User's Manual

20
SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System Instruction Manual Date Code 20170317
System Installation
Follow the guidelines in the following table to ensure the right pickup levels
while avoiding false pickups.
This selection method guarantees that the SEL-FT50 picks up
for any fault
that the protective device element can see. This setting method will perform
well in locations where fault current is much higher than load current.
The protection planner must consider the load current range on the
distribution system at each node, combined with the SEL-FT50 accuracy
rating at currents below the fault pickup. High load current levels may result in
false assertion of the SEL-FT50 fault detector. In most cases, this is acceptable
because the supervising overcurrent element will not assert. However, the
more frequent a load condition is mistaken for a fault, and the longer each
instance persists, the higher the chance of the SEL-FT50 missing an actual
fault. To avoid this situation, consider selecting the SEL-FT50 Fault Pickup
setting to be one step higher.
Field Installation
Wireless Considerations
Radio Path. Spread-spectrum radios operating in the 900-MHz band are
limited by line-of-sight. Obstructions in the line-of-sight will impact the
performance of the radio, because the strongest radio signal is communicated
directly along the radio line-of-sight. The line-of-sight between two antennas
is shaped like an ellipse (called the Fresnel zone). The point exactly halfway
between the two antennas is the widest part of the ellipse, as shown in
Figure 16. At 900 MHz and 304.8 m (1000 ft) apart, the Fresnel zone is 4.9 m
(16 ft) in diameter. At 32 km (20 mi) apart, it is 51.8 m (170 ft) in diameter.
Anything within the Fresnel zone will obstruct and reduce signal strength and
availability (the ground, buildings, vegetation, etc.). Table 9 depicts the
maximum Fresnel zone diameter and path loss for some typical path
distances.
Table 8 Fault Pickup Accuracy Considerations
Fault Pickup Level
Host Relay Pickup
Setting
Load Behind FT50
50A > 75A < 25A
100A > 130A < 70A
200A > 260A < 140A
400A > 480A < 320A
600A > 720A < 480A
800A > 960A < 640A
1000A > 1200A < 800A
1200A > 1440A < 960A
NOTE: The SEL-FT50 current
measurement circuitry is not as
precise as a protective relay
overcurrent element. Refer to
Specifications on page 25 for fault
pickup accuracy, response
characteristics, and device ratings.