GE 745 Instruction Manual page 296

AUXILIARY PROTECTION AND MONITORING FUNCTIONS
7.6.2.2 Operating Time
7.6.3 Transformer Overload
7.6.3.1 Operating Level
7–58
 Lower the DC component until the element resets.
The reset value should be approximately 2% larger than the operate
value.
 Verify that the Pickup and Alarm LEDs reset if the target function is set
to Self-resetting.
The Trip LED should remain latched.
To measure the basic operating time of this element,
 Preset a fundamental and DC component composite current signal to
cause the element to operate.
 Using the setup of FIGURE 7–1: Test setup on page 7–4, apply the
current suddenly, at the same time the timer is triggered.
The measured operating time should correspond to the time delay
setting for the element.
The transformer overload element uses the phase A current of each winding to compute a
transformer loading. The computation assumes a rated voltage on the wInding, hence the
loading is effectively a percent of rated load current.
 Inject a fundamental-frequency current into phase a of winding 1.
 Increase the current signal to a value just above the transformer
overload pickup setting (take into account the CT ratio and the rated-
MVA phase current to set the current correctly).
The element should operate after its set time delay.
 Check that the Trip, Pickup (and Alarm if selected) LEDs are on, and
one of the following is displayed:
LATCHED: Xformer Overload or OPERATED: Xformer Overload
 Lower the current until the element resets.
The reset value should be approximately 97% of the operate value.
 Verify that the Pickup and Alarm LEDs reset if the target function is set
to Self-resetting.
The Trip LED should remain latched.
CHAPTER 7: COMMISSIONING
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL