June-Seok Lee and Kyo-Beum Lee, "Time-Offset Injection Method for Neutral-Point AC Ripple Voltage Reduction in a Three-Level Inverter," IEEE Transactions on Power Electronics, vol. 31, no. 3, pp. 1931–1941, Mar. 2016. -SCI > Paper

The purpose of neutral-point voltage balancing algorithms in the three-level neutral-point clamped (3L-NPC) topology is to eliminate

the voltage unbalance of top- and bottom-side dclink voltages (neutral-point voltage). These algorithms are divided into dc 

unbalance and ac unbalance compensation methods. The dc unbalance causes distortion of output currents; therefore, this problem 

has to be solved. The ac unbalance implies that there is an ac ripple in the top- and bottom-side dc-link voltages. In applications 

using 3L-NPC topology, a large dc-link capacitor value is required to mitigate this ac unbalance problem. As a result, the size and 

the cost of applications become large and expensive. In this paper, a compensation method for mitigating the ac unbalance of 

the neutral-point voltage is proposed, which causes the dc-link capacitor value to be small, when a carrier-based pulse width

modulation method is applied. The proposed method compensates the ac unbalance of the neutral-point voltage by adding an 

optimal compensation value to the reference duty signals. This compensation value is calculated simply by currents and reference 

duty signals. The calculation process of the optimal compensation value is discussed, and the effectiveness of the proposed method 

is verified with simulations and experiments.