Analysis of high boost ratio Y-source inverter with DC chain voltage spike suppression

. Y-source inverter is a power electronic converter that can achieve an excessive voltage boost with a weak through dutyfactor. Due to the addition of three-winding coupling inductance, the inverter is more flexible in choosing turns ratio and has higher reliability. This study examines the history of Y-source inverter (YSI) development as well as the evolution of its topology, and identifies advantages with regard to Z-source inverters and conventional inverters. Then state analysis and mathematical analysis of I-YSI, a typical topology of Y-source inverter, are carried out to obtain the current situation of each part of the inverter, and the problem of DC chain voltage maximum caused by leakage induction is pointed out. To solve the voltage peak problem of DC chain, two solutions are summarized, including adding energy absorption loop and changing control strategy, and the two strategies are analysed in detail. Finally, the possible development direction of YSI is discussed. The development of the YSI will benefit greatly from the research implications of this study.


Introduction
Y-source inverter is a power converter topology proposed by Poh Chiang Loh and other scholars in 2014.By modifying the turns ratio of coupling inductors at low through-duty ratios, substantial voltage gain can be achieved, and its application involves all aspects of new energy power generation [1].The ability to change the DC bus voltage of inverters is a feature of both conventional and Z-source inverters.Traditional voltage source inverters and ZSI have evolved into Y-source inverters.Compared with conventional inverters, Zsource converters are more reliable and require fewer individual components, while Y-source inverters use lower straight-through dutyfactor to achieve to gain excessive voltage [2].The selection of winding turns is more flexible [3].Y-source inverter has received full attention from researchers since it was proposed, and academics have worked very hard on their study to improve the topology structure of coupled inductive inverters based on the defects of conventional Y-source inverters.Through state analysis and mathematical analysis of I-YSI topology, it is discovered that paired leakage inductance is the root cause of the DC chain voltage issue.This problem reduces the reliability of the inverter and causes the damage of the inverter.To tackle what is causing the problem of voltage peak in the DC chain, scholars put forward the solution of energy recovery by changing the control strategy of inverter.In this paper, the development history of Y-source inverter, topology structure, and the solution of DC chain voltage peak are studied.

Development vein
In 2002, Professor Peng Fangzheng presented a Z-source inverter with the ability to alter the inverter's DC bus voltage [4].Compared with conventional inverters, Zsource inverters use fewer components and have improved reliability [5].However, its two inductors and capacitors occupy a lot of space and weight, and the voltage boost effect is weak, and there are certain limitations.Therefore, in 2014, Professor P. C. Loh proposed the Y-source inverter, which, when a modest straight-through duty cycle is used, can achieve substantial voltage gain [1].The YSI is evolved from the ZSI.Compared with the ZSI, the selection of winding turns is more flexible [6].For the purpose of trying to resolve the issues with DC chain voltage peaks present in Y-source inverters, academics have come up with numerous derived topologies of these devices.Figure .1 depicts the Y-source inverter's evolution throughout time.

Z-source inverter topology
The Z-source inverter contains a total of two inductors and two capacitors [1].By switching the three-phase bridge arm between states, the charge and discharge control of inductors and capacitors is completed, allowing the DC chain voltage to be pushed up in the inverter bridge's non-straight condition for the purpose of boosting voltage [2].The Z-source inverter allows the voltage type inverter bridge arm to short circuit, and the bridge arm of the present-day inverter to open, improving reliability.In the meantime, it allows passthrough, essentially eliminating the impact of dead zones.However, there are two inductors and capacitors, which occupy a lot of space and weight, and the voltage boost effect is weak.In Figure .2, the ZSI topology is displayed.In order to minimize the quantity of capacitors and inductors, and decrease the weight and space of inverters, scholars proposed a derivative structure of Z-source inverter Γ-ZSI, which produces a substantial gain and a high modulation ratio while using fewer parts and coupling transformers [7].By altering the transformer's turn ratio, Gamma-ZSI can change the gain throughout a constrained range.Compared to other comparable topologies, the inverter can operate with fewer windings at high gain [3].However, the turns ratio has a significant impact on the voltage gain., and the application scenario has limitations.The topology of Γ-ZSI is shown in Figure 3.

Y-Source inverter topology
In 2014, scholars proposed the Y-source inverter [6].The YSI employed a decreased through-duty ratio and additional three-winding coupling inductance in comparison to the Z-source inverter to achieve significant voltage boost.There were additional variations in the winding's number of turns, and by shifting the turn ratio, the boost ratio could be enhanced.Unfortunately, Y-source inverters also have shortcomings, these linked inductors leak precisely at the time the through state changes to the non-through state, causing DC chain voltage spikes.Leakage detection and DC chain voltage peak issues must be resolved, scholars have proposed the derived topology of Y-source inverter.On the basis of the YSI, the advanced inverter contains a capacitor, which can reduce the starting current, the input current continuity, the influence of coupling inductance and the voltage spikes [8].The topology of I-YSI is depicted in Figure 5.

Analysis of DC chain voltage spike problem 3.1 Analysis of the operating principle of classical Y-source inverter (I-YSI)
The impedance network inverter containing coupled inductance can obtain greater voltage gain capability with fewer devices, and has the advantages of small output voltage waveform distortion, strong antiinterference ability, and single-stage structure; However, compared, the inverter also has the DC chain voltage spike problem caused by the coupled inductance leakage [9].This paper will analyse the working principle of the existing typical YSI -improved Y source inverter (Improved Y-Source Inverter, I-YSI), in addition study how the leakage caused by coupled inductance causes the voltage spike in the DC chain.
Figure 6 is the topological circuit diagram about the modified YSI.Compared with the conventionall Y source inverter, the starting current of I-YSI is smaller, which is able to effectively resolve the issue of intermittent input current, and minimize coupled inductance leakage's negative effects, and has a more stable DC chain voltage.The bridge of the inverter and the alternating load are effective in the form of a switch and a current source Ic parallelly.The direct through and non-direct through of the bridge of the inverter are equivalent to the opening and closing of switch SW, respectively.
In the direct through state, as depicted in Figure 7, the two IGBT tubes of the same inverter bridge are connected in the meantime, equivalent to the switch SW closure, causing the inverter bridge to short circuit, then current flowing through diode D is gradually reduced to 0, making D in the reverse deviation state, which is equivalent to the open circuit.At this point, the Kirchhoff's law: Where: C , and the relevant current expression is: The subsequent relation is able to be derived: Where, When SW in the circuit is off, the circuit enters the non-direct through state as depicted in Figure 8.In the non-straight-through state, the inverter bridge is in a normal working mode or a continuous flow mode.At this time, the three-winding coupling inductance 1   L and 3 L is controlled by the capacitor 2 C .
After the analysis, the relevant mathematical expression is obtained: It can also be deduced that: For excitation inductance M L and input inductance in L , on the basis of the second balance principle, the following equation can be acquired: Where is the boost ratio.Therefore, peak value of the direct current chain voltage dc V and output AC voltage O V is: Where M is expressed as the modulation factor of the inverter, also known as the modulation ratio, and in any case, M is always less than 1.At the same time, M and d also have the following mutual checks and balances relationship: It is able to be seen that the boost gain of I-YSI is closely related to the pass-through dutyfactor d and the winding coefficient K. Compared with I-YSI, the conventional Y-source inverter's boost ratio is only The interaction between the excitation current, the output current and the input current is able to be obtained by the equation ( 13): The boost ratio B of I-YSI at different three-winding coupled inductive turns rate is shown in table 1.

I-YSI DC chain voltage spike analysis
In I-YSI, the leakage of the coupled inductance will greatly impact the effectiveness of the whole circuit.
When the circuit state is switched from the pass-through state to the non-straight-through state, due to the leakage of the coupling inductance, the winding current will change, producing a huge voltage peak, which is be applied to the DC side switch pipe, causing excessive voltage stress to the switch pipe, and eventually leading to damage to the switch pipe.At the same time, the characteristics of the coupled inductance can also affect the voltage peak of the DC chain.According to the experimental evidence, the voltage spike amplitude in I-YSI is usually much larger than the normal DC chain voltage average.The I-YSI circuit with coupled inductance leakage sense can be divided into straight-through state and nonstraight-through state.Its effective circuit is depicted in Figure 9 and 10 below.
In the non-straight through state, the diode D is work and the current streaming through the coupling When the circuit state from direct state to direct state, according to formula ( 22) -( 26), the coupling inductance current violent mutations, and because each winding inductance must have a certain amount of leakage, according to the inductance voltage and current relationship , the current will make the leakage at both ends of the huge voltage difference, then the dc chain on the great voltage spike phenomenon.

The Solution of DC Link Voltage Spike Suppression
Even though the three-winding coupled inductance inverter embodied by the Y-source inverter has many benefits, this type of inverter generates a significant voltage spike when the through state transitions to the non-through state, leading to excessive switching tube voltage stress and other issues.Therefore, this chapter summarizes two current effective strategies to solve this problem.

Join the Energy Absorption Loop Strategy
In the early days, Marek Adamowicz proposed two energy absorption circuits that can effectively improve the problem of DC link voltage spikes, and applied them in a T-source inverter among coupled inductance impedance source inverters [10].Later, some scholars applied the absorption circuit to the Y source inverter and made a series of improvements [11][12][13][14][15].
One type of absorbing circuit is a passive energy absorbing circuit, that is, adding a passive component such as a capacitor or an inductor to the circuit and using its energy storage properties to absorb or store electrical energy.When the input voltage spikes, the passive components absorb or release the stored energy, smoothing the voltage waveform.
Another type of absorption circuit is an active energy recovery circuit, that is, an active component such as a transistor or an operational amplifier is added to the circuit to absorb or absorb part of the current or power.When a spike occurs in the input voltage, the active element absorbs current or power to reduce the magnitude of the spike.At present, the typical energy absorption circuit is active-switched Y-source boost inverter (ASYS-BI) [15].Its topology is depicted in Figure 11.The architecture combines traditional quasi switching boost inverters (QSBI) and quasi-T-source inverters (QTSI), ensuring that the inductor current is incessantly streaming and that the problem of straight-link voltage spikes is resolved.

Change the impedance source inverter's control strategy
As a result of the through state of YSI, the coupling inductor's leakage current quickly changes, which in turn causes a spike in the voltage of the direct current link.In this regard, a strategy is proposed that does not use the straight-through voltage state to achieve step-up and step-down, but uses the topology combination of capacitors, inductors, diodes, and inverter bridges to achieve the effects of step-up and step-down.Through this topology, connected inductors may be charged and discharged inductively, and the voltage is able to be changed based on the volt-second counterbalance.The principle is very similar to the buck-boost conversion circuit.
A split-source inverter architecture (SSI) that initially implements this strategy is an inverter independent of the YSI architecture, which solves the problem of DC link voltage spikes [16][17].However, its actual voltage gain is not high enough, so some scholars subsequently proposed the Split-Y-source Inverter (SYSI) and the improved Split-Y-source Inverter (ISYSI) optimized on YSI structure [18][19].As shown in Figure 12, the ISYSI topology is based on the IYSI structure with a voltage clamping unit added.

Conclusion
The Y-source inverter has benefits of less components, elevated voltage gain, and flexible selection of winding turns.In this paper, through the state analysis and voltage and current analysis of I-YSI topology, it is known that a significant direct current link voltage spike is possible to brought on by the connected inductor's leakage inductance.The article summarizes two strategies to solve direct current link voltage spikes, explains the principles of the strategies adopted, and presents the most authoritative models for each strategy.The proposed strategies effectively solve the problem of direct current link voltage spikes, increases the circuit system's dependability, and makes the YSI better used in photovoltaic grid-connected, electric vehicles and other fields.Through the research on the YSI and its direct current link voltage spike problem, author believes that the future research work can be carried out from the following two aspects: (1) Compare the use of two strategies to solve direct current link voltage spikes in different occasions, and summarize the advantages and disadvantages of the two strategies and their applicable occasions.(2) To conduct further research on the effects of typical topological structures derived from different strategies in the application of other types of inverters.

Fig. 6 .
Fig. 6.Improved Y-source inverter (Photo/Picture credit: Original)The running states of I-YSI can be separated into straight-through states and non-straight-through states, as depicted in Figure7and 8 below.

capacitors 1 C 2 N 、 3 N 2 C
charges the coil and the coupling inductance , while the input inductor in L is clamped by the capacitance and the DC voltage source in V .Relevant mathematical expressions for

1 C 1 N - 3 N 1 L - 3 L
and 2 C ; Lin  and LM v are the voltage at both ends of the input inductance in L and the excitation inductance M L ; is the number of turns of coupling inductance .In the meantime, due to the short circuit reason, the input inductance in L limits the flow of current into the capacitor 2

,
and the I-YSI can undoubtedly achieve greater boost gain.At the same time, I-YSI also has an advantage: in the circuit start moment, the input current in I , almost all to the capacitance 2 C charge, resulting in almost no current in the coupling inductance flow, thus protecting the main circuit from the current impact.
at different three-winding coupled inductive turns rate.

Fig. 12 .
Fig. 12. Topology of ISYSI (Photo/Picture credit: Original) The magnetizing inductance in IYSI has an intermediate state during the switching process of charging and discharging states, which keeps the current in a continuous state, thus solving the problem of DC link voltage spikes.In addition, It is possible to stop the voltage oscillation brought on by resonance and generated by the parasitic capacitance of the diode D1 and the leakage inductance of the Y source impedance network when the magnetizing inductance enters the charging moment, and further improves the gain in voltage.

,
Lx representing M