1) Full bridge inverter
Bipolar PWM modulation mode is adopted. As shown in Figure 1.11, in a switching cycle, when S1 and S4 are on, S2 and S3 are off, S1 and S4 are off, and S2 and S3 are on, it can be seen that as long as VPV remains constant, the common mode voltage VCM can remain unchanged, so as to ensure that the common mode leakage current is zero.
According to the PWM control mode, the four power devices are operated by high-frequency switches, so the switching loss is large. At the same time, the output voltage is bipolar, which also increases the loss of AC filter inductance, resulting in a slightly lower efficiency of the whole circuit, and the maximum efficiency is generally not more than 95%, but this circuit is suitable for reactive power compensation.
2) Half bridge inverter
It can be seen from figure 1.12 that the midpoint of the capacitance bridge arm of the half bridge inverter is directly connected to the zero line of the power grid, so at this time, the common mode voltage on the parasitic capacitance to ground is independent of the power switch bridge arm and is only determined by v2n [112]. If the capacitance C1 and C2 are equal and large enough, vcm=v2n=0.5vpv remains unchanged. Therefore, the half bridge inverter basically does not produce common mode current. However, the output voltage of half bridge inverter is only half of the input voltage, and it is bipolar, with low voltage utilization and large AC filter inductance loss. Under the same grid voltage condition, the input voltage is required to be twice that of the full bridge inverter, which leads to high voltage withstand selection of power switching devices, a large number of photovoltaic modules in series, low conversion efficiency of the whole device, and the maximum efficiency is generally not more than 96%.
3) Three level half bridge inverter (NPC circuit)
It can be seen from figure 1.13 that NPC circuit generally consists of two identical photovoltaic branches and split capacitors in parallel as a bridge arm, and the other bridge arm, composed of four power switches, is connected to the midpoint of the split capacitor through two clamping diodes [112.113]. Since l2=0, at this time, the common mode voltage can be obtained from equation (1.3):
Its working principle is: when the power grid is in the positive half cycle, S2 is always on, and S1 and S3 high-frequency complementary switches act. When S1 is on and S3 is off, the energy of the upper half bridge photovoltaic branch is transmitted to the power grid through S1, S2 and filter. If S1 is off and S3 is on, the current flows through diodes vd5 and S2. When the power grid is in negative half cycle, S3 is always on, and S2 and S4 high-frequency complementary switches act. When S4 is turned on and S2 is turned off, the energy of the lower half bridge photovoltaic branch is transmitted to the power grid through S3, S4 and filter. If S4 is turned off and S2 is turned on, the current flows through diodes vd6 and S3. It can be seen from figure 6.14 that the common mode voltage fluctuates with a small amplitude of 50Hz, resulting in a very small common mode current, which basically overcomes the influence of common mode current. Reference [112] has improved NPC circuit, and its maximum efficiency can reach 98.16%.
4) Herric circuit (highly efficient and reliable inverter concept)
It can be seen from figure 6.15 that the herric circuit adds an AC switch composed of S5 and S6 to provide a freewheeling path for the inverter output current when s1~s4 are turned off.
The specific working mode is: when the power grid is in the positive half cycle, S1 and S4 are switched on at the same time, S2 and S3 are turned off, S5 is always on, and S6 is always off. If S1 and S4 are connected at the same time, the energy of the photovoltaic array is transmitted to the power grid through S1, S4 and filter. The waveform of the inverter output voltage, common mode voltage and common mode current of the herric circuit is shown in Figure 1.16.
From the above analysis, it can be seen that the common mode voltage vcm=0.5vpv remains unchanged, so the circuit topology basically does not generate leakage current to the ground. At the same time, it can be seen that due to the existence of the freewheeling path on the AC side, there are only two high-frequency switching losses in the positive and negative half cycles of the power grid, which is half the switching loss of the full bridge inverter. The output voltage is the same as the unipolar modulation method, and also reduces the loss of the filter inductance, with the maximum efficiency of about 97.8%[114]. Of course, the AC switch composed of S5 and S6 can also be replaced by a switch and a diode rectifier bridge, but because the replaced AC switch works in high-frequency mode, there are dead time problems and slightly low efficiency problems [115].
5) Herric derivative circuit
The idea of this circuit comes from the herric circuit. As shown in Figure 1.17, its efficiency is slightly lower than that of the herric circuit.
When the grid voltage is positive for half a cycle, switch S5 is always on, switches S2, S3 and S6 are always off, and switches S1 and S4 are turned on and off in PWM mode at the same time
When switches S1 and S4 are turned on at the same time, the current generated by the photovoltaic array flows through anti reverse charging diode VD1, switch S1, switch S5, filter inductor L1, single-phase power grid, filter inductor L2 and switch S4.
When switches S1 and S4 are turned off at the same time, the current on the line continues to flow through the anti parallel diode of switch S5, filter inductor L1, single-phase power grid, filter inductor L2 and switch S6. At this time, there is no electrical connection between the photovoltaic array and the single-phase power grid, thus avoiding the high-frequency voltage component on the DC bus, so the high-frequency common mode current is effectively suppressed.
When the grid voltage is negative for half a cycle, switch S6 is always on, switches S1, S4 and S5 are always off, and switches S2 and S3 are turned on and off in PWM mode at the same time.
When switches S2 and S3 are turned on at the same time, the current generated by the photovoltaic array flows through anti reverse charging diode VD1, switch S3, filter inductor L2, single-phase power grid, filter inductor L1 and switch S2.
Similarly, when switches S2 and S3 are turned off at the same time, the current on the line continues to flow through the anti parallel diode of switch S6, filter inductor L2, single-phase power grid, filter inductor L1, and switch S5. At this time, the electrical connection between the photovoltaic array and the single-phase power grid is cut off, thus avoiding the high-frequency voltage component from appearing on the DC bus, so the high-frequency common mode current is also effectively suppressed at this time.
6) H6 circuit
It can be seen from figure 6.18 that it is a full bridge inverter with two DC block switches. Switches s1~s4 operate at grid frequency, while S5 and S6 operate at high frequency at the same time. Its specific working mode is: when the power grid is in the positive half cycle, S5 and S6 are switched on at the same time, S1 and S4 are always on, and S2 and S3 are always off. When S5, S6, S1 and S4 are turned on at the same time, the energy of the photovoltaic array is transmitted to the power grid through the filter, and the waveform of the inverter output voltage, common mode voltage and common mode current of the H6 circuit is shown in Figure 1.19.
From the above analysis, it can be seen that the common mode voltage vcm=0.5vpv remains unchanged, so the circuit topology does not generate leakage current to the ground. The forward voltage of switches S5 and S6 is 0.5vpv. Although it is a high-frequency switch, the switching loss is greatly reduced. At the same time, switches s1~s4 have only conduction loss, and almost no switching loss. The output voltage of the inverter is also unipolar, and the filter loss is also very small, so the efficiency of the whole circuit is high, and the maximum efficiency can reach 97.4%[116].
7) H5 circuit
H5 circuit can be simplified from H6 circuit, as shown in Figure 6.20. It is a patented circuit applied by German SMA Technology Co., Ltd. S1 and S3 act at the grid frequency, and S5, S2 or S5 and S4 act at high frequency. Its specific working mode is: when the power grid is in the positive half cycle, S5 and S4 are switched on at the same time, S1 is always on, and S2 and S3 are always off. When S5, S1 and S4 are turned on at the same time, the parasitic capacitance discharges very slowly, so theoretically v1n and v2n are close to 0.5vpv. In fact, the common mode voltage fluctuates within a certain range, but the resulting common mode current meets the German DIN VDE 0126-1-1 standard, as shown in Figure 1.21.
Similarly, the commutation mode and common mode voltage analysis of the negative half cycle of the power grid are similar to the positive half cycle of the power grid. Its advantages are the same as those of H6 circuit, but the maximum efficiency can reach 98.1%[117]. H5 circuit has the least power devices and the lowest cost of main circuit.
8) Quasi H5 circuit
In order to improve the common mode voltage fluctuation of H5 circuit, H6 circuit is simplified into two circuits with clamping diodes, which are called quasi H5 circuit [118], as shown in Figure 6.22. While having many advantages of H5, the common mode voltage will also be better suppressed. As shown in Figure 6.23, the voltage on capacitors cde1 and cde2 is not unbalanced, and the common mode voltage is well clamped at 0.5vpv.
9) Hybrid bridge arm circuit
It consists of an NPC circuit bridge arm and a two level bridge arm [119120], as shown in Figure 6.24. Switches s1~s4 work in high frequency mode, and S5 and S6 work in grid frequency. Its specific working mode is: when the power grid is in the positive half cycle, S1 and S4 are switched on at the same time, S5 is always on, and S2, S3 and S6 are always off. When S1, S4 and S5 are connected at the same time, the energy of the photovoltaic array is transmitted to the power grid through the filter,
10) Three switch bridge arm circuit
The design idea of this circuit also comes from herric circuit [121], as shown in Figure 6.26 (a) and (b). Obviously, the efficiency of this circuit is reduced because the conduction loss is higher than that of the herric circuit. The efficiency of the full load machine is about 97% [122], and its working principle is similar to that of the herric derivative circuit, which will not be repeated here.
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