Instantaneous Oc/Ef (50/50G/50N); Time Delayed Oc/Ef (51/51G/51N); Selection Of Over-Current Characteristics - Siemens 7SR242 Duobias Technical Manual

2.2

Instantaneous OC/EF (50/50G/50N)

Instantaneous overcurrent can be applied to protect the HV terminals against high fault currents. The current
setting applied must be above the maximum 3-phase through fault level of the transformer to ensure grading with
the LV overcurrent protection.
Where the setting applied is below the magnetising inrush current of the transformer then inrush blocking
(81HBL2) should be enabled.
A sensitive instantaneous HV earth fault setting can be considered where the transformer winding connections do
not allow the through flow of zero sequence current. Time delayed (DTL) operation can also be considered where
the transformer switch on magnetising inrush current may cause mal-operation. This will be identified during
commissioning. An increased operate current or high impedance operation can also be considered.
2.3

Time Delayed OC/EF (51/51G/51N)

The time delayed element can provide a number of shaped characteristics. The selectable Inverse definite
minimum time lag (IDMTL) and Definite Time Lag (DTL) characteristics provide protection for phase and earth
faults.
As these protections are used as back-up protections discrete relays are often installed. To reduce cost and
complexity it may be considered acceptable to implement the backup protection using elements within the main
protection relay. The relay is self supervised and this can be used as justification for allowing the backup
protection to be included as part of the main differential protection relay.
The following elements can be included:
Three phase over current with IDMTL (IEC or ANSI) or DTL operate characteristic (51)
Derived earth fault with IDMTL (IEC or ANSI) or DTL operate characteristic (51N)
Measured earth fault with IDMTL (IEC or ANSI) or DTL operate characteristic (51G)
Each of the above elements can be selected to winding 1 or winding 2 CT inputs.
The time delayed characteristics are used to provide grading with other relays or fuses.
Earth fault elements can be used to provide system protection or standby earth fault protection of an earthing
resistor.

2.3.1 Selection of Over-current Characteristics

Where the relay operate time must be co-ordinated with other time delayed relays on the system, the operating
characteristic is selected to be the same type as the other relays. Often a normally inverse (NI) characteristic is
applied, however extremely inverse curves (EI) can provide improved grading with fuses or moulded case circuit
breakers.
To optimise the grading capability of the relay additional time multiplier, 'Follower DTL' (Figure 2-14) or 'Minimum
Operate Time' (Figure 2-15) settings can be applied.
©2015 Siemens Protection Devices Limited Chapter 7 Page 25 of 64