Gases analysis

The analysis of gases dissolved in oil has proved to be a highly practical method for the field monitoring of power Transformers.

 This method is very sensitive and gives an early warning of incipient faults. It is indeed possible to determine from an oil sample of about one litre the presence of certain gases down to a quantity of a few mm3 , i.e., a gas volume corresponding to about 1 millionth of the volume of the liquid (ppm).

 The gases (with the exception of N2 and O2) dissolved in the oil are derived from the degradation of oil and cellulose molecules that takes place under the influence of thermal and electrical stresses. Different stress modes, e.g., normal operating temperatures, hot spots with different high temperatures, partial discharges and flashovers, produce different compositions of the gases dissolved in the oil.

The relative distribution of the gases is therefore used to evaluate the origin of the gas production and the rate at which the gases are formed to assess the intensity and propagation of the gassing. Both these kinds of information together provide the necessary basis for the evaluation of any fault and the necessary remedial action.

This method of monitoring power Transformers has been studied intensively and work is going on in international and national organizations such as CIGRE, IEC and IEEE.

APPLICATION.

 The frequency with which oil samples are taken depends primarily on the size of the Transformer and the impact of any Transformer failure on the network.

Some typical cases where gas analysis is particularly desirable are listed in the following:

  1 - When a defect is suspected (e.g., abnormal noise).

  2 - When a Buchholz (gas-collecting) relay or pressure monitor gives a signal.

  3 - Directly after and within a few weeks after a heavy short circuit

  4 - In connection with the commissioning of Transformers that are of significant importance to the network, followed by a further test some months later.

 Different routines for sampling intervals have been developed by different utilities and in different countries.

One sampling per year appears to be customary for large power Transformers (Rated >= 300 MVA >= 220 kV).

 The routine that has been used over a long period of time of checking the state of the oil every other year by measuring the breakdown strength, the tan value, the neutralization coefficient and other physical quantities is not replaced by the gas analysis.

 Extraction and analysis

 To be able to carry out a gas analysis, the gases dissolved in the oil must be extracted and accumulated.

 The oil sample to be degassed is sucked into a pre-evacuated degassing column. A low pressure is maintained by a vacuum pump. To assure effective degassing (> 99 per cent), the oil is allowed to run slowly over a series of rings which enlarge its surfaces.

 An oil pump provides the necessary circulation. The gas extracted by the vacuum pump is accumulated in a vessel.

Any water that may have been present in the oil is removed by freezing in a cooling trap to ensure that the water will not disturb the vacuum pumping.

The volumes of the gas and the oil sample are determined to permit calculation of the total gas content in the oil. The accumulated gas is injected by means of a syringe into the gas chromatograph, which analyses the gas sample.

 The result is plotted on a recorder in the form of a chromatogram.

Using calibration gases it is possible to identify the different peaks on a chromatogram. Recalculation of the height of a peak to the content of this gas is done by comparison with chromatogram deflections from calibration gases.

With the composition of the gas mixture and the total gas content in the oil sample known; the content (in ppm) of the individual gases in the oil is obtained. The following gases are analyzed:

          1 - CARBON MONOXIDE    CO

          2 - CARBON DIOXIDE         CO2

          3 - HYDROGEN                     H2

          4 - ETHANE                            C2H6 

          5 - ETHENE                            C2H4     

          6 - ACETYLENE                    C2H2

          7 - METHANE                        CH4

          8 - PROPANE                         C3H6

The detection limits depend partly on the total gas content; for hydrocarbons (except methane) the limit lies below 0,5 ppm, for hydrogen, methane and carbon monoxide about 5 ppm and for carbon dioxide about 2 ppm.

This high sensitivity is necessary in those cases where it is desired to determine a trend in the gas evolution at short sampling intervals, e.g., during a heat run test or when oil samples are taken at intervals of only a few days.

Identification of faults.

The fault types that can and should be identified are corona, electrical discharges, excessively hot metal surfaces and fast degradation of cellulose. It is possible to obtain an idea of the type of fault by using a diagnosis scheme.

A number of different schemes of this type have been prepared.

To avoid having to deal with the contents of the individual gases, one frequently uses quotients between different gases.

Some schemes give an appearance of great precision, but certain care should be observed when making assessments, until all factors influencing the gassing rate are known.

GAS ANALYSIS OF TRANSFORMER

Gas concentration limits used in the

Interpretation of DGA data

 A statistical survey concerning gas concentrations in Transformer

Oil using the results of that survey the following limits have been set:

 The limits above are for a Transformer which are open with a breather and have no OLTC or has a separate conservator for the OLTC.

 If the Transformer tank and the OLTC have a common conservator the warning and fault limits are 30 ppm and 100 ppm respectively for C2H2