阅读说明：本技术 一种igct-mmc换流阀子模块箝位电路损耗计算方法和装置 (IGCT-MMC converter valve submodule clamping circuit loss calculation method and device ) 是由 郭明珠 赵彪 周兴达 白睿航 唐博进 周文鹏 郝峰杰 余占清 张若愚 曾嵘 于 2020-12-31 设计创作，主要内容包括：本发明公开了一种IGCT-MMC换流阀子模块箝位电路损耗计算方法和装置,计算方法包括如下步骤：建立二极管反向恢复过程的电流源等效模型；通过电流源等效模型对IGCT-MMC换流阀子模块投切进行等效；分析IGCT-MMC换流阀子模块投切引起的暂态过程；对暂态过程进行仿真；分析仿真结果,得到箝位电路损耗计算公式,计算出箝位电路损耗能量。本发明有效求解出IGCT-MMC换流阀子模块箝位电路损耗能量计算公式,其中包括二极管关断后反向恢复电流产生的能量损耗,有助于箝位电路损耗能量的精确定量分析,便于系统优化设计。(The invention discloses a method and a device for calculating the loss of a clamping circuit of an IGCT-MMC converter valve submodule, wherein the calculating method comprises the following steps: establishing a current source equivalent model of a diode reverse recovery process; carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model; analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module; simulating a transient process; and analyzing the simulation result to obtain a clamping circuit loss calculation formula, and calculating the clamping circuit loss energy. The invention effectively solves the energy consumption calculation formula of the clamping circuit of the submodule of the IGCT-MMC converter valve, wherein the energy consumption is generated by reverse recovery current after the diode is turned off, which is beneficial to the accurate quantitative analysis of the energy consumption of the clamping circuit and is convenient for the optimization design of the system.)

1. The method for calculating the loss of the clamping circuit of the submodule of the IGCT-MMC converter valve is characterized by comprising the following steps of:

establishing a current source equivalent model of a diode reverse recovery process;

carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

simulating a transient process;

and analyzing the simulation result to obtain a clamping circuit loss calculation formula, and calculating the clamping circuit loss energy.

2. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of claim 1, wherein the current source equivalent model specifically includes the following equivalent processes:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the current of the diode is reduced to a certain threshold value from the peak value of the reverse recovery current with the same current change rate di/dt;

the third equivalent process: the diode current changes from a certain threshold value to 0 over a longer period of time below the above-mentioned rate of change of current di/dt.

3. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of claim 2, wherein the simulation of the diode turn-off process is implemented according to the switching of the equivalent process control current source.

4. The IGCT-MMC converter valve sub-module clamp loss calculation method of claim 1, wherein the transient process comprises four transient processes:

a first transient process: submodule current direction i_SM>0, switching on before switching, switching off after switching, and anode reactance current i_LsThe change process is as follows: i is_b→-I_rr→0；

And a second transient process: submodule current direction i_SM>0, cut-off state before switching, put-in state after switching, and anode reactance current i_LsThe change process is as follows: 0 → I_b；

And a third transient process: submodule current direction i_SM<0, the input state before switching, the cut-off state after switching, yangPole reactance current i_LsThe change process is as follows: -I_b→0；

A fourth transient process: submodule current direction i_SM<0, cut-off state before switching, put-in state after switching, and anode reactance current i_LsThe change process is as follows: 0 → - (I)_b+I_rr)→-I_b；

Wherein, I_bFor switching the bridge arm current at the moment I_rrFor the diode forward current is I_bThe reverse recovery current peak in time.

5. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of claim 4, wherein the IGCT-MMC converter valve sub-module switching comprises the following four switching combinations:

a first switching combination: switching the first transient process to a second transient process;

and a second switching combination: the second transient process is switched to the first transient process;

and (3) third switching combination: the third transient process is switched to a fourth transient process;

a fourth switching combination: the fourth transient process is switched to the third transient process.

6. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of claim 4, wherein the clamp resistance dissipated energy during the first and fourth transients is approximately calculated by:

W_Rs＝0.28LsI_rr ²

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_rrFor the diode forward current is I_bThe reverse recovery current peak in time.

7. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of claim 4, wherein the clamp resistance dissipated energy in the second and third transient processes is approximately calculated by:

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_fIs the bridge arm current.

8. The IGCT-MMC converter valve sub-module clamp circuit loss calculation method of any of claims 5-7, wherein the clamp circuit loss energy of the four switching combinations is approximately calculated by the following formula:

wherein W is the loss energy of the clamping circuit, Ls is the anode reactance, I_rrFor the diode forward current is I_bTime reverse recovery current peak value, I_fIs the bridge arm current.

9. An IGCT-MMC converter valve sub-module clamp circuit loss determination device, the determination device comprising:

an equivalent model establishing unit: the current source equivalent model is used for establishing a diode reverse recovery process;

an equivalent analysis unit: the device is used for carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

a transient process analysis unit: the system is used for analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

a simulation unit: for simulating a transient process;

a simulation analysis unit: and analyzing the simulation result to obtain a clamping circuit loss calculation formula and calculate the clamping circuit loss energy.

10. The IGCT-MMC converter valve sub-module clamp circuit loss determination device of claim 9, wherein the equivalent model establishing unit establishes the current source equivalent model by an equivalent process of:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the current of the diode is reduced to a certain threshold value from the peak value of the reverse recovery current with the same current change rate di/dt;

the third equivalent process: the diode current changes from a certain threshold value to 0 over a longer period of time below the above-mentioned rate of change of current di/dt.

Technical Field

The invention belongs to the technical field of power electronics, and particularly relates to a method and a device for calculating the loss of a clamping circuit of an IGCT-MMC converter valve submodule.

Background

The direct current transmission technology has attracted extensive attention due to the advantages of good electric energy quality, large transmission capacity, high system stability, convenience for distributed energy access and the like. The converter is a core component of the direct current transmission technology, and the performance of the converter directly influences whether the direct current transmission technology can be popularized and applied on a large scale. The Modular Multilevel Converter (MMC) has good application prospect in the DC power transmission technology due to the outstanding advantages of high electric energy quality, good reliability, high output waveform quality, low power loss and the like. At present, an Insulated Gate Bipolar Transistor (IGBT) is adopted as a main power switch device of the MMC, and the IGBT has the characteristics of easiness in driving, large peak current capacity, self-turn-off, high switching frequency and the like. Compared with an IGBT, an Integrated Gate Commutated Thyristor (IGCT) has the advantages of lower on-state voltage drop, higher reliability, lower manufacturing cost, compact structure, higher blocking voltage and higher through-current capacity, and is expected to remarkably improve the performance of a voltage control device in the application of a high-voltage high-capacity flexible direct-current power transmission MMC converter valve. However, the IGCT device adopts a hard drive method, which cannot control the turn-on speed, and in order to limit the reverse recovery current when the IGCT is turned on and off, an anode reactance needs to be added between the dc capacitor and the dc bus, and in addition, in order to absorb the energy when the IGCT is turned off, an absorption loop composed of a clamping diode, a clamping resistor, and a clamping capacitor is used. The anode reactance, the clamping resistor, the clamping diode and the clamping capacitor are collectively referred to as a clamping circuit of the IGCT-MMC sub-module, and the selection of the elements of the clamping circuit plays a decisive role in the IGCT switching characteristics, particularly the on-current and the off-voltage, and is a necessary condition for the IGCT to exert high-power processing capacity. The conventional clamping circuit loss calculation method only considers the conduction loss and the disconnection loss of main devices, and does not calculate the energy loss generated by reverse recovery current after a diode is switched off, so that the obtained clamping circuit loss energy is not accurate, certain deviation is caused to the heat dissipation design of an MMC converter, the long-term stable operation of equipment is influenced, and the integral performance of a direct current transmission system is also adversely influenced.

Disclosure of Invention

Aiming at the problems, the invention discloses a method for calculating the loss of a clamping circuit of an IGCT-MMC converter valve submodule, which comprises the following steps:

establishing a current source equivalent model of a diode reverse recovery process;

carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

simulating a transient process;

and analyzing the simulation result to obtain a clamping circuit loss calculation formula, and calculating the clamping circuit loss energy.

Further, the current source equivalent model specifically includes the following equivalent processes:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the current of the diode is reduced to a certain threshold value from the peak value of the reverse recovery current with the same current change rate di/dt;

the third equivalent process: the diode current changes from a certain threshold value to 0 over a longer period of time below the above-mentioned rate of change of current di/dt.

Furthermore, the switch of the current source is controlled according to the equivalent process, so that the simulation of the turn-off process of the diode is realized.

Still further, the transient process includes the following four transient processes:

a first transient process: submodule current direction i_SM>0, switching on before switching, switching off after switching, and anode reactance current i_LsThe change process is as follows: i is_b→-I_rr→0；

And a second transient process: submodule current direction i_SM>0, cut-off state before switching, put-in state after switching, and anode reactance current i_LsThe change process is as follows: 0 → I_b；

And a third transient process: submodule current direction i_SM<0, switching on before switching, switching off after switching, and anode reactance current i_LsThe change process is as follows: -I_b→0；

A fourth transient process: submodule current direction i_SM<0, cut-off state before switching, put-in state after switching, and anode reactance current i_LsThe change process is as follows: 0 → - (I)_b+I_rr)→-I_b；

Wherein, I_bFor switching moment bridge armCurrent, I_rrFor the diode forward current is I_bThe reverse recovery current peak in time.

Furthermore, the switching of the IGCT-MMC converter valve sub-module comprises the following four switching combinations:

a first switching combination: switching the first transient process to a second transient process;

and a second switching combination: the second transient process is switched to the first transient process;

and (3) third switching combination: the third transient process is switched to a fourth transient process;

a fourth switching combination: the fourth transient process is switched to the third transient process.

Further, the clamp resistance dissipation energy during the first and fourth transients is approximately calculated using the following equation:

W_Rs＝0.28LsI_rr ²

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_rrFor the diode forward current is I_bThe reverse recovery current peak in time.

Further, the clamp resistance dissipation energy during the second and third transients is approximately calculated using the following equation:

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_fIs the bridge arm current.

Further, the clamp circuit loss energy of the four switching combinations is approximately calculated by the following formula:

wherein W is the loss energy of the clamping circuit, Ls is the anode reactance, I_rrFor the diode forward current is I_bPeak value of time reverse recovery current，I_fIs the bridge arm current.

Still further, an IGCT-MMC converter valve sub-module clamp circuit loss determining apparatus, the determining apparatus comprising:

an equivalent model establishing unit: the current source equivalent model is used for establishing a diode reverse recovery process;

an equivalent analysis unit: the device is used for carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

a transient process analysis unit: the system is used for analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

a simulation unit: for simulating a transient process;

a simulation analysis unit: and analyzing the simulation result to obtain a clamping circuit loss calculation formula and calculate the clamping circuit loss energy.

Further, the equivalent model establishing unit establishes the current source equivalent model by the following equivalent process:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the current of the diode is reduced to a certain threshold value from the peak value of the reverse recovery current with the same current change rate di/dt;

the third equivalent process: the diode current changes from a certain threshold value to 0 over a longer period of time below the above-mentioned rate of change of current di/dt.

Compared with the prior art, the invention has the beneficial effects that: establishing a current source equivalent model of a diode reverse recovery process, carrying out equivalence on switching of the IGCT-MMC converter valve sub-module through the current source equivalent model, analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module, simulating the transient process, analyzing a simulation result, and concluding a more accurate IGCT-MMC converter valve sub-module clamping circuit loss energy calculation formula. The clamping circuit loss energy calculation formula comprises energy loss generated by reverse recovery current after the diode is turned off, so that accurate quantitative analysis of the clamping circuit loss energy is facilitated, and system optimization design is facilitated.

Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and those skilled in the art can also obtain other drawings according to the drawings without creative efforts.

FIG. 1 illustrates a diode reverse recovery ideal waveform in accordance with an embodiment of the present invention;

FIG. 2 shows a voltage-current reference direction diagram according to an embodiment of the invention;

fig. 3 shows a simulated waveform diagram of a first transient process according to an embodiment of the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The method for calculating the loss of the clamping circuit of the submodule of the IGCT-MMC converter valve, provided by the embodiment, comprises the following steps of:

establishing a current source equivalent model of a diode reverse recovery process;

carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

simulating a transient process;

and analyzing the simulation result to obtain a clamping circuit loss calculation formula, and calculating the clamping circuit loss energy.

Fig. 1 shows a schematic diagram of a diode reverse recovery ideal waveform according to an embodiment of the invention. As shown in fig. 1, the diode reverse recovery process mainly includes the following three stages:

the first stage is as follows: t0-t1 is the forward current descending stage of the diode, and the current descending stage is carried out at a fixed current change rate, and the current change rate is determined by the direct current bus voltage and the commutation inductance;

and a second stage: t1-t2 is the diode reverse current rise phase where the diode reverse voltage is low and the current change rate is generally considered to remain constant. However, as the carrier is extracted, a depletion layer is formed, the reverse voltage of the diode is gradually increased, the current change rate is gradually reduced, and when the reverse current is increased to be equal to the bus voltage, the reverse current reaches a peak value;

and a third stage: t2-t3 is the reverse current decreasing stage of the diode, the reverse recovery current decreases due to the decreasing carrier concentration gradient, and the current change rate also gradually decreases.

Wherein i_D(t) is the waveform of the current flowing during the diode reverse recovery, v_ak(t) is the voltage waveform across the diode during reverse recovery of the diode, I_fRepresenting the steady state current, I, passing when the diode is conducting in the forward direction_rmFor the diode reverse recovery current peak value, V_DCThe reverse steady-state voltage experienced by the diodes (which may also be considered the dc bus voltage for the IGCT-MMC converter valve sub-module).

The embodiment focuses on the influence of the transient process of the power device switch on the characteristics of the clamping resistance loss, the maximum voltage of the clamping capacitor and the like in the buffer circuit, and the influence is mainly realized by coupling the transient current of the power device to the anode reactance to generate a voltage on the anode reactance. For the three processes of IGCT turning-on, IGCT turning-off and diode conducting, the speed is high, and the reverse recovery phenomenon does not exist, so that the three processes can be regarded as an ideal switching process. In the embodiment, only the turn-off process of the diode is equivalent through the constant current source.

The current source equivalent model specifically includes the following equivalent processes:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current (IGCT-MMC converter valve sub-module current) and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the diode current decreases from the reverse recovery current peak to 1/3 at the same rate of current change di/dt;

the third equivalent process: the diode current changes from 1/3 reverse current peak to 0 over a longer time at a rate di/dt lower than the current change rate.

And controlling the switch of the constant current source according to the equivalent process to realize the simulation of the turn-off process of the diode.

FIG. 2 shows a voltage-current reference direction diagram according to an embodiment of the invention. As shown in fig. 2, in normal operation of the MMC, a transient process caused by switching of the IGCT-MMC converter valve sub-module specifically includes the following four transient processes:

a first transient process: submodule current direction i_SM>0, switched-on state before switching (T1 is on, T2 is off, the same applies below), switched-off state after switching (T1 is off, T2 is on, the same applies below), and anode reactance current i_LsThe change process is as follows: i is_b→-I_rr→0；

And a second transient process: submodule current direction i_SM>0, cut-off state before switching, put-in state after switching, and anode reactance current i_LsThe change process is as follows: 0 → I_b；

And a third transient process: submodule current direction i_SM<0, switching on before switching, switching off after switching, and anode reactance current i_LsThe change process is as follows: -I_b→0；

A fourth transient process: submodule current direction i_SM<0, cutting off before switching, and cutting off after switchingOn state, anodic reactance current i_LsThe change process is as follows: 0 → - (I)_b+I_rr)→-I_b；

The above transient processes are not in sequence. Wherein, I_bFor switching the bridge arm current at the moment I_rrFor the diode forward current is I_bTime reverse recovery current peak, D₁Is a diode, T on an IGCT-MMC converter valve submodule₁Is an IGCT switch D on an IGCT-MMC converter valve submodule₂Is a lower diode, T, of an IGCT-MMC converter valve submodule₂Is an IGCT switch under an IGCT-MMC converter valve submodule, D_sAs a clamping diode, C_sAs a clamping capacitance, L_sIs an anode reactance, R_sTo clamp resistance, i_SMIs the input current i of an IGCT-MMC converter valve submodule_LsIs an anodic reactive current u_LIs the voltage across the anode reactance.

Due to the diode reverse recovery process, there is an anodically reactive current i in all four cases described above_LsAnd a positive increase stage, in which the anode reactance voltage is positive, the voltage of the clamping diode is positively biased, and the clamping circuit acts.

The switching of the IGCT-MMC converter valve sub-module comprises the following four switching combinations:

a first switching combination: switching the first transient process to a second transient process;

and a second switching combination: the second transient process is switched to the first transient process;

and (3) third switching combination: the third transient process is switched to a fourth transient process;

a fourth switching combination: the fourth transient process is switched to the third transient process.

FIG. 3 illustrates a first transient process (i.e., IGCT-MMC converter valve sub-module current i) according to an embodiment of the present invention_SM>0, the input sub-module is cut off). As shown in FIG. 3, at 1.000ms, the sub-module current i_SM>0, the sub-module is switched off, and the diode current drops at the current change rate di/dt and rises in the reverse direction to the peak value of the reverse recovery current. During this transient, the anodeThe reactance current varies synchronously therewith. After 1.0025ms, the diode reverse current decays to 0 according to the rule. Due to the clamping circuit, the anode reactance current does not decay synchronously with it, but is partially transferred to the clamping capacitor through the clamping diode, and the voltage of the clamping capacitor is raised, so that the clamping resistance current starts to increase. In the whole transient process, the electrical parameter waveforms of all devices are smooth.

Wherein i (D) is the current of the main circuit diode D1, i (ls) is the anode reactance current, u (cs) is the clamp capacitor voltage, i (Rs) is the clamp resistor current, w (Rs) is the energy emitted by the clamp resistor Rs, w (D) is the energy emitted by the main circuit diode D1, and negative values represent the dissipated energy.

And (4) observing energy transfer in the transient process, wherein before the reverse recovery current reaches a peak value, the clamping circuit does not act, and energy is exchanged between the direct current capacitor and the anode reactance. During the decay of the reverse recovery current, the DC capacitor loses energy, and the anode reactance energy also decreases as the current thereof decreases. Part of this energy is stored in the clamp capacitor and part is dissipated in the main circuit diode as turn-off loss energy. As the voltage of the clamping capacitor gradually rises, the resistance current increases and gradually exceeds the anode reactance current, a part of energy is dissipated in the clamping resistor, and a part of energy is fed back to the direct current capacitor. The clamp capacitor only temporarily stores a portion of the energy during the entire transient. Numerically, all energy lost by the direct current capacitor and a small part of energy stored in the anode reactance are dissipated in the main circuit diode, and the rest energy in the anode reactance is dissipated in the clamping resistor.

In this embodiment, four transient processes caused by switching of the sub-module are subjected to simulation analysis under the experimental conditions that the bridge arm current is 3000A and the diode reverse recovery peak current is 3500A, and the simulation results of the clamp circuit in each transient process are shown in table 1. In the simulation model, the parameters in the circuit shown in fig. 2 are: the anode reactance is 0.8uH, the clamping capacitance is 6uF, and the clamping resistance is 0.4 omega.

TABLE 1 simulation results of clamping circuit at specific current

The clamp resistor dissipated energy is related only to the anode reactance stored energy. According to the simulation result, in the first transient process and the fourth transient process, the relation between the dissipated energy of the clamping resistor and the bridge arm current is not large, but is in direct proportion to the square of the reverse recovery current, and the following formula is used for approximate calculation:

W_Rs＝0.28LsI_rr ²

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_rrFor the diode forward current is I_bThe reverse recovery current peak in time.

The dissipation energy of the clamping resistor in the second transient process is similar to that in the third transient process, is approximately equal to the stored energy of the anode reactance, is in direct proportion to the square of the bridge arm current, and is approximately calculated by the following formula:

wherein, W_RsThe clamping resistance dissipates energy, Ls being the anodic reactance, I_fIs the bridge arm current.

According to the above results, for single pulse switching, the loss energy of the clamp circuit of four switching combinations of the IGCT-MMC converter valve sub-modules can be approximately calculated by the following formula:

where W is the clamp loss energy (unit J), Ls is the anode reactance (unit uH), I_rrFor the diode forward current is I_bTime reverse recovery current peak (in kA), I_fBridge arm current (in kA).

An IGCT-MMC converter valve submodule clamp circuit loss determination device comprises:

an equivalent model establishing unit: the current source equivalent model is used for establishing a diode reverse recovery process;

an equivalent analysis unit: the device is used for carrying out equivalence on the switching of the IGCT-MMC converter valve sub-module through a current source equivalent model;

a transient process analysis unit: the system is used for analyzing a transient process caused by switching of the IGCT-MMC converter valve sub-module;

a simulation unit: for simulating a transient process;

a simulation analysis unit: and analyzing the simulation result to obtain a clamping circuit loss calculation formula and calculate the clamping circuit loss energy.

The equivalent model establishing unit establishes the current source equivalent model through the following equivalent processes:

a first equivalent process: the current change rate di/dt determined by a circuit except the diode current is reduced to 0 from the forward current and continuously and reversely increased to the reverse current peak value;

a second equivalent process: the diode current decreases from the reverse recovery current peak to 1/3 at the same rate of current change di/dt;

the third equivalent process: the diode current changes from 1/3 reverse current peak to 0 over a longer time at a rate di/dt lower than the current change rate.

Although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.