Joe Perez

Fundamental principles of transformer thermal loading and protection

Power transformers are traditionally protected by differential protection schemes that use voltages and currents to detect abnormalities in the differential zone of protection. For this type of scheme, a short circuit or high magnitude current must be present to initiate a trip. However, this scheme might not be ideal when transformers need to be overloaded to mitigate contingency conditions. Using the IEEE Guide for Loading of Oil-Immersed Power Transformers C57.91-1995, one can thermally rate transformers beyond their nameplate conditions to a level that is safe for operation. Using the guide, engineers can establish continuous, emergency and short term emergency transformer ratings. Operators can use these ratings until the contingency conditions are mitigated. However, once the transformer has surpassed the short term emergency ratings, the transformer might reach critical temperatures and could possibly sustain damage. Protection engineers can avoid further transformer damage by using the thermal protection principles of the IEEE standard. This paper discusses the fundamental thermal principles of power transformers, philosophies of operations and the implementations of thermal protection.

A guide to digital fault recording event analysis

Proper interpretation of fault and disturbance data is critical for the reliability and continuous operation of the power system. A correct interpretation gives you valuable insight into the conditions and performance of various power system protective equipment. Analyzing records is not an intuitive process and requires system protection knowledge and experience. Having an understanding of the fundamental guidelines for the event analysis process is imperative for new power engineers to properly evaluate faults. As senior power engineers retire, knowledge of how to decipher fault records could be lost with them. This paper addresses aspects of power system fault analysis and provides the new event analyst with a basic foundation of the requirements and steps to analyze and interpret fault disturbances.

The effects of neutral shifts on protective relays

This paper has shown the benefits of viewing and determining neutrals shift by graphically showing it in the voltage triangle along with the phasor diagram. We have demonstrated how neutral shifts are observed on solid grounded, ungrounded and semi-ungrounded systems. Finally, it was determined that neutral shifts can have a direct impact on the fault phase voltage and therefore affecting protective relay performance.

Preventing transformer mis-operations for external faults

These new test methods are a new tool that can be used to verify the stability of differential relays for external faults. By simulating real time events, one can discovered errors that were not possible using single phase test methods. As a result, it is encouraged that new microprocessor relays be tested as close to real system events as possible.

Novel approach to relay setting development

The 2015 NERC State of Reliability Report states that 31% of mis-operations are attributed to incorrect settings/logic/design errors [1]. According to the report, one major cause of mis-operations is human error. We know today that the current relay settings development process is laborious and difficult to complete in an accurate manner with todays tools. This process requires the setting engineer to manually transfer vast amounts of data from short circuit programs to calculation sheets and finally to relay setting files. In addition, this process requires extensive resources in peer-peer review programs to minimize human error. Depending on the internal process of the utility, preventing errors might not be feasible with the current process. This paper outlines these deficiencies and presents an innovative novel method that unifies as well as drastically simplifies the relay setting process. This new approach proposes a new way to create intelligent calculation templates, eliminates human error due to data transfer, and automates the relay setting development process.

Analysis of a differential and overcurrent operation on a 345KV high voltage line reactor

Since differential relays offer solutions for multiple applications, one can conclude that the relay engineer must deeply understand not only the element behavior, but also how each relay calculates its protection functions for the given application. This paper has described the behavior of a reactor energization and the response of two different differential relays. In addition, this paper has provided information that equips the customer and settings engineer with the necessary information to properly avoid operations during in-rush conditions.

Relay loadability challenges experienced in long lines

As discussed throughout this paper, the employment of load encroachment and OOS blinders make it difficult to ensure that the relay will operate for symmetrical faults when these two functions are enabled and still comply with the standard. The use of current differential relays solves this issue when these type of relays are available. The relay engineer can also decide to disable the OOS function based on the utility engineer best judgment.

Analysis of a flashover operation on two 138kV transmission lines

Flashover operations due to a change in the physical positioning of a foreign object with an energized conductor are a common occurrence seen throughout many transmission and distribution systems. The inception of a fault from a flashover between two stationary conductors, however, is not as prevalent. This is primarily due to extensive protection system coordination and insulation design specifications that, under anticipated fault conditions, will allow the necessary protection devices to operate, thereby clearing the fault. For Bryan Texas Utilities during the summer of 2013, a particular flashover operation brought about a series of atypical events, which resulted in several operations involving two 138kV transmission lines as well as portions of the 12.5kV distribution system. As presented in the following analysis, it is evident that, while the fault conditions experienced by the system were not within anticipated limits, there are some steps that can be taken to mitigate future operations of this nature.

Calculating loadability limits of distance relays

Typically, distance relays protect transmission lines from power system faults by using the method of step distance protection. This method uses the line impedance as the basis to form zones of protection and each zone is calculated by a predetermined percentage of the line impendence. The impedance setting will establish the relays impedance characteristic, which is graphically displayed as circles or quadrilaterals in the R-X plane. A distance relay is capable of detecting faults, indicated by a drop in the impedance of the line, when the observed impedance is inside the relays defined impedance characteristic.

A guide to transformer digital fault recording event analysis

Proper interpretation of fault and disturbance data is critical for the reliability and continuous operation of the power system. A correct interpretation gives you valuable insight into the conditions and performance of various power system protective equipment. Analyzing records is not an intuitive process and requires system protection knowledge and experience. Having an understanding of the fundamental guidelines for the event analysis process is imperative for new power engineers to properly evaluate faults. As senior power engineers retire, detailed knowledge of how to decipher fault records has the potential to be lost with them. This paper addresses fundamentals of power system fault analysis and provides the new event analyst with a basic foundation of the requirements and steps to properly analyze and interpret fault disturbances.