21)- Distance Relay:
The relay which will function when the circuit admittance, impedance, or reactance increases or decreases beyond predetermined current limits.
Example:
Transmission lines are protected with this type of relay. They are covered in different zones of protection like 3 zones or 5 zones. And depending on the value set for each zone, protection will operate.
Question may arise here, why distance protection? Isn’t over current protection capable to take care of transmission line?
This can be understood by following figure. There are two different conditions shown. First condition is showing, fault at F1. So, total impedance will be Z= 5+4 = 9.
So on I= 110*1000/rt3*9= 7064 Amps.
Now consider another condition. As in this condition I=110*1000/(rt3*10) = 6358.
So over current range will be 6358 to 7064 Amps. This is unpractical.
In distance relay, relay will be operates on based on the impedance settings. So if impedance seen by relay is below set value, it will operate for that zone.
However, traditional relays don’t measure accurate impedance. These relays will measure fault voltage and compare it with the voltage derived from fault current and zone impedance setting. And it will determine whether fault is within zone or out of zone.
22) – Equalizer Circuit Breaker:
It is a circuit breaker that serves to control or to make and break equalizer or the current balancing connection for a machine field, or for regulating equipment in a multiple-unit installation.
23)- Temperature Control Device:
It is a device that functions to raise or lower the temperature of a machine or other apparatus, or of any medium, when its temperature falls blow or rises above, predetermined value.
Example: Thermostat in Switchgear assembly. When temperature falls below set value, it will switch on heater and after reaching desired value it will disconnect heater circuit.
24)- Reserved for Future Application: OVER EXCITATION/OVER FLUXING Device
It is a device which operates in response to the over fluxing of a machine.
Example:
Transformer Over-fluxing or generator over-fluxing problem
Short Description:
Over excitation or over fluxing protection depends on ration of V/f (V= Voltage, & f = Frequency). So when Voltage increases or frequency decreases over excitation or over fluxing introduce.
During a power system emergency disturbance, for instance, a load rejection or load shedding, the transformers may be subjected to overvoltage or under frequency resulting in a condition usually called over excitation, also known as over fluxing.
E= 4.44*f*N*a*B
Where E is the sinusoidal root mean square voltage of the winding, f is the frequency in hertz, N is the number of turns of wire, a is the cross-sectional area of the core and B is the peak magnetic flux density in tesla. The value 4.44 collects a number of constants required by the system of units.
So from EMF equation it is clear that B is inversely proportional to frequency.
So in short when rated flux density in the transformer core exceeding the maximum allowed limit set by the equipment designer.
So term Over-fluxing is more appropriate than Over-excitation.
Now consider a generator, sudden full load rejection. So there will be Power frequency high voltage at nominal frequency. However this condition would not be sustained for prolong time. So there will not be any effect other than a problem related to transformer differential protection stability.
Consider, a generator in run up condition. AVR is in service and if field excitation is applied to early, then over voltage condition would be sustain and resulting in to over fluxing.
Or excitation still applied even though generator rundown.
It is important that VT reference for AVR and Over fluxing protection must be different.
25) – Synchronizing or Synchronism Check Device:
It is a device which checks the synchronism condition before paralleling two A.C. devices. It will operate on when frequency, phase angle & voltage of these two A.C. Circuits will be equal, and it will permit to close the closing circuit.
Example: Paralleling of Two generators or connecting a generator with bus.
It ensures when switching a line onto a Bus bar, that the stability of system is not endangered.
26) – Apparatus Thermal Device:
It is a device that functions when the temperature of the shunt field or the amortiseur winding of a machine or that of load limiting or load shifting resistor or of a liquid or other medium, exceeds a predetermined value; or if the temperature of the protected apparatus, such as a power rectifier, or of any medium decrease below a predetermined value.
27) – Under voltage Relay:
It is relay which functions on a given value of under voltage.
When a load is connected suddenly to a source, the load will start to draw a current causing voltage drop. This may lead to under voltage condition. And if it will continue in the absence of UV protection, it may lead to system shutdown, or it may damage the equipment.
For some equipments, UV protection is used for tripping purpose and for some equipment UV protection used for interlock &/or annunciation purpose.
Like in Motor, which is drawing high current from source, motor can get tripped from thermal overload relay in the sense of overload setting, but if source voltage is getting more dip, it will get trip for UV protection.
In generator, tripping for UV protection rarely applied, however it may be used for interlock purpose for another protection schemes, like field failure protection or inadvertent energisation protection or other cases where the abnormality is directly leads / connected to under voltage condition.
In transmission system, in the event of insufficient reactive power generation, which is necessary to maintain system profile, UV condition may arise. Such condition must be removed, in order to avoid possible condition of system voltage collapse. But in such condition one should not trip generation. Generally UV occurs due to overloading or AVR failure or any other reason.
Even though if there is requirement of UV protection trip, it should be comprise with time delay, in order to avoid mal-operation or clearance of another cause behind UV.
27P: Phase under voltage Protection. UV relay will operates for the UV in phase only.
27N: Neutral/Ground Under voltage protection: UV conditions in ground only.
27A: Auxiliary under Voltage: UV conditions arises in Auxiliary circuits, or the circuits used
28) – Flame Detector:
It is a device that monitors the presence of pilot or main flame of such apparatus as a gas turbine or steam boiler.
29) – Isolating contactor: It is a device that used expressly for disconnecting one circuit from another for the purpose of emergency operation, maintenance or test.
30) – Annunciator Relay: It is a device which gives a number of separate visual indication of the function of protective devices.
Still Working on ANSI devices Description, Will publish it soon as soon as I will complete it.
Still Working on ANSI devices Description, Will publish it soon as soon as I will complete it.
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