When the
second tank and its associated CTs are added to the arrangement,
the power system conditions may have changed so that the tank
and CT will probably be different from the first tank and CTs that
were originally installed. Hence, percentage errors caused by
unbalance current may occur. To avoid this problem, a percentage
differential characteristic with restraint current (Ires) is applied to
the current balance relay. The characteristic of the current balance
relay is shown in Figure 4.