Series resonant direct-current transformer and control method for magnetic bias suppression of series resonant direct-current transformer

Abstract

The invention discloses a series resonant DC transformer and a control method for magnetic bias suppression thereof, and the DC transformer consists of a low-voltage-side circuit breaker, a low-voltage-side DC support capacitor, a low-voltage-side full-bridge inverter, a resonant capacitor, an isolation transformer, a high-voltage-side DC support capacitor and a rectification circuit. During starting, pre-charging is carried out, a low-voltage side direct-current supporting capacitor is charged, and after charging is completed, progressive phase-shifting control is carried out on a low-voltage side full-bridge inverter in the series resonant direct-current transformer; a normal operation stage is carried out after pre-charging is completed, when shutdown occurs, resonance current flowing through leakage inductance of an isolation transformer is gradually reduced by controlling a low-voltage side circuit breaker and a low-voltage side full-bridge inverter, and non-magnetic-bias shutdown is carried out; and when a fault occurs, the series resonant direct-current transformer is locked according to the exciting current. According to the invention, magnetic bias suppression can be achieved without external demagnetization equipment or an auxiliary circuit.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of direct current power distribution, and more particularly relates to a series resonant direct current transformer and a control method for suppressing magnetic bias thereof. BACKGROUND

[0002] In a high-voltage direct current (HVDC) power distribution system, a direct current transformer (DCT) is a core device for realizing voltage level conversion and electrical isolation. However, the DCT often faces the problem of isolated transformer magnetic bias in actual operation, that is, the residual magnetism of the transformer core accumulates due to the direct current magnetizing current, resulting in unbalanced magnetic flux. The magnetic bias phenomenon not only causes the excitation inrush current, but also can cause the core saturation, thereby threatening the safety of the switching device and even causing system failure. The main reasons for the magnetic bias include: asymmetric driving signals, leading to unbalanced positive and negative half excitation; driving circuit delay or pulse distortion, causing magnetic flux excitation difference; switching element conduction characteristics are inconsistent, forming a direct current bias. The traditional demagnetization method has the following disadvantages: first, the special demagnetization device has high cost and complex structure, and second, the application of instantaneous reverse magnetic energy may introduce additional stress. In the prior art, although the series resonant direct current transformer (SRC-DCT) has the advantages of high energy transmission and soft switching, it is easy to cause excitation inrush current due to magnetic bias during startup and shutdown, threatening the safety of the switching device. SUMMARY

[0003] To solve the problems in the prior art, the application provides a series resonant direct current transformer and a control method for suppressing magnetic bias thereof.

[0004] The application adopts the following technical solutions.

[0005] The first aspect of the application provides a series resonant direct current transformer, which comprises a low-voltage side circuit breaker, a low-voltage side direct current support capacitor, a low-voltage side full-bridge inverter, a resonant capacitor, an isolation transformer, a high-voltage side direct current support capacitor and a rectifier circuit, and comprises:

[0006] The low-voltage side circuit breaker comprises two contacts, and the two contacts are simultaneously closed when the low-voltage side circuit breaker is closed or opened. The positive and negative poles of the low-voltage bus voltage are connected to the two input ends of the low-voltage side full-bridge inverter through one contact. The low-voltage side direct current support capacitor is connected in parallel between the two input ends of the low-voltage side full-bridge inverter. When the series resonant direct current transformer starts, the low-voltage side circuit breaker is closed. When shutdown occurs, the low-voltage side circuit breaker is opened, and the low-voltage side full-bridge inverter remains in the working state of the normal operation stage during shutdown;

[0007] The first output terminal of the low-voltage side full-bridge inverter is connected to the second output terminal of the low-voltage side full-bridge inverter in sequence through a resonant capacitor, the leakage inductance of the isolation transformer, and the primary winding of the transformer; the two ends of the secondary winding of the transformer are connected to the two input terminals of the rectifier circuit respectively; the DC support capacitor on the high-voltage side is connected in parallel between the two output terminals of the rectifier circuit.

[0008] Preferably, the low-voltage side full-bridge inverter includes an anode reactor, a clamping resistor, a clamping diode, a clamping capacitor, and an IGCT full-bridge inverter circuit;

[0009] The first input terminal of the low-voltage side full-bridge inverter is connected to the second input terminal of the low-voltage side full-bridge inverter in sequence through an anode reactor, a clamping diode, and a clamping capacitor. The positive terminal of the clamping diode is connected to one end of the anode reactor, and the negative terminal of the clamping diode is connected to one end of the clamping capacitor. One end of the clamping resistor is connected to the other end of the anode reactor, and the other end of the clamping resistor is connected to the negative terminal of the clamping diode. The two input terminals of the IGCT full-bridge inverter circuit are connected to the positive terminal of the clamping diode and the other end of the clamping capacitor, respectively.

[0010] Preferably, the rectifier circuit is a diode uncontrolled rectifier circuit, which includes four bridge arms. Each bridge arm is composed of multiple rectifier units connected in series, and each rectifier unit is composed of a diode and a voltage equalizing capacitor connected in parallel.

[0011] A second aspect of the present invention provides a control method for suppressing bias magnetization in a series resonant DC transformer as described in the first aspect of the present invention, comprising:

[0012] When starting the series resonant DC transformer, pre-charging is performed, which involves charging the DC support capacitor on the low-voltage side. After charging is completed, progressive phase-shift control is applied to the low-voltage side full-bridge inverter in the series resonant DC transformer.

[0013] After pre-charging is completed, the normal operation phase begins. During the normal operation phase, the low-voltage bus voltage is maintained at the rated voltage of the low-voltage side, and the energy transfer is controlled by the low-voltage side full-bridge inverter through a fixed frequency and duty cycle.

[0014] When a shutdown occurs, the resonant current flowing through the leakage inductance of the isolation transformer is gradually reduced by controlling the low-voltage side circuit breaker and the low-voltage side full-bridge inverter, thus achieving a non-biased shutdown.

[0015] When a fault occurs, the series resonant DC transformer is locked out according to the excitation current.

[0016] Preferably, the charging of the low-voltage side DC support capacitor is performed by closing the low-voltage side circuit breaker, and the low-voltage side DC support capacitor is charged by the low-voltage bus voltage. The low-voltage bus voltage rises with time according to a set exponential curve until the low-voltage side DC support capacitor reaches the low-voltage side rated voltage.

[0017] Preferably, the progressive phase-shift control specifically includes:

[0018] Phase-shift modulation is performed on all upper bridge arms of the low-voltage side full-bridge inverter. The initial phase shift angle of all upper bridge arms is set to 0. The phase shift angle of all upper bridge arms is gradually adjusted according to the set step size until the set pre-charge condition is reached. The step size is less than 1 / 100T, where T is the period of the switching frequency. The phase of all lower bridge arms is out of phase with the phase of the upper bridge arms.

[0019] Preferably, the pre-charging condition is that the high-voltage side voltage reaches 1.03 times the turns ratio of the isolation transformer in the series resonant DC transformer, and then multiplied by the current low-voltage bus voltage.

[0020] Preferably, during the normal operation phase, the low-voltage bus voltage is maintained at the rated voltage on the low-voltage side, and energy transfer is performed by controlling the low-voltage side full-bridge inverter through a fixed frequency and duty cycle, specifically as follows:

[0021] The low-voltage side circuit breaker remains closed, and the low-voltage bus voltage is always the low-voltage side rated voltage.

[0022] The low-voltage side full-bridge inverter operates normally as follows: it operates at a frequency lower than a fixed frequency, where the fixed frequency is... Where f is a fixed frequency; L r The leakage inductance integrated in the isolation transformer of a series resonant DC transformer; C r The capacitance value of the resonant capacitor in the series resonant DC transformer; the phase shift angle of all upper bridge arms is kept at 180°; the phase of all lower bridge arms is out of phase with the phase of the upper bridge arms, and the duty cycle is set to 50%.

[0023] Preferably, the step of gradually reducing the resonant current flowing through the leakage inductance of the isolation transformer by controlling the low-voltage side circuit breaker and the low-voltage side full-bridge inverter specifically involves:

[0024] When a shutdown occurs, the low-voltage side circuit breaker is tripped, and the low-voltage side bus voltage decreases over time until it eventually drops to 0V; the low-voltage side full-bridge inverter maintains its normal operating state.

[0025] Preferably, when a fault occurs, the low-voltage bus voltage is the rated voltage of the low-voltage side, the excitation current is obtained, and when the excitation current crosses zero, the drive signals of all power switching devices in the low-voltage side full-bridge inverter are blocked to achieve the blocking of the series resonant DC transformer.

[0026] The beneficial effects of this invention are as follows: Compared with the prior art, this invention charges the DC support capacitor on the low-voltage side through an exponentially increasing low-voltage bus during pre-charging, which is simple, efficient, universal, and practical. It also performs progressive phase-shift control on the low-voltage side transformer, ensuring that the transformer flux and high-voltage side voltage Us are smoothly established from zero through dynamic modulation, while suppressing bias magnetism and improving system reliability. During shutdown, demagnetization is achieved by alternately applying DC current in the forward and reverse directions at both ends of the transformer high-voltage winding and gradually reducing the current to reduce the hysteresis loop of the iron core. In case of a fault, the residual magnetism is reflected by the excitation current, and the timing of the lockout is controlled so that the residual magnetism of the transformer is zero at the moment of lockout. Reducing the residual magnetism can significantly reduce the current and voltage overshoot of the IGCT and LC resonant elements. This invention is implemented in software, eliminating the need for external demagnetization equipment or auxiliary circuits, thus reducing costs and losses. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the topology of the DC transformer of the present invention;

[0028] Figure 2 The phase shift angle and voltage waveform of the low-voltage side inverter output voltage Vab during the pre-charging phase;

[0029] Figure 3 The voltage waveforms of the drive pulses and the low-voltage side inverter output voltage Vab during normal operation.

[0030] Figure 4 This refers to the drive pulses and the voltage waveform of the low-voltage side inverter output voltage Vab during the shutdown operation phase.

[0031] Figure 5 This is a schematic diagram of the excitation current at the moment of fault lockout. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0033] like Figure 1As shown, Embodiment 1 of the present invention proposes a series resonant DC transformer, including a low-voltage side circuit breaker QF, a low-voltage side DC support capacitor Cp, a low-voltage side full-bridge inverter, a resonant capacitor Cr, an isolation transformer T, a high-voltage side DC support capacitor Cs, and a rectifier circuit, including:

[0034] The low-voltage side circuit breaker QF includes two contacts. When the low-voltage side circuit breaker QF is closed or closed, both contacts are closed simultaneously. The positive and negative terminals of the low-voltage bus voltage are respectively connected to the two input terminals of the low-voltage side full-bridge inverter through one contact. The low-voltage side DC support capacitor Cp is connected in parallel between the two input terminals of the low-voltage side full-bridge inverter. When the series resonant DC transformer starts, the low-voltage side circuit breaker QF is closed. When a shutdown occurs, the low-voltage side circuit breaker QF is opened, and the low-voltage side full-bridge inverter maintains the normal operating state during the shutdown.

[0035] The first output terminal of the low-voltage side full-bridge inverter is connected to the second output terminal of the low-voltage side full-bridge inverter in sequence through the resonant capacitor Cr, the leakage inductance Lr of the isolation transformer T, and the primary winding of the transformer; the two ends of the secondary winding of the transformer are connected to the two input terminals of the rectifier circuit respectively; the DC support capacitor Cs on the high-voltage side is connected in parallel between the two output terminals of the rectifier circuit.

[0036] In this preferred embodiment, the low-voltage side full-bridge inverter includes an anode reactance Lbuf, a clamping resistor Rbuf, a clamping diode Dbuf, a clamping capacitor Cbuf, and an IGCT full-bridge inverter circuit.

[0037] The first input terminal of the low-voltage side full-bridge inverter is connected to the second input terminal of the low-voltage side full-bridge inverter in sequence through the anode reactor Lbuf, the clamping diode Dbuf, and the clamping capacitor Cbuf. The positive terminal of the clamping diode Dbuf is connected to one end of the anode reactor Lbuf, and the negative terminal of the clamping diode Dbuf is connected to one end of the clamping capacitor Cbuf. One end of the clamping resistor Rbuf is connected to the other end of the anode reactor Lbuf, and the other end of the clamping resistor Rbuf is connected to the negative terminal of the clamping diode Dbuf. The two input terminals of the IGCT full-bridge inverter circuit are connected to the positive terminal of the clamping diode Dbuf and the other end of the clamping capacitor Cbuf, respectively.

[0038] The IGCT full-bridge inverter circuit consists of a full-bridge structure composed of four IGCTs (T1, T2, T3, T4), with a diode connected in parallel to each IGCT.

[0039] In this preferred embodiment, the rectifier circuit is a diode uncontrolled rectifier circuit, which includes four bridge arms. Each bridge arm consists of multiple rectifier units connected in series, and each rectifier unit consists of a diode and a voltage-equalizing capacitor connected in parallel. Specifically, each bridge arm in this embodiment consists of four rectifier units connected in series.

[0040] Embodiment 2 of the present invention proposes a control method for suppressing bias magnetization applied to the series resonant DC transformer described in Embodiment 1 of the present invention, comprising:

[0041] It should be noted that the initial state before starting the series resonant DC transformer is as follows: the system is in a locked state, the low-voltage side circuit breaker QF is open, and the high-voltage side voltage Us = 0. The initial voltages of the low-voltage side and high-voltage side supporting capacitors Cp and Cs are zero.

[0042] When starting the series resonant DC transformer, pre-charging is performed. The pre-charging is to charge the low-voltage side DC support capacitor Cp. After the charging is completed, the low-voltage side full-bridge inverter in the series resonant DC transformer is subjected to progressive phase-shift control.

[0043] After pre-charging is completed, the normal operation phase begins. During the normal operation phase, the low-voltage bus voltage Up is maintained at the rated voltage of the low-voltage side, and the energy transfer is controlled by the low-voltage side full-bridge inverter through a fixed frequency and duty cycle.

[0044] When a shutdown occurs, the resonant current flowing through the leakage inductance Lr of the isolation transformer T is gradually reduced by controlling the low-voltage side circuit breaker QF and the low-voltage side full-bridge inverter, thus achieving a non-biased shutdown.

[0045] When a fault occurs, the series resonant DC transformer is locked out according to the excitation current.

[0046] In this preferred embodiment, the charging of the low-voltage side DC support capacitor Cp is performed by closing the low-voltage side circuit breaker QF. The low-voltage side DC support capacitor Cp is charged by the low-voltage bus voltage Up. The low-voltage bus voltage Up rises with time according to a set exponential curve until the low-voltage side DC support capacitor Cp reaches the low-voltage side rated voltage.

[0047] In this preferred embodiment, the progressive phase-shift control specifically refers to:

[0048] Phase-shift modulation is applied to all upper arms of the low-voltage side full-bridge inverter. The initial phase shift angle of all upper arms is set to 0, and the phase shift angle of all upper arms is gradually adjusted according to a set step size until the set pre-charge condition is reached. The step size is less than 1 / 100T, where T is the period of the switching frequency. The phases of all lower arms are out of phase with the phases of the upper arms. The phase shift angle during the pre-charge phase and the voltage waveform of the low-voltage side inverter output voltage Vab are shown below. Figure 2 As shown.

[0049] It should be noted that the step size is less than 1 / 100T to ensure that it is small enough to avoid sudden current changes. During the entire pre-charging process, the excitation inductor voltage and time integral are approximately 0, which can avoid inrush current and ensure that the transformer flux and high-voltage side voltage Us are smoothly established from zero, while suppressing bias magnetization.

[0050] In this preferred embodiment, the pre-charging condition is that the high-voltage side voltage reaches 1.03 times the turns ratio of the isolation transformer T in the series resonant DC transformer, and then multiplied by the current low-voltage bus voltage Up.

[0051] In this preferred embodiment, the normal operation phase involves maintaining the low-voltage bus voltage at the rated voltage on the low-voltage side, and controlling the low-voltage side full-bridge inverter for energy transfer through a fixed frequency and duty cycle. Specifically:

[0052] The low-voltage side circuit breaker QF remains closed, and the low-voltage bus voltage Up is always the low-voltage side rated voltage.

[0053] The low-voltage side full-bridge inverter operates normally as follows: it operates at a frequency lower than a fixed frequency, where the fixed frequency is... Where f is a fixed frequency; and L is a constant frequency. r The leakage inductance integrated in the isolation transformer T of a series resonant DC transformer; C r The capacitance value of the resonant capacitor Cr in the series resonant DC transformer is given; the phase shift angle of all upper bridge arms is maintained at 180°; the phase of all lower bridge arms is out of phase with the phase of the upper bridge arms, and the duty cycle is set to 50% to ensure ZCS (zero current switching). The drive pulse and voltage waveform of the low-voltage side inverter output voltage Vab during normal operation are shown below. Figure 3 As shown.

[0054] In this preferred embodiment, the resonant current flowing through the leakage inductance Lr of the isolation transformer T is gradually reduced by controlling the low-voltage side circuit breaker QF and the low-voltage side full-bridge inverter, specifically as follows:

[0055] When a shutdown occurs, the low-voltage side circuit breaker QF is tripped, and the low-voltage side bus voltage decreases over time until it eventually drops to 0V; the low-voltage side full-bridge inverter maintains its normal operating state.

[0056] It should be noted that during normal shutdown, the voltage source can be cut off by opening the circuit breaker on the input bus. Since the primary side of the series resonant DC transformer in this embodiment is a controllable full-bridge, and the secondary side is a diode-uncontrolled rectifier circuit, the transmission is unidirectional. In the unlocked state and with the power cut off, energy is transferred to the high-voltage side, and the low-voltage side voltage gradually decreases. Furthermore, the low-voltage side full-bridge inverter maintains its normal operating state, meaning the H-bridge on the primary side is still operating normally. The current in the resonant circuit gradually decreases, resulting in alternating forward and reverse DC currents with gradually decreasing amplitudes, reducing the core hysteresis loop, and achieving a non-biased shutdown. The drive pulses and the voltage waveform of the low-voltage side inverter output voltage Vab during the shutdown operation phase are shown below. Figure 4 As shown.

[0057] In this preferred embodiment, when a fault occurs, the low-voltage bus voltage is the rated voltage on the low-voltage side. The excitation current is obtained, and when the excitation current crosses zero, the drive signals of all power switching devices in the low-voltage side full-bridge inverter are blocked, thereby achieving the blocking of the series resonant DC transformer. In this embodiment, the power switching device is an IGCT.

[0058] It should be noted that when a transformer fault requires immediate blocking, the voltage on the low-voltage side of the transformer is the rated voltage on the low-voltage side, and the residual magnetism in the transformer is the integral of the excitation inductance voltage over time, which can be obtained from the excitation current I in the diagram. m This is reflected in the following: Once Im crosses zero (at 0.25T or 0.75T), immediately block all IGCT drive signals; Figure 5 It can be seen that the ideal blocking time is 0.25T or 0.75T, and the residual magnetism of the transformer is 0 at the moment of blocking.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention.

Claims

1. A series resonant DC transformer, comprising a low-voltage side circuit breaker, a low-voltage side DC support capacitor, a low-voltage side full-bridge inverter, a resonant capacitor, an isolation transformer, a high-voltage side DC support capacitor, and a rectifier circuit, characterized in that: The low-voltage side circuit breaker includes two contacts. When the low-voltage side circuit breaker is closed or closed, both contacts are closed simultaneously. The positive and negative terminals of the low-voltage bus voltage are respectively connected to the two input terminals of the low-voltage side full-bridge inverter through one contact. The low-voltage side DC support capacitor is connected in parallel between the two input terminals of the low-voltage side full-bridge inverter. When the series resonant DC transformer starts, the low-voltage side circuit breaker is closed. When a shutdown occurs, the low-voltage side circuit breaker is opened. During the shutdown, the low-voltage side full-bridge inverter maintains the normal operating state. The first output terminal of the low-voltage side full-bridge inverter is connected to the second output terminal of the low-voltage side full-bridge inverter in sequence through a resonant capacitor, the leakage inductance of the isolation transformer, and the primary winding of the transformer; the two ends of the secondary winding of the transformer are connected to the two input terminals of the rectifier circuit respectively; the DC support capacitor on the high-voltage side is connected in parallel between the two output terminals of the rectifier circuit.

2. The series resonant DC transformer according to claim 1, characterized in that: The low-voltage side full-bridge inverter includes an anode reactor, a clamping resistor, a clamping diode, a clamping capacitor, and an IGCT full-bridge inverter circuit. The first input terminal of the low-voltage side full-bridge inverter is connected to the second input terminal of the low-voltage side full-bridge inverter in sequence through an anode reactor, a clamping diode, and a clamping capacitor. The positive terminal of the clamping diode is connected to one end of the anode reactor, and the negative terminal of the clamping diode is connected to one end of the clamping capacitor. One end of the clamping resistor is connected to the other end of the anode reactor, and the other end of the clamping resistor is connected to the negative terminal of the clamping diode. The two input terminals of the IGCT full-bridge inverter circuit are connected to the positive terminal of the clamping diode and the other end of the clamping capacitor, respectively.

3. A series resonant DC transformer according to claim 1, characterized in that: The rectifier circuit is a diode uncontrolled rectifier circuit, which includes four bridge arms. Each bridge arm consists of multiple rectifier units connected in series, and each rectifier unit consists of a diode and a voltage equalizing capacitor connected in parallel.

4. A control method for suppressing bias magnetization applied to the series resonant DC transformer according to any one of claims 1-3, characterized in that, include: When starting the series resonant DC transformer, pre-charging is performed, which involves charging the DC support capacitor on the low-voltage side. After charging is completed, progressive phase-shift control is applied to the low-voltage side full-bridge inverter in the series resonant DC transformer. After pre-charging is completed, the normal operation phase begins. During the normal operation phase, the low-voltage bus voltage is maintained at the rated voltage of the low-voltage side, and the energy transfer is controlled by the low-voltage side full-bridge inverter through a fixed frequency and duty cycle. When a shutdown occurs, the resonant current flowing through the leakage inductance of the isolation transformer is gradually reduced by controlling the low-voltage side circuit breaker and the low-voltage side full-bridge inverter, thus achieving a non-biased shutdown. When a fault occurs, the series resonant DC transformer is locked out according to the excitation current.

5. The method for controlling bias suppression according to claim 4, characterized in that: The charging of the low-voltage side DC support capacitor is achieved by closing the low-voltage side circuit breaker. The low-voltage side DC support capacitor is charged by the low-voltage bus voltage. The low-voltage bus voltage rises with time according to a set exponential curve until the low-voltage side DC support capacitor reaches the low-voltage side rated voltage.

6. The method for controlling bias suppression according to claim 4, characterized in that: The progressive phase-shift control is specifically as follows: Phase-shift modulation is performed on all upper bridge arms of the low-voltage side full-bridge inverter. The initial phase shift angle of all upper bridge arms is set to 0. The phase shift angle of all upper bridge arms is gradually adjusted according to the set step size until the set pre-charge condition is reached. The step size is less than 1 / 100T, where T is the period of the switching frequency. The phase of all lower bridge arms is out of phase with the phase of the upper bridge arms.

7. The method for controlling bias suppression according to claim 6, characterized in that: The pre-charging condition is set as follows: the high-voltage side voltage reaches 1.03 times the turns ratio of the isolation transformer in the series resonant DC transformer, and then multiplied by the current low-voltage bus voltage.

8. The method for controlling bias suppression according to claim 4, characterized in that: During the normal operation phase, the low-voltage bus voltage is maintained at the rated voltage on the low-voltage side, and energy transfer is performed by controlling the low-voltage side full-bridge inverter through a fixed frequency and duty cycle. Specifically: The low-voltage side circuit breaker remains closed, and the low-voltage bus voltage is always the low-voltage side rated voltage. The low-voltage side full-bridge inverter operates normally as follows: it operates at a frequency lower than a fixed frequency, where the fixed frequency is... Where f is a fixed frequency; L r The leakage inductance integrated in the isolation transformer of a series resonant DC transformer; C r The capacitance value of the resonant capacitor in the series resonant DC transformer; the phase shift angle of all upper bridge arms is kept at 180°; the phase of all lower bridge arms is out of phase with the phase of the upper bridge arms, and the duty cycle is set to 50%.

9. The method for controlling bias suppression according to claim 4, characterized in that: The resonant current flowing through the leakage inductance of the isolation transformer is gradually reduced by controlling the low-voltage side circuit breaker and the low-voltage side full-bridge inverter, specifically as follows: When a shutdown occurs, the low-voltage side circuit breaker is tripped, and the low-voltage side bus voltage decreases over time until it eventually drops to 0V; the low-voltage side full-bridge inverter maintains its normal operating state.

10. The method for controlling bias suppression according to claim 4, characterized in that: When a fault occurs, the low-voltage bus voltage is the rated voltage of the low-voltage side, and the excitation current is obtained. When the excitation current crosses zero, the drive signals of all power switching devices in the low-voltage side full-bridge inverter are blocked, thereby blocking the series resonant DC transformer.

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