Active thermal balancing for modular multilevel converters in HVDC applications

Abstract

The modular multilevel converter (MMC) has become a very attractive solution for interfacing high voltages 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 (NLM). However, in order to limit failures of the power modules, the thermal stress of the submodules (SMs) should be properly studied. For NLM a capacitor voltage balancing algorithm is required and this algorithm, as demonstrated in this paper, offers already good thermal balance among the cells of the MMC. However, at low power factor, operation which could occur in case of low-voltage ride through and of reactive power injection, the mentioned algorithm is not effective anymore. This paper proposes an active thermal balancing algorithm which is embedded in the previously mentioned capacitor voltage balancing algorithm. The purpose of the active balancing is to achieve an equal heat distribution among the submodules to enhance the lifetime. The junction temperatures with and without active thermal balancing are studied in simulation for an HVDC application. The paper proves that thermal balance of MMC can be significantly improved.

Thermal Analysis and Balancing for Modular Multilevel Converters in HVDC Applications

“…As a problem resulting from the very low switching frequency with NLM, the turn on times are not similar for the different power semiconductors in the different SMs and can result in uneven thermal stress distribution [15]. This uneven stress affects the lifetime and can result in failures of power semiconductors.…”

“…The power transmission and the average capacitor voltages are separately controlled by the AC current and the differential current, respectively [15]. The energy stored in the upper and the lower arm are equalized [21].…”

“…Active thermal balancing algorithms are presented in [15] and [16] with the purpose of an equalized heat and stress distribution among all SMs. Unbalanced stress distribution, affecting the semiconductors' lifetimes, can occur at very low switching frequencies in HVDC applications [15] or due to parameter variations as demonstrated for fast switching mediumvoltage applications [16].…”

“…The control was described in Section II and in [15] and NLM is applied to achieve high efficiency. (1.3) A centralized method is applied for capacitor voltage balancing due to the huge number of SMs.…”

“…For application in the MMC, it was proposed to circulate reactive power for reducing the thermal fluctuations [14]. However, this thermal control approach is not addressed to the thermal imbalances among the different submodules (SMs) which can occur at low switching frequencies or due to parameter variations, as it has been done instead in [15] and [16], respectively.…”