Siemens Ohmega 406 7SG16 Manual page 3

Distance protection relays

1 Loss of Load Protection

The Loss of Load protection scheme in the relay is used to give faster fault clearance time for an end zone fault

(i.e between the Zone 1 boundary and the line end) when there is no signalling channel available. This allows a

faster clearance time than waiting for the operation of the zone 2 elements. The logic diagram for this scheme is

shown below;

For a Loss Of Load trip to occur:

There must be a loss of load current in at least one phase.

LOLUCA

LOLUCB

LOLUCC

Fault current must continue to flow in at least one other phase for at least 20ms after the

loss of load, to allow time for all the poles of the breaker at the start of the next line to open.

LOLUCA

LOLUCB

LOLUCC

LOL

The Zone 2 protection must pickup within 40ms of the loss of load.

LOL

Z2G

Z1D

Z2D

Z3D

Consider a fault occurring near to the remote end, i.e outside of the Zone 1 reach, but within the line length. In a

normal time stepped distance scheme, the remote end relay would trip in Zone 1 time, and the local end relay

would trip after the Zone 2 time delay. The Loss of Load scheme monitors the current in the healthy phases and

can remove the Zone 2 time delay, speeding up the local end trip, when the remote end trip occurs.

If the relay detects a drop in current in one or two phases below the Loss of Load Level, with the current on the

remaining phases above this level, it will remove the time delay from zone 2, for a fixed time delay (the LOL

Time Limit) to allow the relay to trip instantaneously. This will allow the relay to trip more quickly for single or

double phase faults, but will not affect operation for three phase faults.

A short time delay (typically 20ms), known as the Loss of Load Pole Scatter Delay or the LOL CB Op Delay is

introduced to prevent the Loss of Load feature picking up during normal breaker operation.

PoleScatterDelay

20 / 0

OperateWindow

40 / 0

SOLOTRIP

7SG16 Ohmega 406 Loss of Load

_Latch1

=2k+1

LOL

AIDTRIP

Chapter 12 Page 3 of 4