TECHNICAL FIELD

The present disclosure relates to a converter, and more particularly to a hybrid modular multilevel converter (HMMC). The present application is based on, and claims priority from an Indian application No. 201821011623 filed on Mar. 28, 2018 the disclosure of which is hereby incorporated by reference herein.

BACKGROUND

High voltage direct current (HVDC) converters convert a high voltage electric power from an alternating current (AC) to a direct current (DC), or vice versa. Modular multilevel converters (MMCs) are a kind of HVDC converters gaining increasingly a prominence for high voltage applications (e.g., power distribution) due to its modular design, lower switching losses and being scalable to any voltage or power level. However, the MMCs require large sized capacitors for each of its sub module (SM). Further, the control of the MMCs may lose when a dc side fault occurs in the MMCs. Conduction loss and circulating current generation are generally higher in the MMCs during a high voltage conversion.

Various existing hybrid topologies such as an alternate arm converter (AAC) and a parallel hybrid converter (PHC) are proposed to address some of the aforementioned limitations. The AAC contains full bridge sub modules (FBSMs) in its circuits which generates a required output voltage. The AAC has a dc side fault blocking capability due to a presence of the FBSMs in its circuit. However, the conduction losses in the AAC increases as a number of IGBTs in a conduction path is higher. The PHC uses a minimum number of SMs and has a lower conduction losses as half bridge sub modules (HBSMs) are not in the a main conduction path. However, the PHC suffers from some major drawbacks such as coupling of the AC and the DC side of the converter, a loss of control during the dc side fault, etc.

The conduction loss in the converter is a major loss component for the MMCs based HVDC systems. The occurrence of the conduction loss mainly depends upon on-state voltage drop at thyristors/insulated gate bipolar transistors (IGBTs) present in the MMCs. The thyristors have a lower on-state voltage drop compared to that of the IGBTs, besides providing a higher power handling capability and a reliability. Moreover, the thyristors have a higher surge current handling capability which increases a short circuit strength of the MMCs. Recently, the MMCs made of hybrid combinations of the IGBTs and thyristors have been developed which makes the converter highly efficient. However, power handling capability of the thyristors are underutilized due to limited power handling capability of the IGBTs in existing hybrid combinations since same current flows through both the IGBTs and the thyristors during the high voltage conversion.

Thus, it is desired to address above mentioned disadvantages or other shortcomings or at least provide a useful alternative.

SUMMARY

Accordingly the embodiments herein provide a hybrid modular multilevel converter. The hybrid modular multilevel converter includes one or more chain links formed by sub modules, one or more high voltage switches formed by one of semi-controlled devices and fully controlled devices. A plurality of inductors are arranged in the one or more chain links to limit circulating current among the one or more chain links. The one or more chain links are configured to enhance a power handling capability of the hybrid modular multilevel converter.

In an embodiment, the one or more chain links is formed by connecting at least one of unipolar sub modules and bipolar sub modules.

In an embodiment, the unipolar sub modules comprises at least one of a direct current source and a storage element, wherein the bipolar sub modules comprises at least one of a direct current source and a storage element.

In an embodiment, the one or more chain links generate an output voltage.

In an embodiment, each phase of the hybrid modular multilevel converter comprises at least three chain links.

In an embodiment, each phase is connected in series or in parallel or in any mixed combination of series and parallel from the dc side with the other phase circuits.

In an embodiment, the one or more chain links in each phase is used for dc side voltage ripple minimization.

In an embodiment, the one or more chain links is controlled in such a way that at least two chain links are connected in parallel.

In an embodiment, at least two chain links are connected in parallel so as to share the current among the two or more chain links to enhance the power handling capability.

In an embodiment, at least two chain links are controlled in such a way that it provides a wide modulation index range, besides eliminating the requirement of any extra SMs.

These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

