# A quasi-Z-source active neutral point clamped inverter topology employingsymmetrical/unsymmetrical boost modulation control scheme for renewableenergy resources

TL;DR: A symmetrical/unsymmetrical boost modulation control technique to mitigate the DC-link unbalance voltage problem in an ANPC inverter to integrate renewable energy resources under their fluctuating DC voltages is proposed.

Abstract: This paper proposes a bipolar quasi-Z-source active neutral point clamped inverter (QZS-ANPCI) topology. It acts as a buck/boost inverter (3-phase, 3-level) to integrate renewable energy resources under their fluctuating DC voltages. We propose a symmetrical/unsymmetrical boost modulation control technique to mitigate the DC-link unbalance voltage problem in an ANPC inverter. This worthwhile control technique exploits voltage-current closedloops on AC and DC sides to regulate the desired parameters. Moreover, the constant boost control (CBC) modulation has provided a switching sequence that generates a symmetrical/unsymmetrical full shoot-through (FST) state for boosting input DC voltage in the proposed inverter. Detailed loss and efficiency analysis is carried out to show its superior performance under the proposed scheme. Furthermore, the total harmonic distortion (THD) of the proposed QZS-ANPCI meets IEEE Standard-519. Simulink/MATLAB (MathWorks, USA) and PSIM (Powersim, USA) software programs are used to simulate the proposed topology. To verify the theoretical proposals and simulation results, we have developed an experimental prototype setup (1 kW). Both simulation results and experimental data show satisfactory agreement and support the theoretical postulates.

## Summary (3 min read)

### 1. Introduction

- Renewable energy resources (RERs) are penetrating into electrical power systems.
- Also, the works in [10, 11] derived a Z-source NPCI structure to decrease the number of passive components and also proposed a modulation scheme.
- This topology multilevel inverter uses a symmetrical boosting control method (FST).
- Also, 4-level operation is achieved instead of 3-level converter operation with the same number of components .

### 2. Proposed control strategy and modulation technique

- Previously, the authors of [6] proposed a constant boost control (CBC) modulation as depicted in Figure 1a.
- It has better performance as compared to other PWM modulation techniques.
- If the upper side ST duty ratio (D0P ) is equal to the lower side ST duty ratio (D0N ), then FST and full nonshoot-through (FNST) states are generated [11].
- The single carrier-based CBC generates ST states with a frequency two times the carrier frequency.
- Therefore, this type of proposed voltage boost is called unsymmetrical boost conversion.

### 3. Proposed quasi-Z-source ANPC inverter (QZS-ANPCI) topology

- Here the authors present the proposed QZS-ANPCI topology and its detailed control scheme for RERs, as illustrated in Figures 2a and 2b.
- An output of the outer loop PI regulator ensures tracking of the desired DC-link voltage.
- The power transfers from the DC input towards the AC load in these states.
- Table 1 gives the equations of the proposed QZS-ANPCI topology during the NST time interval.

### 3.3. Case 1: Unsymmetrical boost operation

- If an ANPC inverter has either different RERs as input sources or unbalanced loads, then unsymmetrical boost conversion becomes necessary.
- By applying the voltage-second principle on inductors over one switching time period (T ), Table 2 formulates the capacitor voltages.
- In addition, this table mathematically denotes the positive side DC-link voltage as VPN and negative side DC-link voltage as VNN .
- Furthermore, the peak DClink voltage V0,NPC across the proposed QZS-ANPCI bridge, in the case of the unsymmetrical boost conversion, is calculated as follows.

### 3.4. Case 2: Symmetrical boost operation

- If the QZS-ANPCI has balanced load and equal input voltage sources, then FST and FNST states are utilized in modulation for symmetrical boost conversion.
- The peak output voltage can be written in buck mode of operation as follows.
- The inductor currents are derived and the following relationship is found.
- Also, iPN and iNN are instantaneous DC-link output currents during NST states.

### 5. Simulation

- Simulink/MATLAB has been used to simulate the developed system.
- The proposed system delivers the rated power to loads as a stand-alone system.
- The waveforms in the case of steady-state and dynamic response are the simulated results as shown in Figure 4.
- Figure 4a illustrates the upper side ST switching signal (VG ), input voltage (Vin(+) ), and capacitor voltages (VC1 , VC2 ).
- In these cases, the control parameters (DC-link voltages, AC voltages) maintain their constant values under step change of either input voltages or load.

### 6. Harmonic analysis

- To analyze the harmonic contents in the proposed topology, Table 4 and Figure 5 show THDs of output voltages and currents under the proposed symmetrical/unsymmetrical modulation technique.
- These show that the THDs are within the acceptable range in symmetrical and unsymmetrical modes, illustrated in Figures 5a and 5b.
- Moreover, the THD and modulation index (M) have an inverse relation between them.
- This shows that the output filter always becomes a necessary requirement whenever low THDs are required for improvement of power quality, irrespective of which modulation technique is used.

### 7. Experimental results and discussion

- A prototype setup of the proposed QZS-ANPCI, built in the research lab, has supported the theoretical formulation and simulation.
- This prototype has shown the steady-state and dynamic response results to verify the proposed topology and its control scheme.
- There are some voltage drops across nonideal components.
- The inductor currents IL1 and IL3 are 5 A, each of which are slightly greater than simulation values.
- These dynamic results confirm that AC and DC side control parameters maintain their voltages level.

### 8. Power loss analysis and comparison of efficiency

- The authors evaluate the power losses and efficiency of the impedance-based QZS-ANPCI topology for both the traditional symmetrical and proposed symmetrical/asymmetrical boost control schemes.
- The same table shows the ideal diode connected in series with its parasitic resistance rD and forward voltage drop VF .
- Similarly, the equivalent series resistances (ESRs) of the passive capacitor (rC ) and inductor (rL ) in series with their ideal lossless components present both passive capacitor and inductor components respectively [29–32].
- The parameters of equipment, used for efficiency and loss calculation, are found from the manufacturer data sheets.

### 8.1.3. Overall efficiency for proposed topology

- Finally, the efficiency for the proposed QZS-NPCI topology can be calculated using the following empirical formula: Efficiency(η) = Pout Pout +.
- It shows that efficiency decreases with increase of the output load.
- The proposed structure has higher efficiency at high input voltage than that at low input voltage, as illustrated in Figure 10b.
- Similarly, if two input voltage sources undergo a low voltage situation as depicted by Figure 10c, then one of the input sources remains unaffected.
- The total losses by all components in the proposed topology are summarized under different input voltages in Figure 10i.

### 9. Conclusion

- This paper has presented a proposed buck/boost QZS-ANPCI topology to integrate independent RERs whose voltages are equal or unequal.
- Moreover, DC and AC side control loops with PI and P regulators are integrated with the topology so that input voltages of RERs or load variation should not distort the rated DC and AC side control parameters.
- The simulation results and experimental setup have satisfactory agreement to prove the proposed system and theoretical postulates.
- M. Imtiaz Hussain, Muhammad Talha Gul, and Muhammad Rehan Usman supervised the research work.

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2,647 citations

### "A quasi-Z-source active neutral poi..." refers background in this paper

...The conventional ZSI topologies have employed modulations in different research works [5, 6]....

[...]

...The Z-source inverter (ZSI) [2], developed in 2003, has a built-in buck/boost characteristic to overcome the above problems....

[...]

...An improved form of the ZSI is the QZSI to overcome its problems....

[...]

...There are four distinct QZSI topologies for RERs....

[...]

...The structure of this article is as follows: Section 1 has described the background history of ZSI topology....

[...]

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### "A quasi-Z-source active neutral poi..." refers methods in this paper

...The conventional ZSI topologies have employed modulations in different research works [5, 6]....

[...]

...The Z-source inverter (ZSI) [2], developed in 2003, has a built-in buck/boost characteristic to overcome the above problems....

[...]

...An improved form of the ZSI is the QZSI to overcome its problems....

[...]

...There are four distinct QZSI topologies for RERs....

[...]

...The structure of this article is as follows: Section 1 has described the background history of ZSI topology....

[...]

272 citations

### "A quasi-Z-source active neutral poi..." refers background or methods in this paper

...Proposed control strategy and modulation technique Previously, the authors of [6] proposed a constant boost control (CBC) modulation as depicted in Figure 1a....

[...]

...The conventional ZSI topologies have employed modulations in different research works [5, 6]....

[...]

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