Filter
Top Products
⎯ 12 ⎯
6 F 2 S 0 8 5 7
2.2 Current Differential Protection
2.2.1 Differential Scheme
Current differential protection DIFT provides an overall transformer protection deriving phase
current from each transformer winding, calculating the differential current on a per phase basis
and detecting phase-to-phase and phase-to-earth faults.
The current differential protection is based on Kirchhoff’s first law that the vector summation of
all currents flowing into a protected zone must be zero. Figure 2.2.1.1 shows the principle of
current differential protection. Differential current (id) is the vector summation of all terminal
current of the transformer. The differential current (id=i1+i2) is zero because the current (i1)
equals current (−i2) during a load condition or an external fault. During an internal fault, the
differential current (id) is not zero because the current (i1) does not equal to the current (−i2), and
the DIFT operates.
DIFT
I1
I2
i1
i2
id=i1+i2
Differential current
detection
Transformer
Primary
Secondary
Figure 2.2.1.1 Current Differential Protection
Scheme logic
Figure 2.2.1.2 shows the scheme logic of the current differential protection. Current differential
element DIFT comprises sub-elements HOC, DIF, 2F and 5F which operate for differential
current on a per phase basis.
Note: For the symbols used in the scheme logic, see Appendix M.
HOC is a high-set overcurrent element operating for differential current. It provides high-speed
protection for heavy internal faults.
DIF is a percentage restraining element and has dual restraining characteristics, a weak restraint in
the small current region and a strong restraint in the large current region, to cope with erroneous
differential current which may be caused due to output imbalance of the CTs in case of an external
fault. (For the characteristics, see Section 2.10.)
The DIF output signal can be blocked when the 2F or 5F elements detect second harmonic inrush
current during transformer energization or fifth harmonic components during transformer
overexcitation. Blocking is enabled by setting scheme switch [2F-LOCK] or [5F-LOCK] to “ON”.
The following two or three blocking schemes are selectable by scheme switch [DIFTPMD].
“3POR”: When any one phase of the 2F or 5F element operates, tripping by the DIF
element is blocked in all 3 phases. “3POR” is recommended for transformers with
large capacity whose second harmonic component may be low. Its blocking
function is stronger than that of the “1P” or “2PAND” below.
“1P”: When any phase of the 2F or 5F elements operate, only the corresponding phase
output of the DIF element is blocked.
“2PAND”: Even if 2F or 5F element operates during manetising inrush, the trip by DIF
element is allowed when any two phases or more of DIF element operate.