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Tech Quiz #78

Protective Relaying

by Jim White Shermco Industries

Protectiverelaysperformacriticalfunctioninpowersystemprotection.Below aresomequestionstotestyourgeneralknowledgeofthesedevices.

1. Identifythedevicenumberwithitsfunction: A.59___________________________ B.27___________________________ C.50/51________________________ D.87T__________________________ 2. Reversepowerrelaysoperateunderwhatconditions? A.Reversalofvoltagepolarityandphaseanglefalling intheoperatezone. B.Reversalofcurrentflowandphaseanglefallingin theoperatezone. C.Bothcurrentandvoltagemustreversepolarity D.Thisrelayisnotpolaritysensitive 3. Electromechanical phase directional overcurrent and powerrelaystypicallyusewhatquantitytopolarizethe relay? A.Voltage B.Residualcurrent C.Neutralcurrent D.Phasecurrent 4. Whatconditionsmusta67deviceseeinordertoclose itscontacts? A.Overcurrent B.Overvoltage C.Overcurrentinapredetermineddirection D.Overcurrentandundervoltage

5.Anovercurrentrelayisconnectedtoa200:5CTwitha minimumpickupof4amperes.Whatprimarycurrent willcausetherelaytorespond? A. 200A B. 180A C. 160A D.120A

www.netaworld.org

1. A.59OvervoltageB.27UndervoltageC.50/51Instantaneousovercurrent/timeovercurrentD.87TTransformerdifferential

JamesR.Whiteisnationallyrecognizedfortechnicalskillsandsafetytrainingintheelectricalpowersystemsindustry.Heiscurrentlythe TrainingDirectorforShermcoIndustries,aNETAAccreditedCompany.Jimhasspentthelasttwentyyearsdirectlyinvolvedintechnical skillsandsafetytrainingforelectricalpowersystemtechnicians.

5. 160amperesisthecorrectanswer.A200:5CThasa40:1 ratio.MultiplytheCTratiobytherelay'sminimumpickup setting. 4. A 67 device is a directional overcurrent relay and must seebothanovercurrentconditionandcurrentflowina predetermineddirection.Thedirectionalcontactstypically splittheshadingcoilsontheovercurrentrelaycontrolling itsoperation. Winter 2006-2007 NETA WORLD 2. Typically,reversepowerrelayslookforareversalofcurrentflow,muchlikedirectionalovercurrentrelaysdo.The phaseanglebetweenvoltageandcurrentmustfallinthe operatezonewhichisfrequently90degreeseithersideof themaximumtorqueangle.However,unlikedirectional overcurrentrelays,reversepowerrelaysdonotuseadirectionalunittotorquecontrolanovercurrentunit'soperation. Thedirectionalelementoperatesthetimingandtripping functionsdirectly.

Answers

3. Phasedirectionalandpowerrelaysusuallyusevoltageas apolarizingquantity.Inorderfortherelayto"see"what directionthecurrentisflowing,itmusthaveastablereferencepointwhichthevoltageprovides.

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