NPC (neutral point clamped converter) is widely used in medium- and high-voltage grid connection because of its small power devices, simple overall structure, and easy control. This paper presents an improved indirect current control strategy for an LCL-type NPC. Firstly, the reference current is compensated to suppress the resonant peak of the LCL filter and improve the tracking accuracy of the output current. Then, with the relationship between state variables, the control structure is optimized by introducing the non-PLL control method, so that the sensors are reduced to only two sets. Finally, the traditional differential operator is improved to compensate the calculation delay in digital control. The experimental results show that the amplitude of thee grid-connected current is consistent with the reference value, the phase deviation is reduced from 7° to about 1°, and the transient response time is shortened to 1/3 of that of the traditional method.

Cascaded H-bridge multilevel converter is widely applied in flexible AC transmission and grid connection of renewable energy, but its current quality decreases as the grid voltage distortion continues to deteriorate. A modified natural frame control method to enhance the harmonic elimination ability of converters is proposed in this paper. A single-phase cascaded delayed signal cancellation operator is applied to deal with the distorted grid voltage signals. Benefit from a system structure without any feedback loops or integral items, the transient response of single-phase cascaded delayed signal cancellation operator is faster than commonly-used second-order generalised integrator. The settling time of single-phase cascaded delayed signal cancellation operator facing grid voltage phase jump is less than one grid period. After immunising the synchronisation reference against harmonic distortion by using the single-phase cascaded delayed signal cancellation operator, the modified natural frame control method can then generate high-quality sinusoidal current. Phaselocked loop and coordinate transformation are unnecessary for the proposed method, which decreases the system complexity. Various simulations and experimental tests based on a down-scaled platform demonstrate the performance of the proposed modified natural frame control method under distorted grid voltage.

In this paper, a control scheme combining grid voltage sensorless control and natural frame control (NFC) has been proposed for single-phase cascaded H-bridge converter (CHB) under distorted grid conditions. Firstly, the virtual flux (VF) signal is estimated by a modified virtual flux estimator (VFE). Then the VF-based NFC is derived, using the estimated sinusoidal VF signal to generate the unit vectors for synchronization, which enables the omitting of the coordinate transformation, current quadrature signal generation, and phase-locked loop (PLL). Aiming at the problem that grid harmonic interference is not well considered in the conventional NFC and conventional voltage sensorless control, a current controller with enhanced harmonic-rejection ability is proposed in this paper. As a result, better steady-state performance and rapid transient response are achieved. The effectiveness of the proposed method has been verified on a prototype of the single-phase 3-cell CHB converter.

Based on a single-phase cascaded H-bridge (CHB) multilevel converter, this paper investigates a control strategy combining power estimation and natural frame control (NFC) with the omission of grid voltage sensors. Besides, this control method, without phase-locked loop (PLL), can meet the continuous operation of the system and also eliminates grid voltage feedforward. First, natural frame control is introduced to derive the implementation process of control strategy without PLL, and the calculation formula of AC reference currents is given. Furthermore, the separate control of active and reactive currents is achieved. Second, the principle of proposed control algorithm is presented, and the sinusoidal voltage unit vector for synchronization can be accurately estimated. Next, considering the errors, the article corrects the method through parameter compensation. In addition, the balanced control method stabilized the DC side voltage of the converter. Finally, the effectiveness of this method is verified by computer simulation and experimental prototype.cascaded H-bridge (CHB), natural frame control (NFC), grid voltage sensors, phase-locked loop (PLL), power estimation