Voltage self-balancing direct-current converter of bipolar direct-current micro-grid and modulation method

Abstract

The invention discloses a voltage self-balancing direct-current converter of a bipolar direct-current micro-grid and a modulation method, and belongs to the technical field of power systems, the voltage self-balancing direct-current converter comprises a # imgabs0 # branch and two dual-active-bridge converters, the grid-connected sides of the two dual-active-bridge converters are connected in series and then connected between a positive bus and a negative bus of the micro-grid, and the grid-connected sides of the two dual-active-bridge converters are connected in series. The common end of the two dual-active bridge converters is connected to a zero-pole bus of the micro-grid, and the load sides of the two dual-active bridge converters are connected to direct current equipment; the # imgabs 1 # branch comprises an inductor # imgabs 2 # and a transformer # imgabs 3 #, the two sides of the transformer # imgabs 4 # are connected to the two midpoints of the grid-connected side full-bridge circuits of the two double-active-bridge converters respectively, and the inductor # imgabs 5 # is connected to any connecting line of the transformer # imgabs 6 # and the double-active-bridge converters in series. According to the invention, two-pole voltage balance can be realized without connecting any direct current device, the device cost and size can be saved, and the power grid current ripple can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a voltage self-balancing DC converter and modulation method for a bipolar DC microgrid. Background Art

[0002] With the rapid development of a new power system mainly based on new energy, a large number of distributed energy sources such as photovoltaic arrays, wind turbines, electric vehicle charging facilities, and battery energy storage systems are connected to the grid. Since distributed energy sources are usually in the form of direct current (DC), compared with an alternating current (AC) microgrid, a DC microgrid does not require an AC-DC converter to connect distributed DC source loads, nor does it require reactive or synchronous control, which improves the energy conversion efficiency of the system and makes the energy supply more flexible and reliable. Therefore, DC microgrids are widely used in the current new power system.

[0003] According to the bus structure, DC microgrids can be divided into: single-pole DC microgrids and bipolar DC microgrids. As shown in the attached Figure 1 figure, there are positive, zero, and negative three power buses in the microgrid system, which can provide two voltage levels of Vdc and 2Vdc. According to different voltage levels, different loads or distributed energy sources such as photovoltaic arrays and data centers can be connected to the microgrid. Compared with a single-pole DC microgrid, a bipolar DC microgrid has many advantages. Although a bipolar DC microgrid has significant advantages in power transmission, its construction cost and technical requirements are relatively high. Due to the uneven distribution of distributed source loads between the two poles, the power injected or absorbed by the two poles is different, resulting in unbalanced bus voltages of the two poles of the bipolar DC microgrid, which affects the normal operation of the system. Therefore, how to maintain the voltage balance between the two poles has become a research hotspot for bipolar DC microgrids. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies in 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 purpose, the present invention is implemented by the following technical solutions: In the first aspect, the present invention provides a voltage self-balancing DC converter for a bipolar DC microgrid, including a branch and two bi-directional active bridge converters. The grid-connected sides of the two bi-directional active bridge converters are connected in series between the positive and negative busbars of the microgrid, the common end of the two bi-directional active bridge converters is connected to the zero busbar of the microgrid, and the load sides of the two bi-directional active bridge converters are both connected to a DC device; the branch includes an inductor and a transformer The transformer Both sides of it are respectively connected to two midpoints of the grid-connected side full-bridge circuits of the two dual-active-bridge converters, and the inductor is connected in series with the transformer and any one of the connecting lines of the dual-active-bridge converter.

[0006] Optionally, the two dual-active-bridge converters are respectively denoted as the first dual-active-bridge converter and the second dual-active-bridge converter; the positive pole of the grid-connected side of the first dual-active-bridge converter is connected to the positive bus of the microgrid, the negative pole of the grid-connected side of the second dual-active-bridge converter is connected to the negative bus of the microgrid, and the negative pole of the grid-connected side of the first dual-active-bridge converter and the positive pole of the grid-connected side of the second dual-active-bridge converter are both connected to the zero-pole bus of the microgrid.

[0007] Optionally, the first dual-active-bridge converter includes a branch, a first load-side full-bridge circuit, and a first grid-connected side full-bridge circuit. The branch includes an inductor and a transformer . The two sides of the transformer are connected to two midpoints of the first load-side full-bridge circuit and the first grid-connected side full-bridge circuit, and the inductor is connected in series with the transformer and any one of the connecting lines of the first load-side full-bridge circuit.

[0008] Optionally, the first load-side full-bridge circuit includes switching tubes , switching tubes , switching tubes , and switching tubes . The switching tubes and the switching tubes are connected in series to form a first load-side bridge arm group, and the switching tubes and the switching tubes are connected in series to form a first load-side bridge arm group. The two first load-side bridge arm groups are connected in parallel; The first grid-connected bridge circuit includes switching tubes , switching tubes , switching tubes , and switching tubes . The switching tubes and the switching tubes are connected in series to form a first grid-connected side bridge arm group, and the switching tubes and the switching tubes are connected in series to form a first grid-connected side bridge arm group. The two first grid-connected side bridge arm groups are connected in parallel.

[0009] Optionally, a capacitor is also connected in parallel on the first load-side bridge arm group., a capacitor is also connected in parallel to the first grid-connected side bridge arm group .

[0010] Optionally, the second dual-active-bridge converter includes a branch, a second load-side full-bridge circuit, and a second grid-connected side full-bridge circuit. The branch includes an inductor and a transformer . The two sides of the transformer are connected to the two midpoints of the second load-side full-bridge circuit and the second grid-connected side full-bridge circuit. The inductor is connected in series to any connection line between the transformer and the second load-side full-bridge circuit.

[0011] Optionally, the second load-side full-bridge circuit includes switching tubes , switching tubes , switching tubes , and switching tubes . The switching tubes and the switching tubes are connected in series to form a second load-side bridge arm group. The switching tubes and the switching tubes are connected in series to form a second load-side bridge arm group. The two second load-side bridge arm groups are connected in parallel; The second grid-connected bridge circuit includes switching tubes , switching tubes , switching tubes , and switching tubes . The switching tubes and the switching tubes are connected in series to form a second grid-connected side bridge arm group. The switching tubes and the switching tubes are connected in series to form a second grid-connected side bridge arm group. The two second grid-connected side bridge arm groups are connected in parallel.

[0012] Optionally, a capacitor is also connected in parallel to the second load-side bridge arm group, and a capacitor is also connected in parallel to the second grid-connected side bridge arm group.

[0013] In a second aspect, the present invention provides a modulation method for a voltage self-balancing DC converter adapted to the bipolar DC microgrid described in claim 6, including: The driving signals of the switching tubes and the switching tubes are the same, denoted as the driving signal ; The driving signals of the switching tubes and the switching tubes The drive signals are the same and are denoted as the drive signal ; Drive signal and drive signal are complementary signals within the same full-bridge circuit, with a duty cycle of 50% each and including a dead zone; drive signal and drive signal are collectively referred to as drive signal ; ; Switch , switch have the same drive signal, denoted as drive signal ; Switch , switch have the same drive signal, denoted as drive signal ; Drive signal and drive signal are complementary signals within the same full-bridge circuit, with a duty cycle of 50% each and including a dead zone; drive signal and drive signal are collectively referred to as drive signal .

[0014] Optionally, drive signal is a given initial PWM signal, and the phase shift angles of drive signal and drive signal are , the phase shift angles of drive signal and drive signal are , the phase shift angles of drive signal and drive signal are , and the phase shift angles , phase shift angle and the phase shift angle of are controlled separately.

[0015] Compared with the prior art, the beneficial effects achieved by the present invention: The present invention provides a voltage self-balancing DC converter and modulation method for a bipolar DC microgrid. The voltage self-balancing DC converter is composed of two dual-active bridge converters and an inductor transformer series-type voltage balancer, which is functionally equivalent to two independent DAB grid-connected converters and an independent voltage balancer. The voltage balancing function and the energy conversion function are integrated so that the voltage at the two poles of the grid can be kept balanced while the port device draws or injects grid energy, and the grid current ripple is significantly reduced compared to the LC_VB structure. At the same time, the LT_VB structure is used to realize device reuse, and the voltage balancing function can be realized without using additional active devices, saving device costs. In addition, the present invention can balance the voltage at the two poles of the bipolar DC microgrid at any point, and overcome the voltage drop phenomenon of the long-distance microgrid. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a structural schematic diagram of a bipolar DC microgrid provided in the background technology of the present invention; Figure 2 Schematic diagram of the structure of a voltage self-balancing DC converter provided by an embodiment of the present invention; Figure 3 is a control strategy diagram of a voltage self-balancing DC converter provided by an embodiment of the present invention; Figure 4 is a working mode diagram of a voltage self-balancing DC converter provided by an embodiment of the present invention; Figure 5 is a signal waveform diagram of the voltage self-balancing DC converter provided by an embodiment of the present invention when it is working; Figure 6 is a structural diagram of LC_VB provided in an embodiment of the present invention; Figure 7 is a comparison diagram of the working modes of the LT_VB branch and the LC_VB branch provided in an embodiment of the present invention; Figure 8 1 is a comparison diagram of grid-connected currents of the LT_VB branch and the LC_VB branch provided in an embodiment of the present invention; Figure 9 is a relationship diagram of the reduced grid-connected current ripple versus transmission power provided by an embodiment of the present invention; FIG. 10( a ) is a bus voltage waveform diagram without any voltage balancing measures in an experiment provided by an embodiment of the present invention; FIG. 10( b ) is a bus voltage waveform diagram when port 1 and port 2 are unloaded during an experiment provided in an embodiment of the present invention; FIG. 10( c ) is a bus voltage waveform diagram when Pport1=100W and Pport2=-100W in an experiment provided in an embodiment of the present invention; Figure 11It is the voltage and current waveform diagram of Port 1 and Port 2 during the experiment provided by the embodiment of the present invention; Figure 12(a) is the grid-connected current waveform diagram of the dual LC branch during the experiment provided by the embodiment of the present invention; Figure 12(b) is the grid-connected current waveform diagram of the single LC branch during the experiment provided by the embodiment of the present invention; Figure 13 It is the voltage and current waveform diagram of switch tubes S1.1 and Q1.1 during zero-voltage turn-on of the present invention. Detailed implementation manners

[0017] 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 cannot be used to limit the protection scope of the present invention.

[0018] Embodiment 1:

[0019] As Figure 2 shown, a voltage self-balancing DC converter for a bipolar DC microgrid provided by an embodiment of the present invention is composed of two dual active bridge converters (Dual Active Bridge, DAB) and an inductor-transformer series voltage balancer (LT Series Voltage Balancer, LT_VB). The dual active bridge converter is as Figure 2 shown in (a) therein, and the inductor-transformer series voltage balancer is as Figure 2 shown in (b) therein. The two dual active bridge converters DAB and an inductor-transformer series voltage balancer LT_VB share 8 switch tubes by using device multiplexing technology, which greatly reduces the hardware cost and control cost.

[0020] The two dual active bridge converters are respectively denoted as the first dual active bridge converter and the second dual active bridge converter ; the positive pole of the grid-connected side of the first dual active bridge converter is connected to the positive bus of the microgrid, the negative pole of the grid-connected side of the second dual active bridge converter is connected to the negative bus of the microgrid, and the negative pole of the grid-connected side of the first dual active bridge converter and the positive pole of the grid-connected side of the second dual active bridge converter are both connected to the zero-pole bus of the microgrid. The load sides of the first dual active bridge converter and the second dual active bridge converter are both connected to DC equipment.

[0021] The first dual active bridge converter includes a branch, a first full-bridge circuit on the load side, and a first full-bridge circuit on the grid-connected side. The branch includes an inductor and a transformer , the transformer has both sides connected to two midpoints of a first load-side full-bridge circuit and a first grid-connection-side full-bridge circuit, and an inductor is connected in series with the transformer and any one of the connection lines of the first load-side full-bridge circuit.

[0022] The first load-side full-bridge circuit includes switching transistors , switching transistors , switching transistors and switching transistors . The switching transistors and the switching transistors are connected in series to form a first load-side bridge arm group, and the switching transistors and the switching transistors are connected in series to form a first load-side bridge arm group. The two first load-side bridge arm groups are connected in parallel; the first grid-connection bridge circuit includes switching transistors , switching transistors , switching transistors and switching transistors . The switching transistors and the switching transistors are connected in series to form a first grid-connection-side bridge arm group, and the switching transistors and the switching transistors are connected in series to form a first grid-connection-side bridge arm group. The two first grid-connection-side bridge arm groups are connected in parallel.

[0023] The second dual-active-bridge converter includes a branch, a second load-side full-bridge circuit, and a second grid-connection-side full-bridge circuit. The branch includes an inductor and a transformer . The two sides of the transformer are connected to two midpoints of the second load-side full-bridge circuit and the second grid-connection-side full-bridge circuit, and the inductor is connected in series with the transformer and any one of the connection lines of the second load-side full-bridge circuit.

[0024] The second load-side full-bridge circuit includes switching transistors , switching transistors , switching transistors and switching transistors . The switching transistors and the switching transistors are connected in series to form a second load-side bridge arm group, and the switching transistors and the switching transistors are connected in series to form a second load-side bridge arm group. The two second load-side bridge arm groups are connected in parallel; the second grid-connection bridge circuit includes switching transistors , switching transistors , a switching transistor and a switching transistor , the switching transistor and the switching transistor are connected in series to form a second grid-connected side bridge arm group. The switching transistor and the switching transistor are connected in series to form a second grid-connected side bridge arm group. The two second grid-connected side bridge arm groups are connected in parallel.

[0025] A capacitor is also connected in parallel to the first load side bridge arm group , and a capacitor is also connected in parallel to the first grid-connected side bridge arm group . A capacitor is also connected in parallel to the second load side bridge arm group , and a capacitor is also connected in parallel to the second grid-connected side bridge arm group .

[0026] The branch includes an inductor and a transformer . The two sides of the transformer are respectively connected to the two midpoints of the grid-connected side full-bridge circuits of two dual-active-bridge converters. The inductor is connected in series to the transformer and any one of the connection lines of the dual-active-bridge converter. Due to the characteristic that the branch transfers energy in the full cycle, compared with the LC branch for voltage equalization, the grid-connected current ripple introduced by the voltage balancing function is greatly reduced.

[0027] As Figure 3 shown, based on the voltage self-balancing DC converter of the bipolar DC microgrid provided in the above embodiment, this embodiment also provides a modulation method, including:

[0028] The drive signals of the switching transistor and the switching transistor are the same, denoted as the drive signal ;

[0029] The drive signals of the switching transistor and the switching transistor are the same, denoted as the drive signal ;

[0030] The drive signal and the drive signal are complementary signals within the same full-bridge circuit, and the duty cycle is 50% for both, including a dead zone; the drive signal and the drive signal are collectively referred to as the drive signal ; ;

[0031] The switching transistor , The drive signals of the switching transistors are the same, denoted as drive signal ;

[0032] The drive signals of the switching transistors and the switching transistors are the same, denoted as drive signal ;

[0033] Drive signal and drive signal are complementary signals within the same full-bridge circuit, with a duty cycle of 50% and including a dead zone; drive signal and drive signal are collectively referred to as drive signal .

[0034] Drive signal is the given initial PWM signal, and the phase shift angles of drive signal and drive signal are , the phase shift angles of drive signal and drive signal are , the phase shift angles of drive signal and drive signal are , and the phase shift angles , phase shift angle and the phase shift angle of are controlled separately. By adjusting the phase shift angle , bus voltage balance control can be achieved, and by adjusting the phase shift angle , phase shift angle of , port power control can be achieved.

[0035] Analyze the principle of the voltage self-balancing DC converter for the bipolar DC microgrid provided in this embodiment. First, define the circuit parameters. v Lk and i Lk are the voltage and current of the inductor L k , v LB and i LB are the voltage and current of the inductor L B , i' Lk and i' LB are the secondary-side currents of the transformers T k and T B . v portk and i portk are the voltage and current of port k. It is stipulated that when the port is connected to the DC power supply, the direction of the current i portk is the positive direction. f s is the switching frequency. V totalis the positive and negative bus voltages of the system, and in the steady state, V dc = V dc+ = V dc- . I + 、I 0 、I - are the currents of the converter incorporated into the positive, neutral, and negative busbars respectively. T 1 、T 2 、T B The turns ratios of the transformers are n 1 、n 2 、n B . I L1 、I L2 、I LB are the magnitudes of the inductors L 1 、L 2 、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 in one cycle is analyzed. The equivalent working modes of the embodiments of the present invention are as shown in Appendix Figure 4 . The drive signals of the switching tubes, the voltages and currents of the inductors L k and L B are as shown in Appendix Figure 5 .

[0036] To simplify the analysis process, the following assumptions are made:

[0037] 1) Operating in the steady state;

[0038] 2) The switching tubes are all ideal devices;

[0039] 3) The turns ratio of the transformer n 1 = n 2 = n B = n, the port voltages V port1 and V port2 remain stable and satisfy with the single-pole bus voltage V of the power grid dc : V portk : V dc = n:1;

[0040] 4) Ignoring the voltage fluctuations of the capacitors.

[0041] Sub-state A (t < t 0 ): Before the moment of t 0 , the drive 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 transmits energy to the power grid, the unbalanced power is transferred from the positive pole to the negative pole, port 2 transmits energy to the power grid, vL1 = v LB = v L2 = 0, i L1 = I L1 , i LB = -I LB , i L2 = I L2 。

[0042] Sub - state B (t 0 <= t < t 1 ): At t = t 0 moment, S 1,1 and S 1,4 turn off. Due to the inductor L 1 current 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 .

[0043] Sub - state C (t 1 <= t < t 2 ): At t = t 1 moment, Q 1,1 and Q 1,4 turn 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, the negative pole returns reactive power 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 .

[0044] Sub - state D (t 2 <= t < t 3 ): At t = t 2 moment, Q 2,1 and Q 2,4 turn off. Q2,3 and Q 2,2 's body diode conducts. 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 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 decreases from I L2 to -I L2 .

[0045] Sub-state E (t 3 <= t < t 4 ): At t = t 3 moment, 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 .

[0046] Sub-state F (t 4 <= t < t 5 ): At t = t 4 moment, 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 .

[0047] Sub-state G(t 5 <=t<t 6 ): At t = t 5 moment, Q 1,3 and Q 1,2 turn off. The body diodes of Q 1,1 and Q 1,4 conduct. After the dead time, Q 1,1 and Q 1,4 achieve zero-voltage turn-on. The circuit operating state is: a1 + b4 + c3. In this state, the power grid returns reactive power to port 1, the negative pole returns reactive power to the positive pole, port 2 transmits energy to the power grid, v L1 = 0, i L1 = I L1 , v LB = -2V dc , i LB decreases from I LB to -I LB .

[0048] Sub-state H(t 6 <=t<t 7 ): At t = t 6 moment, 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 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 = 2nV dc , i L2 increases from -I L2 to I L2 .

[0049] At t = t 7 moment, 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 2,4Zero-voltage turn-on is achieved. 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 switching tubes can achieve zero-voltage turn-on.

[0050] After the above analysis, the inductor current I Lk and I LB amplitudes satisfy Equation (1-1):

[0051] (1-1)

[0052] Analyze the power transfer principle of the voltage self-balancing DC converter for the bipolar DC microgrid provided in this embodiment:

[0053] In the 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 reflux stages of DAB k . The relationship between i portk and i Lk can be described as:

[0054] (1-2)

[0055] According to the state-space averaging method, the DABk port current and power satisfy Equation (1-3):

[0056] (1-3)

[0057] where P portk is the transmitted power of port k. When P portk >0, port k is connected to the DC power supply; when P portk <0, port k is connected to the DC load.

[0058] For the unbalanced power transmitted by LT_VB, the b1 and b3 power transfer 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 Equation (1-4):

[0059] (1-4)

[0060] Analyze the grid-connected current ripple of the voltage self-balancing DC converter for the bipolar DC microgrid provided in this embodiment:

[0061] From the analysis of the technical principle, the grid-connected currents I + 、I 0 and I - of the product are for DAB1 and the linear superposition of the grid-connected currents of DAB and LT_VB. Utilizing the characteristic of the interleaved turn-on of the full-bridge circuit, LT_VB can achieve continuous output of the voltage-balancing current. For a bipolar DC microgrid, the LC_VB structure is commonly used to balance the bus voltages of the two poles. As shown in the appendix 2 Figure 6 , its C B has a relatively large value, and the capacitor voltage remains almost unchanged. For LC_VB, the expression for the unbalanced power it transmits is:

[0062] (1-5)

[0063] where L B2 is the inductance of the LC voltage-balancing branch. When the port is open-circuited, compare the grid-connected current ripples of the voltage-balancing branches of LC_VB and LT_VB. Under the condition of the same grid voltage, if the two structures transmit the same unbalanced power, then . Taking the transmission of unbalanced power from the positive pole to the negative pole as an example, the comparison of the voltage-balancing operating states of the two structures is shown in the appendix Figure 7 , and the comparison of the grid-connected current waveforms at this time is shown in the appendix Figure 8 .

[0064] (1) Analysis of LT_VB grid-connected current

[0065] State b1: Power is transmitted from the positive pole to the negative pole, i + = i - = i LB = -I LB , i 0 = i + + i - = -2I LB .

[0066] State b2: Reactive power flows back from the negative pole to the positive pole, i + = -i - = -i LB , i 0 = 0.

[0067] State b3: Power is transmitted from the positive pole to the negative pole, i + = i - = -i LB = -I LB , i 0 = i + + i - = -2I LB .

[0068] State b4: Reactive power flows back from the negative pole to the positive pole, i + = -i - = i LB , i 0 ​= 0;

[0069] As can be seen from the above analysis, when the LT_VB transfers unbalanced power, the grid-side full-bridge circuit conducts alternately. Within one cycle, the positive and negative grid-connected currents are in a continuous working state, and the current output is relatively smooth.

[0070] (2)LC_VB grid-connected current analysis

[0071] State b1: Capacitor C B discharges to the negative terminal. i + = 0, i - = i 0 = i LB = -2I LB .

[0072] State b2: The negative terminal returns reactive power to the positive terminal. i + = -i - = -i LB , i 0 = 0;

[0073] State b3: The positive terminal charges capacitor C B . i + = -i 0 = -i LB1 = -2I LB , i - = 0.

[0074] State b4: The negative terminal returns reactive power to the positive terminal. i + = i - = i 0 = 0;

[0075] From the current waveform and state analysis, it can be seen that the positive and negative grid-connected currents generated by the LC_VB transferring unbalanced power are discontinuous within one cycle, and the grid-connected current fluctuates greatly. Under the condition of transmitting the same unbalanced power, the device stress in the LC_VB is higher.

[0076] Under the condition of transmitting the same unbalanced power, the percentage of the grid-connected current fluctuation reduced by the LT_VB varies with the per-unit value of the transmitted unbalanced power (P B ) as shown in the appendix Figure 9 . It can be seen that the LT_VB structure can reduce the grid-connected current fluctuation by at least 68.42%.

[0077] Analysis of the control strategy of the voltage self-balancing DC converter for the bipolar DC microgrid provided in this embodiment:

[0078] The phase-shift control strategy is applicable to all power transmission processes of the four-port converter proposed in this paper. Because d 1 , d 2 and d BThey are mutually independent, so the control variables of port 1, port 2 power control and bipolar voltage balance control are mutually independent. According to formula (1-3), by adjusting d k the power demand of port k can be satisfied, and then the grid connection function of any DC device can be realized. Similarly, according to formula (1-4), by adjusting d B the magnitude and direction of the unbalanced power transmitted by LT_VB can be adjusted, which means that the voltage balance problem will be solved. The control block diagram of the proposed converter is as shown in the appendix Figure 3 as follows.

[0079] An experiment was conducted on the voltage self-balancing DC converter of the bipolar DC microgrid provided in this embodiment:

[0080] The experimental situation is introduced as follows: when 0≤t<0.1s, the positive and negative loads are equal, R load+ =R load- =2.5Ω; when 0.1≤t<0.15s, the positive pole is no-load, R load- =2.5Ω; when 0.15≤t<0.2s, the negative pole is no-load, R load+ =2.5Ω. Three working conditions were set in the experiment. Working condition 1: No voltage equalization measures were adopted. Working condition 2: Ports 1 and 2 were no-load, and only DLCVB was used to achieve voltage balance. Working condition 3: Port 1 was connected to a DC power supply (P port1 =100W), and port 2 was connected to a DC load (P port2 =-100W).

[0081] (1) Voltage balance function

[0082] Figures 10(a), 10(b), and 10(c) in the appendix are the waveforms of the system voltage under the three working conditions when the bipolar load is switched according to the above process. As shown in Figure 10(a), when no voltage equalization measures are adopted, the bipolar voltages are seriously unbalanced. Figures 10(b) and 10(c) show that the present invention can maintain voltage balance whether it is no-load or connected to any DC device.

[0083] (2) Power transmission function

[0084] Appendix Figure 11 shows the voltage and current waveforms of ports 1 and 2 under working condition 3. It can be seen that there is only a certain fluctuation in the port current of the proposed converter during the load switching process. At steady state, the converter can achieve port power transmission through constant current control. The power transmission function and voltage balance function of the proposed invention can operate simultaneously.

[0085] (3) Comparison of grid-connected currents of LC_VB and LT_VB

[0086] Under working condition 2, the LT_VB structure proposed in the present invention is compared with the LC_VB structure (appendixFigure 6 )Conduct experimental comparisons. As shown in Figures 12(a) and 12(b), 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 the grid-connected current fluctuations and decrease the volume of the filtering components.

[0087] (4) Zero-voltage turn-on

[0088] Appendix Figure 13 shows the voltage and driving signal waveforms of switch S 1,1 and Q 1,1 under Condition 3. Before the driving signal g S1,1 rises, the anti-parallel diode of S 1,1 conducts, and the voltage across both ends v S1,1 drops to zero, enabling S 1,1 to achieve zero-voltage turn-on. Q 1,1 can also achieve zero-voltage turn-on. Since the four-port converter structure is symmetric, all the switching tubes of the proposed converter can achieve zero-voltage turn-on.

[0089] In summary, the voltage self-balancing DC converter for a bipolar DC microgrid provided by the embodiments of the present invention does not require additional active power devices and realizes voltage balance through device multiplexing technology, which not only saves costs but also simplifies the control strategy. At the same time, the invention can balance the voltages of the two poles at any location in the microgrid and overcome the voltage drop phenomenon in a long-distance microgrid. The proposed LT_VB structure topology effectively reduces the grid current fluctuations caused by the voltage balancer, decreases the volume of the filtering components, and all the switching tubes can achieve zero-voltage turn-on. In addition, the invention integrates the voltage balancing function and the energy conversion function. While the port devices draw or inject energy into the grid, the voltages of the two poles of the grid can also be kept balanced, making the proposed converter more in line with the actual usage requirements. Through experimental verification, the proposed invention can effectively maintain the voltage balance of the bipolar bus, realize the energy conversion of the port devices, and reduce the grid current ripple caused by the voltage balancer.

[0090] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A voltage self-balancing DC converter for a bipolar DC microgrid, characterized in that: include The two dual active bridge converters are connected in series and then connected between the positive and negative busbars of the microgrid. The common ends of the two dual active bridge converters are connected to the zero-pole busbar of the microgrid. The load sides of the two dual active bridge converters are connected to the DC device. The branch includes the inductor and transformer , the transformer The two sides of the inductor are respectively connected to the two midpoints of the grid-connected full-bridge circuits of the two dual active bridge converters. Connected in series to the transformer And any one connecting line of the dual active bridge converter.

2. The voltage self-balancing DC converter of the bipolar DC microgrid according to claim 1 is characterized in that: The two dual active bridge converters are respectively recorded as the first dual active bridge converter and the second dual active bridge converter; the grid-connected positive pole of the first dual active bridge converter is connected to the positive bus of the microgrid, the grid-connected negative pole of the second dual active bridge converter is connected to the negative bus of the microgrid, and the grid-connected negative pole of the first dual active bridge converter and the grid-connected positive pole of the second dual active bridge converter are both connected to the zero-pole bus of the microgrid.

3. The voltage self-balancing DC converter of the bipolar DC microgrid according to claim 2 is characterized in that: The first dual active bridge converter comprises branch, a first load-side full-bridge circuit and a first grid-connected side full-bridge circuit, the The branch includes the inductor 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 side full-bridge circuit. Connected in series to the transformer and any one connecting line of the first load-side full-bridge circuit.

4. The voltage self-balancing DC converter of the bipolar DC microgrid according to claim 3 is characterized in that: The first load-side full-bridge circuit includes a switch tube , switch tube , switch tube And switch tube , the switch tube And switch tube are connected in series to form a first load-side bridge arm group, the switch tube And switch tube The first load-side bridge arm groups are connected in series, and the two first load-side bridge arm groups are connected in parallel; The first parallel bridge circuit includes a switch tube , switch tube , switch tube And switch tube , the switch tube And switch tube The switch tube is connected in series to form a first grid-connected bridge arm group. And switch tube The first bridge arm groups are connected in series to form a first grid-connected side bridge arm group, and the two first bridge arm groups are connected in parallel.

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

6. The voltage self-balancing DC converter of the bipolar DC microgrid according to claim 4, characterized in that: The second dual active bridge converter comprises branch, a second load-side full-bridge circuit and a second grid-connected side full-bridge circuit, the The branch includes the inductor 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-connected side full-bridge circuit. Connected in series to the transformer and any one connecting line of the second load-side full-bridge circuit.

7. The voltage self-balancing DC converter of the bipolar DC microgrid according to claim 6, characterized in that: The second load-side full-bridge circuit includes a switch tube , switch tube , switch tube And switch tube , the switch tube And switch tube The switch tube is connected in series to form a second load side bridge arm group. And switch tube 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 parallel bridge circuit includes a switch tube , switch tube , switch tube And switch tube , the switch tube And switch tube The switch tube is connected in series to form a second grid-connected bridge arm group. And switch tube The two second grid-connected side bridge arm groups 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 converter of the bipolar DC microgrid according to claim 7, characterized in that: The second load side bridge arm group is also connected in parallel with a capacitor The second grid-connected side bridge arm group is also connected in parallel with a capacitor .

9. A modulation method for a voltage self-balancing DC converter of a bipolar DC microgrid according to claim 6, characterized in that: include: Switching tube , switch tube The driving signal is the same as ; Switching tube , switch tube The driving signal is the same as ; Drive signal and drive signal Complementary signals in the same full-bridge circuit have a duty cycle of 50%, including dead zone; drive signal and drive signal Referred to as drive signal ; ; Switching tube , switch tube The driving signal is the same as ; Switching tube , switch tube The driving signal is the same as ; Drive signal and drive signal It is a complementary signal in the same full-bridge circuit, with a duty cycle of 50%, including dead zone; the driving signal and drive signal Referred to as drive signal .

10. The modulation method of the voltage self-balancing DC converter of the bipolar DC microgrid according to claim 9, characterized in that: Drive signal For a given initial PWM signal, the drive signal and drive signal The phase shift angle is , driving signal and drive signal The phase shift angle is , driving signal and drive signal The phase shift angle is , phase shift angle , Phase shift angle And the phase shift angle is Individually controlled.

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