Protection Functions
136 PRS-7367
Once initiation logic is fulfilled and no blocking signal is activated, the 51NA_Str signal is set.
The timer model is determined by IDMT curves for over quantity protection.
The operation is activated after the operation timer has reached the calculated value. However,
51NA_Min_Op_T defines the minimum desired operate time for IDMT. If a drop-off situation
happens, that is, a fault suddenly disappears before the operate delay is exceeded, the operation
will reset after the set value 51NA_Reset_T is exceeded.
NOTICE!
The 51NA_Min_Op_T setting should be used with great care because the operation
time is according to the IDMT curve, but always at least the value of the
51NA_Min_Op_T setting.
The binary input 51NA_Blk can be used to block the function. The activation of the 51NA_Blk input
deactivates all outputs and resets internal timers. The binary input 51NA_Blk_Str can be used to
block the start signals. The binary input 51NA_Blk_Op can be used to block the operation signals.
3.24.3 51NA Application Scope
51NA is designed for the protection against internal faults in Double-Y-connected capacitor banks.
This unbalance protection detects an asymmetry in the capacitor bank caused by blown fuses or
short circuits across bushings or between capacitor units and the racks in which they are mounted.
Normally, the capacitor units are designed to withstand 110 percent of the nominal voltage
continuously. When an element inside a capacitor bank fails, the remaining healthier elements
experience an increase in voltage across them. If the voltage exceeds the 110 percent value of the
nominal voltage, it can lead to a failure of the healthier elements of the bank and in turn fail the
entire capacitor bank.
3.24.4 51NA Settings
In the following table, i is the protection stage number, it can be set according to the requirements.
Table 3.24-3 51NA settings
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