Answer / vel

differential relay works on the principle of kirchoff
current law. i.e incoming current = outgoing current if it
is not equal relay will give trip signal.it is used for
transformer and generator protection.

distance relay works on the principle of z=v/i. when z
value of the line fall below the set value relay will give
trip signal. it is used for tranmission line.

Answer / mohanraj

Differential protection is a unit protection, relay works
on the principle of Kirchoff's current Law.
Measuring element is Current Transformer.
The differential current measured between the incoming
current and Outgoing current must be negligible current
during stable and through fault condition. In case of
inzone fault or unstable condition (due to CT saturate)
relay will sense the differential current and issue the
trip signal.
Application: Transformer, Generator and Cable Protection.
**********************************************************
Distance protection is widely used in the transmission
network, its also called impedance protection because relay
operates with respect to fault impedance of the
transmission line. (Z=V/I)
Measuring element is Current Transformer and Voltage
Transformer.
Usually relay having 4 zones of the transmission line.
Zone1,Zone2,Zone3 will be forward zones (Towards the Line)
and zone 4 will be Reverse zone (Towards the Source).

Answer / jacob

A differential protection compares the currents flowing
into and
out from the transformer. Auxiliary current
transformer “aux.ct:s”,
for adjusting the phase angle and ratio are necessary.
Ratio correction
is normally calculated for tap changer at middle position.

Distance protection relays are the most common relays on
transmission
lines. The reason for this is the simple measuring
principle,
the built-in back-up and the low requirement on
communication with remote end.
The Distance protection relay is a directional
underimpedance
relay. Normally two to four measuring zones are available.
Basically the Distance protection relays measures the
quotient
“U/I”, considering also the phase angle between the
voltage “U”
and the current “I”. The measured “U/I” is then compared
with the
set value. The relay will trip when the measured value is
less than
the value set

Answer / raju t

Differential Protection:
1. It simultaneously compares the phaser difference & magnitude of the current entering & leaving the protected zone.
2. It works on kirchoff's current law
3. It is a unit protection scheme i.e. it needs current inputs from minimum two different sources.
4. Differential current = |I1+I2| --> Vector sum
5. It cannot be used to provide backup to other relays
6. %Slope = (Id2-Id1)/(Ib2-Ib1)

Distance Protection:
1. It calculates the apparent impedance of a line with the help of voltage & current input connected to the relay. If measured impedance falls below set impedance trip command is issued to clear the fault.
2. It works on impedance measuring principle
3. It is a non-unit protection but can be easily used as an unit protection by co-ordinating distance protection with PLCC
4. Z=V/I
5. It can be set to provide backup to adjoining lines & bus bar depending on the requirement.

Answer / gopal1986

define thermal overload protection and how to calculate tripping time in ABB relays.

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