GE P642, Technical Manual

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5 APPLICATION NOTES
5.1 SETTING GUIDELINES
The differential setting, Configuration/Diff Protection, should be set to
Enable
.
The basic pick up level of the low set differential element, Is1, is variable between
0.1 pu
and
2.5 pu
in
0.01
pu
steps. The setting will be dependant on the item of plant being protected and by the amount of differential
current that might be seen during normal operating conditions. When the device is used to protect a transformer,
we recommend a setting of
0.2 pu
.
When protecting generators and other items of plant, where shunt magnetizing current is not present, a lower
differential setting would be more typical. We recommend
0.1 pu
.
The P64x percentage bias calculation is performed 8 times per cycle. A triple slope percentage bias characteristic
is implemented. Both the flat and the lower slope provide sensitivity for internal faults. Under normal operation
steady state magnetizing current and the use of tap changers result in unbalanced conditions and hence
differential current. To accommodate these conditions the initial slope, K1, may be set to 30%. This ensures
sensitivity to faults while allowing for mismatch when the power transformer is at the limit of its tap range and CT
ratio errors. At currents above rated, extra errors may be gradually introduced as a result of CT saturation, Hence,
the higher slope may be set to 80% to provide stability under through fault conditions, during which there may be
transient differential currents due to saturation effect of the CTs. The through fault current, in all but ring bus or
mesh fed transformers, is given by the inverse of the per unit reactance of the transformer. For most transformers,
the reactance varies between 0.05 to 0.2 pu, therefore typical through fault current is given by 5 to 20 In.
The wide matching factor range is provided to allow the designer to trade off between the CT selection and the
scheme sensitivity. This is useful for applications such as busbar protection where a wide range of CT ratios may
be encountered. You should also note that the matching factor check should be carried out for all ends. One end
alone is not sufficient. The maximum sensitivity achieved in this product depends on the type of analog input and
is given in the CT requirements.
Note:
Differential protection alone may not achieve the full sensitivity required, and other protection functions such as REF may
have to be incorporated in conjunction with the differential protection.
The number of biased differential inputs required for an application depends on the transformer and its primary
connections. We recommend, where possible, that a set of biased CT inputs is used for each set of current
transformers. According to IEEE C37.110-2007, separate current inputs should be used for each power source to
the transformer. If the secondary windings of the current transformers from two or more supply breakers are
connected in parallel, under heavy through fault conditions, differential current resulting from the different
magnetizing characteristics of the current transformers flows in the IED. This current only flows through one
current input in the device and can cause maloperation. If each CT is connected to a separate current input, the
total fault current in each breaker provides restraint. You should only connect CT secondary windings in parallel
when both circuits are outgoing loads. In this condition, the maximum through fault level is restricted solely by the
power transformer impedance.
The P64x IED achieves stability for through faults in two ways, both of which are essential for correct relay
operation. The first consideration is the correct sizing of the current transformers. The second is by providing a bias
characteristic as shown below:
P64x Chapter 6 - Transformer Differential Protection
P64x-TM-EN-1.3 121