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Transformers experience magnetizing inrush that creates an operating current in the transformer differential protection. Early transformer differential relays had to address the limits of analog technology. Analog filter circuits could extract the second-and fourth-harmonic components of the differential current, and these were used as a surrogate measure for determining if the operating current was due to inrush. Today, we can design algorithms that distinguish directly between inrush characteristics and fault current characteristics. By using the new time-domain algorithms, we can improve sensitivity and speed of transformer differential protection. We can also maintain protection security for transformers built using improved core steels.

This paper reviews technical, safety, and economical merits of adaptive autoreclosing based on fault location calculated in real time. These applications include preventing reclosing for faults on cable sections of hybrid lines comprising overhead and cable sections, faults located close to large generating stations, faults on line sections crossing densely populated areas or fire-prone terrain, or faults on line sections near airports that receive small airplanes. The paper explains principles of fault locating based on traveling waves and introduces an adaptive autoreclosing control logic to allow or cancel reclosing based on the location of the fault. The paper includes examples that explain and illustrate these principles. The paper also describes several methods of using operational data — internal and external faults as well as switching events — to further improve the fault-locating accuracy of a commissioned fault locator.

This paper explains the asymmetrical short-circuit interrupting current rating for high-voltage circuit breakers. The paper teaches how the decaying dc component in the asymmetrical fault current affects the breaker, and it explains how the X/R ratio and the relay operating time affect the asymmetrical current breaker rating. The paper briefly introduces, and illustrates with field cases, several ultra-high-speed protection principles that can operate in just a few milliseconds. The paper then explains how to derate a breaker for the relay operating time that is shorter than the standard reference value of 0.5 cycle. The paper calculates the “rating loss” due to fast tripping and suggests that applying customary margins when selecting breakers may be sufficient to mitigate the effect of ultra-high-speed relays without the need to replace breakers.

We look back at the history of distance protection, explain the first principles, and discuss why our industry settled on designs we know and appreciate today. We look at why, after a century of refinements, a typical distance element still uses heavily filtered voltages and currents and operates on the order of one power cycle. In the second part of the paper, we explain the principles of time-domain distance protection based on incremental quantities, and operating by processing samples of voltages and currents without band-pass filtering to retrieve phasors. We discuss various choices for a time-domain distance element and present test results and field cases of an implementation with operating times of just a few milliseconds. In the third part of the paper, we discuss the feasibility of a distance element based on traveling waves and operating even faster.

This paper describes a single-ended traveling-wave-based fault-locating method that works with currents only. Unlike current transformers, coupling-capacitor voltage transformers do not have a frequency bandwidth that is wide enough to allow measuring of voltage traveling waves for this application. The key to a robust single-ended traveling-wave fault-locating method is to correctly identify reflections from the fault point. For this purpose, the method uses additional information, such as the impedance-based fault location and reflections from the remote terminal and external network elements. This paper presents the single-ended traveling-wave-based fault-locating method in detail and explains how to perform fault locating manually using ultra-high-resolution fault records from any recording device. This paper also presents laboratory test results as well as field cases in which line crews found the actual faults.

Ultra-high-speed line protection is becoming a reality today, giving the industry a way to trip line faults in a few milliseconds. One relay described in this paper that uses time-domain principles incorporates incremental-quantity (TD32) and traveling-wave (TW32) directional elements in a communications-assisted tripping scheme, incremental-quantity distance element (TD21), and traveling-wave differential element (TW87). This paper introduces these time-domain line protection elements, shares key details of their implementation in hardware, and illustrates their operation using real-world faults and digital simulations. By comparing the performance of the time-domain line protection elements with the traditional phasor-based elements of the in-service relays that captured the fault records, we demonstrate the performance of the time-domain line protection elements.

This paper analyzes the performance of current transformers (CTs) under the presence of geomagnetically induced currents (GICs). Our intent is to qualify the impact of GICs on CT performance in the context of protection security and dependability. The paper proposes a simple method to analyze the GIC problem and applies it to explain and quantify the impact of GICs on CTs. The paper uses carefully selected simulation tools, as well as laboratory tests on a physical CT to derive and support our key findings. We summarize our conclusions as follows: GIC impact is negligible in steady states during load conditions or faults but is significant in the first few milliseconds of a fault. As such, the GIC impact is very similar to the impact of the CT remanent flux on the performance of CTs during fault conditions. This impact is significant but short-lived. The component of CT saturation caused by high remanent flux or a preexisting GIC disappears very quickly.

This paper explains the principles of negative-sequence differential (87Q) protection, its basis for excellent sensitivity and speed, and the need for securing it with external fault detectors to deal with the saturation of current transformers. The paper reviews applications of 87Q elements to lines and transformers. It explains why 87Q elements cannot be used for turn-to-turn fault protection in shunt reactors and stators of generators and motors. The paper presents applications of negative-sequence directional elements for turn-to-turn fault protection in reactors and stators. Finally, it derives two new protection principles based on negative sequence for generator turn-to-turn fault protection.

Today's relays are predominantly based on phasors, and as such, they incur a delay associated with the full-cycle observation window required for accurate phasor estimation. Considerable improvement in speed is possible by using information in the transients of voltages and currents. We review a number of protection techniques, including directional elements, direct tripping underreaching elements, and differential elements that significantly speed up line protection.

This paper reviews the protection requirements for generators during periods of off-nominal frequency operation. It investigates the behavior of instrument transformers during these periods as well as the impact of frequency on the accuracy of protection operating quantities. Traditional methods for frequency tracking and compensation are reviewed. A novel approach, which provides excellent performance for very significant frequency deviations and can accommodate multiple frequency islands, is presented.

Transmission power lines can feed distribution networks or either serve loads or interconnect generation directly via tapped transformers. In some utilities, as many as three to five taps may be used on some lines. As a part of the bulk electric system, tapped transmission lines must be protected with instantaneous fault-clearing times and proper selectivity and sensitivity. This paper reviews various approaches to protecting tapped lines, including pilot protection schemes with distance and zero- and negative-sequence directional elements, time-overcurrent elements, line current differential schemes, and stepped distance backup. In addition, this paper discusses protection applications for in-line transformers where a transmission line terminates on a transformer and a single protection zone includes both the line and the transformer.

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.

Faults on overhead transmission lines cause transients that travel at the speed of light and propagate along the power line as traveling waves (TWs). This paper provides an overview of TWs and TW fault locators. It explains the physics, reviews the theory of TWs, explains the foundations of various types of TW fault locators, and provides an in-depth discussion on a number of TW fault locating implementation challenges. Finally, it discusses integration of TW fault locating in microprocessor-based relays and presents Bonneville Power Administration's (BPA's) field experience using these relays.

It is common knowledge that the line current differential protection principle is not susceptible to unusual system conditions. This is true and straightforward regarding protection security but is not immediately obvious when considering dependability of protection. As a result of dismissing the impact of unusual system conditions, the effect of series compensation, power swings, and inverter-based short-circuit current sources on line current differential protection is not well understood. This paper briefly touches on protection security but focuses on the dependability of line current differential protection under a number of specific system conditions, including series compensation, power swings and off-nominal frequency, and inverter-based sources. This paper reviews the impact of each of the mentioned conditions and provides illustrations using simulations.

The globally used COMTRADE standard was initially developed by IEEE and later adopted by IEC. The first IEEE version was published in 1991 and was later revised in 1999. The IEC version was adopted in 2001. The 2013 revision of the COMTRADE standard is an IEEE/IEC Dual Logo standard planned for publication during the first quarter of 2013. The main motivations for the current revisions are: 1) to remove restrictions that were only relevant for computing technologies of the 1990s and 2) to satisfy the requirement of universal time information in COMTRADE files. The second need was identified during the 2003 Northeast Blackout analysis to time synchronize data from different substations. The working group has also addressed other issues, including the availability of a single file. Industry users feel very strongly about this need to easily exchange and manage COMTRADE files, and the working group's recommendations address these concerns.

Testing line current differential (87L) schemes is more complicated than testing directional comparison schemes. The latter exchange simple on/off (permission or block) signals that can be conveniently checked at the output of a given relay and conveniently forced at the input to a given relay in the scheme. Line current differential schemes exchange synchronized current values that cannot be easily intercepted or forced at the 87L communications port of the relay. Moreover, the communications channel is an integral part of the 87L scheme, and its characteristic can impact some fundamental aspects of the scheme, such as sensitivity and speed. Therefore, it is preferable that the actual channel be in use when testing the scheme. Line current differential schemes that apply external time sources for synchronization need to be tested with time sources as well. Often, 87L schemes are tested with multiple crews dispatched to two or more line terminals using either test sets synchronized via the Global Positioning System or other methods of controlling the relationship and timing between current sources at the line terminals. This paper presents several approaches to testing 87L schemes. These approaches include verification of relay hardware, firmware and settings, channel performance, and, if used, external time sources. The paper also describes features of line current differential relays and multiplexers to aid testing.

This paper discusses merits, advantages, and disadvantages of integrating breaker failure (BF) protection with zone protection relays (ZPRs). In this context, the paper considers cost savings, security and dependability, simplicity, the danger of human errors when testing bus configurations, overall relaying philosophy, and reliability of applied protection devices. Several ways of integrating BF protection are proposed, allowing different tradeoffs between the mentioned factors. This paper also reviews methods to improve the security of BF protection.

The IEEE/PSRC Substation Protection Subcommittee Working Group (WG) K5 on Ancillary Protective and Control Functions Common to Multiple Protective Relays have produced a document that addresses the considerations in applying the ancillary protection, control and monitoring functions that are commonly available in multiple relays and the integration of these functions into the overall protection system. Modern protection schemes are designed with fully integrated protection, control and monitoring functions to accommodate the implementation of many different design requirements. These functions can be used to achieve reliable protection and control solutions. Implementing these designs can be an exhilarating and very satisfying challenge to the engineer's imagination. This summary paper addresses subjects related to specific protection and control topics with application examples which were covered in the special PSRC publication.

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.

Distance and directional overcurrent protection can provide 100 percent coverage of transmission or subtransmission feeders through the use of teleprotection (communications-assisted tripping) schemes. We briefly review in this paper various types of teleprotection schemes and then present principles to successfully apply a directional comparison blocking scheme (DCB). We amplify these principles by examining the reasons why incorrect tripping occurred on feeders using DCB schemes in five field cases. Further, we alert readers to the conundrum of using DCB schemes in emerging electricity markets — dependability of protection versus less security of supply. We propose that protection engineers can apply the advantages of digital communications systems, functionality of numerical relays, and increased electricity network infrastructure to intelligently design line protection systems with high dependability and high security.

This paper concerns the justification of the use of pilot protection on transmission lines.

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.

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.

The IEC 61850-9-2 standard [1] focuses on transparency and standardization of data communications. Implementation issues such as suitable architectures, reliability, time synchronization, data sharing, maintainability, testability, and scalability remain outside the scope of the standard. Process bus architecture is a missing element on the road to implementing the next generation of Protection and Control (P&C) systems. In this paper, architecture refers to the definition and structure of the process interface points, partitioning and allocation of functions to the devices, the underlying structure of time synchronization, settings and firmware management, failure-tolerant communication framework, required data throughputs and latency considerations, data traffic patterns, and other related aspects. Careful analysis of the rules and symmetries occurring in topologies of high voltage substations allow for identification of process bus data traffic patterns, origins, destinations, and throughput required to accomplish a simple, robust, scalable and flexible IEC 61850 process bus architecture. The primary equipment itself drives a logical and natural architecture for a communication-based protection and control scheme. This paper presents a practical process bus architecture conforming to IEC 61850-9-2 that fits the task of protection and control of substations by drawing from the universal topology rules of substations.

After two decades of phasor network deployment, phasor measurements are now available at many major substations and power plants. The North American SynchroPhasor Initiative (NASPI), supported by the US Department of Energy and the North American Electric Reliability Corporation (NERC), provides a forum to facilitate cultivating the efforts in phasor technology in North America and globally. Phasor applications have been explored and some are in today's utility practice. The IEEE C37.118 Standard is a milestone in standardizing phasor measurements and defining performance requirements. To comply with the IEEE C37.118 and to better understand the impact of phasor quality on applications, the NASPI Performance and Standards Task Team (PSTT) has prepared two comprehensive documents which leverage prior industry work (esp. in WECC) and international experience. The first document describes PMU testing based on both IEEE C37.118 requirements and required dynamic performance tests. The second document describes characterization of PMUs and instrumentation channels based on practical information. This paper summarizes the accomplished PSTT work and presents the methods for phasor measurement evaluation to assure consistent PMU system performance.

This paper presents an operational experience with incipient faults: it has been observed that 10 to 15% of cable faults are preceded by incipient faults. Practically all incipient faults become permanent faults in the period between a few seconds to few weeks. Incipient faults occurring in fast successions create considerable over-voltages and induce faults on other feeders. A method has been presented to detect incipient faults in a secure and reliable way. The method is secure by checking consistency of the load before and after the event, checking if the event is a single phase event, and checking for duration and consistency between the superimposed fault component and the ground current. The presented method has been implemented and tested using recorded field cases and on a digital simulator. Simplified variants of the method can be implemented by using programmability and flexibility of modern microprocessor based relays. Recommendations are given as to the trip vs alarm applications of the incipient cable fault detection functions. In many cases tripping on the first incipient fault is a prudent application.

This paper presented two algorithms for enhanced numerical BF protection. The fast reset algorithm for current-based BF detector is based on differentiating between the rotating pattern of a fault current and the decaying pattern of a subsidence current. The algorithm is simple, yet very fast and robust. It uses few factory constants, but they have well defined meanings and can be easily fine tuned in any particular implementation that follows our approach. The algorithm can be further enhanced by recognizing that when the current vector stops rotating and starts decaying due to subsidence, it actually rotates at a significantly lower rate, but in the opposite direction compared with an alternating current. This reversal of the angular speed can be used to strengthen and speed up the algorithm

Shunt capacitor banks (SCB) are installed to provide capacitive reactive compensation and power factor correction. This paper derives technically accurate operating equations for capacitor bank short circuit protection that are derived assuming both inherent capacitor bank and system unbalance. The presented relay protection methods allow compensating simultaneously for the bank inherent unbalance and system unbalance increasing both sensitivity and security of protection. The presented methods also facilitate auto-setting and self-tuning applications

This paper derives correct balance equations for short circuit protection of shunt capacitor banks taking into account inherent unbalances in the protected bank. Four methods are derived: voltage differential, compensated neutral voltage unbalance, phase current balance, and neutral current balance. As can be seen from key equations the proper way of balancing the bank (or banks) involves instantaneous values of currents or voltages. Subtracting the residual unbalance as a time-delayed signal (a historical, or a constant value), and responding to the delta changes does not constitute a proper, sensitive and secure operating equation for protective relaying purposes. The methods presented in this paper compensate for both bank and system unbalances. Therefore they are insensitive to major system events such as close-in faults. Presently used relaying techniques might misoperate on such system conditions, as they typically disregard system unbalances and compensate for the bank unbalance assuming no, or minor system unbalances.

This paper discusses several protection and control aspects in relation to dual-breaker line terminals. First, breaker failure and reclosing functions are discussed as applied to dual-breaker configurations. Second, the paper talks about protection security as related to saturation of current transformers (CTs) under fault currents flowing locally through the two breakers. New generation of microprocessor-based line relays support dual CT inputs to monitor both breakers individually, and three voltage points to provide for the main line protection, and synchrocheck across both breakers. The paper points to advantages and disadvantages of integration protection, and provides some guidance regarding dual-breaker line applications

Although the concept and definition of Synchrophasors dates back to 1980, the combination of 2nd generation IED platforms and power system needs has brought the technology into high-visibility in the electric power industry. As synchrophasor technology has matured, nuances of the measurement of a synchronized phasor have been identified and the details of ldquohowrdquo a phasor is defined, synchronized to absolute time, reported, and communicated have subsequently been re-codified int he recently revised IEEE standard: Synchrophasors for Power Systems - C37.118. This paper reviews the concept of the synchronized phasor in light of the IEEE C37.118 standard. Specifically, details of an ldquoun-correctedrdquo Fourier based phasor during off-nominal frequency conditions are presented as well as techniques for correction to meet the Synchrophasor standard. The need for IED input transformer and filter characterization / correction is presented. Simulations of various system transients and their phasor response are presented - specifically, the response of synchrophasor calculation to dynamic system conditions. Finally, this paper reviews application of synchrophasors on the SRP system today as well as plans and needs in the future.

More and more often series compensation, non-standard generation and power electronic devices co-exist in a vicinity of EHV transmission lines. A typical scenario is a relatively long transmission line already with elements of HVDC and/or series-compensation, interconnected to a new wind farm with significant capacity driven by non-standard machines. In some cases single-pole tripping is applied in order to maximize power transfer during reclosing, and to avoid synchronization of the islanded generation. During transients, in such systems complex interactions occur between series capacitors, multiple machines, and HVDC devices. This includes both the primary equipment, and the associated controls. As a result the system responds differently compared to a traditional network fed from synchronous generators. Some protection techniques that are well-recognized and successfully applied in traditional configurations may perform inadequately under such complex conditions. The paper lists protection concepts that must be carefully examined or tested before using in such a difficult application environment. It also proposes practical supervision methods that could be implemented via custom logic on a modern multi-function relay in order to solve some of the performance problems

This paper reviews concepts proposed by the IEC 61850 standard; Protection and Control (P&C) aspect in particular. The standard describes building blocks meant to facilitate the general idea of migrating traditionally hard-wired P&C connections and configurations into the realm of software, while opening considerable opportunities for engineering and labor savings. The underlying principle is that the newly introduced complexity resides in software, and therefore, new software tools will reduce or eliminate some of it, simplifying both the engineering and deployment, thus resulting in a substantial net gain. IEC 61850 has received considerable attention in recent years, but vast majority of publications are written around the theme of the Intelligent Electronic Device (IED), and rest at introducing the concept, reviewing selected details of IED implementations, and focusing on anticipated advantages. This paper is written for a traditional P&C engineer and reviews important practical questions that need to be answered, solutions worked out, and tools developed before the IEC 61850 concept could see its all-inclusive implementation.

This paper defines underlying performance quality measures for designing, optimizing, setting and evaluating the protective relaying algorithms and equipment. The paper starts with evaluation of digital measuring algorithms, and gradually introduces the performance indices for the operating principles, relays and protection systems. Multi-objective formal ranking methods based on fuzzy set theory are used to combine diverse measures into composite performance indices enabling evaluating and optimizing various functions of protective equipment.

An application of artificial neural networks (ANNs) to the protection of three-phase power transformers is presented in this paper. The emphasis is placed on selecting the appropriate type of an ANN as well as pre- and post-processing algorithms. More than 30 essentially different architectures have been developed and tested in this paper: from natural relaying signals to highly pre-processed logic signals as far as the pre-processing is considered; from a single perceptron to "winner takes all" neural nets as far as the type of an ANN is considered; and from natural output from an ANN to median filters as far as the post-processing is considered. The comparative analysis of the selected designs is performed and the optimal ANN-based classifier is selected. The included operation examples illustrate high security and dependability of the optimized ANN-based protection for power transformers.