TRANSFORMER HEALTH CHECK AND ON-SITE SERVICES
A. Transformer IR test
What it is?
Insulation resistance is the ratio of applied voltage to the resulting current at a specified time after the voltage is applied. Direct voltage is used for measuring insulation resistance values. Insulation resistance test of transformer is very crucial since this test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer.
Why it is important?
Since over 80% of electrical maintenance testing involves evaluating insulation integrity, Transformer IR test is a very important test and this is because electrical insulation begins to age as soon as it’s manufactured and aging causes deterioration in the performance of the insulation. Moreover, harsh operating environments also cause further deterioration, especially where the electrical insulation is exposed to extreme operating temperatures, moisture, and chemical contamination. As a result, workplace safety and operating reliability can both be compromised. Hence, it’s extremely critical to always know the electrical condition (IR) of the insulation in your equipment at all times.
In another way, every unprotected wire carrying live electricity is a threat and needs to be carefully wrapped with some type of electrical insulation. Electrical insulation is a crucial part of any electrical maintenance. When it comes to workplace safety standards, nothing can be compromised. If the insulation wears off after some time and its performance deteriorates then it becomes essential that the safety of the workplace is tested. Insulation deterioration can result in dangerous conditions for power reliability and personal safety. So, before it is too late, and to ensure no electrical hazard, it is important to identify the deterioration and take corrective measures towards it. Because IR testers are portable, the IR test is used in the field as the final check of equipment insulation and also to confirm the reliability of the circuit and that there are no leakage currents from unintended faults in the wiring (e.g. a shorted connection would be obvious from the test results).
How it is done?
The insulation resistance (IR) test (also commonly known as a Megger test) is a spot insulation test which uses an applied DC voltage to measure insulation resistance in either KΩ, MΩ or GΩ. The measured resistance is intended to indicate the condition of the insulation or dielectric between two conductive parts, where the higher the resistance, the better the condition of the insulation. Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.
B. Transformer Oil Dehydration
What it is?
The process by which moisture can be removed from the transformer oil is called as dehydration of transformer oil. In this process, we can eliminate the solid particles, dissolved gases, dissolved water, moisture and sludge from the transformer oil. Dehydration method is also used to maintain the temperature of the transformer oil. Approximately 75 % of the transformers fail due to moisturization in the transformer oil.
How it is done?
Transformer oil dehydration process is based on use of synthetic absorbents. Removal of water as one of the main promoters of the oxidation process significantly extends the life of the transformer and increases operation reliability on the network. Power transformer Insulating oil dehydration is achieved by absorption of water molecules, i.e. by contact of oil and granulated absorbent. Since main characteristic of synthetic adsorbents is possibility of their reactivation after saturation, absorbents can be reactivated a virtually unlimited number of times without losing of their absorption capacity. Moreover, Synthetic absorbents are absolutely neutral to all kinds of mineral oils hence while drying of insulating oil there is no inhibitor absorption. Therefore, it is possible to implement this treatment on transformers as often as required, as basic process of maintaining and extending their lifetime.
Benefits of Transformer oil dehydration
- It is applicable in all weather conditions without oil heating and vacuuming.
- It can be performed in ON-LOAD mode, i.e. while the power transformer is energized hence no loss of production.
- No transformer vacuuming is required.
- No natural inhibitor removal is required.
- No fault gases removal is required.
- Cost effective
- Ecology friendly (zero gas emission and no disposal of waste oil)
- Transformer washing not required
- No exposure of transformer core to atmosphere
- Reactivation of synthetic adsorbents for few hundred times
- Simple machinery construction
- Simple handling
- Easy installation
- Minimum supervision
- Energy consumption is significantly lower in comparison with other technologies. (0.5-3 kW)
C. Transformer Di-electric strength
What it is?
The dielectric strength of transformer oil is mainly determined by the presence of acids, water, and other contaminates. It is therefore, important to keep the transformer oil free from such contaminates as much as possible. The dielectric strength of the oil will decrease with time and based on the service conditions where the transformer is located.
How it is done?
The Dielectric Strength of transformer oil is very important since it is essential in maintaining the reliable operation of power transformers. The Dielectric strength of oil is extremely sensitive to hydration or introduction of water and moisture thus under the action of the electric field of the emulsified oil, droplets of water are drawn to places where the field strength is particularly high. This process begins the process of oil breakdown. There can be an even more dramatic reduction of dielectric strength when the oil becomes contaminated by other impurities other than water such as acids and particulate matter. Some particulate matter, in the form of fibre, can be arranged in the form of lines known as “lines of force” and under the influence of the field forces, the wet fibres are not only drawn into places where the field is stronger, but are arranged in a way that greatly facilitates the breakdown of the transformer oil.
The Dielectric strength of transformer oil is also known as the breakdown voltage of the transformer oil. “Breakdown Voltage” (BDV) is measured by observing the voltage that is required to jump a spark between two electrodes immersed in the oil separated by a specific gap or distance. Also known as the sparking strength, the higher the voltage required to jump the spark, the higher the BDV will be in the oil. The lower the voltage required to jump the spark, the BDV will be lower indicating the presence of moisture content and other conducting substances in the oil. Clean oil with a low moisture content gives higher BDV results than oil with high moisture content and other conducting impurities.
Benefits of Transformer Di-electric Strength
- Determination of the electrical strength of oil
- Determination of the oil’s moisture content.
- Determination of the oil’s gas content. Performed using an absorption meter.
- Determination of mechanical impurities.
D. Measuring Polarization Index
What it is?
Polarization Index Test (PI Value Test) is conducted on HV electrical machines to determine service condition of the insulation. PI test is conducted specially to determine the dryness and cleanliness of the insulation. The polarization index test in a transformer has been regarded as a useful tool in evaluating its windings for build-up of dirt or moisture, deterioration of the insulation, fitness for high potential tests and suitability for further operation.
How it is done?
Polarization Index (PI) test is a universally accepted diagnostic test for insulation tests. It has been in use for more than 75 years. PI is the ratio of IR measured after voltage has been applied for 10 minutes to the IR measured after one minute, i.e. A low value of PI indicates that the windings may have been contaminated with oil, dirt etc or absorbed moistures. In this test, a relatively high DC voltage is applied between the copper conductor and the stator or rotor core usually between the winding and ground as the machine core & body are grounded).
Why it is important?
PI test was developed to make interpretation of results less sensitive to temperature. PI is the ratio of two IR at two different times. Temperature of the winding does not rise during the test period of 10 minutes. So, it is fairly assumed that both IRs are measured at same winding temperature. Then the temperature correction factor will be same for both cases and will be cancelled during the calculation of Pl. Thus, PI is relatively insensitive to temperature.
E. Checking and servicing of OLTC
OLTC or On Load Tap Changers’ servicing includes cleaning of contacts, Oil sludge removal, adjustment of springs, checking of motor drive operation. As the only moving parts in a typical power transformer, on-load tap changers (OLTCs) are among the most vulnerable components, and their regular testing and maintenance is an important part of transformer asset management. Visual inspection can be used to detect degradation of accessible OLTC parts at regular intervals. However, many mechanical parts are not easily accessible for inspection, and it is recommended to perform additional tests to establish the OLTC condition.
F. Other Services
The other Transformer related on-site services we provide are as follows-
- Checking of panel annunciations
- Calibration of gauges
- Checking of tripping & control circuit
