Xiu Yao

Characteristic Study and Time-Domain Discrete- Wavelet-Transform Based Hybrid Detection of Series DC Arc Faults

DC arc fault introduces major safety concerns in a wide variety of components in dc networks. However, the randomness and instability of dc arc makes it difficult to be detected. In this paper, an experimental system was designed and tested at different load current, dc source voltage, and gap length to evaluate the impact of each parameter to the dc arc. Based on the experimental results, characteristics in the electrical behavior were studied and fault detection oriented analysis was conducted. A detection algorithm utilizing both time and time-frequency domain characteristics was proposed to differentiate between dc arc fault and normal condition. The detection algorithm was then realized on a digital signal processing board and tested to verify the effectiveness. Experimental results show that the proposed algorithm can detect arc fault in a timely manner and is free of nuisance trip from normal circuit operations such as load change condition.

FPGA-Based Detailed Real-Time Simulation of Power Converters and Electric Machines for EV HIL Applications

Real time simulation can play a fundamental role in the design and validation of hardware and software for power electronic converters and electric drives. In this paper, an FPGA based real time platform for simulation of power converters and electric machines is presented. Strategies for modeling power electronic devices are explained and compared. In addition, a Thevenin based equivalent model of a switch is enhanced to model the on characteristics of these devices. By using these on properties, an algorithm for electro-thermal modeling of converters is presented. With respect to electric machines, different methods for modeling of the saturation characteristics are evaluated for their use in real time simulation. Lastly, a Hardware in the Loop (HIL) model of an Electric Vehicle (EV) drive is presented.

The detection of DC arc fault: Experimental study and fault recognition

DC arc fault introduces major safety concerns in a wide variety of power electronic applications. However, the randomness and instability of dc arc makes it difficult to be detected. In this paper, an experimental system to study the characteristics of series dc arc is designed and different tests were conducted in order to determine the influence of different factors to the arc such as gap length, current, etc. During the experiments, the load current was varied from 6 A to 30 A, and dc source voltage was changed from 75 V to 300 V. Also, dc arc test with fixed power supply voltage and load current but changing gap length were conducted to examine the influence of arc length. Based on the experimental results, a primary V-I characteristics study was carried out. Current variation analysis was performed to investigate the pulse patterns of the arc current for detection purposes. High frequency arc impedance under different test conditions was also investigated. Lastly, time frequency analysis was applied to the different arc current signals in order to examine the impacts of factors such as load current, dc bus input voltage to the arc current with respect to frequency domain. The results of this paper provide insights of the dc arc characteristics as well as methods for dc arc fault detection.

Characteristics and Development Mechanisms of Partial Discharge in SF 6 Gas Under Impulse Voltages

It is well known that impulse voltages strongly affect the reliability of gas-insulated switchgear (GIS) under actual operational conditions. The alternating-current (ac) withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. However, the bulkiness and inefficiency of aperiodic impulse voltage generator make it unsuitable for on-site application. Therefore, in practical tests, oscillating impulse voltage (OIV) is generally adopted instead of aperiodic impulse voltage. In this paper, we measure the PD inception voltage (PDIV) and the breakdown characteristics of a needle-plate gap in SF6 gas under four types of impulse voltages by electrical and optical methods. The 50% probability PDIV is analyzed as a function of gas pressure. The physical mechanisms of the PD under the four impulse voltages are discussed. The PD characteristics under ac voltage are investigated for comparison. Experimental results reveal that the PD development process under the two aperiodic impulse voltages was mainly dominated by the precursor mechanism which consists of streamer and leader stages, while the PD propagation under the two OIVs was affected by the displacement current produced by oscillation. The difference of the PD characteristics under the aperiodic impulse voltage and OIV, and ac voltage are also presented.

DC arc fault: Characteristic study and fault recognition

DC arc fault introduces major safety concerns in a wide variety of power electronic applications. However, the randomness and instability of dc arc makes it difficult to be detected. In this paper, an experimental system to study the characteristics of series dc arc is designed and different tests were conducted in order to determine the influence of different factors to the arc such as gap length, current, etc. During the experiments, the load current was varied from 6 A to 30 A and dc source voltage was changed from 75 V to 300 V. Also, dc arc fixed power supply voltage and load current but changing gap length were conducted to examine the influence of arc length. Based on the experimental results, a primary V-I characteristics study was carried out. Current variation analysis was performed to investigate the pulse patterns of the arc current for detection purposes. The results of this paper provide insights of the dc arc characteristics as well as methods for dc arc fault detection.

Impact evaluation of series dc arc faults in dc microgrids

This paper evaluates possible impacts of series dc arc faults to dc microgrids. Series dc arc can happen at various locations of a dc microgrid and is associated with high impedance and low fault current level, which adds difficulty to detection and protection. This paper firstly presents dc arc fault modeling of both dc component and high frequency noise component. With the dc arc model, the impact of dc arc faults at various locations of a dc microgrid are then studied. Two different controllers are adopted to study the interaction between dc arc faults and microgrid controllers. It is found that current and voltage control strategies can have adverse impact to dc arc faults and dc arc faults at certain locations can be very difficult to detect with existing dc microgrid protection strategies. Additional dc arc fault detectors are of necessity to improve the overall safety of the microgrid.

Analysis and Suppression of Circulating Harmonic Currents in a Modular Multilevel Converter Considering the Impact of Dead Time

This paper focuses on analysis and suppression of circulating harmonic currents in a modular multilevel converter (MMC) considering the impact of dead time in medium-voltage applications. A continuous equivalent model of the MMC containing two ideal transformer models is presented. Using this model, the impact of a harmonic voltage upon the dc side is analyzed and the production mechanism of circulating harmonic currents is elucidated. At the same time, the impact of dead time and insulated gate bipolar transistor (IGBT) voltage drop (DTVD) is studied, which indicates that capacitor voltages, output harmonics, and circulating harmonic currents are influenced. Based on this analysis, an open-loop control strategy to suppressing circulating harmonic currents caused by the output current and DTVD is presented. Finally, all these conclusions are verified using a simulation platform with 14 modules per arm fed by a 14-kV dc voltage source and a downscaled experimental platform with four modules per arm fed by a 560-V dc voltage source.

Partial discharge characteristics over SF 6 /epoxy interfaces under impulse voltage

Gas insulated switchgears (GIS) have been widely used due to their high reliability, low maintenance and compact size. The field experiment is an important step to ensure the safety of GIS. The AC withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. This paper reports on experiments to study surface discharge over SF6/epoxy under impulse voltage. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD). The PD signals under four types of impulse voltages were detected by electrical methods. The 50% probability PD inception voltage (PDIV50), time lags and maximum discharge magnitude were analyzed as a function of gas pressure. The PD characteristics under AC voltage were investigated for comparison. The results indicate that the PD signals under impulse voltage can be detected using electrical method, the number of discharges under oscillating impulse voltage was significantly larger, which was due to the influence of source oscillation. The oscillating lightning impulse has the lowest impulse ratio and it is recommended as the best test waveform choice for the field impulse withstand test with PD detection.

Partial discharge characteristics and mechanism in voids at impulse voltages

Partial discharge (PD) characteristics and mechanism in artificial cavities in an epoxy plate have been investigated for different void dimensions and impulse voltage waveforms. A differential measurement system was developed in order to detect PD current pulses effectively. Experimental results showed that the 50% probability PD inception voltage (PDIV50) increases initially as the cavity diameter decreases at constant depth for double exponential impulses as well as oscillating impulses, but after aging, it becomes independent of the cavity diameter. Moreover, some distinctive characteristics of PD (e.g. main discharge and reverse discharge during the rise and fall phases of the applied voltage) were also investigated. The differences of the PD propagation and the mechanism between double exponential impulses and oscillating impulse were discussed.

Modeling and circulating current control of MMC

Modular Multilevel Converters (MMC) offer several advantages over common three level inverters. For example, MMCs have lower switching frequency and lower dv=dt due to their multilevel output. For this reason, MMCs are typically used in High Voltage DC (HVDC) applications. In this paper, different approaches for (real time) simulation of MMC based systems are presented. A Thevenin based approach with a constant admittance matrix is proposed to lower the complexity and computation time. In addition, a circulating current suppression controller is formulated based on time domain analysis and validated through simulations.