Normann Fischer

Application guidelines for power swing detection on transmission systems

Power swing detection on transmission systems is becoming more critical. Traditionally, setting relays for power swing blocking (PSB) or power swing tripping applications has been very complex and time consuming. In many cases, the settings are not correct, which is discovered when the relay operates incorrectly. This paper provides the reader with practical setting and application guidelines for traditional impedance-based PSB schemes. It shows how to set a PSB scheme without stability studies. Highlighted are some problem areas when setting and applying power swing detection elements. Application of these setting guidelines will be demonstrated using a power system modeled on a real-time digital simulator

Modern line current differential protection solutions

Line current differential protection creates challenges for relay design and application. From a design perspective, the distributed nature of the line current differential system imposes limits on the amount of data that can be exchanged between the system terminals and calls for data alignment schemes to enable the differential protection principle. From the application perspective, line current differential schemes are concerned with CT saturation, particularly in dual-breaker applications; in-zone reactors and line-charging current; in-line and tapped transformers; sensitivity to high-resistive faults; single-pole tripping; security on channel impairments; application to lines with more than three terminals; and so on. This paper reviews technical solutions to the line current differential design and application, addressing the common design constraints and utility-driven application needs. The paper is a tutorial in this challenging area where protection principles and applications mix with communications and signal processing.

Deterministic High-Impedance Fault Detection and Phase Selection on Ungrounded Distribution Systems

Downed conductors, tree branches touching conductors, and failing insulators often cause high-impedance faults in overhead distribution systems. The fault currents of these faults are much smaller than detection thresholds of traditional ground fault detection devices, so reliable detection of these high-impedance faults is challenging. Although fault currents can be much smaller in ungrounded systems than fault currents in multi-grounded systems given similar fault conditions, fault detection for ungrounded systems is nevertheless easier. This paper contrasts the differences between high-impedance fault detections for ungrounded and multigrounded systems. The paper explains fault detection of ungrounded distribution systems and the issue of fault detection sensitivity. The paper also introduces a recent advance in faulted phase selection on these ungrounded systems and demonstrates this advance through a staged fault test example from a utility

Single-Phase Transformer Inrush Current Reduction Using Prefluxing

Power transformers can experience large inrush currents upon energization, the severity of which depends on the source strength, the leakage impedance and residual flux of the transformer, and the angle of the applied voltage at energization. A novel inrush current reduction strategy has been implemented which involves setting a single-phase transformers residual flux to a known polarity after the transformer has been de-energized, a process called “prefluxing,” and controlling the instant of transformer energization based on the flux polarity, seeking not to eliminate inrush current but to substantially reduce it. Unlike a popular suggested solution, this strategy does not require prior knowledge of the transformers flux. The device used for prefluxing is simple in construction and operates at substantially lower voltage levels when compared to the transformers rated voltage. The presented strategy has been successfully implemented on an 18-kVA laboratory transformer with inrush current levels reduced below the rated current of the transformer even when accounting for typical breaker deviations. This paper describes the operation of the reduction strategy, including theory, device sizing, and implementation, and presents the successful laboratory results, all of which provide the basis for implementing inrush current reduction in three-phase transformers using a three-pole circuit breaker.

Improvements in transformer protection and control

This paper describes protection elements that detect transformer faults quickly and avoid unnecessary transformer disconnections. The paper introduces a differential element that combines the security and dependability of harmonic restraint with the speed of harmonic blocking to optimize relay performance. An additional negative-sequence differential element improves sensitivity for internal turn-to-turn faults under heavy load conditions. External fault detection supervision adds security to this negative-sequence differential element during external faults with CT saturation. The paper also describes a dynamically configurable overcurrent element that improves protection coordination for different operating conditions but does not require settings group changes. In addition, the paper discusses an underload tap changer control system that uses time-synchronized phasor measurements to minimize loop currents and losses in parallel transformer applications.

Fundamentals of short-circuit protection for transformers

This paper reviews principles of protection against internal short circuits in transformers of various constructions. Transformer fundamentals are reviewed as pertaining to protection. In particular, the electromagnetic circuit of a transformer is reviewed that links the terminal currents, winding currents, fluxes, and ampere-turns (ATs) in a set of balance equations for a given transformer. These balance equations are used to explain the sensitivity of protection to various types of transformer faults. The paper shows that the classical transformer differential compensation rules have roots in the first principles — they reflect the AT balance of the protected transformer. The rule of building transformer differential protection equations following the AT balance is used in this paper to derive differential equations for autotransformers; power zig-zag, Scott-T, and Le-Blanc transformers; and phase shifters. The restricted earth fault (REF) and negative-sequence transformer differential (87TQ) functions are explained as a means to detect ground faults near the neutral and turn-to-turn faults, respectively.

Do system impedances really affect power swings — Applying power swing protection elements without complex system studies

One of the traditional techniques for detecting power swings uses a dual-quadrilateral characteristic. It is based on the measurement of the time interval it takes the positive-sequence impedance to cross two blinders. Another technique monitors the variation of the swing center voltage approximation. This paper presents a performance comparison between applications of these two techniques in cases derived from a sample network transient simulation and in cases recorded during real operations in the field.

A fresh look at limits to the sensitivity of line protection

This paper considers the sensitivity of essential line protection elements: ground distance and ground directional overcurrent elements applied as time-coordinated functions or in pilot-assisted protection schemes and line current differential schemes. Factors discussed include fault resistance, line unbalance and charging currents, impact of in-line reactors, system short-circuit capacity, load encroachment and swings, sequential tripping and weak feed terminals, steady-state and transient errors of instrument transformers, impact of current transformers (CTs) in dual-breaker line terminals, and single-pole-open conditions. Protection element design improvements and application principles enhancing sensitivity are included.

Low second-harmonic content in transformer inrush currents - Analysis and practical solutions for protection security

This paper addresses the security of transformer differential protection with low levels of second harmonic during magnetizing inrush conditions. The paper explains the phenomenon of ultrasaturation causing the second harmonic to drop below the traditional 15 to 20 percent setting levels and points to possible causes of and conditions for ultrasaturation. A number of field cases are presented and discussed in addition to the engineering analysis of the problem. The paper outlines several simple solutions to address the security problem while minimizing the adverse impact on dependability. Further, the paper presents a new method for inrush detection that considerably improves security without diminishing dependability. Finally, a method to accelerate operation of transformer differential protection is presented.

Analysis of an Autotransformer Restricted Earth Fault Application

Restricted earth fault, or zero-sequence differential protection, is beneficial in transformer applications. Because it does not respond to load current, it offers a significant improvement in sensitivity over traditional differential protection. Ground current in the transformer neutral is used as a reference and is compared to zero-sequence current at the terminals to determine if a fault is internal to the transformer. The predictability of the neutral current phase angle is critical to it being a stable reference. It is not well understood how the magnitude and direction of circulating zero-sequence current in a delta tertiary relates to the zero-sequence current in the autotransformer neutral. This technical paper derives that relationship, explains restricted earth fault protection, and uses a real- world, unexpected relay operation to demonstrate these concepts and make relay settings recommendations.