A New Improved Trans Z Source Inverter based Three Phase UPS

Objective: In applications like Wind Energy Conversion System, electric cell power conversion and Grid-connected PV generation, a small DC source voltage should be boosted to induce fascinating AC output voltage. Methods/Statistical Analysis: Z Structured Impedance Source Inverter (ZSI) is well-liked for its buck to boost operations. The high voltage gain will be obtained from the normal Z Structured Impedance Source Inverter at low modulation index state. On the other side, a little modulation index state produces high voltage stress at the inverter side switches. To defeat this problem, a Trans Z Structured Impedance Source Inverter (TZSI) is developed. In TZSI, transformer winding turns demand is high and the windings get burnt because of high instant current throughout shoot through length. Findings: A new topology of Trans Z Structured Impedance Source Inverter is introduced in this paper to boost the voltage gain fair enough. In this new topology the winding turn’s usage is smaller amount and consequently inverter gain is lift up by lower the modulation index. To validate the ideas planned in theories, experimental results were taken and bestowed in this paper. Application: This inverter is utilized for three faze Uninterruptable Power Supply. The proposed inverter is designed and simulated in MATLAB/Simulink podium for 3 KVA load capacity and it will sustain the specified output voltage with the substantial drop in voltage at battery bank side.


Introduction
Uninterruptible Supply scheme is used to offer uninterrupted, consistent and prime quality power to critical loads. It offers protection against irregular power-line voltage or power outage to critical loads. Three different UPS are there traditionally in three wire system. The primary type stepped up transformer needs enhanced inverter side output voltage 1 , as in Figure 1(a). At this time the current in inverter is much higher than the current at load side. Owing to this, current stress raise happens on the switches in inverter and conjointly increases transformer weight, size and volume. The second kind requires DC/ DC boost converter, inverter as in Figure 1(b). During this type, the extra boost converter results in high prize, difficult boost management and low efficient output. The inverter with dead time requires protection for short circuit of inverter in the same leg. Also dead time distorts the output voltage wave shape. The third sort UPS, the necessity of booster circuit is eliminated and it provides single stage power translation during ZSI 2-6 as like Figure  1(c). The voltage plane of ZSI isn't ample with low modulation index (M) condition from [7][8][9][10] . In addition ZSI leads high voltage stress in switching mechanism [11][12][13] .
To overcome listed troubles in the TZSI is introduced. In TZSI transformer winding turns demand is high.
This study presents a unique topology of Trans Z Structured Impedance Source Inverter so as to produce improved performance in TZSI. This improved TZSI is implemented in UPS system, as shown in Figure 1 (d). This new suggested UPS offers the subsequent deserves than the traditional UPSs: • One step power conversion by mingling DC/DC converter with the inverter operation. • Elimination of dead time interval within the PWM signal results reduction of AC output-voltage wave shape distortion. • The output voltage level inverter is high enough throughout low modulation index condition by means of voltage stress reduction across the switching elements. • The transformer winding turns demand is smaller amount and it needs less shoot through duration • Quick transient response and sensible steady state performance is achieved by adopting twin-loop control.

System Arrangement and Description
Figure 1(d) shows a brand new topology of the UPS through a proposed inverter. During regular operation, the battery bank and inverters are powered by the rectifier unit. In case of power failure, the battery bank energises the inverter as like Figure 2. It consists of a DC supply (Rectifier, battery V DC, Capacitor C4 and diode D), Novel impedance network (W1, W2, C1, C2 and C3), PWM inverter switch (S1-S6), Inductive Filter Ls and Capacitive filter Cs. The gate pulses to the inverter are given as Z Structured Impedance Source Inverter triggering. The Novel impedance network makes the projected UPS to produce the desired AC output voltage, in spite of the battery powered DC voltage. While improved TZSI in T1 time interval (non shootthrough state), on condition of diode D, starts to charging of capacitors C1 and C2 in one direction while C3 in conflicting direction.
The improved TZSI equivalent circuit on non shootthrough interval (T0) is viewed in Figure 3.
The consequent voltage expression is written as: Vc Vc Vin The C2 capacitor voltage is remains same as of winding W2 voltage. Once Improved TZSI is Functioning on shoot-through state (T0 time Interval), D the diode is in reverse bias condition. C1 and C2 Capacitors discharge their stored energy to the transformer. Improved TZSI on shoot-through time interval is bestowed in Figure 4. The respective voltage expressions are written as:

Vw Vc Vc
Vin Peak DC link voltage, B boost factor is acquired at shoot through interval. Vac The peak phase voltage is expressed as Where M is modulation index?

Proposed UPS with Improved RZSI Control
In this Figure 6 the triangular carrier wave height is represented as HT and DC line voltage height is represented as HP. W and T0 are the Width of triangular carrier wave and shoot through rate is represented as W and T0.
The calculation of shoot through interval can be articulated as: The Control methodology of simple boost is revealed in Figure 7. The evaluation of shoot through interval and non shoot through interval is feasible with this flow diagram. The firing pulse for devices is often programmed victimization the Figure 7. The modulation index control utilizes twin loop control as expressed in Figure 8. The inner load current (i 0j ) is match up to reference current (I Lj ). The result is match up with capacitor current (I Cj ) and it's given to the proportional gain controller (K p) . Reference output voltage V oj estimation involves the (I Lj ) and (I Cj ) prediction. The reference output voltage (V oj ) given to feedback loop and conjointly given to the gate circuit wherever the proportional modulating sine wave voltage calculable.
By referring Figure 4 the Laplace Transform equations for T0 interval is expressed below: L sI s C s I s C s I s By referring Figure 3 the Laplace Transform equations for T1 is written below: To simplify the analysis assume C1 = C2 = C3 = C and L1 = L2 = L. The dynamic Equation for the T1 interval is written as: The dynamic Equation for T0 interval is written as: 1 1 The response of each state variable expressed as a linear combination of perturbation variables by solving the above Equations the DC side voltage gain GV i as follows: For the AC-side control management, the voltage regulation explored in traditional control method for the individual system. In order to get a good control system, AC side dynamics should be designed more quickly than the DC side to prevent oscillation. The control parameter change in DC side control will limit the AC side, as the shoot through interval is limited within the zero state. The controller generally will achieve good performance with higher end of input voltage.
The AC side controller consists of inner and outer control loops. Stabilized output and quicker response for a current disturbance is achieved via proportional controller that is employed in the midst of inner loop current. PI compensator, the glorious control loop that produces the stabilized variations and quick reference tracking. The Mason's gain rule applied to estimate the system transfer function as follows:

Simulation Results and Discursions
The suggested UPS is compared with traditional Z Structured Impedance Source Inverter based UPS and with Trans Z Structured Impedance Source Inverter (TZSI) based UPS. MATLAB/Simulink plat form kind is effectively utilized to replicate the system. The circuit parameter for simulation is exposed in Table 1. The simulation is done at the switching frequency rate of 10 KHz, switching period of time Ts is 5 ¥ 10 -5 sec and T1 interval is 0.08 ¥ 10 -4 sec. M, T0/T1 is set to 0.84 and 0.16. The system intention is to generate a three phase AC output voltage of 415 V from DC voltage supply of 154.5 V. From the above analysis the subsequent theoretical calculations are done.  (33) and (34) we are able to perceive that any modification in supply voltage will built variant in DC link side voltage and alteration with boost factor able to maintain AC output side voltage constant. DC link set to 571 V and AC line voltage set to 415 V. The assessment between reference and actual value produces the error signal. These error signals initiate the closed loop system operation and it's given to PI controller.
PI controller transfer function is: The Proportional and Integral gain ( K p and K i ) ranges are evaluated by using Zieglers-Nichols scheme. This technique is incorporated in step response system. This system nature is normalized by two different parameters, time constant (Tp) and delay time (Lp). These parameters are calculated by sketching a tangent at the point of intersection in the step response and intersections with time axis. Thus the steady state value noted is shown in Figure  9. In the same way the error signal generated in output side voltage is processed through PI controller.
Step function to the system applied to predict the output response and L p and T p has been calculated. The calculated L p = 1 and T p = 0.004, for this the K p and K i are 0.3 and 0.1 respectively. Bode plot has been drawn to examine stable condition and it is revealed in Figure 10.
Here gain margin posses positive value and the phase margin shows infinite value. Thus the system is a stable system. In conventional UPS 20% of variation in battery voltage makes wave shape distortion in inverter output voltage. The suggested UPS showing glorious voltage regulation even with 50% voltage droop in battery bank voltage. At 100% battery bank voltage (i.e. 154.5 V) the UPS produces inverter side output AC voltage of 415 V and it is exposed in Figure 11. In 50% of battery voltage (78 V), the AC side output voltage wave shape is not distorted because of Improved TZSI action and it's shown in Figure 12. Total Harmonic Distortion of UPS voltage is revealed in Figure 13. The UPS voltage THD is restricted to 0.60%. Figure 12 showing some second order harmon-  ics, this will be eliminated by selecting higher rating of capacitive filter.
The proposed UPS competency level with the conventional UPS has been calculated for different KVA ratings.
It is plotted in Figure 14. It is noted that the suggested UPS has exhibited 10% greater efficiency than that of conventional UPS.

Hardware Implementation
To validate the simulation result, experimental setup for NTZSI has been deliberated for 3 KVA load capacity. This setup consists of NTZSI impedance network, power circuit and feedback circuit shown in Figure 15 The Hardware implementation outcomes are shown in Figure 13 (b) and (c). During this obtained DC link voltage is 564 V and AC line voltage is 406 V. Table 2 shows the comparative results between theoretical estimation and experimental results of NTZSI. It is observed that Experimental results are closely are closely intact with the theoretical estimation.

Conclusion
A brand new improved TZSI has been introduced in this paper. This inverter finds its appliance in three phase UPS for the load capacity of 3 KW. Once the load of traditional UPS raises the drop across the battery bank voltage directs to deficient AC voltage generation at the inverter. The ZSI based UPS resolves this trouble by regulating the shoot through time interval at the inverter side. However this inverter cannot produce enough voltage under high modulation index conditions and it desires long shoot through time interval. This raises stress across the device. In contrast to traditional ZSI UPS, the planned UPS can sustain at high modulation index condition with terribly less shoot through time interval amount and also the transformer winding turns demand is very less with this inverter. This reduces the prize and weight of the system. Here quick transient response and superior steady state performance is achieved by twin-loop control management. Simulation has been executed in MATLAB/ Simulink platform.