Cane type hybrid polarity dc distribution system

Abstract

The application provides a K-type hybrid polarity direct current power distribution system, comprising: a high-voltage side bipolar bus, a low-voltage side bipolar bus, a converter part for connecting the high-voltage and low-voltage power distribution networks, and a high-voltage and low-voltage bipolar direct current shared neutral line; the converter part is composed of a four-port active bridge (QAB) converter and a power direct transmission line; a positive 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, and a low-voltage side active bridge 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 a turns ratio n of 1:1, the equivalent leakage inductance of the transformer is Lr1, and the negative pole has the same structure as the positive pole. The application has the ability to interconnect direct current systems of different voltage levels and transmit part of the power of high-voltage and low-voltage direct current systems, and has the load access selection of five voltage levels of ±2VDC, 4VDC and ±VDC at the same time, and the operation mode is flexible.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of direct current power distribution system architecture, in particular, to a Cane type hybrid polarity direct current power distribution system, and more particularly to a Cane type hybrid polarity direct current power distribution system architecture. BACKGROUND

[0002] With the distributed access of renewable energy, frequent faults of power transmission and distribution network and large-scale fluctuations of power consumption side load, the traditional alternating current power grid is difficult to meet the demand of coping with the rapid change of power grid operation state in terms of power flow, load distribution, voltage regulation and various power quality.

[0003] The emergence of direct current system (DC Microgrid) has revolutionized the concept of power transmission and distribution, especially in the context of distributed power generation and renewable energy. DC microgrid brings a new solution to the construction of power distribution network. Compared with alternating current power distribution, the most important advantages of direct current power distribution include higher reliability and efficiency, simpler control, and natural interface with renewable energy, electronic load and energy storage system. With the rapid emergence of these components in modern power systems, the importance of DC microgrid 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 due to their high power transmission and distribution efficiency, good power reliability and easy control method.

[0004] According to the different number of voltage levels and busbars that the microgrid can provide, the DC microgrid is divided into the following three operation architectures: single polarity structure, true bipolar structure and pseudo bipolar structure. The single polarity structure has the disadvantages of providing only one voltage level, less flexible load access, lower power supply reliability, and the whole line being affected by the failure of a certain place in the microgrid, and large line loss. The advantage is that only one pole line network needs to be built, and the construction cost is lower.

[0005] Compared with the single pole metal loop operation mode, the pseudo bipolar access mode has stronger power supply flexibility and supports positive and negative pole access.

[0006] The true bipolar structure has the advantages of providing three voltage levels, ±UDC and 2UDC, facilitating flexible access of loads, and the other pole bus can continue to operate when a certain pole fails, which is more reliable than the single polarity architecture. However, the disadvantage is that two pole transmission lines need to be built, which has high initial construction cost and relatively complex control strategy.

[0007] The bipolar design is one of the main DC system configurations due to its specific wiring, although it has many desirable characteristics, it requires a large number of DC-DC converters to connect the "sources" and "loads" to the microgrid, at this time, it is easy to appear the uneven distribution of load and power between the two poles, and the voltage and current imbalance, which may reduce the operation performance of the DC system. Therefore, specific power electronic-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

[0009] In view of the defects in the prior art, the purpose of the present application is to provide a type hybrid polarity DC power distribution system.

[0010] According to the present application, a type hybrid polarity DC power distribution system is provided, comprising: a high-voltage side bipolar bus, a low-voltage side bipolar bus, a converter part connecting the high and low voltage power distribution networks with each other, and a high and low voltage bipolar 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 switches S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, and the low-voltage side is composed of switches 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 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 positive bridge and the negative bridge are interconnected through a transformer Te between the AC sides, and the equivalent leakage inductance of the interconnection 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, C6 are connected at the interface of the high and low voltage sides and the converter part, the high voltage side active bridge of the positive bridge is Hh1, the low voltage side is Hl1, the high voltage side active bridge of the negative bridge is Hh2, the low voltage side is Hl2, the rated voltage of the high voltage side bipolar bus is ±2UDC, and the voltage levels of ±2UDC and 4UDC are provided, the rated voltage of the low voltage side bipolar bus is ±UDC, and the voltage levels of ±UDC and 2UDC are provided.

[0014] Preferably, the system adopts a hub that collects and distributes energy with a high-voltage side ±2UDC DC bus as the energy sink, takes the high-voltage side as the source side, and transmits power from the high-voltage side to the low-voltage side in the form of partial power transmission through the four-port converter of the converter part and the power direct transmission line, so that the high-voltage DC and the low-voltage DC realize partial power transmission.

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

[0016]

[0017] Wherein, vph is the high-voltage side positive bus voltage, vNh is the low-voltage side positive bus voltage, imid is the current of the system neutral line Zh, Rdroop is the droop control coefficient; ε is the maximum bus voltage deviation allowed value of the bipolar system, and the droop control coefficient Rdroop and the voltage reference value have the following relationship formula:

[0018]

[0019]

[0020] Wherein, vN is the line rated voltage, and imax is the maximum value of the neutral current; The balanced positive and negative pole voltages are controlled by voltage-current double-loop control, the input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship, the current reference value iref is obtained after the PI controller is adjusted, and the input signal of the current inner loop is composed of the leakage current imid, and the output phase shift angle φ2 is obtained after PI adjustment, thereby controlling the power flow between the high-voltage side positive and negative poles.

[0021] Preferably, the average value of the leakage current on the positive and negative pole interconnection transformer Te is:

[0022]

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

[0024]

[0025] Wherein, 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 pole bridge is:

[0027] U o1 =U DC (5)

[0028] The input signal of the voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low voltage sampling value vPl, and the current reference value iref1 is obtained after being adjusted by a PI controller, and the current inner loop input signal is composed of the current inner loop input signal and the leakage current iLr1, and the phase shift angle φ1 for the positive bridge control output is obtained after PI adjustment.

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

[0030]

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

[0032] Preferably, the power delivered from the high-voltage side active bridge Hh1 to the low-voltage side Hl1 of the positive bridge is obtained by 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 the voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low voltage sampling value vPl, and the current reference value iref1 is obtained after being adjusted by a PI controller, and the current inner loop input signal is composed of the current inner loop input signal and the leakage current iLr1, and the phase shift angle φ1 for the positive bridge control output is obtained after PI adjustment.

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

[0038]

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

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

[0041]

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

[0043] 1. The present application has the ability to interconnect DC systems of different voltage levels, and at the same time has the selection of ±2VDC, 4VDC, ±VDC, five voltage levels of load access, and flexible operation mode;

[0044] 2. The neutral line is shared between different voltage levels of the direct current system, partial power transmission is realized based on the proposed QAB circuit, and the total power loss and converter cost of the system are reduced;

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

[0046] 4. The four-port converter is used to undertake the role of a bridge for partial power transmission, and compared with a bipolar direct current network interconnected by a DC-DC converter, the transmission efficiency is significantly improved;

[0047] 5. The proposed When the mixed polarity direct current power distribution system is applied to power transmission and distribution, the four-port converter of the converter part and the power direct transmission line are used to transmit power from the high-voltage side to the low-voltage side of each power equipment in the form of partial power transmission, and the positive and negative power transmission channels adopt the architecture of partial power transmission, which reduces the loss and improves the efficiency while connecting two power grids with different voltage levels;

[0048] 6. When the positive and negative lines of the bipolar microgrid are overloaded, causing the bus voltage to drop and deviate from the allowed voltage fluctuation range, the traditional method of introducing a voltage equalizer is not needed, but the phase shift control can be used to change the phase shift angle φ, and the power is transmitted from the line with lighter load to the line with heavier load through the interconnection transformer between the positive bridge and the negative bridge, thereby solving the power shortage of the overloaded line and balancing the positive and negative voltages; The cost of installing additional equipment is saved, and the economy is improved. BRIEF DESCRIPTION OF DRAWINGS

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

[0050] Figure 1 For Architecture diagram of the Kan-type direct current power distribution system;

[0051] Figure 2 For Interconnection four-port active bridge diagram in the Kan-type power distribution system;

[0052] Figure 3 For Control architecture diagram of the Kan-type direct current power distribution system;

[0053] Figure 4 High voltage side positive and negative bus voltage difference chart for power balance control not enabled;

[0054] Figure 5 High voltage side DC power distribution system power regulation chart for scenario 1;

[0055] Figure 6 Chart for high voltage side load change affecting low voltage side power regulation;

[0056] Figure 7 Key waveform chart for positive pole bridge in scenario 1;

[0057] Figure 8 Key waveform chart for negative pole bridge in scenario 1;

[0058] Figure 9 Key waveform chart for interconnection transformer in scenario 1;

[0059] Figure 10 Low voltage side DC power distribution system power regulation chart for scenario 1;

[0060] Figure 11 Chart for low voltage side load change affecting high voltage side power regulation;

[0061] Figure 12 Key waveform chart for positive pole bridge in scenario 2;

[0062] Figure 13 Key waveform chart for negative pole bridge in scenario 2;

[0063] Figure 14 Key waveform chart for interconnection transformer in scenario 2. DETAILED DESCRIPTION

[0064] The application will be described in greater detail with reference to specific embodiments. The following examples are provided to further assist those skilled in the art in understanding the application, but are not intended to limit the application in any way. It should be noted that, for those skilled in the art, several changes and improvements can be made without departing from the concept of the application. These all fall within the scope of the present application.

[0065] Example 1

[0066] According to the application, a A (Kan) type mixed polarity DC power distribution system comprises: a high voltage side bipolar bus, a low voltage side bipolar bus, a converter part connecting the high and low voltage power distribution networks, and a 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 feed line, and the power direct feed line of the converter part is equivalent to a single-polarity power transmission network.

[0068] The positive high-voltage side active bridge of the converter part is composed of four MOSFET switching tubes S11, S12, S13 and S14 and four anti-parallel diodes D11, D12, D13 and D14, and the low-voltage side is composed of switching tubes S21, S22, S23, S24 and diodes D21, D22, D23 and 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 of 1:1, and the equivalent leakage inductance of the transformer is Lr1. The positive bridge and the negative bridge on the negative bus have the same structure. The positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnection 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 interface of the high-voltage side, the low-voltage side and the converter part, the high-voltage side active bridge of the positive bridge is Hh1, the low-voltage side is Hl1, the high-voltage side active bridge of the negative bridge is Hh2, the low-voltage side is Hl2, the rated voltage of the high-voltage side bipolar bus is ±2UDC, and the voltage levels of ±2UDC and 4UDC are provided, and the rated voltage of the low-voltage side bipolar bus is ±UDC, and the voltage levels of ±UDC and 2UDC are provided.

[0070] Preferably, the system adopts a hub that collects and distributes energy with the high-voltage side ±2UDC DC bus as the energy hub, takes the high-voltage side as the source side, and transmits power 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 feed line of the converter part, so that the high-voltage DC and the low-voltage DC realize partial power transmission.

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

[0072]

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

[0074]

[0075]

[0076] Wherein, vN is the line rated voltage, imax is the maximum value of the neutral current; the balanced positive and negative electrode voltage adopts voltage and current double loop control, the input signal of the voltage outer loop is the voltage reference value vref obtained from the droop relationship, the current reference value iref is obtained after PI controller adjustment, and the input signal of the current inner loop is composed of the leakage current imid, and the PI adjustment is performed to output the phase shift angle φ2, and the power flow between the positive and negative electrodes on the high voltage side is controlled.

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

[0078]

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

[0080]

[0081] Wherein, 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 electrode bridge is:

[0083] U o1 =U DC (5)

[0084] The input signal of the voltage outer loop is the positive electrode voltage reference value v*ref1 and the positive bus low voltage side voltage sampling value vPl, the current reference value iref1 is obtained after PI controller adjustment, and the input signal of the current inner loop is composed of the leakage current iLr1, and the control amount phase shift angle φ1 for the positive electrode bridge is output after PI adjustment.

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

[0086]

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

[0088] Preferably, the power transmitted from the high voltage side active bridge Hh1 of the positive electrode 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 electrode bridge is:

[0091] U o2 =U DC(8)

[0092] The input signal of the voltage outer loop is the positive voltage reference value v*ref2 and the negative bus low voltage sampling value vNl, and the current reference value iref2 is obtained after being adjusted by the PI controller, and the input signal of the current inner loop is composed of the leakage current iLr2, and the control amount phase shift angle φ3 for the negative bridge is output after PI adjustment.

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

[0094]

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

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

[0097]

[0098] Example 2

[0099] The technical scheme applied in the present application is a kind of (Can) type mixed polarity DC power distribution system topology, and the corresponding control strategy is proposed for the topology.

[0100] Topology structure: the (Can) type mixed polarity DC power distribution system proposed in the present application is as shown in the figure, and the mixed polarity DC system can be used in the scene of interconnecting two different voltage grade bipolar lines. As shown in the structure in the figure, the rated voltage grade of the line is represented by the low voltage side unipolar line rated voltage amplitude UDC, and the unipolar rated voltage of the high voltage side microgrid is 2UDC. The mixed polarity in the present application is reflected in that it is divided into three parts, one is the high voltage side bipolar bus, the second is the low voltage side bipolar bus, and the third is the converter part connecting the high and low voltage power distribution networks, which is as shown in the dashed box in Figure 1 The converter part is composed of four-port active bridge (QAB) converter and power direct transmission line, and this part is the core of the mixed polarity power distribution system, and the power direct transmission line can be equivalent to unipolar power transmission network. As Figure 1 Figure 2 ​As shown, the positive bridge on the positive bus, the high-voltage side active bridge is composed of four MOSFET switches S11, S12, S13, S14 and four anti-parallel diodes D11, D12, D13, D14, the low-voltage side is also composed of switches S21, S22, S23, S24 and diodes D21, D22, D23, D24, the high-voltage side active bridge and the low-voltage side active bridge are connected through a high-frequency transformer with a turns ratio of 1:1, 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 positive bridge and the negative bridge are interconnected through a transformer Te, and the equivalent leakage inductance of the interconnection transformer Te is LT.

[0101] Basic working principle: The proposed hybrid polarity DC power distribution system is composed of high-voltage side, low-voltage side, converter / power direct line three parts, the converter part plays the role of system high, low voltage side power transmission and voltage transformation. The positive and negative poles of this system have the same structure, and capacitors C1, C2, C3, C4, C5, C6 are connected at the interface of the high-voltage side and the low-voltage side with the converter part. The high-voltage side active bridge of the positive bridge is Hh1, 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, 4UDC. The rated voltage of the low-voltage side bipolar bus is ±UDC, providing voltage levels of ±UDC, 2UDC, which can meet the demand of flexible access of various types of DC power equipment with different voltage levels. This system uses the high-voltage side ±2UDC DC bus as the hub of energy collection and distribution, takes the high-voltage side as the source side, and transmits power from the high-voltage side to the low-voltage side through the four-port converter and the power direct line in the form of partial power transmission, realizing partial power transmission between high-voltage DC and low-voltage DC. Figure 3 As shown, nodes ①, ②, ③ represent the part directly connected by high-voltage and low-voltage lines, which is the power direct line, the rest of the 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 efficient transmission of voltage reduction. The control structure and electrical connection of the hybrid polarity power distribution system are shown in Figure 3 As shown, the unbalanced power of the high-voltage side positive and negative poles is controlled by the phase shift angle φ2, and the droop control with high reliability is used to realize it, and the control target is as formula (1):

[0102]

[0103] Wherein, vph is the high voltage side positive bus voltage, vNh is the low voltage side positive bus voltage, imid is the current of the system neutral line Zh, Rdroop is the droop control coefficient. Assuming that ε is the maximum bus voltage deviation allowed value of the bipolar system, then the droop control coefficient Rdroop and the voltage reference value have the following relationship:

[0104]

[0105]

[0106] Wherein, vN is the line rated voltage, imax is the maximum value of the neutral current. The balanced positive and negative voltage adopts voltage current double loop control, the input signal of the voltage outer loop is the voltage reference value vref obtained by the droop relationship, the current reference value iref is obtained after PI controller adjustment, and the input signal of the current inner loop is composed of the leakage current imid, and the output phase shift angle φ2 is obtained after PI adjustment, the power flow between the high voltage side positive and negative is controlled.

[0107] The average value of the leakage current on the positive and negative interconnection transformer Te is:

[0108]

[0109] Wherein, Uin1 is the high voltage side positive input voltage, ω = 2πf, f is the switching frequency, and LT is the equivalent leakage inductance on the interconnection transformer. The balanced positive and negative power transmitted by the interconnection transformer can be obtained from equation (3):

[0110]

[0111] Wherein, 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 the voltage outer loop is the positive voltage reference value v*ref1 and the positive bus low voltage side voltage sampling value vPl, the current reference value iref1 is obtained after PI controller adjustment, and the input signal of the current inner loop is composed of the leakage current iLr1, and the control amount of the positive bridge is output after PI adjustment, the phase shift angle φ1.

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

[0116]

[0117] Wherein, Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge. From equation (6), the power delivered by 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 the voltage outer loop is the positive voltage reference value v*ref2 and the low-voltage side voltage sampling value vNl of the negative bus, and the current reference value iref2 is obtained after adjustment by the PI controller, which is the input signal of the current inner loop together with the leakage current iLr2. After PI adjustment, the control amount phase shift angle φ3 for the negative bridge is output.

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

[0123]

[0124] Wherein, Uin2 is the high-voltage side positive input voltage, and Lr2 is the equivalent leakage inductance of the transformer contained in the negative bridge. From equation (9), the power delivered by the high-voltage side active bridge Hh2 of the negative bridge to the low-voltage side Hl2 can be obtained as:

[0125]

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

[0127] The microgrid system is running normally, and the power balancing function between the positive bridge and the negative bridge is not started. The duty ratio of each converter valve of the four-port converter is d = 0.5. Since Rp_high = 100Ω, Rn_high = 200Ω, the power imbalance between the high-voltage side positive and negative buses already exists, and the voltage difference between the high-voltage side positive and negative buses at t = 0s to 0.25s is as follows: Figure 4As shown, at the moment of t = 0.35s, the imbalance degree of 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 is increased from-10V to-13V.

[0128] Scenario 1: Start the power balancing function between the positive bridge and the negative bridge. As shown in Figure 5 As shown in (a) and (b), the voltages of the high-voltage side positive bus and the high-voltage side negative bus are respectively shown. At the moment of t = 0.35s, the imbalance degree of the positive and negative load resistances is increased, that is, the size of the load resistance between the high-voltage side positive buses is changed. As can be seen from the figure, the influence of the load change of the positive bus on the voltage of the negative bus is relatively small. Figure 5 As shown in (c), after starting the power balancing function between the positive bridge and the negative bridge, the power imbalance degree of the positive and negative buses is-2V at the moment of t = 0s to 0.35s, and is increased to-2.5V after changing the load resistance at t = 0.35s. Compared with the case where the power balancing function between the positive bridge and the negative bridge is not started, the voltage difference is significantly reduced, which proves the effectiveness of the proposed power balancing control between the positive and negative buses of the bipolar direct current system. Figure 6 As shown in (a) and (b), the power imbalance of the high-voltage side has a relatively small influence on the voltage fluctuation of the low-voltage side bipolar bus. Figure 6 As shown in (c), the change of the load resistance of the high-voltage side has no influence on the power imbalance degree between the positive and negative buses of the low-voltage side, and the voltage difference between the positive and negative buses can be ignored.

[0129] Scenario 2: Start the power balancing function between the positive bridge and the negative bridge, and the load impedance of the low-voltage side changes while the load of the high-voltage side remains unchanged. As shown in Figure 10 As shown in (a) and (b), the voltages of the low-voltage side positive bus and the low-voltage side negative bus are respectively shown. At the moment of t = 0.35s, the imbalance degree of the positive and negative load resistances is increased, that is, the size of the load resistance between the low-voltage side positive buses is changed. As can be seen from the figure, the influence of the load change of the positive bus on the voltage of the negative bus is relatively small. Figure 10 As shown in (c), the power imbalance degree of the positive and negative buses is maintained at around 0 at the moment of t = 0s to 0.35s, and can still maintain good voltage balancing degree after changing the load resistance at t = 0.35s. Figure 11 As shown in (a) and (b), the power imbalance of the low-voltage side has a relatively small influence on the voltage fluctuation of the high-voltage side bipolar bus. Figure 11 As shown in (c), the change of the load resistance of the low-voltage side has almost no influence on the power imbalance degree between the positive and negative buses of the high-voltage side. The power imbalance degree is-1.9V at the moment of t = 0s to 0.35s, and is increased to-2.1V after changing the load resistance of the low-voltage side at t = 0.35s.

[0130] Those skilled in the art can understand the present embodiment as a more specific description of embodiment 1.

[0131] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essential content of the present application. The embodiments of the present application and the features in the embodiments can be combined with each other at will without conflict.

Claims

1. A ☵-type hybrid polarity DC power distribution system, characterized in that, include: High-voltage side bipolar busbar, low-voltage side bipolar busbar, converter section connecting high and low voltage distribution networks, and high and low voltage bipolar DC common neutral line; The converter section consists of a four-port active bridge QAB converter and a power transmission line. The positive high-voltage side active bridge of the converter section consists of four MOSFET switches S11, S12, S13, and S14 and four anti-parallel diodes D11, D12, D13, and D14. The low-voltage side consists of switches 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 by a high-frequency transformer with a turns ratio n of 1:

1. 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 by a transformer Te. The equivalent leakage inductance of the interconnecting transformer Te is LT. The converter section performs high- and low-voltage side power transmission and voltage conversion; capacitors C1, C2, C3, C4, C5, and C6 are connected to the interfaces between the high-voltage side, the low-voltage side, and the converter section. The active bridge on the high-voltage side of the positive bridge is Hh1, and the active bridge on the low-voltage side is Hl1. The active bridge on the high-voltage side of the negative bridge is Hh2, and the active bridge on 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. The system uses the ±2UDC DC bus on the high-voltage side as the hub for energy collection and distribution. The high-voltage side is used as the source side. Through the four-port converter of the converter section and the power direct transmission line, electrical energy is sent from the high-voltage side to the low-voltage side in the form of partial power transmission. The high-voltage DC and low-voltage DC achieve partial power transmission.

2. The ☵-type hybrid polarity DC power distribution system according to claim 1, characterized in that, 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 droop control, with the control objective as shown in equation (1): （1） Where vph is the high-voltage side positive bus voltage, vNh is the low-voltage side positive bus voltage, imid is the current in the line Zh of the system, Rdroop is the droop control coefficient; ε is the maximum allowable deviation of the bipolar system bus voltage, and the droop control coefficient Rdroop and the voltage reference value are determined by the following relationship: （2） Where vN is the line rated voltage and imax is the maximum value of the neutral current; the voltage balance between the positive and negative poles adopts a dual-loop control of voltage and current. The input signal of the voltage outer loop is the voltage reference value vref obtained from the droop relationship. After being adjusted by the PI controller, the current reference value iref is obtained. Together with the leakage inductance current imid, they form the input signal of the current inner loop. After being adjusted by the PI controller, the output phase shift angle φ2 is used to control the power flow between the positive and negative poles on the high voltage side.

3. The ☵-type hybrid polarity DC power distribution system according to claim 2, characterized in that, The average leakage inductance current on the positive and negative interconnecting transformer Te is: （3） 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 interconnecting transformer; the balanced power transmitted by the positive and negative poles of the interconnecting transformer is obtained from equation (3): （4） Where m is the turns ratio of the transformer in the four-port converter.

4. The ☵-type hybrid polarity DC power distribution system according to claim 3, characterized in that, The control objective for the output voltage Uo1 of the positive bridge is: （5） The input signals of its outer voltage loop are the positive voltage reference value v*ref1 and the positive bus low-voltage side voltage sampling value vPl. After being adjusted by the PI controller, the current reference value iref1 is obtained. Together with the leakage inductance current iLr1, they form the input signal of the inner current loop. After PI adjustment, the output is the phase shift angle φ1 of the control quantity for the positive bridge.

5. The ☵-type hybrid polarity DC power distribution system according to claim 4, characterized in that, The average output current of the positive bridge is expressed by the following formula: （6） Where Lr1 is the equivalent leakage inductance of the transformer contained in the positive bridge.

6. The ☵-type hybrid polarity DC power distribution system according to claim 5, characterized in that, From equation (6), the power transferred from the high-voltage side active bridge Hh1 of the positive bridge to the low-voltage side Hl1 is: （7） The control objective for the output voltage Uo2 of the negative bridge is: （8） The input signals of its outer voltage loop are the positive voltage reference value v*ref2 and the negative bus low-voltage side voltage sampling value vNl. After being adjusted by the PI controller, the current reference value iref2 is obtained. Together with the leakage inductance current iLr2, they form the input signal of the inner current loop. After PI adjustment, the output is the phase shift angle φ3 of the control quantity for the negative bridge.

7. The ☵-type hybrid polarity DC power distribution system according to claim 6, characterized in that, The average output current of the negative bridge is expressed by the following formula: （9） Wherein, 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.

8. The ☵-type hybrid polarity DC power distribution system according to claim 7, characterized in that, From equation (9), the power transferred from the high-voltage side active bridge Hh2 of the negative pole bridge to the low-voltage side Hl2 is: （10）。

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