This paper proposes a new modulation strategy that balances the neutral-point voltage for three-level neutral-clamped inverter systems. The proposed modulation replaces the P-type or N-type small switching states with other switching states that do not affect the neutral-point voltage. The zero and medium switching states are employed to help the neutral-point voltage balancing. This method little bit increases the switching events and output total harmonic distortion. However, this method has a strong balancing ability at all regions. Further, it is very simple to implement in both space vector modulation and carrier-based PWM methods. Simulation and experimental results verify the validity and feasibility of the proposed new modulation strategy.

New Modulation Strategy to Balance the Neutral-Point Voltage for Three-Level Neutral-Clamped Inverter Systems

A new pulsewidth modulation (PWM) strategy which is an alternative approach of the discontinuous PWM (DPWM) for a three-level neutral-point-clamped (NPC) inverter is proposed in this paper. A three-level NPC inverter is completely mature and very well-established topology in high-power applications. However, the three-level NPC inverter has an inheritance problem of the neutral-point voltage unbalancing due to the split dc-link capacitors. This structure can cause large neutral-point voltage ripple. Furthermore, output currents of the three-level NPC inverter are distorted by the neutral-point voltage ripple. Therefore, the neutral-point voltage must be controlled. In this paper, a new DPWM method using two different offsets for the neutral-point ripple reduction is proposed and the effectiveness of the proposed DPWM method is verified by using the simulation and the experimental results.

Novel Discontinuous PWM Method of a Three-Level Inverter for Neutral-Point Voltage Ripple Reduction

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 dc-link 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.

Time-Offset Injection Method for Neutral-Point AC Ripple Voltage Reduction in a Three-Level Inverter

The nongenerative-boost type rectifier, which is called the Vienna rectifier, is advantageous in terms of efficiency and current total harmonic distortion. Therefore, the Vienna rectifier is used in grid-connected applications such as telecommunication systems and wind turbine systems. Owing to the structural characteristics of the Vienna rectifier, various switching methods exist. Among these methods, a carrier-based discontinuous pulse-width modulation (CB-DPWM) method guarantees high efficiency compared to continuous pulse-width modulation methods; moreover, it is simple compared to a DPWM method based on space vectors. In this paper, the performance of the CB-DPWM method, its applicable region depending on the power factor and modulation index ( $M_{a}$ ), and its efficiency are analyzed in detail. Then, based on the analysis, a neutral-point voltage balancing method for the CB-DPWM method is proposed. The effectiveness and performance of the proposed neutral-point voltage balancing method are verified by simulation and experiment.

Performance Analysis of Carrier-Based Discontinuous PWM Method for Vienna Rectifiers With Neutral-Point Voltage Balance

Multilevel inverters are now-a-days being applied in electric vehicles (EV). Considering the dynamic operating conditions of an EV propelled by an induction motor, this paper proposes a flexible discontinuous modulation strategy for three-level neutral-point-clamped (NPC) inverter that can shift modulation pattern seamlessly from sinusoidal pulsewidth modulation (SPWM) to discontinuous PWM (DPWM). The modulation scheme is generalized at each degree of discontinuity to ensure minimum switching losses for variable load power factors. Additionally, a hybrid voltage balancing strategy, which comprises two sets of compensating offset signals with complementary features, has been introduced to mitigate voltage deviation problem at neutral point of the inverter. The unique feature of the proposed hybrid balancing approach is that it generates uniform compensating neutral current regardless of the level of discontinuity. This results in identical and predictable voltage balancing dynamics for the entire modulation transition covering SPWM and DPWM. It has been shown that the proposed hybrid balancing strategy does not lose its unbalance compensation strength even during transition from one modulation pattern to another. The effectiveness of the proposed scheme for wide variations in operating conditions is studied through extensive simulation, and validated in experimentation using a prototype NPC inverter with induction motor loads.

A Flexible Discontinuous Modulation Scheme With Hybrid Capacitor Voltage Balancing Strategy for Three-Level NPC Traction Inverter

Neutral-point (NP) balancing issues in the neutral-point-clamped three-level converters have been widely discussed in the last decades. Numbers of prior NP control algorithms have been proven working well in terms of NP potential balancing. However, it seems that the impact of these algorithms on the ac side did not draw much attention in the past, e.g., some algorithms inject additional common mode voltages with considerable amplitude that may saturate the duty cycles or inject low-order harmonic currents into the ac load. As a consequence, compared with the original system without NP controls, the total harmonic distortion (THD) performance at the ac side becomes worse, i.e., the THD standards are violated and a redesign of the system (e.g., ac filter) may be essential. Therefore, particularly for industrial applications, low distortion and high margins for duty cycles are typically important criteria for designing an NP controller. In this paper, a novel NP controller with full power-factor (PF) range and low distortion is proposed, which has the minimum common voltage injection at no unbalance or slight unbalance conditions. On the other hand, at heavy unbalance conditions, the controller has the maximum balancing ability at any PFs. Its superior performance is summarized via a comparison between different NP controls in this paper.

A Neutral-Point Balancing Controller for a Three-Level Inverter With Full Power-Factor Range and Low Distortion

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A Neutral-Point Balancing Controller for a Three-Level Inverter With Full Power-Factor Range and Low Distortion