Unbalanced loads cause the imbalance of the grid current and the resulting imbalance of the grid voltage when the imbalance rate of the grid current increases considerably. To cope with this problem flexibly at low cost, we modify the circuit structure from previously installed Δ-Y transformer 3-phase 3-leg inverter to split dc-link capacitor 3-leg inverter eliminating the transformer. And then we propose two kinds of compensation algorithms for balancing the grid current that can basically utilize a preinstalled control board. It is divided into reactive compensation, where dc-link of the inverter does not supply effective power to the grid but compensates for the imbalance of grid current, and effective compensation, where dc-link of the inverter supplies effective power to the grid for compensating the unbalanced grid current. We design an additional power calculator board to obtain power information, and apply it to a 10kW prototype, and then verify its validity through PSIM simulation and experiments.

This paper uses a DAB converter and a non-isolated DC-DC converter as modules. It analyzes the differences in circuit characteristics and reliability due to the redundancy effect by combining them in series and parallel. In particular, reliability is analyzed considering the redundancy effect when the rated power of the DAB converter and non-isolated DC-DC converter are designed to be 50% and 100% of the module-rated power, respectively. The conventional FTA method can analyze failures that reflect the operational risks of modules but cannot analyze partial failures when failures occur in some modules. To solve this problem, this paper predicts the failure rate and MTBF of the converter by modeling state changes and partial failures between modules of a modular converter based on the Markov model. The impact of the redundancy effect on reliability is presented as a quantitative value by calculating the failure rate of major components from the MIL-STD-217F fault library and substituting it into the Markov model.

The 4-leg inverter can adjust the load current or output voltage even under unbalanced load conditions, but it is known that the additional switch arm to the 3-leg inverter can increase the overall cost and the failure rate. This paper aims to analyze the failure rate and mean time between failures (MTBF) of 3-leg inverters and 4-leg inverters using part count failure analysis (PCA) and fault-tree analysis (FTA), and to compare the price of the inverters. The FTA can analyze the failure rate, including the type, number and connection status of the circuit components, and moreover the redundancy effect of the 4-leg inverter. For more accurate failure-rate prediction, the failure rate and MTBF of the 4-leg inverter according to the lifecycle of the controller are analyzed. Finally, by comparing the price of 3-leg inverters and 4-leg inverters using the cost model of major parts, the degree of reliability improvement against price increase is quantitatively analyzed.

We propose a circuit topology suitable as a battery charge/discharge tester with a DAB converter and a non-isolated dc-dc converter as a module structure. The module structure can be configured to have a wide input and output voltage range because it is easy to expand. DAB converters are used for bidirectional power transfer and galvanic isolation. By controlling the phase difference of the voltages across the DAB to be almost identical, the generation of circulating current due to the phase difference is minimized, and the ZVS is guaranteed. In addition, by switching at a phase angle that flattens the DAB switch current, the peak current is reduced to diminish the switch’s conduction loss, lowering the switch current rating, and reducing the magnetic core size. The non-isolated dc-dc converter is used to control the battery output voltage. Through the interleaved structure, good dynamic characteristics and current ripple can be reduced, so the effect of improving the battery lifecycle can be expected. We present four series-parallel structures of the proposed converter and compare them with the case of generating an output voltage only with a DAB converter. Analyze the characteristics of automatic voltage balancing due to the difference between the input capacitors’ initial voltage and capacitance connected in series. We analyze the features of the proposed system through simulation and verify the technical feasibility and excellence of the proposed battery charge/discharge tester through charge/discharge experiments using supercapacitors instead of batteries.

A cascaded H-bridge multilevel inverter (CHBMI) can increase the number of output voltage levels by increasing the number of H-bridge modules, thereby achieving low dv/dt stress and good total harmonic distortion (THD). However, as switching devices increase due to increased H-bridge modules, control becomes complicated, and the failure rate may rise. This paper analyzes the reliability of a CHBMI that can generate 27 levels, including failures in some H-bridge modules. The Markov model of a method with 13 H-bridge modules using a swapping control algorithm (Type 1) and a form with 3 H-bridge modules with input voltage sources of V _dc , 3V _dc , and 9V _dc (Type 2) was obtained, and reliability was calculated. To analyze the effect of redundancy on reliability, a reliability analysis is also performed for the two cases where two H-bridge modules are added to Type 1. Finally, we aim to help select a 27-level inverter method by comparing economic feasibility and switching loss based on the cost model.

xEV batteries are not easy to accurately estimate and measure aging parameters for reuse due to continuous charging and discharging. In addition, existing AC-IR measurement methods require high-performance micro-controller units (MCUs) to handle complex operations for parametric estimation. As a cost-effective way to solve this problem, we propose AC-IR estimation techniques and implementation circuits to estimate internal battery parameters using AC ripple with DC offset wave in low and high frequencies. It estimates electrolyte resistance (R _i ) by using band-pass filter (BPF) to extract AC components of a particular frequency voltage and current and by entering very high frequencies that have the effect of shorting the equivalent capacitance by the electrical double layer (C _d ) of the Randls model. The charge transfer resistance (R _d ) and electrical double layer capacitance (C _d ) are estimated by Discrete Fourier transform (DFT), detecting effective and reactive current by inserting low frequencies. To realize the proposed approach, we propose analog power measurements and peak detection circuits, and a circuit configuration of converter supplying AC ripple with DC offset wave to estimate internal parameters at the same time to charge battery. To verify the feasibility and high-performance of the proposed AC-IR estimation method, we carry out PSIM simulations and experiments, and compare results with commercial battery parameter measuring instrument. As a result, we found that the error rate of the proposed method is lower than that of the commercial instrument as R _i is 0.24% and R _d is 1.18%, respectively.