Optimization-Based Capacitor Balancing Method with Selective DC Current Ripple Reduction for CHB Converters

Abstract

From its introduction to the present day, Cascaded H-Bridge multilevel converters were employed on numerous applications. However, their floating capacitor, while advantageous for some applications (such as photovoltaic) requires the usage of balancing methods by design. Over the years, several such methods were proposed and polished. Some of these methods use optimization techniques or inject a zero-sequence voltage to take advantage of the converter redundancies. This paper describes an optimization-based capacitor balancing method with additional features. It can drive each module DC-Link to a different voltage for independent maximum power point tracking in photovoltaic applications. Moreover, the user can specify the independent active power set points to modules connected to batteries or any other energy storage systems. Finally, DC current ripple can be reduced on some modules, which can extend the lifespan of any connected ultra-capacitors. The method as a whole is tested on real hardware and compared with the state-of-the-art. In its simplest configuration, the presented method shows greater speed, robustness, and current wave quality than the state-of-the-art alternative in spite of producing about 1/3 fewer commutations. Its other characteristics provide additional functionalities and improve the adaptability of the converter to other applications.

Optimization-Based Capacitor Balancing Method with Customizable Switching Reduction for CHB Converters

“…The proposed method is based on a previously disclosed method [34] with the aim to improve it. The original method from [34] was designed for a CHB with three phases and modules per phase.…”

“…The proposed method is based on a previously disclosed method [34] with the aim to improve it. The original method from [34] was designed for a CHB with three phases and modules per phase. It selects all modules' output voltages so that they meet a required phase-to-phase voltage while aiming to maximize a certain objective function.…”

“…The objective function evaluates how much a LOP solution helps in achieving certain goals. Some of these goals were introduced in [34]. In this paper, one more objective is considered: the reduction of the switching losses.…”

“…When designing new control methods, the effective switching frequency must be considered. In this regard, the present paper proposes an improvement over the method disclosed in [34] to reduce switching losses. In particular, it modifies the objective function of the method to penalize unnecessary commutations between control cycles.…”

“…Section 3 describes the materials and methods used to test the upgraded method. The tests include a comparison with the original method from [34] with different configurations. Section 4 describes these tests and shows their results.…”