Karl Zimmerman

Impedance-based fault location experience

Accurate fault location reduces operating costs by avoiding lengthy and expensive patrols. Accurate fault location expedites repairs and restoration of lines, ultimately reducing revenue loss caused by outages. In this paper, we describe one- and two-ended impedance-based fault location experiences. We define terms associated with fault location and describe several impedance-based methods of fault location (simple reactance, Takagi, zero-sequence current with angle correction, and two-ended negative-sequence). We examine several system faults and analyze the performance of the fault locators given possible sources of error (short fault window, non-homogeneous system, incorrect fault type selection, etc.). Finally, we show the laboratory testing results of a two-ended method, where we automatically extracted a two-ended fault location estimate from a single end.

Fundamentals and improvements for directional relays

Phase and ground directional elements are relied on for fast and secure protection throughout the power system. Although directional relays have been applied successfully for many years, several new and unique applications and power system disturbances present challenges. Using field and laboratory data, this paper reviews fundamentals, discusses the limits to sensitivity, and shows how and why directional element designs have progressed. The paper also describes how directional elements are applied during loss of voltage conditions. In addition to design basics, we show several practical field examples that illustrate problems and solutions, while providing guidance on applying and setting modern directional relays.

Trip and restore distribution circuits at transmission speeds

In this paper, the authors show how to reduce tripping and restoration times for faults on distribution circuits. Using relays and/or controls with improved communications capabilities, linked together by optical fiber or other communication channels, one can detect faults and restore load to unaffected line sections faster.

Commissioning of Protective Relay Systems

Performing tests on individual relays is a common practice for relay engineers and technicians. Most utilities have a wide variety of test plans and practices. However, properly commissioning an entire protection system, not just the individual relays, presents a challenge. This paper suggests a process for performing consistent and thorough commissioning tests through many sources: breaking out relay logic into schematic drawings; using SER, metering, and event reports from relays; simulating performance using end-to-end testing and lab simulations; and utilizing other tools, including synchrophasor measurements. We examine and suggest approaches for commissioning several applications: distribution bus protection, short line protection using communications-aided tripping, main-tie-main scheme, line and transformer differential protection. Finally, we propose that, while 100% commissioning certainty may not be possible, we can approach 100% by integrating event report analysis to validate our commissioning strategy.

CVT transients revisited — Distance, directional overcurrent, and communications-assisted tripping concerns

Several classic papers explain the fundamentals of capacitive voltage transformer (CVT) design, operation, and transient response. Distance elements can overreach, particularly in high source-to-line impedance ratio (SIR) applications, which can result in undesired Zone 1 operations. Because this continues to be a problem in real applications, this paper revisits documented field cases using event data in hopes of shedding new light on this known problem. Solutions to distance element overreach are shared, from modified reach and time delays to modern solutions such as CVT transient detection logic. How does the protection engineer know what type of CVT is used? How can the protection engineer calculate the SIR from real-world event data? This paper gives practical guidance for the user to answer these fundamental questions. New data and research included in this paper update the topic. We investigate the CVT transient effect on directional element stability, directional overcurrent applications, and various communications-assisted protection schemes. We also share field cases of directional element and directional comparison blocking scheme misoperations and solutions and practical recommendations for mitigating the problems in all cases.

Determining the faulted phase

In August 1999, a lightning strike caused a misoperation of a relay installed in the late 1980s. The relay misoperation caused a two-minute outage at a petrochemical plant and led to an exhaustive root-cause analysis. The misoperation can be attributed to incorrect fault type selection in a distance element-based, 1980s-era relay. Two separate events in different locations, one in December 2007 and another in March 2009, highlight additional incorrect operations that occurred due to the same problem and root cause. The recent events remind us that this topic is still important and should be reviewed. This paper shares details about three challenging case studies and their root causes. Methodical root-cause analysis techniques are used, including mathematical simulation and testing of old and newer relay designs. This paper contrasts distance and fault identification algorithms, demonstrates methodical analysis techniques, and proposes solutions. Fault type selection logic is discussed, and the evolution and improvement of faulted phase selection logic over several decades is demonstrated. A newer relay design, available since 1993, is proven to have improved performance, namely better security, for these challenging cases.

Reducing arc-flash hazards

In this article, we include some important industry definitions of arc flash and ways of measuring arc-flash hazards. We then examine the use of existing technologies, including digital relays and communications capabilities, to implement reduced trip times using instantaneous overcurrent relays, a fast bus-trip scheme, and differential schemes. We use a typical industrial switchgear lineup as an example of how to implement these schemes. Finally, we quantify the levels to which we can reduce arc-flash energy and its impact on safety.

Lessons learned from commissioning protective relaying systems

Commissioning protective relays has changed with the increased use of microprocessor-based relays. Many relays have multiple functions, and logic that used to be contained in wiring diagrams or control schematics now resides in relay settings.

Protection Solutions to Reduce Arc-Flash Hazards

In this paper, we include some important industry definitions of arc flash and ways of measuring arc-flash hazards. We then examine the use of existing technologies, including digital relays and communications capabilities, to implement reduced trip times using instantaneous overcurrent relays, a fast bus trip scheme, differential schemes, and light detection. We use a typical industrial switchgear lineup as an example of how to implement these schemes. Finally, we quantify the levels to which we can reduce arc-flash energy and its impact on safety.

Maximizing line protection reliability, speed, and sensitivity

This paper describes several commonly applied line protection schemes, including distance schemes, directional comparison schemes using distance and directional elements, and line current differential schemes. Using analysis tools like fault trees, power system studies, and event analysis, we evaluate and compare these protection schemes in terms of speed, sensitivity, dependability, security, and selectivity. The paper considers the use of various communications channels, including direct relay-to-relay fiber-optic channels and multiplexed digital fiber-optic networks. The paper also discusses some practical considerations for evaluating line protection schemes when faced with complications like series compensation, mutual coupling, single-pole tripping and reclosing, three-terminal lines, and short lines.