Siemens 7SR23 DAD, Manual

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Chapter 7) 7SR23 DAD Applications Guide
©2013 Siemens Protection Devices Limited Chapter 7 Page 8 of 42
The remaining current transformers maintain their ratio.
The resistance of the secondary winding of the saturated CT together with the leads connecting it to the
relay circuit terminals constitute the only burden in parallel with the relay.
The minimum required relay operate voltage setting (Vs) is given by:
T ) R (R I V
L CT F S
(1)
To ensure high speed relay operation the relay circuit operating voltage should be selected in accordance with the
stability requirement above (equation 1), also, the operate voltage should not exceed 0.5 x CT knee point voltage
(Vk).
2
V
V
K
S
(2)
2.1.1.2 Operation Requirement
For internal faults the relay will operate at the calculated ‘Voltage Setting’ Vs. This operating voltage will also be
applied across the CT secondary windings of all the CT secondaries connected in parallel with the relay. This
voltage will drive a magnetising current in each of the CT secondary windings and this must be added to the relay
operate current when calculating the operate current of the high impedance protection scheme.
In general:
/T I I I P.O.C.
MAG NLR S
(3)
2.1.1.3 Consideration of Component Thermal Ratings
When the relay circuit operates for an internal fault the circuit breakers are opened and the flow of fault current
ceases.
Where a CB fails to trip then fault current will flow in the high impedance circuit until the fault is cleared by the
operation of CB failure or back up protection. The thermal rating of the relay circuit components should then be
considered.
Alternatively the high impedance circuit can be arranged to short circuit the external components after operation.