GE 745 Instruction Manual page 259

CHAPTER 7: COMMISSIONING
7.5.2.10 Fifth Harmonic Restraint
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
FIGURE 7–7: Second harmonic restraint testing
 Close switch S1. Set the AC current, I
I to obtain harmonic content above the 2nd harmonic restraint setting
DC
under
HARMONIC INHIBIT LEVEL
 Calculate the percent second harmonic content from the following
equations. If the current is measured with average-responding/reading
meters:

%2nd
 If the current is measured with RMS-responding/reading meters, then:

%2nd
 Open and reclose S1. The relay should not operate.
 Decrease I
harmonic at this point using the equations above. The calculated percent
harmonic value should equal the relay setting.
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 generate the
appropriate signal.
 Connect the test setup as below to supply the phase A element. Set the
fundamental current level to the CT rated secondary value. The
harmonic restraint differential element of phase A should be operated.
S4 ELEMENTS DIFFERENTIAL

.
100 0.424 I × ×
DC
------------------------------------------ -
=
I 0.9 × I
+
DC AC
100 0.424 I × ×
DC
------------------------------------------ -
=
I 1.414 × I
+
DC AC
until the element operates. Calculate the percent of second
DC
PROTECTION SCHEMES
to 2 × rated CT secondary. Set
AC
HARMONIC INHIBIT
 
7–21