Distance Relay

[kookypedia]

Guys,
Distance relay can only detect LG or LLG faults and it can not detect LL faults right?

 

[davidbeach]

Phase distance relay responds to 3LG, 2LG, and LL faults. Ground distance relay responds to SLG faults.

 

[submonkey]

Incorrect - What gave you that idea?
Full scheme distance relays can see
faults in all phase combinations.

 

[Mbrooke]

They can detect LL faults, though some choose an MHO for phase-phase faults and quads for L-G faults. Don't forget the line's K factor when making the settings.

 

[HamburgerHelper]

Phase distance relay can see any fault. It acts on positive and negative sequence components so it can pick up on a slg fault. It should be a lot less sensitive than the ground distance relay for normal k values for ground faults.

a = 1 <120 deg

VA = V0 + V1 + V2

IA = I0 + I1 + I2

VC = V0 + a*V1 +a^2*V2

IC = I0 + a*IC + a^2*I2

VAC = VA - VC = V1-a*V1 + V2 - a^2*V2 >>>>> Line to line quantities have no zero sequence dependency

IAC = IA - IC = I1 - a*I1 + I2 - a^2*I2

ZAC = VAC/IAC

Ground distance relays can see all faults but its sensitivity to ground faults is dependent on the k value.

VA = (V0 + V1 + V2), IA = (I0 + I1 + I2)

ZA = VA/(IA + 3*k*I0) = (V0 + V1 + V2) / (I0 + I1 + I2 +3*k*I0) = (V0 + V1 + V2)/ (I1 + I2 + (3*K+1)*I0)


I think though ground distance relay will get the directionality wrong if it has negative sequence directionality and picks up for a phase to phase fault and vice versa. One of the GE relays I think the L90 has a note about DLG fault potentially causing the relay to the get the directionality wrong but I could never figure out from the manual why for sure that was.

 

[davidbeach]

The SEL relay use a fault type selection logic to block the phase distance elements from responding to SLG faults that fall within the characteristic and to keep the ground distance elements from responding to faults that involve more than one phase. Faults can show up within the characteristic that don't cause the element to operate; prevents confusion when you expect a particular response to a particular fault.