GE Multilin 745 Instruction Manual

Protection Schemes 745Transformer Management RelayCommissioninghttp://www.GEindustrial.com/multilin 7ñ21GE MultilinNeutral TimeOvercurrenta) DescriptionThis procedure verifies that the Neutral Time Overcurrent performance matches thein-service settings. Since these elements can have any one of a multitude of timingcurves, a table of expected operating times versus applied current should beprepared prior to testing. The neutral element measures the derived zero-sequencecurrent signal as an input. Refer to Time Overcurrent Curves on page 5ñ50 forinformation on timing curves.If the relay elements are set for the ìLinearî reset characteristic when measuringthe operating times, ensure there is sufficient time between test current injectionsfor the element to reset fully. Otherwise, erroneous timing measurements will beobtained.The settings for these elements are found under the S4 ELEMENTS "# NEUTRAL OCsetpoints menu. Note that there can only be one or two Neutral Time Overcurrentelements in service at the same time.b) Winding 1 ElementTo ensure that only the Neutral Time Overcurrent element under test operates thetrip relays (and any other output relays) selected by the logic, disable all protectionfeatures except Neutral Time Overcurrent. Use the general test setup shown inFIGURE 7ñ9: General Test Setup on page 7ñ18.Connect the current supply to terminals X = H1 and Y = G1 to test the Winding 1neutral element. Monitor the appropriate output relays as per the relay FlexLogicôsettings.c) Pickup Level1. With the interval timer disabled, apply the current signal and slowly increase itsmagnitude until the trip relay (and all the selected auxiliary relays) operate.2. If the relay under test has a very inverse time characteristic, it is easier andmore accurate to increase the current far above the pickup level until the triprelay operates, then reduce the current to just above the expected operatelevel. Slowly reduce the current below the operate level and observe for a resetaction on the trip relay. This current reset level should be approximately 98% ofthe pickup level setting. Once the relay drops out, slowly increase the currentuntil the trip contact closes. The operate level should correspond to the S4ELEMENTS !" NEUTRAL OC ! W1 NTRL TIME OC !" W1 NEUTRAL TIME OC PICKUPsetpoint: Since current is being introduced into one phase only, the inputcurrent signal is equal to the 3Io signal used by the element.3. When the element operates, check that the Trip, Pickup, and Phase LEDs are onand one of the following messages is displayed:LATCHED a: W1 Ntrl Time OC or OPERATED a: W1 Ntrl Time OC4. Reduce the current until the element resets. The reset level should be 97% ofthe operate level. When the element resets, the Trip and Message LEDs shouldremain on if the W1 NEUTRAL TIME OC TARGET was selected as ìLatchedî.Otherwise only the Trip LED remains on.5. Reset indicators and clear messages.d) Operating TimeUsing a table like the one shown below, select three (3) or four (4) values of currentmultiples at which timing is to be measured. Enter the expected operating timesfrom the timing curve applied in the settings. Using the setup in FIGURE 7ñ9:General Test Setup on page 7ñ18 and the Interval Timer enabled, set the currentlevel to the desired value and apply suddenly by closing the double-pole switch.Record the operate time and compare to the expected value. Repeat for all desiredvalues of current.