Diagnostic Experience on HV Bushings Retired from Service MANUEL GUZMÁN-V

Diagnostic Experience on HV Bushings Retired from Service MANUEL GUZMÁN-V

Diagnostic experience on HV bushing retired from service. Manuel Guzmán; Fernando Elizarraraz; Ricardo Montes Comisión Federal de Electricidad MEXICO

Introduction. In order to attend the electricity demand in Mexico, CFE operates and maintains more than 2700 power transformers in 470 substations with a near capacity of 163 000 MVA. In the substations at generation plant, CFE uses three-phase power transformers. In transmission substations, uses single phase auto transformers The nominal voltages are 400 kv, 230 kv and < 230 kv. Most of bushings installed are Oil Impregnated Paper (OIP) and some are of Resin Bonded Paper (RBP) and Resin Impregnated Paper (RIP). CFE Substations Substations 470 MVA in operation 163 572.13 MVA instaled 189 546.03 Power Transformers 2 762

According with statistical analysis about failures on power transformers, since 2000 to 2014, 27.7% were due to bushing failures. It s the second cause of power transformer failures Main causes of Power Transformer Failures

The percentage of bushing failures related with the nominal voltage is: Bushing failures related with nominal voltage

And 53% of total failures with transformer burning was due to bushing failure. Power Transformer burning causes

Findings about failures on polymeric housing bushings were discussed in previous document (INMR 20013).

Technical reference for laboratory test. CFE buys power transformers and the manufacturers are in charge of select the standard for bushings, however the CFE technical specification [1] is based on the international standard IEC 60137 [2]. This standard indicates the bushings reference values for C1 in laboratory test: Tan δ of 0.007 Max Δtan δ 0.001 for OIP (at Um & 1.05Um/ 3) Partial discharges of 10 pc @ Um and 5 pc @ 1.05Um/ 3 But for Tan δ, most of the manufacturers have lower values. Tan δ 0.005 for OIP

In the diagnostic process these values are used only as a reference, then a technique that combines several tests is applied, such as measuring C1, tanδ, (before and after) a power frequency (80% to 85%) withstand voltage test and the measurement of partial discharges. There are no tests on insulating oil.

The CFE substations manual recomends to make diagnostic test on bushing due the following causes:! Lost of oil level.! When occurs a failure on power transformer.! On the other two bushings if one fails (three phase power transformer).! If the bushing remains on warehouse during years and is necesary to install it (spare parts).! If the results of on site test of Power Factor is high.! When the power transformer is repaired and its necesary to verify the old bushing condition.! After severe earthquake.! When the bushing is retired from the power transformer because it is relocated in other substation.! In order to prevent a failure at the end of its useful life.! When the bushing is refurbished. Laboratory test set up

Technical reference for on site test Taking in account previous experiences and different recommendations from technical groups, CFE developed a bushing test on site procedure. The technical document is a substation maintenance manual named Manual de Transformadores y Reactores de potencia, Tomo I y Tomo II, junio 2007 [3] wich is also referenced on CFE Technical Spec [1]. For bushing test on site the reference values are: FP: Up to 0.5% at 20 C. C1: ± 2% of original value C2: ± 2% of original value

In some cases if there is not enough information from test results, a dielectric spectroscopy test could give us more information about insulation condition.

Findings during test on laboratory Always with the time of operation in mind, and the Std. of reference. " The bushing presents changes in C1 and in tan δ values. " Presents a fault (perforation) during the power frequency withstand test. " Values of partial discharges from 50 pc up to 500 PC. " All the previous results have been presented either alone or in combinations, which helps us to give a recommendation, for example: " If the bushing has DP values near to 100 pc and tanδ values near to 0.008, but the C1 and power frequency withstand test shows good results, a regeneration of the insulating oil is recommended, and re-testing after the regeneration.

Findings during test on site According to [3] the period of maintenance on transformers is two years, then the most common findings between periods are: Power Factor differences up to 1% Exponential differences on C1 measurements due to foils degradation. Corrosion on capacitive tap connections to the outside, leads false connections. Movement of section of the porcelain from flange.

Most common failed parts on bushings

Degradation of sealing and moisture ingress.

Corrosion on Capacitive tap materials. This kind of failure is common on bushings. This connection is one of the weakest part of bushings. The material used in the connection to outside suffer corrosion and its severity depends on: Material Weather In may cases the cost of spare parts is expensive. If the bad connection is not detected, the results would be catastrophic.

Capacitive foil degradation. Degradation of insulating material by ageing. Degradation of insulating material by moisture.

Rupture or separation of porcelain from flange.

Bushing repair. If there is not bushing for substituting one failed, a common practice is to repair it. The repair includes: " Repair the connection, installation and sealing of C1 Tap. " Regeneration or change of insulating oil. " Drying of insulating parts. " Re-tightening of flanges, connectors and bushing terminals. " Rewinding of the last paper foils without modifying the capacitive foils. " Alignment of porcelain hollow core insulator and flange. " Re-sealing between porcelain sections and flange. " Minor repairs on external porcelain parts. " Oil level recovery.! The C1 is not repaired # After bushing repairing activities, a diagnostic test routine is performed. If the test results are satisfactory, according with criteria mentioned before, the bushing is installed.

Discussion about standards. CFE bases its bushing technical specification on IEC [2] standard and the last edition takes flange dimensions from ANSI standard [4]. It is necessary to add a technical criteria considering C1 increase that indicates incipient fail (1% every 5 years). There is not technical criteria that consider the increase of tanδ (as CT standard indicates). A sampling valve to test de insulating oil might be necessary. IEC standard [2] don t include flange dimensions.

Final remarks. The bushing is an accessory for the power transformer but bushing contributes importantly to power transformer failure. Its important to have diagnostic tools in order to prevent bushing failures and power transformer failures. Depending on transformer importance, it is possible to carry out high voltage diagnostic tests on site, mainly with 400 kv and above, avoiding bushing risk damage due movement to laboratory facilities. Have bushings as spare parts is expensive because of lack of definitions about flange dimensions and their different designs. It s important to have experience in bushing rebuild and diagnostic test. It is helpful for bushing recovery in a short time.

Final remarks. Online monitoring systems have not been shown effectiveness yet, there are some doubts about operation and performance in service: Furthermore, monitoring systems, in some cases are cause of failure in power transformers, rather than being a stand for reliability. Regeneration of bushing is a technical and economic option to reenergize the power transformer in a short time, while it is possible to buy a new one. CFE has been done these activities in México since 20 years ago, with successful results.

Bibliography. [1] Esp CFE 53000-95, Boquillas de alta tensión de 15 kv a 420 kv para transformadores y reactores de potencia, 2011 [2] International Standard IEC 60137, Insulated bushings for alternating voltages above 1000 V, 2008. [3] Manual Transformadores y Reactores de potencia, Tomo I y Tomo II, junio 2007. [4] ANSI/IEEE C57.19.00, General requirements and test procedures for Aparatus Bushings, 2004.

Thanks for your kind attention!! Manuel Guzmán manuel.guzman01@cfe.gob.mx Fernando Elizarraraz fernando.elizarraras@cfe.gob.mx Ricardo Montes ricardo.montes@cfe.gob.mx http://www.cfe.gob.mx/ingles/services/lapemtechnicalservices/pages/lapemtechnicalservices.aspx