ABSTRACT:

Flying capacitor and cascaded H-bridge multilevel inverters are two of the main multilevel inverter topologies; each has its distinct advantages and drawbacks. Regarding the latter, cascaded H-bridge inverters require multiple separate dc sources, whereas (semi-active) flying capacitor inverters contain capacitors that require a means to balance their voltages. This paper investigates a hybrid-topology inverter, comprising a single-phase five-level flying capacitor inverter and a single-phase cascaded H-bridge inverter with their outputs connected in series, as one way to mitigate the drawbacks of each topology. The proposed control scheme for this inverter operates the switches at fundamental frequency to achieve capacitor voltage-balancing while keeping the switching losses low. Moreover, the step-angles are designed for the 13-level and 11-level output voltage waveform cases for a fixed modulation index to achieve optimal total harmonic distortion. The proposed system is simulated in MATLAB/Simulink software.

INTRODUCTION:

Dual-use ac power generators that are operated to provide peak shaving on a regular basis and to provide backup/emergency power on an as needed basis are beneficial in several ways. For customers, using these on-site generators in a dual role maximises their value, especially since peak shaving can often help them avoid paying (the higher) demand charges. For utilities, this helps them to possibly avoid the higher costs of bringing additional capacity online to meet the peak demand. However, most backup generators currently available are powered by diesel engines, while the rest are fuelled by natural gas or a combination of diesel and gas. Thus, to use them for peak shaving regularly would result in undesirable emissions that are strictly regulated by governmental agencies. Therefore, efforts are being undertaken to develop alternative dual-use ac power supplies. This paper investigates a multilevel converter – possibly supplied by a combination of fuel cells, solar arrays and batteries – that could be used in such applications. One example, but not limited to this, is placing the ac power supply in a hospital's parking garage (these will likely be key power nodes as electrified cars proliferate) and connecting it to adjacent buildings, where a higher supply voltage would lower the distribution losses. In 1975, the cascaded H-bridge converter was introduced as the first topology of multilevel inverters (MLIs). This was followed by the diode-clamped (neutral-clamped) topology, which utilised a bank of series capacitors for supplying the input dc voltages. Subsequently, the flying capacitor (capacitor-clamped) topology was also introduced, employing floating capacitors rather than series capacitors for maintaining dc voltages. The common concept underlying these topologies is to have various input dc voltages ‘added’ together at the inverter output, via judicious switching of power semiconductor devices, so as to produce an ac waveform that is sinusoidal together with some acceptable amount of higher harmonics. This approach is advantageous due to the higher voltage capability obtained from lower-rated devices, reduced switching losses, better electromagnetic compatibility and less required filtering. However, drawbacks of MLIs include the need for separated dc sources (in the cascaded H-bridge case), and the need for capacitor voltage balancing (in the diode-clamped and flying capacitor cases); in particular, for the diode-clamped MLI, it is unable to balance its capacitors’ voltages during real power conversion without sacrificing output voltage performance. Due to the need for diode-clamped and capacitor-clamped MLIs to require additional circuits to balance their capacitors’ voltages and avoid impaired performance, a generalised three-phase MLI topology using diodes, capacitors and transistors for clamping purposes was proposed by Peng. This subsumes two topologies that use fewer transistors, one with only passive clamping devices proposed by Suh and Hyun, and the other with both passive and active clamping devices proposed by Chen and He. However, although the MLI topologies can keep their capacitor voltages balanced during purely reactive conversions, the MLI topology cannot. Then, a hybrid nine-level inverter consisting of a threephase three-level diode-clamped inverter, with a two-level Hbridge in series with each phase was proposed for drive application. The H-bridges are connected to capacitors instead of power sources, thus can only supply reactive power. A complex non-linear model-predictive controller was proposed to stabilise the floating capacitor voltages of the H-bridges and the diode-clamped inverter by deliberately varying the common-mode voltage of the drive's three-phase output. The same topology was also studied by the authors and others, with various control algorithms offered to regulate the floating dc links to desired values while striving to minimise the lowest harmonics present in the converter's output. On the other hand, work on a single-phase asymmetric seven-level diode-clamped inverter with its output connected in series with a three-level H-bridge inverter was described. However, a multi-output boost converter connected to the dc link's capacitors was the means for regulating the capacitors’ voltages, rather than a process ‘internal’ to the inverter. Regarding capacitor voltage balancing, a key principle relied on is the existence of inverter redundant states, i.e. different combinations of the inverter transistors’ on or off states yielding the same output voltage level, whether it be a phase voltage or a line voltage. This has motivated a substantial amount of research on various modulation schemes for both single-phase and threephase inverters that rely on per-phase redundancy and/or joint phase redundancy, respectively, to achieve capacitor voltage balancing. Interestingly, although several single-phase and three-phase MLI circuits are presently known to possess redundant states, the single-phase diode-clamped MLI (1ϕ-DCMLI) has been characterised as not having such states. To obtain an advantageous embodiment of a MLI with a reduced number of separate dc sources, one possibility is to use the ‘mixed-level hybrid multilevel cells’ approach, with multilevel diode-clamped or capacitor-clamped inverters replacing one or more H-bridge cells in a cascaded inverter. However, a means of maintaining the capacitor voltages at the desired levels is needed, otherwise the waveform will become increasingly distorted over time, especially with smaller-sized capacitors. Recently, Diong et al. had proposed replacing a subset of cascaded Hbridge inverter cells by a 1ϕ-FCMLI to reduce the required number of separate dc sources. In addition, they had proposed a means of re-charging the 1ϕ-FCMLI's inner capacitors for the semi-active front-end case, based on the discovery that a 1ϕ-FCMLI with its output connected in series with a voltage source can exhibit redundant states, depending on this source's magnitude and its polarity; a feature referred to as forced redundant states, which studied this behaviour in greater detail.

PROBLEM STATEMENT:

In this paper, the series connection of a single phase cascaded H-bridge MLI (1ϕ-CHBMLI) and a five level 1ϕ-FCMLI can balance its capacitors’ voltage, which improves upon. In this, CHBMLI-only circuit of having reduced number of dc sources, and the advantage over a FCMLI-only circuit of having capacitor voltage balancing capability and how this behaviour corresponds to being able to recharge its capacitors. The new hybrid 1ϕ-CHBMLI plus 1ϕ-FCMLI topology, and a way to operate it to maintain balanced capacitor voltages while minimising transistor switching losses. 11 and 13 level output voltage waveforms are achieved and %THD is compared for different levels.