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6.4.7.6 Internal earth-fault protection structure
M13941-157 v5
The protection is internally divided into the following parts:
1. Four residual overcurrent steps.
2. Directional supervision element for residual overcurrent steps with integrated
directional comparison step for communication based earth-fault protection
schemes (permissive or blocking).
3. Second harmonic blocking element with additional feature for sealed-in
blocking during switching of parallel transformers.
4. Switch on to fault feature with integrated Under-Time logic for detection of
breaker problems during breaker opening or closing sequence.
6.4.7.7 Four residual overcurrent steps
M13941-166 v8
Each overcurrent step uses operating quantity Iop (residual current) as the
measuring quantity. Each of the four residual overcurrent steps has the following
built-in facilities:
• Directional mode can be set to Off / Non-directional / Forward / Reverse . By this
parameter setting the directional mode of the step is selected.
• Residual current start value.
• Type of operating characteristic (inverse or definite time). By this parameter
setting it is possible to select inverse or definite time delay for the earth-fault
protection. Most of the standard IEC and ANSI inverse characteristics are
available. For the complete list of available inverse curves, please refer to
section
"Inverse characteristics" .
• Type of reset characteristic ( Instantaneous / IEC Reset / ANSI Reset ). By this
parameter setting it is possible to select the reset characteristic of the step. For
the complete list of available reset curves, please refer to section
"Inverse
characteristics" .
• Time delay related settings. By these parameter settings the properties like
definite time delay, minimum operating time for inverse curves, reset time
delay and parameters to define user programmable inverse curve are defined.
• Supervision by second harmonic blocking feature ( On / Off ). By this parameter
setting it is possible to prevent operation of the step if the second harmonic
content in the residual current exceeds the preset level.
• Multiplier for scaling of the set residual current pickup value by external
binary signal. By this parameter setting it is possible to increase residual
current pickup value when function binary input ENMULTx has logical value
1.
• The operation current value INx> , is limited to be between INx>Max and
INx>Min . The default values of the limits are the same as the setting limits for
INx>, and the limits can only be used for reducing the allowed range of INx>.
This feature is used when remote setting of the operation current value is
allowed, making it possible to ensure that the operation value used is
reasonable. If INx> is set outside INx>Max and INx>Min, the closest of the
limits to INx> is used by the function. If
INx>Max is smaller then INx>Min ,
the limits are swapped. The principle of the limitation is shown in Figure 42 .
1MRK 505 384-UEN A
Section 6
Current protection
Breaker protection REQ650 2.2 IEC 117
Technical manual