For the W2 side:
Zero sequence removal is calculated for the W2 side, if the W2con value is set to yn or zn.
A zero sequence current can only flow:
1. If the vector group is odd;
2. If the neutral is connected to ground on the W2 side; and
3. The grid on the W2 side is grounded as well
After setting the values for the percentage restrained characteristic curve, the settings for harmonic and transient
restraining have to be defined. Both the harmonic and transient restraining settings depend on many parameters:
• Transformer type;
• Transformer material;
• Operational parameter of the grid; and
• Time of energizing relative to the sinusoidal phase.
Therefore it is very difficult to give “one for all” settings in this area and to find a compromise between making a
differential relay extremely fast and extremely reliable in its trip decisions.
Beginning with the static characteristic curve, typical slopes of 25% and 50% for both sections are recommended.
They will be obtained by the settings
»Id(IS0)«
,
»Id(IS1)«
,
»Id(IS2)«.
In case of harmonic or transient restraint, the curve will be added by a static offset d(H,m)
To be able to withstand magnetizing inrush currents of typical values, the setting »d(H,m)« = 8 is recommended.
In case that harmonic restraint threshold is reached, this value will be added to the characteristic curve.
It is important to estimate the necessary harmonic threshold to obtain stability against magnetizing inrush, CT
saturation, and over-excitation. The harmonics seen under different operational conditions like magnetizing inrush
and CT saturation depend on many different parameters.
Magnetizing inrush:
Basically, harmonics can be observed and monitored. Due to this fact, the 2
nd
and 4
th
harmonic are monitored.
Inrush currents depend on the time of energizing, the remnant magnetizing compared to phase of sinusoidal curve,
the voltage (low voltage energizing produce less harmonic), the core material and the core geometry among others.
It is recommended generally to set the harmonic restraint as active. This is done (for each harmonic separately) by
setting the respective parameter »Stab H2«, »Stab H4«, »Stab H5« to “active”.
To operate very stably under stationary circumstances, it can be distinguished between a stationary value of
harmonic thresholds and a transient harmonic threshold directly after energizing. This transient period is always
started if the differential as well the restraining current is below 5% of the base current
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