Transformer general arrangement. This topic covers the general arrangement of a transformer. The transformer tank houses the core and windings. The transformer's core material is made of cold rolled grain-oriented, CRGO steel and can be either shell or core type of construction. Core clamping is earthed to the body of the tank, outside the tank. Copper conductor is used in the transformer windings. Other forms of conductors include copper sheet, aluminum foil, and standard copper. Kraft paper is used for insulating the windings when immersed in transformer oil. It provides Class A insulation. Tank shall be well mounted for large transformers. The tank can be plain rectangular, or corrugated type. Tank shall withstand specified vacuum and pressure during testing. Porcelain bushings are insulating bodies through which the leads are bored out of the tank for connecting the transferable to the external conductor. Based on the pollution level, specific creepage distance of bushings shall be 16, 20, 25 or 31 mm per kilo volt. Recall what you have learned in the basic design parameters of a substation module. Cable boxes are used if external cables are required to be terminated at the transformer. Cable box is normally provided with disconnecting link. Insulators may be of epoxy, casteism or porcelain. The schematic diagram of connections in cable box is displayed for reference. A typical gender arrangement drawing of a transformer is displayed on the screen. On load tap changer that this OLTC, is an integral part of the main tank or the separate tank based on the voltage level, current, and the number of taps. It regulates the output voltage of the transformer while the transformer delivers the load or if there is an under voltage on the HV side. OLTC is located on the HV side of the transformer, for ease of handling the current, which is small compared to LV side. The operation of the tap changes, which is motorized. The motorized mechanism is attached with the OLTC compartment of the transformer. OLTC tap raise or low operation can be carried out from the local OLTC mechanism manually as well as electrically. Tap raise or low operation can also be carried out from a remote panel called remote tap changer control. That is RTCC panel. OLTC motor, power contactors, tap raise or lower push buttons, indicating lamps are provided as part of tap changer mechanism control box on the transformer. Leading manufacturers of OLTC are, MR Germany, ABB, and EASUN MR. Tap selector switch, diverter switch, transition resistor and motor for OLTC are the important components of OLTC. Number of steps and value per step for OLTC are dependent on the voltage variation foreseen in the system. In general, the voltage variation of up to plus or minus 10 percent is allowed. Hence, in this case, the OLTC has 17 taps, each of 1.25 percent for finer control. However, the probability of grid variations on the positive side is very rare. Hence, practically tap changer is specified for 5 percent above and 15 percent below the primary voltage in steps of 2.5 percent. Off circuit tap changer can be operated only when the transformer is de-energized. Off circuit tap changer is specified for voltage variation of plus or minus 5 percent in steps of 2.5 percent. RTCC, that is remote tap changer panel, is located inside the substation building, in the lineup of control panels. It enables operation of tap changer. It acts as interfacing between remote automation system and transformer tap changer mechanism. With respect to tap changing functionality, it houses AVR, that is automatic voltage regulator relay. It senses the secondary voltage and adjust the tap position to stay closer to the set point or reference voltage. It also houses manual tap raise or lower commands, tap position indicator, tap change in progress indication lamp. Necessary circuits for running the tap changer as master or slave or in independent mode are included, so that when the transformers are operated in parallel, tap change shall be consistent. Cooler fan or pump controls, tap changer auto and manual mode selections are provided at the RTCC Panel. For ONAF rated transformers, radiator banks are fitted with cooler fans. Cooler fans and oil pumps are the first cooling equipment available in an OFAF rated transformer. The cooler fans on radiator banks are motor driven, and are required to operate with minimum noise and vibration. It is preferable to have the number of fans follow n minus 1 operation philosophy, so that the failure of one fan does not affect loading of the transformer. Wire mesh guards are provided to avoid accidental contact. Oil pumps out of submersible axial flow type. Axial flow type construction permits oil circulation even when the pump is idle. The oil pump will be installed with flanges on both sides for easy removal and replacement. The controls for cooler fans are normally available in the transformer marshaling kiosk or remote tap changer control panel, that is RTCC. The major accessories of a transformer is displayed on screen. Buchholz relay is mounted in the pipe connecting the main tank of transformer to the conservator. It serves as the main protection for any minor or major electrical faults, such as inter- turn faults, shots circuit faults, core faults, and incipient faults that may develop inside a transformer. The mercury switch in the relay, is actuated by the surging of gas from the main tank to conservator, due to internal faults. It has two contacts, the top contact and the bottom contact. The top contact of this relay initiates an alarm and bottom contact trips the transformer. Oil temperature indicator, generally indicates the maximum temperature of the tap oil. Winding temperature indicator, indicates the maximum temperature of the winding. These indicators work on the principle of liquid expansion. They operate an alarm and trip contacts, at preset values, and switch on or off the cooling fans and pumps at preset temperatures. The marked features provided on the disk make or break connections as per the calibrated values. As it is not cost-efficient to directly measure the winding temperature, it is carried out by an imaging process. The transformers can have embedded fiber optic heat sensors to directly measure the winding temperatures. However, this arrangement is expensive. Hence as a standard, the winding hotspot temperature is measured by combining the top oil temperature and imaging of winding temperature. In the imaging process, the winding temperature indicator is provided with an additional heating element. This element is fed by a current proportional to the load current flowing through transformer winding, detected by a bushing current transformer. This picture clearly shows the mounting location of oil temperature indicator and winding temperature indicator in a transformer. Marshalling box is a steel enclosure, which is weather and vermin proof with a degree of protection IP 55. All internal wirings within transform is taken up to the marshalling kiosk for extending external connections. It accommodates oil and winding temperature indicators, terminals for the external connection of the alarms and drips from the OTA and WTI, terminals for transformer neutral CT, motor starters for fans. In case of a forced air cooled transformer that is ONAF, and anti-condensation heater. A major fault within the transformer and consequential vaporization of the oil leads to the buildup of pressure inside the transformer tank. If this pressure is not relieved and the transformer is not isolated from the supply within a few milliseconds, the transformer tank will be ruptured and leads to a major fire or electrical accident due to the spillover of oil. The pressure relief valve generally mounted on the top cover, provides instantaneous relieving of dangerous pleasure and sends signals for tripping of circuit breakers. The pressure relief valve has a port seal by stainless steel diaphragm and kept pressurized against an O ring with heavy-duty springs. Whenever the pressure exceeds the preset value, the diaphragm is lifted and the pressure is relieved. A flag indicator and micro switch are provided on the pressure relief valve. The micro switch has got one normally open contact and one normally closed contact. In smaller size transformers, explosion vent is provided instead of PRV. The vent has diaphragm which ruptures when pressure inside the tank builds up to release pressure. The conservators in smaller capacity transformers are provided with conventional glass eye level indicators. Plain glass type, and magnetic oil level gauge, that is MOG, fixed on the sidewall of the conservator are used to indicate the oil level in the conservator tank. MOG is provided with contact with alarm for low oil level condition. Oil level indicator is separately provided for OLTC conservator compartment. For transformers with the common conservator, the main conservator will be divided transversely, internally into two paths. The smaller part will be used as conservator for OLTC. Oil surge delay, that is OSR, is an oil actuated relay installed in the pipe connection between OLTC tank and its conservator. In the event of fault within the OLTC or the divertor switch of OLTC, the oil surge relay trips due to an oil surge from OLTC towards the OLTC conservator. Since gas is produced at each tab change operation, a [inaudible] relay is not suitable for this operation. Transformer oil is mostly based on mineral oil. Primary function of transformer oil is insulation and cooling of a transformer. Ideally, transformer oil shall have high dielectric strength, thermal conductivity, and chemical stability continuously at all operating temperatures of the transformer. Usage of polychlorinated biphenyls, that is PCB-based transformer oil is banned due to environmental issues. The purpose of a rating plate is to provide the actual design and tested parameters of a transformer. It contains details such as manufacturer's name, year of manufacture, serial number, voltage, power, current ratings, percentage impedance as per tested values, etc. Valve schedule plate identifies all valves installed on the transformer, with their status open or closed during normal operation of the transformer. There are several valves provided on the transformer to facilitate ease of maintenance and minimizing downtime. Filter valves, one at top and one at bottom to enable oil filtration. They are closed during normal operation. Sample valve, to enable extraction of oil sample for the purpose of periodical testing of oil breakdown voltage. This valve is normally closed. Drain valve is used to enable draining of oil from the tank. In some cases, drain valve may double up as bottom filter valve, normally closed during operation. Drain plug. This plug is located at a level lower than drain valve to enable removal of oil sludge after draining the oil during large-scale maintenance. This plug is normally closed. Shut-off valve is provided in the pipe between the main tank on both sides of Buchholz relay to enable maintenance work on the relay or replacement. Sometimes it may be provided on only one side. These valves are normally kept open. Radiator valves are provided at the entry and exit of removable type of radiators. They're always kept open during normal operation to allow oil circulation through radiators. Other constructional aspects of transformer are line-side CTs are mounted inside the turrets of the bushings. Neutral CTs are mounted around neutral conductors. Neutral conductor is brought out through neutral bushing. Online monitoring systems are normally installed on very large transformers to analyze the gas that may be dissolved in oil and to monitor a few parameters like temperature of oil and winding, humidity level, ambient temperature, load, and functioning of cooler fans. Some of the transformer manufacturers, are ABB, Siemens, Schneider Electric, Bharat Bijlee, Gujarat Transformers, Century Transformers, Essener, Hammond, Toshiba, Voltamp, EMCO, Crompton Greaves, TELK, and EASUN. Here is a quick recap of what you have learned this far. Based on the pollution level, specific creepage distance of bushings shall be 16,20,25, or 31 mm per kilovolt. Cable boxes are used if external cables are required to be terminated at the transformer. On Load Tap Changer, that is OLTC is an integral part of the main tank or the separate tank based on the voltage level, current, and number of taps. Radiator banks may be fitted with cooler fans. Air blowers and oil pumps are the forced cooling equipment available in a transformer. Buchholz relay protects the transformer from electrical faults such as inter-turn faults, short circuit faults, core faults, and incipient faults that may develop inside a transformer. Oil and winding temperature indicators indicate maximum oil and winding temperatures. The conservators in smaller capacity transformers are provided with conventional glass oil level indicators. Oil Surge Relay, that is, OSF is an oil actuated relay installed in the pipe connection between OLTC tank and its conservator. Transformer oil shall have, high dielectric strength, thermal conductivity, and chemical stability. The purpose of a rating plate is to provide the actual design parameters of a transformer. The percentage impedance is engraved on the name plate after testing of transformer. Marshalling box is provided to terminate all external connections from transformer to external switchgear or panel. Valve schedule plate identifies all valves installed on the transformer with their status open or closed during normal operation of the transformer.
