Latest Research on Voltage Engineering : April 21
 High-voltage engineering
The topics covered in this book include gas discharge, insulating materials, system earthing, overvoltage and insulation coordination, and high-voltage equipment and testing techniques. In two chapters, the principles of design of high-voltage busbars are discussed, together with their insulation and ampacity, whether they are of conventional air-insulated type or the metal-clad GIS types now widely used at the HV and EHV levels. The various types of circuit breakers and cables are discussed including mention of solid-state breakers and superconducting cables. The authors present a treatment of power system grounding, external and internal overvoltages imposed on system insulation, and techniques adopted for insulation coordination. The last three chapters focus on the area of insulation testing, covering the topics of high-voltage generation, measurements, and standard specifications.
 Threshold Voltage Engineering in GaN-Based HFETs: A Systematic Study With the Threshold Voltage Reaching More Than 2 V
One of the key challenges for the adoption of gallium nitride (GaN)-based heterostructure field effect transistors (HFETs) in power-switching applications is obtaining enhancement mode behavior. A large variety of methods have been applied to shift the threshold voltage V th of HFETs. However, most of the time, approaches were discussed individually, neglecting the effects of combinations. Hence, in this paper, a comprehensive study of four different approaches to shift Vth well into the positive range is presented. We show the effects of different gate metallizations, of a backbarrier, of a gate oxide, and of a gate recess. Each approach is discussed individually, and special focus is on the insulator/semiconductor interface, which is apparently different with and without gate recess. The final device exhibits a Vth of +2.3 V, which is shown to be stable when applying OFF-state stress during dynamic characterization.
 Zap [extreme voltage for fighting diseases]
Bioelectrics is an emerging field of research that combines the disciplines of high voltage engineering and cell biology to gain a better understanding of the effects of powerful, ultrashort voltage pulses on living tissue. Bioelectrics depends on advanced pulsed power technology, which is the ability to switch on and off thousands of amperes of current and just as many volts in mere nanoseconds. The most promising and practical result so far has been the recent discovery that certain pulsed electric fields can wipe out skin tumors in mice. Although many years will pass before it will even be worth testing on humans, bioelectrics offers a totally new therapeutic avenue – one that could lead to a therapy free of the debilitating side effects of chemotherapy drugs and tissue damage of radiation.
 Defective Barrier on Voltage Optimization for Small Airgap
Aim: To investigate the effect of defective barrier on the optimum breakdown voltage, using positive and negative needle electrodes in an air medium of 10cm gap distance.
Methodology: The barriers for the tests were placed at 2.5cm from the point electrode for each test. The defective barriers were created by having holes of 6mm, 8mm, 12mm and 20mm diameter at the centre of the barrier. For each barrier position the breakdown test for positive and negative polarity for needle electrodes were carried out. Also, tests were carried out with non defective barrier and with point-plane airgap (without barrier).
Result: From the test without barrier the negative point electrode offered higher breakdown voltage (1.8 times), than the positive point. When with plain barrier the positive point was optimized to 1.6 times, while the negative point was lowered.
The optimum breakdown voltage decreased gradually as the hole diameter increased and at 20mm hole diameter the effect was like the plain barrier.
Conclusion: From the results, optimization is only effective with positive point’s electrode and it endures even with small opening within the ionization zone. It is necessary to check this in practical situations because the specified optimum voltage of an equipment may be lowered.
 Design of Reactive Power and Voltage Controllers for Converter-interfaced ac Microgrids
This paper aims at presenting design of two controllers for the study of a microgrid testbed. The response of the microgrid testbed to different short circuits would be investigated under these two control regimes, namely, reactive power and voltage controls. This paper therefore presents design of active power, reactive power and voltage regulators for a converter-interfaced ac microgrid. The design was performed using Simulink Control Design® in the Department of Electrical and Computer Engineering, Curtin University, Sarawak, Malaysia between May 2015 and December 2015. The microgrid consists of two 5.5kW, 575V wind turbines based on doubly-fed induction generators (DFIGs). The systems designed are pitch control system, active power regulator, reactive power regulator, grid ac voltage regulator, dc bus voltage regulator, grid-side converter current regulator and rotor-side converter current regulator. The time-domain step response analysis for each modeled plant indicated stable performance but poor response. Therefore, regulators were realized in closed-loop feedback architecture. Each regulator was designed using small signal frequency response analysis, resulting in stable systems with satisfactory response. The regulators have been combined to implement two mutually exclusive control regimes: the active power-voltage (PV) control and the active-reactive power (PQ) control. Microgrid short circuit studies have been performed while the effect of control is decoupled, a highly simplified method which does not sufficiently mimic real systems. While attempting to study the microgrid short circuit response under different control regimes in a project which is still ongoing, this paper presents an attempt to design two control regimes for the ac microgrid testbed.
 Khalifa, M., 1990. High-voltage engineering.
 Hahn, H., Benkhelifa, F., Ambacher, O., Brunner, F., Noculak, A., Kalisch, H. and Vescan, A., 2014. Threshold voltage engineering in GaN-based HFETs: A systematic study with the threshold voltage reaching more than 2 V. IEEE Transactions on Electron Devices, 62(2), pp.538-545.
 Schoenbach, K.H., Nuccitelli, R. and Beebe, S.J., 2006. Zap [extreme voltage for fighting diseases]. IEEE Spectrum, 43(8), pp.20-26.
 Afa, J.T., 2013. Defective Barrier on Voltage Optimization for Small Airgap. Current Journal of Applied Science and Technology, pp.1301-1310.
 Aminu, M.A., 2016. Design of reactive power and voltage controllers for converter-interfaced ac microgrids. Current Journal of Applied Science and Technology, pp.1-14.