Protection Relays made by Siemens
The distance protection relay with reactance method (RMD) function (ANSI 21/21 N):
Is a selective short-circuit protection for lines and cables supplied from one or more ends in radial,
looped, or meshed systems,
Is used as a backup protection for busbars, transformers, and other lines
Works only in systems with a grounded neutral point
Is suitable for use at all voltage levels
Under extreme conditions, load currents and high fault resistances can influence the selectivity. The distance protection relay with reactance method (RMD) function reduces the unfavorable influence of high fault resistances at high loads. If the electrical power system shows inhomogeneities, for example, different impedance angles of the infeeds, the reach of the distance protection can be affected.
The reactance method compensates this influence via adjustable compensation angles. Structure of the Function The RMD function can be placed in function groups that provide voltage and current measured values as well as line parameters.
Adaptive loop selection
Distance zone with quadrilateral characteristic curve
AREC zone (not preconfigured)
The RMD function is preconfigured with 4 zones. The zones are structured identically.
The RMD function monitors the phase current. If the phase currents exceed the set Min. phase-current thresh , the impedances of the 3 phase-to-phase loops and of the positive-sequence loop (phase-phasephase) are calculated. Ground-fault detection checks whether any ground fault has occurred. If a ground fault is detected, the impedances of the phase-to-ground loops are also calculated. The calculated impedance is assigned to the operate curve (quadrilateral characteristic curve) of the zones. For long lines with high loads, there is a risk that the positive-sequence impedance is reflected in the operate curve of the distance protection. For this case, a load cutout can be configured that eliminates unwanted pick-ups due to overload. For all picked up loops, the direction is then determined. The adaptive loop selection determines which loops are actually selected and which loops lie within the operate curves of the zones (loop pickup). All other loops are not considered by the adaptive loop selection. For the picked up loops, the time delay in the zone logic is started. The zone logic forms the pickup and trip signals of the loops and phases for the zone. The output logic processes the pickup and trip signals of the zones and forms the pickup and trip signals of the distance protection.
Ground-fault detection checks whether a ground fault has occurred. If a ground fault has occurred, the phase to-ground measurement loops are released.
Monitoring of the zero-sequence current 3I0
Monitoring of the zero-sequence voltage V0
Current and voltage criteria complement one another (see Figure below). If the ratio of zero-sequence impedance to positive-sequence impedance is greater, zero voltage increases. If the ratio of zero-sequence impedance to positive-sequence impedance is small, the ground current increases. If a phase current leads to current-transformer saturation, the voltage criterion must be met for ground-fault detection. Uneven saturation of current transformers can lead to a secondary zero-sequence current without a primary zero-sequence current flowing. Automatic scanning of the voltage criterion in the event of current transformer saturation prevents unwanted ground-fault detection. Ground-fault detection alone does not lead to the general pickup of the distance protection; it only controls other pickup modules. Ground-fault detection is also not indicated on its own.
For each zone, an operate curve is defined in the R-X plane (quadrilateral characteristic curve). You can set the zones as basic or as advanced zone. The Advanced setting provides you additional parameters in the zone. This allows you to adapt the zone better to the specific conditions.
You can define for each zone whether the zone should work forwards, backwards, or non-directional. For directional zones, you define the direction in the R-X plane. When assigning the impedance phasors in the R-X plane, the direction is analyzed additionally.
Adaptive Loop Selection
The RMD function works with adaptive loop selection. Different loop-selection criteria are processed and
weighted at the same time.
The output logic of the RMD function links the output information of the zones. The output logic forms the
accumulative pickup indications and operate indications of the function.