Thd Level Scheme - GE 745 TRANSFORMER MANAGEMENT RELAY Instruction Manual

10 COMMISSIONING
10.7 AUXILIARY PROTECTION/MONITORING FUNCTIONS
The THD settings are under:
SETPOINTS/S4 ELEMENTS/HARMONICS/W1 THD LEVEL/....
a) MINIMUM PICKUP
1. Testing of this element uses with the same setup used in testing the harmonic restraint percent differential
elements (see Figure 10–1: TEST SETUP on page 10–3).
2. To test the Winding 1 THD element, connect the composite current signal to terminals H1 and G1. Since
the DC component actually consists of a half-wave rectified signal, it contains all even harmonics which the
relay measures and operates on. Note that the fundamental component is required to prevent saturation of
the input CTs. Monitor the output relays as per the relay FlexLogic assignment.
3. Set the fundamental component to rated CT secondary (1 or 5 A). Gradually increase the DC component
to produce even harmonics until the THD Level element operates. Display the total harmonic content
under:
ACTUAL/A2 METERING/HARMONIC CONTENT/THD/W1(%fo) THDa:....
4. The displayed value of THD at which operation took place should be the same as the programmed value.
5. Check that the TRIP, PICKUP (and ALARM if selected) LEDs are on and the following trip message dis-
played:
SS LATCHED (OPERATED):
SS W1 THD Level
6. Lower the DC component until the element resets. The reset value should be approximately 2% less than
the operate value. Verify that the Phase, Pickup and Alarm LEDs reset if the target function is set to Self-
resetting . The trip LED should remain latched.
b) OPERATING TIME
To measure the basic operating time of this element, preset a fundamental and DC component composite cur-
rent signal to cause the element to operate. Using the setup of Figure 10-1, apply the current suddenly, at the
same time as you trigger the timer. The measured operating time should correspond to the time delay setting
for the element.
c) MINIMUM OPERATING CURRENT
The THD elements will only operate if the amplitude of the fundamental component is above the threshold set-
ting. To verify this threshold, initially set the fundamental component above the threshold, with a harmonic con-
tent high enough to cause the element to operate. Now reduce the fundamental component only. This will have
for effect to increase the THD level. When the fundamental component reaches a value below the set thresh-
old, 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 ACTUAL VALUES/A2
METERING/CURRENT/W1 RMS CURRENT. These values represent the fundamental component only.
CAUTION
d) OTHER THD ELEMENTS
A THD element can be programmed for each winding of the transformer. Use the above procedure to verify the
element(s) on the other winding(s).
GE Power Management
10.7 AUXILIARY PROTECTION/MONITORING FUNCTIONS
745 Transformer Management Relay

10.7.1 THD LEVEL SCHEME

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