4.46
SEL-700G Relay
Instruction Manual
Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
Field Ground Protection Elements
The SEL-700G works with the SEL-2664 Field Ground Module to protect the
generator field winding. Connect the SEL-2664 directly to two ends of the
generator field winding and the rotor ground brush. When the SEL-2664
calculates the insulation resistance value between the field winding and
ground, it uses a fiber-optic cable with ST connectors and a transceiver
(SEL-2812MR or MT) to transmit the insulation resistance value to the
SEL-700G. Consult the SEL-2664 Instruction Manual for detailed instructions
on setting up the SEL-2664.
Set the EIA-232 port in the SEL-700G to SEL protocol and change the port
settings to 9600 bps, 8 data bits, no parity, and 1 stop bit to start receiving the
insulation resistance value from the SEL-2664.
The SEL-700G compares the insulation resistance value to the pickup settings
(64F1P and 64F2P) of the field ground protection elements (64F1T and
64F2T, see Figure 4.23). Set two different pickup levels to alarm and/or trip
when the insulation resistance value causes the elements (64F1T or 64F2T) to
assert.
If there is no insulation deterioration, there is no leakage path between the
field winding to ground; the insulation resistance value is extremely high. In
this situation, however, because of sensitivity limits, the SEL-2664 calculates
a very large insulation resistance value of 20 megohms. As soon as the field
winding insulation develops a breakdown to the rotor iron (assuming that the
generator rotor iron is connected to ground through a grounding brush), the
SEL-2664 detects a sharp drop in insulation resistance.
The MET command response includes the value of the insulation resistance
Rf in kilohms when the element is enabled and functional. The STA command
response includes the status of the field ground module and the associated
communications link.
The technology the SEL-2664 uses does not discriminate between one point
of insulation breakdown and multiple points of insulation breakdown. A
single point of insulation breakdown does not harm the generator. Multiple
points of insulation breakdown could lead to serious generator damages
because the distribution of magnetic flux in the rotor is substantially altered.
When a different device (such as a generator vibration detector) is used to
detect multiple points of insulation breakdown, SEL recommends using the
SEL-2664 to generate an alarm only and using the other device under
supervision of the SEL-2664 to trip. If you do not use an additional device, it
is a good idea to alarm and trip with the SEL-2664.
Figure 4.23
Field Ground Protection (64F) Elements Logic
4*
0
4*
0
64F1
64F1T
64F2
64F2T
Rf
64F1C
Relay
Word
Bits
64FTC
SELOGIC
Setting
64FFLT
Relay
Word
Bits
64F2P
Setting
64F1P
Setting
64F1D
0
64F2D
0
*Delay of 4 Samples
64F2C
Rf = Field ground insulation resistance in kilohms
Summary of Contents for SEL-700G Series
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