1.Refusol three-phase photovoltaic grid-connected power generation circuit:
Refusol three-phase photovoltaic grid-connected power generation circuit consists of three single-phase circuits, which share two symmetric BOOST converters, and their common connection nodes are V1, V2, Vn, V3 and V4, as shown in FIG. 6.29. Each single phase has BUCK and BOOST working modes in half A grid cycle (if VPV/2 is always higher than the absolute value of the instantaneous voltage of the grid, then each single phase only has BUCK working mode). Taking phase A as an example, its working principle is explained as follows:
①When VPV / 2 > | Va |, is that half of the pv array output voltage has been more than the absolute value of the grid voltage instantaneous value, A phase circuit work in BUCK mode, it is single stage type working mode.
When the grid voltage is positive half cycle, switch Sa1 works at high frequency, switch Sa6 is always on, and other switches are off. When switch Sa1 is on, the current generated by the PV array flows into the midpoint B through switch Sa1, filter inductor La, and single-phase grid. When switch Sa1 is turned off, the current on the line is continued through filter inductor La, single-phase grid and switch Sa6 loop.
During the negative half cycle of the grid voltage, the switch Sa2 works at high frequency, the switch Sa5 is always turned on, and the other switches are turned off. When the switch Sa2 is turned on, the current generated by the photovoltaic array flows out from the midpoint B, and reaches the negative output end of the photovoltaic array through the single-phase power grid, the filter inductor La, and the switch Sa2. When the switch Sa2 is turned off, the current on the line is freewheeling through the filter inductor La, the single-phase power grid, and the switch Sa5 loop.
②When VPV/2<lVal, that is, the output voltage of half of the photovoltaic array is always not higher than the absolute value of the instantaneous value of the grid voltage, the A-phase circuit works in the BOOST mode, which is a two-stage working mode.
During the positive half cycle of the grid voltage, the switches Sa3 and S1 work at high frequency, the switch Sa6 is always on, and the other switches are off. The BOOST converter controlled by the switch S1 works independently, and the high-frequency switch of the switch S1 ensures that the corresponding BOOST converter outputs a stable DC voltage. When the switch Sa3 is turned on, the output current of the BOOST converter flows into the midpoint B through the switch Sa3, the filter inductor La, and the single-phase power grid. When the switch Sa3 is turned off, the current on the line is freewheeling through the filter inductor La, the single-phase grid, and the switch Sa6 loop.
During the negative half cycle of the grid voltage, the switches Sa4 and S2 work at high frequency, the switch Sa5 is always turned on, and the other switches are turned off. The BOOST converter controlled by the switch S2 works independently, and the switch S2 works at a high frequency to ensure that the corresponding BOOST converter outputs a stable DC voltage. When the switch Sa4 is turned on, the current generated by the photovoltaic array flows out from the midpoint B, and reaches the negative output end of the photovoltaic array through the single-phase power grid, the filter inductor La, the switch Sa4, and the BOOST converter. When the switch Sa4 is turned off, the current on the line is freewheeling through the single-phase power grid, the filter inductor La, and the switch Sa5 loop.
It can be seen from the above working state analysis that during the current freewheeling period, there is no energy transmission between the photovoltaic array and the single-phase power grid, which avoids the occurrence of high-frequency voltage components at the output end of the photovoltaic array, thus suppressing the generation of common mode current. The maximum efficiency of the circuit is 98.6%.
2.Steca single-phase photovoltaic grid-connected power generation circuit:
Steca single-phase photovoltaic grid-connected power generation circuit is shown in Figure 6.30. It consists of two symmetrical buck converters and a power frequency polarity control full-bridge inverter circuit. The buck converter 1 is composed of C1, S1, VD1, and L1, and the buck converter 2 is composed of C2, S2, VD2, and L2. It has basically the same starting point as the Refusol single-phase photovoltaic grid-connected power generation circuit, but the Steca single-phase photovoltaic grid-connected power generation circuit requires that the output voltage of the photovoltaic array is always higher than the absolute value of the instantaneous value of the grid voltage, so it only has the BUCK working mode, the photovoltaic array The input voltage range is much smaller than the Refusol single-phase photovoltaic grid-connected power generation circuit.
①Traditional BUCK working mode
The traditional BUCK working mode is similar to the H6 circuit. Switches S3~S6 operate at grid frequency, S1 and S2 operate at high frequency at the same time. The specific working mode is: during the positive half cycle of the power grid, S1 and S2 switch at high frequency at the same time, S3 and S6 are always on, and S4 and S5 are always off. When S1, S2, S3 and S6 are on at the same time, the energy of the photovoltaic array is transmitted to the grid through the filter. When S1 and S2 are turned off at the same time, the current on the line will continue to flow through the inductor L1, switch S3, filter inductor L3, single-phase power grid, filter inductor L4, switch S6, inductor L2, diode VD2 and VD1 circuits.
Similarly, when the power grid has a negative half cycle, S1 and S2 work at high frequency at the same time, S4 and S5 are always on, and S3 and S6 are always off. When S1, S2, S3 and S6 are on at the same time, the energy of the photovoltaic array is transmitted to the grid through the filter. If S1 and S2 are shut off at the same time, the current on the line will continue to flow through the circuits of inductor L1, switch S5, filter inductor L4, single-phase power grid, filter inductor L3, switch S4, inductor L2, diode VD2 and VD1.
According to the above analysis, there is no energy transmission between the photovoltaic array and the single-phase grid during the current continuation period, so the path of common mode current generation is cut off.
②Steca time-sharing mode
In the traditional BUCK mode, S1 and S2 always operate at high frequency, so the switching loss is high. Steca’s time-sharing mode improves this. In half a cycle, when VPV / 2 > | Vout |, is that half of the pv array output voltage has been more than the absolute value of the grid voltage instantaneous value, Steca circuit of a BUCK converter working in high frequency, another BUCK converter to stop; When VPV / 2 | or less Vout | < VPV, is that the absolute value of the grid voltage instantaneous value has been more than half the output voltage of the pv array, and below the output voltage of pv arrays, Steca circuit of a BUCK converter working in high frequency, the switch has been conducting a BUCK converter. Grid is half a cycle, when VPV / 2 > | Vout |, S2 high frequency work, have been conducting S3, S6, S1, S4, S5 straight off. When S2, S3 and S6 are on at the same time, half of the energy of the photovoltaic array is transmitted to the power grid through inductance L2, capacitors C2 and C3, switches S3 and S6, and filter inductors L3 and L4. When S2 shut off, the circuit of electric current through the C3, switch S3, filtering inductance L3, single-phase power grid, filtering inductance L4, switch S6, inductor L2, diode VD2 fly-wheel circuit, when VPV / 2 | or less Vout | < VPV, high frequency work S1, S2, S3, S6 has been conducting, S4, S5 shut off all the time. When S1, S2, S3 and S6 are on at the same time, the energy of photovoltaic array is transmitted to the power grid through inductors L1 and L2, switch tubes S3 and S6 and filter inductors L3 and L4. When S1 is turned off, the current on the line is continued through circuits C2, VD1, inductor L1, switch S3, filter inductor L3, single-phase power grid, filter inductor L4, switch S6, inductor L2, and switch S2. Grid negative half cycle, when VPV / 2 > | Vout |, S1 high-frequency work, have been conducting S4, S5, S2, S3, S6 straight off. When S1, S4 and S5 are on at the same time, half of the energy of the photovoltaic array is transmitted to the power grid through inductor L1, capacitor C1, C4, switch S4 and S5, and filter inductor L4 and L3. When S1 is turned off, the current on the line goes through C4, diode VD1, inductor L1, switch tube S5, filter inductor L4, single-phase grid, filter inductor L3, switch S6 loop for continuous flow. When VPV / 2 | or less Vout | < VPV, S2 high frequency work, have been conducting S1, S4, S5, S3, S6 have been shut off. When S1, S2, S4 and S5 are on at the same time, the energy of the photovoltaic array is transmitted to the power grid through inductors L1, L2, switches S4 and S5 and filtering inductors L3 and L4. When S2 is turned off, the current on the line continues to flow through VD2, C1, switch S1, inductor L1, switch S5, filter inductor L4, single-phase grid, filter inductor L3, switch S4, inductor L2 loops.
According to the above working state analysis, there is also no energy transmission between the photovoltaic array and the single-phase power grid during the current continuous flow, which avoids the occurrence of high-frequency voltage components at the output end of the photovoltaic array, thus reducing the common-mode current. The maximum efficiency of Stecagrid 3600 circuit is 98.6%
