Numerical line differential protection with 2 pilot wires (178 pages)
Summary of Contents for Siemens 7PG2114
Page 1
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Section 1: Pilot Wire Current Differential Scheme 1.1 General The 7PG2113 and 7PG2114 relays provide both pilot wire current differential protection and overcurrent and earth fault protection as well as other additional functions. The 7PG2115 and 7PG2116 additionally provide directional functionality for the overcurrent and earth fault protection.
The calculation of the value of the Stability Resistor is based on the worst case where one CT fully saturates and the other balancing CT does not saturate at all. A separate Siemens Protection Devices Limited Publication is available covering the calculation procedure for REF protection. To summarise this: The relay Stability (operating) Vs voltage is calculated using worst case lead burden to avoid relay operation for through-fault conditions where one of the CTs may be fully saturated.
A 3-phase CT failure is considered so unlikely (these being independent units) that this condition is not tested for. Current Transformer Supervision (60CTS – 7PG2114/6) When a CT fails, the current levels seen by the protection become unbalanced. A large level of NPS current is therefore detected - around 0.3 x In for one or two CT failures.