New Junction Temperature Balancing Method for a Three Level Active NPC Converter

The three-level Neutral Point Clamped Voltage Source Converter (3L-NPC VSC, 3L-ANPC VSC) has many attractive features like its high reliability and availability. Nowadays, this technology is mature and can be found in several industrial applications [1], [2]. Some common applications are pumps, fins, compressors, mixers, extruders, crushers, rolling mills, mine hoist drives and excavators. The three-level Active NPC VSC (3L-ANPC VSC) was introduced in 2001 to overcome the drawbacks of the conventional 3L-NPC VSC [3]. The 3L-ANPC VSC includes additionally active switches parallel to the NPC diodes for clamping the neutral tap of the converter It features 3 extra switch states in contrast to the conventional 3L-NPC VSC, which enable the possibility to reduce the temperature imbalance and/or to increase the output frequency or the converter power In order to use the potential of the 3L-ANPC VSC and to balance the losses among the semiconductors, the implementation of a temperature balancing strategy is necessary [4] and [5]. The medium voltage 3L-ANPC VSC is especially advantageous in the following applications: High power applications, where the required output power cannot be archived without a serial/parallel connection of devices. Medium voltage converters, where the switching frequency should be increased without decreasing the converter power (e.g. applications which require a sine filter or high speed applications). Applications where the nominal converter current is required at low modulation index and low fundamental frequencies (e.g. zero speed operating points, hot and cold rolling mill applications, converters for doubly fed induction generators, etc.). The starting point for the derivation of the new balancing algorithm was the Active Loss Balancing (ALB) system, which is reported in [4] and [5]. The new temperature balancing scheme profits from the features of Predictive Control [6] in order to precalculate the conduction and the switching losses and finally the temperature of each semiconductor up to the future commutation to the zero state, i.e. over the next period of the switching frequency. The precalculated maximal junction temperature will be used for the control strategy in order to determine the optimal switch state to be applied. Some additional features of the new balancing algorithm with respect to the ALB algorithm are: consideration of the conduction losses fed into the balancing algorithm, regard of the temperature ripple between two consecutive commutations and the use of a control criteria (cost function) in the selection of the zero state [6]. The algorithm is implemented in Matlab and compared with the 3L-NPC VSC using experimental data of a 4.5 kV Press-pack IGBT and Diode for the calculation of the losses. The comparison shows a potential to increase the output power of the 3L-ANPC VSC.