Neutral Point Clamped Grid-connected Inverter Research Articles

An improved CB-DPWM strategy with NP voltage balance and switching loss reduction for 3-L NPC converter

Abstract The three-level (3-L) neutral-point-clamped (NPC) converter has found widespread applications in various fields, due to its simple structure, low failure rate and high efficiency. However, the 3-L NPC converter suffers the issue of unbalanced neutral-point (NP) voltage due to the divided two dc-side capacitors. The large NP voltage ripple and offset Based on the detailed analysis of the effects of vectors and clamping patterns on the NP voltage, a discontinuous optimized modulation method for NPC grid-connected inverter with both low switching loss and ability to balance NP voltage is presented. By measuring NP voltage and utilizing the property that superfluous clamping patterns have opposing effects on the NP voltage, the modulation method adjusts the NP voltage with the injection of various zero sequence voltages (ZSV) into the original sinusoidal waves. The results of this study demonstrate that the CB-DPWM can control the NP voltage balanced and decrease switching loss compared to conventional DPWM and SVPWM strategies. Moreover, the CB-DPWM strategy does not need to identify sectors and subsectors or calculate dwelling times. This modulation strategy effectively suppresses low-frequency fluctuations and deviations of the NP voltage with the injection of a simple ZSV, and improves the power quality of the output voltages and currents. Compared to the traditional SVPWM scheme, the suggested CB-DPWM has a 27 % lower overall IGBT loss. Moreover, this method is implemented through carrier waves, which is simpler than modulation schemes based on space vector, greatly shortens the program running time, and is conducive to engineering implementation.

Parameter co-design of active damping loop and grid current controller for a 3-phase LCL-filtered grid-connected inverter

Under non-ideal grid conditions, especially the harmonic grid condition, the proportional resonant controller with multiple harmonic resonant controllers (HR-PR controller) is recommended to provide high quality grid currents for the 3-phase LCL-filtered grid-connected inverter. Besides, in order to solve the resonance problem, the active damping loop with the grid-side currents feedback only is preferred. However, parameters of the HR-PR controller and the active damping loop both affect the performance of LCL filter resonance suppression and stability of the inverter, which is analyzed in this paper. Moreover, the stability constraint and the resonance suppression constraint is derived to maintain the good ability of LCL filter resonance suppression and satisfy the stability requirements of the inverter. Considering the wide control bandwidth for the HR-PR controller, a co-design method of the active damping loop and HR-PR controller is proposed in this paper. By introducing the Lagrange multiplier method, the proposed co-design method obtains the optimized parameters for the active damping loop and grid currents controller in z-domain appropriately. A prototype of 3-phase LCL-filtered three-level neutral point clamped grid-connected inverter is established to validate the proposed method.

Fault diagnosis in grid‐connected PV NPC inverters by a model‐based and data processing combined approach

This study presents a fault detection and isolation (FDI) method for open-circuit faults (OCFs) in the switching devices of a grid-connected neutral-point-clamped (NPC) inverter for photovoltaic (PV) applications. The proposed methodology addresses the fault diagnosis problem by a combined model-based and data processing perspective to study single and simultaneous faults in the NPC inverter. For the model-based scheme, a bank of sliding-mode proportional-integral observers is suggested to estimate the fault profiles under an additive model. Thus, from the estimated fault profiles, and by performing a directional residual evaluation in a fixed reference frame, single and simultaneous fault scenarios can be isolated in the NPC inverter. However, for some fault classes, there is some ambiguity by just the model-based approach that is overcome by employing the average line currents to construct extra fault signatures. The proposed FDI scheme only requires the measurements of line currents and grid voltages in the diagnosis media and can isolate 6 x 2 single OCFs and 12 x 4 simultaneous OCFs in the order or lower than a fundamental period of the grid frequency. Our new FDI methodology is validated through experimental data from a practical PV system in a closed-loop grid-connected NPC inverter under single and simultaneous OCF conditions.

An adaptive PI control scheme to balance the neutral-point voltage in a solar PV fed grid connected neutral point clamped inverter

In the context of current harmonics mitigation in inverter, neutral point clamped (NPC) inverter is the promising one because of its robustness. Owing to the advantages of NPC inverter, this paper considers a PV fed grid connected NPC inverter. However, the main concern with the NPC inverter is the unbalance in the DC link capacitor voltage due to non-idealities such as DC offset in modulating signals, dissimilarities in the DC link capacitors and asymmetric gating signals, which lead to non-zero neutral point voltage. To address this issue, this paper proposes a simple adaptive proportional integral (API) controller and operates in conjunction with the dq axes power controller for PV fed Grid connected NPC inverter. Comprehensive mathematical expressions are derived for the design of API controller and the control parameters are adjusted continuously based on maximum PV power, PV voltage and modulation index for achieving the faster dynamic response. The closed loop stability of the proposed API controller is verified under the deviation in neutral point voltage using root locus plot. The effectiveness of the API with dq axes power controller is analysed and validated through MATLAB simulations and experimentation under the various dynamic operation of the PV- grid interactive system.

Multi-Step Predictive Current Control for NPC Grid-Connected Inverter

Aiming at improving the quality of output current when the inverter is connected to the grid, this paper proposes a control strategy of neutral point clamped (NPC) grid-connected inverter based on radial basis function neural network (RBFNN) multi-step predictive control. Firstly, the predictive control model for output current of NPC inverter is constructed by $\alpha \beta $ coordinate transformation. Then the future values of output current are predicted for each voltage vectors, using the RBFNN prediction model. After the predicted values are obtained, a cost function $f$ is calculated for each voltage vectors and the optimal voltage vector which minimizes cost function is selected. The switch state corresponding to the optimal voltage vector is applied to the inverter in the next sampling period. Thereby the control of grid-connected output current can be achieved. The simulation results show that the RBFNN multi-step predictive control method improves the capability of current tracking and reduces the harmonic rate of output current. The proposed method can balance output current under the condition of grid voltage imbalance.

Adaptive perturb and observe maximum power point tracking with current predictive and decoupled power control for grid-connected photovoltaic inverters

In order to improve maximum power point tracking (MPPT) performance, a variable and adaptive perturb and observe (P&O) method with current predictive control is proposed. This is applied in three-phase three-level neutral-point clamped (NPC) photovoltaic (PV) generation systems. To control the active power and the reactive power independently, the decoupled power control combined with a space vector modulation block is adopted for three-phase NPC inverters in PV generation systems. To balance the neutral-point voltage of the three-phase NPC grid-connected inverter, a proportional and integral control by adjusting the dwell time of small voltage vectors is used. A three-phase NPC inverter rated at 12 kVA was established. The performance of the proposed method was tested and compared with the fixed perturbation MPPT algorithm under different conditions. Experimental results confirm the feasibility and advantages of the proposed method.

Design and Implementation of a Three-Phase Active T-Type NPC Inverter for Low-Voltage Microgrids

This paper presents the design and implementation of a 3 kVA three-phase active T-type neutral-point clamped (NPC) inverter with GaN power devices for low-voltage microgrids. The designed inverter is used in a battery-based energy system (BESS) for power conversion optimization in applications to low-voltage microgrids. A modular design method has been developed for the design and implementation of the AT-NPC inverter. Experimental verification has been carried out based on a 3-kW three-phase T-Type NPC grid-connected inverter. FPGA based digital control technique has been developed for the current control of the three-level three-phase grid inverter. A maximum efficiency of 98.49% has been achieved within a load range from 50% to 75%.

Model predictive control strategy for NPC grid‐connected inverters in unbalanced grids

A model predictive control (MPC) strategy for a three-phase three-level neutral point clamped (NPC) grid-connected inverter is discussed. Conventional two-level inverters are effective when a symmetric fault occurs. But when a three-phase unbalanced fault occurs, they will generate errors and introduce delay in the power system because of the complex design or the neglected negative-sequence component of the grid. To solve this problem, a new method to control a NPC grid-connected inverter is proposed, which combines an MPC strategy with a decoupling double synchronous reference frame phase-locked loop technique. The proposed method reduces the use of PI controllers and enhances the system's response speed. Using simulations and experiments, the proposed MPC strategy for a three-level NPC grid-connected inverter is proven to be accurate and reliable.

Open-Circuit Fault Diagnosis and Fault-Tolerant Control for a Grid-Connected NPC Inverter

This paper presents an open-circuit fault detection method for a grid-connected neutral-point clamped (NPC) inverter system. Further, a fault-tolerant control method under an open-circuit fault in clamping diodes is proposed. Under the grid-connected condition, it is impossible to identify the location of a faulty switch by the conventional methods which usually use the distortion of outputs because the distortion of the outputs is the same in some fault cases. The proposed fault detection method identifies the location of the faulty switch and the faulty clamping diode of the NPC inverter without any additional hardware or complex calculations. In the case of the clamping diode faults, the NPC inverter can transfer full rated power with sinusoidal currents by the proposed fault-tolerant control. The feasibility of the proposed fault detection and the fault-tolerant control methods for the grid-connected NPC inverter are verified by simulation and experimental results.