CN110011317A - A Dynamic Voltage Restorer Based on Soft Switching Circuit - Google Patents

Abstract

The present invention discloses a kind of dynamic electric voltage recovery device based on soft switch circuit, respectively constitutes A, B, C three-phase system including three groups of single-phase full bridge inverter bridge legs in parallel.The resonant branch of auxiliary switch of the access comprising anti-paralleled diode, clamping capacitance and resonant inductance composition between the DC bus and DC voltage source of three groups of full-bridge inverting bridge arms, the output of three groups of full-bridge inverting bridge arms pass through transformer series in three-phase distribution net after LC filter respectively.It is open-minded that the present invention by the movement of auxiliary circuit may be implemented all main switch no-voltages, realizes circuit gamut Sofe Switch in power frequency period.The invention effectively reduces switching loss caused by hard switching process and EMI interference, conducive to the power density for improving dynamic electric voltage recovery device.

Description

A Dynamic Voltage Restorer Based on Soft Switching Circuit

technical field

The invention relates to the technical field of dynamic voltage restorer circuits, in particular to a dynamic voltage restorer topology based on a soft switching circuit and a modulation method thereof.

Background technique

At present, part of the dynamic voltage restorer topology is shown in Figure 1, which includes a first bridge arm composed of a fully-controlled main switch (S _a1 , S _a2 ) composed of anti-parallel diodes (D _a1 , D _a2 ), which is composed of anti-parallel diodes (D a1 , D a2 ) The fully controlled main switches (S _a3 , S _a4 ) of (D _a3 , D _a4 ) constitute the second bridge arm, and the fully controlled main switches (S _b1 , S _b2 ) of anti-parallel diodes (D _b1 , D _b2 ) The third bridge arm is formed, and the fourth bridge arm is formed by the fully controlled main switch (S _b3 , S _b4 ) of the anti-parallel diodes (D _b3 , D _b4 ), and the fully controlled by the anti-parallel diodes ( D _c1 , D _c2 ) The main switches (S _c1 , S _c2 ) form the third bridge arm, and the fully controlled main switches (S _c3 , S _c4 ) with anti-parallel diodes (D _c3 , D _c4 ) form the fourth bridge arm, and the DC source (U _dc ) is connected in parallel with all the above bridge arms on the same DC bus, and the midpoints of the bridge arms of the first bridge arm and the second bridge arm pass through the LC filter formed by the filter inductor L ₁ and the filter capacitor C ₁ and the transformer A phase winding. The low-voltage side is connected, and the midpoints of the bridge arms of the third bridge arm and the fourth bridge arm are connected to the low-voltage side of the B-phase winding of the transformer through the LC filter formed by the filter inductor L ₂ and the filter capacitor C ₂ . The midpoint of the bridge arm of the bridge arm is connected to the low voltage side of the C-phase winding of the transformer through the LC filter formed by the filter inductor L ₃ and the filter capacitor C ₃ . The high-voltage sides of the three-phase transformers are respectively connected in series between the three-phase distribution network and the load. Since the above-mentioned fully-controlled switches all work in the hard switching state, the anti-parallel diodes all have reverse recovery phenomenon, resulting in large switching losses, large voltage and current stress of the fully-controlled switches, low operating frequency of the dynamic voltage restorer, and EMI interference. high power density and low power density.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a dynamic voltage restorer topology and a modulation method based on a soft switching circuit that reduces switching loss and improves circuit efficiency.

In one aspect of the present invention, a dynamic voltage restorer based on a soft switching circuit is provided, which includes two fully controlled type with parallel resonant capacitors (C _a1 , C _a2 ) and anti-parallel diodes (D _a1 , D _a2 ) The main switches (S _a1 , S _a2 ) constitute the first bridge arm, which consists of two fully-controlled main switches (S _a3 , C a4 ) with parallel resonant capacitors (C _a3 , C _a4 ) and anti-parallel diodes (D _a3 , D _a4 ) S _a4 ) constitutes the second bridge arm, and is composed of two fully-controlled main switches (S _b1 , S b2 ) with parallel resonant capacitors (C _b1 , C _b2 ) and anti-parallel diodes (D _b1 , D _b2 ) (S b1 , S _b2 ) to form the third The bridge arm consists of two fully controlled main switches (S _b3 , S _b4 ) with parallel resonant capacitors (C _b3 , C _b4 ) and anti-parallel diodes (D _b3 , D _b4 ) to form the fourth bridge arm, which consists of two The fully controlled main switch (S _b1 , S _b2 ) with parallel resonant capacitors (C _c1 , C _c2 ) and anti-parallel diodes (D _c1 , D _c2 ) constitutes the fifth bridge arm, which consists of two parallel resonant capacitors (C _c3 , C _c4 ) and fully controlled main switches (S _c3 , S _c4 ) of anti-parallel diodes (D _c3 , D _c4 ) constitute the sixth bridge arm, and a parallel _resonant capacitor (C _a ) Auxiliary circuit composed of a circuit composed of a fully controlled auxiliary switch (S _a ) with an anti-parallel diode (D _a ) and a series clamping capacitor (C _c ); the DC source (U _dc ) passes through the auxiliary circuit and all the above-mentioned bridge arms Connected in parallel on the same DC bus, the midpoints of the bridge arms of the first bridge arm and the second bridge arm are connected to the low-voltage side of the A phase winding of the transformer through the LC filter formed by the filter inductor L ₁ and the filter capacitor C ₁ , and the third bridge The midpoint of the bridge arm of the arm and the fourth bridge arm is connected to the low voltage side of the B phase winding of the transformer through the LC filter formed by the filter inductor L ₂ and the filter capacitor C ₂ , and the midpoint of the bridge arm of the fifth bridge arm and the sixth bridge arm The LC filter formed by the filter inductor L ₃ and the filter capacitor C ₃ is connected to the low voltage side of the C-phase winding of the transformer; the high voltage side of the three-phase transformer is respectively connected in series between the three-phase distribution network and the load.

Another aspect of the content of the present invention provides a modulation method of a dynamic voltage restorer circuit based on a soft switching circuit, wherein the first bridge arm fully controlled main switch (S _a1 , S _a2 ), the second bridge arm fully controlled type main switch (S _a3 , S _a4 ), the third bridge arm fully controlled main switch (S _b1 , S _b2 ), the fourth bridge arm fully controlled main switch (S _b3 , S _b4 ), the fifth bridge arm fully controlled main switch (S b3 , S b4 ) The control-type main switches (S _c1 , S _c2 ) and the sixth-arm fully-controlled main switches (S _c3 , S _c4 ) are controlled by the zero-voltage sinusoidal pulse width modulation method; in each switching cycle, the above six The switching action of the anti-parallel diode to the fully-controlled main switch existing in the bridge arm is synchronized at the same time, and the control signal of the fully-controlled auxiliary switch (S _a ) of the auxiliary branch is synchronized with the above-mentioned switching action timing, that is, when the current The fully-controlled auxiliary switch (S _a ) is turned off before the commutation time from the anti-parallel diode of the fully-controlled main switch to the fully-controlled main switch, and the voltage on the DC bus immediately resonates to zero. The switch provides zero-voltage turn-on conditions. When all bridge arms complete the commutation, it is determined whether to join the bridge arm through process according to the power flow of the _{DC source (U dc )} . The bridge arm pass-through process needs to be added. After the commutation process of all the above diodes to the fully controlled main switch is completed, the voltage on the resonant capacitor C _a will resonate to zero, and the fully controlled auxiliary switch S _a will be turned on at zero voltage; when the power flows out of the DC When the source (U _dc ) needs to be added to the bridge arm pass-through process, at this time, the above-mentioned fully-controlled main switches (S _a1 ～S _a4 , S _b1 ～S _b4 , S _c1 ～S _c4 ) are all turned on to realize the resonant inductance. (L _r ) is charged, and the voltage on the fully-controlled auxiliary switch (S _a ) immediately resonates to zero after the charging is completed, providing a zero-voltage turn-on condition for the fully-controlled auxiliary switch (S _a ) to be turned on.

Compared with the prior art, the present invention has the following beneficial effects:

The invention can realize zero-voltage opening of all main switch tubes by turning off the auxiliary switch tubes of the auxiliary circuit, and charge the resonant inductance of the auxiliary circuit through the short-circuit pulse of the bridge arm, so as to realize the full-range softness of the three-phase system in the power frequency cycle. switch. The invention effectively reduces the switching loss, larger voltage, current stress and EMI interference caused by the hard switching process, and is beneficial to improve the power density of the dynamic voltage restorer.

Description of drawings

Figure 1 shows a traditional dynamic voltage restorer topology.

Figure 2 shows a dynamic voltage restorer topology based on a soft-switching circuit.

Fig. 3 is the six regions divided by the filter inductor currents i _La , i _Lb , i _Lc and zero in the power frequency cycle of the dynamic voltage restorer based on the soft switching circuit under the three-phase ground fault.

4 to 21 are respectively the working equivalent circuits of the present invention in each stage of a switching cycle in region 2.

FIG. 22 shows the main operating voltage and current waveforms of the present invention for one switching cycle in region 2. FIG.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those skilled in the art, several modifications and improvements can be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention. The present invention will be described in detail below with reference to the accompanying drawings.

Referring to FIG. 2 , the dynamic voltage restorer topology based on the soft switching circuit includes two fully controlled main switches (S _a1 , C a2 ) with parallel resonant capacitors (C _a1 , C _a2 ) and anti-parallel diodes (D _a1 , D _a2 ) (S a1 , D a2 ). S _a2 ) constitutes the first bridge arm, and two fully-controlled main switches (S _a3 , S _a4 ) with parallel resonant capacitors (C _a3 , C _a4 ) and anti-parallel diodes (D _a3 , D _a4 ) constitute the second bridge arm The bridge arm consists of two fully controlled main switches (S _b1 , S _b2 ) with parallel resonant capacitors (C _b1 , C _b2 ) and anti-parallel diodes (D _b1 , D _b2 ) to form the third bridge arm, which consists of two The fully-controlled main switch (S _b3 , S _b4 ) with parallel resonant capacitors (C _b3 , C _b4 ) and anti-parallel diodes (D _b3 , D _b4 ) constitutes the fourth bridge arm, which consists of two parallel resonant capacitors (C _c1 , C _c2 ) and anti-parallel diodes (D _c1 , D _c2 ) fully controlled main switches (S _b1 , S _b2 ) constitute the fifth bridge arm, which consists of two parallel resonant capacitors (C _c3 , C _c4 ) and The fully controlled main switch (S _c3 , S _c4 ) of the anti-parallel diodes (D _c3 , D _c4 ) constitutes the sixth bridge arm, and a parallel resonant capacitor (C _a ) and the anti-parallel diode are connected in parallel by the resonant inductor (L _r ). The auxiliary circuit formed by the fully controlled auxiliary switch (S _a ) of (D _a ) is connected in series with the clamping capacitor (C _c ); the DC source (U _dc ) passes through the auxiliary circuit and all the bridge arms are connected in parallel to the same DC bus On the upper side, the midpoints of the bridge arms of the first bridge arm and the second bridge arm are connected to the low voltage side of the transformer A phase winding through the LC filter formed by the filter inductor L ₁ and the filter capacitor C ₁ , the third bridge arm and the fourth bridge arm The midpoint of the bridge arm is connected to the low voltage side of the B-phase winding of the transformer through the LC filter formed by the filter inductor L ₂ and the filter capacitor C ₂ , and the mid point of the bridge arm of the fifth bridge arm and the sixth bridge arm passes through the filter inductor L ₃ , _The LC filter formed by the filter capacitor C3 is connected with the low-voltage side of the C-phase winding of the transformer; the high-voltage side of the three-phase transformer is respectively connected in series between the three-phase distribution network and the load.

A modulation method of a dynamic voltage restorer based on a soft switching circuit, the first bridge arm fully controlled main switch (S _a1 , S _a2 ), the second bridge arm fully controlled main switch (S _a3 , S _a4 ), the first bridge arm fully controlled main switch (S a3 , S a4 ), The three-arm fully-controlled main switch (S _b1 , S _b2 ), the fourth-arm fully-controlled main switch (S _b3 , S _b4 ), the fifth-arm fully-controlled main switch (S _c1 , S _c2 ), The fully-controlled main switches (S _c3 , S _c4 ) of the sixth bridge arm are controlled by the zero-voltage sinusoidal pulse width modulation method; At the same time, the control signal of the full-controlled auxiliary switch (S _a ) of the auxiliary branch is synchronized with the above-mentioned switching action, that is, when the current flows from the anti-parallel diode of the full-controlled main switch to the full-controlled main switch The fully-controlled auxiliary switch (S _a ) is turned off before the commutation time of the control-type main switch, and the voltage on the DC bus resonates to zero immediately, which provides a zero-voltage turn-on condition for the fully-controlled main switch to be turned on. At the moment of commutation, it is determined whether to join the bridge arm through process according to the power flow of the DC source (U _dc ); when the power flow is to flow into the DC source (U _dc ), there is no need to join the bridge arm through process, and all the above diodes are directed to the full After the commutation process of the controlled main switch is completed, the voltage on the resonant capacitor C _a will resonate to zero, and the fully controlled auxiliary switch S _a will be turned on at zero voltage; when the power flows out of the DC source (U _dc ), the bridge arm pass-through process needs to be added , at this time, the above-mentioned fully-controlled main switches (S _a1 ～S _a4 , S _b1 ～S _b4 , S _c1 ～S _c4 ) are all turned on to realize the charging of the resonant inductor (L _r ). The voltage on the fully-controlled auxiliary switch (S _a ) immediately resonates to zero, providing a zero-voltage turn-on condition for the fully-controlled auxiliary switch (S _a ) to be turned on.

Referring to FIG. 3 , for the dynamic voltage restorer based on the soft switching circuit, the working area can be divided into six parts according to the comparison of the currents i _La , i _Lb , i _Lc of its filter inductors L ₁ , L ₂ , L ₃ and zero. area. Taking the filter inductor currents i _La , i _Lb , and i _Lc in region 2 as an example, the working process of the circuit in one switching cycle is analyzed.

Since in region 2, i _La and i _Lb are greater than zero, and i _Lc is less than zero, there are six diodes commutating to the main switch tube, namely D _a2 commutating to S _a1 , D _a3 commutating to S _a4 , D _b2 commutates to S _b1 , D _b3 commutates to S _b4 , D _{c1 commutes} to S _c2 , and D _{c4 commutes} to S _c3 .

Figures 4 to 21 are 18 working equivalent circuits of one switching cycle in this area 2. The main voltage and current waveforms of its operation are shown in Figure 22, and the reference direction of the circuit's voltage and current is shown in Figure 2. The working process of the circuit working in other areas is similar.

The specific stage analysis is as follows:

Stage 1 (t ₀ ~ t ₁ ):

As shown in Figure 4, the main switches _{Sa2, Sa3, Sb2, Sb3, Sc1, Sc4 and the auxiliary switches Sa of the six groups of bridge arms are turned on, and the clamping capacitor Cc is discharged , which is the resonant} inductance Recharge.

Stage two (t ₁ ~ t ₂ ):

As shown in FIG. 5 , the main switches _{Sa2 , S a3 ,} S _b2 , S _b3 , S _c1 , and S _c4 of the six groups of bridge arms are turned off at the same time, and the anti-parallel diodes D _a2 , D _a3 , D of the six groups of bridge arms are turned off at the same time. _b2 , D _b3 , D _c1 , and D _c4 are turned on, the auxiliary switch S _a continues to be turned on, and the clamping capacitor C _c is discharged to charge the resonant inductor.

Stage three (t ₂ ~ t ₃ ):

As shown in FIG. 6 , the auxiliary switch S _a is turned off at time t ₂ , and the resonant inductor L _r makes the parallel connection of the main switches S _a1 , S _a4 , S _b1 , S _b4 , S _c2 , and S _c3 of the six groups of bridge arms The capacitors C _a1 , C _a4 , C _b1 , C _b4 , C _c2 , and C _c3 are discharged, and at the same time the parallel capacitor C _a of the auxiliary switch S _a is charged. At time t ₂ , the main switches S _a1 , The voltages of the parallel capacitors C _a1 , C _a4 , C _b1 , C _b4 , C _c2 , and C _c3 of S _a4 , S _b1 , S _b4 , S _c2 , and S _c3 resonate to zero, and this stage ends.

Stage four (t ₃ ~ t ₄ ):

As shown in FIG. 7 , after time _t3 , the anti-parallel diodes D _a1 , D _a4 , D _b1 , D _b4 , D _c2 and D _c3 of the six groups of bridge arms are turned on, and the main switch transistors S _a1 , S _a4 , The voltages of S _b1 , S _b4 , S _c2 , S _c3 are clamped at zero. Turn on the main switch tubes S _a1 , S _a4 , S _b1 , S _b4 , S _c2 , and S _c3 to realize zero-voltage turn-on of the main main switch tubes S _a1 , S _a4 , S _b1 , S _b4 , S _c2 , and S _c3 , This phase ends.

Stage five (t ₄ ~ t ₅ ):

As shown in FIG. 8 , after the main switches Sa1, _Sa4 , Sb1, _Sb4 , Sc2, and _Sc3 are _{turned on, the bridge arm current begins to commutate, and the current iLa of the filter inductor L1 is determined by the diodes D a2 , S c2 ,} and S _c3 . D _a3 commutates to the main switch tubes S _a1 and S _a4 , the current i _Lb of the filter inductor L ₂ is commutated to the main switch tubes S _b1 and S _b4 by the diodes D _b2 and D _b3 , and the current i _Lc of the filter inductor L ₃ is given by The diodes D _c1 and D _c4 commutate to the main switch tubes S _c2 and S _c3 . _At time t5, the above-mentioned commutation processes are all completed, and this stage ends.

Stage six (t ₅ ~ t ₆ ):

As shown in FIG. 9 , at time t ₅ , all switch tubes S _{a1 ˜S a4} , S _{b1 ˜S b4} , and S _{c1 ˜S c4} of the six groups of bridge arms are turned on. The bridge arm of the circuit is short-circuited, and the DC voltage source U _dc supplements the resonant energy for the resonant inductor L _r .

Stage seven (t ₆ ~ t ₇ ):

As shown in Figure 10, at time t6, the main switches _{Sa2, S a3 ,} S _b2 , S _b3 , S _c1 and S _c4 of the _six groups of bridge arms are turned off, and the resonant inductor L _r makes the auxiliary switch S _a The parallel capacitor C _a of the main switch tube S _a2 , S _a3 , S _b2 , S _b3 , S _c1 , S _c4 is discharged, and the parallel capacitor main switch tubes C _a2 , C _a3 , C _b2 , C _b3 , C _c1 , C _c4 is charged, and at time t ₇ , the voltage on the parallel capacitor C _a of the auxiliary switch tube _Sa resonates to zero, and this stage ends.

Stage 8 (t ₇ ~ t ₈ ):

As shown in FIG. ₁₁ , after time t7, the anti-parallel diode D _a of the auxiliary switch S _a will be turned on to clamp the voltage on the auxiliary switch S _a to zero. The auxiliary switch tube _Sa is turned on to realize zero-voltage turn-on of the auxiliary switch tube _Sa , and this stage ends.

Stage nine (t ₈ ~ t ₉ ):

As shown in Figure 12, the auxiliary switch S _a is turned on, the clamping capacitor C _c clamps the voltage of the resonant inductor L _r , and the resonant inductor current i _Lr increases linearly at the rate of U _Cc /L _r .

Stage ten (t ₉ ~ t ₁₀ ):

As shown in FIG. ₁₃ , at time _t9 , the main switches Sa1 and S _a4 of the first bridge arm and the second bridge arm are turned off, their parallel capacitors C _a1 and C _a4 are charged, and the first bridge arm and the second bridge arm are charged. The parallel capacitors C _a2 and C _a3 of the arms _Sa2 and S _a3 are discharged. At time t ₁₀ , the voltages on the parallel capacitors C _a1 and C _a4 rise to U _Cc +U _dc , while the voltages on the parallel capacitors C _a2 and C _a3 rise to U Cc +U dc . The voltage drops to zero and the phase ends.

Stage eleven (t ₁₀ ~ t ₁₁ ):

As shown in FIG. ₁₄ , at time t10, the anti-parallel diodes D _a2 and D _a3 of the first bridge arm and the second bridge arm are turned on, and the current i _La of the filter inductor L ₁ is completed by the main switch transistors S _a1 and S _a4 The diodes D _a2 and D _{a3 are} commutated.

Stage 12 (t ₁₁ ~ t ₁₂ ):

As shown in FIG. ₁₅ , at time t11, the main switch tubes _{Sa2 and Sa3 of the first bridge arm and the second bridge arm are turned on, and the currents of the anti-parallel diodes D a2 and D a3 are transferred} to the main switch tube _Sa2 , S _a3 .

Stage 13 (t ₁₂ ~ t ₁₃ ):

As shown in FIG. 16 , at time _t12 , the main switches S _b1 and S _b4 of the third bridge arm and the fourth bridge arm are turned off, their parallel capacitors C _b1 and C _b4 are charged, and the second bridge arm and the third bridge arm are charged. The parallel capacitors C _b2 and C _b3 of the arms S _b2 and S _b3 are discharged. At time t ₁₃ , the voltages on the parallel capacitors C _b1 and C _b4 rise to U _Cc +U _dc , while the voltages on the parallel capacitors C _b2 and C _b3 rise to U Cc +U dc . The voltage drops to zero and the phase ends.

Stage Fourteen (t ₁₃ ~ t ₁₄ ):

As shown in Figure 17, at time t ₁₃ , the anti-parallel diodes D _b2 and D _b3 of the third bridge arm and the fourth bridge arm are turned on, and the current i _Lb of the filter inductor L ₂ is completed by the main switch transistors S _b1 and S _b4 The diodes D _b2 and D _{b3 are} commutated.

Stage 15 (t ₁₄ ~ t ₁₅ ):

As shown in FIG. 18 , at time _t14 , the main switch tubes S _b2 and S _b3 of the third bridge arm and the fourth bridge arm are turned on, and the currents of the anti-parallel diodes D _b2 and D _b3 are transferred to the main switch tube S _b2 , S _b3 .

Stage sixteen (t ₁₅ ~ t ₁₆ ):

As shown in FIG. 19 , at time _t15 , the main switches S _c2 and S _c3 of the fifth bridge arm and the sixth bridge arm are turned off, the parallel capacitors C _c2 and C _c3 are charged, and the fifth bridge arm and the sixth bridge arm are charged. The parallel capacitors C _c1 and C _c4 of the arms S _c1 and S _c4 are discharged. At time t ₁₆ , the voltages on the parallel capacitors C _c2 and C _c3 rise to U _Cc +U _dc , while the voltages on the parallel capacitors C _c1 and C _c4 rise to U Cc +U dc . The voltage drops to zero and the phase ends.

Stage seventeen (t ₁₆ ~ t ₁₇ ):

As shown in Figure ₂₀ , at time t16, the anti-parallel diodes D _c1 and D _c4 of the fifth bridge arm and the sixth bridge arm are turned on, and the current i _Lc of the filter inductor L ₃ is completed by the main switch tubes S _c2 and S _c3 The diodes D _c1 and D _{c4 are} commutated.

Stage 18 (t ₁₇ ~ t ₀ '):

As shown in FIG. ₂₁ , at time t17, the main switch tubes S _c1 and S _c4 of the fifth bridge arm and the sixth bridge arm are turned on, and the currents of the anti-parallel diodes D _c1 and D _c4 are transferred to the main switch tube S _c1 , S _c4 .

Claims (2)

1. A dynamic voltage restorer based on a soft-switching circuit, characterized in that: the dynamic voltage restorer circuit based on a soft-switching circuit comprises two parallel resonant capacitors (C _a1 , C _a2 ) and an anti-parallel diode ( The fully-controlled main switches (S _a1 , S _a2 ) of D _a1 , D _a2 ) constitute the first bridge arm, which consists of two parallel resonant capacitors (C _a3 , C _a4 ) and anti-parallel diodes (D _a3 , D _a4 ) The fully-controlled main switches (S _a3 , S _a4 ) constitute the second bridge arm, which consists of two fully-controlled main switches with parallel resonance capacitors (C _b1 , C _b2 ) and anti-parallel diodes (D _b1 , D _b2 ) (S _b1 , S _b2 ) constitute the third bridge arm, which consists of two fully-controlled main switches (S _b3 , S _b4 ) with parallel resonant capacitors (C _b3 , C _b4 ) and anti-parallel diodes (D _b3 , D _b4 ) ) constitutes the fourth bridge arm, and the fifth bridge arm is constituted by two fully controlled main switches (S _b1 , S _b2 ) with parallel resonant capacitors (C _c1 , C _c2 ) and anti-parallel diodes (D _c1 , D _c2 ) , the sixth bridge arm is composed of two fully controlled main switches (S _c3 , S _c4 ) with parallel resonant capacitors (C _c3 , C _c4 ) and anti-parallel diodes (D _c3 , D _c4 ), and the resonant inductor (L _r ) Parallel an auxiliary circuit consisting of a parallel resonant capacitor (C _a ) and a fully controlled auxiliary switch (S _a ) of an anti-parallel diode (D _a ) and a series clamping capacitor (C _c ) circuit; DC source (U _dc ) ) through the auxiliary circuit and all the above bridge arms are connected in parallel on the same DC bus, the midpoints of the bridge arms of the first bridge arm and the second bridge arm pass through the LC filter formed by the filter inductor L ₁ and the filter capacitor C ₁ and the transformer A The low-voltage sides of the phase windings are connected, and the midpoints of the bridge arms of the third bridge arm and the fourth bridge arm are connected to the low-voltage side of the transformer B phase winding through the LC filter formed by the filter inductor L ₂ and the filter capacitor C ₂ , and the fifth bridge arm is connected to the low-voltage side of the transformer B phase winding. The midpoint of the bridge arm of the sixth bridge arm is connected to the low-voltage side of the C-phase winding of the transformer through the LC filter formed by the filter inductor L ₃ and the filter capacitor C ₃ ; the high-voltage side of the three-phase transformer is connected in series between the three-phase distribution network and the load, respectively. between. 2. A modulation method of a dynamic voltage restorer based on a soft switching circuit, characterized in that: the first bridge arm fully controlled main switch (S _a1 , S _a2 ), the second bridge arm fully controlled main switch ( S _a3 , S _a4 ), the third arm fully controlled main switch (S _b1 , S _b2 ), the fourth arm fully controlled main switch (S _b3 , S _b4 ), the fifth arm fully controlled main switch (S _c1 , S _c2 ), the sixth bridge arm fully-controlled main switches (S _c3 , S _c4 ) are controlled by the zero-voltage sinusoidal pulse width modulation method; At the same time, the control signal of the full-control auxiliary switch (S _a ) of the auxiliary branch is synchronized with the above-mentioned switching action time, that is, when the current changes from the fully-controlled type The anti-parallel diode of the main switch turns off the fully-controlled auxiliary switch (S _a ) before the commutation time of the fully-controlled main switch, and the voltage on the DC bus immediately resonates to zero, providing zero voltage for the fully-controlled main switch to be turned on. Turn-on condition, when all bridge arms complete the commutation, according to the power flow of the DC source (U _dc ) to decide whether to join the bridge arm through process; when the power flow direction is to flow into the DC source (U _dc ), there is no need to add the bridge arm In the straight-through process, after the commutation process of all the above diodes to the fully-controlled main switch is completed, the voltage on the resonant capacitor C _a will resonate to zero, and the fully-controlled auxiliary switch S _a will be turned on at zero voltage; when the power flows out of the DC source (U _dc ), the bridge arm pass-through process needs to be added. At this time, the above-mentioned fully-controlled main switches (S _a1 ～S _a4 , S _b1 ～S _b4 , S _c1 ～S _c4 ) are all turned on to realize the resonant inductance (L _r ) After the charging is completed, the voltage on the fully-controlled auxiliary switch (S _a ) immediately resonates to zero, providing a zero-voltage turn-on condition for the fully-controlled auxiliary switch (S _a ) to be turned on.