Monday: Transformers Track

Monday |  Transformers Track

9:30 AM–10:15 AM 
Transformer Winding Resistance Measurement: Field Challenges

Speakers: Daniel Carreno Perez and Dinesh Chhajer, Megger

A DC winding resistance measurement is a common diagnostic test performed on distribution and power transformers. This test can be used to detect internal issues such as shorted turns, burnt or open windings, broken strands, poor connections, and problems associated with On-Load Tap Changers (OLTC) and De-Energized Tap Changers (DETC). Although this test looks simple, in practice, it presents several technical complexities that can cause trouble for field technicians to obtain accurate readings.

This paper covers some of the less known facts associated with the DC winding resistance measurements. The paper dives deep into the topics such as selection of the correct test current, importance of compliance voltage during the test, core saturation, current stabilization, the influence of winding inductance on readings, application of dual injection testing technique, the effect of temperature, improving test efficiency for select winding configurations such as secondary delta windings and auto transformers with tertiary,  importance of safe discharge of energy stored in an inductor, and the importance of the demagnetization process that need to be followed after the completion of the winding resistance test.

10:30 AM–11:15 AM
Understanding the Revisions to PC57.104 – the Guide for Gas Generation in Mineral-Oil Immersed Transformers

Speaker: Scott Reed, MVA Diagnostics, Inc.

The Guide for the Interpretation of Gases Generated in Mineral Oil-Immersed Transformers is undergoing significant changes that affect the fault gas analysis within the transformer.

 The purpose of this presentation is to review how transformer faults gases are to be interpreted under the new guide, the establishment of the new gas limits, how the age and transformer design affect acceptable gas limits within a transformer and how the revised gas thresholds were determined. 

 The presentation will review the interpretation of test reports under the old method and new method, as well as demonstrate how the ratio of gases assists in the fault identification within the transformer. 

11:30 AM–12:15 PM
Testing and Commissioning of Current Transformer Circuit in Substation: Different Practices Among US Utilities

Speaker: Morteza Talebi, TRC Solutions

There have been many incident and outage reports in our industry caused by protective relay miss-operation during the normal load condition or not operation during fault condition because of incorrect phasing, and polarity of CTs, excessive burden in CT's secondary, CTs wiring error, shorted CTs in CT's shorting block, spurious CT's circuit ground, loos ground or multiple ground  and open circuited CT under the load. All these make the correct CT circuit testing before putting back to the service  very important. This presentation provides a theory and simple explanation of the common practices used for correct CT circuit testing among US utilities and compares the benefits and drawbacks of each method for commissioning engineers and protective relay technicians. These methods include current transformer primary and secondary injection testing, high impedance testing, light bulb testing, and through fault testing.

2:15 PM–3:00 PM
Challenges Associated with Diagnostic Testing of PT/VT Instrument Transformers

Speaker: Charles Sweetser, OMICRON electronics Corp. USA

Testing voltage transformers (VTs) or also referred to as potential transformers (PTs), has always been a challenging task. Application and technology applied can vary. Test plans, test procedures, and analysis of the results must be executed correctly if we are to determine and continually track the condition of VT/PT instrument transformers. It is important to understand the need, value, and application of comprehensive testing of PT/VT instrument transformers.

This paper will focus on both inductive and coupling capacitor (CCVT) PT/VT instrument transformers, including theory and practice. Both L-L and L-G configurations with be included, as well as recommended test voltages and settings. We will investigate the best practices regarding test procedure, test preparation, and the analysis of the results.

The following standard electrical field tests will be included in the discussion:

  • Power Factor
  • Cross Check
  • Exciting Current
  • Voltage Ratio

The audience will be provided with an understanding of the application, and analysis of these best practices, supported by specially selected case studies validating the value that these diagnostic tests bring to testing, and finally assessing, PT/VT instrument transformers.

3:15 PM–4:00 PM
The Testing Side of IEEE Std. C37.113TM – 2015

Speaker: Ruben Dario Corrales, POWER Testing and Energization, Inc.

ANSI/IEEE C37.113TM-2015 Guide for protective relay applications to transmission lines is intended mostly to serve as a guide for Transmission Line protection design; nonetheless, the proper understanding of the concepts and applications that it covers is fundamental to accomplish the goals set forth for testing and commissioning endeavors that aim to end with a reliable transmission line protection system. The standard presents concepts that translate into practical applications with different interpretations from different utilities. Concepts like reliability, dependability and security have their implementation in the drawings and settings we deal with in our day-to-day tasks as P&C engineers or technicians. Having the big picture clear is even more relevant if we consider the expectations presented in NETA ECS standard where a system approach takes importance. This paper describes the main aspects presented in this standard and their application to the testing side of business. The approach selected to test the different components of a protection system and its overall commissioning will have an everlasting effect on the reliability of the system. At the end all concepts come down to earth with physical representation of P&C assets that need to be installed, tested and maintained. This paper is intended to create interest and awareness on this must-read standard whose understanding can elevate the quality of the commissioning process for transmission line protection systems.

4:15 PM–5:00 PM
Diagnostic of Mechanical Integrity of High Voltage Power Transformer Using Sweep Frequency Response Analysis

Speaker: Jay Gosalia and Jiten Jesing, Power Diagnostic Instrument Company/Utility Power Test

Power transformer mechanical integrity is of prime importance in power delivery system. During faults on the power system, transformers sees the fault current. The fault current induces destructive mechanical forces on the transformer structure. The core, windings, terminations and associated mounting structure experiences these forces. These internal forces weakens or alters the mechanical structure of the transformer. If not attended or diagnosed the subsequent faults may result in the failure of the transformer. Sweep Frequency Response Analysis (SFRA) technique enables user to diagnose the mechanical integrity of the transformer following the events like fault, relocation of the transformer as well as acceptance of the transformer received from the manufacturer. SFRA technique allows user to compare the frequency based response from the previous one to detect any possible mechanical changes in the mechanical structure of the transformer. This paper describes theory of the SFRA, standards requirements, existing industry practices with number of field case study.