Ridge type mixed polarity direct current power distribution system

Abstract

The invention provides a ridge-shaped mixed-polarity direct-current power distribution system. The ridge-shaped mixed-polarity direct-current power distribution system comprises a high-voltage side bipolar bus, a low-voltage side bipolar bus, a current converter part for connecting a high-voltage power distribution network with a low-voltage power distribution network, and a high-voltage and low-voltage bipolar direct-current common neutral line, the converter part is composed of a four-port active bridge QAB converter and a power direct transmission line. A positive electrode high-voltage side active bridge of the converter part is composed of four MOSFET switch tubes S11, S12, S13 and S14 and four anti-parallel diodes D11, D12, D13 and D14, a low-voltage side is composed of switch tubes S21, S22, S23 and S24 and diodes D21, D22, D23 and D24, the high-voltage side active bridge and the low-voltage side active bridge are connected through a high-frequency transformer with the turn ratio n being 1: 1, the equivalent leakage inductance of the transformer is Lr1, and the negative electrode and the positive electrode have the same structure. According to the invention, the direct current systems with different voltage levels are interconnected, the partial power transmission capability of the high-voltage and low-voltage direct current systems is realized, the load access selection of the five voltage levels of + / -2VDC, 4VDC and + / -VDC is realized, and the operation mode is flexible.

Description

Technical Field

[0001] The present invention relates to the technical field of DC power distribution system architecture, in particular to a DC power distribution system with mixed polarity, and in particular to a DC power distribution system with mixed polarity. Type mixed polarity DC distribution system architecture. Background Art

[0002] With the distributed access of renewable energy, frequent failures in the power transmission and distribution network, and large-scale fluctuations in the load on the power consumption side, traditional AC power grids are facing problems with power flow, load distribution, voltage regulation and various power quality issues, and can no longer meet the needs of responding to rapid changes in the operating status of the power grid.

[0003] The emergence of direct current systems (DC Microgrid) has revolutionized the concepts of power transmission and distribution, especially in the context of distributed generation and renewable energy. DC microgrids have brought new solutions to the construction of distribution networks. Compared with AC distribution, the most important advantages of DC distribution include higher reliability and efficiency, simpler control, and natural interfaces with renewable energy, electronic loads, and energy storage systems. With the rapid emergence of these components in modern power systems, the importance of DC microgrids in today's society is gradually increasing. Traditional DC applications, such as traction, telecommunications, vehicles, and distributed power systems, have become an important development direction for future power grids with their advantages of high transmission and distribution efficiency, good power reliability, and easy control methods.

[0004] According to the number of voltage levels and busbars that the microgrid can provide, the DC microgrid is divided into the following three operating architectures: unipolar structure, true bipolar structure, and pseudo bipolar structure. The disadvantage of the unipolar structure is that it only provides one voltage level, the load access is not flexible, the power supply reliability is low, and a fault occurs somewhere in the microgrid, the entire line is affected, and the line loss is large. Its advantage is that only one pole line network needs to be built, and the construction cost is low.

[0005] Pseudo-bipolar structure, compared with the unipolar metal loop operation mode, the pseudo-bipolar access mode has stronger power supply flexibility and supports positive and negative pole access.

[0006] The advantage of the true bipolar structure is that it can provide three voltage levels, namely ±UDC and 2UDC, which is convenient for flexible access to loads. If a fault occurs in one pole, the busbar in the other pole can continue to operate. Its reliability is higher than that of the unipolar architecture. However, its disadvantage is that it requires the construction of a two-pole transmission line, with high initial construction costs and relatively complex control strategies.

[0007] Bipolar design has become one of the main DC system configurations with its unique wiring method. Although it has many ideal characteristics, it requires a large number of DC-DC converters to connect the "source" and "load" to the microgrid. At this time, it is easy to have uneven distribution of loads and power between the two poles, resulting in voltage and current imbalance, which may reduce the operating performance of the DC system. Therefore, specific power electronics-based solutions are needed to ensure the balance of these DC systems.

[0008] Therefore, it is necessary to propose a new technical solution to improve the above technical problems. Summary of the invention

[0009] In view of the defects in the prior art, the purpose of the present invention is to provide a Type mixed polarity DC distribution system.

[0010] According to the present invention, a The hybrid polarity DC power distribution system includes: a high-voltage side bipolar bus, a low-voltage side bipolar bus, a converter part connecting the high-voltage and low-voltage distribution networks, and a high-voltage and low-voltage bipolar DC common neutral line;

[0011] The converter part is composed of a four-port active bridge QAB converter and a power direct transmission line, and the power direct transmission line of the converter part is equivalent to a unipolar power transmission network;

[0012] The positive high-voltage side active bridge of the converter part is composed of four MOSFET switch tubes S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, and the low-voltage side is composed of switch tubes S21, S22, S23, S24 and diodes D21, D22, D23, D24. The high-voltage side active bridge is connected to the low-voltage side active bridge through a high-frequency transformer with a turns ratio n of 1:1, and the equivalent leakage inductance of the transformer is Lr1. The negative bridge on the negative bus has the same structure as the positive bridge; the AC sides of the positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnected transformer Te is LT.

[0013] Preferably, the converter part performs high- and low-voltage side power transmission and voltage conversion; capacitors C1, C2, C3, C4, C5, and C6 are connected at the interfaces between the high-voltage side, the low-voltage side, and the converter part; the high-voltage side active bridge of the positive bridge is Hh1, and the low-voltage side is Hl1; the high-voltage side active bridge of the negative bridge is Hh2, and the low-voltage side is Hl2; the rated voltage of the high-voltage side bipolar bus is ±2UDC, and voltage levels of ±2UDC and 4UDC are provided; the rated voltage of the low-voltage side bipolar bus is ±UDC, and voltage levels of ±UDC and 2UDC are provided.

[0014] Preferably, the system adopts the ±2UDC DC bus on the high-voltage side as the hub for energy collection and distribution, takes the high-voltage side as the source side, and transmits electric energy from the high-voltage side to the low-voltage side in the form of partial power transmission through the four-port converter and the power direct transmission line of the converter part, so that partial power transmission is achieved between high-voltage DC and low-voltage DC.

[0015] Preferably, the unbalanced power of the positive and negative electrodes on the high-voltage side is controlled by the phase shift angle φ2 and is implemented by droop control, and its control target is as shown in formula (1):

[0016]

[0017] Among them, vph is the positive bus voltage on the high-voltage side, vNh is the positive bus voltage on the low-voltage side, imid is the current of the system intermediate line Zh, and Rdroop is the droop control coefficient; ε is the maximum allowable bus voltage deviation of the bipolar system. The droop control coefficient Rdroop and the voltage reference value are determined by the following relationship:

[0018]

[0019]

[0020] Among them, vN is the rated voltage of the line, and imax is the maximum value of the neutral current. The voltage and current dual-loop control is used to balance the positive and negative voltages. The input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship. After adjustment by the PI controller, the current reference value iref is obtained, which together with the leakage inductance current imid constitutes the input signal of the current inner loop. After PI adjustment, the phase shift angle φ2 is output to control the power flow between the positive and negative electrodes on the high-voltage side.

[0021] Preferably, the average value of the leakage inductance current on the positive and negative electrode interconnected transformer Te is:

[0022]

[0023] Where Uin1 is the positive input voltage on the high-voltage side, ω=2πf, f is the switching frequency, and LT is the equivalent leakage inductance on the interconnection transformer. The positive and negative balanced power transmitted by the interconnection transformer is obtained from formula (3):

[0024]

[0025] Where m is the turns ratio of the transformer in the four-port converter.

[0026] Preferably, the control target of the output voltage Uo1 of the positive bridge is:

[0027] U o1 =U DC (5)

[0028] The input signal of its voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low-voltage side voltage sampling value vPl. After adjustment by the PI controller, the current reference value iref1 is obtained, which together with the leakage inductance current iLr1 constitutes the input signal of the current inner loop. After PI adjustment, the control amount phase shift angle φ1 for the positive bridge is output.

[0029] Preferably, the average output current of the positive bridge is expressed by the following formula:

[0030]

[0031] Among them, Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge.

[0032] Preferably, the power transmitted from the high-voltage side active bridge Hh1 of the positive bridge to the low-voltage side Hl1 is obtained from formula (6):

[0033]

[0034] The control target of the output voltage Uo2 of the negative bridge is:

[0035] U o2 =U DC (8)

[0036] The input signal of its voltage outer loop is the positive voltage reference value v*ref2 and the negative bus low-voltage side voltage sampling value vNl. After adjustment by the PI controller, the current reference value iref2 is obtained, which together with the leakage inductance current iLr2 constitutes the input signal of the current inner loop. After PI adjustment, the control quantity phase shift angle φ3 for the negative bridge is output.

[0037] Preferably, the average output current of the negative bridge is expressed by the following formula:

[0038]

[0039] Among them, Uin2 is the positive input voltage on the high-voltage side, and Lr2 is the equivalent leakage inductance of the transformer contained in the negative bridge.

[0040] Preferably, the power transmitted from the high-voltage side active bridge Hh2 of the negative bridge to the low-voltage side Hl2 is obtained from formula (9):

[0041]

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. The present invention has the ability to interconnect DC systems of different voltage levels, and has load access options of five voltage levels: ±2VDC, 4VDC, and ±VDC, with flexible operation modes;

[0044] 2. The present invention shares the neutral line between DC systems of different voltage levels, and based on the proposed QAB circuit, partial power transmission is achieved, thereby reducing the total power loss of the system and the converter cost;

[0045] 3. When there is a large deviation between the voltage and current of the positive and negative electrodes, the power exchange function between the two positive and negative electrodes QAB is enabled to solve the problem of unbalanced power between the positive and negative electrodes through phase shift modulation; and droop control is used to achieve power balance between the positive and negative electrodes, without the need to install an additional controller, which is simple and reliable;

[0046] 4. The present invention adopts a four-port converter to play the role of a bridge for partial power transmission, which significantly improves the transmission efficiency compared to a bipolar DC network directly interconnected by a DC-DC converter;

[0047] 5. The present invention proposes When the hybrid polarity DC distribution system is applied to power transmission and distribution, the four-port converter and the direct power transmission line of the converter part can be used to transmit electric energy from the high voltage side to various electrical equipment on the low voltage side in the form of partial power transmission. Both the positive and negative power transmission channels adopt the partial power transmission architecture, which reduces losses and improves efficiency while connecting two power grids with different voltage levels.

[0048] 6. In the present invention, when one of the positive and negative lines of the bipolar microgrid is overloaded, causing the bus voltage to drop and deviate from the allowable voltage fluctuation range, there is no need to adopt the traditional method of introducing a voltage equalizer. Instead, the phase shift angle φ can be changed through phase shift control to transmit power from the lightly loaded line to the heavily loaded line via the interconnected transformer between the positive bridge and the negative bridge, thereby solving the power shortage of the heavily loaded line and balancing the positive and negative voltages; saving the cost of installing additional equipment and improving economy. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0050] Figure 1 for Kan type DC distribution system architecture diagram;

[0051] Figure 2 for Diagram of interconnected four-port active bridge in a Kan-type power distribution system;

[0052] Figure 3 for Kan type DC distribution system control architecture diagram;

[0053] Figure 4 This is the voltage difference diagram of the positive and negative busbars on the high-voltage side when power balancing control is not enabled;

[0054] Figure 5 This is the power regulation diagram of the high-voltage side DC distribution system in scenario 1;

[0055] Figure 6 This is the diagram showing the impact of high-voltage side load changes on low-voltage side power regulation;

[0056] Figure 7 This is the key waveform diagram of the positive bridge in scenario 1;

[0057] Figure 8 This is the key waveform diagram of the negative bridge in scenario 1;

[0058] Fig. 9 It is the key waveform diagram of the interconnected transformer in scenario 1;

[0059] Fig.10 This is the power regulation diagram of the low-voltage side DC distribution system in scenario 1;

[0060] Fig.11 This is the diagram showing the impact of load changes on the low-voltage side on power regulation on the high-voltage side;

[0061] Fig.12 This is the key waveform diagram of the positive bridge in scenario 2;

[0062] Fig.13 This is the key waveform diagram of the negative bridge in scenario 2;

[0063] Fig.14 It is the key waveform diagram of the interconnected transformer in scenario 2. DETAILED DESCRIPTION

[0064] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0065] Embodiment 1:

[0066] According to the present invention, a (Kan) type mixed polarity DC power distribution system, including: high voltage side bipolar bus, low voltage side bipolar bus, converter part connecting high and low voltage distribution networks, high and low voltage bipolar DC common neutral line;

[0067] The converter part is composed of a four-port active bridge QAB converter and a power direct transmission line, and the power direct transmission line of the converter part is equivalent to a unipolar power transmission network;

[0068] The positive high-voltage side active bridge of the converter part is composed of four MOSFET switch tubes S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, and the low-voltage side is composed of switch tubes S21, S22, S23, S24 and diodes D21, D22, D23, D24. The high-voltage side active bridge is connected to the low-voltage side active bridge through a high-frequency transformer with a turns ratio n of 1:1, and the equivalent leakage inductance of the transformer is Lr1. The negative bridge on the negative bus has the same structure as the positive bridge; the AC sides of the positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnected transformer Te is LT.

[0069] Preferably, the converter part performs high- and low-voltage side power transmission and voltage conversion; capacitors C1, C2, C3, C4, C5, and C6 are connected at the interfaces between the high-voltage side, the low-voltage side, and the converter part; the high-voltage side active bridge of the positive bridge is Hh1, and the low-voltage side is Hl1; the high-voltage side active bridge of the negative bridge is Hh2, and the low-voltage side is Hl2; the rated voltage of the high-voltage side bipolar bus is ±2UDC, and voltage levels of ±2UDC and 4UDC are provided; the rated voltage of the low-voltage side bipolar bus is ±UDC, and voltage levels of ±UDC and 2UDC are provided.

[0070] Preferably, the system adopts the ±2UDC DC bus on the high-voltage side as the hub for energy collection and distribution, takes the high-voltage side as the source side, and transmits electric energy from the high-voltage side to the low-voltage side in the form of partial power transmission through the four-port converter and the power direct transmission line of the converter part, so that partial power transmission is achieved between high-voltage DC and low-voltage DC.

[0071] Preferably, the unbalanced power of the positive and negative electrodes on the high-voltage side is controlled by the phase shift angle φ2 and is implemented by droop control, and its control target is as shown in formula (1):

[0072]

[0073] Among them, vph is the positive bus voltage on the high-voltage side, vNh is the positive bus voltage on the low-voltage side, imid is the current of the system intermediate line Zh, and Rdroop is the droop control coefficient; ε is the maximum allowable bus voltage deviation of the bipolar system. The droop control coefficient Rdroop and the voltage reference value are determined by the following relationship:

[0074]

[0075]

[0076] Among them, vN is the rated voltage of the line, and imax is the maximum value of the neutral current. The voltage and current dual-loop control is used to balance the positive and negative voltages. The input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship. After adjustment by the PI controller, the current reference value iref is obtained, which together with the leakage inductance current imid constitutes the input signal of the current inner loop. After PI adjustment, the phase shift angle φ2 is output to control the power flow between the positive and negative electrodes on the high-voltage side.

[0077] Preferably, the average value of the leakage inductance current on the positive and negative electrode interconnected transformer Te is:

[0078]

[0079] Where Uin1 is the positive input voltage on the high-voltage side, ω=2πf, f is the switching frequency, and LT is the equivalent leakage inductance on the interconnection transformer. The positive and negative balanced power transmitted by the interconnection transformer is obtained from formula (3):

[0080]

[0081] Where m is the turns ratio of the transformer in the four-port converter.

[0082] Preferably, the control target of the output voltage Uo1 of the positive bridge is:

[0083] U o1 =U DC (5)

[0084] The input signal of its voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low-voltage side voltage sampling value vPl. After adjustment by the PI controller, the current reference value iref1 is obtained, which together with the leakage inductance current iLr1 constitutes the input signal of the current inner loop. After PI adjustment, the control amount phase shift angle φ1 for the positive bridge is output.

[0085] Preferably, the average output current of the positive bridge is expressed by the following formula:

[0086]

[0087] Among them, Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge.

[0088] Preferably, the power transmitted from the high-voltage side active bridge Hh1 of the positive bridge to the low-voltage side Hl1 is obtained from formula (6):

[0089]

[0090] The control target of the output voltage Uo2 of the negative bridge is:

[0091] U o2 =U DC(8)

[0092] The input signal of its voltage outer loop is the positive voltage reference value v*ref2 and the negative bus low-voltage side voltage sampling value vNl. After adjustment by the PI controller, the current reference value iref2 is obtained, which together with the leakage inductance current iLr2 constitutes the input signal of the current inner loop. After PI adjustment, the control quantity phase shift angle φ3 for the negative bridge is output.

[0093] Preferably, the average output current of the negative bridge is expressed by the following formula:

[0094]

[0095] Among them, Uin2 is the positive input voltage on the high-voltage side, and Lr2 is the equivalent leakage inductance of the transformer contained in the negative bridge.

[0096] Preferably, the power transmitted from the high-voltage side active bridge Hh2 of the negative bridge to the low-voltage side Hl2 is obtained from formula (9):

[0097]

[0098] Embodiment 2:

[0099] The technical solution applied for by the present invention is a (Kan) type mixed polarity DC distribution system topology is proposed, and the corresponding control strategy is proposed for this topology.

[0100] Topology: The present invention proposes (Kan) type mixed polarity DC distribution system such as Figure 1 As shown, this mixed polarity DC system can be used in the scenario of interconnecting two bipolar lines of different voltage levels. As shown in the structure in the figure, the rated voltage level of the line is uniformly represented by the rated voltage amplitude UDC of the unipolar line on the low-voltage side, and the unipolar rated voltage of the microgrid on the high-voltage side is 2UDC. The mixed polarity in the present invention is reflected in that it is divided into three parts, one is the bipolar bus on the high-voltage side, the second is the bipolar bus on the low-voltage side, and the third is the converter part that interconnects the high and low voltage distribution networks. This part is as shown in FIG. Figure 1 As shown in the dashed box. The converter part consists of a four-port active bridge (QAB) converter and a power direct transmission line. This part is the core of the mixed polarity distribution system, and its power direct transmission line is equivalent to a single polarity transmission network. Figure 2As shown, the positive bridge on the positive busbar has an active bridge on the high voltage side composed of four MOSFET switch tubes S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, and the low voltage side is also composed of switch tubes S21, S22, S23, S24 and diodes D21, D22, D23, D24. The high voltage side active bridge is connected to the low voltage side active bridge through a high frequency transformer with a turns ratio n of 1:1, and the equivalent leakage inductance of the transformer is Lr1. The negative bridge on the negative busbar has the same structure as the positive bridge. The AC sides of the positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnected transformer Te is LT.

[0101] Basic working principle: The proposed hybrid polarity DC power distribution system consists of three parts: high voltage side, low voltage side, and converter / power direct transmission line. The converter part is responsible for power transmission and voltage conversion on the high and low voltage sides of the system. The positive and negative poles of this system have the same structure. Capacitors C1, C2, C3, C4, C5, and C6 are connected at the interface between the high and low voltage sides and the converter part. The high voltage side active bridge of the positive bridge is Hh1, and the low voltage side is Hl 1. The high voltage side active bridge of the negative bridge is Hh2, and the low voltage side is Hl2. The rated voltage of the high voltage side bipolar bus is ±2UDC, providing voltage levels of ±2UDC and 4UDC. The rated voltage of the low voltage side bipolar bus is ±UDC, providing voltage levels of ±UDC and 2UDC, which can meet the needs of flexible access to various types of DC power equipment with different voltage levels. This system uses the ±2UDC DC busbar on the high-voltage side as the hub for energy collection and distribution, and uses the high-voltage side as the source side. Through the four-port converter and the power direct transmission line of the converter part, the electric energy is sent from the high-voltage side to the low-voltage side in the form of partial power transmission, and the high-voltage DC and low-voltage DC realize partial power transmission. Figure 3 As shown in the figure, nodes ①, ②, and ③ represent the parts directly connected to the high and low voltage side lines, which are direct power transmission lines. The remaining power is transmitted by the positive bridge and the negative bridge, and the amplitude of the output voltage Uo of the four-port converter is controlled to be UDC, which realizes the function of stepping down and efficiently transmitting electric energy. The control structure and electrical connection of the mixed polarity distribution system are shown in the figure. Figure 3 As shown in Figure 1, the unbalanced power of the positive and negative electrodes on the high-voltage side is controlled by the phase shift angle φ2, and is achieved by using a droop control with higher reliability. The control target is as shown in formula (1):

[0102]

[0103] Among them, vph is the positive bus voltage on the high-voltage side, vNh is the positive bus voltage on the low-voltage side, imid is the current of the system line Zh, and Rdroop is the droop control coefficient. Assuming that ε is the maximum allowable bus voltage deviation of the bipolar system, the droop control coefficient Rdroop and the voltage reference value are determined by the following relationship:

[0104]

[0105]

[0106] Among them, vN is the rated voltage of the line, and imax is the maximum value of the neutral current. The voltage and current dual-loop control is used to balance the positive and negative voltages. The input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship. After adjustment by the PI controller, the current reference value iref is obtained, which together with the leakage inductance current imid constitutes the input signal of the current inner loop. After PI adjustment, the phase shift angle φ2 is output to control the power flow between the positive and negative electrodes on the high-voltage side.

[0107] The average leakage inductance current on the positive and negative interconnected transformer Te is:

[0108]

[0109] Among them, Uin1 is the positive input voltage on the high-voltage side, ω=2πf, f is the switching frequency, and LT is the equivalent leakage inductance on the interconnection transformer. From formula (3), we can get the positive and negative balanced power transmitted by the interconnection transformer as:

[0110]

[0111] Where m is the turns ratio of the transformer in the four-port converter.

[0112] The control target of the output voltage Uo1 of the positive bridge is:

[0113] U o1 =U DC (5)

[0114] The input signal of its voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low-voltage side voltage sampling value vPl. After adjustment by the PI controller, the current reference value iref1 is obtained, which together with the leakage inductance current iLr1 constitutes the input signal of the current inner loop. After PI adjustment, the control amount phase shift angle φ1 for the positive bridge is output.

[0115] The average output current of the positive bridge can be expressed as follows:

[0116]

[0117] Where Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge. From formula (6), the power transmitted from the high-voltage side active bridge Hh1 of the positive bridge to the low-voltage side Hl1 can be obtained as:

[0118]

[0119] The control target of the output voltage Uo2 of the negative bridge is:

[0120] U o2 =U DC (8)

[0121] The input signal of its voltage outer loop is the positive voltage reference value v*ref2 and the negative bus low-voltage side voltage sampling value vNl. After adjustment by the PI controller, the current reference value iref2 is obtained, which together with the leakage inductance current iLr2 constitutes the input signal of the current inner loop. After PI adjustment, the control quantity phase shift angle φ3 for the negative bridge is output.

[0122] The average output current of the negative bridge can be expressed as follows:

[0123]

[0124] Among them, Uin2 is the positive input voltage on the high-voltage side, and Lr2 is the equivalent leakage inductance of the transformer contained in the negative bridge. From formula (9), it can be obtained that the power transmitted from the high-voltage side active bridge Hh2 of the negative bridge to the low-voltage side Hl2 is:

[0125]

[0126] according to Figure 3 Plecs simulation model was built. The positive input voltage on the high-voltage side is Vin_high_P = 220V, the negative input voltage is Vin_high_N = 220V, the unbalanced load resistance is Rp_high = 100Ω, Rn_high = 200Ω, Rp_low = 100Ω, Rn_low = 100Ω. The switching frequency is fs = 20kHz, the transformer ratio is n = 1, the capacitors are C1 = 0.5mF, C2 = 0.5mF, C3 = 0.5mF, C4 = 0.5mF, C5 = 0.25mF, C6 = 0.25mF, the leakage inductance is Lr1 = 0.4uF, the leakage inductance is Lr2 = 0.4uF, and the leakage inductance is LT = 2uF.

[0127] The microgrid system operates normally, and the power balancing function between the positive bridge and the negative bridge is not started. The duty cycle of each converter valve of the four-port converter is set to d = 0.5. Since Rp_high = 100Ω, Rn_high = 200Ω, the power imbalance between the positive and negative buses on the high-voltage side already exists. At t = 0s to 0.25s, the voltage difference between the positive and negative buses on the high-voltage side is as follows: Figure 4As shown, at t=0.35s, the imbalance between Rp_high and Rn_high is increased, that is, Rp_high=50Ω, Rn_high=200Ω. At this time, the voltage difference between the positive and negative bus bars increases from the original -10V to -13V.

[0128] Scenario 1: Enable the power balance function between the positive bridge and the negative bridge. Figure 5 (a) and (b) show the voltages of the positive and negative busbars on the high-voltage side, respectively. At t = 0.35s, the imbalance of the positive and negative load resistances is increased, that is, the load resistance between the positive electrodes on the high-voltage side is changed. It can be seen from the figure that the change in the positive load has a smaller impact on the negative voltage. Figure 5 As shown in (c), after the power balancing function between the positive bridge and the negative bridge is activated, the power imbalance between the positive and negative electrodes is -2V from t = 0s to 0.35s, and increases to -2.5V after the load resistance is changed at t = 0.35s. Compared with the case where the power balancing function between the positive bridge and the negative bridge is not activated, the voltage difference is significantly reduced, which proves the effectiveness of the proposed power balancing control between the positive and negative electrodes of the bipolar DC system. Figure 6 As shown in (a) and (b), the power imbalance on the high-voltage side has a relatively small impact on the voltage fluctuation of the bipolar bus on the low-voltage side. Figure 6 (c) It can be obtained that the change of load resistance on the high-voltage side has no effect on the power imbalance between the positive and negative busbars on the low-voltage side, and the difference between the positive and negative voltages can be ignored.

[0129] Scenario 2: When the power balancing function between the positive bridge and the negative bridge is activated, the load on the high voltage side remains unchanged, but the load impedance on the low voltage side changes. Fig.10 (a) and (b) show the voltages of the positive and negative busbars on the low-voltage side, respectively. At t = 0.35s, the imbalance of the positive and negative load resistances is increased, that is, the load resistance between the positive electrodes on the low-voltage side is changed, Rp_low = 100Ω, Rn_low = 50Ω. It can be seen from the figure that the change in the positive load has a small impact on the negative voltage. Fig.10 As shown in (c), the positive and negative power imbalance fluctuates around 0 from t = 0s to 0.35s. After changing the load resistance at t = 0.35s, the voltage balance can still be maintained well, as shown in Fig.11 As shown in (a) and (b), the power imbalance on the low-voltage side has a relatively small impact on the voltage fluctuation of the bipolar bus on the high-voltage side. Fig.11 (c) It can be obtained that the change of the load resistance on the low-voltage side has almost no effect on the power imbalance between the positive and negative busbars on the high-voltage side. It is -1.9 V from t = 0s to 0.35s, and increases to -2.1 V after changing the load resistance on the low-voltage side at t = 0.35s.

[0130] Those skilled in the art may understand this embodiment as a more specific description of Embodiment 1.

[0131] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A A mixed polarity DC power distribution system, characterized in that: include: High voltage side bipolar busbar, low voltage side bipolar busbar, converter part connecting high and low voltage distribution networks to each other, high and low voltage bipolar DC common neutral line; The converter part is composed of a four-port active bridge QAB converter and a power direct transmission line; The positive high-voltage side active bridge of the converter part is composed of four MOSFET switch tubes S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, and the low-voltage side is composed of switch tubes S21, S22, S23, S24 and diodes D21, D22, D23, D24. The high-voltage side active bridge is connected to the low-voltage side active bridge through a high-frequency transformer with a turns ratio n of 1:1, and the equivalent leakage inductance of the transformer is Lr1. The negative bridge on the negative bus has the same structure as the positive bridge; the AC sides of the positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnected transformer Te is LT.

2. according to claim 1 A mixed polarity DC power distribution system, characterized in that: The converter part performs power transmission and voltage conversion on the high and low voltage sides; capacitors C1, C2, C3, C4, C5, and C6 are connected at the interfaces between the high voltage side, the low voltage side, and the converter part; the active bridge on the high voltage side of the positive bridge is Hh1, and the low voltage side is Hl1; the active bridge on the high voltage side of the negative bridge is Hh2, and the low voltage side is Hl2; the rated voltage of the high voltage side bipolar bus is ±2UDC, and voltage levels of ±2UDC and 4UDC are provided; the rated voltage of the low voltage side bipolar bus is ±UDC, and voltage levels of ±UDC and 2UDC are provided.

3. According to claim 2 A mixed polarity DC power distribution system, characterized in that: The system adopts the ±2UDC DC bus on the high-voltage side as the hub for energy collection and distribution, takes the high-voltage side as the source side, and transmits electric energy from the high-voltage side to the low-voltage side in the form of partial power transmission through the four-port converter and the power direct transmission line of the converter part, so that partial power transmission is achieved between high-voltage DC and low-voltage DC.

4. according to claim 3 A mixed polarity DC power distribution system, characterized in that: The unbalanced power between the positive and negative electrodes on the high-voltage side is controlled by the phase shift angle φ2 and is achieved by droop control. The control target is as follows: Among them, vph is the positive bus voltage on the high-voltage side, vNh is the positive bus voltage on the low-voltage side, imid is the current of the system intermediate line Zh, and Rdroop is the droop control coefficient; ε is the maximum allowable bus voltage deviation of the bipolar system. The droop control coefficient Rdroop and the voltage reference value are determined by the following relationship: Among them, vN is the rated voltage of the line, and imax is the maximum value of the neutral current. The voltage and current dual-loop control is used to balance the positive and negative voltages. The input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship. After adjustment by the PI controller, the current reference value iref is obtained, which together with the leakage inductance current imid constitutes the input signal of the current inner loop. After PI adjustment, the phase shift angle φ2 is output to control the power flow between the positive and negative electrodes on the high-voltage side.

5. According to claim 4 A mixed polarity DC power distribution system, characterized in that: The average leakage inductance current on the positive and negative interconnected transformer Te is: Where Uin1 is the positive input voltage on the high-voltage side, ω=2πf, f is the switching frequency, and LT is the equivalent leakage inductance on the interconnection transformer. The positive and negative balanced power transmitted by the interconnection transformer is obtained from formula (3): Where m is the turns ratio of the transformer in the four-port converter.

6. According to claim 5 A mixed polarity DC power distribution system, characterized in that: The control target of the output voltage Uo1 of the positive bridge is: IN o1 =U DC (5) The input signal of its voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low-voltage side voltage sampling value vPl. After adjustment by the PI controller, the current reference value iref1 is obtained, which together with the leakage inductance current iLr1 constitutes the input signal of the current inner loop. After PI adjustment, the control amount phase shift angle φ1 for the positive bridge is output.

7. According to claim 6 A mixed polarity DC power distribution system, characterized in that: The average output current of the positive bridge is expressed as follows: Among them, Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge.

8. According to claim 7 A mixed polarity DC power distribution system, characterized in that: From formula (6), the power transmitted from the high-voltage side active bridge Hh1 of the positive bridge to the low-voltage side Hl1 is: The control target of the output voltage Uo2 of the negative bridge is: IN o2 =U DC (8) The input signal of its voltage outer loop is the positive voltage reference value v*ref2 and the negative bus low-voltage side voltage sampling value vNl. After adjustment by the PI controller, the current reference value iref2 is obtained, which together with the leakage inductance current iLr2 constitutes the input signal of the current inner loop. After PI adjustment, the control quantity phase shift angle φ3 for the negative bridge is output.

9. The method according to claim 8 A mixed polarity DC power distribution system, characterized in that: The average output current of the negative bridge is expressed as follows: Among them, Uin2 is the positive input voltage on the high-voltage side, and Lr2 is the equivalent leakage inductance of the transformer contained in the negative bridge.

10. The method according to claim 9 A mixed polarity DC power distribution system, characterized in that: From formula (9), the power transmitted from the high-voltage side active bridge Hh2 of the negative bridge to the low-voltage side Hl2 is:

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