June-Seok Lee

New Modulation Techniques for a Leakage Current Reduction and a Neutral-Point Voltage Balance in Transformerless Photovoltaic Systems Using a Three-Level Inverter

Transformerless topologies of many topologies are widely used in photovoltaic (PV) systems because these topologies have many advantages in terms of the weight, size, and efficiency. A three-level inverter has an outstanding performance and is advantageous in the switching device selection than a two-level inverter. In the transformerless PV systems using the three-level inverter, the PV systems should suffer from the leakage current and generate the neutral-point voltage unbalance. To solve two problems, this paper proposes modulation techniques to reduce the leakage current and balance the dc-link voltages. The cause of the leakage current in the three-level inverter is analyzed. The proposed technique LMZVM using the large, medium, and zero vectors reduces the common mode voltage that causes the leakage current than that of the conventional PWM. Moreover, the proposed technique LMSVM using the large, medium, and small vectors balances the dc-link voltages with reduced CMV as the same in LMZVM. The effectiveness of the proposed techniques is verified by comparing its results with those of the convectional PWM. The results are obtained through simulations and experiments.

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

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.

An Open-Switch Fault Detection Method and Tolerance Controls Based on SVM in a Grid-Connected T-Type Rectifier With Unity Power Factor

Multilevel converter topologies are widely used in applications of wide-power range. The T-type topology, which is one of many three-level topologies, has an advantage in terms of efficiency compared to the neutral-point-clamped (NPC) type. In applications using the T-type topology, interest in reliability has been increased recently. Therefore, the open-switch fault detection method and tolerance control for T-type rectifiers, which are different from those of T-type inverters, are necessary to improve the reliability of applications. When the open-switch fault of switches connected to a neutral point occurs, an NPC-type rectifier cannot restore distorted input currents; however, a T-type rectifier is able to eliminate input current distortion completely. In this paper, an open-switch fault detection method is proposed and can find a position of the open-switch fault. Moreover, two tolerance controls based on space vector modulation are proposed and their characteristics are analyzed. The effectiveness and performance of the proposed open-switch fault detection method and two tolerance controls are verified by simulations and experiments.

Open-Switch Fault Detection Method of a Back-to-Back Converter Using NPC Topology for Wind Turbine Systems

In wind turbine generation (WTG) systems, a back-to-back converter with a neutral-point-clamped (NPC) topology is widely used because this topology has more advantages than a conventional two-level topology, particularly when operating at high power. There are 12 switches in the NPC topology. An open-switch fault in the NPC rectifier of the back-to-back converter leads to the distortion of the input current and torque vibration in the system. Additionally, an open-switch fault in the NPC inverter of the back-to-back converter causes the distortion of the output current. Furthermore, the WTG system can break down in the worst case scenario. To improve the reliability of WTG systems, an open-switch fault detection method for back-to-back converters using the NPC topology is required. This study analyzes effects of inner and outer open-switch faults of the NPC rectifier and inverter and describes a novel open-switch fault detection method for all possible open-switch faults in the back-to-back converter.

Open-Switch Fault Tolerance Control for a Three-Level NPC/T-Type Rectifier in Wind Turbine Systems

Three-level topologies are widely used in back-to-back converters of wind turbine generation (WTG) systems. In comparison with a conventional two-level topology, three-level topologies have more advantages, particularly for the high power. Neutral-point clamped (NPC) and T-type are typical three-level topologies. An open-switch fault of a three-level rectifier that is connected to a permanent-magnet synchronous generator (PMSG) leads to the current distortion and torque vibration in WTG systems. Furthermore, WTG systems can break down in the worst case. Therefore, a tolerance control for open-switch faults is required to improve the reliability of WTG systems. WTG systems by applying the tolerance control can be stopped safely and prevent additional problems. Depending on the situation, it can keep the operation of WTG systems under open-switch fault. The effects of outer open-switch faults of three-level rectifiers are analyzed by considering PMSG specifications in this paper. Moreover, a tolerance control for outer open-switch faults of the NPC and T-type three-level rectifiers are proposed with existing tolerance controls for inner open-switch faults. The proposed tolerance control is to inject the exact

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

d

Carrier-Based Discontinuous PWM Method for Vienna Rectifiers

-axis current value to eliminate current distortion. The simulation and experiment are conducted to prove the feasibility of the proposed tolerance control and to identify its performance.

On-line Parameter Estimation of Interior Permanent Magnet Synchronous Motor using an Extended Kalman Filter

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.

Tolerance Control for the Inner Open-Switch Faults of a T-Type Three-Level Rectifier

Vienna rectifiers, which are nongenerative-boost types of rectifiers, are used in grid-connected applications with a unity power factor such as telecommunication systems and wind turbine systems. These rectifiers are advantageous in that they have low total harmonic distortion and efficiency. There are many switching methods, which are different from those of threelevel topologies, for Vienna rectifiers. In this paper, we propose a carrier-based discontinuous pulse width modulation (CB-DPWM) method. This method is simple compared to the discontinuous PWM (DPWM) method for Vienna rectifiers based on space vectors and guarantees normal rectifier operation for all modulation index (Ma) values. We perform experiments to verify the suitability and performance of the proposed CB-DPWM method.

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

This paper presents estimation of d-axis and q-axis inductance of an interior permanent magnet synchronous motor (IPMSM) by using an extended Kalman filter (EKF). The EKF is widely used for control applications including the motor sensorless control and parameter estimation. The motor parameters can be changed by temperature and air-gap flux. In particular, the variation of the inductance affects torque characteristics like the maximum torque per ampere (MTPA) control. Therefore, by estimating the parameters, it is possible to improve the torque characteristics of the motor. The performance of the proposed estimator is verified by simulations and experimental results based on an 11kW PMSM drive system.