Comparative Evaluation of Three-Phase Inverter Topologies Based on Voltage Boosting Features

Voltage source inverter (VSI) is commonly used in industrial due to its stable operation and low cost. However, VSI needs to operate with an extra converter stage which is a DC-DC converter for voltage boosting purposes. In contrast, current source inverter (CSI) inherits voltage boosting features may become an alternative option to VSI. Yet, there were minimal research on CSI that dedicates to the voltage boosting features. This research focuses on comparing the voltage boosting features of CSI and VSI in both open-loop and closed-loop conditions. The performance of VSI and CSI are simulated using MATLAB/Simulink. Under open-loop operation, CSI produces a voltage boosting capability at approximately 55% higher than VSI. Yet, CSI suffers high THD percentage as compared to VSI for the same switching frequency. This high THD shortcoming can be easily resolved by using a simple CL filter. For closed-loop operation, VSI and CSI with voltage-controlled synchronous frame PI control systems are proven to have good reference tracking and harmonic rejection and are suitable to be implemented for household applications or for a standalone system. Interestingly, CSI closed-loop system can achieve a wider range of output due to the voltage boosting capability and provide a better quality of output waveform as compared to VSI.


Introduction
As a country locates at the Earth's equatorial, Malaysia is rich in solar energy.This encourages the development of solar energy power generation in Malaysia.The Sustainable Energy Development Authority (SEDA) Malaysia also play an important role in the renewable energy industry.SEDA has introduced lots of programmes to promote the renewable energy industry, for example the Solar Rooftop Programme (under Feed-in Tariff scheme), Net Energy Metering (NEM) programme, and various technical training programmes [1].These programmes promote Malaysia's solar industry has 2 grown tremendously over the past decade and it extends to rural or remote areas by means of standalone system.
Voltage source inverters (VSIs) are the most widely used topology in photovoltaic applications for Distributed Generation (DG).However, VSIs have some drawbacks, such as a short lifetime of the dclink capacitor and inherent buck characteristics.Consequently, researchers and manufacturers are exploring the development of alternative topologies.Current source inverter (CSI) on the other and gains attention due to its implicit boost characteristics, longer lifetime of the energy storage unit and direct control of the output current [2,3,4].The voltage boosting features has increased interest in CSI as an alternative option to VSI with a DC-DC boost converter.Yet, there are minimal research on CSI, particularly on the analysis of its voltage boosting features.This intrigues a deep interest in investigating the features of CSI [11].
VSI and CSI control strategies are primarily on voltage regulation and current regulation respectively.Thus, the comparison of the performance between voltage-controlled VSI and currentcontrolled CSI cannot be done in a straightforward approach.This is because the output controllable parameter of VSI is voltage, but for CSI, its controllable parameter is current.It is interesting to investigate the performance of voltage-controlled CSI which may increase the competitiveness feature of CSI in industrial fields and standalone system.

Theoretical background of inverter circuit and modulation strategies
Three-phase VSI topology with a configuration of six power switches and six antiparallel freewheeling diodes as shown in Figure 1(a).These power switches are opened and closed periodically according to valid switching arrangement to produce the desired waveforms.On the other hand, CSI converts a DC input current to a three-phase current output, where its amplitude, phase and frequency should always be controllable [4].Figure 1(b) shows the standard three-phase CSI topology with a configuration of six power switches and six reverse blockage diodes.Since three-phase CSI input behaves as a current source, its DC current input can be obtained from a DC voltage source by connecting a large series inductor.This large series inductor enables changes of inverter voltage to be accommodated at low values of di/dt, thus the current ripple is negligible [5].At the output AC side of three-phase CSI, it normally requires a three-phase capacitor, Cf filter to suppress the output voltage spikes in order to assist the commutation of the switching devices.Cf filter also functions as a harmonic filter which can improve the load current and voltage waveform [5,6].Pulse width modulation (PWM) is the most common and efficient method of controlling the output voltage which can easily meet THD requirements with minimised filtering requirements.Two criteria characterize its behaviour which are frequency modulation index, mf and amplitude modulation index, ma [7].mf is defined as the ratio of the switching and reference signals' frequencies as in (1).
For a three-phase inverter, the value of mf should be an odd multiple of three to eliminate the presence of even harmonic and odd multiples of three harmonic in output.The amplitude of fundamental frequency of the output voltage, vo is linearly proportional to amplitude modulation index, ma as shown in (2).
In this work, three modulation strategies namely sinusoidal PWM (SPWM), third harmonic injection PWM (THIPWM) and space vector modulation (SVM) are used as the modulator for both inverter topologies.A three-phase reference signal is compared to high frequency carrier signal to produce set of gating pulses in SPWM technique.THIPWM uses similar modulating signals with injection of selected harmonics in order to produce higher fundamental amplitude of 15.5% than SPWM, thus able to fully utilize the dc-link voltage at the inverter's input side.THIPWM modulating signal, vmy is defined as in (3): Where  V to 6 V , zero vector 0 V and the reference vector, ref V .
These vectors rotate in space at an angular velocity as the fundamental frequency of the inverter.[8,10].For CSI, the relationship of fundamental inverter current, Im and ma can be expressed as [11]: The maximum fundamental current using SPWM is only makes SPWM suffers poor dc-link current utilization.However, this drawback can be improved by using THIPWM and SVM as modulator [4,11].Table 1 lists the parameter used to simulate the three-phase inverter system in MATLAB/Simulink software.Table 2 presents the boosting features of CSI and VSI while Figure 3 illustrates the boosting features of CSI for three discussed modulation strategies.With reference to Malaysia's grid voltage of 230Vrms [9], CSI always has higher fundamental output at about 55 to 56% than VSI, as presented in Figure 4.It can be claimed that CSI has an average 55% voltage boosting features in fundamental components as compared to VSI.This study found that CSI can reduce the need for a DC-DC converter, thus simplifying the system.However, CSI output has higher THD than VSI at the same switching frequency.CSI has a voltage boosting advantage, yet it also has higher output THD.This can be minimized using an AC power-CL filter.It can be concluded that, using the same modulation strategy and ma, the voltage boosting capability of CSI can be best featured as in ( 6) and ( 7   The voltage boosting capability of CSI is further investigated with voltage-controlled synchronous frame PI control system.Table 3 presents the comparison of ma for VSI and CSI based on Vd,ref.As summarized in Table 3, both inverters produce the same Vø,rms under Vd,ref.However, the ma required to generate the Vø,rms is different.It can be clearly found that the ma required for CSI to produce the Vø,rms is always smaller as compared to VSI.This had presented the boosting features of CSI that in line with the finding of Table 3.

Comparison verification of the inverters in open and closed-loop system
Figures 5 and 6 present the tracking response between reference and measured signals of the closed-loop system for the inverters.Both inverters are able to track the reference, Vd,ref until it is over the system limitation.For VSI, when Vd,ref = 1.3pu, the Vd,in signals is continued oscillating and cannot reach a stable state as shown in Figure 5.It can be observed that the voltage-controlled PI control in VSI reached the system limitation at Vd,ref = 1.2pu (green line in Figure 5).As for CSI, Vd,in signal cannot reach a stable state and continued oscillating when Vd,ref = 2.0pu as shown in Figure 6.When PI control generates an output, ma >1, overmodulation occurred, thus ma signal is oscillating and output waveform of CSI are no longer in pure sinusoidal waveforms.This determined CSI reached the system limitation at Vd,ref = 1.9pu as shown in Figure 6 (light blue line).The appropriate modulation index and switching frequency are identified in this condition.Closedloop simulation is conducted to further study the inverter's performance using a voltage-controlled synchronous frame PI control system.It is worth to mention both inverters are suitable for Malaysia's household applications to operate under off-grid mode and a standalone system.Throughout the openloop steady-state simulation and closed-loop simulation, CSI has implicit the voltage boosting capability that can provide a fundamental output on average 55% higher than VSI.Besides, the voltage boosting capability led that CSI with voltage-controlled synchronous frame PI control system have a wider bandwidth of the system limitation, hence its output range also have a wider bandwidth.

Figure 1 .
Topology of three-phase VSI and CSI in stand-alone application y = phase a, b or c.

Figure 2
Figure 2 shows six stationery active vectors, 1

Figure 2 .
Figure 2. Space vector diagram in SVM techniqueThe dwell times for the active vectors represent the duty-cycle time of the selected switches during a sampling period Ts can be calculated in (4):

Figure 4 .
Figure 4. Comparison of voltage boosting features of two inverter system at Vref,grid with selected ma

Figure 5 .
Figure 5. Closed-loop tracking of reference and measured signals of VSI (a) Vd,in; (b) Vq,in.

Figure 6 .
Figure 6.Closed-loop tracking of reference and measured signals of CSI (a) Vd,in; (b) Vq,in.

Figures 7 (Figure 7 .
Figures 7(a) to (f) presents a sample of voltage, current and output power when SVM is used as a modulator in dedicated synchronous frame PI closed-loop system for both inverters.The phase current and line-to-line voltage waveforms in Figures 7(a) to (d) become steady-state balanced sinusoidal waveforms after one cycle.The output active power of VSI in Figure 7(e) shows an overshoot at the start-up while a smooth gradual build-up for CSI is shown in Figure 7(f).Both inverters are able to reach steady-state and successfully track the reference signals of Vd,ref and Vq,ref respectively.
VSI line-to-line voltage (d)CSI line-to-line voltage (e) VSI output power (f) CSI output power 8th International Conference on Man Machine Systems 2023 Journal of Physics: Conference Series 2641 (2023) 012021

Table 1 .
Parameter used for verification

Table 2 .
Boosting features of the inverter topologies

Table 3 .
Comparison of ma for VSI and CSI based on Vd,ref.