Impact of Variable Frequency Drive and Cyclic Loading on Multi-Function Motor Protection Relays

7/21/2019 Impact of Variable Frequency Drive and Cyclic Loading on Multi-Function Motor Protection Relays

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Impact of VariableFrequency Drive andCyclic Loading on Multi-

Function Motor ProtectionRelaysmar !"an - #$ Digital $nergy

%"omas $rnst - #$ Digital $nergy&tan $is'ant - %os"iba International Corporation

%e(as )*M Relay Conference +,.



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IntroductionProtection C"allenges )ssociated 0it"Variable &peed Drives 1also 'no0n as

)d2ustable &peed Drives3 Motor)pplications

Protection C"allenges )ssociated 0it"Cyclic or Reciprocating Load Motor

)pplicationsConcluding Remar's



4o0 Motor driven by VariableFrequency Drives 1VFDs3 andReciprocating or Cyclic Loading

applications can impact t"eperformance of t"e today5smultifunctional motor protection relay6- C"allenges and Issues

Proposition of various solutions to"andle and solves t"ese c"allenges

and issues in order to ac"ieve propero eration of t"e rela s 0it" reater

%"is is all about7788



Multifunctional motor protection relays e(perience various

c"allenges and issues 0"en applied to protect medium-voltage VFD and cyclic loaded motor applications8 %"esec"allenges may include9

VFD&igni:cant distortion in t"e motor input signals due to"armonics generated by t"e VFD operation

/;-nominal and varying frequency during motor start

<ormal load running at o;-nominal frequency

Impact of VFD on P"asors $stimation %ec"niques - RM& vs DF%

Variation in t"e s"aft driven fan cooling

Cyclic LoadVariation in t"e current magnitude in cyclic load applications

Varying duty cycle&ecurity vs sensitivity of various protection and monitoring

Introduction-C"allenges



Installations don5tal0ays

matc" t"emanual



Protection C"allenges

)ssociated 0it" VFDDriven Motor)pplications



Distortion in t"e motor input signals

VFD generatessigni:cant "armonicsin t"e input signals

4armonics results inadditional copper and

iron losses

More "eat

Deterioration inoverall life of a motor



Important factors9Lo0 frequency or o;-nominal frequency duringstarting

Rate of c"ange of frequency to reac" t"e desired level )cceleration=Deceleration

e8g8 starting at +,4> and accelerating to t"enominal frequency of ?,4>

VFDs allo0 programming of t"e total time requiredto accelerate from , to ?,4> in a range from ,8sec to @,,, sec in MV drives and ,8, to ?,,, sec

in LV drives8 )lt"oug" t"e drive is capable ofacceleratin t"e motor 0it" t"e de:ned rateA t"e

&tarting of t"e motor fed t"roug" VFD



Illustration of motor startingcurrent




Impact of varying frequency input on multifunctionmotor relay


ses :(ed sampling DF%-type 1discreteFourier transform3 and=or RM&-type 1rootmean square3 tec"niques to estimate t"e

p"asors 1magnitude and p"ase3 of inputsignal $stimation of t"e o;-nominal frequencysignal using t"e :(ed samplingfrequency estimators results in asigni:cant error in t"e measurement



&olution Correction of t"e sampling frequency byproperly trac'ing t"e signal frequency


Fi(ed sampling

frequencyestimation

&amplingfrequencyad2ustment by

trac'ing frequency



%"e problem is not yet fully solved9 magnitude of t"e signals

still s"o0s measurement error during t"e :rst fe0 cycles


RM& magnitude s"o0s large oscillations in measurement 0"ile DF%magnitude s"o0s calculation delay until magnitude reac"es t"eactual magnitude level




This error is due to:

Processing time ta'en by t"e frequency trac'ingalgorit"m plus t"e intentional security delay of a fe0cycles8

&o "o0 to overcome t"is error due to algorit"mprocessing time6




&o "o0 to reduce measurement error due toalgorit"m processing time6

&olution is to force t"etrac'ing frequency tostart trac'ing from t"epre-de:ned Starting-

Frequency

<ormallyA samplingfrequency is ad2usted

based on t"e nominalfrequency 1e8g8A ?,4>3until trac'ing frequencystarts providing t"eactual frequency




Starting Frequency AdjustmentFor e(ampleA in t"e application of a motor fed t"roug" aVFD t"at starts at frequency of , 4>A t"e relay engineermust use t"e Starting Frequency feature and program

t"e starting frequency value to , 4>



%ypically RM&- or DF%-type estimators are used to calculate p"asors for t"e

current based s"ort circuit protection functions of modern microprocessorbased relays

%"e actual cause of t"e lo0er estimation of t"e magnitude using DF%-typeAcompared to RM&-typeA is varying frequency rat"er t"an "armonics

DF%-type calculates lo0er magnitude t"an RM&-type estimator

RM& vs DF%



Current based t"ermal protection can fail to accuratelydetermine t"e "eating replica 0"en t"e motor is driven by t"e

VFD

Motor t"ermal calculations must ta'e into account t"e e(tra

"eat generated by t"e VFD 0aveform and reduction of t"e

amount of "eat removed in t"e VFD driven motors 0it" s"aft

driven cooling fanA especially if t"e application is constant

torque RM& currents must be used

Distortion in motor input voltages are more signi:cant t"an

currents and p"ase voltages are measured at t"e bus side

1source side3 of t"e VFDA 0"ic" don5t reBect t"e actual motor

input voltages8 %"ereforeA voltages measured at t"e bus side

of t"e VFD are not suitable to be used for t"e motor voltage

RM& vs DF%



In any motor t"at is not forced cooledA t"e cooling fan isattac"ed to t"e motor s"aft and cooling of t"e motor varies

0it" frequency or speed of motor

Current based t"ermal protection can fail to calculate t"e

reduced cooling t"at results from t"e varying fan speed since

t"e given motor manufacturer damage curves are based on

full speed operation only

Variation in t"e fan cooling must be ta'en into account in

order to do best estimation of t"e motor "eating




&olution %o correct t"e t"ermal capacity by usingt"e temperature measurement device li'e R%Diasing




Protection C"allenges

)ssociated 0it" Cyclicor Reciprocating Load

Motor )pplications



Cyclic9 <ote(actly0"at I "adin mind



Variation in the current magnitude - cyclic orreciprocating load

In cyclic load applicationsA load varies bet0een lig"t and aboveallo0able load level during a load cycle8 %"e motor starts "eatingup 0"en it is running overload and cools do0n during t"e lig"t loadperiods8 %"ereforeA t"e t"ermal model must be intelligent and fast

enoug" to properly reBect t"e motor "eating and coolingt"rou "out t"e load c cle8

Cyclic Load



Large duty cycle load

Current based t"ermal model 1)ppendi( )3 properlyreBects t"e "eating and cooling during t"e cyclicload running condition

ased on duty-cycle-ratioA load can be categori>edas balanced duty 1DE,,3A "eavy duty 1DG,,3and lig"t duty 1DH,,3

Cyclic Load



Large duty cycle load

%"ermal capacity must remain belo0 ,, t"res"old for t"e balanceduty cycle load

4o0everA t"ermal model must operate 0"en t"e motor is running

"eavy duty cyclic loadt"res"old

Cyclic Load

0.0 0.5k 1.0k 1.5k 2.0k 2.5k 3.0k

0

1020

3040

50

6070

80

90100

Heavy duty load

Time(s)

T

C U

!ala"#ed duty load



&mall Duty-Cycle Load

For t"e motor applications 0"ere t"e load cycle "as a smallduty-cycle lengt" 1@+ po0er cycles3 but is "eavy duty innature 1DG,,3 and t"e motor engineer decides to run t"e

motor close to overload conditionsA t"e t"ermal model can becomplemented 0it" an additional )veraging Load Filtering1)LF3 feature8

%"e )LF feature "elps to determine t"e average of t"e cyclic

overload current over t"e duty-cycle using t"e runningaverage tec"nique

4o0everA t"e t"ermal model must operate to protect t"emotor from over"eating for t"e cyclic load application 0it""eavy duty-cycle-ratioA 0"ic" can over"eat t"e motor8

Cyclic Load



Cyclic Load



Varying frequency results in signi:cation error in t"e p"asormeasurementJproper trac'ing of t"e frequency can "elp to reduce

measurement error

P"asor measurement error can be furt"er reduced by using t"e

&tarting Frequency )d2ustment feature

For VFD applicationsA t"ermal protection must ta'e into account

additional motor "eating due to current "armonics content and

varying fan cooling8 RM& based magnitude must be used8

sing RM& currents and t"e R%D biasing feature can "elp to estimate

t"e additional "eating in t"e motor due to "armonics and varying fan

coolingA respectively

Conclusions



For cyclic load motor applicationsA t"e t"ermal model must properlyreBect motor "eating and cooling as load c"anges bet0een t"e

ma(imum and minimum load levels during cyclic load period8

4o0everA t"e t"ermal model must protect t"e motor from

over"eating 0"en running "eavy duty cyclic loadingA 0"ic" can

over"eat t"e motor8

In small duty-cycle applicationsA if t"e motor is required to run "eavy

duty load 1duty-cycle-ratio G ,,3A an additional )LF 1averaging

load :lter3 feature in t"e t"ermal model can "elp to 'eep t"e motor

running

Conclusions



%"an' Kou

uestions6

Documents

Impact of Variable Frequency Drive and Cyclic Loading on Multi-Function Motor Protection Relays