Investigation of improved Differential Y-source inverter

Y-typed networking with three winding-coupled inverter is a brand-new DC-DC and DC-AC inverter. The inverter has a simple topology structure. However, it owns powerful functions. The inverter, with two Y-shaped structures, has not adopted two independent circuits, which are replaced by some new topology parts, such as the switches, adding up to control the current. Obviously, it has now become a single-stage inverter. The improved inverter has also been called DYSI, the Differential Y-source Inverter, for it combines the Differential Boost Inverter with the Y-source Inverter. The improved inverter would produce high AC output voltage, solving the problems caused by the low output voltage in photovoltaic inverters while increasing voltages to connect with the national grid. The modulation of the improved inverter is the half-sine wave, which drives one coupled and two switches in the branch to conduct at high times. The improved inverter also has some defects. The inverter would produce abrupt energy leakage because of the incomplete coupling of inductances. Similar to other inverters, the process during which the switches turn on and turn off would inevitably generate power loss, leading to a much lower output than the ideal design. In contrast, the deficiency could be decreased by adding two diodes without loss to the circuit topology.


Introduction
In this age of change, the development of modern photovoltaic techniques has been increasingly prosperous, with the impacts of both the development of innovation of new energy and the declination of the cost of electricity.In other words, the new energy of innovation renders entrepreneurs, businesses, and individual users well-accepted using the photovoltaic (PV) technique.In addition, the state supports new energy power generation to be connected to the national grid, reducing the cost of photovoltaic generation enterprises by subsidizing enterprises.Both measures mentioned above have made some breakthroughs, further developing the technique to link the PV part in the photovoltaic circuit to the distributed AC national grid.In the whole photovoltaic system, as is known in common practice that the photovoltaic panels could only emerge the unstable and lower direct output voltage and current.Hence, adding an inverter into the topology is necessary to stabilize the voltage and smoothly connect the photovoltaic part with the state grid [1,2].
In order to improve the input voltage remarkedly, such as at least four to five times, to connect to the output AC circuit, it is a normal way to consider, called centralized power generation technology.It first aims to link the photovoltaic in series because of a series boosting theory.For the next step, make the obtained high voltage go through the inverter to reach the suitable AC voltage that the big grid acquires [3].But in this way, the photovoltaic panels must be connected to produce a high output, which always brings the problem of mismatching, causing the issues of lower efficiency as well as checking and fixing frequently in daily life.To sum up, using it is not an ideal method [4,5].
Another common pattern is adding boosting circuits to every photovoltaic converter, which means the two independent circuit structures of DC-DC and DC-AC are connected.The former stage circuit achieves a voltage boost through DC-DC, and the latter stage achieves direct flow to alternating flow through DC-AC.However, there are inevitably many active switches and driving circuits in this topology, so there would be a lot of switching and device losses.At the same time, the inverter bridge circuit is more susceptible to interference and leads to abnormal work.Thus the efficiency is greatly reduced [6].In conclusion, both of the patterns discussed above are not ideal in a sense.
Apart from this, Differential Boost Inverters (DBI) and Z-source inverters are often used to achieve this function.A Differential Boost Inverter consists of double independent bidirectional boost converters, as shown in figure 1, which could generate an AC output by connecting both sides of the load to the output part [7].The supposed method above is widely used in other DC-DC topologies and connected with other inverters.This inverter is a single-stage inverter; instead of the former dual-stage boost inverter, circuit devices have relatively fewer structures that are easy to understand and operate and have fewer components-more reliable in practice [8].Whereas, for the parasitic resistance from some components, the method could only achieve a very limited voltage boost and could not even meet the common standard of connecting to the large grid, especially when it conducts at a high on-off ratio.
For this reason, the coupled inductors are added to the topology to boost the voltage.However, the abrupt change of the current generated by the leakage inductance in the coupling inductors leads to voltage spikes in current going through the switches.At this point, to solve the leakage inductance problem, it is usually the better way to add the buffers.As for the Z-source inverter, as shown in figure 2, it has the function of single-stage in boost/buck circuits.And its impedance network is responsible for voltage boost.Besides, it regulates the bus line voltage by controlling the pass-through zero vector through the legs of the bridge [9].What is more, the life span of the input power source of the Z-source inverter would be shortened because of the problems in the structure of the circuit itself [10].
Figure 2. The Z-source inverter [1].One solution to achieve the supposed output voltage is adding more energy storage components like inductors and capacitances, but this method would increase circuit losses and could not improve the situation well.In addition, the Z-source inverter often produces a huge current when it is started.In addition, the input current could not be fairly continuous, so the Quasi-Z-source inverter is proposed in the latest research, as shown in figure 3.

Operation and modulation
In real life, the coupled inductors of any circuit cannot be completely ideal, so the several inductors used in the circuit have their impedance and some functions which would be affected by the parasitic resistance, which would finally cause the voltage gain reduction in any case.Therefore, the parasitic resistance of the coupling inductance and the resistance of the switch during the on-off should be considered when calculating some values and analysis.At the same time, the size of the DC capacitor (C1, C2) could be ignored when a thin film capacitor rather than an electrolytic capacitor is applied in practice (figure 4).

During the state 2
After state 1, there is a shot dead zone in which both S1 and S2 are off, which is similar to state 3, S1 body diode is on in state 2, and S1 itself is on in state 3, so only state 3 needs to be analyzed.

During the state 3
Lm discharges through winding an N1 and indirectly to N3 through coupling inductors.The two windings in series then transfer the energy to C1.Values could be calculated during the above analysis.Similarly, the values of DC capacitor C2 could be derived in a negative half period by the same analysis procedures.
The modulation adopted by the DYSI is the half-sine modulation.The common DBI modulation is to switch four switches continuously at high frequencies, thus resulting in extremely high switching losses.And DYSI uses half-sine modulation so that the boost converter has intermittent work; that is, DYSI generates a half-sine wave on a phase leg in each half-basic period (figure 5).Besides, the output of another phase leg is controlled to a certain voltage value.The two groups generate a difference of 180 degrees with a DC bias of a 'steamed bun-shaped wave' [11].Then, according to the theory of KVL, the output voltage would be like a complete sine wave.In mode I-III, leakage Lk1, Lk2, and Lk3 are much smaller than magnetized Lm.Therefore, their presence does not significantly affect DYSI's voltage gain.However, when in mode IV, the current through the windings N2 and N1 cannot be interrupted by switching off S2.Therefore, the leakage energy from Lk1 must continue to flow through 1's bulk diode.At the same time, the current through the leakage Lk2 flows through the parasitic capacitor of switch S2, which is usually too small for the clamping voltage.Thus, voltage spikes caused by sudden changes in Ik2 appear at both ends of S2.As mentioned earlier, mode four in figure 6 above causes a sudden interruption of the leakage current through Lk2 during the positive semi-fundamental period.The small parasitic capacitance of switch S2 cannot fully absorb the leakage energy.As a result, a voltage spike will occur unless an alternative path for leakage current flow is formed.Figure 6.DYSI [11].As shown in the picture above, adding a lossless diode D1 near the winding N2 of each coupled inductor is possible.The resulting path marked in the diagram (dotted red line) allows the leakage of energy from Lk2 to condenser C1 when S2 is turned off.Since Lk2 is small, this continuation only lasts for a short time, but its presence can effectively clamp the voltage at both ends of S2 to Vc1 rather than withstand voltage spikes [11].

Conclusion
Compared with the DBI, the Z-source inverters, and the Quasi-Z-source inverters, the discussed DYSI is much more reliable to be used in real life.The most significant characteristic is that it adopts a Yshaped structure, dramatically improving output.Besides, it has applied coupled inductors in topologies to improve the output voltage, which the different ratios can flexibly regulate among three windings per coupled inductor.On top of that, a unique modulation of half-sine modulation has been introduced to this inverter instead of the common modulation, which deals with high losses.And the added diodes are used to cope with the issues of leakage inductance.However, DYSI is a topology with many components compared with common inverters, leading to inevitable losses when conducting.And how to avoid the common problem of current discontinuity must also be discussed in more depth.
Further research can be carried out from the following aspects.First, to study whether a newer analysis method is available to replace the original switches with soft switches to extend the working life and deal with the problem of current discontinuity.Second, to explore whether the topology can be replaced by another simpler one to lower the losses brought by components.
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Figure 1 .
Figure1.The DBI circuit[6].As for the Z-source inverter, as shown in figure2, it has the function of single-stage in boost/buck circuits.And its impedance network is responsible for voltage boost.Besides, it regulates the bus line voltage by controlling the pass-through zero vector through the legs of the bridge[9].What is more, the life span of the input power source of the Z-source inverter would be shortened because of the problems in the structure of the circuit itself[10].

Figure 4 .
Figure 4. DYSI [11].2.1.During the state 1 Lm (the three windings' coupled inductance values) is charged by the voltage generated by winding N2 and DC power supply in series; S2 and S4 are disconnected, and S2 and S3 are connected.The AC output voltage of state 1 is the voltage difference between C1 and DC input.
During Mode 1, Mode 1: S1 and S3 switches are closed, and current flows; S2 and S4 switches are off.The leakage inductors Lk1 and Lk3 of windings N1 and N3 and the energy of the DC power supply are released to capacitor C1 and load.During Mode 2, S1 is off before S2 is on-S1 and S2 are disconnected, S3 is on, and S4 is disconnected.This model is the same as mode1 in that the leakage inductors Lk1 and Lk3 of windings N1 and N3 continue to discharge through the body diode of S1.Thus, the input energy continues to flow to the output, and the voltage across the capacitor C1 remains constant.During Mode 3, the mode in which S2 is on and S1 is off, S1 and S4 are disconnected, S2 and S3 are switched on, windings N1 and N2 and their leakage inductors Lk1 and Lk2 start charging, while capacitor C1 discharges to the load.During Mode 4, this working state creates leakage problems.The mode in which S2 is turned off before S3 is turned on, S1 and S2 are disconnected, and S3 and S4 are closed and switched on, leaking the energy of Lk1 to load through S1's body diode.At the same time, the current leaking Lk2 flows through S2's parasitic capacitance.