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Diagnostic methods for assessing the state of transformer insulation

Diagnostic methods for assessing the state of transformer insulation

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Transformers play an essential role in providing a reliable and efficient electricity supply. Hence it represents the most critical equipment in the electric power transmission and distribution systems. With increasing age, there are more possibilities of unexpected failures and outages of Power Transformers. Therefore the assessment of the transformer insulation system has become one of the most critical issues faced by maintenance engineers. The main objectives are: to extend their service life and optimize their performance through increased availability hence reducing the operating cost.

Transformers need maintenance from time to time to ensure reliable service. Inspection has to be done at regular intervals, and corrective measurements need to be taken timely to ensure the most satisfactory operation of this equipment. The frequency of inspection depends on the operating environment conditions. For clean, dry areas, an inspection annually or after a long period may be fine. For other areas where the air is polluted with dust or chemical fumes, an inspection at three or six-month intervals may be needed. Other elements such as motor,
fan etc. should also be inspected and maintained.

The trouble-free service of the equipment depends on proper installation, operation, and maintenance. The goal of the transformer maintenance is to control and treatment of serious oil and to wind insulation deterioration. The insulating materials used in the Transformers like mineral oil and winding paper(cellulose paper) are impacted by moisture, oxygen, heat, and other agents such as copper, iron, and electric stress, etc. Moisture will decrease the dielectric strength of both the oil and the winding insulation systems. Oxidation that takes place in the oil leads to Transformer sludging. Therefore, the Transformer should be constructed in such a way that moisture and oxygen are kept out.

Insulation System of a Transformer :
The insulation system of a power transformer consists mainly of transformer oil (hydrocarbon oil) and Cellulose paper (winding insulation). These materials degrade at higher operating temperatures in the presence of oxygen and moisture. The frequently used Chemical diagnostic methods to access the insulation condition of aged transformers are 1. Moisture analysis in transformer oil 2. Dissolved gas analysis(DGA). 3. Degree of polymerization(DP) measurement. 4. Furan analysis by high-performance liquid chromatography (HPLC ).The electrical diagnostic methods are mainly based on either time-domain polarisation or frequency domain polarisation.

Chemical techniques:

Moisture Analysis
Water content in insulation materials increases electric conductivity and dissipation Factor. This results in a decrease in the electric strength of the material. Hence, it is necessary to measure the moisture content of the transformer oil and cellulose paper. Several direct measurement methods are available for moisture analysis. A thin film capacitive humidity sensor was inserted for moisture sensing in the transformer oil in cold and warm weather conditions. Also, near-infrared spectroscopy(NIR), along with their developed modeling, has proved well and could estimate small changes in the moisture content of the paper insulation.

Dissolved Gas Analysis
This method has gained worldwide acceptance as a diagnostic method for the detection of incipient Alt gases are produced by the degradation of transformer oil and solid installation materials like paper made up of cellulose. A healthy transformer should have less than 0.05 ml of combustible gases(Hydrocarbon gases.- Methane, ethane, ethylene, acetylene, and hydrogen) per 100 my of transformer oil. The levels of carbon dioxide and carbon monoxide should be. 0.4 ml per 100 ml of oil after 15 years of service. The causes of fault gases are Corona or partial discharge thermal heating and Arcing. The diagnostic methods for gases in the oil are

1. Doernenberg method
2.Rogers method.
3. Duval’s triangle method

The Doernenberg method and rogers method are based on the comparison of ratios of the number of hydrocarbons determined from the oil with their limiting values proposed by IEEE standards. Duval’s method is based on total dissolved Key gas concentration and alarm values( Maximum acceptable values), and the typical rate of increase of fault gases for power transformer are given in IEC standard. The Transformer can be tripped when the values fall outside the standard values and can be protected.

Degree of polymerization measurements
The solid insulation used in the Transformer is a sheet of material made of cellulose (kraft paper). Cellulose is a linear polymer composed of anhydrous glucose units linked through Glucosidic bonds. The utility of cellulose paper is due to its polymeric and fibrous nature. The number of monomer units in the polymer is known as the degree of polymerization measured by the Viscometric method (DPv). New kraft paper has an average chain length of 1000 – 1500, and after a long period of service, DPv falls to 200 – 250. For a Kraft paper with a DPv of 150 – 200, the mechanical strength of paper can be reduced to 20% of its initial strength, and this point is regarded as the end of life criteria for transformer insulation. It is also reported that with an increase of .5% of water content in an aging transformer, the DPv value of paper would be halved. Also, the tensile strength of the paper depends on the final molecular properties of the cellulose in the paper and is independent of the degradation temperature. The rate of degradation increases discontinuously with the increasing temperature above 140 degrees Celsius.

Molecular weight studies by gel permeation chromatography (GPC) have been found to be more useful for assessing the degradation of the kraft paper. GPC gives the accurate molecular distribution of the polymer, and hence any small change in it is easily observable through the chromatogram. GPC measurement has been performed to investigate oxidation and thermal aging in Transformers. DPV gives no more than an approximate estimate of the average chain length of the cellulose, while GPC has the potential to give a detailed analysis of molecular weight distribution changes during cellulose aging.

Furan Analysis by high-performance liquid chromatography
Some transformers in use are found to contain by-products of Kraft paper insulation degradation – those by-products are furans. 2 – Furfuraldehyde is a major compound formed from the degradation of cellulose insulation papers. It Is found that an approximate logarithmic relationship existed between the concentration of 2 – Furfuraldehyde and the degree of polymerization of the cellulose in the paper. It is observed that the decrease in the tensile strength of the paper corresponded to an increase in the concentration of furans in the oil. The formation of 2- Furfuraldehyde considerably increases when the DP value goes below 400, and when it approached the critical value of 200, the paper losses all the mechanical strength and becomes susceptible to damage.

Electrical diagnostic methods:
With the aging of cellulose insulation in a transformer, the dielectric properties do not change drastically. Insulation resistance and polarization index methods have been used to ascertain the transformer moisture condition. The measurement of dielectric strength of the insulation paper using polarization techniques has been promoted in recent years.

Time Domain Polarization Method
When a dielectric material is exposed to an electric field, the material becomes polarized. The current density of the material J(t) Is the summation of both the displacement current and the conduction current. When a direct voltage is applied to a dielectric for a long period of time and short-circuited for a short period, then after opening the short circuit, the voltage will build up between the electrodes on the dielectric. This phenomenon is called return voltage(RV). During the return voltage measurement, the current density function will approach zero since the circuit being without any supply. The return voltage can be calculated through the respective function by knowing the permittivity and conductivity of the material. It is reported that the initial slope of return voltage is directly proportional to the polarization conductivity, and the maximum value of return voltage is proportional to the intensity of polarization phenomena. Also, it is found that shorter the Central time Constant (charging time to the peak of the return voltage) worse is the condition of the Transformer.

Based on various comprehensive experiments, it is suggested that the return voltage(RV) maxima appear and move towards lower charge times with increasing moisture content. The Central time constant tends to decrease with the increase in aging and moisture content. The return voltage method is also useful for checking the effectiveness of drying treatment. The return voltage method was tested on a 330/11 KV transformer. It is observed that capacitance and insulation resistance change due to thermal aging and moisture Ingress. Thus the return voltage method is extensively used for analyzing the condition of aged cellulose insulation. Among return voltage parameters, the Central time constant is found to be most sensitive to aging. The return voltage parameters are found to very at a much slower rate in the samples aged in the presence of nitrogen than compared to samples aged under air. It is suggested that the polarization parameters measured by the return voltage method very significantly and consistently with the aging condition of the insulation systems.

It is also found that the geometry of the insulation has a strong impact on the RV parameters. Therefore it is equally important to consider the oil quality and geometry of the insulation system to correctly interpret the dielectric diagnostic results.

Frequency Domain Polarization Method
In frequency domain polarization measurement, the dissipation factor is measured as a function of the frequency of the test voltage. In the frequency domain, the dielectric response function f(t) is the dielectric susceptibility. The susceptibility is a Complex function of frequency and is related to relative permittivity.

The insulation object is tested by supplying a sinusoidal voltage, and it is seen that the conductivity, the high frequency. The complex relative permittivity and the dielectric susceptibility Characterize the behavior of dielectric material. If the material is linear, Homogeneous, and isotropic, the measured values in the time domain and frequency domain is the same. The ratio of the imaginary part to the real part of the complex permittivity is called the Dissipation factor, and it is independent of the geometry of the material. This makes the dissipation factor important when the geometric composition of the material is unknown.
The low-frequency dielectric spectroscopy study for a number of Transformers are performed in both and time and frequency domain and compared with the values of the RV method. It is absolutely noted that change in oil quality and moisture content of the solid insulation, has significantly affected the RV parameters.

The RV method is useful but more sensitive to systematic errors. The Dissipation factor measured in the frequency domain describes the Spectrum of energy absorption by the dielectric. The aging of cellulose insulation has an influence on its polarization properties, which can be detected by the frequency domain polarization characteristics. Hence it is suggested that low-frequency dielectric spectroscopy is the best measurement method. That is also in good agreement between the frequency domain dielectric spectroscopy measurements with other chemical and electrical techniques.

Conclusion
No single method is considered as best. Among chemical methods, the DGA method is widely used for determining the fault gases. For analyzing the cellulose aging, the Furan analysis by HPLC technique is mostly used. The mechanical strength of the cellulose paper is better measured by the degree of polymerization method. It is also proposed that Polarization/depolarization measurement is superior as it monitors oil and paper conditions separately. The time domain and frequency domain predicts the same condition of the insulation. However, the advancement in measurement technologies and data analysis makes the polarization-based Electrical diagnostic methods more attractive.

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