CN103116080A - Circuit and method for measuring stray inductance of current conversion circuit of three-level converter - Google Patents

Abstract

The invention discloses a measurement circuit and a test method for the miscellaneous inductance of a commutation circuit of a three-level converter. The circuit includes: a first IGBT, a second IGBT, a third IGBT, a fourth IGBT, a first clamping diode, The second clamping diode, the first resonant capacitor, the second resonant capacitor, the first DC support capacitor, the second DC support capacitor, laminated bus bars, DC power supply and freewheeling reactor. The method includes: forming different freewheeling circuits through freewheeling reactors, applying double pulses to the corresponding IGBTs, taking the average value of the resonant current frequency at the time of turning on and turning off, using the LC parallel resonance characteristics formed by the resonant capacitor and the stray inductance of the circuit, Accurately calculate the stray inductance of the commutation circuit. The invention is beneficial to practical operation, can accurately measure the stray inductance of the commutation circuit of the three-level converter, and is convenient for accurately grasping the overvoltage level of the power device under different current levels of the three-level converter, so as to guide the system Development of control strategies.

Description

Measuring circuit and method for measuring stray inductance of commutation circuit of three-level converter

technical field

The invention relates to the application field of power electronics, in particular to a measuring circuit and a measuring method for the stray inductance of a commutation circuit of a three-level converter.

Background technique

With the improvement of the power level of the power conversion system and the development of power electronics technology, the power density of the IGBT (insulated gate bipolar transistor) module is getting higher and higher, and the turn-on and turn-off performance is getting better and better, which means that the turn-off current and its slope is getting bigger and bigger. At this time, a voltage will be induced on the stray inductance of the commutation circuit of the three-level converter, and the voltage, together with the DC bus voltage, will be directly added to both ends of the turned-off IGBT, which may exceed the rated voltage of the IGBT and cause the device damage. Therefore, accurately grasping the value of the stray inductance of the commutation circuit can effectively infer the level of IGBT turn-off overvoltage under different power levels, which is beneficial to the design of the system protection scheme to ensure the safe and stable operation of the system in the transient process.

There are two large commutation circuits and two small commutation circuits in the three-level converter. Since the commutation circuit contains multiple power devices, and the equivalent stray inductance of power devices is an order of magnitude different from that of laminated busbars Therefore, relying only on the stray inductance value of the laminated busbar, it is not possible to accurately judge the overvoltage level of the IGBT at a certain current level. The traditional stacked busbar stray inductance test method relies on the voltage drop of the device at the moment the IGBT is turned on or off, so the measured results do not include the equivalent stray inductance of the IGBT itself, and the measurement process requires the current to be stable during the voltage drop period. The rate of change, the accuracy of the measurement is closely related to the test conditions.

Contents of the invention

The object of the present invention is to provide a measuring circuit and method for measuring the stray inductance of the commutation circuit of the three-level converter, which can conveniently and accurately measure the stray inductance of the commutating circuit of the three-level converter.

The technical scheme for realizing the above-mentioned purpose is:

A measurement circuit for the stray inductance of a commutation circuit of a three-level converter according to one of the present invention, comprising a first IGBT, a second IGBT, a third IGBT, a fourth IGBT, a first clamping diode, and a second clamping diode , the first resonant capacitor, the second resonant capacitor, the first DC support capacitor, the second DC support capacitor, laminated busbars, DC power supply and freewheeling reactor, wherein:

The emitter of the first IGBT is connected to the collector of the second IGBT; the emitter of the second IGBT is connected to the collector of the third IGBT; the emitter of the third IGBT is connected to the fourth IGBT the collector;

The positive terminal of the laminated busbar is respectively connected to the collector of the first IGBT, the positive pole of the first DC support capacitor and the positive pole of the DC power supply; the negative terminal of the laminated busbar is respectively connected to the The emitter of the fourth IGBT, the negative pole of the second DC support capacitor and the negative pole of the DC power supply;

The negative pole of the first DC support capacitor is connected to the positive pole of the second DC support capacitor;

The first resonant capacitor is connected in parallel with the first DC support capacitor;

The second resonant capacitor is connected in parallel with the second DC support capacitor;

The cathode of the first clamping diode is connected to the emitter of the first IGBT; the anode of the first clamping diode is connected to the cathode of the second clamping diode; the anode of the second clamping diode is connected to the the collector of the fourth IGBT;

The connecting ends of the first clamping diode and the second clamping diode are connected to the connecting ends of the first DC supporting capacitor and the second DC supporting capacitor;

The freewheeling reactor is connected between the collector of the first IGBT and the emitter of the second IGBT, or between the cathode of the second clamping diode and the collector of the fourth IGBT;

The laminated busbars include: positive busbars, negative busbars, zero busbars, AC busbars, first connecting busbars and second connecting busbars, wherein:

The positive busbar is connected to the collector of the first IGBT and the positive pole of the first DC support capacitor;

The negative busbar is connected to the emitter of the fourth IGBT and the negative pole of the second DC support capacitor;

The zero busbar connects the anode of the first clamping diode and the cathode of the second clamping diode;

The AC busbar is connected to the emitter of the second IGBT and the collector of the third IGBT;

The first connecting busbar is connected to the emitter of the first IGBT and the cathode of the first clamping diode;

The second connecting busbar is connected to the collector of the fourth IGBT and the anode of the second clamping diode. The commutation circuit of the three-level converter includes a first commutation circuit and a second commutation circuit, wherein:

The first commutation circuit includes the positive busbar, the first IGBT, the second IGBT, the AC busbar, the third IGBT, the second connecting busbar, the second clamping diode, the zero busbar and the first resonant capacitor;

The second commutation circuit includes a second resonant capacitor, a zero busbar, a second clamping diode, a second connecting busbar, a fourth IGBT and a negative busbar.

The measurement circuit for the stray inductance of the commutation circuit of the above-mentioned three-level converter, wherein,

When testing the first commutation circuit, the freewheeling reactor is connected between the collector of the first IGBT and the emitter of the second IGBT;

When testing the second commutation circuit, the freewheeling reactor is connected between the cathode of the second clamping diode and the collector of the fourth IGBT.

The method for measuring the stray inductance of the commutation circuit of the three-level converter based on the measuring circuit described in the first aspect of the present invention according to the second aspect of the present invention comprises the following steps:

Test the first commutation loop:

Step 1, the freewheeling reactor is connected between the collector of the first IGBT and the emitter of the second IGBT; keep the first IGBT and the fourth IGBT off, keep the second IGBT on, and turn on the third IGBT at time t0 , turn off at time t1, turn on again at time t2, and turn off again at time t3;

Step 2: Obtain the frequency f of the parallel resonance current formed by the first resonance capacitor and the stray inductance of the first commutation circuit through the turn-on process from t2 to t3, and obtain the first commutation current according to the characteristics of LC parallel resonance The stray inductance L ₁ of the loop =1/(4Cs1π ² f ² ); where, Cs1 represents the capacitance value of the first resonant capacitor;

Test the second commutation circuit:

Step 3, the freewheeling reactor is connected between the cathode of the second clamping diode and the collector of the fourth IGBT; so that the first, second and third IGBTs are all kept off, and the fourth IGBT S4 is turned on at time t0' , turn off at time t1', turn on again at time t2', and turn off again at time t3';

Step 4: Obtain the frequency f' of the parallel resonant current formed by the second resonant capacitor and the stray inductance of the second commutation circuit through the turn-on process from t2' to t3', and obtain the first The stray inductance L ₂ of the second commutation circuit =1/(4Cs2π ² f ² ); wherein, Cs2 represents the capacitance value of the second resonant capacitor.

The beneficial effects of the present invention are: the present invention utilizes LC parallel resonance to test the commutation circuit miscellaneous inductance of the three-level converter, does not rely on the voltage and current at the moment of IGBT turn-on or turn-off, but utilizes the whole process of device turn-on, and only uses The frequency value of the collector current, so that the test results include the equivalent stray inductance of all power devices in the circuit and the stray inductance of the laminated busbar, which is more accurate and reliable. At the same time, the present invention only needs to measure the current of the device, has few variables, is beneficial to practical operation, and is convenient to accurately grasp the overvoltage level of the power device under different current levels of the three-level converter, so as to guide the formulation of the system control strategy.

Description of drawings

Fig. 1 is a test circuit schematic diagram of the present invention;

Fig. 2 is the waveform diagram of the first commutation circuit experiment of the present invention;

Fig. 3 is the experimental waveform diagram of the first commutation circuit of the traditional test method;

Fig. 4 is an experimental waveform diagram of the second commutation circuit of the present invention.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

Please refer to Fig. 1, the measuring circuit of the stray inductance of the commutation circuit of one of the three-level converters of the present invention, including the first IGBT S1, the second IGBT S2, the third IGBT S3, the fourth IGBT S4, the first clamp Diode D1, second clamping diode D2, first resonant capacitor Cs1, second resonant capacitor Cs2, first DC support capacitor C1, second DC support capacitor C2, laminated busbar (not shown in the figure), DC Power supply DCs and freewheeling reactor L, where:

The first IGBT, the second IGBT, the third IGBT and the fourth IGBT are connected in series in the same direction, that is, the emitter of the first IGBTTS1 is connected to the collector of the second IGBT S2; the emitter of the second IGBT S2 is connected to the collector of the third IGBT S3 electrode; the emitter of the third IGBT S3 is connected to the collector of the fourth IGBT S4;

The positive terminal DC+ of the laminated busbar is respectively connected to the collector of the first IGBT S1, the positive pole of the first DC support capacitor C1 and the positive pole of the DC power supply DCs; the negative terminal DC- of the laminated busbar is respectively connected to the fourth IGBT S4 The emitter, the negative pole of the second DC support capacitor C2 and the negative pole of the DC power supply DCs;

The negative pole of the first DC support capacitor C1 is connected to the positive pole of the second DC support capacitor C2;

The first resonant capacitor Cs1 is connected in parallel with the first DC support capacitor C1;

The second resonant capacitor Cs2 is connected in parallel with the second DC support capacitor C2;

The cathode of the first clamping diode D1 is connected to the emitter of the first IGBT S1; the anode of the first clamping diode D1 is connected to the cathode of the second clamping diode D2; the anode of the second clamping diode D2 is connected to the collector of the fourth IGBTS4 ;

The connecting ends of the first clamping diode D1 and the second clamping diode D2 are connected to the connecting ends of the first DC supporting capacitor C1 and the second DC supporting capacitor C2;

The freewheeling reactor L is connected between the collector of the first IGBT S1 and the emitter of the second IGBT S2, or between the cathode of the second clamping diode D2 and the collector of the fourth IGBT S4;

The first IGBT S1, the second IGBT S2, the third IGBT S3, the fourth IGBT S4, the first clamping diode D1 and the second clamping diode D2 form a single-phase three-level circuit;

Laminated busbars include: positive busbar, negative busbar, zero busbar, AC busbar, first connecting busbar and second connecting busbar, among which:

The positive busbar is connected to the collector of the first IGBT S1 and the positive pole of the first DC support capacitor C1;

The negative busbar is connected to the emitter of the fourth IGBT S4 and the negative pole of the second DC support capacitor C2;

The zero busbar is connected to the anode of the first clamping diode D1 and the cathode of the second clamping diode D2;

The AC busbar is connected to the emitter of the second IGBT S2 and the collector of the third IGBT S3;

The first connecting busbar is connected to the emitter of the first IGBT S1 and the cathode of the first clamping diode D1;

The second connecting busbar connects the collector of the fourth IGBT S4 and the anode of the second clamping diode D2; the commutation circuit of the three-level converter includes the first commutation circuit, the second commutation circuit, and the third commutation circuit loop and the fourth commutation loop, wherein:

The first commutation circuit consists of positive busbar, first IGBT S1, second IGBT S2, AC busbar, third IGBT S3, second connecting busbar, second clamping diode D2, zero busbar and first resonant capacitor Cs1 composition;

The second commutation circuit consists of a second resonant capacitor Cs2, a zero busbar, a second clamping diode D2, a second connecting busbar, a fourth IGBT S4 and a negative busbar;

The third commutation circuit consists of the second resonant capacitor Cs2, the zero busbar, the first clamping diode D1, the first connecting busbar, the second IGBT S2, the AC busbar, the third IGBT S3, the fourth IGBT S4 and the negative busbar row composition;

The fourth commutation circuit is composed of the first resonant capacitor C s1, the positive busbar, the first IGBT S1, the first connecting busbar, the first clamping diode D1 and the zero busbar;

The two large commutation circuits (the first commutation circuit and the third commutation circuit) are symmetrical in structure, and the equivalent noise is equal; the two small commutation circuits (the second commutation circuit and the fourth commutation circuit) are structurally The distribution is also symmetrical, and the equivalent noise is equal; therefore, it is enough to test one of the large commutation loops and one of the small commutation loops, and the present invention tests the first commutation loop and the second commutation loop.

When testing the first commutation circuit (large commutation circuit), the freewheeling reactor L is connected between the collector of the first IGBT S1 and the emitter of the second IGBT S2; commutation circuit) for testing, the freewheeling reactor L is connected between the cathode of the second clamping diode D2 and the collector of the fourth IGBT S4, that is, the first DC support capacitor C1 and the second DC support capacitor C2 Between the connecting terminal and the connecting terminal of the third IGBT S3 and the fourth IGBT S4.

In addition, in Figure 1, L _σ1 , L _σ2 , L _σ3 , L _σ4 , L _σ5 , and L _σ6 are respectively positive busbar, negative busbar, zero busbar, AC busbar, upper connection busbar and lower connection busbar The equivalent stray inductance of L _s1 , L _s2 , L _s3 , and L _s4 are the equivalent stray inductance of the first IGBT S1, the second IGBT S2, the third IGBT S3, and the fourth IGBT S4 respectively; L _d1 , L _d2 is the equivalent stray inductance of the first and second clamping diodes D1 and D2 respectively; the DC power supply DCs is applied to the positive terminal DC+ and the negative terminal DC- of the laminated busbar; where DC0 represents the first DC support capacitor C1 and The intersection point of the second DC support capacitor C2 is the DC neutral line terminal; AC means the output AC.

The working principle of the test circuit of the present invention, that is, the test method of the commutation circuit spurious inductance of the three-level converter of the present invention (based on the present invention), is as follows:

When testing the first commutation circuit:

The freewheeling reactor L is connected between the collector of the first IGBT S1 and the emitter of the second IGBT S2; at this time, the first IGBT S1 and the fourth IGBT S4 are kept off, and the second IGBT S2 is kept on. The third IGBT S3 applies double pulses, that is, the third IGBT S3 is turned on at t0, turned off at t1, turned on again at t2, and turned off again at t3; as shown in Figure 2, it is the first commutation of the present invention Circuit experiment waveform diagram; in Figure 2, Vpulse is the driving voltage of the third IGBT S3, Ic is the current flowing through the collector of the third IGBT S3; Vce represents the voltage between the collector and the emitter of the third IGBT S3, that is, the third IGBT The terminal voltage of S3;

At time t0, the third IGBT S3 is turned on, the positive terminal DC+ of the laminated busbar discharges to the terminal DC0 through the freewheeling reactor L, and Ic rises linearly; it can be seen from Figure 1 that at this time Cs1 and L _σ1 + L + L _σ4 + L _s3 + L _σ6 + L _d2 + L _σ3 form the LC parallel resonance, which shows damped oscillation on Ic. At t1 and t3, the third IGBT S3 is turned off, and the current flowing through the freewheeling reactor L is freewheeling through the antiparallel diodes of the first and second IGBTs S1 and S2. Ic is the energy storage in the stray inductance and the first resonance Capacitor Cs1 resonates and eventually decays to zero. At t2, the third IGBT S3 is turned on again, the current in the freewheeling reactor L flows to the terminal DC0 through the third IGBT S3, and the anti-parallel diode of the first IGBT S1 enters the reverse recovery process, at this time, the first IGBT S1 is equivalent to turning on ; From this we can get the stray inductance L ₁ =L _σ1 +[L//(L _s1 +L _s2 )]+L _σ4 +L _s3 +L _σ6 +L _d2 +L _σ3 in the LC resonant circuit, where L// (L _s1 +L _s2 ) means that the equivalent stray inductance of the first and second IGBTs S1 and S2 are connected in series and then connected in parallel with L. Generally, the difference between L _s1 +L _s2 and L is about three orders of magnitude, so L//(L _s1 +L _s2 )=L _s1 +L _s2 , that is, L ₁ =L _σ1 +L _s1 +L _s2 +L _σ4 +L _s3 +L _σ6 +L _d2 +L _σ3 All stray inductances are included: the respective stray inductances of positive busbar, first IGBT S1, second IGBT S2, AC busbar, third IGBT S3, second connecting busbar, second clamping diode D2 and zero busbar.

则得到整个第一换流回路的杂散电感L₁=1/（4Cs1π²f²）=216nH。It can be obtained from Figure 2 that the frequency of the Ic current within the time period Δt is, f=4.5/Δt=730kHz, Δt represents the duration of 4.5 cycles shown in the figure; f is the first resonant capacitor Cs1 and the first commutation circuit The frequency of the parallel resonance current formed by the stray inductance; the known first resonance capacitance Cs1=0.22uF, according to the characteristics of LC parallel resonance

Then the stray inductance L ₁ of the entire first commutation circuit is obtained =1/(4Cs1π ² f ² )=216nH.

Please refer to Figure 3, which is the experimental waveform diagram of the first commutation circuit of the traditional test method, that is, the waveform when the third IGBT S3 is turned on at time t2 in Figure 2, and Vge in Figure 3 is the gate voltage of the third IGBT S3; ΔV _ce is the change value of S3 emitter and collector voltage; ΔI _c is the change value of S3 collector current; Δt' indicates the length of time that V _ce remains stable. According to Lenz's law, L _σ1 and L _σ6 in the equivalent stray inductance of laminated busbars will induce left positive and right negative voltages, and L _s1 , L _s2 , L _σ4 , L _s3 , L _d2 , and L _σ3 will induce Out of the positive and negative voltage. At this time, the terminal voltage Vce of the third IGBT S3 is lower than the DC bus voltage DCs, and the voltage difference is L _σ1 , L _s1 , L _s2 , L _σ4 , L _σ6 , L _d2 , The pressure drop of L _σ3 does not include the pressure drop caused by the stray inductance of L _s3 itself. At the same time, in order to measure the accurate value of stray inductance, it should be ensured that Vce has a stable and obvious voltage ladder. It is necessary to ensure that Vce obtains a stable voltage within the period from t20 to t21, which requires the current in the freewheeling reactor L to be large enough, which is bound to increase the supply voltage of the DC power supply DCs or greatly reduce the inductance value of the freewheeling reactor L. This requires the IGBT to withstand excessive voltage and current during the test, which may cause damage to the test device. According to the dynamic characteristics of the inductor, the value of the stray inductance is obtained, L1=ΔV _ce /(ΔI _c /Δt')=300V/(297A/198ns)=199nH. Compared with the method of the present invention, the difference of 17nH between the two is the stray inductance of the third IGBT S3 itself, which is consistent with the stray inductance value of 18nH in the data sheet of the used IGBT ABB 5SNA 1200G450300.

When testing the second commutation circuit:

The freewheeling reactor L is connected between the cathode of the second clamping diode D2 and the collector of the fourth IGBT S4; at this time, the first, second, and third IGBTs S1, S2, and S3 are kept turned off, and the second The four IGBTs S4 apply double pulses, that is, the fourth IGBT S4 is turned on at t0', turned off at t1', turned on again at t2', and turned off again at t3'. As shown in Figure 4, it is the second commutation circuit experiment waveform diagram of the present invention; Among Fig. 4, Vpulse' is the drive voltage of the 4th IGBT S4, and Ic' is the electric current flowing through the collector of the 4th IGBT S4; Vce' Indicates the voltage between the collector and emitter of the fourth IGBT S4, that is, the terminal voltage of the fourth IGBT S4; Vge' indicates the gate voltage of the fourth IGBT S4:

At time t0', the fourth IGBT S4 is turned on, the terminal DC0 discharges to the negative terminal DC- of the laminated busbar through the freewheeling reactor L, and Ic' rises linearly. It can be seen from Fig. 1 that Cs2 and L+L _s4 +L _σ2 form LC parallel resonance at this time, which shows damped oscillation on Ic'. At t1' and t3', the fourth IGBT S4 is turned off, and the current flowing through the freewheeling reactor L is freewheeling through the second clamping diode D2. Ic' is the energy storage in the stray inductance and resonates with the second resonant capacitor Cs2. and eventually decays to 0. At t2', the fourth IGBT S4 is turned on again, and the current in the flow reactor L flows through the fourth IGBT S4 to the negative terminal DC- of the laminated busbar, and the second clamping diode D2 enters the reverse recovery process. At this time, the second clamping diode D2 Bit diode D2 is equivalent to open. From this we can get the stray inductance L ₂ =L _σ3 +[L//(L _d2 +L _σ6 )]+L _s4 +L _σ2 in the LC resonant circuit, where L//(L _d2 +L _σ6 ) represents the second The clamping diode D2 and the equivalent stray inductance of the second connected busbar are connected in series and then connected in parallel with L. Generally, the difference between L _d2 + L _σ6 and L is about three orders of magnitude, so L//(L _d2 + L _σ6 ) = L _d2 +L _σ6 , that is, L ₂ =L _σ3 +L _d2 +L _σ6 +L _s4 +L _σ2 , indicating that all stray inductances in the second commutation circuit are included in this process: zero busbar, second clamping diode D2, the noise inductance of the second connecting busbar, the fourth IGBT S4 and the negative busbar.

则整个第二换流回路的杂散电感L₂=1/（4Cs2π²f²）=177nH。对比第一换流回路的杂感有39nH的差值，主要是因为第一换流回路比第二换流回路多包含2只IGBT和交流母排的杂感，其中2只IGBT大约为36nH（由IGBTABB 5SNA 1200G450300数据手册得出），进一步说明了本发明所公开方法的准确性。It can be obtained from Figure 4 that the frequency of Ic' current within Δt" is f'=7/Δt"=801kHz, Δt" represents the duration of 7 cycles shown in the figure; f' is the second resonant capacitor Cs2 The frequency of the parallel resonance current formed with the stray inductance of the second commutation circuit; the known first resonance capacitance Cs2=0.22uF, according to the characteristics of LC parallel resonance

Then the stray inductance L ₂ of the whole second commutation circuit =1/(4Cs2π ² f ² )=177nH. Compared with the noise inductance of the first commutation circuit, there is a difference of 39nH, mainly because the first commutation circuit contains two more IGBTs and the noise inductance of the AC busbar than the second commutation circuit, and the two IGBTs are about 36nH (by IGBTABB 5SNA 1200G450300 data book), which further illustrates the accuracy of the method disclosed in the present invention.

The above embodiments are only for the purpose of illustrating the present invention, rather than limiting the present invention. Those skilled in the relevant technical fields can also make various changes or modifications without departing from the spirit and scope of the present invention. Therefore, all Equivalent technical solutions should also belong to the scope of the present invention and should be defined by each claim.

Claims (3)

1. the metering circuit of the commutation circuit random thoughts of a three-level current transformer, it is characterized in that, comprise an IGBT, the 2nd IGBT, the 3rd IGBT, the 4th IGBT, the first clamp diode, the second clamp diode, the first resonant capacitance, the second resonant capacitance, the first DC support electric capacity, the second DC support electric capacity, stack bus bar, direct supply and afterflow reactor, wherein:

The emitter of a described IGBT connects the collector of described the 2nd IGBT; The emitter of described the 2nd IGBT connects the collector of described the 3rd IGBT; The emitter of described the 3rd IGBT connects the collector of described the 4th IGBT;

The anode of described stack bus bar connects respectively the positive pole of the anodal and described direct supply of the collector of a described IGBT, described the first DC support electric capacity; The negative terminal of described stack bus bar connects respectively the negative pole of the emitter of described the 4th IGBT, described the second DC support electric capacity and the negative pole of described direct supply;

The negative pole of described the first DC support electric capacity connects the positive pole of described the second DC support electric capacity;

Described the first resonant capacitance and described the first DC support Capacitance parallel connection;

Described the second resonant capacitance and described the second DC support Capacitance parallel connection;

The negative electrode of described the first clamp diode connects the emitter of a described IGBT; The negative electrode of described the second clamp diode of the anodic bonding of described the first clamp diode; The collector of described the 4th IGBT of the anodic bonding of described the second clamp diode;

Described the first clamp diode is held the end that joins that connects described the first DC support electric capacity and the second DC support electric capacity with being connected joining of clamp diode;

Described afterflow reactor is connected between the emitter of the collector of a described IGBT and the 2nd IGBT, perhaps is connected between the collector of the negative electrode of described the second clamp diode and described the 4th IGBT;

Described stack bus bar comprises: positive busbar, negative busbar, zero busbar, alternating current bus bar, first connect busbar and connect busbar with being connected, wherein:

Positive busbar connects the collector of an IGBT and the positive pole of the first DC support electric capacity;

Negative busbar connects the emitter of the 4th IGBT and the negative pole of the second DC support electric capacity;

Zero busbar connects the anode of the first clamp diode and the negative electrode of the second clamp diode;

Alternating current bus bar connects the emitter of the 2nd IGBT and the collector of the 3rd IGBT;

First connects busbar connects the emitter of an IGBT and the negative electrode of the first clamp diode;

Second connects busbar connects the collector of the 4th IGBT and the anode of the second clamp diode.The commutation circuit of described three-level current transformer comprises the first commutation circuit and the second commutation circuit, wherein:

The first commutation circuit comprises that described positive busbar, an IGBT, the 2nd IGBT, alternating current bus bar, the 3rd IGBT, second connect busbar, the second clamp diode, zero busbar and the first resonant capacitance;

The second commutation circuit comprises that the second resonant capacitance, zero busbar, the second clamp diode, second connect busbar, the 4th IGBT and negative busbar.

2. the metering circuit of the commutation circuit random thoughts of described three-level current transformer according to claim 1 is characterized in that,

When the first commutation circuit was tested, the afterflow reactor was connected between the emitter of the collector of an IGBT and the 2nd IGBT;

When the second commutation circuit was tested, the afterflow reactor was connected between the collector of the negative electrode of the second clamp diode and the 4th IGBT.

3. the measuring method based on the commutation circuit random thoughts of the three-level current transformer of the described metering circuit of claim 1 is characterized in that, comprises the following steps:

The first commutation circuit is tested:

Step 1, afterflow reactor are connected between the emitter of the collector of an IGBT and the 2nd IGBT; Make an IGBT and the 4th IGBT keep turn-offing, it is open-minded that the 2nd IGBT keeps, and the 3rd IGBT is constantly open-minded at t0, and t1 turn-offs constantly, and t2 is constantly again open-minded, and t3 turn-offs constantly again;

Step 2 by t2 to t3 opening process constantly, is tried to achieve the frequency f of parallel resonance electric current of the stray inductance formation of the first resonant capacitance and the first commutation circuit, and according to the characteristic of LC parallel resonance, obtains the stray inductance L of the first commutation circuit ₁=1/(4Cs1 π ²f ²); Wherein, Cs1 represents the capacitance of the first resonant capacitance;

The second commutation circuit is tested:

Step 3, afterflow reactor are connected between the collector of the negative electrode of the second clamp diode and the 4th IGBT; So that first, second, and third IGBT all keeps turn-offing, the 4th IGBT S4 is constantly open-minded at t0 ', and t1 ' turn-offs constantly, and t2 ' is constantly again open-minded, and t3 ' turn-offs constantly again;

Step 4 is passed through t2 ' to t3 ' opening process constantly, tries to achieve the frequency f of parallel resonance electric current of the stray inductance formation of the second resonant capacitance and the second commutation circuit ', and according to the characteristic of LC parallel resonance, obtain the stray inductance L of the second commutation circuit ₂=1/(4Cs2 π ²f ²); Wherein, Cs2 represents the capacitance of the second resonant capacitance.

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