Handbook of Electrical Installation Practice - BeKnowledge

Fig. 22.7 Grading between two relays.
Protective Systems 607
High-set relays
If high-set relays are used they must be set so that they can be included in the
co-ordination of the relays. Setting of the high-set relays must take account of the
stability requirements for a fault at the next relay point with due allowance made
for the 50% overreach of simple electromechanical high-set elements. If twice
the setting requirement for operate current is also introduced, it is often difficult to
achieve viable settings for the high-set elements.
Having selected the high-set settings, the operation point of the relays for grading
is obtained using the lowest value of current multiple as determined by the highset
setting or the fault current. If the value is less than twice the current setting the
relay current is too low to ensure operation and a change in CT rating should be
considered.
The high-set setting of relay 1 or the fault level at relay 1 is then chosen to
determine the grading on the basis of which the lowest multiple of the plug setting
at relay 2 is obtained. With the lowest multiple of plug setting for relay 2 given by
the high-set setting of relay 1 or a fault at relay 1, the time multiplier or relay 2 is
determined. If a time multiplier setting of greater than 1 is called for, the plug
setting must be increased.
Grading by computer
The grading of relays is a relatively simple task which can easily be done by a computer;
however, the problem of collecting data and authenticating the data remains.