Project
Abstract
Premature failure of large transformers is costly and threatens the transmission of safe and reliable power. Recent tests conducted as a part of Arizona Public Service (APS) routine transformer maintenance have shown the development of hot spots and abnormal levels of gasses within a transformer’s mineral oil at the Cholla Power Plant in Joseph City, Arizona. We examined possible sources of these problems. To accomplish this, we researched several transformer failure case studies, developed a program to rapidly analyze years of dissolved gas in oil data, and carefully examined other crucial data gathered during the life of this transformer. Our assessment of the transformer is a tool for APS to indicate if the transformer is near the end of its operational life. The results of this work will help APS continue to serve its customers with a reliable source of power.

Project Definition
Arizona Public Service Company has requested that our team analyze the current condition of the Cholla Unit 2 Generation Step Up transformer. As part of APS’s routine maintenance for the transformer, you check for dissolved gasses within the oil of the transformer. In oil samples taken during the summer of 2012 there have been certain gasses present which indicate possible degrading of the transformer’s health. Along with these gasses found in the oil, two of the four oil cooling pumps have also been taken out of service due to high vibrations. These concerns have led to a planned outage on the transformer during the spring of 2013. During this outage the transformer will be drained of its oil so that the failed pumps can be replaced. An internal visual inspection will also be performed during this outage.

It is important to predict a failure before it happens with this transformer, because a transformer of this size can take over 9 months to acquire a replacement. There is no spare transformer, which is ready for service, to use in its place.  This means that unit 2 at Cholla would be offline for several months. Having a unit which is unable to generate power for several months would result in a huge economic loss for APS.

Our team is to research predictive analysis, reasons for failure, and a variety of tests which will help you better understand the current condition of your transformer. The reasons for failure portion of our research will include case studies of previous failures. These case studies may show a correlation between the problems in other transformers before they failed and the current problems with the transformer at Cholla. The several tests which will be included in our research will assist us with predictive analysis, and allow our team to better understand the current health of the transformer. These tests will include oil testing that can be done with the unit online and a recommendation of electrical test that can be performed after the unit is not in service. Our final product will be a report to APS which will assist you on determining the current condition of this transformer as well as additional recommendations for future monitoring.

System Diagram

 

Requirements and Specification

Mechanical

APS has a Westinghouse Generation Step Up transformer currently in service. The transformer has shown signs suggesting it is near its end of the service.  Mechanical problems have already occurred in the two failing pumps used to circulate cooling oil. This improper oil flow has started to cause unwanted hotspots throughout the transformer.

  • 21200 gallons of oil
  • Weight total: 753,000 lbs

Electrical

APS needs our team to assess the integrity of electrical components making up a transformer and determine their current state of repair.  APS wants the assessment to give an estimated time for the remaining life of the transformer, as well as a suggested course of action.

  • 3 phase 60 hertz
  • Class F0A
  • Impedance 17% at 305000 KVA
  • 65° C average rise

Environmental

The environmental factors of the APS transformer are minimal for most of the transformer’s life. There are some factors that we have to take in account for the life of the transformer such as oil temperature, winding temperature, noise, and vibration.

  • Average 65 degrees Celsius in oil
  • Average 70 degrees Celsius in copper windings
  • 60 Hz hum and vibration

Documentation

APS need all documentation to be professional and submitted in a timely manner.  Siemens, an outside testing company, is scheduled to investigate the transformer in April of 2013.  Because of this, APS needs our report before that investigation occurs.  APS wants the documentation to be technical, but still assessable to non-engineers. The health assessment report should contain a predictive analysis, reasons for failure, and a variety of tests which will help APS better understand the current condition of their transformer.

  • Create a condition assessment by May 2013
  • Cholla Unit 2 Generation Step Up Transformer serial number: 7002213
  • APS’ instruction book: EBM1057-12 (is available)
  • APS’ wire diagram: EBM1057-04 (is available)

Testing

In order to accurately determine the health of the transformer several tests will need to be performed. APS continues performing test that can be done while the unit is online such as oil dissolved gas analysis and acoustic testing. Our research is to include more online testing procedures and also tests that can be done during an upcoming outage.

  • Dissolved Gas Analysis
  • Water Content Test
  • Dielectric Strength Test
  • Acidity Number
  • Furaldehyde Test
  • Turns Ratio Test
  • Winding Resistance Test
  • Excitation Current Test
  • Frequency Response Analysis
  • Power Factor Test

               

General

The general aspects of the transformer being investigated are safety and reliability. Safety is a prime concern to APS as well as our team. Reliability is linked to the safety of transformers, because it governs the properties that decide if the transformer will fail under certain scenarios

  • Electromagnetic radiation level as identified in ANSI std. C95.1
  • Approximate life of the transformer is 40 years.