PERFORMANCE EVALUATION OF SYMMETRICAL AND ASYMMETRICAL CASCADED H – BRIDGES OPERATED WITH MULTICARRIER SINUSOIDAL PWM AND NEAREST LEVEL CONTROL (NLC) TECHNIQUE

Cascaded H – Bridge (CHB) is one of the multilevel inverter topologies which is most used in the medium and high-power applications. To increase the output power and the voltage levels hybrid cascaded H bridges with unequal input voltages have been proposed in the various publications. The number of stages i.e. series connected H -bridges are reduced. It also reduces the distortion and the Electromagnetic Interferences (EMI). The analysis of the hybrid Cascaded H bridges operated with level Shifted (LSPWM), Phase shifted (PSPWM) and Nearest Level Control (NLC) have been studied. Total Harmonic Distortion (THD), fundamental output voltage and the first harmonic spectrum are the performance parameters considered. The comprehensive simulations of three stage CHB, and new hybrid multi-level inverter topologies have been carried out in MATLAB/SIMULINK software.


Introduction
Multilevel inverters have been established their place in various applications such as HVDC, drives and FACTS controllers etc. They have the advantages such as low THD, low EMI, and reduction in rating of the semiconductor devices over the two -level inverter. The various multilevel inverter topologies have been proposed and discussed in the publications. But very few of them have been gained commercial status. The topologies which have been found in commercial use are Neutral Point Clamped (NPC), Flying Capacitor (FC) and Cascaded H-Bridge (CHB) inverters. The NPC inverter requires more number of diodes for increased levels and the capacitor voltage of NPC inverter cannot be balanced easily. For FC inverter large number of capacitors are required and also balancing of them is critical. The CHB inverter requires a smaller number of components to get the same number of levels in the output voltage as compared to the NPC and FC. And because of its structure it is more suitable to medium and high voltage applications especially high -power medium voltage (MV) drives. It is basically a series connection of the multiple units of H -bridges. And in practice the selection of the number of H -bridges depend on operating voltage and the manufacturing cost. The symmetric and asymmetric hybrid CHB are having equal and unequal input voltages to the seriesconnected H -bridges [1][2].
The modulation techniques play an important role in the dynamics of CHB inverters. The SPWM has two methods Level shifted PWM(LSPWM) and Phase Shifted PWM(PSPWM), Selective Harmonic   [3][4]. The SHE and SHE-PWM can be used for the high-power applications because of having the low switching frequency. But calculations of the switching angles at which the gate pulses are generated of the nonlinear equations are cumbersome and tedious [5]. Due large switching states space vector , the application of it is limited to the low voltage level [6][7]. Recently, introduced Nearest Level Control (NLC) does not require carrier signals and it is low switching frequency method [8]. Therefore, it is most suitable for high power applications. The application of NLC to the Modular Multilevel Converter (MMC) has been found in publications [9]. But as per best of author's knowledge, the analysis and applications of LSPWM, PSPWM & NLC to symmetrical and asymmetrical CHB have so far not been found in the existing literature. In this paper, the comprehensive analysis of their applications to the symmetrical and asymmetrical CHB have been carried out.

Symmetric H -Bridge
The symmetric topology having the equal input voltages and is called CHB. The fundamental circuit of H-bridge is shown in Fig.1(a) in which the two switches (S1S2 & S3S4) of the cross arm are ON at a time gives two levels of the output voltage +VDC and -VDC. Redundancy is available for the zero level i.e.it can be generated by switching S1S3 or S2S4 as given in Table   I. The gate pulses for corresponding switches are shown in Fig.1(b). The switches S2 and S4 in the Fig.1(a) are complementary to S1 and S3. The switches can be IGBT, IGCT, which can be turned ON or turned OFF by gate pulse.

Asymmetric cascaded topologies
The input voltages in the asymmetric topologies are unequal. Hybrid, quasilinear and new hybrid multilevel inverter are considered for the analysis. The number of stages(p) is the number of H -bridge connected in series. Two H -bridges are connected in series for p = 2. Similarly, for three stages p = 3 three H -bridge are connected in series.

C. New hybrid multilevel inverters [15]
New hybrid multilevel inverter topology for the p th stages is shown in the Fig. 2

Level Shifted PWM (LSPWM)
In this modulation method the carrier and the modulation or reference waveforms are required. Carriers are shifted by the amplitude determined by the number of levels. N is the number of voltage levels. Total N-1 carriers are required with same amplitude and frequency. The modulating or reference waveform is a sinusoidal wave form with the fundamental frequency 50Hz. The gate pulses are generated by comparing the modulating with the carrier waveforms. The amplitude of the carrier and the modulating waveform are determined from the modulation index which are defined by Table III. The principle is explained in the Fig. 3(a). If all the carriers are in phase is called the phase disposition (PD). In Phase Opposition Disposition (POD) all the carriers above the zero are in phase and in opposition with the carriers below the zero. If all the carriers are alternatively in opposition is called the Alternate Phase Opposition Disposition (APOD) [3]. Fig. 3 (b), (c) & (d) show the PD, POD and APOD schemes. The inverter switching frequency is given as

Phase Shifted PWM(PSPWM)
Here the carriers are shifted by the phase angle. The principle is shown in the Fig. 4. The phase shift between any two carriers is   0 360 1 N  .The amplitude of the carrier and the modulating waveform is same i.e. between the +1 and -1. The gate pulses are generated by comparing the modulating with the carrier waveforms. The modulation index is given in the Table I. The switching frequency of the inverter is given by [7]   And the device switching frequency is as follows

Nearest Level Control (NLC)
NLC is also known as round method. The three phases are controlled independently by having 120 0 phase differences in the reference waveforms. The closest voltage level is used to normalise the reference waveform vr and is given by Where VDC is difference between the two levels. The normalised value is then evaluated by round function. The function returns the nearest integer of the input number (e.g., round (2.4) = 2, round (2.6) = 3) [8]. This nearest integer multiplied by VDC corresponds to the closed level to the reference that is generated by the inverter. The operating principle for N level is shown in the Fig. 5 6. Simulation results Simulation parameters are listed in the Table IV

Conclusion
Three stage CHB and new hybrid multilevel inverter operated with LSPWM, PSPWM and NLC have been simulated comprehensively in MATLAB/SIMULINK software. The effective frequency of the inverter in the LSPWM is equal to the carrier frequency while it is depended on voltage levels in the PSPWM. Therefore, the first harmonic in the PSPWM for the same frequency is shifted to high order. NLC does not have the defined harmonic spectrum and hence low order harmonics present in the output voltage and current. In new hybrid multilevel inverter, the output voltage levels are highest compared to the CHB, quasilinear and hybrid multilevel inverter topologies. This paper may find useful in knowing the modulations methods for low, medium and high-power cascaded bridge applications