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By adding Buck-type AC/AC converter to conventional power capacitor, dynamic capacitor (D-CAP) can be formed to compensate variable rather than fixed reactive power. However, some nonlinear distortion factors, namely grid harmonic voltage, voltage drop of switches and dead-time, would cause harmonics in its compensation current, meanwhile, its capacitance may interact with the grid inductance to cause series resonance. Both nonlinear distortion factors and series resonance would distort the waveform of its compensation current and make the grid current failed to meet the power quality standard IEEE Std.519. This paper focuses on the output current distortion of Buck-type D-CAP in reactive power compensation. For harmonic distortion, through establishing the equivalent circuit of Buck-type D-CAP, the generation of low-order harmonic current is investigated under three nonlinear factors. A current reshaping method to simultaneously diminish the harmonics caused by the three nonlinear distortion factors is thus adopted, which is a feedback control of output current harmonics. Then for series resonance, based on the block diagram of Buck-type D-CAP within the grid, transfer function from grid voltage to the output current is deduced and the corresponding Bode diagram is depicted. An active damping method is therefore suggested to shrink the low-order series resonance peak, by detecting power capacitor's voltage for feedback control. Finally, a combined control, reactive current reshaping with series resonance damping is proposed in this paper for three-phase Buck-type D-CAP. A wide variety of experimental results from a 33kVar/220V laboratory prototype are provided to demonstrate the validity of the combined control.INDEX TERMS Buck-type dynamic capacitor, reactive power compensation, waveform distortion, current reshaping, series resonance damping.

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Reactive power and negative-sequence current generated by inductive unbalanced load will not only increase line loss, but also cause the malfunction of relay protection devices triggered by a negative-sequence component in the power grid, which threatens the safe operation of the power system, so it is particularly important to compensate reactive power and suppress load imbalance. In this paper, reactive power compensation and imbalance suppression by a three-phase star-connected Buck-type dynamic capacitor (D-CAP) under an inductive unbalanced load are studied. Firstly, the relationship between power factor correction and imbalance suppression in a three-phase three-wire system is discussed, and the principle of D-CAP suppressing load imbalance is analyzed. Next, its compensation ability for negative-sequence currents is determined, which contains theoretical and actual compensation ability. Then an improved control strategy to compensate reactive power and suppress imbalance is proposed. If the load is slightly unbalanced, the D-CAP can completely compensate the reactive power and negative-sequence currents. If the load is heavily unbalanced, the D-CAP can only compensate the positive-sequence reactive power and a part of the negative-sequence currents due to the limit of compensation ability. Finally, a 33 kVar/220 V D-CAP prototype is built and experimental results verify the theoretical analysis and control strategy.

Background Medical education is characterized by numerous features that are different from other higher education programmes, and evaluations of teaching quality are an integral part of medical education. Although scholars have made extensive efforts to enhance the quality of teaching, various issues unrelated to teaching that interfere with the accuracy of evaluation results remain. The purpose of this study is to identify the barriers that prevent objective and reliable results from being obtained during the evaluation process. Methods This study used mixed methods (3 data sources) to collect opinions from different stakeholders. Based on purposive sampling, 16 experts familiar with teaching management and 12 s- and third-year students were invited to participate in interviews and discussions, respectively. Additionally, based on systematic random sampling, 74 teachers were invited to complete a questionnaire survey. All qualitative data were imported into NVivo software and analysed using thematic analysis in chronological order and based on grounded theory. Statistical analyses of the questionnaire results were conducted using SPSS software. Results Sixty-nine valid questionnaires (93.24%) were recovered. A total of 29 open codes were extracted, and 14 axial codes were summarized and divided into four selective codes: evaluation preparation, the index system, the operation process, and the consequences of evaluation. The main barriers to obtaining reliable evaluation results included inadequate attention, unreasonable weighting, poor teaching facilities, an index without pertinence and appropriate descriptions, bad time-points, incomplete information on the system, lagged feedback, and disappointing result application. Almost all participants suggested lowering the weight of students as subjects, with a weight of 50–60% being appropriate. Students showed dissatisfaction with evaluation software, and the participants disagreed over the definition of good teaching and the management of student attendance. Conclusions This study reveals the difficulties and problems in current evaluations of teaching in medical education. Collecting data from multiple stakeholders helps in better understanding the evaluation process. Educators need to be aware of various issues that may affect the final results when designing the evaluation system and interpreting the results. More research on solutions to these problems and the development of a reasonable evaluation system is warranted.