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10.6 PROTECTION SCHEMES
h) 2nd HARMONIC RESTRAINT
To measure the percentage of second harmonic required to block the operation of the harmonic-restraint differ-
ential elements, use the connection diagram shown below. Current is supplied as an operating current to the A-
phase element.
1. Close switch S1. Set the AC current, I
content above the 2nd harmonic restraint setting under:
SETPOINTS/S4 ELEMENTS/DIFFERENTIAL/HARMONIC INHIBIT/HARMONIC INHIBIT LEVEL
2. Calculate the percent second harmonic content from the equations below:
If the current is measured with average-responding/reading meters:
%2nd
=
b) if the current is measured with rms-responding/reading meters:
%2nd
=
3. Open and reclose S1. The relay should not operate.
4. Decrease I
DC
equations above. The calculated percent harmonic value should equal the relay setting.
i) 5th HARMONIC RESTRAINT
Verifying the operation of the 5th harmonic restraint requires test equipment capable of generating a current
signal containing a fundamental and 5th harmonic. Most modern dedicated relay test instruments, such as
Powertec's (or Manta) DFR, Doble, or Multiamp instruments are capable of generating appropriate signals. A
power operational amplifier with a suitably rated output, or a power audio amplifier, may also be used to gener-
ate the appropriate signal.
10
10-18
Figure 10–7: 2ND HARMONIC RESTRAINT TEST
AC
×
×
100 0.141
I
DC
--------------------------------------------- -
×
I
0.9 I
+
DC
AC
×
×
100 0.141
I
DC
--------------------------------------------- -
×
I
I
+
1.414
DC
AC
until the element operates. Calculate the percent of second harmonic at this point using the
745 Transformer Management Relay
to twice rated CT secondary current. Set I
10 COMMISSIONING
to obtain harmonic
DC
GE Power Management
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