THE UNCERTAINTY IN POWER TRANSFORMER FAULT DIAGNOSTICS USING CONVENTIONAL TESTING METHODS Yogendra Dev Vashishtha*, Paul Ascione* and Qi Su** * Power and Water Authority ** Monash University Abstract The average age of power transformers in Australia is about 25-35 years. Conventional oil tests, dissolved gas analysis (DGA) of oil, dielectric loss angle (DLA or tanδ) of winding and recovery voltage monitoring (RVM) are among many diagnostic techniques available. Power and Water Authority (PAWA) of Northern Territory refurbished five 66 / 11 kV, 10—30 MVA, 24—33 years old transformers. From the test results it is observed that a transformer which has very good results from these diagnostic tests and even has minimum moisture content in the winding (from RVM) may still have very poor mechanical integrity. Winding cellulose paper may be aged to such an extent (assessed from degree of polymerisation; DP) that any sudden force or transient would cause failure. DP test is very difficult to perform regularly to ascertain mechanical integrity of winding paper. Furan analysis is another alternative to DP. Most of the times Furans are not detected and there is no universal correlation available for DP and Furans. Also, both ageing and moisture content influence Furans. Frequency response analysis (FRA) gives information regarding winding movement but it does not give much information about mechanical integrity of a winding which is essential for transformer reliability. This paper also compares moisture contents in the winding assessed from RVM and estimated from oil PPM (using Norris diagram) and estimated from winding DLA (based on Gussenbauer’s relationship). It appears that moisture content from RVM needs further validation and research to separate out ageing and moisture effects. 1.
INTRODUCTION
Transformers represent a high capital investment in a transmission substation at the same time as being a key element determining the loading capability of the station within the network. With appropriate maintenance, including insulation reconditioning at the appropriate time, The technical life of a transformer can be in excess of 60 years. The end of life, however, can be strategic or economic. Quantitative, risk based approach can be used to aid costly investment decisions involving transformer life, otherwise made from a subjective viewpoint. Diagnostic systems have been listed under three categories as portable / periodic, continuous on-line and specialist support [1]. There are various on-line monitors for load, operating conditions, gas-in-oil and moisture-in-oil are available these days [5]. Dissolved gas analysis (DGA) is the main condition monitoring technique used at Power and Water Authority (PAWA) in the Northern Territory. Recovery Voltage Monitoring (RVM) has been found as a better technique to estimate moisture content in the winding paper insulation and its ageing. However, RVM does not distinguish between moisture content and degradation of paper. Some experiments have been reported correlating charging current with paper ageing. Due to very limited work, these are not very
conclusive. There is a possibility that charging current may also correlate with insulation resistance (IR) as we found RVM following polarisation index (PI).
2.
BACKGROUND
2.1
The major factors involved in transformer insulation life are:
•
Insulation moisture content
•
Extent of oil oxidation
•
Extent of cellulose degradation—water, CO, CO2 and furans
•
Insulation Structure compression
MOISTURE, OXYGEN and HEAT are the three ageing accelerators.
2.2
Transformer Life Assessment Indicators:
Insulation moisture content in oil provides a simple method of estimating residual life. The following guide may be useful (based on equilibrium water content of paper as a function of water content of impregnating transformer oil): 3%
Maximum acceptable
4%
Entering “ at risk “ zone
5 %--6 %
Considerable risk of failure
7%
Failure imminent
For life assessment, emission associated with the insulation ageing process is of interest, principally the levels of CO, CO2 and furans in the oil. Cellulose degradation can be assessed by furan (HPLC) analysis and preferably supported by degree of polymerisation (DP) for a more definitive assessment. DP test involves the removal of samples of paper from the winding insulation. Due to the need for internal access and expensive analysis, the test is only recommended on suspect transformer on the basis of DGA and other tests. DP is correlated to the tensile strength. The paper is considered to have no mechanical strength at a DP of 150 to 200. New transformers have DP above 900.
2.3
a)
b)
The following methods of measurement are available:
2.4
Condition Monitoring Methods:
Method WINDING:
Suitable for
IR & PI
Moisture & Contamination Moisture & Contamination Faulted turns Conductor damage Winding movement Poor dielectric Poor dielectric Detect and locate PD
DDF & Capacitance Winding Ratio Winding Resistance Impedance & Losses Applied HV Induced HV & PD PD ( acoustic & electrical) Frequency Response Analysis (Transfer Function) Voltage Recovery
Moisture in paper And ageing of paper Off-Line Test
Vibration Analysis
Slack winding & Mechanical faults Insulation degradation
DP & Gel Permeation OIL: DGA & Ratio analysis Furan Analysis
moisture
Direct measurement of the water content in paper on a layer by layer basis by means of Karl Fisher moisture meter (KFMM) using ‘four bar probe’. Water content in winding established from measurement of tanδ using Gussenbaeur’s characteristics [4].
c)
Water content in winding established from measurement of oil PPM using Norris Diagram.
d)
Water content in winding established from RVM [6].
Winding displacement, possible loose winding, and core faults
Water content Resistivity, Acidity, IFT and DDF BUSHINGS: Tan Delta (DLA)
Detecting incipient faults Overheating and ageing of paper Dryness Ageing of oil
Moisture & contamination
Tan δ or dielectric loss angle (DLA) is a measure of overall quality of insulation system in terms of moisture and contamination.
2.5 Recovery Voltage Method:
If the DGA analysis is performed correctly (proper sampling, storage and calibration), most of the incipient faults in the oil may be detected. The paper insulation is responsible for containing most of the moisture due to ageing and thermal stress. The paper insulation may fail under high electrical stress or may
release moisture into the oil insulation. To detect ageing or moisture content it is necessary to analyse low frequency part of polarisation spectrum of dissipation factor. A tanδ would have been sufficient but finding a sinusoidal source voltage of 0.001 Hz is very difficult [6]. The alternative is the recovery voltage measurement. It was found that IR & PI do not reflect complete information on polarisation process. Cases were reported where electrical motors having good PI were found to have contaminated windings and also motors having poor PI had no problems in the winding insulation [2]. To resolve this, an one thousand seconds charging and discharging test (dc absorption) was developed. Recovery Voltage Method for transformer seems to be developed from this test.
2.6
In RVM, winding is charged for known time and then shorted to ground for pre-decided time. The recovery voltage is then measured and dominant time constant is achieved which is essentially a polarisation time constant.
Charging current is given as the sum of displacement current, the polarisation current and the conduction current. Polarising current is dependent on material property and state of ageing. The polarisation of dielectric can be expressed as sum of various slow polarisation phenomena like ion migration, slow relaxation and interfacial polarisation. Care must be taken in the interpretation of results of RVM, in particular the relative effects of moisture, genuine ageing and temperature [3].
Condition Monitoring Model [1]
Insulation Ageing Products In Oil
DP of Insulation Paper Sample RVM
Data Base Of Test Results
Failure Rate Data
Expert Assessment
Statistical Analysis
INSULATION AGE
FAILURE PROBABILITY
RELIABILITY ANALYSIS
Remaining Reliable Life Of Transformer
3.
REFURBISHMENT DATA Table 4
Five 66/11kV, 10-30MVA transformers of 24 --33 years old transformers were refurbished. Some of the important parameters are given in table 1, 2, 3 & 4.
Table 1 PI
1.09 1.79 1.80 2.34 2.40
% Moisture (from RVM) 2.52 2.10 1.74 1.56 1.42
% Mechanical Strength (from DP) 35—55 35—55 35—55 35—55 25—35
% Moisture estimated from winding DLA (Cigre)
% Moisture estimated from RVM
2 3 2 2.8 < 0.5
1.74 1.53 2.52 1.42 2.10
DP
2.09 1.53 1.21 2.38 2.18
250—350 250—350 200—250 250—350 250—350
% Mechanical Strength (from DP) 35—55 35—55 25—35 35—55 35—55
200—250 250—350 200—250 250—350
DISCUSSION AND RESULTS
•
It is observed from table 1 that RVM tends to follow PI in this case. However, a conclusion can not be drawn based on this limited number of observations. Essentially RVM does have DC absorption phenomenon and may / may not follow IR / PI.
•
It is also observed that DP values (or percentage retention of mechanical strength) do not correlate with any of the electrical parameters. DP reflects ageing and remaining mechanical strength of paper but it is not a diagnostic test as such.
•
From table 3, it is evident that moisture percentage in the winding does not correlate to DP or mechanical strength retention.
•
From table 2, it is evident that percentage of moisture estimated from Norris diagram (based upon oil PPM and temperature) and percentage moisture estimated from Cigre publication (based upon winding DLA) tend to agree to some extent. However, these do not correlate with moisture percentage estimated from RVM. This is due to the fact that RVM is a total reflection of ageing of cellulose paper and moisture content in the winding. There are no established methods to separate these components.
•
It appears from table 4 that Furan analysis may, in some cases, give indication about paper ageing. However, most of the times, Furans are not detected and there is no universal correlation available for DP and Furans. Also, both ageing and moisture content influence Furans.
Norris diagram gives estimated percentage of moisture in paper based upon moisture content in oil (PPM) and oil temperature. Cigre paper has established a method to estimate moisture in paper based upon winding tan δ (DLA) and temperature.
1.10 0.49 0.39 0.54
% Mechanical Strength (from DP) 25—35 35—55 25—35 35—55
4.
Table 3 % Moisture (from RVM)
DP
2-Furfurylalcohol, 2-Acetylfuran, 5-Methyl-2-Furfural and 5-Hydroxymethyl-2-Furfural were not detected in all four cases.
Table 2 % Moisture estimated from Oil PPM (Norris Diagram) 2 4 2.5 3.2 1.5
2-Furfural (PPM)
5.
CONCLUSION
Mechanical integrity of paper is very important to ensure reliability of transformers. However at this stage this is not conclusively reflected in any of the electrical diagnostic tests available. There is a need for further research to validate RVM results and establish more effective method of estimating moisture content in the cellulose paper winding.
6.0
REFERENCES:
1.
D.ALLAN, “Condition Monitoring & Life Assessment Of Aged Transmission / SubTransmission Plant”, Lecture notes of the short course program on Insulation Condition Monitoring And Reliability Centred Maintenance Of Electrical Plant. February 1997, Monash University.
2.
CAJETAN PINTO, “An Improved Method of detecting Contamination of H.V. Stator Winding in the Field", Proceedings of the 20th Electrical Electronic Insulation Conference Boston, 1991, pp. 55-59.
3.
R.E.JAMES, “Assessment Of Electrical Insulation”, Proceedings of the 1998 Residential School in Electrical Power Engineering, Tasmania.
4.
I.GUSSENBAUER, “Examination of Humidity Distribution in Transformer Models by Means of Dielectric Measurements”, Cigre paper 15-02, International Conference on Large High Voltage Electric System, 1980.
5.
THOMAS LEIBFRIED, “On-line Monitors Keep Transformers in Service”, IEEE Computer Application in Power, July 1998, pp 36-42.
6. SYED MOFIZUL ISLAM, “ Power Transformer Condition Monitoring – Pacific Power International and University of Newcastle Developments”, Lecture notes of the short course program on Insulation Condition Monitoring And Reliability Centred Maintenance Of Electrical Plant. February 1997, Monash University.
