Voltage self-balancing dc converter and modulation method of bipolar dc microgrid

Abstract

The application discloses a voltage self-balancing direct-current converter and a modulation method of a bipolar direct-current micro-grid, and belongs to the technical field of power systems. The voltage self-balancing direct-current converter comprises a branch and two double active bridge converters. The grid-connected sides of the two double active bridge converters are connected in series and then connected between the positive and negative bus bars of the micro-grid. The common ends of the two double active bridge converters are connected to the zero-pole bus bar of the micro-grid. The load sides of the two double active bridge converters are connected to direct-current devices. The branch comprises an inductor and a transformer. The two sides of the transformer are respectively connected to two midpoints of the full-bridge circuit of the grid-connected side of the two double active bridge converters. The inductor is connected in series on any connection line between the transformer and the double active bridge converter. The application can balance two-pole voltage without connecting any direct-current device, can save the cost and volume of devices, and can reduce the grid current ripple.

Description

Technical Field

[0001] This invention relates to the field of power system technology, and in particular to a voltage self-balancing DC converter and modulation method for a bipolar DC microgrid. Background Technology

[0002] With the rapid development of new power systems dominated by new energy sources, distributed energy sources such as photovoltaic arrays, wind turbines, electric vehicle charging facilities, and battery energy storage systems are being connected to the grid in large quantities. Since distributed energy sources are typically in direct current (DC) form, compared to alternating current (AC) microgrids, DC microgrids do not require AC-DC converters or reactive power or synchronization control when connecting distributed DC loads. This improves the system's energy conversion efficiency and makes energy supply more flexible and reliable. Therefore, DC microgrids are widely used in current new power systems.

[0003] Based on bus structure, DC microgrids can be divided into: unipolar DC microgrids and bipolar DC microgrids. The structure of a bipolar DC microgrid is shown in the attached figure. Figure 1 As shown, the microgrid system has three power buses: positive, zero, and negative, providing two voltage levels: Vdc and 2Vdc. Depending on the voltage level, the microgrid can also connect to different loads or distributed energy sources, such as photovoltaic arrays and data centers. Compared to monopolar DC microgrids, bipolar DC microgrids have many advantages. Although bipolar DC microgrids have significant advantages in power transmission, their construction costs and technical requirements are relatively high. Due to the uneven distribution of distributed source loads between the two poles, the power injected into or absorbed by the poles differs, leading to voltage imbalance on the two pole buses of the bipolar DC microgrid, affecting the normal operation of the system. Therefore, how to maintain voltage balance between the poles has become a research hotspot for bipolar DC microgrids. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a voltage self-balancing DC converter and modulation method for a bipolar DC microgrid, which can achieve voltage balance between the two poles without connecting any DC devices.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0006] In a first aspect, the present invention provides a voltage self-balancing DC-DC converter for a bipolar DC microgrid, comprising: The branch circuit and two dual active bridge converters are connected in series on their grid-connected sides and then connected between the positive and negative buses of the microgrid. The common terminal of the two dual active bridge converters is connected to the zero-pole bus of the microgrid. The load sides of both dual active bridge converters are connected to DC equipment. Branch circuits include inductors and transformer The transformer The primary winding ends are respectively connected to the two midpoints of the grid-connected full-bridge circuit of the dual active bridge converter, and the transformer The two ends of the secondary winding are respectively connected to the two midpoints of the grid-connected full-bridge circuit of another dual active bridge converter, and the inductor Connected in series with the transformer And any connection line of any of the dual active bridge converters.

[0007] Optionally, the two dual active bridge converters are referred to as the first dual active bridge converter and the second dual active bridge converter, respectively. The grid-connected positive terminal of the first dual active bridge converter is connected to the positive bus of the microgrid, and the grid-connected negative terminal of the second dual active bridge converter is connected to the negative bus of the microgrid. Both the grid-connected negative terminal of the first dual active bridge converter and the grid-connected positive terminal of the second dual active bridge converter are connected to the zero bus of the microgrid.

[0008] Optionally, the first dual active bridge converter includes The branch, the first load-side full-bridge circuit, and the first grid-connected full-bridge circuit, the Branch circuits include inductors and transformer The transformer The two sides of the inductor are connected to the two midpoints of the first load-side full-bridge circuit and the first grid-connected full-bridge circuit, respectively. Connected in series with the transformer And any connection line of the first load-side full-bridge circuit.

[0009] Optionally, the first load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a first load-side bridge arm group. and switching transistor The first load-side bridge arm group is connected in series, and the two first load-side bridge arm groups are connected in parallel.

[0010] The first grid-connected full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a first grid-connected side bridge arm group. and switching transistor The first grid-connected side bridge arm group is connected in series, and the two first grid-connected side bridge arm groups are connected in parallel.

[0011] Optionally, a capacitor is also connected in parallel on the first load-side bridge arm assembly. A capacitor is also connected in parallel on the first grid-connected side bridge arm assembly. .

[0012] Optionally, the second dual active bridge converter includes The branch, the second load-side full-bridge circuit, and the second grid-connected full-bridge circuit, the Branch circuits include inductors and transformer The transformer The two sides of the inductor are connected to the two midpoints of the second load-side full-bridge circuit and the second grid-side full-bridge circuit. Connected in series with the transformer And any connection line of the second load-side full-bridge circuit.

[0013] Optionally, the second load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a second load-side bridge arm group. and switching transistor The two load-side bridge arm groups are connected in parallel to form a second load-side bridge arm group;

[0014] The second grid-connected full-bridge circuit includes switching transistors. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistor is connected in series to form a second grid-connected side bridge arm group. and switching transistor They are connected in series to form a second grid-connected side bridge arm group, and the two second grid-connected side bridge arm groups are connected in parallel.

[0015] Optionally, a capacitor is also connected in parallel on the second load-side bridge arm assembly. A capacitor is also connected in parallel on the second grid-connected side bridge arm assembly. .

[0016] Secondly, the present invention provides a modulation method for a voltage self-balancing DC converter adapted to the above-mentioned bipolar DC microgrid, comprising:

[0017] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0018] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0019] drive signal and drive signal Complementary signals within the same full-bridge circuit all have a 50% duty cycle, including dead time; drive signals and drive signal Collectively referred to as drive signals ; ;

[0020] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0021] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0022] drive signal and drive signal These are complementary signals within the same full-bridge circuit, both with a 50% duty cycle, including dead time; drive signals. and drive signal Collectively referred to as drive signals .

[0023] Optional, drive signal Given an initial PWM signal, the drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is Phase shift angle Phase shift angle and the phase shift angle is Individual control.

[0024] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0025] This invention provides a voltage self-balancing DC-DC converter and modulation method for a bipolar DC microgrid. The voltage self-balancing DC-DC converter consists of two dual active bridge converters and an inductor-transformer series-connected voltage balancer, functionally equivalent to two independent DAB grid-connected converters and an independent voltage balancer. By integrating voltage balancing and energy conversion functions, the voltage across the grid poles remains balanced while the port devices draw or inject energy from the grid, and the grid current ripple is significantly reduced compared to the LC_VB structure. Furthermore, the LT_VB structure enables device reuse, achieving voltage balancing without additional active components, thus saving device costs. In addition, this invention can balance the voltage across any point in the bipolar DC microgrid, overcoming voltage drop phenomena in long-distance microgrids. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the structure of a bipolar DC microgrid provided in the background art of this invention;

[0027] Figure 2 This is a schematic diagram of the voltage self-balancing DC-DC converter provided in an embodiment of the present invention;

[0028] Figure 3 This is a control strategy diagram of the voltage self-balancing DC-DC converter provided in an embodiment of the present invention;

[0029] Figure 4 This is a working mode diagram of the voltage self-balancing DC-DC converter provided in the embodiment of the present invention;

[0030] Figure 5 This is a signal waveform diagram of the voltage self-balancing DC-DC converter provided in the embodiment of the present invention during operation;

[0031] Figure 6 This is a structural diagram of LC_VB provided in an embodiment of the present invention;

[0032] Figure 7 This is a comparison diagram of the working modes of the LT_VB branch and the LC_VB branch provided in the embodiments of the present invention;

[0033] Figure 8 This is a comparison diagram of the grid-connected currents of the LT_VB branch and the LC_VB branch provided in an embodiment of the present invention;

[0034] Figure 9 This is a graph showing the relationship between reduced grid-connected current ripple and transmission power provided by an embodiment of the present invention;

[0035] Figure 10 shows the bus voltage waveforms under different operating conditions during the experiment provided in the embodiment of the present invention;

[0036] Figure 11This is a voltage and current waveform diagram of port 1 and port 2 during the experiment provided in the embodiment of the present invention;

[0037] Figure 12 is a waveform diagram of the grid-connected current of the dual LC branch and the single LC branch during the experiment provided in the embodiment of the present invention;

[0038] Figure 13 This is a voltage and current waveform diagram of the switching transistors S1.1 and Q1.1 when the present invention is turned on at zero voltage. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0040] Example 1:

[0041] like Figure 2 As shown in the figure, the voltage self-balancing DC-DC converter for a bipolar DC microgrid provided in this embodiment of the invention consists of two dual active bridge (DAB) converters and one inductor-transformer series voltage balancer (LT_VB). The dual active bridge converter is as follows... Figure 2 As shown in (a), the inductor-transformer series voltage balancer is as follows: Figure 2 As shown in (b), the two dual active bridge converters (DAB) and the inductor-transformer series voltage balancer (LT_VB) share eight switching transistors using device multiplexing technology, which greatly reduces hardware and control costs.

[0042] The two dual active bridge converters are denoted as the first dual active bridge converter. Second dual active bridge converter First dual active bridge converter The grid-connected positive terminal is connected to the positive bus of the microgrid, and the second dual active bridge converter... The grid-connected negative terminal is connected to the negative bus of the microgrid, the first dual active bridge converter. The grid-connected negative terminal and the second dual active bridge converter The positive terminals on the grid-connected side are all connected to the zero-pole bus of the microgrid. First dual active bridge converter. Second dual active bridge converter The load side of each device is connected to a DC device.

[0043] The first dual active bridge converter includes Branch circuit, first load-side full-bridge circuit and first grid-connected full-bridge circuit, Branch circuits include inductors and transformer ,transformer The two sides of the inductor are connected to the two midpoints of the first load-side full-bridge circuit and the first grid-connected full-bridge circuit, respectively. Connected in series with transformer On any of the connection lines of the full-bridge circuit on the first load side.

[0044] The first load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors Switching transistor and switching transistor The first load-side bridge arm is connected in series, and the switching transistors are... and switching transistor Two first load-side bridge arm groups are connected in series to form a first load-side bridge arm group, and two first load-side bridge arm groups are connected in parallel; the first grid-connected full-bridge circuit includes switching transistors. Switching transistor Switching transistor and switching transistors Switching transistor and switching transistor The first grid-connected bridge arm group is connected in series, and the switch tubes are connected in series. and switching transistor They are connected in series to form a first grid-connected side bridge arm group, and two first grid-connected side bridge arm groups are connected in parallel.

[0045] The second dual active bridge converter includes Branch circuit, second load-side full-bridge circuit and second grid-connected full-bridge circuit, Branch circuits include inductors and transformer ,transformer The two sides of the inductor are connected to the two midpoints of the second load-side full-bridge circuit and the second grid-connected full-bridge circuit. Connected in series with transformer And any connection line of the second load-side full-bridge circuit.

[0046] The second load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors Switching transistor and switching transistor The switching transistors are connected in series as a second load-side bridge arm group. and switching transistor The two second load-side bridge arm groups are connected in series to form a second load-side bridge arm group, and the two second load-side bridge arm groups are connected in parallel; the second grid-connected full-bridge circuit includes switching transistors. Switching transistor Switching transistor and switching transistors Switching transistor and switching transistor Series connection to form a second grid-connected bridge arm group, switch tube and switching transistor They are connected in series to form a second grid-connected side bridge arm group, and the two second grid-connected side bridge arm groups are connected in parallel.

[0047] A capacitor is also connected in parallel on the first load-side bridge arm assembly. A capacitor is also connected in parallel on the first grid-connected side bridge arm group. A capacitor is also connected in parallel on the second load-side bridge arm assembly. A capacitor is also connected in parallel on the second grid-connected side bridge arm. .

[0048] Branch circuits include inductors and transformer ,transformer The primary winding is connected to the two midpoints of the grid-connected full-bridge circuit of a dual active bridge converter. The two ends of the secondary winding are respectively connected to the two midpoints of the grid-connected full-bridge circuit of another dual active bridge converter, and the inductor Connected in series with transformer And any connection line of any dual active bridge converter.

[0049] because The characteristic of energy transmission throughout the entire cycle of the branch greatly reduces the grid current ripple introduced by the voltage balancing function compared to the voltage balancing of the LC branch.

[0050] like Figure 3 As shown, based on the voltage self-balancing DC-DC converter for bipolar DC microgrids provided in the above embodiments, this embodiment also provides a modulation method, including:

[0051] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0052] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0053] drive signal and drive signal Complementary signals within the same full-bridge circuit all have a 50% duty cycle, including dead time; drive signals and drive signal Collectively referred to as drive signals ; ;

[0054] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0055] Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ;

[0056] drive signal and drive signal These are complementary signals within the same full-bridge circuit, both with a 50% duty cycle, including dead time; drive signals. and drive signal Collectively referred to as drive signals .

[0057] drive signal Given an initial PWM signal, the drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is Phase shift angle Phase shift angle and the phase shift angle is Individual control. This is achieved by adjusting the phase shift angle. It can achieve bus voltage balance control by adjusting the phase shift angle. The phase shift angle is Port power control can be achieved.

[0058] The principle of the voltage self-balancing DC-DC converter for the bipolar DC microgrid provided in this embodiment is analyzed, starting with the definition of the circuit parameters. Lk and i Lk It is an inductor L k voltage and current, v LB and i LB It is an inductor L B voltage and current, i' Lk and i' LB For transformer T k and T B Secondary current. portk and i portk These are the voltage and current at port k. When a DC power supply is connected to port k, the current i is specified. portkThe direction is the positive direction. f s is the switching frequency. V total is the positive and negative bus voltages of the system. In the steady state, V dc = V dc+ = V dc- . I + , I0, I - are the currents of the converter connected to the positive, neutral, and negative busbars respectively. T1, T2, T B The turns ratios of the transformers are n1, n2, n B . I L1 , I L2 , I LB are the magnitudes of the inductors L1, L2, L B . Taking the case where port 1 is connected to the DC power supply, port 2 is connected to the DC power supply, and the unbalanced power of the system is transferred from the positive pole to the negative pole as an example, the working state of the proposed invention within one cycle is analyzed. The equivalent working modes of the embodiments of the present invention are as shown in Appendix Figure 4 . The driving signals of the switching tubes, the inductors L k and L B The voltage and current waveforms are as shown in Appendix Figure 5 .

[0059] To simplify the analysis process, the following assumptions are made:

[0060] 1) Operating in a steady state;

[0061] 2) The switching tubes are all ideal devices;

[0062] 3) The turns ratio of the transformer n1 = n2 = n B = n, the port voltages V port1 and V port2 remain stable and satisfy the grid monopole bus voltage V dc : V portk : V dc = n:1;

[0063] 4) Ignoring the voltage fluctuations of the capacitors.

[0064] Sub-state A (t < t0): Before the moment t0, the driving signals g S1,1 , g S2,1 , g Q1,1 and g Q2,1 are at high level. The working state of the circuit is a1 + b1 + c1. In this state, port 1 transfers energy to the grid, the unbalanced power is transferred from the positive pole to the negative pole, port 2 transfers energy to the grid, v L1 = v LB = v L2 = 0, i L1 = I L1 , i LB=-I LB , i L2 =I L2 .

[0065] Sub - state B (t0 <= t < t1): At t = t0, S 1,1 and S 1,4 are turned off. Since the current of inductor L1 cannot change abruptly, the body diodes of S 1,3 and S 1,2 conduct. After the dead - time, S 1,3 and S 1,2 achieve zero - voltage turn - on. The circuit operating state is: a2 + b1 + c1. In this state, the power grid returns reactive power to port 1, the unbalanced power is transferred from the positive pole to the negative pole, port 2 transmits energy to the power grid, v L1 =-2nV dc , i L1 decreases from I L1 to - I L1 .

[0066] Sub - state C (t1 <= t < t2): At t = t1, Q 1,1 and Q 1,4 are turned off. The body diodes of Q 1,3 and Q 1,2 conduct. After the dead - time, Q 1,3 and Q 1,2 achieve zero - voltage turn - on. The circuit operating state is: a3 + b2 + c1. In this state, port 1 transmits energy to the power grid, reactive power flows back from the negative pole to the positive pole, port 2 transmits energy to the power grid, i L1 =-I L1 , v L1 =0, v LB =2V dc , i LB increases from - I LB to I LB . [[ID=,64]]

[0067] Sub - state D (t2 <= t < t3): At t = t2, Q 2,1 and Q 2,4 are turned off. The body diodes of Q 2,3 and Q 2,2 conduct. After the dead - time, Q 2,3 and Q 2,2 achieve zero - voltage turn - on. The circuit operating state is: a3 + b3 + c2. In this state, port 1 transmits energy to the power grid, the unbalanced power is transferred from the positive pole to the negative pole, the power grid returns reactive power to port 2, v LB =0, i LB =I LB , v L2 =-2nVdc , i L2 From I L2 decreases to -I L2 .

[0068] Sub - state E (t3 <= t < t4): At t = t3, S 2,1 and S 2,4 turn off. The body diodes of S 2,3 and S 2,2 conduct. After the dead - time, S 2,3 and S 2,2 achieve zero - voltage turn - on. The circuit operating state is: a3 + b3 + c3. In this state, port 1 transfers energy to the power grid, the unbalanced power transfers from the positive pole to the negative pole, port 2 transfers energy to the power grid, v L1 = v LB = v L2 = 0, i L1 = -I L1 , i LB = I LB , i L2 = -I L2 .

[0069] Sub - state F (t4 <= t < t5): At t = t4, S 1,3 and S 1,2 turn off. The body diodes of S 1,1 and S 1,4 conduct. After the dead - time, S 1,1 and S 1,4 achieve zero - voltage turn - on. The circuit operating state is: a4 + b3 + c3. In this state, the power grid returns reactive power to port 1, the unbalanced power transfers from the positive pole to the negative pole, port 2 transfers energy to the power grid, v L1 = 2nV dc , i L1 increases from -I L1 to I L1 .

[0070] Sub - state G (t5 <= t < t6): At t = t5, Q 1,3 and Q 1,2 turn off. The body diodes of Q<00​​​​​​​​​​​​​​​dc , i LB decreases from I LB to -I LB .

[0071] Sub - state H (t6 <= t < t7): At t = t6, Q 2,3 and Q 2,2 turn off. The body diodes of Q 2,1 and Q 2,4 conduct. After the dead - time, Q 2,1 and Q 2,4 achieve zero - voltage turn - on. The circuit operating state is: a1 + b1 + c4. In this state, port 1 transfers energy to the power grid, the unbalanced power transfers from the positive pole to the negative pole, the power grid returns reactive power to port 2, v LB = 0, i LB = -I LB , v L2 = 2nV dc , i L2 increases from -I L2 to I L2 .

[0072] At t = t7, S 2,3 and S 2,2 turn off. The body diodes of S 2,1 and S 2,4 conduct. After the dead - time, S 2,1 and S 2,4 achieve zero - voltage turn - on. The circuit operating state is: a1 + b1 + c1, which is the same as state A. In summary, within one cycle, the product realizes the energy transfer process between the port and the power grid, balances the voltages at both poles of the power grid, and all the switching tubes can achieve zero - voltage turn - on.

[0073] After the above analysis, the amplitudes of the inductor currents I Lk and I LB satisfy Equation (1 - 1):

[0074] (1 - 1)

[0075] Analyze the power transfer principle of the voltage - self - balancing DC converter for the bipolar DC micro - grid provided in this embodiment:

[0076] In Appendix Figure 5 , sub - states a1, a3, c1, c3 are the power transfer stages of DAB k , and sub - states a2, a4, c2, c4 are the reactive - power return stages of DAB k . The relationship between i portk and i Lk can be described as:

[0077] (1-2)

[0078] According to the state-space averaging method, the DABk port current and power satisfy formula (1-3):

[0079] (1-3)

[0080] Among them, P portk Let P be the transmission power of port k. portk >0, port k is connected to a DC power supply; when P portk <0, port k is connected to a DC load.

[0081] For the unbalanced power transmitted by LT_VB, b1 and b3 are the power transmission stages, and b2 and b4 are the current commutation stages. According to the state-space averaging method, the unbalanced power transmitted by LT_VB satisfies formula (1-4):

[0082] (1-4)

[0083] The grid-connected current ripple of the voltage self-balancing DC-DC converter of the bipolar DC microgrid provided in this embodiment is analyzed:

[0084] Based on the technical principle analysis, the grid-connected current I of the product is... + , I0 and I - This represents the linear superposition of the grid-connected currents of DAB1, DAB2, and LT_VB. Utilizing the interleaved switching characteristic of the full-bridge circuit, LT_VB can achieve continuous output of equalizing current. For bipolar DC microgrids, the LC_VB structure is commonly used to balance the voltages of the two busbars, as shown in the attached diagram. Figure 6 As shown, its C B When the value is large, the capacitor voltage remains almost constant. For LC_VB, the expression for its transmitted unbalanced power is:

[0085] (1-5)

[0086] Among them, L B2 This is the inductance of the LC equalizing branch. With the port unloaded, compare the grid current ripple of the LC_VB and LT_VB voltage balancing branches. Under the condition of the same grid voltage, if the two structures transmit the same unbalanced power, then... Taking the transfer of unbalanced power from the positive to the negative electrode as an example, a comparison of the voltage equalization working states of the two structures is shown in the appendix. Figure 7 As shown, the grid-connected current waveform at this time is compared in the attached figure. Figure 8 As shown.

[0087] (1) LT_VB grid-connected current analysis

[0088] State b1: Power is transferred from the positive terminal to the negative terminal, i + = i - =i LB =-I LB , i0=i + +i - =-2I LB .

[0089] State b2: Reactive power is lost when the negative electrode flows back to the positive electrode, i + =-i - =-i LB , i0=0.

[0090] State b3: Power is transferred from the positive terminal to the negative terminal, i + = i - =-i LB =-I LB i0 = i + +i - =-2I LB .

[0091] State b4: Reactive power is lost when the negative electrode flows back to the positive electrode, i + =-i - =i LB i0=0;

[0092] As the above analysis shows, when LT_VB transmits unbalanced power, the grid-side full-bridge circuit is switched on alternately. Within one cycle, the positive and negative grid-connected currents are in a continuous operating state, and the current output is relatively smooth.

[0093] (2) LC_VB grid-connected current analysis

[0094] State b1: Capacitor C B Discharge towards the negative electrode. + =0, i - =i0=i LB =-2I LB .

[0095] State b2: Reactive power is lost as the flow returns from the negative terminal to the positive terminal. + =-i - =-i LB i0=0;

[0096] State b3: Positive terminal towards capacitor C B Charging. + =-i0=-i LB1 =-2I LB i - =0.

[0097] State b4: Reactive power is lost as current flows back from the negative terminal to the positive terminal. + =i- =i0=0;

[0098] Analysis of the current waveform and state reveals that the positive and negative grid-connected currents generated by the LC_VB during unbalanced power transmission operate intermittently within one cycle, exhibiting large fluctuations. Under the same unbalanced power transmission conditions, the device stress in the LC_VB is higher.

[0099] Under the same unbalanced power transmission conditions, the percentage reduction in grid current fluctuation of LT_VB increases with the transmitted unbalanced power (P). B The per-unit value variation curve is shown in the attached figure. Figure 9 As shown in the figure, it can be seen that the LT_VB structure can reduce grid-connected current fluctuations by at least 68.42%.

[0100] Analysis of the control strategy for the voltage self-balancing DC-DC converter of the bipolar DC microgrid provided in this embodiment:

[0101] The phase-shift control strategy is applicable to the entire power transfer process of the four-port converter proposed in this paper. Because d1, d2, and d... B Since they are independent of each other, the control variables for power control at port 1 and port 2, and the voltage balance control at the two poles, are independent of each other. According to formula (1-3), by adjusting d... k This satisfies the power requirement of port k, thus enabling grid connection of any DC device. Similarly, according to formula (1-4), by adjusting d... B This allows adjustment of the magnitude and direction of the unbalanced power transmitted by LT_VB, meaning the voltage balance problem will be solved. The proposed converter control block diagram is attached. Figure 3 As shown.

[0102] An experiment was conducted on the voltage self-balancing DC-DC converter of the bipolar DC microgrid provided in this embodiment:

[0103] The experimental conditions are as follows: When 0 ≤ t < 0.1 s, the positive and negative loads are equal, and R... load+ =R load- =2.5Ω; in 0.1≤t<0.15s, with the positive electrode unloaded, R load- =2.5Ω; in 0.15≤t<0.2s, negative pole unloaded, R load+ =2.5Ω. Three operating conditions were set up in the experiment: Condition 1: No voltage equalization measures were used. Condition 2: Ports 1 and 2 were unloaded, and voltage balancing was achieved solely by the DLCVB. Condition 3: Port 1 was connected to a DC power supply (P... port1 =100W), port 2 connects to a DC load (P port2 =-100W).

[0104] (1) Voltage balancing function

[0105] Figure 10 shows the system voltage waveforms under three operating conditions when the two-pole load is switched according to the above process. As shown in Figure 10(a), without any voltage equalization measures, the voltage between the two poles is severely unbalanced. Figures 10(b) and 10(c) show that the present invention can maintain voltage balance whether under no-load or when connected to any DC device.

[0106] (2) Power transmission function

[0107] Appendix Figure 11 The waveforms of voltage and current at ports 1 and 2 are shown under operating condition 3. It can be seen that the port current of the proposed converter only fluctuates during load switching. In steady state, the converter can achieve port power transfer through constant current control. The proposed power transfer function and voltage balancing function can operate simultaneously.

[0108] (3) Comparison of grid-connected currents of LC_VB and LT_VB

[0109] Under operating condition 2, the LT_VB structure and LC_VB structure proposed in this invention (attached) Figure 6 Experimental comparisons were conducted. As shown in Figure 12, when transmitting the same unbalanced power, the grid-connected current ripple of the LT_VB structure (Figure 12(b)) is significantly smaller than that of the LC_VB structure (Figure 12(a)). Therefore, the proposed converter topology can effectively reduce grid-connected current fluctuations and decrease the size of filter components.

[0110] (4) Zero voltage turn-on

[0111] Appendix Figure 13 Demonstrates the switching transistor S under operating condition 3 1,1 and Q 1,1 Voltage and drive signal waveforms. In the drive signal g... S1,1 Before the rise, S 1,1 The anti-parallel diode is turned on, and the voltage across it is v. S1,1 Reduced to zero, S 1,1 Achieve zero-voltage turn-on. Q 1,1 Zero-voltage turn-on can also be achieved. Because the four-port converter has a symmetrical structure, all switches in the proposed converter can achieve zero-voltage turn-on.

[0112] In summary, the voltage self-balancing DC-DC converter for bipolar DC microgrids provided in this invention eliminates the need for additional active power devices, achieving voltage balancing through device multiplexing technology, thus saving costs and simplifying control strategies. Furthermore, this invention can balance the voltage at any point on the microgrid, overcoming voltage dips over long distances. The proposed LT_VB topology effectively reduces grid current fluctuations caused by the voltage balancer, reduces the size of filter components, and allows all switches to achieve zero-voltage turn-on. Moreover, this invention integrates voltage balancing and energy conversion functions, ensuring that the grid voltage remains balanced while port devices draw or inject energy from the grid, making the proposed converter more suitable for practical applications. Experimental verification shows that the proposed invention can effectively maintain voltage balance on the bipolar bus, achieve energy conversion for port devices, and reduce grid current ripple caused by the voltage balancer.

[0113] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A voltage self-balancing DC converter for a bipolar DC microgrid, characterized in that, include The branch circuit and two dual active bridge converters are connected in series on their grid-connected sides and then connected between the positive and negative buses of the microgrid. The common terminal of the two dual active bridge converters is connected to the zero-pole bus of the microgrid. The load sides of both dual active bridge converters are connected to DC equipment. Branch circuits include inductors and transformer The transformer The two sides of the inductor are respectively connected to the two midpoints of the grid-connected full-bridge circuit of the two dual active bridge converters. Connected in series with the transformer And any of the connection lines of the dual active bridge converter.

2. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 1, characterized in that, The two dual active bridge converters are referred to as the first dual active bridge converter and the second dual active bridge converter, respectively. The grid-connected positive terminal of the first dual active bridge converter is connected to the positive bus of the microgrid, and the grid-connected negative terminal of the second dual active bridge converter is connected to the negative bus of the microgrid. Both the grid-connected negative terminal of the first dual active bridge converter and the grid-connected positive terminal of the second dual active bridge converter are connected to the zero bus of the microgrid.

3. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 2, characterized in that, The first dual active bridge converter includes The branch, the first load-side full-bridge circuit, and the first grid-connected full-bridge circuit, the Branch circuits include inductors and transformer The transformer The two sides of the inductor are connected to the two midpoints of the first load-side full-bridge circuit and the first grid-connected full-bridge circuit, respectively. Connected in series with the transformer And any connection line of the first load-side full-bridge circuit.

4. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 3, characterized in that, The first load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a first load-side bridge arm group. and switching transistor The first load-side bridge arm group is connected in series, and the two first load-side bridge arm groups are connected in parallel. The first grid-connected bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a first grid-connected side bridge arm group. and switching transistor The first grid-connected side bridge arm group is connected in series, and the two first grid-connected side bridge arm groups are connected in parallel.

5. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 4, characterized in that, A capacitor is also connected in parallel on the first load-side bridge arm assembly. A capacitor is also connected in parallel on the first grid-connected side bridge arm assembly. .

6. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 4, characterized in that, The second dual active bridge converter includes The branch, the second load-side full-bridge circuit, and the second grid-connected full-bridge circuit, the Branch circuits include inductors and transformer The transformer The two sides of the inductor are connected to the two midpoints of the second load-side full-bridge circuit and the second grid-side full-bridge circuit. Connected in series with the transformer And any connection line of the second load-side full-bridge circuit.

7. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 6, characterized in that, The second load-side full-bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistors are connected in series to form a second load-side bridge arm group. and switching transistor The two load-side bridge arm groups are connected in parallel to form a second load-side bridge arm group; The second grid-connected bridge circuit includes a switching transistor. Switching transistor Switching transistor and switching transistors The switching transistor and switching transistor The switching transistor is connected in series to form a second grid-connected side bridge arm group. and switching transistor They are connected in series to form a second grid-connected side bridge arm group, and the two second grid-connected side bridge arm groups are connected in parallel.

8. The voltage self-balancing DC-DC converter for a bipolar DC microgrid according to claim 7, characterized in that, A capacitor is also connected in parallel on the second load-side bridge arm assembly. A capacitor is also connected in parallel on the second grid-connected side bridge arm assembly. .

9. A modulation method for a voltage self-balancing DC-DC converter adapted to the bipolar DC microgrid of claim 6, characterized in that, include: Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ; Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ; drive signal and drive signal Complementary signals within the same full-bridge circuit all have a 50% duty cycle, including dead time; drive signals and drive signal Collectively referred to as drive signals ; ; Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ; Switching transistor Switching transistor The drive signals are the same, denoted as drive signal. ; drive signal and drive signal These are complementary signals within the same full-bridge circuit, both with a 50% duty cycle, including dead time; drive signals. and drive signal Collectively referred to as drive signals .

10. The modulation method for the voltage self-balancing DC-DC converter of the bipolar DC microgrid according to claim 9, characterized in that, drive signal Given an initial PWM signal, the drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is drive signal and drive signal The phase shift angle is Phase shift angle Phase shift angle and the phase shift angle is Individual control.

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