Transformer Condition Assessments

GTR – Utilising Advanced Testing Technology

Transformer Condition Assessment.

Knowledge ‘is’ Power


“GTR believe extending the useful life of a power transformer is the single most important strategy for increasing life of power transmission and distribution infrastructure”

Determining the present condition of a power transformer is an essential step in analysing the risk of failure.

Reasons for Assessment


For providing reliable electrical energy, it is very necessary to have highly reliable associated electrical equipment. The transformer, being a key element in the transmission and distribution of electrical energy, improving its reliability is of utmost importance. System abnormalities, loading, switching and ambient condition normally contributes towards accelerated aging and sudden failure, Hence, it is, all the more essential, to employ on-site diagnostics followed by quality maintenance for trouble-free and reliable operation with minimum outages.

Given that the average age of power transformers in most countries is around 25–30 years and considering the increase of the typical failure probability with the transformer age, action clearly needs to be taken to ensure their continued reliability and functionality.

 

 

 

 

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The cost of failure

Power transformers are often situated at strategically critical locations in power supply systems, and as a result the financial consequences of their failure can easily exceed their actual asset value. The real challenge lies in implementing the right action at the right time.

Given that the average age of power transformers in most countries is around 25–30 years and considering the increase of the typical failure probability with the transformer age, action clearly needs to be taken to ensure their continued reliability and functionality.

Evaluation for Analysis

GTR Transformers are able to evaluate previous Transformer maintenance records including oil analysis samples along with a visual external inspection to determine which units are showing signs of degradation and vulnerability.

When a unit has been selected for analysis our experienced engineers equipped with the latest industry leading technology will arrive at your facility and perform onsite tests and conduct oil sampling from various locations using expert methods to ensure accuracy of reporting.

GTR transformers will analyse the collected measurements in house providing the customer with a life expectance in present state and recommendations of extending life. Identify weaknesses to manage and control or plan for mitigating measures.

Our software delivers comprehensive reports to ensure future condition monitoring has an accurate reference to benchmark.

OMICRON CPC100

Winding Resistance measurement

Winding Insulation Resistance IR

This basic insulation resistance test is used to determine the integrity of windings and the condition of windings insulation. Transformer windings are exposed to environmental factors such as dirt, grease, temperature, stress, and vibration which can lead to insulation failure. The test is performed after the initial installation of transformer and as part of the routine maintenance.

Core Ground Resistance test

The transformer core is intentionally grounded through one connection.  The core-to-ground resistance test can detect if this connection is loose.  It can also detect whether there are other undesired and inadvertent, grounds.  If the intentional core ground is intact, the resultant resistance should be very low.  To check for unintentional core grounds, remove the intentional ground and megger between the core and the grounded transformer tank.  This test should produce very high resistance, indicating that an unintentional ground is not present.  This test is to supplement dissolved gas analysis that shows generation of hot metal gases (i.e., methane, ethane, and ethylene) and to indicate if a spurious, unintentional core ground is the problem. Experience can help locate the source of the problem.

Frequency Response Analysis

Frequency Response Analysis (FRA) or Sweep Frequency Response Analysis (SFRA) can determine if windings of a transformer have moved or shifted.  It can be done as a factory test prior to shipment and repeated after the transformer is received on site to determine if windings have been damaged or shifted during shipping.  This test is also helpful if a protective relay has tripped or a through fault, short circuit, or ground fault has occurred

A sweep frequency is generally placed on each of the high voltage windings and the signal is detected on the low-voltage windings.  This provides a picture of the frequency transfer function of the windings.  If the windings have been displaced or shifted, test results will differ markedly from prior tests.  Test results are kept in transformer history files so they can be compared to later tests.  Results are determined by comparison to baseline or previous measurements or comparison to units of similar design and construction.

Dielectric Frequency Response

DFR also known as ‘Polarisation Spectrum Measurement’ it is used to assess the moisture content of the cellulose insulation located between the primary and secondary winding. This test is normally carried out as part of the maintenance routine to assess insulation conditions. Insulation performance can be affected by paper aging or increased moisture level due to broken seals. This measurement is also used for new transformers to prove the low moisture content after the drying process.

A sweep frequency is generally placed on each of the high voltage windings and the signal is detected on the low-voltage windings.  This provides a picture of the frequency transfer function of the windings.  If the windings have been displaced or shifted, test results will differ markedly from prior tests.  Test results are kept in transformer history files so they can be compared to later tests.  Results are determined by comparison to baseline or previous measurements or comparison to units of similar design and construction.

OMICRON DIRANA –

Dielectric Frequency Response (DFR)

Polarisation Spectrum Measurement

Oil Analysis

 

Transformer Condition Indicator – Insulating Oil

Dissolved gas analysis is the most important factor in determining the condition of a transformer.  Being performed more frequently than other tests, it may be the first indication of a problem.  Insulating oil analysis can identify internal arcing, bad electrical contacts, hot spots, partial discharge, or overheating of conductors, oil, tank, or cellulose.  The “health” of the oil reflects the health of the transformer itself.

Oil analysis

Transformer Condition Indicator – Insulating Oil

Dissolved gas analysis

 

Dissolved gas analysis (DGA) consists of drawing transformer insulating oil samples from the transformer tank and sending the samples to a commercial laboratory for analysis.  The most important indicators are the individual and total dissolved combustible gas (TDCG) generation rates, based on IEC and IEEE standards.  Although gas generation rates are not the only indicator, they are reasonable for use in determining the condition.

Trafo Assessment areas

Common Failure areas

The following are the major components, which have a direct bearing on reliability of the transformer

  • Windings and electrical circuit
  • Core and clamping structure
  • Bushings and external connections
  • OLTC On Load Tap Changers
  • Cooler equipment and cooling medium
  • Control and supervisory equipment

Transformer diagnostic techniques

Several transformer diagnostic techniques are used by GTR onsite to determine the matured transformer life expectance & health. They can be grouped into Electrical tests applied to the transformer active part on-site and Insulating Oil extracted for laboratory analysis.

OMICRON DIRANA –

Dielectric Frequency Response (DFR)

Polarisation Spectrum Measurement

Transformer Condition indicator Electrical Tests


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Turns Ratio Test

The transformer turns ratio (TTR) test detects shorts or severe tracking between turns of the same coil, which indicates insulation failure between the turns.  These tests are performed with the transformer de-energised and may show the necessity for an internal inspection or removal from service.

 

Short Circuit Impedance

Percent Impedance or Leakage Reactance, these tests are conducted in the field and compared to nameplate information, previous tests, and similar units to detect deformation of the core or windings caused by shipping damage, through faults, or ground faults.  Some difference may be expected between nameplate and field test results because factory tests are conducted at full load current.  Field connections, test leads and jumpers also play a significant role in test results so considerable attention must be paid.

Winding DC Resistance Measurement

Careful measurement of winding resistance can detect broken conductor strands, loose connections, and bad contacts in the tap changer.  Results from these measurements may indicate the need for an internal inspection.  This information supplements dissolved gas analysis (DGA) and is useful when DGA shows generation of heat gases (i.e., ethane, ethylene, methane).  Test results are compared between phases or with factory tests. When comparing to factory tests, a temperature correction must be employed.  Furthermore, OLTC dynamic resistance can be measurement to determine the contact resistance and wear.

OMICRON CPC100  / TD1

Tan Delta

Insulation power factor test of windings and bushings

 

Power factor is a ratio of dielectric loss (or watt loss) to the charging volt-amperes (or apparent power input). It is a property of the electrical insulation system and a measure of the electrical losses in the insulation when subjected to an applied alternating voltage. Power factor testing is the single most valuable method of obtaining data for determining insulation quality. A high loss may indicate problems in the insulation structure. Normal ageing of an insulating material will also cause dielectric loss to increase. Contamination of insulation by moisture or chemical substances may cause losses to be higher than normal.

The power factor value of 0.5% was used as acceptance criteria for new transformers. Sudden increases in value of power factor over time are taken as a sign of deterioration of the insulation condition. The power factor on serviced-aged transformer in good condition should be in the range of 0.5% to 1.5%

OMICRON CPC100  / TD1

Tan Delta

Degree of Polymerisation

Winding insulation (cellulose) deterioration can be quantified by analysis of the Degree of Polymerization (DP) of the insulating material.  This test gives an indication of the remaining structural strength of the paper insulation and is an excellent indication of the remaining life of the paper and the transformer itself.  This requires analysing a sample of the paper insulation in a laboratory to determine the deterioration of the molecular bonds of the paper.

Partial Discharge

Partial discharge (PD) can cause irreversible damage to power transformer insulation, long before the insulation actually fails. Even upon detection and analysis, it is essential to know exactly where insulation defects are located in the transformer.

GTR Transformers perform PD Online diagnostics without the need for outage using the latest technology.

Omicron MPD 600

High-end measuring and analysis

for partial discharges

Oil analysis

Transformer Condition Indicator – Insulating Oil

Dissolved gas analysis

 

Dissolved gas analysis (DGA) consists of drawing transformer insulating oil samples from the transformer tank and sending the samples to a commercial laboratory for analysis.  The most important indicators are the individual and total dissolved combustible gas (TDCG) generation rates, based on IEC and IEEE standards.  Although gas generation rates are not the only indicator, they are reasonable for use in determining the condition indicator score.

Furanic analysis may indicate a problem with the paper insulation which could affect transformer longevity.  A baseline furanic analysis should be made initially and repeated if the transformer is overheated, overloaded, aged, or after changing or processing the oil.

Physical tests such as interfacial tension (IFT), acidity, moisture content, and dielectric strength usually indicate oil conditions that can be remedied through various reclamation processes. Therefore, they are not indicative of overall transformer condition that would lead to replacement.

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Transformer Condition Assessments

GTR - Utilising Advanced Testing Technology