ABB Relion REG670, Technical Manual

Page: 150 / 1298

It is of utmost importance to insure that only one earthing point
exists in such protection scheme.
3. shows the setting (stabilizing) resistor RS.
4. shows the over-current measuring element.
The series connection of stabilizing resistor and over-current
element is designated as measuring branch.
5. shows the non-linear resistor (that is, metrosil).
6. U is the voltage across the CT paralleling point (for example, across the
measuring branch).
7. I is the current flowing through the measuring branch.
U and I are interrelated in accordance with the following formula
U=RS × I.
Due to the parallel CT connections the high impedance differential relay can only
measure one current and that is the relay operating quantity. That means that there is
no any stabilizing quantity (that is, bias) in high-impedance differential protection
schemes. Therefore in order to guaranty the stability of the differential relay during
external faults the operating quantity must not exceed the set pickup value. Thus, for
external faults, even with severe saturation of some of the current transformers, the
voltage across the measuring branch shall not rise above the relay set pickup value. To
achieve that a suitable value for setting resistor RS is selected in such a way that the
saturated CT secondary winding provides a much lower impedance path for the false
differential current than the measuring branch. In case of an external fault causing
current transformer saturation, the non-saturated current transformers drive most of
the spill differential current through the secondary winding of the saturated current
transformer and not through the measuring brunch of the relay. The voltage drop
across the saturated current transformer secondary winding appears also across the
measuring brunch, however it will typically be relatively small. Therefore, the pick-
up value of the relay has to be set above this false operating voltage.
See the application manual for operating voltage and sensitivity calculation.
6.2.7.1 Logic diagram
The logic diagram shows the operation principles for the 1Ph High impedance
differential protection function HZPDIF, see Figure
46 .
The function utilizes the raw samples from the single phase current input connected to
it. Thus the twenty samples per fundamental power system cycle are available to the
HZPDIF function. These current samples are first multiplied with the set value for the
used stabilizing resistor in order to get voltage waveform across the measuring branch.
Section 6
1MRK502052-UEN B
Differential protection
144
Technical manual