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Power hardware-in-the-loop (PHIL) simulations need to be accurate to truly reflect the behavior of the systems under test. However, a PHIL simulation may result in errors or even instability due to imperfections (e.g., time delay, noise injection, phase lag, and limited bandwidth) in the power interface, particularly in high-power applications. Additionally, it is usually difficult to determine the accuracy of a simulation because there is no reference available for people to know the 'should-be' system responses in advance. Therefore, a method is demanded to predict the accuracy of PHIL simulations. In this paper, an effective method for evaluating the PHIL accuracy is proposed. This method provides a means to justify the result of a PHIL simulation analytically and quantitatively instead of empirically. While the method is based on linear system analysis, it is shown to be also applicable for nonlinear PHIL systems. Index Terms--Interface perturbation, power hardware-in-the-loop (PHIL) simulation, simulation accuracy.

The closed-loop stability and the simulation accuracy are two paramount issues in power hardware-in-the-loop (HIL) simulation in regard to the operational safety and the experiment reliability. In this paper, the stability issue of the power HIL simulation is first introduced with a simple example. A stability analysis and accuracy estimation method based on the system's loop transfer function is later given. Five different interface algorithms are described, and their respective characteristics with respect to the system stability are compared. Through Matlab simulations and field experiments of two representative power HIL examples, it is revealed that certain interface algorithms exhibit higher stability and accuracy than the others under the given conditions. A recommendation for selecting appropriate interface algorithms is finally proposed at the end of this paper. Index Terms--Interface algorithm, power hardware-in-the-loop (HIL) simulation, simulation stability.

Resistance welders are a source of voltage fluctuations and flicker in industrial power distribution systems. The high-reactance welding transformer that limits the welding current creates a low-power-factor (pf) load. With hundreds of welders, factories may use synchronized welding cycles that lead to annoying flicker. Other facilities use random timing to prevent flicker, but suffer deep voltage sags due to simultaneous welding operations. Severe voltage variations reduce the power delivered to the welders, causing reduced heating and poor-quality welding joints. The paper examines the design and application of a mini-static var compensator (SVC) to improve the voltage quality on the welding circuits of industrial power distribution systems. The compact range of reactive-power demands for robotic welders suggest the use of thyristor-switched capacitors (TSCs). The compensation provides the necessary voltage control for consistent welds with the added benefit of reduced load currents throughout the industrial distribution system. The compensator control is coupled with the welder controls to mitigate the random reactive-power mismatches that are often seen with other SVC welding and arc-furnace applications. Index Terms--Harmonic filtering, reactive-power compensation, resistance welding, voltage regulation.

With the recent initiatives to modernize large ship power systems, ship builders are integrating technologies with the aim of an all-electric energy system. New control strategies utilizing distribution automation and network reconfiguration provide greater flexibility, reliability, and quality of service. These automated controls require efficient and accurate load flow solutions. A critique of classical and distribution power-flow methods is conducted for shipboard distribution automation. Three classical radial-network power-flow methods are reviewed. A modification of the forward-backward sweep method is presented, which enhances the computation time and solution convergence. Comparative tests are conducted on a new benchmark test system of a ship's distribution network. The results demonstrate that the modified forward-backward sweep method succeeds in being the fastest method with good convergence on networks with low X/R ratios. Index Terms--Distribution power flow, radial load flow, shipboard power system.

With better understanding of arc flashes, the electrical safety industry continues to develop new safety procedures for electrical workspaces. To assess the potential arc hazards of a workspace, workers must rely on engineering fault studies to provide vital fault-current data. An instrument, based on a network impedance analyzer, determines the maximum flash-arc incident-energy exposure at a worksite within a few seconds. The digital analyzer measures the power system source impedance, X/R ratio, and the system voltage to predict the bolted fault current and incident energy, while the power distribution system is energized and in normal operation. The instrument computes the incident energy for standard electrical workspaces of an open-air arc and an enclosed box with one open side. Experiments have been conducted to verify the accuracy of the impedance and X/R ratio measurements. Index Terms--Arc hazard, electric arc, electrical safety, fault current.

Locating ground faults is a difficult and challenging problem for low-voltage power systems that are ungrounded or have high-impedance grounding. Recent work in pilot signals has renewed efforts in developing fault location methodologies. This paper presents a method for directional ground-fault indication that utilizes the fundamental frequency voltages and currents. Although the ground-fault current is small and usually less than the load currents, the fault has zero-sequence components that distinguish it from the load. Signal processing techniques are used to identify and compare the fault signals to determine the fault direction. The process takes advantage of the currents flowing from the distributed grounding capacitance. An experimental microprocessor-based directional indicator unit is tested in an industrial power distribution system. Directional indication of ground faults is applied near tap-off branch circuit connections. Promising results from field test conducted in a harmonic-noisy setting are presented. Directional indicator units simplify the search process on large networks, thus reducing the time and effort necessary to locate and remove the fault, and thereby significantly reduces the probability of a second ground fault with its destructive currents. Index Terms--Directional relay, fault location, high-resistance grounding, ungrounded systems.

High-resistance grounding and ungrounded power systems have many advantages, including continuous operation after a ground fault. However, locating ground faults is inherently difficult and is a major disadvantage with these systems. The problem stems from the lack of a good signal, such as a large fault current, that points to the fault. Classical search methods use injected locating signals, but they have shortcomings with intermittent faults, multiple 'same-phase' faults, and closed-loop/mesh power networks. New location methods attempt to overcome these previous problems by providing signals that can be used during system operation and with microprocessor-based detectors. This paper explores some of the key technical issues surrounding locating signals. Of concern are the signal flows through the power network including the uniqueness of the signal path. Other issues that are addressed are signal levels and power requirement, network impedances, and the impact of grounding capacitances and grounding resistances that divert the locating signal. Index Terms--Ground-fault location, high-resistance grounding, ungrounded system.

One of the most common and difficult problems to solve in industrial power systems is the location and elimination of the ground fault. Ground faults that occur in ungrounded and high-resistance grounded systems do not draw enough current to trigger circuit breaker or fuse operation, making them difficult to localize. Techniques currently used to track down faults are time consuming and cumbersome. A new approach developed for ground-fault localization on ungrounded and high-resistance grounded low-voltage systems is described. The system consists of a novel ground-fault relay that operates in conjunction with low-cost fault indicators permanently mounted in the circuit. The ground-fault relay employs digital signal processing techniques to detect the fault, identify the faulted phase, and measure the electrical distance away from the substation. The remote fault indicators are used to visually indicate where the fault is located. The resulting system provides a fast, easy, economical, and safe detection system for ground-fault localization. Index Terms--Ground-fault location, high-resistance grounding, ungrounded system.

Power hardware-in-the-loop (PHIL) simulations need to be accurate to truly reflect the behavior of the systems under test. However, a PHIL simulation may result in errors or even instability due to imperfections (e.g., time delay, noise injection, phase lag, and limited bandwidth) in the power interface, particularly in high-power applications. Additionally, it is usually difficult to determine the accuracy of a simulation because there is no reference available for people to know the "should-be" system responses in advance. Therefore, a method is demanded to predict the accuracy of PHIL simulations. In this paper, an effective method for evaluating the PHIL accuracy is proposed. This method provides a means to justify the result of a PHIL simulation analytically and quantitatively instead of empirically. While the method is based on linear system analysis, it is shown to be also applicable for nonlinear PHIL systems. [ABSTRACT FROM AUTHOR]