field testing program for power transformers jeffrey short, doble engineering

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Field Testing Program for Power Transformers

Jeffrey Short, Doble Engineering

2005 APPA Conference

Introduction

What are the benefits of starting a testing program for transformers?

What is the motivation for having the program?

What tests can be performed to assess the condition of transformers?


Benefits of Testing Program

Enhance System Reliability Minimize Damage to Apparatus Enhances Safety to Personnel Minimize Loss of Revenue Extension of Apparatus Life

Degradation of Insulation, if detected before failure, can generally be restored to its original condition

Defer replacement costs



Better Utilization of Resources Acceptance of New Apparatus

Verify that new apparatus meets purchased specification and agrees with factory test reports

Assures proper field Assembly



Field Testing provides the maintenance engineer a group of tools to assess the condition of the transformer

Benefit for each test must be clear and results able to be interpreted Data may relate to different areas

Dielectric Thermal Mechanical

Engineer should choose the right tool for the right job


Motivation for Field Testing

Acceptance Testing Establish a Baseline Routinely to Establish a Condition Trend Determine Dryness of Insulation Assess Condition after Electrical Disturbance Isolate a Problem Area Assess Condition after a Relocation Asset Management - Ranking and Prioritizing Reduce Catastrophic Failures


Reduce Catastrophic Failure


Transformer Failures

Cause of Failure for Utility Transformers

29%

18%16%

13%

13%

7%

2%1%

0%1%

Elect Dist

Insulation

Lightning

Maintenance

Elect Connection

Moisture

Overload

Sabotage

Workmanship

Other

Hartford Steam Boiler Insurance Co. Statistics


Motivation for Field Testing

Average age of failure is 17.7 years


Condition Assessment Tests

Dielectric tests - Power Factor and Capacitance Perform on

Overall Windings Bushings Liquid Insulation Surge Arresters -Field test on insulation portion of

arrester

Assess the condition of the insulation and physical properties of the transformer


Dielectric Power Factor Example


FPE, 3-, 2-winding , -Y transformer13.8/4.3 kV, 7 MVA

High CL % P.F. & disagreement between HV & LV CHL %P.F.s

Localized moisture/contamination in the L.V. winding

Problem Revealed in LV Winding


Dielectric CapacitanceExample


Significance of Measured Capacitance

Capacitance detects movement and deformation of transformer windings.

Interwinding (CHT) capacitance of anautotransformer.

Test Date 20C % PF Cap (pF)1965 0.20 2,6501968 0.29 2,7561974 0.29 3,7101982 0.32 5,100


Capacitance change detects movement and deformation of transformer windings.

Significance of Measured Capacitance

C =A

4d


Excitation Currents


Field Testing

Excitation Current and Loss Factory Tests at Rated Voltage. Field Tests at the Lesser of Rated Voltage or Highest

Capability of the Test Set. Simple measurement of single-phase current on one side

of the transformer, usually the HV side, with the other side left floating (with the exception of a grounded neutral).

Magnetic Circuit and Winding Tests

Types of Problems found with Excitation Current

Windings (includes high, low, tertiary, preventative auto and series windings) Turn-to-turn winding insulation failure causing a short or

high resistance connection. Winding to ground short for a grounded winding. Open windings (main, tap, reactor). High resistance conductor connections. Phase to phase electrical tracking.

• These conditions result in a change in the effective reluctance of the magnetic circuit, which affects the current required to force a given flux through the core.

Types of Problems found with Excitation Current

Tap Changer Misalignment, mechanical problems, coking and wear

of LTC and DETC contacts.

Core Abnormal circulating currents in the core, clamping

components or through multiple core grounds. Core laminations insulation damage. Core joint dislocations.


Field Testing

Winding Turns Ratio

Ratio of the number of turns in a higher voltage winding to that in a lower voltage winding

Factory and Field• Low Voltage Method• High Voltage Capacitance Reference Method

Purposes

Confirm nameplate ratios

Detect short-circuited turn-to-turn insulation

Find open-circuited windings

Find problems with tap changer connections


Mechanical Assessment

Purpose

Assess Mechanical Condition (physical distortions) Detect Core and Winding Movement

Due to large electromagnetic forces from fault currents Winding Shrinkage causing release of clamping pressure Transformer Relocations or Shipping


Mechanical Assessment

Current Techniques

Frequency Response Analysis (FRA) Leakage Reactance Capacitance Excitation Current These independent diagnostic methods have their place in

ascertaining transformer condition


Frequency Response Analysis Simply Stated: To measure the frequency response of

passive elements (RLC) for various types of power apparatus. The result is a transfer function which produces a fingerprint related to the mechanical geometry for a given apparatus (i.e. transformers, reactors, generators, and motors).

Sweep Frequency Response Analysis (SFRA) Apply a 10 volt peak to peak sinusoidal signal from a network analyzer

(M5100) to a winding and measure the signal input/output from other end of same winding, or from another winding as function of frequency

Low-Voltage Impulse Method (LVI) Apply impulse to a winding and measure input & response

signals using high speed digitizer. Perform FFT calculation to convert time domain into frequency domain


Frequency Response Analysis

Experience to date shows that certain frequency bands indicate different problem conditions 2kHz scan sensitive to core deformation, open circuits,

shorted turns & residual magnetism 20kHz scan is sensitive mainly to bulk winding

movement relative to each other 200kHz and 2MHz scans are sensitive to deformation

within the windings 10MHz scan is sensitive to movement of winding leads


Leakage Reactance Testing

Similar to Short-Circuit Impedance Test Obtain benchmark impedance data Confirm nameplate impedance Investigate winding deformations


Transformer Test Summary

Each test is sensitive to a specific set of problems. Motivation for testing should always be

determined before arbitrarily performing a variety of tests

Routine testing should provide owner with a high level of comfort with transformer condition

Investigative testing needs to be more focused and thorough

Test results should always be scrutinized and taken seriously


Transformer Test Summary

Planning discussions for contingencies are important in making good decisions

There are cases where some tests will fail to identify a problem.

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Thank You!!!

field testing program for power transformers jeffrey short, doble engineering

Documents

field tests

condition of transformers

test set

test date20c

problem areaassess condition

lossfactory tests

hv lv chl

chl ust1021