ABB Relion 670 series, Техническое руководство

Страница: 153 / 1432

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. V is the voltage across the CT paralleling point (for example, across the measuring branch).
7. I is the current flowing through the measuring branch.
V and I are interrelated in accordance with the following formula V=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
M13075-9 v5
The logic diagram shows the operation principles for the 1Ph High impedance differential
protection function HZPDIF (87), see Figure 49 .
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. The voltage waveform is then filtered in order
to get its RMS value. Note that used filtering is designed in such a way that it ensures complete
removal of the DC current component which may be present in the primary fault current. The
voltage RMS value is then compared with set Alarm and Trip thresholds. Note that the
TRIP
signal
is intentionally delayed on drop off for 30 ms within the function. The measured RMS voltage is
available as a service value from the function. The function has block and trip block inputs
available as well.
1MRK 502 066-UUS B Section 6
Differential protection
147
Technical manual