Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers


The most important component of the substation is the transformer and it typically represents the single most expensive component. A transformer gives many signals which can be used to predict its performance. Depending on the type of transformer, several different tests should be performed during the full maintenance program inspection:

  • insulation resistance,
  • insulation power factor and capacitance test
  • transformer turns ratio test.

Further to the electrical testing performed, oil samples should be drawn from the transformer and analysed for its electrical insulation integrity and gas content.

Most transformers are either liquid filled or of the dry type. A very brief explanation of both is given below.

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Liquid Filled Transformers

A liquid filled transformer usually has the purpose of decreasing a primary line voltage ( e.g. 27.6 kV ) to a safer voltage potential, such as 600 V. Like any transformer, the liquid filled transformer is not 100% efficient in energy conversion, and most of this efficiency loss results in heating. Unlike the dry transformer, the coils of this transformer operate immersed in a special insulating oil or silicon. Natural heat convection allows the liquid to cycle through the transformer windings and the cooling fins attached to the transformer casing.

Common problems with this equipment include oil leaks, (possibly due to defective welds, but usually from an aged gasket ), inadequate air ventilation about the cooling fins and even corrosion about the exterior of the transformer. Primary and secondary bushings of these transformers require close visual inspection for cracks and/or gasket leaks, as well as torquing of the connections on a regular basis.

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Dry Transformers

A dry transformer usually has the purpose of decreasing a secondary line voltage ( e.g. 600V ) to a safer and more usable distribution voltage potential, such as 120 V. Like any transformer, the dry transformer is not 100% efficient in energy conversion, and most of this efficiency loss results in heating. It is for this reason that adequate ventilation must be supplied for this equipment.

When inspecting these transformers, problem areas can be detected through abnormal or inconsistent heat distribution, discoloration of the coil insulation, and perhaps even from a persistent yet tangible odour. Visual signs and unusual noises may also indicate defective transformer operation.

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Insulation Resistance Measurement by Megohmmeter
A method of measuring insulation resistance is by megohmmeter. This instrument is very convenient and indicates resistance directly. A megohmmeter applies a consistent voltage (usually between 500 and 5000 V) to an electrical component and then measures the current which the megohmmeter must supply to the component to keep the voltage at the required level. In effect, this supplied current is equivalent to the leakage current of the electrical component, allowing the meter to calculate the resistivity of the electrical components between the two points of measurement - a useful measurement of equipment quality.

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Resistance should be measured between winding and all other windings, with all windings grounded except the one being measured. Windings are never left floating for insulation resistance testing.

Grounded windings must have the ground removed so the insulation resistance can be tested. If the ground cannot be removed, as with solidly grounded neutrals, the test cannot be made. Such a winding is treated as part of the grounded circuit.
Tests should be made from windings to ground and between windings as follows:
Two winding transformers

  1. High to low and ground
  2. Low to high and ground
  3. High and low to ground

Three winding transformers

  1. High to low, tertiary, and ground
  2. Low to high, tertiary, and ground
  3. Tertiary to high, low, and ground
  4. High and low to tertiary and ground
  5. Low and tertiary to high and ground
  6. Low and tertiary to high and ground
  7. High, low, and tertiary to ground

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Permanently connected windings, such as autotransformers and regulators should be considered as single winding.

The insulation resistance measuring of control wiring has to be made with a 500 volt instrument.

Power Factor and Capacitance Test
The quality of an electrical apparatus’ insulation is something normally taken for granted. However, that same insulation can unnecessarily be the cause of equipment failure resulting in costly replacement or repairs and downtime. To avoid this situation, one can monitor the quality of the insulation by periodically testing the apparatus. One such test is a insulation power factor and capacitance test which can detect moisture, contamination or deterioration of the insulation.

Knowing that capacitance values can be heavily affected through changes in the dielectric constant, we can therefore see that if the insulating material has become saturated with water, there will be a considerable increase in capacitance of the equipment.

Power factor in insulating testing is basically the same as the power factor in measurement. A piece of insulation applied in this test is practically the equivalent to a capacitor in parallel with a resistor. Mathematically, power factor is the cosine of the phase angle between the total current and the resistance component of the current. In insulation measurement the numerical value of the power factor is usually small, in the order of a few percent, zero being perfect.

Because normal power factor is so low measurements may be greatly distorted by dirt or moisture on the insulation surfaces. Incorrect readings can also result if tests are made near wet or grounded surfaces. Due to that, before making a power factor test on a piece of insulation, it has to be cleaned of all foreign matter.

For full and correct interpretation of measurement results in context of absolute withstand capability of insulation systems, it is necessary to establish a statistical correlation between power factor, voltage withstand capability, and service life for the kind of system being tested. In that case properly made and evaluated, power factor tests are useful in detecting signs of insulation deterioration.

Transformer Ratiometer
One of the most important parameters of a transformer is its no-load transformation ratio ( the ratio of the high voltage winding to low voltage winding ). This ratio is measured by a ratiometer in a bridge circuit and at low flux density in the core. The ratiometer instrument provides a bridge excitation voltage, either 120 or 12 V AC, which is isolated from the main supply. When balanced, the ratio of the transformer windings is indicated as a digital readout on the instrument. This ratiometer allows an inspector to compare the theoretical voltage ratio(s) against a ratio detected through this practical test. Tap setting tests are normally tested through such a meter as described here.

Transformer Oil Sample Analysis

Constant monitoring of the quality of the insulating oil is required in order to ensure a long service life for oil filled transformers. There are tests designed specifically to evaluate the condition of insulating oil. Each of these tests can provide a quantitative evaluation of the insulating capabilities of the liquid. The tests described are those established by the American Society for Testing and Materials (ASTM) and are accepted by the electrical industry as standard.

Acidity, ASTM D-974
This test determines when service aged oil should be reclaimed. The total acid number of the oil sample is measured and compared to the value of unused oil of the same type. A significant increase in the total acidity number is an indicator that the oil should be replaced.

Colour, ASTM D-1500, ASTM D-1524
This test is designed to detect oil deterioration. Colour is measured on a scale of 0 to 8, with 8 being the darkest. When compared to unused oil, an increase in the colour number represents contamination and deterioration of the oil sample.

Dielectric Breakdown, ASTM D-877, ASTM D-1816
This test measures the oil’s insulating strength. When low dielectric breakdown values are found, this test can be used to detect the presence of oil contaminants.

Interfacial Tension, ASTM D-971, ASTM D-2285
This test allows the equipment maintenance personnel to monitor contamination of new and service aged oils with soluble polar contaminants and oxidation products. A high value for new insulating oil indicates the absence of undesirable polar contaminants.

Density (Specific Gravity), ASTM-1298
This test determines the density of the oil that must be within specific limits.

Visual Inspection of Oil
This inspection is for the presence of visible particles in oil. If the oil is cloudy, this indicates that sludge and/or water may be present. The oil sample can also be analysed for sediments and free water.

Transformer Gas-in-Oil Analysis

The analysis of gases dissolved in insulating oil can be used to predict a problem in a transformer that may eventually evolve into an incipient fault. The type, volume and proportion of the gases produced in oil will depend on the materials damaged, the condition responsible for the damage and the energy levels involved.

Oil samples are taken from the transformer in glass syringes. These syringes have individual mated pistons and barrels and are fitted with a three-way stop cock. When taking these samples, it is essential that every precaution be taken to avoid introduction of gases (air) into the liquid being sampled. The gas in oil analysis will report concentrations of Hydrogen H2, Oxygen O2, Nitrogen N2, Carbon Monoxide CO, Methane CH4, Carbon Dioxide CO2, Ethylene C2H4, Ethane C2H6, Acetylene C2H2 and the total gas content. Of more than 2800 liquid hydrocarbon compounds present in transformer oil, only these nine affect the ultimate transformer performance.

Under normal operating conditions, oil-insulated power transformers generate gases very slowly. These gases eventually dissolve into the unit's insulating oil without serious damage. This natural chemical reaction occurs as result of the transformer ageing and experiencing a relatively constant load.

During abnormal operating conditions, combustible gas production increases in direct relation to the severity of the electrical or thermal stress. Each type of fault produces different types of gases. In general, the kind of fault gases which are generated depend upon the type of insulation which is being degraded and the temperature at the fault site.

  •  Faults involvingoverheating cellulose insulation generate mainly carbon monoxide and carbon dioxide
  • Faults involvingoverheating cellulose insulation generate mainly carbon monoxide and carbon dioxide

At low temperatures, CO2 predominates with increasing amounts of CO as the temperature rises. Under normal operating conditions, there is continual production of CO2 and CO in a ratio of about 3:1 and relatively large amounts of these gases will be found in a normally operating transformer. Very high levels of both gases with CO approaching or exceeding the CO2 are required before suspecting a localized fault involving cellulose.

  • Faults which result in breakdown of the oil

At the low temperature and energy dissipation of partial discharges or corona, virtually the only gas produced is hydrogen H2. Low temperature and generalized overheating produces methane CH4 and ethane C2H6 and some hydrogen. As a temperature increases, ethylene becomes the predominant gas. At the very high temperature of an arc, acetylene and hydrogen predominate.

Interpretations of gas analysis results depend on three related factors: the specific gas being generated, the total volume of the gas, and the rate of gas production. Possible fault conditions can be predicted by detecting changes in any or all of these areas.
Analysis of dissolved gases is just one important tool used to determine the operating condition of a transformer. Test results should be analyzed with additional information that is available about the transformer, including the results of all previous gas analyses and oil quality tests, electrical tests, transformer loading profile, and all abnormal operating conditions.

At the end, don’t forget, without the oil's full dielectric strength and cooling efficiency, the transformer cannot operate safely and efficiently.


Cable Testing : Circuit Breakers : Co-ordination Studies : Low Voltage : Maintenance :
MV Switchgear : MV Testing : Substation Structure : Transformers