Energy Pulsation Reduction in Modular Multilevel Converters Using Optimized Current Trajectories

Abstract

In power electronics, the modular multilevel converter (MMC) is an easily scalable topology with an high output voltage quality. It is suitable for the transmission of large amounts of electrical power over long distances, which supports the realization of the ongoing energy transition. State-of-the-art methods require a comparatively large total cell capacitance in the system for energy pulsations during operation. In the present paper, in order to minimize this total capacitance, first a new method is developed to model the system, and second, by help of this model, optimal current trajectories are calculated. These currents are used for control to reduce the energy pulsation over the complete operating range, and thus, to better utilize the hardware. The new method independent on the Clarke transformations is implemented on a laboratory scale setup, and measurement results are presented which validate the new method. Furthermore, the new method is compared to the state-of-the-art method of the compensation of the 2nd harmonic and outperforms the latter significantly. This applies to the entire operating range for different power factors. A total reduction of up to 44% of the energy pulsations is achieved.

Reducing Losses and Energy Storage Requirements of Modular Multilevel Converters With Optimal Harmonic Injection

“…Therefore, even similar numerical optimization approaches (such as e.g. [8], [10], [11], [12]) cannot achieve the same reduction in required module capacitance as proposed in this paper.…”

“…In order to further decrease the amplitude of the arm energy/capacitor voltage fluctuation, many examples that inject circulating currents are proposed in literature. The amplitude and phase of the injected harmonics are either determined analytically [4], [5], [6] or using offline optimizations [7], [8], [9], [10]. In some cases, the circulating current injection is also combined with additional harmonics in the CM voltage v cm at the grid side of the MMC [11], [12].…”

“…Almost all listed references ([4], [5], [6], [8], [9], [10], [11], [12] to be precise) aim to decrease the fluctuation of the capacitor energy and do not consider the actually required capacitance value or the trajectories of the arm output voltage and the available arm voltage. An exception is the procedure described in [7], which aligns the peak of the available arm voltage with the peak of the required arm output voltage using a second harmonic circulating current.…”

A modular signal processing platform for grid and motor control, HIL and PHIL applications

“…As mentioned above, the ETI-SoC-System is applicable in wide areas of research [14], for motor and grid control [15,16] as well as for HIL and PHIL [9,17] applications. Although the current performance allows a use in a wide research area, a continuous further development of the system is planned and has already started.…”

Submodule Capacitor Voltage Balancing Through High-Frequency-Side Control for Multiport MMC-Based Solid-State Transformers

“…The number of state variables is proportional to the number of capacitors and inductors in the system. Thus, it is preferred to replace all MMC submodules in the upper arm with a dependent voltage source, as well as those in the lower arm [19], [20], [21], [22], [23]. However, this technique cannot be applied to MMC-based SST systems since the DC/DC converters are connected to each of the submodule capacitors.…”