Features of the transformer oil testing process

Why is transformer oil testing necessary?

Oil is used in transformers to perform several functions - the most important of which is to ensure the proper level of insulation quality.
In addition, it cools the installation and promotes normal heat dissipation to the outside, despite the increased level of protection. Transformer oil testing is designed to determine whether it can withstand the effects of an electric arc, as well as penetration into parts of the fiber insulation of the windings without loss of performance. Also, connecting to electricity requires that, even during long-term operation, no solid impurities are found in the oil, which act as good conductors of current. If certain defects are detected, the oil must be changed before continuing to operate the device.

4. AREA OF APPLICATION AND PROCEDURE FOR MIXING TRANSFORMER OILS

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Table 3

TKp and TAp oils can be used for topping up current and voltage measuring transformers, oil-filled bushings with voltage up to 500 kV inclusive, and TSp oil up to 220 kV inclusive. For economic reasons, T-1500 and T-750 oils are recommended for use in power transformers and reactors with voltage 220 kV and higher, and for current and voltage measuring transformers and oil-filled bushings with a voltage of 110 kV and higher.

All transformer oils can be used in oil switches. For economic reasons, it is not advisable to use GK, T-1500 and T-750 oils for these purposes. It is most advisable to use T-750 oil in sealed oil-filled bushings. It is allowed to reuse oil drained from oil switches during major repairs, after cleaning it from mechanical impurities, coal and water to the standards for these indicators for fresh dry oil.

4.2. Oils manufactured according to different standards and specifications are recommended to be stored separately and used, as a rule, in accordance with the area of ​​application. It is recommended to use HA hydrocracking oil, as a rule, without mixing with other oils. If necessary, to mix GK oil with other oils, it is most rational to use oils of the GB, T-I500 and T-750 brands.

If necessary, it is allowed to mix domestic inhibited oils in any ratio, taking into account their scope of application. If the mixture contains oils for different voltage classes, then the mixture is used in electrical equipment of the lowest voltage class. If at 90 ° C the mixture exceeds the component with the highest dielectric losses, then such a mixture of oils can only be used for filling into oil switches.

Domestic transformer oils containing ionol, manufactured in accordance with previously valid GOSTs and TUs, can also be used as oils of similar brands produced in accordance with current standards. Mixing uninhibited oils, previously produced and in use, with fresh inhibited oils is not allowed.

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If it is necessary to mix such oils, the ionol additive should be added in such an amount that the intended mixture contains at least 0.25% of the additive weight, and the mixture should be tested for stability against oxidation. The stability of the mixture must be higher than the stability of uninhibited oil. Imported oils containing the antioxidant additive DBK (ionol) or after its introduction at the point of consumption in a concentration of at least 0.3% by weight and meeting the requirements of GOST 10121-76 and having a flash point of at least 135 °C and a sulfur content of no more than 0.35% by weight, can be mixed in any ratio with TKp and TAp oils and used in electrical equipment for voltages up to 500 kV inclusive, in case of mixing with TSp oil - up to 220 kV inclusive.

With a sulfur content of more than 0.35%, imported oils can be used in electrical equipment for voltages up to 220 kV inclusive. As an exception, it is possible to mix imported oil containing no more than 0.35% sulfur with T-750 and T-1500 oils and use the mixture in electrical equipment for voltages up to 500 kV inclusive;

operating oil with an acid number of no more than 0.05 mg KOH/g, meeting the operating oil standards for the reaction of water extract, the content of dissolved sludge, mechanical impurities and having a breakdown voltage 10 kV higher than the operating standard and at 90 ° C no more than 6% ;mixtures of operating oil with any fresh, dry transformer oil, unless there is a sharp deterioration in the quality of the oils.

Adding oil to electrical equipment must be carried out taking into account the area of ​​application of the oil. Topping up power transformers filled with oil of other brands is allowed. Topping up sealed bushings can be done with oil from a transformer tank equipped with film protection. Topping up must be done with prepared dry oil with the quality indicators required for fresh oils poured into new equipment.

Standard Test Procedure for Transformer Oil

All laboratories use a standard method for testing the quality of process fluids, which involves determining the voltage at which instantaneous breakdown occurs. The normal test procedure for transformer oil involves using a glass or porcelain vessel into which a small amount of oil is poured. Next, the electrodes are immersed in it so that the oil covers them by at least 1.5 cm. Before immersion, the electrodes are completely washed with oil, and the bottle is turned over three times to eliminate the presence of air bubbles in the oil, which significantly improves the conductivity of the material.

Further electrical maintenance involves turning on the voltage and gradually increasing it by 1-2 thousand volts per second until it reaches the value at which breakdown occurs. This state is clearly distinguishable by the creation of a powerful electric arc between the rods. Transformer oil testing does not stop - the first breakdown is not documented. Next, the voltage is turned off and then increased 5 more times to the breakdown point. The final result is calculated as the average of the five measurements. Experts say that it should be 25-40% higher than the standard operating voltage of the transformer to ensure proper reliability and safety.

Change in physical properties

The physical characteristics of the operating oil directly determine how reliably the electrical equipment will function. Therefore, during the testing process, close attention is paid to the following properties of transformer oil:

• Acceptable value of density (specific gravity) . It is important that this parameter is inferior to ice. This is due to the fact that when ice formed in an idle installation (in winter), it formed at the bottom of the tank, without creating obstacles to free circulation in the oil cooling system. The norm is considered to be a density in the range of 860-880 kg/m3 at a temperature of 20.0°C. According to the laws of physics, specific gravity indicators change depending on temperature (when heated, they increase, and when cooled, they decrease).
• Critical heating of the oil to the ignition temperature (flash point ). This parameter must be high enough to prevent fire when the transformer, operating in overload mode, is subjected to extreme heat. The normal temperature is considered to be between 125-135°C. Over time, under the influence of frequent overheating, the oil begins to decompose, which leads to a sharp decrease in the flash point.
• Oxidation index (acid number) of a transformer liquid dielectric . Since the presence of acids leads to damage to the insulation of the transformer windings, it is important to determine their presence. The acid number displays the amount (in mg) of potassium hydroxide (KOH) required to remove traces of acid in 1 gram of product.

Additional methods for testing transformer oil

A similar figure is entered into the passport of the transformer itself - in addition to it, there are many other indicators of the suitability of the technical fluid for use. In particular, it is necessary to determine the content of water and solid impurities in the oil. For this purpose, specialized devices are used, which are equipped in professional electrical laboratories.

The test method for transformer oil involves analyzing its acidity and density - this is also included in the protocol. Along with the breakdown voltage, the documentation also contains another purely technical indicator - it is represented by the dielectric loss tangent, which is determined at a voltage of 2/3 of the recommended test voltage.

Example of a technical report

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e) surface tension

The determination method is set out in ISO 6295, GOST 5985-79. The determination is to estimate the force (in mN/m) required to break through the oil-water interface in a metal ring under prescribed conditions. This force, associated with capillarity properties, changes depending on the composition of the oil and under the influence of oil decomposition products.

Surface tension depends on the degree of aging and the acid number and indicates changes occurring in the oil. The table shows the recommended minimum values ​​for the oil in operation. A decrease in surface tension below the prescribed minimum values ​​indicates profound changes in the physical and chemical properties of the oil due to its aging. In these cases, it is preferable to replace the oil rather than regenerate it.

Minimum surface tension values ​​for oil in operation

Analysis of data, documentation and test results

Effective control in any environment cannot be carried out without monitoring over periods of time of a particular item, its control and cutting of parameters and controlled values. Storing them in a safe place with quick access allows you to implement an archive block, which, when taking readings of a device or unit again, must be analyzed, compared with the received data, and the trim between them quickly identified. These actions provide direction for subsequent actions on an item or equipment.

It is on this that the first point of diagnostics of power transformers is based, the main thrust of which is either to create the first parametric database of converter elements - if this is control, testing of new, just released equipment or analysis of indicators from the archive, comparison with current values ​​of diagnostic results, checking testing of components energy unit.

The conclusions of the analysis allow us to determine further actions for the device:

1. In the case of a nominal delta between the current and past diagnostic parameters of the device, new experimental values ​​are recorded and dated in the same archive until the next transformer test.
2. As a result of the emergence of high discrepancies and wide trims between the parameters of a particular piece of equipment, timely adjustment, repair or replacement of transformer parts is carried out in order to return to the basic values ​​and normal operation of electrical equipment.

The documentation in the archive for the device must contain all the necessary regulations, passports of the transformer elements, conclusions and test values ​​of the previous check of the necessary components of the converter. Currently, it is possible to access such an archive using computer technology and the Internet.

Electronic storage of the parametric base of the power device allows you to perform the analysis quickly, with high accuracy, and therefore carry out the current diagnostics in full format, drawing the correct conclusions upon its completion. In Russia, control checks are regulated by special software - “Diagnostics +”, with the help of which it is even easier and more accurate to gain access to the archive, evaluate its data and compare it with the current state of the transformer.

Methods for comprehensive oil analysis

A set of experimental and evaluation activities with power unit oil is the first practical tests as part of the diagnostics of a power transformer. This analysis is carried out using a special physical and chemical laboratory and research equipment.

To implement it, it is necessary to take oil samples from the device under test in several parts of different heights.

The conclusions that can be drawn from the results of the experiments can indicate the state of many characteristics of the power unit, its serviceability of internal mechanisms and hidden devices. After all, transformer oil not only plays the role of an insulating component of the converter, but is also its coolant.

There are two main methods for diagnosing an electrical device:

Chromatographic analysis

Determined by means of complex technical equipment for taking samples and carrying out control procedures and testing of liquid substances. An analysis of experience will be able to tell about the presence of defects at an early stage of their development, experienced diagnostic elements will describe in detail the degree of development of the malfunction, the causes of damage to the transformer oil, and therefore, projecting them onto the unit itself, first correct the condition of the device, prevent serious damage and even more serious losses , equipment downtime. There is control over overheating of current-carrying parts and the magnetic circuit of the transformer.

The main purpose of analysis using this method is to evaluate and diagnose gases in the composition of liquid oil, determine their types, and their quantitative level of contamination. The main gases that not only worsen the condition and performance of the liquid oil of the voltage converter are hydrogen, acetylene, ethane, methane. Their analysis and comparison with the previous state of the device allows us to make quite accurate forecasts on the general state of survivability of the power generating unit, as well as suggest a solution taking into account all possible factors.

Physico-chemical diagnostics

By taking test tube samples of part of the transformer oil fluid and diagnosing it in laboratory conditions, the test resolution reveals the whole truth about the insulating component of the device. The dielectric properties of the oil in the transformer deteriorate due to an increase in the amount of impurities in its composition, from daily operation or stressful operating conditions of the device.

Analysis by physical and chemical tests helps to record accurate data on the condition of the solid insulation of the converter, the operation of the cooling system, the aging of areas with paper insulation, and assess the risks of complete failure of the operating process of the entire device.

The estimated oil parameters are entered into the work log of routine diagnostics, compared with previous archived data, leaving at the end of the transformer diagnostics one of the conclusions for monitoring the cooling and insulating liquid:

• if the condition of the dielectric is completely of poor quality, the presence of gas structures exceeds all permissible equivalents, but the transformer itself has not yet failed - you should urgently change the oil in the device in order to avoid more serious and technically complex problems with the converter;
• if the analysis of the oil structure does not reveal a large number of gas impurities and there are no deviations in the studied composition of the coolant, a written conclusion is drawn that confirms the equipment’s approval for the oil diagnostic parameter.

Assessing the composition of transformer oil also makes it possible to identify the presence of electrical discharges in its composition, which arise as a result of constantly occurring arc discharges in the design of equipment or distribution systems.

Why do you need to test transformers?

The power transformer is an important transmission unit as part of a powerful and complex power system that provides power to a significant number of industrial and household energy consumers. Such a unit must be reliable and serviceable for a long time, so that there is no failure in the useful work of industrial consumers, and there is no shortage of electricity consumption in the everyday life of ordinary people.

That is why oil and dry power voltage converters are constantly tested:

• At manufacturing plants, numerous inspections and performance tests are carried out - in order to guarantee that a complex technical device for converting voltage from one class to another after manufacturing is fully operational and ready for further work at the site;
• When installing a supply system in an ensemble, testing according to a special acceptance testing methodology - in order to understand that during the transportation and subsequent installation of power equipment, no defects or installation errors occurred or were created that would not be able to provide proper, stable power to the required number of consumers;
• Periodically during the operation of electrical installations and components, as a result of which certain failures or defects of complex transmission equipment may also occur - to prevent pre-emergency or emergency conditions. To identify defects at early stages and timely eliminate them in operational mode with minimal losses for all energy consumers.

Such monitoring and checks of the operation of power transmission devices ensure maximum quality of operation of power systems as a whole, which means that the maximum quantity and quality of electricity is obtained in industry and in the domestic sector, which has a beneficial effect on the level of their efficiency.

Delivery of samples to the laboratory

Delivery of samples is a step that is often overlooked. The intake occurs when the transformer is operating or after a few seconds after shutdown. The documents record not only the technical characteristics of the device that will be examined, but also the ambient temperature, time of sampling and humidity indicators.

The faster the delivery, the better. It is optimal if the samples are delivered the next day or the same. But in extreme cases, you can wait up to a week - be sure to comply with the storage conditions for the composition. If you bring samples after the specified period, you cannot vouch for the effectiveness of the study. Transportation is carried out in containers, excluding spraying and sudden movements of oil.

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j) control of soluble oxidation products - soluble sludge

Experience shows that there is virtually no soluble sludge in the oil while the adsorbing filter is operating. The guidance document RD 34.43.105-89 requires periodic monitoring of this parameter. This takes advantage of the fact that the sludge becomes insoluble when the oil is diluted with H-heptane, but dissolves in a mixture of equal amounts of toluene and 95% ethyl alcohol. A series of chemical reactions allows you to determine the amount of sludge. There should be no more than 0.005% of its mass in the operating oil. There should be no soluble sediment in fresh and regenerated oils.