2 Motor Protection Alstom.no Iet Logo

7/30/2019 2 Motor Protection Alstom.no Iet Logo

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Copyright of AREVA T&D UK Limited

This document is the exclusive property of Alstom Grid and shall not betransmitted by any means, copied, reproduced or modified without the prior

written consent of Alstom Grid Technical Institute. All rights reserved.

GRID

Technical Institute

Motor Protect ion


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2 > Motor Protection

In t roduct ion

Many different applications

Different motor characteristics

Difficult to standardise protection

Protection applied ranges from

FUSES to RELAYS


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In t roduct ion

COST & EXTENT POTENTIAL

OF PROTECTION HAZARDS

SIZE OF MOTOR,TYPE & IMPORTANCE

OF THE LOAD

=


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SYSTEM

Voltage Dips

Voltage Unbalance

Loss of supply

Faults

Motor Protect ion

MOTOR CIRCUIT

Insulation failure

Open circuits

Short circuits

Overheating

LOAD

Overload

Locked rotor

Coupling faults

Bearing faults


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For Induc t ion motors

The main types to be considered :-

Overtemperature

Thermal overload

Stall / Locked rotor

Phase unbalance and single phasing

Short Circuit

Earth Fault

Undercurrent


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Motor Protect ion Appl ication

Voltage Rating Switching ProtectionDevice

< 600V < 11kW Contactor (i) Fuses

(ii) Fuses + direct acting

thermal O/L + U/V

releases

< 600V 11 - 300kW Contactor Fuses+ Electronic O/L

3.3kV 100kW - 1.5MW Contactor + Time delayed E/F

Options :- Stalling

6.6kV 1MW - 3MW Contactor Undercurrent

6.6kV > 1MW Circuit

Breaker As above

+ Instantaneous O/C

11kV > 1MW Circuit + DifferentialBreaker


7/727 > Motor Protection

Protection must be able to :-

Operate for abnormal conditions

Protection must not :-

Affect normal motor operation

Considerations :-

Starting current

Starting time

Full load current

Stall withstand time (hot & cold)

Thermal withstand

In t roduct ion



Motor Currents

INDUCTION MOTOR

Define Slip, S, as the per unit difference in speed betweenthe stator and rotor fields

Slip S = f- fr

f

Speed of stator field relative to rotor

f - fr = sf

frStator

Field f



Motor Currents

INDUCTION MOTOR

Magnitude of induced volts :

Proportional to sf

Frequency of induced rotor current :

Equal to sf

R2

kVS

X2 = 2fL(STAND STILL)



Rotor Equ ivalent Circui ts

STANDSTILL :

R2

kVS

X2 = 2fLROTOR

REACTANCE

AT

STANDSTILL

RUNNING :

R2

SkVS2sfL

= sX2

1/2

XS

R

kV

1/2XSR

skVCurrentRotor

22

22

2s

222

22

s

R2

SkVS X2



Rotor Equ ivalent Circui ts

R2

SkVS2sfL

= sX2

1/2

XS

R

kV

1/2XSR

skV

CurrentRotor

2

2

222

s

222

22

s

R2

SkVS X2



Motor Start ing Character ist ics

1/2

XS

R

kV

1/2XSR

skVphaseperCurrentRotor

22

222

s

22222

s

Time

Start

Time

CurrentFull Load Current

X2 >> R2Therefore R2 >> X2when s is small



Abou t to Start

Phase Loss

Low Volts



Reverse Phase Sequence Start ing

Protection required for lift motors, conveyors

Instantaneous I2 unit

Time delayed thermal trip

Separate phase sequence detector for low loadcurrent machines


15/72Copyright of AREVA T&D UK Limited



GRID

Technical Institute

Undervol tage

Protect ion



Undervol tage

Causes low output torque

machine cannot reach rated speed

draws high stator current

Use time delayed undervoltage protection



47 Three Phase Voltage Check

V2Start Low V Setting

Avoids excessive start times on DOL machines caused byinadequate voltage



Undervo l tage Considerations

Reduced torque

Increased stator current

Reduced speed

Failure to run-up

Form of undervoltage condition :-

Slight but prolonged (regulation)

Large transient dip (fault clearance)

Undervoltage protection :-

Disconnects motor from failed supply

Disconnects motor after dip long enough to preventsuccessful re-acceleration



Undervo l tage Tripp ing

Means of undervoltage tripping :-

AC holding coil for fused contactor

Undervoltage release

Undervoltage relay for shunt trip

Definite time

Inverse time

Considerations:-

U/V tripping should be delayed for essential motorsso that they may be given a chance to re-acceleratefollowing a short voltage dip (< 0.5s)

Delayed drop-out of fused contactor could bearranged by using a capacitor in parallel with the ACholding coil


20/72Copyright of AREVA T&D UK Limited



GRID

Technical Institute

Mechanical

Overload



Mechanical Overload

OVERLOAD

HEATING

INSULATION

DETERIORATION

OVERLOAD PROTECTION

THERMAL REPLICAFUSES



Motor Heating

HEAT STORED

INCREASES THEMOTOR TEMPERATURE

HEAT DISSIPATED AT A

RATE PROPORTIONAL TO

MOTOR TEMPERATURE

HEAT DEVELOPED AT A

CONSTANT RATE DUE TO

CURRENT FLOW


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Moto r Heating

MOTOR TEMPERATURE

T = Tmax (1 - e-t/)

or as temp rise (current)2T = KI2max (1 - e-t/)

Rate of rise depend on motorthermal time constant

Time

TMAX


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Motor Heating

Time

TMAX

T1T2

t2 t1

I2

I22I1

2

IR2

Time

Current

IR I1 I2

t1t2

Thermal Withstand


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Motor Heating

-

-1nt

2eq

2

m2

eq2

22

22

1-K

a-K1nt

I2 - I2m = (I2eq - I2m) (1 - e-t/)

Rearranging this expression in terms of time

or alternatively

Time

I

2eq

I2

I2

m

tTRIP

Current2

M t C l i


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Motor Cool ing

COOLING EQUATION :

I2m' = I2m e

-t/r

After time t equivalent motor current is reduced from Im to Im.

Time

Im

Current2

Im'

t0

M t H ti


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Motor Heating

t1 = Motor restart not possible

t2 = Motor restart possible

Time

Tmax

t2t1

Trip

Temp

Cooling timeconstant r

T


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GRIDTechnical Institute

Start / Stal l Pro tectio n

St l l i P t t i


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Stal l ing Protect ion

Required for :-

Stalling on start-up (locked rotor)

Stalling during running

With normal 3 supply :-

ISTALL = ILOCKED ROTOR @ ISTART

Cannot distinguish between STALL and START by currentalone.

Most cases :- tSTART < tSTALL WITHSTAND

Sometimes :- tSTART > tSTALL WITHSTAND

L k d R t P t t i


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Locked Rotor Protect ion

Start Time < Stal l Withs tand Time

Where Starting Time is less than Stall Withstand Time :

Use thermal protection characteristic

Use dedicated locked rotor protection

St l l P t t i


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Stal l Protec t ion :-

tSTART< tSTALL

Thermal relay provides protection against 3 stall.

Thermal

StallWithstand

Start

t

tS

LtS

T

IFLISTISL

I

If St l l With t d I B l Th l C


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If Stal l Withstand Is Below Thermal Curve

Separate stalling relay required :- Definite time O/C.

tSTART

Thermal

StallWithstand

tS

LtS

ISISTISL

Definite Time

Trip

(tS)T

O/C (IS)

tSL > tS > tSTART

Stal l Protect ion


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Stal l Protect ion

Tstart < Tstall

Use of motor start contact and 2 stage definite time overcurrent relay.

Current

+ -

TD1MSD

TD1 O/C

TD2

TRIP

TD2

86

Time

TD1+TD2

starttime

TD1

tST

Cold Stall tSL (COLD)

TD2

Full loadCurrent

Io/c

Hot Stall tSL (HOT)

TD1 > tST (TD1 + TD2) < tSL(COLD)

TD2 < tSL(HOT)

Loc ked Rotor Protect ion


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Motors with high inertia loads may often take longer to startthan the stall withstand time

However, the rotor is not being damaged because, as therotor turns the skin effect reduces, allowing the current to

occupy more of the rotor winding

This reduces the heat generated and dissipates the existingheat over a greater area

Detect start using tachometer input

Loc ked Rotor Protect ion

Start Time > Cold Stal l Withs tand Time

Stal l Protect ion


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Stal l Protect ion

Tstart > Tstall

Use of tachoswitch and definite time overcurrent relay.

+ -

TDO/C

TD

TRIP

86

TACHOTime

StartTime

TD

Full loadCurrent

CurrentIo/c

Stall - Tstall

Tacho opens at

10% speedTD < Tstall

> Tacho opening


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Unbalanced Supply Protect ion

Motor Currents


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Motor Currents

NEGATIVE SEQUENCE CURRENT

Relative frequency of stator field = f + fr

But fr = (1-s)f

Therefore f + fr = (2-s)f

fr

Stator

Field f

Operat ion on Supply Unbalance


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Operat ion on Supply Unbalance

At normal running speed

POSITIVE SEQ IMP STARTING CURRENT

NEGATIVE SEQ IMP NORMAL RUNNING CURRENT

Negative sequence impedance is much less than positivesequence impedance.

Small unbalance = relatively large negative sequence current.

Heating effect of negative sequence is greater than equivalentpositive sequence current because they are HIGHERFREQUENCY.

Equivalent Moto r Current


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Equivalent Moto r Current

Heating from negative sequence current greater than positivesequence

take this into account in thermal calculation

Ieq = (I12 + nI2

2)

where : n = typically 6

small amount of I2 gives large increase in Ieq andhence calculated motor thermal state.

Loss of 1 Phase While Start ing


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Loss of 1 Phase While Start ing

A

ANABA

ANA

x0.8662z

3V

2z

V'

openphase1With

z

V

currentstartingNormal

A2

A2

B2

A2

A1

ABA1

2

1

')a-(13

1)'a'(

3

1

21

'a)-(13

1)'a'(

3

1

STAR DELTA

normalx0.866

2z3xV

openPhase1

z

3VNormal

AB

AB

A

z

z

z

BC

A

B

C

1 winding carries twice

the current in the other 2.

Sing le Phase Stal l ing Protect io n


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Sing le Phase Stal l ing Protectio n

Loss of phase on starting motor remains stationary

Start Current = 0.866 normal start I

Neg seq component = 0.5 normal start I

Clear condition using negative sequence element

Typical setting ~ 1/3 I2

i.e. 1/6 normal start currenti.e. Rated Current

Single Phasing Wh ile Runn ing


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Single Phasing Wh ile Runn ing

Difficult to analyse in simple terms

Slip calculation complex

Additional I2 fed from parallel equipment

Results in :- I2 causes high rotor losses.

Heating considerably increased.

Motor output reduced.

May stall depending on load. Motor current increases.


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Insu lat ion Failure

Insulat ion Failure


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Insulat ion Failure

Results of prolonged or cyclic overheating

Instantaneous Earth Fault Protection

Instantaneous Overcurrent Protection

Differential Protection on some large machines

Stator Earth Fault Protect ion


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Stator Earth Fault Protect ion

M

50

M

50

Rstab

(A) Residually connected CTs

(B) Core Balance (Toroidal)CT

Note: * In (A) CTs can also drive thermal protection

* In (B) protection can be more sensitive

and is stable

Typical Core-Balance CT Appl icat ion


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Typical Core Balance CT Appl icat ion

50


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50

Short Circui t

Due to the machine construction internal phase-phase faultsare almost impossible

Most phase-phase faults occur at the machine terminals oroccasionally in the cabling

Ideally the S/C protection should be set just above the maxIstart (I>>=1.25Istart), however, there is an initial start currentof up to 2.5Istart which rapidly reduces over 3 cycles

Increase I>> or delay tI>> in small increments accordingto start conditions

Use special I>> characteristic


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This document is the exclusive property of Alstom Grid and shall not be

transmitted by any means, copied, reproduced or modified without the prior



Differen tial Protect ion

High-Impedance Wind ing Dif ferent ial Protect ion


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High Impedance Wind ing Dif ferent ial Protect ion

A

B

C

87A

87B

87C

Note: Protection must be stable with starting current.

Self-Balance Winding Differential Protect ion


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g

A

87A

B

C

87A

87B

87C

Ins tantaneous Earth Fault or Neg. Seq.


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g q

Tr ipping is not Perm it ted w ith Contactors

TRIP

MPR

M

TIME

Ts

Is Icont CURRENT

FUSE

MPR

ELEMENT

Ts > Tfuse at Icont.


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transmitted by any means, copied, reproduced or modified without the prior



Bearings

Bearing Failure


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g

Electrical Interference

Induced voltage

Results in circulating currents

May fuse the bearings

Remember to take precautions - earthing

Mechanical Failure

Increased Friction

Loss or Low Lubricant

Heating

Bearing Failure


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g

Ball or Roller Bearings

Immediate standstill

Cannot protect bearing

Stall protection for machine

Sleeve Bearings

Failure rare

Temperature rise, vibration, increase in current

Temperature sensor in bearing

Thermal overload for motor - does not protect bearing

Use of RTDs


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RTD sensors at known stator hotspots

Absolute temperature measurements to bias the relaythermal characteristic

Monitoring of motor / load bearing temperatures

Ambient air temperature measurement

49 Thermal pro tect ion


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p

w ith ambient compensat ion

0%

25%

50%

75%

100%

125%

0C 20C 40C 60C 80C 100C

Temperature

Ith(%settin

g)

26 Stato r RTDs


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RTDs cannot provide protection to the rotor

The protection provided during overloads is dependant upontheir positioning in the Stator windings and the insulationaround them.

In general current measurement is considered superiorunder large overload conditions

They do, however, respond to other conditions not present inthe current measurement (blocked cooling etc)

Undercurrent Protect ion


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Detects loss of load

e.g. Pumps & Conveyors

Submersible down hole pump

Is cooled by pumped liquid

Motor overheats if it runs dry even though

current reduces

Setting current 40% IFL

37 Loss of load


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In most applications it is desirable to stop the motor ifthe mechanical coupling is lost.

In addition a pump can be damaged if it becomesunprimed

No load current is normally about 50-60% ofIfl

On lightly loaded machines underpower provides betterdiscrimination between low load and load loss

No load power about 10%

May need to inhibit during start

Normal Shutdowns


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On machines where the thermal element is limited duringstart, it is critical to ensure that restarts do not damagethe machine

Selectable thermal start inhibit

Jogging

Selectable number of hot starts, cold starts, period andinhibit time

Selectable time between 2 starts

49 Thermal start inh ibit


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PROHIBIT START

Motor halt

Newrestart Prohibit START

Threshold

66 Lim ited starts/period


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Cold start Hot start Hot start

Treference

2 hot starts

Information on StartProhibited

Tprohibit

66 Time between 2 starts


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Tbetween 2 starts

Signal Minimum time between 2 restarts

Tbetween 2 starts Tbetween 2 starts

86 Lockout


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Some trips require maintenance before the machine canbe restarted. A latched trip may be applied for thefollowing conditions

Short circuit

Earth faults

Loss of Phase

Emergency Restart


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In certain applications, such as mine exhaust and shippumps, a machine restart is required knowing that itwill result in reduced life or even permanent damage.

All start up restrictions are inhibited

Thermal state limited to 90%


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OUT OF STEP PROTECTION

Inadequate field or excessive load can cause the machineto fall out of step. This subjects the machine toovercurrent and pulsating torque leading to stalling

Field Current Method

Detect AC Cu rrent Induced In Field Circuit .

Power Factor Method

Detect Heavy Current A t Low Power Factor.



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LOSS OF SUPPLY

On loss of supply motor should be disconnected ifsupply could be restored automatically.

Avoids supply being restored out of phase.

Overvoltage & Underfrequency

Underpower & Reverse Power



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Overvoltage

> Busbar & motor unloaded:

Motor terminal voltage may rise instantaneously

to 20 - 30% on loss off supply due to open circuit

regulation of the motor



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Underfrequency

> Motor loaded:

Decelerate fairly quickly & frequency ofterminal voltage will fall.



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Underpower

Only applicable when power reversals do not occurunder normal operating conditions

Arranged to look into the machine; applicable whenthere is A possibility of no load connected on loss ofsupply.

Time delay required to overcome momentary powerreversals due to faults



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Reverse Power

Only Applicable When Power Reversals Do Not OccurUnder Normal Operating Conditions

Arranged To Look Away From The Machine; ApplicableWhere There Is Aways Load Connected.

Time Delay Required To Overcome Momentary Power

Reversals Due To Faults

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