems of different voltage levels are realized, and real-time adjustment of power and flow direction is realized. The converter has the fault-tolerant operation capability, when a certain port breaks down, the port can be isolated from other non-fault ports, other non-fault ports can continue to transmit power, and the power supply reliability of a bipolar system is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of active bridge converter circuits, and in particular relates to a fault-tolerant four-port active bridge converter for interconnecting bipolar DC systems and a control method thereof. Background Art

[0002] With the development of renewable energy technologies, the application of various distributed renewable energy sources in integrated power systems is becoming increasingly widespread, leading to a proliferation of corresponding electronic loads and energy storage systems. DC distribution systems can improve efficiency by reducing losses in the conversion stage and inverters, thereby facilitating power system integration. They are expected to replace traditional AC networks and become a more promising technical solution. Among various DC distribution systems, bipolar DC systems offer advantages in flexibility, reliability, and safety over unipolar DC systems. Bipolar DC systems allow for higher voltage levels while maintaining a lower voltage to ground. Furthermore, bipolar DC systems offer a degree of fault tolerance. In the event of a single-pole fault, they can ensure normal operation of other lines to a certain extent. This allows for the distribution of power and loads across different lines, improving overall system reliability.

[0003] With the continuous development of DC systems, multi-voltage DC distribution systems will become a trend, and the interconnected application of DC systems is expected to become increasingly widespread. However, current research focuses primarily on unipolar DC systems and DC-AC hybrid systems, with less attention and research on bipolar DC systems. Some of the existing problems of bipolar DC systems have not yet been effectively addressed. A key challenge facing bipolar DC systems is the occurrence of unipolar faults. When a unipolar fault (usually a short circuit) occurs, the power system aims to minimize the impact of the fault on the interconnection of the entire bipolar DC system, ensuring that the remaining unfaulted poles can still supply and transmit power normally. This requires the power topology to be able to isolate the faulted pole, prevent the fault from affecting adjacent unfaulted poles, and restore normal operation after the fault is repaired. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a fault-tolerant four-port active bridge converter and a control method for interconnecting a bipolar DC system, which solves the problem of fault tolerance of the bipolar DC system in the prior art when a unipolar fault occurs.

[0005] The present invention adopts the following technical solutions to solve the above technical problems:

[0006] A fault-tolerant four-port active bridge converter for bipolar DC system interconnection, wherein the four ports are two low-voltage ports and two high-voltage ports, each port is connected to a full-bridge converter, and the full-bridge converters are respectively connected to two three-winding transformers Tr1 and Tr2 to form two AC circuits. Converters FB3 and FB4 corresponding to the low-voltage ports are respectively connected to the primary windings of the three-winding transformers, and the secondary windings of the three-winding transformers are respectively connected in series with converters FB1 and FB2 corresponding to the high-voltage ports. The same-named ends of the primary winding of the three-winding transformer Tr1 are in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB1, and in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB2; the same-named ends of the primary winding of the three-winding transformer Tr2 are in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB1, and in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB2.

[0007] The four-port active bridge converter includes a normal operation mode and a fault-tolerant operation mode.

[0008] In normal operation mode, the four-port active bridge converter operation objectives include power conversion and bipolar system balancing.

[0009] Phase-shift control is used to adjust the switching mode, port voltage, and loop current waveforms of the four-port active bridge converter.

[0010] When a single-pole short-circuit fault occurs on the low-voltage side, the four-port active bridge converter isolates the faulty pole through blocking protection, so that the low-voltage side voltage corresponding to the fault level is 0.

[0011] When a component or system connected to converter FB3 or FB4 fails, the four-port active bridge converter operates in a fault-tolerant operation mode, receiving power from converter FB1 and outputting power to converter FB2.

[0012] The number of low-power conversion stages of bipolar DC systems with different voltage levels is determined by the transformer ratio, and real-time adjustment of power size and flow direction is achieved by adjusting the phase shift angle.

[0013] A control method for a fault-tolerant four-port active bridge converter for bipolar DC system interconnection sets the voltages, transformer ratio, and phase-shift angle of the four ports according to the voltage required by actual operating conditions. The power level of each port is obtained, and the phase-shift angle of each power switch in the converter is adjusted based on the power level of each port, ultimately achieving the goals of bipolar DC system interconnection and fault tolerance.

[0014] The voltages of the four ports are set to two high voltage levels of equal size, one positive and one negative, and two low voltage levels of equal size, one positive and one negative.

[0015] A medium voltage DC power distribution system includes the fault-tolerant four-port active bridge converter for bipolar DC system interconnection.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. This solution can interconnect bipolar systems with different voltage levels, achieve integrated design, and provide bidirectional power flow and fault-tolerant operation capabilities. When a short circuit occurs in a single pole, the entire system can operate in unipolar mode and transmit power through the DC bus to isolate the faulty pole from other non-faulty poles.

[0018] 2. Based on four full-bridge converters and two three-winding transformers, low-power conversion levels and high-reliability interconnection of bipolar DC systems with different voltage levels are achieved, and real-time adjustment of power size and flow direction is realized.

[0019] 3. The converter has the ability to operate in a fault-tolerant manner. When a port fails, it can be isolated from other non-faulty ports. Other non-faulty ports can continue to transmit power, greatly improving the power supply reliability of the bipolar system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a topological circuit diagram of the four-port active bridge converter of the present invention.

[0021] Figure 2 This is the equivalent circuit of the four-port active bridge converter topology of the present invention.

[0022] Figure 3 1 is the operating waveform of the four-port active bridge converter of the present invention in the normal operating mode.

[0023] Figure 4 1 is the operating waveform of the four-port active bridge converter of the present invention in the fault-tolerant operation mode.

[0024] Figure 5 This is a diagram of the current and voltage simulation results of the four-port active bridge converter under normal operating conditions of the present invention.

[0025] Figure 6 This is a diagram of the current and voltage simulation results of the fault-tolerant operation of the four-port active bridge converter of the present invention.

[0026] Figure 7 This is a diagram of the current and voltage simulation results of the four-port active bridge converter of the present invention when a fault occurs at the time of 0.5s. DETAILED DESCRIPTION

[0027] The structure and working process of the present invention will be further described below with reference to the accompanying drawings.

[0028] The purpose of this solution is to innovatively propose a fault-tolerant four-port active bridge converter topology with forward and reverse power flow capabilities for the interconnection of bipolar DC systems. This solution can establish the interconnection of bipolar systems with different voltage levels, achieving an integrated design, and has bidirectional power flow and fault-tolerant operation capabilities. When a short circuit occurs in a single pole, the entire system can operate in a unipolar mode, and power can be transmitted through the DC bus to isolate the faulty pole from other non-faulty poles. Compared with existing technical solutions, this invention has the following features:

[0029] Equipped with two high-voltage ports and two low-voltage ports, it can realize the interconnection of bipolar DC systems;

[0030] With the ability of power flow in both forward and reverse directions, the power of each port can be switched between forward and reverse transmission modes in real time;

[0031] Based on four full-bridge converters and two three-winding transformers, electrical isolation between the four DC ports is achieved, and the system has fault-tolerant operation capability when one DC port fails.

[0032] A fault-tolerant four-port active bridge converter for bipolar DC system interconnection, wherein the four ports are two low-voltage ports and two high-voltage ports, each port is connected to a full-bridge converter, and the full-bridge converters are respectively connected to two three-winding transformers Tr1 and Tr2 to form two AC circuits. Converters FB3 and FB4 corresponding to the low-voltage ports are respectively connected to the primary windings of the three-winding transformers, and the secondary windings of the three-winding transformers are respectively connected in series with converters FB1 and FB2 corresponding to the high-voltage ports. The same-named ends of the primary winding of the three-winding transformer Tr1 are in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB1, and in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB2; the same-named ends of the primary winding of the three-winding transformer Tr2 are in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB1, and in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB2.

[0033] Specific embodiments, such as Figures 1 to 7 As shown,

[0034] A topology scheme of a fault-tolerant four-port active bridge converter for bipolar DC system interconnection, such as Figure 1As shown, in the four-port active bridge converter (QAB), the first port (HP) and the second port (HN) are high-voltage ports, while the third port (LP) and the fourth port (LN) are low-voltage ports. The four DC ports are connected to corresponding full-bridge converters FB1-FB4, and each full-bridge consists of four switches S11-S14 (S21-S24, S31-S34, and S41-S44). Converters FB3 and FB4 corresponding to the low-voltage ports are connected to the primary windings of three-winding transformers Tr1 and Tr2, respectively. The secondary windings of the three-winding transformers are connected in series with converters FB1 and FB2 corresponding to the high-voltage ports, forming two AC circuits.

[0035] In addition, in this topology, the same-named end of the primary winding of Tr1 is in the same direction as the same-named end of the secondary winding on the AC side of FB1, and in the opposite direction to the same-named end of the secondary winding on the AC side of FB2; the same-named end of the primary winding of Tr2 is in the opposite direction to the same-named end of the secondary winding on the AC side of FB1, and in the same direction to the same-named end of the secondary winding on the AC side of FB2. 、 、 are the leakage inductances of the three-winding transformer Tr1 at the FB3, FB1, and FB2 ports, respectively. 、 、 They are the leakage inductances of the three-winding transformer Tr2 at the FB4, FB1 and FB2 ports respectively.

[0036] The topology proposed in this solution can realize the interconnection of bipolar DC systems, power can flow bidirectionally between high-voltage ports and low-voltage ports, and can also ensure a high step-up ratio. Its specific working principle is as follows. To simplify the calculation process of its power characteristics, it is assumed that the voltage of each port is close to constant and , , ,but , , .

[0037] Operation Control and Power Characteristics of Four-Port Active Bridge Converter

[0038] In order to better study and analyze the power characteristics, Figure 1 The topology shown is formally simplified and the inductor is transformed from star to triangle to form the circuit equivalent to Figure 2 The equivalent circuit shown in Figure 1 is shown in Figure 2. The calculation results of the star-delta transformation are shown in equations (1) to (3):

[0039] (1)

[0040] (2)

[0041] (3)

[0042] Next, the power characteristics of the QAB converter will be analyzed in both normal operation mode and fault-tolerant operation mode.

[0043] A. Normal operating mode current, voltage and power characteristics

[0044] In normal operation mode, the operation objectives of the QAB converter include power conversion and bipolar system balance. Phase shift control is adopted here, and the switching mode, port voltage, and loop current operating waveforms of the four-port active bridge converter are as follows: Figure 3 As shown, 、 、 Respectively represent the phase shift angle of the output voltage of FB2, FB3, and FB4 relative to FB1. For example, TS represents a switching cycle, is the transformation ratio of the two three-winding transformers.

[0045] The following will derive the loop current and the power of each port in different switching modes. Due to the symmetry of the QAB converter, only Figure 2 The equivalent circuit in normal mode is shown in the figure to analyze half of the switching mode. For the sake of simplicity, the loss in the circuit will not be considered in the following derivation, and , , Formulas (4)-(6), (7)-(9), (10)-(12), and (13)-(15) are respectively 、 、 、 The corresponding 、 、 The calculation formula is is the angular frequency corresponding to the full-bridge switch, that is , is the full-bridge switching frequency, i1, i2, and i3 are the currents on the leakage inductors L1, L2, and L3 respectively:

[0046] (a) :

[0047] (4)

[0048] (5)

[0049] (6)

[0050] (b) :

[0051] (7)

[0052] (8)

[0053] (9)

[0054] (c) :

[0055] (10)

[0056] (11)

[0057] (12)

[0058] (d) :

[0059] (13)

[0060] (14)

[0061] (15)

[0062] Furthermore, based on equations (4)-(15), the average power transmitted by each port in one cycle can be derived. The output power of the high-voltage side positive electrode and the high-voltage side negative electrode are respectively as shown in equations (16) and (17). 、 As shown:

[0063] (16)

[0064] (17)

[0065] If the switch and transformer losses are ignored, the total transmission power is as follows: As shown:

[0066] (18)

[0067] Similarly, the received power of the positive electrode on the low voltage side and the negative electrode on the low voltage side are respectively as shown in equations (19) and (20): 、 As shown:

[0068] (19)

[0069] (20)

[0070] B. Current, voltage and power characteristics of fault-tolerant operation mode

[0071] In bipolar DC systems, single-pole short-circuit faults are common. When a single-pole short-circuit fault occurs on the low-voltage side, the four-port active bridge converter can isolate the faulty pole while ensuring uninterrupted operation of other non-faulty poles. Without loss of generality, it is assumed that the component connected to the negative pole FB4 on the low-voltage side is short-circuited and isolated through latching protection, so that Phase shift control is still used here. The switching mode, voltage of each port and loop current waveform of the four-port active bridge converter are as follows: Figure 4 As shown, 、 Respectively represent the phase shift angle of FB2 and FB3 output voltage relative to FB1, here For example, TS represents a switching cycle, Turns ratio of two three-winding transformers.

[0072] According to the same method as above, we can calculate 、 、 The corresponding 、 、 The calculation formulas are shown in Equations (21)-(23), (24)-(26), and (27)-(29), respectively, where is the angular frequency corresponding to the full-bridge switch, that is , is the full-bridge switching frequency, i1, i2, and i3 are the currents on the leakage inductors L1, L2, and L3 respectively:

[0073] (a) :

[0074] (twenty one)

[0075] (twenty two)

[0076] (twenty three)

[0077] (b) :

[0078] (twenty four)

[0079] (25)

[0080] (26)

[0081] (c) :

[0082] (27)

[0083] (28)

[0084] (29)

[0085] Based on equations (21)-(23), (24)-(26), and (27)-(29), the transmission power of the three non-faulty ports can be calculated. The transmission power of the high-voltage side positive electrode and the high-voltage side negative electrode is as shown in equations (30) and (31). 、 As shown:

[0086] (30)

[0087] (31)

[0088] If the switch and transformer losses are ignored, the total transmission power under fault-tolerant conditions can be calculated as follows: As shown:

[0089] (32)

[0090] Similarly, the received power of the positive electrode on the low-voltage side can be calculated as follows: As shown:

[0091] (33)

[0092] In summary, when a component or system connected to FB3 or FB4 fails, the proposed four-port active bridge converter has fault-tolerant operation capability and can receive power from FB1 and output power to FB2.

[0093] In order to further illustrate the beneficial effects of the scheme and verify the feasibility of the proposed four-port active bridge converter, a simulation example is constructed in the Plecs simulation environment. Figure 1 The four-port active bridge converter shown in FIG1 can realize the interconnection of bipolar DC systems. Each port is equivalent to a DC voltage source. The circuit parameters are shown in Table 1.

[0094] Application scenario: bipolar system interconnection

[0095] According to the four-port active bridge converter topology proposed in this solution and actual engineering requirements, the rated power of the four-port active bridge converter is set to 400kW.

[0096] Table 1 Simulation circuit parameters

[0097]

[0098] A. Normal working condition simulation verification

[0099] During the simulation, set the phase shift angle 、 、 Make , At this time, FB1 and FB2 emit power, and FB3 and FB4 absorb power. The simulation results of current and voltage are as follows: Figure 5 As shown, is the current on the AC side, is the DC side current of ports FB1 to FB4.

[0100] B. Fault-tolerant operation condition simulation verification

[0101] When a short circuit occurs in the components on the side of the converter FB4, the FB4 is locked to protect the , thereby isolating the fault. During the simulation, set the phase shift angle 、 , making And it is half of the normal working condition. Same as normal working condition. At this time, FB1 and FB2 send out power, FB3 absorbs power, and the simulation current and voltage results are as follows: Figure 6 As shown, is the current on the AC side, is the DC side current of ports FB1~FB4.

[0102] The four-port active bridge converter originally operated under normal working conditions, if the FB4 port short-circuit fault occurs at 0.5s, the simulation current and voltage results are as follows Figure 7 As shown, is the current on the AC side, is the DC-side current of ports FB1 to FB4. When a short-circuit fault occurs on one port of the four-port active bridge converter, it can quickly switch from normal operating mode to fault-tolerant operating mode. The entire switching process takes only about 2ms, minimizing the impact of the short-circuit fault on the interconnection of the entire bipolar DC system.

[0103] The simulation results demonstrate that the proposed four-port active bridge converter for bipolar DC system interconnection can establish connections between ports of varying voltage levels and control the magnitude and direction of power flow through phase-shifting control of the internal full-bridge converter. This converter is suitable for connecting components and DC buses with varying voltages. If a component or DC system fails, it can rapidly switch from normal operation to fault-tolerant operation, enabling power to be transferred from the DC bus to other surviving components and systems.

[0104] The solution also discloses a control method for a fault-tolerant four-port active bridge converter for bipolar DC system interconnection. The voltage, transformer ratio and phase shift angle of the four ports are set according to the voltage required under actual working conditions. The power size of each port is obtained, and the phase shift angle of each power switch in the converter is adjusted according to the power size of each port, ultimately achieving the goal of bipolar DC system interconnection and fault tolerance.

[0105] Specifically, the voltages of the four ports are set to two high voltage levels of the same magnitude, one positive and one negative, and two low voltage levels of the same magnitude, one positive and one negative.

[0106] This solution features a fault-tolerant four-port active bridge converter for bipolar DC system interconnection. It focuses on fault isolation and system reconfiguration in medium-voltage DC distribution systems. By reducing losses in the conversion stage and inverter, it improves efficiency, thereby facilitating power system integration and potentially replacing traditional AC networks. This technology will significantly enhance the reliability and flexibility of DC grids, making it particularly suitable for applications such as renewable energy power plants, microgrids, and marine power systems.

[0107] Those skilled in the art should understand that they can implement variations by combining the prior art and the above embodiments. Such variations do not affect the essence of this solution and are not described in detail here.

[0108] It should be understood that this solution is not limited to the specific implementation methods described above. Devices and structures not described in detail should be understood to be implemented in a common manner in the art. Any person skilled in the art can, without departing from the scope of this solution, use the methods and technical content disclosed above to make many possible changes and modifications to this solution, or modify it into equivalent embodiments with equivalent changes, without affecting the essence of this solution. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of this solution without departing from the content of this solution are still within the scope of protection of this solution.

Claims

1. A fault-tolerant four-port active bridge converter for bipolar DC system interconnection, wherein: The four ports are two low-voltage ports and two high-voltage ports, and each port is connected to a full-bridge converter. The full-bridge converters are respectively connected to two three-winding transformers Tr1 and Tr2 to form two AC circuits. The converters FB3 and FB4 corresponding to the low-voltage ports are respectively connected to the primary windings of the three-winding transformers, and the secondary windings of the three-winding transformers are respectively connected in series with the converters FB1 and FB2 corresponding to the high-voltage ports. It is characterized in that: the same-named ends of the primary winding of the three-winding transformer Tr1 are in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB1, and in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB2; the same-named ends of the primary winding of the three-winding transformer Tr2 are in the opposite direction to the same-named ends of the secondary winding on the AC side of the converter FB1, and in the same direction as the same-named ends of the secondary winding on the AC side of the converter FB2.

2. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 1, characterized in that: The four-port active bridge converter includes a normal operation mode and a fault-tolerant operation mode.

3. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 2, characterized in that: In normal operation mode, the four-port active bridge converter operation objectives include power conversion and bipolar system balancing.

4. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 3, characterized in that: Phase-shift control is used to adjust the switching mode, port voltage, and loop current waveforms of the four-port active bridge converter.

5. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 1, characterized in that: When a single-pole short-circuit fault occurs on the low-voltage side, the four-port active bridge converter isolates the faulty pole through blocking protection, so that the low-voltage side voltage corresponding to the fault level is 0.

6. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 1, characterized in that: When a component or system connected to converter FB3 or FB4 fails, the four-port active bridge converter operates in a fault-tolerant operation mode, receiving power from converter FB1 and outputting power to converter FB2.

7. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 5, characterized in that: The number of low-power conversion stages of bipolar DC systems with different voltage levels is determined by the transformer ratio, and real-time adjustment of power size and flow direction is achieved by adjusting the phase shift angle.

8. A control method for a fault-tolerant four-port active bridge converter for bipolar DC system interconnection, characterized in that: According to the voltage required by the actual working conditions, the voltage of the four ports, the transformer ratio and the phase shift angle are set; the power size of each port is obtained, and the phase shift angle of each power switch in the converter is adjusted according to the power size of each port, ultimately achieving the goal of bipolar DC system interconnection and fault tolerance.

9. The fault-tolerant four-port active bridge converter for bipolar DC system interconnection according to claim 8, characterized in that: The voltages of the four ports are set to two high voltage levels of equal size, one positive and one negative, and two low voltage levels of equal size, one positive and one negative.

10. Medium voltage DC power distribution system, characterized by: A fault-tolerant four-port active bridge converter for bipolar DC system interconnection comprising any one of claims 1 to 7.

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