CONDITION MONITORING OF 132 kV TRANSFORMERS M K Domun
EA Technology, UK
Approximately 3 0 % of the 132 kV grid transformers in the UK are over 40 years old. Some 8 0 % are over 20 years old. To date, performance has been satisfactory with negligible occurrence of serious failure. However, transformer loads are increasing and reserved capacities are decreasing. Consequently, it is imperative to have some simple and practical means of assessing the insulation integrity of high voltage power transformers. Historically, diagnostic tests include dc resistance and ac dielectric loss of the winding insulation. The electric strength and moisture content of the oil are also measured. When a fault occurs samples of oil are sent to a laboratory for dissolved gas analysis (DGA). This technique is effective in identifying fault characteristics from the measurement of concentrations of hydrogen, hydro-carbon and carbon oxide gases (1). It is also possible to define the type of fault by means of gas ratio codes (2). The three gas ratios, acetylene/ethylene (discharge fault), ethylene/ethane (high temperature thermal fault) and methane/hydrogen (low temperature thermal fault) have been very useful parameters in determining the cause of fault
acids and moisture. The hydrolytic effect is to cleave the glucosidic bond, yielding free glucose. Cellulose is susceptible to oxidation, which leads to the formation of carbonyl and carboxyl acid groups. Secondary reactions can eventually occur, to molecular chain scission. leading The degradation of oil results in high acid content, which exacerbates the chemical deterioration of the paper insulation even further. Consequently, condition monitoring of the properties of the paper insulation is more important than an analysis of the properties of the oil. Methods of Monitoring Insulating Paper Properties of
The direct relation between mechanical strength and the degree of polymerisation (DP) has been established long ago (3,4). The main disadvantage of these two methods is the necessity for paper samples to be obtained from transformer windings. Perhaps the most stable residues of paper degradation are those which contain aramatic type ring systems, for example furan and some its derivatives. Furfuraldehyde is one of those derivatives whose presence in oil can be readily detected by a high performance liquid chromatograph (HPLC). The relation between DP and furfuraldehyde has also been established for paper insulation ( 5 ) . The degradation of paper insulation, therefore, can be monitored by measuring the concentration of furfuraldehyde in the oil of an oil-paper composite. 2. DETAILS OF INVESTIGATION
The objectives of the present investigation were (a) to determine an acceptable level of insulation integrity for the large population of aged transformers, and (b) to develop or implement a diagnostic technique which could be used to monitor the rate of deterioration of the oil-paper composite insulation. In general, failures of aged power transformers are due to time-dependent thermal ageing which leads to degradation of the oil-paper insulation. The degraded oil can readily be refurbished or replaced. The degradation of the insulating paper, however, is non-reversible. The effects of thermal ageing on the paper matrix are accentuated by the presence of air and moisture. Insulating paper is a linear polymer made of glucose molecules linked together a chainlike manner by hydrogen bonds involving the hydroxyl groups. The higher the average chain length the higher the mechanical strength of the paper. Insulating paper consists of very long chain molecules, each containing approximately 1,200 monomer units.
The investigation was divided into three groups : a) Establishing a "norm" from the analysis of oil samples from 500 transformers. b) Eight-year monitoring of old, 132 kV transformer. a 35
c) Accelerated ageing of a 4 0 year old, 132 kV...