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.
Each part is described separately in the following sections.
6.4.7.6 Four residual overcurrent steps
Each overcurrent step uses operating quantity Iop (residual current) as 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. It shall be noted that
the directional decision (Forward/Reverse) is not made within each residual
overcurrent step itself. The direction of the fault is determined in a directional
element common for all steps.
• Residual current start value.
• Type of operating characteristic. By this parameter setting it is possible to select
inverse or definitive time delay for step 1 and 4 separately. Step 2 and 3 are always
definite time delayed. All of the standard IEC and ANSI inverse characteristics
are available. For the complete list of available inverse curves please refer to
section
"Inverse time characteristics".
• Time delay related settings. By these parameter settings the properties like
definite time delay, minimum operating time for inverse curves and reset time
delay 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.
Simplified logic diagram for one residual overcurrent step is shown in figure
37.
1MRK 511 287-UEN A Section 6
Current protection
103
Technical manual
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