A high-power hydrogen production converter for a medium-voltage direct-current bus and a control method thereof

Abstract

The application discloses a kind of high-power hydrogen production converters facing medium voltage DC bus and control method thereof, belong to power electronic conversion technical field, including three-phase NPC three-level inverter bridge, three-phase high-frequency isolation transformer and current triple rectifier.Three-phase NPC three-level bridge DC input end connects medium voltage DC bus, and the primary side of three-phase high-frequency isolation transformer is connected with the AC output end of three-phase NPC three-level bridge.Current triple rectifier includes three center-tapped full-wave rectification branches, each branch is formed by two rectifier diodes and an output filter inductance, and the common connection of three center taps is used as output negative terminal, and the common connection of the output end of three output filter inductances is used as output positive terminal.The application reduces the voltage stress of switch tube through three-level structure, and three-phase structure and three-way rectification branch share power and current stress, and three-phase staggered operation reduces output current ripple.

Description

Technical Field

[0001] This invention belongs to the field of power electronic conversion technology, and in particular relates to a high-power hydrogen production converter for medium-voltage DC buses and its control method. Background Technology

[0002] With the advancement of the "dual carbon" target and the rapid growth of installed capacity of renewable energy sources such as wind power and photovoltaics, the randomness, intermittency, and volatility of new energy output in the power system are becoming increasingly prominent. Electrolysis of water to produce hydrogen can convert surplus renewable electrical energy into hydrogen energy, enabling long-term storage and cross-domain utilization of electrical energy into chemical energy. This is an important technological path to improve the absorption capacity of new energy sources and build a low-carbon energy system. As a key interface between the new energy power source and the electrolyzer, the power electronic converter needs to convert the fluctuating new energy electrical energy into low-voltage, high-current DC power that meets the requirements of the electrolyzer. In existing technologies, DC / DC converters for hydrogen production are mainly divided into two categories: non-isolated and isolated. Among non-isolated converters, the interleaved parallel Buck converter is suitable for hydrogen production scenarios. The interleaved parallel technology can improve the output current capability and provide a certain degree of fault tolerance. In the field of isolated converters, traditional single-phase full-bridge and phase-shifted full-bridge topologies are mature, and electrical isolation and voltage matching can be achieved by introducing high-frequency transformers. Dual active bridge (DAB) converters can provide high voltage conversion ratios. Three-phase full-bridge structures can distribute power and reduce the current load on single-phase power devices and transformers. Three-level topologies (such as NPC three-level) can reduce the voltage stress on switching transistors to half that of the bus voltage. The above-mentioned existing technologies have been widely studied and applied in various application scenarios.

[0003] However, existing technologies still have the following shortcomings. First, under high input voltage and high power conditions, traditional single-phase full-bridge and phase-shifted full-bridge isolated converters bear a large current in a single bridge arm, a single transformer, and a secondary rectifier, resulting in concentrated voltage and current stress on power devices. Second, although two-level three-phase full-bridge converters can distribute power, the switching devices still need to withstand a high bus voltage. Third, the interleaved Buck structure is difficult to directly provide high-frequency isolation, and the withstand voltage and duty cycle design of devices are limited under hundreds of volt high-voltage bus. Fourth, in existing isolated hydrogen production converters, the current stress on a single diode, secondary winding, and output inductor is large on the low-voltage, high-current output side. Fifth, the output current ripple of existing converters continuously impacts the inside of the electrolyzer, resulting in a shortened electrolyzer life and reduced hydrogen production efficiency. Sixth, the midpoint potential balance of existing midpoint clamping multilevel converters largely relies on the modulation strategy itself, resulting in high control complexity and difficulty in simultaneously achieving midpoint balance and output regulation under various operating conditions. Seventh, the soft-switching implementation of existing converters often relies on complex resonant circuits or additional auxiliary switches, which increases system cost and control difficulty. Therefore, there is an urgent need for an isolated DC / DC converter solution for hydrogen production that can meet the requirements of high-voltage input and low-voltage high-current output, reduce voltage and current stress on power devices, and simultaneously possess low output ripple and high reliability. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a high-power hydrogen production converter and its control method for medium-voltage DC buses, thereby resolving the issues present in the prior art.

[0005] To achieve the above objectives, in a first aspect, the present invention provides a high-power hydrogen production converter for medium-voltage DC buses, comprising: The system comprises a three-phase NPC three-level bridge, a three-phase high-frequency isolation transformer, and a current tripler rectifier. The DC input terminal of the three-phase NPC three-level bridge is used to connect to the medium-voltage DC bus, and the primary side of the three-phase high-frequency isolation transformer is connected to the AC output terminal of the three-phase NPC three-level bridge. The current tripler rectifier includes three center-tapped full-wave rectifier branches. Each center-tapped full-wave rectifier branch includes two rectifier diodes and one output filter inductor. The three center taps of the three center-tapped full-wave rectifier branches are connected together to form the negative output terminal. The output terminals of the three output filter inductors are connected together to form the positive output terminal. Each phase arm of the three-phase NPC three-level bridge includes four main switching transistors and two clamping diodes. The DC input terminal of the three-phase NPC three-level bridge is provided with a split DC bus consisting of two split bus capacitors connected in series. The common terminal of the two split bus capacitors serves as the clamping midpoint of the three-phase NPC three-level bridge.

[0006] Preferably, it further includes a midpoint potential balancing device, which consists of an upper switch, a lower switch, and a balancing inductor. One end of the upper switch is connected to the positive bus of the split DC bus, and the other end of the upper switch is connected to one end of the lower switch. The other end of the lower switch is connected to the negative bus of the split DC bus, and the balancing inductor is connected between the common terminal of the upper switch and the lower switch and the clamping midpoint.

[0007] Preferably, an inductor is connected in series between the AC output terminal of each phase arm of the three-phase NPC three-level bridge and the primary side of the three-phase high-frequency isolation transformer. The inductor is the equivalent inductance of the leakage inductance of the three-phase high-frequency isolation transformer and the external series inductor.

[0008] Preferably, the three-phase high-frequency isolation transformer is composed of three single-phase high-frequency transformers. Each single-phase high-frequency transformer has one primary winding and two secondary half windings. The primary side of the three-phase high-frequency isolation transformer is connected in a star configuration, and the secondary side is also connected in a star configuration.

[0009] Preferably, in each center-tap full-wave rectifier branch, the anodes of the two rectifier diodes are respectively connected to the outer ends of the two secondary half-windings of the corresponding single-phase high-frequency transformer, and the cathodes of the two rectifier diodes are connected together and then connected to the input end of the corresponding output filter inductor.

[0010] Preferably, a common output capacitor is connected in parallel between the positive output terminal and the negative output terminal, and the common output capacitor is used to connect in parallel with the electrolytic cell.

[0011] Secondly, the present invention provides a control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus, applied to the high-power hydrogen production converter oriented towards a medium-voltage DC bus as described in the first aspect, comprising the following steps: Step 1: Apply the same symmetrical duty cycle pulse width modulation signal to each phase arm. The pulse width modulation signals of each phase arm are staggered by 120° phase angle. The inner switches of each phase arm operate in a complementary manner with a fixed duty cycle, and the outer switches generate active pulses in the positive half-cycle and negative half-cycle respectively. Step 2: Acquire the output voltage, compare the output voltage with the given voltage, and then perform proportional-integral calculation to output the total current given value; Step 3: Collect the branch current of the three center-tap full-wave rectifier branches, compare the sum of the three branch currents with the given total current value, and then adjust the duty cycle by proportional-integral calculation; Step 4: Collect the voltage of the upper capacitor and the voltage of the lower capacitor. When the voltage of the upper capacitor is not equal to the voltage of the lower capacitor, control the upper or lower switch of the midpoint potential balancing device to operate, so that energy is transferred between the upper and lower capacitors until the voltages are equal.

[0012] Furthermore, in step one, the fixed duty cycle of the inner switch is 50%, and the width of the active pulse generated by the outer switch is DTs / 2, where D is the duty cycle and Ts is the switching period.

[0013] Furthermore, in step three, when the electrolytic cell current reaches a preset upper limit value, current limiting is performed first, allowing the output voltage to be temporarily lower than the given voltage.

[0014] Furthermore, the method also includes a soft-switching coordination step: acquiring the primary current polarity, commutation current amplitude, and collector-emitter voltage state before the switching transistor is turned on for each phase arm; selecting the commutation sequence according to the primary current polarity; adjusting the dead time according to the commutation current amplitude; and maintaining the average value of the duty cycle unchanged and extending the zero-level dwell time when the commutation current is lower than the natural zero-voltage turn-on threshold.

[0015] Compared with the prior art, the present invention has the following advantages and technical effects: In this invention, each phase arm of the three-phase NPC three-level bridge includes four main switching transistors and two clamping diodes. The DC input terminal is equipped with a split DC bus consisting of two series-connected split bus capacitors, with the common terminal of the two split bus capacitors serving as the clamping midpoint. Based on this structure, the AC output terminal of each phase arm can be clamped to one of three states: positive bus level, midpoint level, or negative bus level. In this three-level operating mode, when each main switching transistor is turned off, it only bears half of the total DC bus voltage, i.e., the voltage across a single split bus capacitor, rather than the full bus voltage. Therefore, the voltage stress on the switching transistors is significantly reduced compared to a two-level structure.

[0016] In this invention, the three-phase NPC three-level bridge is a three-phase bridge arm structure. Its AC output terminal is connected to the primary side of a three-phase high-frequency isolation transformer, distributing the total transmission power to three bridge arms with a phase difference of 120° and the three-phase transformer. Compared to a single-phase full-bridge structure, a single bridge arm and a single transformer only need to bear one-third of the total power, effectively reducing the current stress on the upstream power switching devices and transformer windings.

[0017] The current tripler rectifier in this invention comprises three center-tapped full-wave rectifier branches. Each branch includes two rectifier diodes and one output filter inductor. The three center taps are connected together to form the negative output terminal, and the output terminals of the three output filter inductors are connected together to form the positive output terminal. This structure allows the total load current to be naturally distributed among the three rectifier branches. The rectifier diodes in each branch only bear approximately one-third of the average total current, significantly reducing the current stress on a single diode compared to a single-channel full-wave rectifier structure.

[0018] In this invention, the primary side of the three-phase high-frequency isolation transformer is connected to the AC output terminal of the three-phase NPC three-level bridge. This transformer achieves electrical isolation between the input and output sides of the converter, and simultaneously achieves voltage matching and voltage reduction through the turns ratio configuration of the primary and secondary windings, meeting the electrical safety and voltage conversion requirements of hydrogen production scenarios.

[0019] In this invention, each of the three center-tapped full-wave rectifier branches is independently configured with an output filter inductor. The output terminals of the three output filter inductors are connected together to form the positive output terminal. The three filter inductors filter the pulsating rectified voltage of each branch, and the combined current supplies power to the load. The output current is smoothed by the combined effect of the multiple branch filter inductors.

[0020] In this invention, all three center-tapped full-wave rectifier branches are composed of rectifier diodes, eliminating the need for any active switching devices on the secondary side. This passive rectifier topology eliminates the need for drive circuits for secondary-side switching devices, resulting in a simple control structure. Furthermore, the diodes themselves possess high overcurrent withstand capability, which helps improve the system's operational reliability under low-voltage, high-current conditions. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of this application. The illustrative embodiments and descriptions of this application are used to explain this application and do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of the power flow direction in an embodiment of the present invention; Figure 2 This is a modulation diagram of the inverter bridge arm signal according to an embodiment of the present invention; Figure 3 This is a block diagram of the voltage and current dual closed-loop control according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the circuit structure of a high-power hydrogen converter according to an embodiment of the present invention. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] It should be noted that the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0024] Example 1 like Figure 1-4 As shown, this embodiment provides a high-power hydrogen production converter for medium-voltage DC buses, including: The system comprises a three-phase NPC three-level bridge, a three-phase high-frequency isolation transformer, and a current tripler rectifier. The DC input terminal of the three-phase NPC three-level bridge is used to connect to the medium-voltage DC bus, and the primary side of the three-phase high-frequency isolation transformer is connected to the AC output terminal of the three-phase NPC three-level bridge. Specifically, such as Figure 1 As shown, the high-power hydrogen converter consists of a split input DC bus, a midpoint potential balancer, a three-phase NPC three-level inverter bridge, three-phase primary-side series inductors, a three-phase high-frequency isolation transformer, three center-tapped full-wave rectifier branches, three output filter inductors, a common output capacitor, and an electrolyzer. Energy enters the NPC bridge from the high-voltage DC bus, and after being stepped down by the high-frequency inverter and transformer isolation, three pulsating DC voltages are formed on the secondary side. The currents of the three inductors are superimposed at the common output terminal, ultimately providing a low-ripple DC current to the electrolyzer.

[0025] Furthermore, the midpoint potential balancing device consists of an upper switch, a lower switch, and a balancing inductor. One end of the upper switch is connected to the positive bus of the split DC bus, and the other end of the upper switch is connected to one end of the lower switch. The other end of the lower switch is connected to the negative bus of the split DC bus, and the balancing inductor is connected between the common terminal of the upper switch and the lower switch and the clamping midpoint.

[0026] Specifically, the input side consists of two series capacitors C 11 and C 12 This forms a split DC bus. The midpoint O of the two capacitors serves as the clamping midpoint of the NPC bridge. The total bus voltage satisfies V dc =VC 11 +VC 12 When ideally balanced, the voltages of both the upper and lower capacitors are approximately V. dc / 2. In addition to providing a three-level voltage source, the bus capacitor also handles high-frequency pulsating power. The midpoint potential balancing device consists of switching transistors S1 and S2 and inductor L. m Composition. The main three-phase bridge arms continue to complete the AC output, and the neutral point potential balancing device only adjusts according to U. c1 with U c2The device operates by measuring the midpoint deviation. A key advantage of this approach is the relative decoupling of midpoint control from the main output modulation. The device achieves midpoint potential balance by adjusting the on-time of switches S1 and S2 based on the midpoint deviation.

[0027] Each phase arm of the three-phase NPC three-level bridge includes four main switching transistors and two clamping diodes. The DC input terminal of the three-phase NPC three-level bridge is provided with a split DC bus consisting of two split bus capacitors connected in series. The common terminal of the two split bus capacitors serves as the clamping midpoint of the three-phase NPC three-level bridge.

[0028] Specifically, such as Figure 4 As shown, the front-end inverter bridge consists of three identical NPC bridge arms: A, B, and C. Each bridge arm contains four main switching transistors; two NPC clamping diodes; and each main switching transistor has its own anti-parallel diode. Taking phase A as an example, the four switching transistors are S... A1 S A2 S A3 S A4 Clamping diodes D1 and D2. S A1 The upper end is connected to the DC positive busbar; S A1 lower end connected to S A2 Top; S A2 lower end and S A3 The common point at the top is the output terminal a of phase A; S A3 lower end connected to S A4 Top; S A4 The lower end connects to the DC negative bus. D1 connects to the DC midpoint O and S. A1 S A2 The nodes between; D2 connects S A3 S A4 The nodes between them and the DC midpoint O. Their function is to clamp the internal nodes of the bridge arm to the midpoint, so that each switch transistor ideally only bears half of the bus voltage. The structure of phases B and C is exactly the same as that of phase A, and the main switch transistor is S. B1 S B2 S B3 S B4 S C1 S C2 S C3 S C4 The clamping diodes are D3, D4, D5, and D6. The output point of phase B is b, and the output point of phase C is c. Each phase arm can form three voltage levels: S... A1 S A2 Activation, S A3 S A4 When turned off, vaO = 1 / 2Vdc, and the current flows from the positive terminal of the voltage through S. A1 ,S A2Flows to the next stage. S A2 S A3 Activation, S A1 S A4 When turned off, phase A forms a freewheeling path with the midpoint through the clamping diode, and the output voltage is 0. Which clamping diode the actual current flows through depends on the direction of the phase A current. A1 S A2 Off, S A3 S A4 When the circuit is on, vaO = -1 / 2Vdc, and the current flows from the negative terminal of the voltage through S. A3 ,S A4 The current flows to the next stage, following a path opposite to the positive level state. Phases B and C are connected to phase A. All 12 switching transistors in the converter are equipped with anti-parallel diodes, enabling positive, zero, and negative bridge arm levels. The power transferred to the transformer can be controlled by adjusting the duty cycle to change the effective voltage duration. Under ideal operating conditions, the voltage drop across all switching transistors does not exceed half of the bus voltage.

[0029] Furthermore, an inductor is connected in series between the AC output terminal of each phase arm of the three-phase NPC three-level bridge and the primary side of the three-phase high-frequency isolation transformer. The inductor is the equivalent inductance of the leakage inductance of the three-phase high-frequency isolation transformer and the external series inductor.

[0030] Specifically, each of the three NPC bridge arm outputs is connected in series with an inductor. Figure 4 Medium inductance L A L B L C These are all equivalent values ​​of transformer leakage inductance and series inductance. Taking phase A as an example, the inductance L... A Connect the output point a of phase A bridge arm to the positive terminal of transformer T1. Inductor L B L C Connect the output points b and c of phase B and C bridge arms to the positive terminals of transformers T2 and T3, respectively. Natural commutation can be achieved with the help of anti-parallel diodes and primary-side inductors, creating conditions for ZVS turn-on and reducing the losses of the switching transistors.

[0031] Furthermore, the three-phase high-frequency isolation transformer is composed of three single-phase high-frequency transformers. Each single-phase high-frequency transformer has one primary winding and two secondary half windings. The primary side of the three-phase high-frequency isolation transformer is connected in a star configuration, and the secondary side is also connected in a star configuration.

[0032] Specifically, a three-phase high-frequency transformer can be composed of three single-phase high-frequency transformers. The high-frequency AC voltage generated by the three-phase inverter bridge is transmitted to the secondary side through the high-frequency transformers, achieving electrical isolation between the input side and the electrolytic cell side. The voltage reduction function is achieved by changing the turns ratio, resulting in a stronger voltage reduction capability compared to non-isolated buck converters. Each high-frequency transformer has one primary winding and two and a half secondary windings, with both the primary and secondary sides connected in a star configuration. The two secondary windings have the same number of turns but opposite polarities. The three center taps on the secondary side are connected together and then connected to the output negative terminal.

[0033] The current tripler rectifier includes three center-tapped full-wave rectifier branches. Each center-tapped full-wave rectifier branch includes two rectifier diodes and one output filter inductor. The three center taps of the three center-tapped full-wave rectifier branches are connected together to form the negative output terminal. The output terminals of the three output filter inductors are connected together to form the positive output terminal. Furthermore, in each of the center-tap full-wave rectifier branches, the anodes of the two rectifier diodes are respectively connected to the outer ends of the two secondary half-windings of the corresponding single-phase high-frequency transformer, and the cathodes of the two rectifier diodes are connected together and then connected to the input end of the corresponding output filter inductor.

[0034] Furthermore, a common output capacitor is connected in parallel between the positive output terminal and the negative output terminal, and the common output capacitor is used to connect in parallel with the electrolytic cell.

[0035] Specifically, T1 corresponds to the first group of center-tapped full-wave rectifiers, with branches consisting of D7, D8, and L1. The anodes of the two diodes are connected to the outer ends of the two half-windings respectively, and the cathodes of the two diodes are connected together to form the A-phase rectifier node. This rectifier node is then connected in series with the output inductor L1. D7 and D8 conduct alternately. When the outer end of the upper half-winding is positive relative to the center tap, D7 conducts, and the current flows through the upper half-winding of T1 to D7, then to L1, to the positive output load, and finally back to the center tap via the negative output C0. When the outer end of the lower half-winding is positive relative to the center tap, D8 conducts, and the lower half-winding of T1 (D8L1) outputs to the positive load and C0 to the negative output center tap. Ideally, the average current in the three output branches is equal. The average current borne by each diode is only 1 / 3 of the load current, and the inrush current borne by the diode does not exceed 1 / 2 of the load current, making it suitable for low-voltage, high-current applications.

[0036] iΣ = iL1 + iL2 + iL3 = iC + Iel ĪL1≈ ĪL2≈ ĪL3≈ Iel / 3 The three rectifier branches are connected in series with their respective inductors before being combined. The input terminals of L1, L2, and L3 are connected to the three rectifier nodes respectively, and the output terminals are connected to the same positive output node. The total output ripple is reduced by interleaving the three phases.

[0037] The junction of the three output inductors is the positive terminal of the power supply, and the common point of the three center taps is the negative terminal. The common output capacitor C0 is connected in parallel with the electrolytic cell between the positive and negative terminals. The output inductors suppress the current ripple of each rectifier branch and maintain continuous power supply, while the output capacitor absorbs the instantaneous difference between the total current of the three inductors and the current of the electrolytic cell, stabilizing the terminal voltage of the electrolytic cell.

[0038] The beneficial effects of this embodiment: The converter in this embodiment employs a front-end three-phase NPC three-level bridge, a three-phase high-frequency isolation transformer, and a rear-end center-tapped current tripler rectifier, suitable for supplying power from a medium-voltage DC bus to a low-voltage, high-current electrolytic cell. The NPC three-level structure reduces the ideal voltage stress of a single switching transistor to half the bus voltage, while also reducing the voltage variation rate at switching nodes and the transformer insulation stress. The 120° interleaved operation of the three phases can share the total power, increase the synthesized ripple frequency, and reduce input and output pulsation. The high-frequency transformer achieves electrical isolation, voltage matching, and power distribution. The three center-tapped rectifier branches in the rear stage are each equipped with an output inductor, allowing the total output current to be shared by all three paths, reducing the current stress on diodes, windings, and inductors, and improving output current continuity through ripple interleaving. This scheme has no active switches on the secondary side, resulting in a relatively simple control and drive structure. Combined with an outer voltage loop, an inner current loop, and a primary-side inductor-assisted soft switch, it balances output stability, efficiency, cost, and engineering implementation difficulty, making it suitable for new energy hydrogen production power applications.

[0039] Example 2 This embodiment provides a control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus, applicable to any of the high-power hydrogen production converters oriented towards a medium-voltage DC bus described in the embodiment, characterized by including the following steps: Step 1: Apply the same symmetrical duty cycle pulse width modulation signal to each phase arm. The pulse width modulation signals of each phase arm are staggered by 120° phase angle. The inner switches of each phase arm operate in a complementary manner with a fixed duty cycle, and the outer switches generate active pulses in the positive half-cycle and negative half-cycle respectively. Furthermore, the fixed duty cycle of the inner switch is 50%, and the width of the active pulse generated by the outer switch is DTs / 2, where D is the duty cycle and Ts is the switching period.

[0040] Step 2: Acquire the output voltage, compare the output voltage with the given voltage, and then perform proportional-integral calculation to output the total current given value; Step 3: Collect the branch current of the three center-tap full-wave rectifier branches, compare the sum of the three branch currents with the given total current value, and then adjust the duty cycle by proportional-integral calculation; Furthermore, when the electrolytic cell current reaches a preset upper limit value, current limiting is performed first, allowing the output voltage to be temporarily lower than the given voltage.

[0041] Step 4: Collect the voltage of the upper capacitor and the voltage of the lower capacitor. When the voltage of the upper capacitor is not equal to the voltage of the lower capacitor, control the upper or lower switch of the midpoint potential balancing device to operate, so that energy is transferred between the upper and lower capacitors until the voltages are equal.

[0042] Step 5: Collect the primary current polarity, commutation current amplitude, and collector-emitter voltage state before the switching transistor is turned on for each phase arm; select the commutation sequence according to the primary current polarity; adjust the dead time according to the commutation current amplitude; when the commutation current is lower than the natural zero voltage turn-on threshold, keep the average value of the duty cycle unchanged and extend the zero-level dwell time.

[0043] In this embodiment, since the three-phase currents cancel each other out, the circuit itself has a certain ability to balance the midpoint potential. However, in order to reduce the complexity of control and ensure that the midpoint potential can still be balanced under various extreme operating conditions, this embodiment still sets up a special midpoint potential balancing device to meet the midpoint potential balance under various operating conditions.

[0044] like Figure 4 As shown, the midpoint potential balancing device consists of an upper switch S1, a lower switch S2, and a bridge arm inductor L. m Composition. S1 and S2 are complementary and conduct, inductor L m It is connected between the midpoint of S1 and S2 and the midpoint O of the DC capacitor. The inductor itself does not store net energy for a long time, but plays the role of energy transfer, redistributing the energy between the upper capacitor C1 and the lower capacitor C2.

[0045] When S1 is on and S2 is off, C1, S1 and L m A circuit is formed. C1 flows to inductor L. m Release energy, U c1 The inductor current decreases; as the inductor's energy storage increases, the inductor current iC gradually increases.

[0046] When S1 is off and S2 is on, inductor L m C2 and S2 form a freewheeling circuit. The inductor releases the energy stored in the previous stage to C2, therefore U c2 As the inductor's energy storage decreases, iC gradually decreases.

[0047] By continuously repeating these two switching states, energy can be transferred from the upper capacitor to the lower capacitor. The principle of control is to always ensure that energy flows from the capacitor with higher voltage to the capacitor with lower voltage.

[0048] When U c1 >U c2 When U... c2 >U c1 At this time, the controller generates adjustments in the opposite direction. After dynamic equilibrium, the average charge and discharge amounts obtained by the two capacitors over a longer time scale are equal, U c1 with U c2 It then stabilizes at nearly the same value.

[0049] Inverter arm signal modulation diagram, as shown Figure 2 As shown, the three-phase inverter bridge arms use the same symmetrical duty cycle PWM. The three-phase gate signals are staggered by 120°. The inner switches of each phase operate complementaryly with a fixed 50% duty cycle, while the outer switches generate active pulses with a width of DTs / 2 in the positive and negative half-cycles, respectively. When the outer switches are on, the bridge arms apply positive or negative effective voltages to the transformer; when the outer switches are off, only the corresponding inner switch gates remain, relying on the primary-side inductor current, the anti-parallel diodes of the switches, and the NPC clamping diodes to form a freewheeling current and a midpoint clamping state. As the duty cycle increases, the duration of the positive and negative effective voltages increases, and the overall effective volt-seconds of the transformer primary side and the output power increase. The converter has multiple modes, and the operating mode of the converter changes accordingly with different duty cycles.

[0050] Block diagram of voltage and current dual closed-loop control, as shown Figure 3 As shown in the figure. This embodiment employs a dual-loop control strategy with an outer voltage loop and an inner current loop. The outer voltage loop stabilizes the electrolytic cell terminal voltage and generates a total inductor current reference based on the voltage deviation. The inner current loop regulates the current output and rapidly controls the transmission power by changing the duty cycle of the preceding NPC bridge. Natural ZVS is achieved by relying on the primary-side series inductor and transformer leakage inductance. The soft-switching range is expanded through current polarity-dependent commutation and adaptive dead time, minimizing IGBT switching losses and turn-off overvoltages without compromising the voltage-current dual-loop steady-state objective.

[0051] Soft-switching control does not replace the steady-state regulation of the dual closed-loop system, but operates as a parallel coordination layer. The soft-switching coordinator adjusts the bridge arm commutation sequence and dead zone based on the polarity of the primary phase current, the amplitude of the commutation current, the terminal voltage of the devices before turn-on, and the conduction state of the anti-parallel diodes. It also issues auxiliary soft-switching branch commands when the natural commutation energy is insufficient. The control priority is set as follows: fault protection is the highest, followed by current limiting, then output voltage regulation, and finally soft-switching optimization, to avoid disrupting the electrolyzer's safe operating point in order to maintain ZVS.

[0052] The outer loop of the output voltage uses the common output capacitor and the voltage Vo across the electrolytic cell as feedback quantities. The output voltage is given by V. ref Generated from the target operating point. Voltage error is defined as: e v (t) = Vref - Vo(t) The outer voltage loop uses a PI controller, whose output serves as the total current reference. The outer voltage loop does not directly output the duty cycle; instead, it indirectly regulates the output power through the current reference. This avoids the PI controller momentarily providing an excessively large duty cycle when the output voltage deviation is large. For electrolytic cell applications, current limiting takes precedence over voltage recovery; when the electrolytic cell current reaches its upper limit, the output voltage is allowed to temporarily drop below the reference value, but the current cannot be further increased.

[0053] The inner current loop applies current to the three-phase high-frequency transformer via a duty cycle-adjustable NPC bridge. As the duty cycle increases, the average effect of the secondary-side rectified voltage increases, and the inductor current in the three branches rises; conversely, as the duty cycle decreases, the total inductor current decreases. Since the inner loop directly controls the energy storage element current, its dynamic speed should be significantly higher than that of the outer voltage loop. The currents in the three branches need to be measured separately to achieve single-branch overcurrent and uneven current monitoring; the common duty cycle can only adjust the total current and cannot actively eliminate branch imbalances caused by transformer turns ratio, diode voltage drop, and inductor parameter deviations.

[0054] Natural Zero Voltage Turn-On (ZVS) primarily utilizes the energy stored in the primary-side series inductance, transformer leakage inductance, and the load current referred to the primary side. After the original conducting device turns off, the inductor current in the commutation circuit cannot change abruptly; therefore, the commutation current continues to flow, charging and discharging the output capacitor of the commutating device and the external commutation capacitor. When the anti-parallel diode of the device to be turned on naturally conducts, clamping its collector-emitter voltage VCE to near zero, applying a gate drive signal achieves zero-voltage turn-on of the device.

[0055] To ensure the completion of the natural commutation process, the energy stored in the equivalent commutation inductor should be no less than the energy required for the equivalent capacitor participating in the commutation to complete charging and discharging. Simultaneously, the dead time should be no less than the shortest time required for the equivalent capacitor participating in the commutation to complete charging and discharging, and should be less than the effective conduction time window of the anti-parallel diode. To improve the soft-switching adaptability under different load conditions, an adaptive dead time can be set according to the real-time commutation current.

[0056] With phase S A1 ~S A4 For example, when transitioning from O to P, keep S2 on and first turn off S2. A3 Waiting for the dead zone to end and S A1 S is turned on after the anti-parallel diode meets the conduction condition. A1 When P→O, first turn off S.A1 Further delay in opening S A3 The O→N and N→O processes occur in S, respectively. A2 With S A4 The same principle applies to commutation between phases. The three-phase switching sequence must remain strictly symmetrical, and any soft-switching correction must not introduce DC volt-second bias or illegal shoot-through.

[0057] The soft-switching coordinator operates in parallel with the voltage and current dual closed loops. Its inputs include the three-phase primary currents IPA, IPB, and IPC, the output voltage and current, the NPC midpoint voltage, and the VCE state before the switch is turned on. The coordinator first selects a commutation sequence that is conducive to the natural conduction of the anti-parallel diodes based on the phase current polarity, and then adjusts the dead zone according to the commutation current amplitude; if the commutation current is lower than the natural ZVS threshold, the average duty cycle of the dual closed loops remains unchanged, and only the zero-level dwell time within the allowable range is extended.

[0058] In this embodiment, the switch numbers of the three-phase NPC bridge can be divided according to phase: phase A is S. A1 ~S A4 Phase B is S B1 ~S B4 C phase is S C1 ~S C4 Each phase arm of the bridge forms a positive level, a midpoint clamping level, and a negative level through the cooperation of the upper switch, the inner switch, the lower switch, and the clamping diode. The three-phase drive signals have the same duty cycle and are staggered by 1 / 3 of a switching cycle. Initially, the current ia of phase A is negative, and the current returns to the phase A arm from the primary side of the transformer through the series inductor. At this time, phase A is still in the lower commutation or freewheeling state. The control system will switch phase A from its original state to the upper positive level state at time t0. Phases B and C are respectively in the conducting or clamping state corresponding to their 120° phase shift positions. Since the primary side of the three-phase transformer adopts a star connection, the three-phase currents satisfy the instantaneous constraints as follows: ia + ib + ic = 0 Phase 1: At time t0, S A3 When turned off, since the current in the series inductor and the transformer leakage inductance cannot change abruptly, the negative direction of i... a It does not disappear immediately, but naturally transfers to the upper switch S. A1 S A2 The anti-parallel diodes D1 and D2. At time T1, S A1 S A2 The gate signal is obtained during the conduction of D1 and D2. Since ia < 0, the current actually flows from the primary side of the transformer to the A-phase bridge arm, and returns to the positive terminal of the DC bus through the upper anti-parallel diode. Its main path is: Transformer TA primary side → L A→a→D2→D1→DC bus positive terminal. Although S... A1 S A2 The gate signal may already be high, but the actual current is carried by D1 and D2. The prior conduction of the diodes will cause S... A1 S A2 The voltage across both terminals is clamped to near zero, creating conditions for zero-voltage turn-on. During this stage, phase B primarily maintains its lower conducting state, while phase C maintains its current clamped or freewheeling state. The three phases together form a closed loop for the primary current. The transformer secondary side D8 is conducting. VL A When the slope is positive, the slope of the A-phase current is positive, and ia gradually increases from a negative value to zero. When ia = 0, the first stage ends.

[0059] Phase Two: The current naturally commutates, and at the moment when the A-phase current crosses zero, S A1 S A2 The gate signal has already been obtained in the previous stage, therefore when i a After the current changes from negative to positive, it will naturally transfer from D1 and D2 to S. A1 S A2 At this time, the forward current path of phase A is the positive terminal of the DC bus → S. A1 →S A2 →a→L A →T A Original Edge Due to S A1 S A2 A gate signal has been applied during the conduction of its anti-parallel diode, and its terminal voltage is close to zero at the moment of turn-on, thus constituting zero-voltage turn-on. As the current in phase A changes from negative to positive, D8 on the secondary side of the transformer turns off, and D7 turns on.

[0060] Phase Three: S B4 Turn off; previously, the B-phase current mainly flowed through S. B3 S B4 This forms the lower conduction path. Since the B-phase current cannot change abruptly, S... B4 After being turned off, the current is transferred to S. B3 The midpoint freewheeling current is formed by the clamping diode D6 in phase B. The path is b→S. B3 →D4→DC midpoint O. Although at this time the S of phase A... A1 S A2 The circuit remains on, but after phase B switches from negative level to midpoint clamping state, the three-phase floating star point potential changes, therefore the voltage vL of the series inductor in phase A changes. A New steps or notches will appear. The current in phase A remains continuous, only its slope changes. The third stage continues until the phase C switch S... C2The circuit is then shut off. Subsequently, phase C undergoes clamping commutation similar to phase B, and the system enters the next operating phase.

[0061] Phase Four: S C2 When switched off, the direction of the C-phase current cannot change abruptly; the current flows from the negative terminal of the busbar through the S-phase. C3 S C4 The freewheeling diode is in operation. When S... C2 After the circuit is turned off, because the energy stored in the transformer leakage inductance and series inductance cannot be released instantaneously, the magnitude and direction of the C-phase current cannot change abruptly, and the original current must continue to flow continuously. At this time, the output node potential of the C-phase bridge arm gradually changes towards the negative terminal of the DC bus under the action of the inductor current, and completes the charging and discharging of the parasitic capacitance of the bridge arm device. When the node potential drops to the negative bus potential, S C3 and S C4 The anti-parallel freewheeling diode naturally conducts in the forward direction, and the C-phase current is transferred from the original switching transistor conduction branch to the diode freewheeling branch.

[0062] The freewheeling current path is: DC bus negative terminal → S C4 anti-parallel diode → S C3 The anti-parallel diodes connect to the output of phase C, then to the series inductor of phase C and the primary winding of the transformer, and finally back to the DC bus. During this process, the primary current of the transformer remains continuous, and the subsequent rectifier continues to transfer energy to the output filter and load through the corresponding conducting diodes. After commutation, the output of phase C is clamped to the negative level of the DC bus. If the total voltage of the DC bus is Udc, then the output voltage of phase C is ucN = -2Udc.

[0063] At this time S C2 The current in S has naturally decreased to zero, and the current has been completely transferred to S. C3 S C4 In the freewheeling diode. If S is subsequently supplied during the conduction period of the freewheeling diode. C3 and S C4 When a drive signal is applied, the voltage across the two switching transistors is close to zero, which creates conditions for them to achieve zero-voltage turn-on, thereby reducing commutation losses and switching voltage spikes.

[0064] Phase 5: When the C-phase current crosses zero from the positive direction and then turns negative, S C3 S C4 The C-phase current is switched on and takes over the conduction of the freewheeling diode. The C-phase current flows from the primary side of the transformer and the inductor L. C Outflow, via S C3 S C4 Returning to the negative terminal of the DC bus, the C-phase bridge arm outputs a negative level. Simultaneously, the polarity of the transformer secondary winding voltage reverses, and D...11 Reverse cutoff, D 12 Forward conduction, secondary current flows through D 12 It transfers energy to the output filter inductor, capacitor and load.

[0065] Phase 6: When S A1 After being turned off, the inductor current of phase A cannot change abruptly; the original forward current is switched by S. A1 The current is transferred to the clamping diode D1 for freewheeling. The current path is midpoint O → D1 → A-phase inductor and primary winding of the transformer, causing the output potential of the A-phase bridge arm to switch from positive level and clamp at the midpoint potential. During this stage, the polarity of the transformer secondary voltage has not changed, so the conduction state of the secondary rectifier diode remains unchanged, and energy continues to be transferred to the output side.

[0066] Phase 7: When S B3 After being turned off, since the inductor current of phase B cannot change abruptly, the original current is transferred to phase S. B1 S B2 The current flows through the anti-parallel diodes. The current flows out from the B-phase inductor and the primary side of the transformer, and is fed back to the positive terminal of the DC bus through the two anti-parallel diodes, causing the output potential of the B-phase bridge arm to gradually rise and be clamped at a positive level, thus completing the commutation process of that bridge arm.

[0067] Phase 8: When the inductance L B After the current in S drops to zero and reverses, B1 S B2 When switched on, the B-phase current starts from the positive terminal of the DC bus and flows through S... B1 S B2 Flow to inductor L B The primary winding of the transformer, and the B-phase bridge arm output a positive level. Simultaneously, the polarity of the secondary voltage of the transformer changes, D... 10 When reverse-biased, D9 is forward-biased, and the secondary current transfers energy to the output filter and load through D9.

[0068] Phase Nine: S C4 When the circuit is closed, current flows through D6 to the midpoint, and the secondary-side switch remains unchanged. When S... C4 After shutdown, since the current in the C-phase inductor cannot change abruptly, the original current is transferred to the clamping diode D6 for freewheeling. The current flows out from the C-phase inductor and the primary side of the transformer, returning to the DC bus midpoint O via D6, causing the C-phase bridge arm output to switch from a negative level and clamp to zero. During this stage, the polarity of the transformer secondary voltage remains unchanged; therefore, the conduction state of the secondary rectifier diodes remains unchanged, continuing to transfer energy to the output filter and load.

[0069] Phase 10: When S A2 After being turned off, due to the inductor L A The current in the middle cannot change abruptly; the current in phase A is transferred to phase S. A3 S A4 The current freewheels through the anti-parallel diodes. The current originates from the negative terminal of the DC bus, flows through the two anti-parallel diodes, and then into the inductor L. A The primary winding of the transformer causes the output of phase A bridge arm to gradually decrease and clamp at a negative level, completing the commutation process from zero level to negative level.

[0070] Phase 11 When the current in phase A changes from positive to negative, the switching transistor S... A3 S A4 The circuit is turned on. Current flows from inductor L. A via S A3 S A4 The current flows to the negative terminal of the bus. At this point, inductor L... A The positive half-cycle ends, and the negative half-cycle is similar to the positive half-cycle, so it will not be described again. When the current in phase A drops to zero and changes from positive to negative, S... A3 S A4 When the circuit is turned on, the current naturally transfers from the anti-parallel diode to the switching transistor. At this time, the current flows from the primary side of the transformer and the inductor L. A Outflow, via S A3 S A4 Returning to the negative terminal of the DC bus, the A-phase bridge arm outputs a negative level. This completes the positive half-cycle of the circuit's operation. The negative half-cycle commutation process is essentially symmetrical to the positive half-cycle, and therefore will not be described further.

[0071] In this embodiment, the split bus pre-charge is completed during startup, and the upper and lower capacitor voltages and midpoint deviation are checked. Then, the three-phase bridge arms are kept at zero level and PWM is enabled. The output voltage command is ramped up, with the outer voltage loop gradually increasing the total inductor current command, while the inner current loop limits the output capacitor charging current and the current of the three branches. The soft-switching coordinator engages the adaptive dead zone after the commutation current is established. If ZVS cannot be met temporarily during the initial startup phase, overcurrent and overvoltage safety should be prioritized, and excessive circulating current should not be artificially injected. During normal operation, total inductor current limiting, single-branch overcurrent, output overvoltage, input over / undervoltage, NPC midpoint imbalance, transformer magnetic bias, IGBT overtemperature, and DESAT short-circuit protection are set. If the soft-switching monitoring finds that VCE has not dropped to a low level for a long time before turn-on, the dead zone within the allowable range can be increased, auxiliary branches can be engaged, or the power command of the operating area can be reduced in sequence. Hardware overcurrent and short-circuit protection must not be disabled for any soft-switching fault.

[0072] The above are merely preferred embodiments of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A high-power hydrogen production converter for medium-voltage DC buses, characterized in that, include: The system comprises a three-phase NPC three-level bridge, a three-phase high-frequency isolation transformer, and a current tripler rectifier. The DC input terminal of the three-phase NPC three-level bridge is used to connect to the medium-voltage DC bus, and the primary side of the three-phase high-frequency isolation transformer is connected to the AC output terminal of the three-phase NPC three-level bridge. The current tripler rectifier includes three center-tapped full-wave rectifier branches. Each center-tapped full-wave rectifier branch includes two rectifier diodes and one output filter inductor. The three center taps of the three center-tapped full-wave rectifier branches are connected together to form the negative output terminal. The output terminals of the three output filter inductors are connected together to form the positive output terminal. Each phase arm of the three-phase NPC three-level bridge includes four main switching transistors and two clamping diodes. The DC input terminal of the three-phase NPC three-level bridge is provided with a split DC bus consisting of two split bus capacitors connected in series. The common terminal of the two split bus capacitors serves as the clamping midpoint of the three-phase NPC three-level bridge.

2. The high-power hydrogen production converter for medium-voltage DC buses according to claim 1, characterized in that, It also includes a midpoint potential balancing device, which consists of an upper switch, a lower switch, and a balancing inductor. One end of the upper switch is connected to the positive bus of the split DC bus, and the other end of the upper switch is connected to one end of the lower switch. The other end of the lower switch is connected to the negative bus of the split DC bus, and the balancing inductor is connected between the common terminal of the upper switch and the lower switch and the clamping midpoint.

3. The high-power hydrogen production converter for medium-voltage DC buses according to claim 1, characterized in that, An inductor is connected in series between the AC output terminal of each phase arm of the three-phase NPC three-level bridge and the primary side of the three-phase high-frequency isolation transformer. The inductor is the equivalent inductance of the leakage inductance of the three-phase high-frequency isolation transformer and the external series inductor.

4. The high-power hydrogen production converter for medium-voltage DC buses according to claim 1, characterized in that, The three-phase high-frequency isolation transformer consists of three single-phase high-frequency transformers. Each single-phase high-frequency transformer has one primary winding and two secondary half windings. The primary side of the three-phase high-frequency isolation transformer is connected in a star configuration, and the secondary side is also connected in a star configuration.

5. The high-power hydrogen production converter for medium-voltage DC buses according to claim 4, characterized in that, In each center-tap full-wave rectifier branch, the anodes of the two rectifier diodes are respectively connected to the outer ends of the two secondary half-windings of the corresponding single-phase high-frequency transformer, and the cathodes of the two rectifier diodes are connected together and then connected to the input end of the corresponding output filter inductor.

6. The high-power hydrogen production converter for medium-voltage DC buses according to claim 1, characterized in that, A common output capacitor is connected in parallel between the positive and negative output terminals, and the common output capacitor is used to connect in parallel with the electrolytic cell.

7. A control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus, applied to the high-power hydrogen production converter oriented towards a medium-voltage DC bus as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Apply the same symmetrical duty cycle pulse width modulation signal to each phase arm. The pulse width modulation signals of each phase arm are staggered by 120° phase angle. The inner switches of each phase arm operate in a complementary manner with a fixed duty cycle, and the outer switches generate active pulses in the positive half-cycle and negative half-cycle respectively. Step 2: Acquire the output voltage, compare the output voltage with the given voltage, and then perform proportional-integral calculation to output the total current given value; Step 3: Collect the branch current of the three center-tap full-wave rectifier branches, compare the sum of the three branch currents with the given total current value, and then adjust the duty cycle by proportional-integral calculation; Step 4: Collect the voltage of the upper capacitor and the voltage of the lower capacitor. When the voltage of the upper capacitor is not equal to the voltage of the lower capacitor, control the upper or lower switch of the midpoint potential balancing device to operate, so that energy is transferred between the upper and lower capacitors until the voltages are equal.

8. The control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus according to claim 7, characterized in that, In step one, the fixed duty cycle of the inner switch is 50%, and the width of the active pulse generated by the outer switch is DTs / 2, where D is the duty cycle and Ts is the switching period.

9. The control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus according to claim 7, characterized in that, In step three, when the current in the electrolytic cell reaches a preset upper limit, current limiting is performed first, allowing the output voltage to be temporarily lower than the given voltage.

10. The control method for a high-power hydrogen production converter oriented towards a medium-voltage DC bus according to claim 7, characterized in that, It also includes soft-switching coordination steps: collecting the polarity of the primary current, the amplitude of the commutation current, and the collector-emitter voltage state before the switching transistor is turned on for each phase arm; selecting the commutation sequence according to the polarity of the primary current; adjusting the dead time according to the amplitude of the commutation current; and keeping the average value of the duty cycle unchanged and extending the zero-level dwell time when the commutation current is lower than the natural zero voltage turn-on threshold.

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