TRANSFORMER PROTECTIONS TRANSFORMER PROTECTION SYSTEM SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 1 TRANSFORMER PROTECTIONS What is a Power Transformer? Power Transformers are the major equipment in the power system network. As we know the Transformers are used to deliver the power to the consumer without changing its power and frequency. The transformer has numerous kinds of protection in order to maintain the transformer healthy at all the conditions of the power system network. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 2 TRANSFORMER PROTECTIONS List of Major Transformer Protection: OTI WTI Buchholz Protection Surge Protection Pressure Relief Valve Protection Differential Protection Restricted Earth Fault Protection SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 3 TRANSFORMER PROTECTIONS OTI: OTI is abbreviated as Oil Temperature Indicator. OTI is used to measure the temperature of the Oil inside the Transformer Main Tank. OTI has the temperature indicator connected with Thermostat along with the probe. This thermostat is mounted at the top of the transformer main tank. Since hot oil will be at top of the tank, it is mounted anywhere at the topmost part of the tank. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 4 TRANSFORMER PROTECTIONS OTI: Normal OTI indicator will have lobe like structure, inside which the mercury is filled along with the contacts. It is called a mercury switch. This switch is fixed in a circular disk along with a marking scale in it. Whenever the temperature increases the mercury starts melting and makes the disk to rotate indicating the temperature. By rotating, the contacts inside the mercury switch/lobe get changed from NO/NC position giving signals to the cooling system. This contacts of OTI is used to control the Air forced/oil forced cooling system in the transformer. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 5 TRANSFORMER PROTECTIONS WTI: WTI is called a Winding Temperature Indicator. WTI indicator is like an OTI indicator. But here the only change is the Thermostat. In WTI, thermostat can’t be used in the winding to measure the winding temperature. Instead of using the Thermostat, a rheostat is being used. This rheostat, having some standard resistance is connected to the control circuit. Based on the level of temperature near the winding area, the resistance of the device will vary. This value is converted into temperature using a separate Current conversion unit and a transducer unit. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 6 TRANSFORMER PROTECTIONS WTI: The Output from the WTI indicator unit will be utilized for activating the Cooling unit. The digital output from the CCU unit will be given to control devices. For both OTI and WTI, Alarm temperature and Trip temperature can be adjusted manually based on the user's needs. When the temperature reaches the fixed alarm state, the device will give an alarm indication. Likewise, if the temperature goes beyond the fixed value of trip temperature, the command will be initiated by the protection system to the transformer connected breaker to isolate the transformer from the grid. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 7 OTI & WTI Image of OTI and WTI Indicators SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 8 TRANSFORMER PROTECTIONS Buchholz Relay: Buchholz relay is an internal protection device of a power transformer. These relays are used to protect the transformer from internal faults. These relays are very fast and sense the internal fault against the transformer. Buchholz relay is fixed in-between the Transformer Main tank and the Conservator Tank. Buchholz relay has two floats inside it. These floats are used to sense the internal faults in a transformer. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 9 TRANSFORMER PROTECTIONS Buchholz Relay Operation: For example, there is a sudden rise in the temperature inside the transformer due to some spark or any other burning inside the tank. Due to this, there will be the formation of some unwanted gases inside the tank. These unwanted gases are collected at the top of the Buchholz relay which is installed at the topmost point of the transformer main tank. This results in the activation of the Buchholz relay giving alarm or trip signals to the protection system. DGA sample collection device is also installed with the Buchholz relay point. Buchholz relay will mal-operate if the level of oil inside the Relay changes, which disturbs the position of floats under normal condition. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 10 TRANSFORMER PROTECTIONS Surge Protection: A surge Protection relay is used in the transformer that is equipped with OLTC’s. Surge relay is installed between the OLTC tank and the OLTC conservator tank. Each phase of the transformer will have a separate OLTC tank. Its working is the same as that of a Buchholz relay, but it has only one float. It is used to monitor the level of oil in the OLTC’s conservator tank and how force the oil enters and leaves the chamber of the OLTC tank. Surge relay will not operate during the normal transformer tap changing process. It will operate only when surge develops inside the OLTC tank. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 11 TRANSFORMER PROTECTIONS Pressure Relief Valve Relay: PRD called a Pressure relief device installed in the transformer main tank. PRD is used to relieve the high pressure inside the transformer tank during a fault condition. PRD has a spring-loaded diaphragm, which is designed to release a huge amount of pressure rapidly under abnormal conditions. Based on the amount of oil, and the rating of the transformer, the number of PRD’s mounted on the transformer tank will differ. The operation of PRD will trip the transformer/ reactor considering a sudden rise in temperature and pressure as a fault occurrence. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 12 TRANSFORMER PROTECTIONS Differential Protection: Differential protection is one of the important protection in the power transformer. Differential protections are employed in order to measure the difference in current between the primary and secondary of the power transformer and take necessary action against the conditions. Differential protection schemes of any transformer, based on its conversion ratio, will have a current transformation ratio. Any difference in this ratio/values can be sensed with the help of neutral CT. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 13 TRANSFORMER PROTECTIONS Differential Protection: Under normal conditions, changes in the ratio will be minimum and current flowing through the Neutral CT will be negligible. If Fault in any one of the phases, the return current will be flowing through the Neutral CT measuring fault current. During this condition, the ratio of current will not be the same and hence the relay operates. This protection is main for transformer, reactor, and bus bar protection. For this protection in the transformer’s, bay CT’s will be used based on the electrical scheme. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 14 TRANSFORMER PROTECTIONS Restricted Earth Fault Protection: This relay is used in the transformer for finding the internal earth faults. This is the same as that of Differential protection, but it is for sensing earth fault. Each transformer will have 6+1 CT mounted in the turret and connected with bushing lead. This relay is connected along with the neutral CT. The common phases of Individual phase Ct will be connected to the secondary of NCT is such a way that secondary unbalanced phase current of phase CT’s will opposes the secondary current of NCT. Under normal conditions, both these currents will nearly be the same and relay will not respond. Under abnormal conditions, the value will change and the relay operates. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 15 TRANSFORMER PROTECTIONS Restricted Earth Fault Protection: In-order to nullify the phase shift at bushings of the power transformer (Primary will be Delta Connected and Secondary will be Star connected with Solid Grounding), the CT's are connected Star side with Delta and vice-versa. By connecting the Delta side(Primary side) CT’s as Star connected, 30degree electrical phase shift can be nullified. This method was employed in olden days while using Electromagnetic relays. Nowadays, numerical relays are equipped and it's not necessary to physically do the wiring as mentioned above. All the numerical relays have an in-built algorithm to sense the differences. SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 16 Thanks for Watching the Video S U B S C R I B E O U R C H A N N E L F O R M O R E E L E C T R I C A L R E L AT E D V I D E O S SUBSCRIBE OUR CHANNEL FOR MORE VIDEOS 17
