ABB Relion REQ650, Руководство по применению

Страница: 129 / 340

When should the sensitive directional residual overcurrent protection be used and
when should the sensitive directional residual power protection be used? Consider the
following facts:
• Sensitive directional residual overcurrent protection gives possibility for better
sensitivity. The setting possibilities of this function are down to 0.25 % of IBase,
1 A or 5 A. This sensitivity is in most cases sufficient in high impedance network
applications, if the measuring CT ratio is not too high.
• Sensitive directional residual power protection gives possibility to use inverse
time characteristics. This is applicable in large high impedance earthed networks,
with large capacitive earth-fault current
• In some power systems a medium size neutral point resistor is used, for example,
in low impedance earthed system. Such a resistor will give a resistive earth-fault
current component of about 200 - 400 A at a zero resistive phase-to-earth fault. In
such a system the directional residual power protection gives better possibilities
for selectivity enabled by inverse time power characteristics.
Phase
currents
Phase-
ground
voltages
IN
UN
IEC13000013-1-en.vsd
IEC13000013 V1 EN
Figure 61: Connection of SDEPSDE to analog preprocessing function block
Over current functionality uses true 3I0, i.e. sum of GRPxL1, GRPxL2 and GRPxL3.
For 3I0 to be calculated, connection is needed to all three phase inputs.
Directional and power functionality uses IN and UN. If a connection is made to
GRPxN this signal is used, else if connection is made to all inputs GRPxL1, GRPxL2
and GRPxL3 the sum of these inputs (3I0 and 3U0) will be used.
6.7.3 Setting guidelines
The sensitive earth-fault protection is intended to be used in high impedance earthed
systems, or in systems with resistive earthing where the neutral point resistor gives an
1MRK 505 291-UEN A
Section 6
Current protection
123
Application manual