Thermal Analysis and Balancing for Modular Multilevel Converters in HVDC Applications

Abstract

Abstract-The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages in hybrid networks. The MMC enables scalability to different power levels, full controllability provided by IGBTs and can achieve very high efficiencies by using a low switching frequency method as the nearest level modulation. However, the nearest level modulation requires a capacitor voltage balancing algorithm, which can result in unbalanced loading for the power semiconductors in the different submodules. Particularly at low power factor operation, which could occur in case of low-voltage ride through and of reactive power injection, the conventional algorithm is not effective anymore. This article provides thermal stress analysis of the MMC in operation and proposes a thermal balancing approach, which is embedded in the capacitor voltage balancing algorithm. The purpose of the thermal balancing is to achieve similar stress distribution among the different submodules to enhance the lifetime. The junction temperatures in the different submodules are studied for HVDC applications and the article proves experimentally, that the thermal balance within the submodules is significantly improved.

Junction Temperature Control for More Reliable Power Electronics

“…In modular multilevel converters (MMC) the modules are stressed unequally in specific conditions, leading to higher thermal stress in particular modules. An algorithm is proposed in [27], [28] and for active balancing Fig. 3: Example of a model-based active temperature control scheme.…”

Challenges and Strategies for a Real-Time Implementation of a Rainflow-Counting Algorithm for Fatigue Assessment of Power Modules

“…An alternative indicator is to look into the historical temperature profile that the semiconductor surfaces have been exposed to, assuming that the mechanical stresses in the packaging structure are a function of the junction-temperature variation. Typically, this method is applied for offline lifetime predictions in the design phase of a product [6], [7]. In this case, the availability of data about the ambient temperature variation is required, which can be combined with a (predicted) mission profile for the targeted application over a certain period of time (e.g.…”

“…Unlike previous efforts, mainly applying rainflow-counting techniques in offline measurements [6], [7], this paper focuses on the efficient implementation of a rainflow counting algorithm, dealing with the constraints an online implementation imposes. A first effort for a similar approach has been presented in [10], [11].…”

Reliability evaluation of modular multilevel converter based on Markov model

“…The offshore wind power has been widely developed in Europe, China, the United States and Australia [1]. Voltage source converter (VSC) based high voltage direct current (HVDC) transmission systems, including multi-terminal HVDC systems, are widely applied to transmit the power [2,3], in which the modular multilevel converter (MMC) topology attracts the most attention of researchers [4]. The MMC topology is also used in DC distribution and power quality control, such as a unified power flow controller (UPFC) and static synchronous compensator (STATCOM) [5,6].…”