There are two output forms of three-phase inverter: one is three-phase three-wire output; the other is three-phase four-wire output. Ordinary inverters are three-phase three-wire output, that is, only line voltage can be output. In order to obtain the phase voltage output, that is, to convert the ordinary three-phase inverter into a three-phase four-wire system output, the following methods are generally used.
①Use the △/Y three-phase transformer output, and use the secondary Y connection method to form the neutral point.
②Use the neutral point to form the transformer (Neutral Forming Transformer, NFT) to form the neutral point. The NFT is actually an autotransformer with a ratio of 1:1.
③ Use the midpoint of the DC input power supply of the three-phase inverter as the neutral point.
④Using three single-phase full-bridge inverters with independent power supplies, and connecting the midpoints of the bridge arms on the same side to form a neutral point.
⑤ Use the midpoint of one bridge arm of the new three-phase four-arm inverter as the neutral point.
The biggest advantage of the three-phase four-leg inverter is that no three-phase △/Y output transformer or NFT neutral point is used to form a transformer, which greatly reduces the volume and quality of the inverter and greatly reduces the cost.
1. The working principle and working mode of the three-phase four-leg inverter
The main circuit of the three-phase four-arm inverter is shown in Figure 1. Among them, the upper picture is a three-phase circuit, and the lower picture is an A-phase circuit. It can be known from the three-phase main circuit that it is composed of a three-phase half-bridge inverter and an additional common bridge arm (composed of S7 and S8). Switch tubes S1, S4 and switch tubes S7, S8 form an A-phase full-bridge inverter; switch tubes S3, S6 and switch tubes S7, S8 form a B-phase full-bridge inverter; switch tubes S5, S2 and switch tubes S7, S8, a C-phase full-bridge inverter is formed; switch tubes S7 and S8 are used to form a neutral point common bridge arm. Since S7 and S8 are common bridge arms, the driving of the inverter switches of the three bridge arms of A, B and C and the excitation of the output current will be mutually restrained. This is an important problem that must be solved in the control circuit of the three-phase four-leg inverter.
For the inverter main circuit shown in Figure 1, when the filter inductor Lf and the filter capacitor Cf are ignored, it can be simplified to the equivalent circuit shown in Figure 2. Among them, each bridge arm has two switching modes, such as the A-phase bridge arm, when the upper tube is turned on and the lower tube is turned off, the voltage at point A is UAg=E, and the switching mode of this bridge arm is defined as SA=1, that is UAg=E(SA)=1E; when the lower tube is turned on and the upper tube is turned off, the voltage at point A is UAg=0, and the switching mode of this bridge arm is defined as SA=0, that is, UAg=E(SA)=0·E= 0. According to this definition, the four bridge arms of A, B, C, O have a total of 24=16 switching modes M0 (SA, SB, SC, SO) ~ M15 (SA, SB, SC, SO). Among them, there are two zero switching modes M0 (0, 0, 0, 0) and M15 (1, 1, 1, 1) and 14 non-zero switching modes M1 (0, 0, 0, 1) ~ M14 (1 , 1, 1, 0), there are 16 different switching modes in total, as shown in the first column in Table 1.
The voltage between the midpoints A, B, C, and 0 of each bridge arm to point g depends on the switching mode of each bridge arm. From the definition of the switching mode of each bridge arm, we can get
The voltage between the midpoint of each bridge arm and the midpoint 0 of the 0 bridge arm, such as the voltage UAO between the A point of the A bridge arm and the O point of the O bridge arm, is equivalent to the inverter composed of the A bridge arm and the 0 bridge arm. The voltage between the midpoints of the two arms of the bridge, so there is
Among the 16 switch modes, the values of UAg, UBg, UCg, UOg, UAo, UBO, UCO (and Uoo=0) are shown in the second and third columns in Table 1.
2. AC filter circuit
The parameters of the AC output filter of the three-phase four-leg inverter directly affect the quality of the output power, such as the distortion rate of the output voltage waveform, the symmetry of the three-phase AC voltage and the ability to resist short circuit. Generally, the AC output filters of three-phase half-bridge inverters are exactly the same three LfCf, low-pass filters. However, for a three-phase four-bridge inverter, since there is a neutral point to form a bridge arm, the output AC filter is based on the above filter and a neutral filter inductor Lfo is added, as shown in Figure 1. shown. There are two functions of adding Lfo: one is to reduce the harmonic content in the neutral current io; the other is to strengthen the overall filtering performance, and on the premise of the same overall filtering effect, try to reduce the overall volume and quality of the filter . When the filter capacitor Cf adopts MLC (MultiLayer Ceramic) multilayer ceramic capacitor, the value of Cf should be selected as large as possible to reduce the overall volume and quality of the filter. As for the selection of the neutral line inductance Lfo, two aspects should be considered: one is the filtering effect of the neutral line current; the other is to minimize the volume and quality of the filter.
1) Select Lfo from Limit Neutral Current Harmonics.
Obtain the equivalent circuit related to 4 filter inductors from Figure 3, as shown in Figure 4.
Assuming Lfa=Lfb=Lfc=Lfφ, Lfo=kLfφ, it can be obtained from Figure 4
Since the control circuit stabilizes the output voltage, there are
For asymmetric loads, there are
Simultaneously solving the above equations, we can get
It can be seen from the expression of dio/dt that the larger the k value is, the smaller the dio/dt is, which indicates that the larger the neutral line inductance Lfo is, the better the speed of io is.
2) Select lfo by reducing AC filter volume and mass.
Assuming Lfo=kLfφ, T.M.Jahns of GE Company studied the influence relationship of choosing different proportional coefficient k on the volume, quality and filtering effect of AC filter, as shown in Table 2.
In Table 2, Lref is the characterizing inductance, and its value can be obtained from the circuit shown in Figure 5, namely
Leq in Table 2 is the equivalent inductance, namely
When k<1, ζ=0.5; when k=1, ζ=0.375.
It can be seen from Table 2 that when k=0.5, the volume and mass of the AC filter can be minimized under the same filtering effect. In addition, Lfo also has a balancing effect on the phase current, the larger the Lfo, the stronger the balancing effect. Lfo also affects the dynamic performance and current tracking speed of the inverter. The larger the L%, the worse the dynamic performance and the slower the current tracking speed. Considering the above factors, it can be considered that when k=0.5, the volume, quality, filtering effect and dynamic performance can be taken into account.
3. The control method of the three-phase four-leg inverter
Since a common bridge arm is added to the three-phase four-arm inverter, the driving of the three-phase four-arm inverter will have a pinning effect, which makes the control method of the three-phase four-arm inverter special. In order to adapt to the asymmetric load, it is more appropriate to use the current hysteresis loop pulse width modulation (Hysteresis Current Pulse Width Modulator, HCPWM). Current hysteresis is divided into two-state hysteresis (hysteresis output two-level), three-state hysteresis (hysteresis output three-level) and multi-state hysteresis (hysteresis output multi-level). is a tri-state hysteresis (tri-state HCPWM).
The block diagram of the A-phase three-state HCPWM control circuit of the three-phase four-arm inverter is shown in Figure 6. Its working process is: the three-state HCPWM periodically samples and holds the filter inductor current iaL and the given current ira (Sample/Hold, S/H), and the filter inductor Lfa acts as the integral link of HCPMW to provide a ramp function for the current closed-loop control. At the sampling point, if ira-iaL=Δi<-h (h is the hysteresis width), then S7 and S4 are turned on, and +E is added to both ends of Ifa to increase iaL; if ira-iaL=Δi> +h, then turn on S8 and S1, add -E to both ends of Lfa to reduce iaL; if ira-iaL=Δi, -h<Δi<+h, then make S7, S1 or S8, S4 lead Turn on, short-circuit Lfa to make the inverter freewheel naturally.
Since the inductor Lfa is both a filter inductor and an integral part of the HCPWM, the selection of Lfa should not only ensure a good filtering effect, but also ensure that iaL can track ira well. The hysteresis width generally takes 10% of iaL.
The three-phase four-bridge-arm inverter can be controlled by three three-state HCPWM control circuits as shown in Figure 6, but the restraint effect caused by the common bridge arm should be removed. The solution is to use the maximum error current to control S7, S8 or A, B, C three-phase control of the conduction of S7, S8 in turn. The former has high control precision and the latter has good fault tolerance. The block diagram of the three-phase three-state HCPWM control circuit is shown in Figure 7. Use ira-iaL, irb-ibL, and irc-iCL for comparison, and take the larger one to control S7 and S8.
For the dual-loop tri-state HCPWM control circuit shown in Figure 7, there is a static difference in the output voltage when the load changes. In order to eliminate the static difference, in addition to the serial connection of the PI regulator, the current positive feedback iaof and the output current ira of the PI, and the output signal of the micro-branch circuit of the uar are added as a “load current feedforward voltage regulator”, Its principle block diagram is shown in Figure 8. Since the transfer function of the voltage regulator is constant, its characteristics are independent of the load, so there is no static voltage difference.
