Over the past decades, when conducting diagnostic studies of transformers, the use of chromatographic oil analysis has become mandatory. First of all, this relates to determining the presence of dissolved gases in it.

Who does the work?

It is extremely important to carry out correct sampling, followed by delivery to specialized laboratories for subsequent testing. Along with the staff serving this equipment, such a procedure (for sampling) can be performed by invited specialists. Moreover, today, along with government agencies, this type services are offered by independent companies.

For example, concluding a service agreement with the ANO Center for Chemical Expertise will allow you to count on timely, high-quality chromatographic analysis of the oil.

Why are the tests carried out?

This is all the more important since it is not always possible to obtain complete and reliable information through conventional physical and chemical tests. Often, only chromatography provides comprehensive information about the degree and types of damage to a power transformer:

• overheating, as a consequence, acceleration of aging processes (related to defects in solid insulation);
• metal overheats, observed partial discharges, etc.

It is intended to avoid or minimize the possibility of creating an emergency situation. additional type studies of chromatographic analysis of oil. Based on its results, it will become much easier to find out the cause of defects and develop timely, appropriate recommendations for elimination.

Flaw detection method based on chromatographic analysis of gases dissolved in oil (CARG)

This method allows you to identify defects in power transformers, as well as in bushings at an early stage of development.

Laboratory studies carried out in a number of countries, as well as analysis of the spectrum of gases in transformers and bushings, made it possible to establish characteristic gases specific to a particular type of damage: hydrogen (H 2), hydrocarbon gases: methane (CH 4); ethylene (C 2 H 4); ethane (C 2 H 6), carbon dioxide (CO 2) and carbon monoxide (CO), acetylene (C 2 H 2). Thus, the type of developing defect can be assumed from the characteristic gases. Gas adsorption chromatography is based on the separation of the components of a gas mixture using various adsorbents - porous substances with a highly developed surface.

The gases released from the oil are usually analyzed by a gas chromatograph with a thermal conductivity detector.

The block diagram of the chromatographic installation is shown in Fig. 3.4.

Fig.4.

1 - cylinder with carrier gas; 2 - device for introducing a sample (dispenser); 3 - separation column; 4 - detector; 5 - recorder; 6 - device for extracting gas from oil.

The gas chromatography process consists of two stages: separation of the analyzed mixture into components ( qualitative analysis) and determination of their concentrations (quantitative analysis).

The analyzed gas mixture (sample) is introduced into the carrier gas flow, which is constant speed passed through a separation column containing an adsorbent. Differences in physical and chemical properties The individual gases of the mixture cause differences in the speed of their movement through the adsorbent (a porous substance with a highly developed surface). Therefore, at the output of the separation column, the components of the analyzed sample (in a mixture with the carrier gas) will appear sequentially. These components have different thermal conductivities, which allows the detector to generate corresponding signals that are recorded by a special device (usually a recording potentiometer).

The sequence (time) of release of specific gases from the separation column is known (for given analysis conditions). This provides information about the composition of the mixture being analyzed. To obtain quantitative data, the integrator determines the area of ​​the chromatogram peaks, which, based on calibration data, is reduced to the concentration values ​​of the corresponding gases. The possibilities for separating the components of a gas mixture are determined by the characteristics of the separation column: its filler (adsorbent), length and temperature conditions.

The carrier gas must be inert with respect to the analyzed substances and the adsorbents used. It must also provide normal work detector.

The purpose of the detector is to convert the individual components of the gas mixture entering its input into electrical signals, which are recorded on the tape of an electronic potentiometer in the form of sequential voltage pulses, called chromatograms.

The operating principle of a frequently used catharometer detector is based on indicating changes in the thermal conductivity of gases passing through it (thermal conductivity detector). The sensitive elements of the katharometer - resistors - are located in the chambers through which the gas flow passes. The two working resistors are flown around by the gas leaving the separating column; the other two resistors are pure carrier gas. Resistors are included in the bridge measuring circuit and are heated by the current flowing through them. When a component of the analyzed mixture appears in the working chamber, which changes the thermal conductivity of the gas in the chamber, the conditions for heat transfer from the working resistors to its wall change. In this case, the resistance of the working resistors changes and the measuring bridge becomes unbalanced. The voltage on the diagonal of the bridge, corresponding to the concentration of a given mixture component, is recorded by the recorder.

The analysis of the extracted gas mixture is carried out according to a method determined by the type of chromatograph used and the composition controlled gases. The results of the analysis are recorded on a chart strip. The composition of the analyzed mixture is determined by the time and sequence of appearance of peaks in the chromatogram. Calibration is performed either with a standard mixture of gases with a known concentration of components, or with one gas (usually nitrogen or air) with appropriate recalculation using sensitivity coefficients.

The method for diagnosing damage by chromatographic analysis of gases dissolved in oil is multi-criteria:

If gas analysis shows a “danger” or “damage” condition, chromatographic control is more often carried out;

the type of developing defect is determined from the characteristic gases;

this defect is specified in relation to gas concentrations;

Based on the rate of increase in gas concentration over a certain period of time, the degree of danger of the developing defect is assessed and recommendations are given.

Advantages of the HARG method: it allows you to detect a fairly wide class of defects, a high probability of coincidence of predicted and actual defects. Currently, HARG is used together with measurement of insulation tgd as the main methods for diagnosing bushings during operation.

Disadvantages: oil sampling under the operating voltage of the bushings is impossible due to the design features of their oil sampling devices. The need for frequent oil sampling is unacceptable, especially for sealed structures.

The small volume of oil in 110-220 kV bushings significantly complicates regular monitoring by taking and analyzing oil samples. Full recoil of bellows compensating temperature change The volume of oil in the designs of serial bushings 110-150 kV is 1.5-2.0 l, so after taking a sample (0.5 l), there is a need for subsequent labor-intensive topping up of oil and the corresponding expensive device. The characteristics of an oil sample do not always correspond to its actual state in the equipment, since some impurities may not be included in the sample.

The method of gas separation significantly affects the accuracy of determining the concentrations of controlled gases. Differences in isolation techniques often cause significant differences in analytical results between different laboratories. In addition, the gas content of the oil of a particular injection and the rate of its change depend on large quantity factors. These include differences structural materials, load modes, voltage class, etc. Therefore, boundary standards should be treated as a value that reflects a trade-off between the desire to detect defects and the costs of control. The high sensitivity of the HARG method increases the likelihood of false rejection, because taking into account the relatively small volume of oil in the bushing, it makes it possible to detect a defect, which, due to its small development, may not lead to emergency damage to the bushing.

The effectiveness of control is largely determined by the experience of the personnel. So, in particular, normal condition input can be stated even if the concentration of a number of gases exceeds the norm, if the rates of change in these concentrations are small. However, when the rate of change in concentration exceeds the normalized limit, a small absolute excess of concentration cannot be a sign of the absence of a defect.

It should also be noted that the complexity and high cost chromatographic installation and the difficulties of its setup and development.

Continuous monitoring of technical condition power transformers for all key parameters includes control:

• current loads;
• oil level and temperature;
• winding temperature;
• alarms, etc.

The proposed transformer monitoring systems can operate both in stand-alone mode and with integration into the industrial control system of the enterprise. Operational work with archives and dynamic data analysis make it possible to optimize the load and extend the service life of power equipment.

We offer implementation the following systems monitoring:

• Qualitrol 509 ITM series (condition monitoring of oil transformers);
• 118 ITM (continuous monitoring of dry power plants);
• 506 VTM/507 ITM (remote recording of fixed equipment parameters);
• T/Guard 408 (fiber optic temperature control system power plants using special sensors).

Chromatographic analysis of transformer oil

Examination of transformer oil for the presence of dissolved gases is also one of the key parameters for monitoring the condition of power oil-filled transformers. Based on the presence of dissolved hazardous gases and their concentration, it is possible to identify faults in the structural components of oil-filled transformers and shunt reactors at an early stage.

The most popular method of continuous diagnostics is online chromatographic analysis of dissolved gases in transformer oil. The BO-ENERGO product line includes in-line online chromatographs "Serveron", monitoring from 2 to 8 key gases, manufactured in accordance with TU-4215-001-70110824-2014 and included in the State Register of Measuring Instruments (certificate No. US. C.31.004.A No. 56677.

What defects are detected by chromatographic analysis of transformer oil?

The condition of the equipment is assessed by the presence of gases, their concentration and the rate of its growth. If hydrogen (H₂) is present in the liquid under study, then electrical defects are likely, namely arc and spark discharges.

Excess ethane (C₂H₆) indicates the occurrence of thermal faults, for example, heating of the insulation to +300...+400°C. The presence of methane (CH₄) in the coolant indicates more high temperature- up to +600°C. If, according to the monitoring results, ethylene gas (C₂H₄) is detected in the transformer oil, then the overheating is severe, above +600 °C.

The presence of dissolved acetylene (C₂H₂) indicates regularly occurring sparking and a passing electric arc. The reason may be a violation of the insulation of tie rods, sheets of technical steel, or incorrect grounding of the magnetic circuit.

If the presence of CO or CO₂ is detected in the test liquid, then this is a signal of accelerated aging or moistening of the solid electrical insulation.

For power units with a power of over 110 kW, chromatographic analysis of transformer oil is recommended to be carried out at least once every six months. The presence of special inputs makes it possible to take samples without stopping the equipment.

Diagnostics of power transformers

The technologies covered, including online chromatographic analysis of transformer oil, are non-destructive methods monitoring the condition of power equipment.

Diagnostics of power transformers using this method provides the following advantages:

• assessment of technical condition without decommissioning;
• identifying faults at early stages;
• monitoring all processes within the system;
• definition optimal timing repair.

Transformers with a voltage of 110 kV with a power of less than 60 MVA and block transformers for auxiliary needs - after 6 months. after switching on and then at least once every 6 months;

Transformers with a voltage of 110 kV with a capacity of 60 MVA or more, as well as all transformers of 220 - 500 kV during the first day, after 1, 3 and 6 months. after switching on and further - at least once every 6 months.

Transformers with a voltage of 750 kV - during the first day, 2 weeks, 1, 3 and 6 months after switching on and then - at least once every 6 months.

The frequency of HARG for transformers with developing defects is determined by the dynamics of changes in gas concentrations and the duration of development of defects. All defects, depending on the duration of development, can be divided into:

instantly developing defects - the duration of development of which is on the order of fractions of a second to minutes,

rapidly developing defects - the duration of development of which ranges from hours to weeks,

slowly developing defects - the duration of development of which ranges from months to several years.

The method of chromatographic analysis of gases dissolved in oil reveals slowly developing defects, possibly rapidly developing defects, and it is impossible to determine instantly developing defects.

If a defect is detected (A i >A g pi . and/or V rel i > 10% per month), it is necessary to perform 2-3 repeated analyzes of dissolved gases (with the frequency of analyzes specified in Section 3) to confirm the type and nature of the defect and making a decision on further operation of the transformer and/or its removal from operation. Where A g pi .- limit concentration i-th gas, %vol; A i - measured concentration value i-th gas, %vol;

The minimum oil re-sampling time (T id) for analysis can be calculated using the formula:

T id = β * M A i / V abs i (9)

Where β is the multiplicity factor of successive measurements (take b = 5); M A i - detection limit in oil i-th gas, %vol;

The detection limit of gases detected in oil (MA i) should be no higher than:

For hydrogen - 0.0005% vol.

For methane, ethylene, ethane - 0.0001% vol.

For acetylene - 0.00005% vol.

For carbon oxide and dioxide - 0.002% vol.

(Guidelines for laboratory and tests according to HARG)

5.1. If, as a result of analysis A i

5.2. If, as a result of the analysis, A i >A g pi and V rel i< 10%в месяц, то провести повторный отбор пробы масла и хроматографический анализ растворенных в нем газов для подтверждения результатов измерения и соответственно:

5.6. If after degassing the gas concentrations are less than the corresponding limit values and do not increase, this indicates the absence of damage. Such a transformer is removed from control, and the further frequency of oil sampling is set to once every 6 months.

5.7. If, after degassing the oil, an increase in the concentration of dissolved gases is observed again with repeated CARG at a rate of:

Vrel i>10% per month, then you should plan to take the transformer out of operation;

V rel i<10% в месяц, то трансформатор остается в работе на учащённом контроле по АРГ.

5.8 If A i >A rpi and V rel i ≤ 0 , then the influence of operational factors should be checked in accordance with Section 4, and if they are absent, it can be assumed that the defect develops “in depth” (burnout of contacts of switching devices, magnetic circuit sheets, metal pins, etc.). In this case, it is necessary to plan to decommission the transformer.

For on-load tap-changers in mounted tanks, in order to determine the possible flow of gases due to a leak in the seal between the contactor and transformer tanks, it is necessary to simultaneously take an oil sample from the contactor and transformer tanks. Examples of problem solving based on the results of HARG are presented in Appendix 1.