10-08-2016, 06:10 PM
hi am rabeena..i would like to know about under frequency relay
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under frequency relay ppt
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hi am rabeena..i would like to know about under frequency relay
11-08-2016, 12:04 PM
Underfrequency Protective Relay
The underfrequency relay (fig. 4-16) is a solidstate device that functions to protect the load in the event generator frequency decreases below preset limits. It actuates when the frequency decreases to 55 hertz for 60-hertz operation and 46 hertz for 50-hertz operation. Upon actuation, contacts within the relay close to signal the annunicator and open to de-energize the generator breaker (contactor), resulting in a display of the fault condition and removal of the load from the generator.
Frequency sensing is accomplished by a tuned circuit consisting of capacitors C1 and C2 and components in the encapsulated base. Zener diodes CR1, CR2, and CR3 limit the peak voltage to the tuned circuit. The ac output of the tuned circuit is rectified by diode CR4 and applied to a voltage divider consisting of resistors R1, R2, R3, and R4. Transistor Q1 compares the voltage at the wiper of potentiometer R3 with the reference voltage established by zener diode CR7. When transistor Q1 conducts, transistor 42 operates as a switch to control the coil voltage on a relay contained in the encapsulated base. Both transistors Q1 and Q2 and the relay in the encapsulated base are energized when the frequency of the input voltage to terminals 1 and 2 is normal frequency (50 to 60 hertz). When an underfrequency condition occurs, the voltage at the base of transistor Q1 is not sufficient for conduction. This causes the relay to be de-energized and its contacts to switch. The underfrequency trip point is adjusted by potentiometer R3.
Appliances in India are designed to operate at 50Hz...if this is not maintained they will damage drawing high currents...
In case of faults or high loads...
power delivered by generator increases --> power input is constant (mechanical)
So the extra power comes from the kinetic enery stored in rotor --> Speed decreases
frequency tends to "fall"
So in order to maitain the frequency the extra load or fault has to be isolated for which under frequency relays are used...They work automatically.
The underfrequency relay (fig. 4-16) is a solidstate device that functions to protect the load in the event generator frequency decreases below preset limits. It actuates when the frequency decreases to 55 hertz for 60-hertz operation and 46 hertz for 50-hertz operation. Upon actuation, contacts within the relay close to signal the annunicator and open to de-energize the generator breaker (contactor), resulting in a display of the fault condition and removal of the load from the generator.
Frequency sensing is accomplished by a tuned circuit consisting of capacitors C1 and C2 and components in the encapsulated base. Zener diodes CR1, CR2, and CR3 limit the peak voltage to the tuned circuit. The ac output of the tuned circuit is rectified by diode CR4 and applied to a voltage divider consisting of resistors R1, R2, R3, and R4. Transistor Q1 compares the voltage at the wiper of potentiometer R3 with the reference voltage established by zener diode CR7. When transistor Q1 conducts, transistor 42 operates as a switch to control the coil voltage on a relay contained in the encapsulated base. Both transistors Q1 and Q2 and the relay in the encapsulated base are energized when the frequency of the input voltage to terminals 1 and 2 is normal frequency (50 to 60 hertz). When an underfrequency condition occurs, the voltage at the base of transistor Q1 is not sufficient for conduction. This causes the relay to be de-energized and its contacts to switch. The underfrequency trip point is adjusted by potentiometer R3.
Appliances in India are designed to operate at 50Hz...if this is not maintained they will damage drawing high currents...
In case of faults or high loads...
power delivered by generator increases --> power input is constant (mechanical)
So the extra power comes from the kinetic enery stored in rotor --> Speed decreases
frequency tends to "fall"
So in order to maitain the frequency the extra load or fault has to be isolated for which under frequency relays are used...They work automatically.
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