ABB CWC, Instruction Leaflet

Page: 9 / 16

Type CWC and CWP Directional Ground Relay 41-242.2F
9
possible for ground faults on the transmission lines
near the breaker. The product available for the relay
in these cases should be large enough to represent a
large multiple of the tap product value so the operat-
ing times can be in range of 0.05 to 0.20 second as
seen from the curves of Figs. 9 and 11.
However, the relays cannot distinguish between a
fault on the line near the remote breaker for which
they should operate, and a similar fault on the bus or
adjacent line for which they should not operate until
the bus differential or adjacent line relays have had
an opportunity to operate and clear the fault. This
requires an increased time setting of the relay for
faults near the remote terminal. The product avail-
able for the relay in these cases will be smaller than
that for the close faults and should represent a
smaller multiple of the tap product previously chosen
so the operating time can be from .4 to .75 second
longer than the remote line or bus relay operating
time. This .4 to .75 second interval is known as the
coordinating time interval. It includes the circuit
breaker operating time plus a factor allowing for dif-
ference between actual fault currents and calculated
values, differences in individual relay performance,
etc. For 8 cycle breakers the value of .4 second is
commonly used, while for 30 cycle breakers,.75 sec-
ond is used.
As an example, a type CWC relay is to be connected
at Station A and set to protect the line running to Sta-
tion B. It must select or coordinate with the type CWP
relay connected at Station B and set to protect the
line running to Station C. The fault current and volt-
age for single line-to-ground faults under minimum
conditions for this system are shown in Fig. 12.
In setting the type CWC and CWP relays, it is conve-
nient to set up Table as shown.
The relay location is shown in Column 1 and the fault
location in Column 2. The primary line residual cur-
rent available for the relay is recorded in Column 3.
The ratio of these current transformers is specified in
Column 4.
The primary fault current or voltage available for the
polarizing winding is shown in Column 5, and the
associated current or potential transformer ratio in
Column 6. All of these fault values are residual val-
ues or three times the zero sequence current of volt-
age. The relay current for the lower pole windings is