Current Transformer Requirements; Current Transformer Location; Cts Overlapping The Circuit Breaker - Siemens 7SG12 DAD-N Manual

4. Current Transformer Requirements

The CTs used in a High-Impedance Differential scheme must follow some simple rules:
•
They must be of the high-accuracy type in accordance with Class 'PX' to IEC 60044.
•
They must have the same turns ratio.
•
The knee point voltage of each CT should be at least 2 x Vs.
The knee point voltage is expressed as the voltage applied to the secondary circuit with the
primary open circuit which when increased by 10% causes the magnetising current to increase by
50%.
It is permissible for the CTs to have different magnetizing characteristics since under load (or through-
fault) conditions the secondary currents will substantially balance. For an in-zone fault, there will be high
levels of secondary current and so any unbalance in the magnetizing characteristics will actually aid
correct tripping.

5. Current Transformer Location

The mounting position of CTs for busbar protection varies according to the type of switchgear. In many
cases, the CTs are built into the Circuit Breakers. In others, they are separate devices located as close to
the Breakers as possible.

5.1 CTs overlapping the Circuit Breaker

This arrangement is common in outdoor bulk oil types. The performance of the protection is considered for
the faults F1 to F4.
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A fault at F1 is a busbar fault which should be cleared by the
busbar protection.
Fault F2 is a circuit fault and should be cleared by the circuit
protection.
F3 is a busbar fault but because of its position should cause
both busbar and circuit protection to operate and the fault will
be cleared, although the circuit breaker at the remote end of the
circuit may also be tripped.
Although F4 is a circuit fault, it may be detected by both circuit
and busbar protection depending on their relative operating
times. Thus, circuit breakers selected to the busbar may be
opened unnecessarily for a circuit fault.
For F3 and F4, the disadvantage may be acceptable in view of
the low incidence of such faults.
Figure 9 – CTs Overlapping the CB
7SG12 DAD N Applications Guide
Chapter 5 Page 9 of 16