W. H. Tang

Switching Control of Buck Converter Based on Energy Conservation Principle

This brief presents a switching control scheme (SCS) based on the energy conservation principle for buck converters. The concept of the SCS is based on the conservation of energy in circuit. It keeps the balance between the energy that is injected into a circuit and the sum of the energy that is consumed by the load and stored in reactive components. The SCS not only regulates the output voltage of the buck converter accurately under static conditions, but also improves its dynamic responses to disturbances of input voltage and load current. Furthermore, the SCS is capable of operating in both the continuous current mode and the discontinuous current mode, and the stability analysis undertaken using the Lyapunov stability criterion shows that the SCS is stable in all cases. Simulation and experimental results confirm the possibility and attractiveness of using this method to control buck converters.

Novel Cascaded Switched-Diode Multilevel Inverter for Renewable Energy Integration

In this paper, a new topology of two-stage cascaded switched-diode (CSD) multilevel inverter is proposed for medium-voltage renewable energy integration. First, it aims to reduce the number of switches along with its gate drivers. Thus, the installation space and cost of a multilevel inverter are reduced. The spike removal switch added in the first stage of the inverter provides a flowing path for the reverse load current, and as a result, high voltage spikes occurring at the base of the stepped output voltage based upon conventional CSD multilevel inverter topologies are removed. Moreover, to resolve the problems related to dc source fluctuations of multilevel inverter used for renewable energy integration, the clock phase-shifting (CPS) one-cycle control (OCC) is developed to control the two-stage CSD multilevel inverter. By shifting the clock pulse phase of every cascaded unit, the staircase-like output voltage waveforms are obtained and a strong suppression ability against fluctuations in dc sources is achieved. Simulation and experimental results are discussed to verify the feasibility and performances of the two-stage CSD multilevel inverter controlled by the CPS OCC method.

Modeling of transient overvoltages in wind power plants

Compared with conventional power plants, wind power plants (WPP) have their own characteristic, which are more vulnerable to transient overvoltages. Transient overvoltages may contribute to insulation failures of step-up transformers. In WPPs, the transient overvoltage is mainly caused by switching operations of vacuum circuit breakers. In this paper, suitable models for the simulation of transient overvoltages are developed. A detailed model of vacuum circuit breaker, which is required to simulate both the capability of high-frequency current interruption and the withstand capability of transient recovery voltage, is developed. Input parameters for cable models are calculated on the basis of the parameters of actual cables. A wide band transformer model is also proposed for studying the transient behaviors in WPPs. Simulations of transient overvoltages during circuit breaker switching operations in WPP have been carried out in PSCAD/EMTDC. In case studies, it considers some influence factors of transient overvoltages, such as the length of cable feeders and the collector system topology adopted in WPPs. The results obtained from this research can be used as a reference for the insulation level designation of electrical components in WPPs.

A novel approach to power transformer fault diagnosis based on ontology and Bayesian network

This paper proposes an application framework to reinforce the existing process for ontology-based transformer fault diagnosis with formal probabilistic semantics using the Bayesian Network. This framework allows users to quantify a certain fault with Bayesian Network, based on the knowledge embedded in a transformer ontology regarding relationships of faults and their features such as causes, symptoms and related diagnostic methods. Firstly, the essential principle of Ontology and Bayesian Network are introduced. Then a transformer ontology, which can capture the knowledge of fault diagnosis for transformers, is described as the basis of the developed framework. This framework has three functionalities, i.e. ontology-based Bayesian network generation, uncertainty assignment, evidence assignment and beliefs update. All of these functionalities are discussed in detail. Finally the proposed framework is exemplified by a nine-node Bayesian Network model for transformer fault diagnosis, which demonstrates the potential of the developed framework for transformer fault diagnosis.

Virtual-flux-based predictive direct power control of three-phase AC/DC converters

In this paper, a novel predictive direct power control (PDPC) strategy is proposed for three-phase AC/DC converters, which is based on a straightforward regulation of instantaneous active and reactive power. The converter controlled by the proposed strategy realizes operations without line-side voltage sensors by introducing the virtual flux (VF) concept and a grid voltage estimation method based on the instantaneous power. A constant converter switching frequency is achieved by employing the space vector modulation, which simplifies the design of the line filter. Simulation results show that the improvement of power ripple elimination and sinusoidal line currents can be observed in the proposed PDPC at a lower switching frequency. Comparative studies reveal that the steady behaviors of the proposed PDPC are superior to that of the conventional look-up-table DPC (LUTDPC) and PDPC, moreover the transient process of which is more shorter compared with that of the conventional LUTDPC.

Half load-cycle one-cycle control for inverter

The conventional one-cycle control (OCC) technique requires the integrator to reset instantaneously. However, it is very difficult to realize by hardware due to the finite time constant of a reset circuitry, which loads to a large DC offset in the output voltage. Therefore, the paper presents a new control scheme to resolve the problem, which is based on considering a singlephase full-bridge inverter as a buck converter in either positive or negative half load-cycle. Then the amplitude and frequency of the output voltage can be adjusted effectively. Furthermore, by using the dual-buck modulation, the inverter keeps one pair of switches shut during a half load-cycle, which reduces switching loss compared with that of the traditional OCC and avoids a waveform distortion brought by adding dead time. Through theoretical analysis and simulation comparison, it is verified that the half load-cycle OCC can solve the problem of DC offset in the output voltage of the inverter using traditional OCC, while the advantages of the OCC such as constant switching frequency, no multipliers, and simple circuitry are preserved.