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Power Transformers Testing
Our Power Transformers Testing Includes:
- Condition Monitoring-Online
- Condition Monitoring-Offline
Thermography is the use of Infrared photography to measure thermal energy from an object. Thermal energy shows us a new picture to us- the one which is not visible with naked eye
Benefits of Infrared Thermography
- Prevent - In one step you can increase equipment efficiency and the safety of your facility- a potentially save your company cost of a major electrical malfunction
- Diagnose – by using thermographic photography, we can look inside at the performance of many different systems and make recommendations that will increase the performance of your business
- Repair – NDL has been providing electrical maintenance and repair since 1990. Safety is our no. 1 priority, so you can assure your electrical work is completed with all the safety standards
Faults can occur at any stage of your transformer’s life and evolve rapidly. Testing your transformers on a regular basis will allow you to detect any unforeseen change in the condition of your transformers.
The recommended tests in this package include:
*Water Content, Tan Delta, Resistivity, Acidity, IFT, Flash Point, Sludge, Dielectric Strength)
Dissolved Gas Analysis
The use of DGA as a powerful diagnostic tool is a critical and important aspect of determining the condition and health of a transformer.
D.G.A. is a very reliable and proven techniques to detect the internal faults in working transformers. The fault diagnosis by the gas chromatography is based upon the types & relative quantities of various hydrocarbon gases which can get dissolved in the oil under various fault. D.G.A has proved to be reliable means of establishing the healthiness of a transformer which have tripped by suspected mal operation of differential protection (due to charging inrush or C.T Circuit problem) or Buchholz relay (due to air suction or control suction problem) can be returned to service with more confidence on the basis of D.G.A results. The accepted methods for performing this test are outlined in the ASTM D3612 and IEC 60567 standards.
A furan test should be included with yearly maintenance and trends developed to monitor the condition of the paper.
Furan Test is used to detect the thermal, oxidative and hydrolytic breakdown of paper insulation. This test, in conjunction with our dissolved fault gas analysis, gives you the best overview of the state of your transformer.
A rough approximation is that, for every 6° to 8° Celsius rise above normal operating temperatures, the life of the insulation is halved. Furans produced from these temperature build-ups are generated in two ways: the first being a localized area of high heat and paper damage, and the second being the general overall heating of the entire insulation system.
Early detection of paper insulation breakdown can prevent major damage or outright failure!
Moisture is destructive to cellulose and even more so in the presence of oxygen.
Acids are formed that attack the insulation and metals which form soaps and more acids, causing a vicious cycle. Oxygen inhibitor is key element for extending the life of transformers.
The inhibitor currently used is Ditertiary Butyl Para cresol (DBPC). This works similar to a sacrificial anode in grounding circuits; oxygen attacks the inhibitor instead of the cellulose insulation. As this occurs and the transformer ages, the inhibitor is used up and needs to be replaced. Replacement of the inhibitor generally requires that the oil also be treated.
Metals in Oil
Metals analysis of transformer oil is used to complement dissolved gas analysis (DGA). When a gas-in-oil analysis indicates the presence of a possible fault, metals analysis will help determine the fault type and pinpoint its location
High energy faults not only degrade the transformer’s insulation (oil, paper, wood etc…) but can generate metal particles that will disperse in the oil.
These particles will be distributed throughout the transformer, mainly due to oil circulation. Certain transformer components produce specific metal particles. These metal particles will be found alone or in different combinations depending on the fault type. Establishing the combinations and concentrations of these metal particles will help to narrow the list of components involved in the fault.
Sulphur, as defined in ASDM D-2864 is “elemental sulphur and thermally unstable compounds in electrical insulating oil, that can cause corrosion of certain transformer metals like copper and sulphur”.
Corrosive Sulphur reacts with copper as it comes in contact with it, and does not requires any heat to promote the reaction. It is more pronounced in sealed systems and leads to decomposition of copper-sulphur compound in insulation paper which leads to a weakened dielectric strength
EFFECT ON COPPER
Frequency Response Analysis
Frequency Response Analysis is a valuable diagnostic tool for the detection of winding movement and other faults that affect the Transformers impedance.
The Sweep frequency Response Analysis (SFRA) diagnosis is made based on the comparison between two SFRA responses and any significant difference in low frequency region, shift of existing resonance, creation of new resonance, change in shape of plot would potentially indicate mechanical or electrical problem with the winding and core of Transformer.
The frequency response of any form of physical damage to the transformer results in the changes of this RLC network. These changes are what we are looking for and employ frequency response to highlight these small changes in the RLC network within the transformer.
An SFRA analysis permits to predict faults such as:
- Core fault – Low frequency variation
- Shorted turns fault – Low frequency variation
- Open circuit or high impedance winding fault – Low frequency variation
- Overall radial shift of the winding – Mid frequency variation
- Axial shift – Mid to High frequency variation
- Core earthing related faults in the transformer/ Tap changer leads & bushing
- Leads related faults – High frequency variation
- Partial Discharge
The electrical stress within the insulation system is never uniform. Material with low dielectric current is more stressed. If the stress increases beyond a limit then a partial insulation failure occurs. Since the discharges occur partially in the insulation system, they are known as partial discharge. Partial Discharge can occur due to the improper design, manufacturing and also the various service conditions.
So we can say when discharges occur not directly between the electrodes but in the part of insulation system, they are called partial Discharge.
The transformer insulation is a combination of oil and paper. Occurrence of PD in the transformer produces various problem. In solid insulation, it can create discharge trees, which reduces the insulation strength. In liquid insulation like oil it creates gas bubbles.
Tan Delta & Capacitance (Winding & Bushing)
Every Power electrical appliances in use, undergo stress from Operating Voltage, Mechanical Vibration, Temperature, Gaseous and Solid Metallic Impurities. Under these stress, the degradation of the insulation takes place, in the Electrical equipment. This stress lead to the continuous ageing of the appliance, with regard to its Electrical Properties. Sometimes there will be an avalanche of Insulation breakdown. In the total complex network of Power Generation, Transmission, Distribution System ageing of a simple insulation may lead to a disastrous breakdown of the system, causing heavy loss.
To avoid such unexpected breakdowns and for the un-interrupted service, from electrical power it is very essential to know the insulation quality of the equipment that is going to be used in the power system. And it is also essential to periodically monitor the Insulation property of equipment, that in use in this Power Network.
In Electrical AC System the Dissipation Factor Tan ? or Power Factor Cos ø is considered as the indicator of quality of insulation.
It is desirable to have Insulation Resistance as high as possible consistent with acceptable mechanical, chemical and heat resisting properties.
This test is to detect the conductive impurities or mechanical imperfections in the dielectric which affect the quality of insulation which may be developed in the dielectric during the manufacturing process.
The insulation resistance test can detect if this connection is loose. It can also detect whether there are other, undesired and inadvertent, grounds. If the intentional core ground is intact, the resultant resistance should be very low. To check for unintentional core grounds, remove the intentional ground and Megger between the core and the grounded transformer tank. This test should produce very high resistance indicating that an unintentional ground is not present. This test is to supplement DGA that shows generation of hot metal gases (methane, ethane, and ethylene) and to indicate if a spurious, unintentional core ground is the problem. Experience can help locate the source of the problem.
Polarization Index test that it can provide an indication of insulation quality in ten minutes for very large equipment that might take very long time to get fully charged.
DC Absorption Test
DC Absorption Test indicates the insulation Condition of the Winding and Supplements to to various other Insulation measurements, therefore, if analyzed, based upon its trend over a period of Time, helps in making decisions better.
Magnetic Balance Test is performed on transformer to check whether the core and winding of the transformer is in balance position or not. Magnetic Balance is a test of Critical use as it helps us to enhance the efficiency and life of transformers
Magnetic Current Test is performed on transformer to check whether the core and winding of the transformer is in balance position or not. Magnetic Current is a test of Critical use as it helps us to enhance the efficiency and life of transformers.
The Leakage Current Test helps us in prediction of Break Down Voltage so as to take the preventive steps well in advance at an early stage of development.
The purpose of this test is to detect short-circuited turns, poor electrical connections, core de-laminations, core lamination shorts, tap changer problems, and other possible core and winding problems.
On three-phase transformers, results are also compared between phases. This test measures current needed to magnetize the core and generate the magnetic field in the windings.
Winding resistances are tested in the field to check for loose connections on bushings or tap changers, broken strands, and high contact resistance in tap changers.
Results are compared to other phases in wye connected transformers or between pairs of terminals on a delta-connected winding to determine if a resistance is too high. Resistances can also be compared to the original factory measurements or to sister transformers.
Turns ratio test performs on transformer to check the voltage ratio between primary and secondary side because the voltage in primary and secondary coil depends on the consequent number of turns in primary and secondary coil.
The transformer turns ratio (TTR) test detects shorts between turns of the same coil, which indicates insulation failure between the turns. These tests are performed with the transformer de-energized and may show the necessity for an internal inspection or removal from service.
A team of our service engineers would visit you, to inspect and analyze the transformers. Inspection helps us to analyze customers requirements and transformers condition so we can plan for the future work which is to be done.
After the inspection and based upon the measured data, suitable course of Overhauling Procedure is recommended to improve the reliability & the Life of equipment. However, at least once in a decade, overhauling is essential to restore the health of equipment.
All the Chemical & Electrical Tests are performed before commissioning of the equipment to ensure the. Integrity and for obtaining the future reference values.