Thomas L. Baldwin

Fault locating in ungrounded and high-resistance grounded systems

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 mounded 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.

An Effective Method for Evaluating the Accuracy of Power Hardware-in-the-Loop Simulations

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, limited bandwidth) in the power interface, especially 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 ldquoshould-berdquo 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.

A generic real-time computer Simulation model for Superconducting fault current limiters and its application in system protection studies

A model for the SCFCL suitable for use in real time computer simulation is presented. The model accounts for the highly nonlinear quench behavior of BSCCO and includes the thermal aspects of the transient phenomena when the SCFCL is activated. Implemented in the RTDS real-time simulation tool the model has been validated against published BSCCO characteristics. As an example for an application in protection system studies, the effect of an SCFCL on a utility type impedance relay has been investigated using a real time hardware-in-the-loop (RT-HIL) experiment. The test setup is described and initial results are presented. They illustrate the effect of how the relay misinterprets the dynamically changing SCFCL impedance as an apparently more distant fault location. It is expected that the new real-time SCFCL model will provide a valuable tool not only for further protection system studies but for a wide range of RT-HIL experiments of power systems.

Directional ground-fault indicator for high-resistance grounded systems

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.

A PLL-Based Multirate Structure for Time-Varying Power Systems Harmonic/Interharmonic Estimation

This paper describes a phase-locked-loop (PLL)-based power systems harmonic estimation algorithm, which uses an analysis filter bank and multirate processing. The filter bank is composed of bandpass filters. The initial center frequency of each filter is purposely chosen to be equal to harmonic frequencies. However, an adaptation strategy makes it possible to track time-varying frequencies as well as interharmonic components. A downsampler device follows the filtering stage, reducing the computational burden, especially because undersampling operations are performed. Finally, the last stage is composed of a PLL estimator which provides estimates for amplitude, phase, and frequency of the input signal. The proposed method improves the accuracy, computational effort, and convergence time of the previous harmonic estimator based on cascade PLL configuration.

An effective method for evaluating the accuracy of Power Hardware-in-the-Loop simulations

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 ldquoshould-berdquo 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.

Study of photovoltaic integration impact on system stability using custom model of PV arrays integrated with PSS/E

This paper focuses on the impact of large solar plants on power systems due to rapid variation in power injection caused by various factors such as the intermittency of solar radiation, changes in temperature and tripping out of power electronic based converters connected to the system. In the first step of this research, incorporating the Maximum Power Point Tracking (MPPT) algorithm, a mathematical model of PV (Photovoltaic) array based solar plant has been developed. The model produces changes in DC power output for changes in its two inputs; (1) solar irradiance and (2) temperature. In the next step, the mathematical model is integrated with the dynamic simulation software PSS/E through user written model integration technique. The dynamic model for the inverter and the electrical controller are used from the PSS/E library. The PV plants are added to the 39-bus New England test system at three different locations. To demonstrate a high penetration of PV, the power generation from PV has been increased up to 20% and was randomly distributed between three plants. The dynamic behavior of the system was studied by changing the solar irradiance, tripping of the PV plant and by simulating a three phase fault at PV connected buses. The responses obtained from these studies indicate that vulnerability of the system increases with the increase in penetration of PV power.

Reactive power compensation for voltage control at resistance welders

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.

A 100 MJ SMES demonstration at FSU-CAPS

The Center for Advanced Power Systems (CAPS) at Florida State University (FSU) was recently established to pursue research and education in power engineering. Development and demonstration of superconducting technologies is one of the cornerstones of the CAPS program. Important aspects of the program are the test of superconducting equipment at power levels up to 5 MW, and the creation of a reconfigurable network that will support pulsed power testing. A 100 MJ SMES system is being completed at BWX Technologies for integration to the CAPS test facility, to allow pulsed power operation of the testbed. The SMES coil, scheduled for completion in 2003, is based on cable-in-conduit technology and NbTi superconductor. The full system (including cryostat and power converter) will be integrated at CAPS and be operational in late 2004.

Dynamic Ward equivalents for transient stability analysis

In an effort to reduce the computing time of transient stability assessment, the paper presents a dynamic equivalent which results from the elimination of the load buses provided with voltage-dependent loads. The elimination is performed through a new version of the Ward equivalencing method. In this approach, the equivalent current injections are expressed in terms of the retained bus angles and a sensitivity matrix W/spl lowbar//spl macr/. The nonlinearity of the load flow model is accounted for through piecewise linear approximations by updating the W/spl lowbar//spl macr/ matrix whenever the operating point moves beyond the validity of the linearization. The paper also derives the expressions of the incremental changes in the generator electric power and the transient energy function for the reduced system. The approach has been tested on several systems with different sizes and characteristics. >