Transformer Accessories for Testing and Protection

Standard Accessories on an Oil Filled Transformer:
Liquid level gauge
Liquid temperature gauge
Pressure-vacuum gauge
Pressure relief device
Winding temperature gauge
Transformer cooling fans
High voltage bushings
Low voltage bushings
De-energized tap-changer
Buchholz relay
galvanized radiators
Breather
Lifting lugs
Jacking facilities
Multi-ratio, current transformers (CTs) for relaying service
Surge arresters
Grounding devices, such as reactors and resistors
De-energized tap changer (DETC)
Load tap changers
Conservator oil preservation systems
Various monitoring devices for gas, oil and winding temperatures; power factor; moisture; etc.
Most customer-specified accessories

1. Transformer Accessories FOR TESTING AND PROTECTION By Durgada Sagar Electrical Engineering Student Reva University Bangalore

2. Standard Accessories on a Oil Filled Transformer Liquid level gauge Liquid temperature gauge Pressure-vacuum gauge Pressure relief device Winding temperature gauge Transformer cooling fans High voltage bushings Low voltage bushings De-energized tap-changer Buchholz relay galvanized radiators Breather Lifting lugs Jacking facilities

3. Multi-ratio, current transformers (CTs) for relaying service Surge arresters Grounding devices, such as reactors and resistors De-energized tap changer  (DETC) Load tap changers Conservator oil preservation systems Various monitoring devices for gas, oil and winding temperatures; power factor; moisture; etc. Most customer-specified accessories

4. Liquid Level Gauge Provides continuous indication of the liquid level inside of a transformer’s main tank, conservator tank or load tap changer compartment. Basically, it consists of a float-arm inside the tank, an indicating pointer. Float travel ranges from min to max markings for various tank configurations

5. Liquid Temperature Gauge known as electromechanical thermometers or OTIs are essential devices on oil-filled transformers, providing accurate temperature measurement of the oil within. The temperature gauge is furnished to indicate the top liquid temperature in the tank in degrees Centigrade. The thermometer can be furnished with two SPDT contacts for a high temperature alarm, for energizing a fan circuit or for a low temperature alarm.

6. Pressure-Vacuum Gauge Designed to balance pressure and vacuum conditions within a sealed vessel by releasing or admitting air and gas. High pressures indicate an overload or trouble inside the tank and should be investigated immediately. A sustained zero pressure reading indicates a leak or a defective gauge. If internal pressure reaches the pre-determined setting, valve will exhaust gas from the tank. If vacuum inside vessel becomes too great, the valve will admit air into the tank Note: Typically the transformer is filled with oil and pressurized with nitrogen to a pressure of 0.2 atm (3 Psi) [Std Atm = 1atm = 14.69595 psi] psi = Pound-Force per square inch 1 pound = 0.45 kg

7. Pressure Relief Device The PRD is used for the relief of excessive pressure buildup in large volumes of gas or fluid in transformers. Its patented dual gasket system provides quick response time and automatically reseals after pressure has subsided. Should the tank pressure increase above that for which the device is set, the gas pressure will lift the diaphragm and let the gas escape quickly. Immediately after the pressure returns to normal, the diaphragm will reset and reseal the transformer.

8. Winding Temperature Gauge All large transformers have an  oil or a winding temperature indicating device  of some type, and most have temperature recorders as well. Indication may be for top-oil temperature or hotspot temperature. Additional temperature-sensing equipment may be installed to provide alarm and control signals needed to activate automatic cooling systems. Indicator operation may be: Purely mechanical (tank-mounted temperature gauge or thermometer), Purely electrical (SCADA transducer and some chart recorders), or A combination of the two (chart recorder and multipoint temp recorder).

9. Transformer Cooling Fans In order  to increase the transformer load  without overheating the windings, a set of fans can be furnished as an optional item .Fan control consists of a contact on either the liquid temperature gauge or the winding temperature gauge (when furnished), and “Manual-Auto” control switch . Recommended Maximum Temperature settings are; A. Oil Temperature Indicator: 85°C B. Winding Temperature Indicator: 105°C

10. High-Voltage Bushings Solid porcelain bushings consist of high-grade porcelain cylinders that conductors pass through. Outside surfaces have a series of skirts to increase the leakage path distance to the grounded metal case. High-voltage bushings are generally  oil-filled  condenser type. Condenser types have a central conductor wound with alternating layers of paper insulation and tin foil and filled with insulating oil. A good indication of insulation deterioration is a slowly rising power factor. The most common cause of failure is moisture entrance through the top bushing seal. Care must be taken in handling the bushing to avoid cracking the porcelain or damaging its surface.

11. Tap-Changer Do not operate the tap-changer while the transformer is energized! Doing so can result in severe personal injury and equipment damage. The tap-changer provides  a means of changing the voltage ratio  of a de-energized transformer without breaking the transformer seal. It is operated by means of a rotatable handle located on the side of the transformer.  The tap-changer is normally provided with five or seven positions.

12. Buchholz’s Relay Buchholz relay  is used for the protection of transformers from the faults occurring inside the transformer. Internal faults such as an insulation faults between turns, breakdown of core of transformer, core heating energize the alarm circuit and one can predict the type of fault in the transformer More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing equipment energize the trip circuit and immediately isolate the transformer from the rest of power system

13. Silica gel Breather The function of breather in transformer is to filter out the moisture from air. Breather consist of silica gel which absorbs the moisture from air. The main reason to remove moisture from air is because the moisture degrades the dielectric strength of the transformer oil.

14. Surge Arrestors Lightning arresters   are protective devices for limiting surge voltages due to lightning strikes or equipment faults or other events, to prevent damage to equipment and disruption of service. Also called surge arresters. Many surge divertors are essentially non-linear resistors which when subjected to high transient overvoltage have their resistance go to zero, thereby creating a short circuit and blowing a fuse. In this way it protects against damage due to serious overvoltage and also disconnects the equipment from the circuit.

15. Current Transformer A  current transformer  ( CT ) is a type of  transformer  that is used to reduce or multiply an  alternating current  (AC). It produces a current in its secondary which is proportional to the current in its primary. The  current transformer  is used to reduce the currents in power systems, producing proportional currents in the unit’s secondary windings that remain isolated from the main circuit. The replica current is then used as an input to a protective relay, which can then automatically isolate part of the power circuit should it experience a fault. A CT for operation on a 110 kV grid

16. References electrical-engineering-portal.com Researchgate.net Library.e.abb.com (ABB Group) Wikipedia.com insulect.com Testguy.net www.qualitrolcorp.com Spxtransformersolutions.com Azom.com Sadtem.com Thank You