GE 745 Instruction Manual page 295

CHAPTER 7: COMMISSIONING
7.6.1.3 Minimum Operating Current
CAUTION
7.6.1.4 Other THD Elements
7.6.2 Harmonic Derating Function
7.6.2.1 Operating level
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
The THD elements will only operate if the amplitude of the fundamental component is
above the threshold setting.
To verify this threshold,
 Initially set the fundamental component above the threshold, with a
harmonic content high enough to cause the element to operate.
 Now reduce the fundamental component only.
This will have the effect of increasing the THD level. When the
fundamental component reaches a value below the set threshold, the
element will reset.
If an RMS-responding meter is used to measure the current signal, the reading is the
total value of current. To determine the fundamental component only, use the relay
values in A2 METERING

fundamental component only.
A THD element can be programmed for each winding of the transformer. Use the above
procedures to verify the element(s) on the other winding(s).
Testing of the harmonic derating function requires that accurate transformer parameters
such as load losses at rated load and winding resistance be entered. This feature makes
use of the harmonic derating factor (HDF) computed by the relay, using the harmonic
content of the current signals and the transformer data (refer to IEEE C57.110-1986 for the
computation method). Once the derating factor falls below a set value, the 745 can trip
and/or alarm.
To verify the correct operation of this element, a current signal containing harmonics must
be introduced into one phase of the relay. Use FIGURE 7–1: Test setup on page 7–4 to
accomplish this.
 Set the fundamental component at rated CT secondary into winding 1
phase A.
 Gradually increase the second harmonic component (and the rest of the
even harmonics) while displaying the
CONTENT
DERATING FACTOR
The element should operate when the displayed HDF equals the element
setting.
 Check that the Trip, Pickup (and Alarm if selected) LEDs are on, and
one of the following is displayed:
LATCHED: W1 Harm Derating or OPERATED: W1 Harm
Derating
AUXILIARY PROTECTION AND MONITORING FUNCTIONS
CURRENT W1 CURRENT. These values represent the

HARMONIC DERATING

value.
A2 METERING HARMONIC

W1(3) HARMONIC

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