GE 745 - 7.5 Protection Schemes; 7.5.1 Precaution; 7.5.2 Harmonic Restrained Percent Differential; 7.5.2.1 Description

7–16 745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION SCHEMES CHAPTER 7: COMMISSIONING
7.5 Protection Schemes
7.5.1 Precaution
CAUTION
Keep track of modifications/changes made to settings during the course of these
commissioning steps and ensure that all settings are returned to the ‘in-service’ values
at the end of the tests.
7.5.2 Harmonic Restrained Percent Differential
7.5.2.1 Description
The harmonic restrained percent differential element setpoints are located in
S4 ELEMENTS
 DIFFERENTIAL  PERCENT DIFFERENTIAL. Disable all other protection elements to
ensure that trip relay(s) and auxiliary relays are operated by element under test only. With
a multimeter, monitor the appropriate output contact(s) per intended settings of the
FlexLogic™. Refer to the relay settings to find out which relay(s) should operate when a
given element operates.
7.5.2.2 Minimum Pickup
The minimum pickup of the phase A element is measured by applying a fundamental
frequency AC current to terminals H1 and G1, winding 1 phase A. Monitor the appropriate
trip and auxiliary contact(s) as the current is increased from 0 A. Compare the current value
at which operation is detected against the
S4 ELEMENTS  DIFFERENTIAL  PERCENT
DIFFERENTIAL
 PERCENT DIFFERENTIAL PICKUP setpoint. Since the operating point is
normally set quite low, fine control of the current signal will be required to obtain accurate
results.
The currents in the winding may be phase shifted or may have the zero-sequence
component removed due to auto-configuration (see Auto-configuration on page 5–6). As
an alternate to calculating to relation of input current to differential current, the differential
current is displayed in A2 METERING
 CURRENT  DIFFERENTIAL. Ensure that the
displayed value is the same as the minimum pickup setting when the element operates.
 Check that the Trip and Message LEDs are flashing and one of the
following trip messages is displayed:
LATCHED a: Percent Differential, or OPERATED a: Percent
Differential
NOTE
Note
The above messages will indicate either OPERATED or LATCHED depending on the S4
ELEMENTS
 DIFFERENTIAL  PERCENT DIFFERENTIAL  PERCENT DIFFERENTIAL
TARGE T setting.
To independently verify that auto-configuration causes the currents to be as measured,
follow the rules outlined in the steps below.
 Look up transformer type in Table 5–1: Transformer types on page 5–
13.