Characteristics Of Measuring Elements - Toshiba GRL100-701B Instruction Manual

2.16 Characteristics of Measuring Elements

2.16.1 Segregated-phase Current Differential Element DIF and DIFSV
The segregated-phase current differential elements DIF have dual percentage restraint
characteristics. Figure 2.16.1.1 shows the characteristics on the differential current (Id) and
restraining current (Ir) plane. Id is a vector summation of the phase current of all terminals and Ir is
a scalar summation of the phase current of all terminals. In these summations, charging current is
eliminated from the phase currents by the charging current compensation function.
Figure 2.16.1.1 Segregated-phase Current Differential Element (I r -I d Plane)
Characteristic A of the DIF element is expressed by the following equation:
where DIFI1 is a setting and defines the minimum internal fault current.
This characteristic has weaker restraint and ensures sensitivity to low-level faults.
Characteristic B is expressed by the following equation:
where DIFI2 is a setting and its physical meaning is described later.
This characteristic has stronger restraint and prevents the element from operating falsely in
response to the erroneous differential current which is caused by saturation or transient errors of
the CT during an external fault. If the CT saturation occurs at the external fault in a small current
region of the characteristics and continues, the element may operate falsely caused by increasing
the erroneous differential current. The DIF prevents the false operation by enhancing the
restraining quantity for the DIF calculation, depending on the magnitude of restraining current in
the large current region characteristic B.
The figure shows how the operation sensitivity varies depending on the restraining current.
The same characteristic can be represented on the outflowing current (I out ) and infeeding current
(I in ) plane as shown in Figure 2.16.1.2.
I d
A
5/6 DIFI1
Small current region
0
− 2 × DIFI2
I d ≥ (1/6)I r + (5/6)DIFI1
I d ≥ I r - 2 × DIFI2
 132 
Operating
Zone
B
Large current region
I r
6 F 2 S 0 8 5 0