The dead-time elimination pulse width modulation (PWM) enables the drive pulses of the upper and the lower switching devices to alternate according to the current polarity, thus abandoning the dead-time and essentially avoiding the dead-time effect in power converters. However, the current zero-crossing will be distorted by the current jump generated by the drive-pulse alternation. And the current zero-crossing distortion will be further intensified if errors exist in the detected current polarity. Thanks to the noise-attenuation and frequency-adaptability characteristics of the double second-order generalized integrator frequency-locked loop (DSOGI-FLL), it can obtain accurate current polarities even in harmonic and unbalanced conditions. A DSOGI-FLL-based dead-time elimination PWM is therefore proposed in this paper, and several improvements are made to minimize the current zero-crossing distortion. An underlap period is added when alternating the upper and lower drive pluses to smooth the current jump at zero-crossing. And a delay compensation term is inserted in the DSOGI-FLL to compensate both the current measurement delay and the control delay. The effectiveness of the proposed DSOGI-FLL-based dead-time elimination PWM has been validated by experiments respectively in the unbalanced-current, power-change, and frequency-variation conditions with a RL load, as well as in a grid-connected converter.

The installation cost of Photovoltaic (PV) plants can be reduced considerably by extending the maximum DC voltage from 1000 V to 1500 V (e.g., with more PV arrays connected in series). However, the increased DC voltage also presents challenges on the design and operation of PV inverters in terms of efficiency and reliability. To ensure an efficient and reliable PV power conversion, an early-stage reliability assessment is of importance in the design phase of the inverter and then the entire system. This paper, thus, evaluates the lifetime of three-level 1500-V PV inverters with respect to their thermal cycling capabilities both at the component level and system level. The evaluation is carried out through a case study on a 160-kW PV system considering the impact of voltage stress, switching frequency, and mission profile. The evaluation reveals that these factors have a significant impact on the inverter reliability, and thus affect the topology selection and the final cost of the entire PV system. More importantly, the exploration provides insights on the design of 1500-V PV systems seen from the reliability perspective.

The symmetrical space vector pulsewidth modulation (SVPWM) has been widely adopted in three-level (3L) T-type inverters due to its low current harmonics and high linear modulation index. However, when faced with unbalanced and oscillated neutral-point (NP) voltages, extra current harmonics will be generated. In this article, a new solution is provided. Other than adopting the existing strategies for balancing the NP voltage, the symmetrical 3L SVPWM is optimized to work with unbalanced and oscillated NP voltages. First, the vectors are idealized, based on which ideal dwell times are obtained by an analytically simplified symmetrical 3L SVPWM. Then, a dc-zero-sequence component is calculated and injected to maximize the linear modulation index, and the real dwell times are further derived after the adjustment according to the “volt–second balance” principle. In addition, a control strategy is investigated to actively adjust the NP voltage. Finally, the optimized symmetrical 3L SVPWM is verified by comparative experiments, showing its low current harmonics with unbalanced and oscillated NP voltages.

There is an increasing demand in integrating energy storage with photovoltaic (PV) systems to provide more smoothed power and enhance the grid-friendliness of solar PV systems. To integrate battery energy storage systems (BESS) to an utility-scale 1500 V PV system, one of the key design considerations is the basic architecture selection between DC- and AC-coupling. Hence, it is necessary to assess the reliability of the power conversion units, which are not only the key system components, but also represent the most reliability-critical parts, in order to ensure an efficient and reliable 1500 V PV-battery system. Thus, this paper investigates the BESS solutions of DC- and AC-coupled configurations for 1500 V PV systems with a comparative reliability analysis. The reliability analysis is carried out through a case study on a 160 kW/1500 V PV-system integrated DC- or AC-coupled BESS for PV power smoothing and ramp-rate regulation. In the analysis, all of the DC-DC and DC-AC power interfacing converters are taken into consideration along with component-, converter-, and system-level reliability evaluation. The results reveal that the reliability of the 1500 V PV inverter can be enhanced with the DC-coupled BESS, while seen from the system-level reliability (i.e., a PV-battery system), both of the DC- and AC-coupled BESSs will affect the overall system reliability, especially for the DC-coupled case. The findings can be added into the design phase of 1500 V PV systems in a way to further lower the cost of energy.

1500-V photovoltaic (PV) configuration is the standard design in the solar PV industry. Extending the maximum dc voltage from 1000 to 1500 V can reduce the installation cost of the entire power plant. However, it may affect the reliability of the corresponding 1500-V PV inverters, due to the increased loading stresses, i.e., voltage stress and thermal loading of power devices. In this context, this article proposes a solution to the reliability enhancement of silicon carbide- mosfet -based 1500-V PV inverters with variable gate resistance. This solution offers a possibility to adaptively adjust the switching speed to make a compromise between the switching power loss and voltage overshoot during commutation, thus enhancing the reliability. The evaluation results based on the mission profile of a 125-kW 1500-V PV system installed in Denmark indicate that the PV inverter with the proposed design, i.e., variable gate resistance, can improve reliability performance compared to the fixed gate resistance solution while ensuring a safer operating voltage margin.

A cascade H-bridge (CHB) stands out for its modular structure and high output voltage among various power converter schemes for battery energy storage systems. While space vector pulsewidth modulation (SVPWM) offers better utilization of the dc-link voltage, it is seldom employed in CHB designs due to the substantial computational burden associated with an increasing number of voltage levels in the converter. This article introduces a novel hybrid SVPWM approach in a multilevel CHB for battery energy storage systems. In this proposed system, the reference vector is decomposed into a low-frequency vector and a high-frequency vector. The low-frequency vector is generated by the low-frequency modules operating at the line frequency, while the high-frequency vector is produced by the high-frequency module working in the pulsewidth modulation mode. It compensates for the disparity between the low-frequency vector and the reference vector. To determine the low-frequency vector, an algorithm based on coordinate transformation is introduced. This significantly reduces the computational complexity, making it independent of the output voltage levels. The switching losses decrease as most power devices operate at the fundamental frequency. An experimental platform of the CHB is constructed to validate the advantages of the proposed modulation algorithm.