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Motor Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Introduction
Many different applications
Different motor characteristics
Difficult to standardise protection
Protection applied ranges from FUSES
2
> Motor Protection
to
RELAYS
Introduction
COST & EXTENT OF PROTECTION
=
POTENTIAL HAZARDS
SIZE OF MOTOR, TYPE & IMPORTANCE OF THE LOAD
3
> Motor Protection
Motor Protection
4
SYSTEM
MOTOR CIRCUIT
LOAD
Voltage Dips
Insulation failure
Overload
Voltage Unbalance
Open circuits
Locked rotor
Loss of supply
Short circuits
Coupling faults
Faults
Overheating
Bearing faults
> Motor Protection
For Induction motors
The main types to be considered :
Overtemperature
Thermal overload Stall / Locked rotor
Phase unbalance and single phasing
5
Short Circuit
Earth Fault
Undercurrent
> Motor Protection
Motor Protection Application Voltage
< 600V
Rating
< 11kW
Switching Device
or
Protection
(i) Fuses (ii) Fuses + direct acting thermal O/L + U/V releases
< 600V
11 - 300kW
or
Fuses + Electronic O/L
3.3kV
100kW - 1.5MW
or
+ Time delayed E/F Options :- Stalling
6.6kV
1MW - 3MW
or
6.6kV
> 1MW
Circuit Breaker
Undercurrent
As above + Instantaneous O/C
11kV
6
> Motor Protection
> 1MW
Circuit Breaker
+ Differential
Introduction
Protection must be able to :-
Operate for abnormal conditions
Protection must not :Affect normal motor operation
Considerations :-
7
> Motor Protection
Starting current Starting time Full load current Stall withstand time (hot & cold) Thermal withstand
Motor Currents
INDUCTION MOTOR
Stator
fr
Field f
Define Slip, S, as the per unit difference in speed between the stator and rotor fields
Slip “S” = f - fr f Speed of stator field relative to rotor f - fr = sf 8
> Motor Protection
Motor Currents INDUCTION MOTOR
Magnitude of induced volts : Proportional to sf
Frequency of induced rotor current :
Equal to sf
R2 kVS
9
> Motor Protection
X2 = 2fL (STAND STILL)
Rotor Equivalent Circuits STANDSTILL :
R2
kVS
RUNNING : X2 = 2fL ROTOR REACTANCE AT STANDSTILL
R2 2sfL = sX2
SkVS
Rotor Current
skVs
R 2 S X 2 2
2 2
kVs
R2 2 2 X 2 S2
1/2
1/2
R2 kVS
10
> Motor Protection
S
X2
Rotor Equivalent Circuits
R2
2sfL = sX2
SkVS
Rotor Current
skVs
R2 2 S2 X22
R2 2 S2
1/2
kVs
1/2 2 X2
R2 kVS
11
> Motor Protection
S
X2
Motor Starting Characteristics Rotor Current per phase Time
Start Time
R22 S2 X22
R 2 2 S2
> Motor Protection
kVs
1/2
1/2
X 22
X2 >> R2 Therefore R2 >> X2 when s is small
Full Load Current 12
skVs
Current
About to Start
Phase Loss Low Volts
13
> Motor Protection
Reverse Phase Sequence Starting
Protection required for lift motors, conveyors
Instantaneous I2 unit Time delayed thermal trip
Separate phase sequence detector for low load current machines
14
> Motor Protection
Undervoltage Protection
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Undervoltage
Causes low output torque
machine cannot reach rated speed draws high stator current
Use time delayed undervoltage protection
16
> Motor Protection
47 Three Phase Voltage Check
V2
Prevents reverse operation of machine Vabc>Start Low V Setting
Avoids excessive start times on DOL machines caused by inadequate voltage
17
> Motor Protection
Undervoltage 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 :-
18
Disconnects motor from failed supply
Disconnects motor after dip long enough to prevent successful re-acceleration
> Motor Protection
Undervoltage Tripping Means of undervoltage tripping :
AC holding coil for fused or
Undervoltage release
Undervoltage relay for shunt trip
Definite time Inverse time Considerations:-
19
U/V tripping should be delayed for essential motors so that they may be given a chance to re-accelerate following a short voltage dip (< 0.5s)
Delayed drop-out of fused or could be arranged by using a capacitor in parallel with the AC holding coil
> Motor Protection
Mechanical Overload
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Mechanical Overload OVERLOAD
HEATING
INSULATION DETERIORATION
OVERLOAD PROTECTION
FUSES 21
> Motor Protection
THERMAL REPLICA
Motor Heating
HEAT STORED INCREASES THE MOTOR TEMPERATURE
HEAT DEVELOPED AT A CONSTANT RATE DUE TO CURRENT FLOW
HEAT DISSIPATED AT A RATE PROPORTIONAL TO MOTOR TEMPERATURE 22
> Motor Protection
Motor Heating MOTOR TEMPERATURE
T = Tmax (1 - e-t/) TMAX
Time or as temp rise
Rate of rise depend on motor thermal time constant (current)2
T = KI2max (1 - e-t/) 23
> Motor Protection
Motor Heating I2 I22
T2 T1
I12 IR2
TMAX
t2 t1
Time
Time
t1
Thermal Withstand
t2
IR I1 I2 24
> Motor Protection
Current
Motor Heating Current2 I2eq I 2 I 2m
tTRIP I2 - I2m = (I2eq - I2m) (1 - e-t/) Rearranging this expression in of time t
or alternatively t 25
> Motor Protection
Ι2eq - Ι2m 1n Ι2eq - Ι2
K2 - a2 1n K2 - 12
Time
Motor Cooling COOLING EQUATION :
I2m' = I2m e-t/r Current2 Im
I m' 0
t
Time
After time „t‟ equivalent motor current is reduced from Im to Im‟.
26
> Motor Protection
Motor Heating Temp
Trip Tmax T
Cooling time constant r
t1
t1 = Motor restart not possible t2 = Motor restart possible 27
> Motor Protection
t2
Time
Start / Stall Protection
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Stalling Protection
Required for : Stalling on start-up (locked rotor) Stalling during running
With normal 3Ø supply :ISTALL = ILOCKED ROTOR @ ISTART
Cannot distinguish between „STALL‟ and „START‟ by current alone.
29
Most cases :-
tSTART < tSTALL WITHSTAND
Sometimes :-
tSTART > tSTALL WITHSTAND
> Motor Protection
Locked Rotor Protection Start Time < Stall Withstand Time
Where Starting Time is less than Stall Withstand Time :
Use thermal protection characteristic Use dedicated locked rotor protection
30
> Motor Protection
Stall Protection :tSTART < tSTALL Thermal relay provides protection against 3Ø stall.
t
Thermal Stall Withstand
tS tLS T
Start
IFL 31
> Motor Protection
IST ISL
I
If Stall Withstand Is Below Thermal Curve Separate stalling relay required :- Definite time O/C.
Definite Time Thermal
tS t LS
O/C (IS)
(tS) T
Stall Withstand
tSTART
Trip tSL > tS > tSTART
IS 32
> Motor Protection
IST ISL
Stall Protection Tstart < Tstall Use of motor start and 2 stage definite time overcurrent relay. Time
-
+ MSD
TD1
TD1
O/C
TD2
TD2 86
TRIP
Cold Stall tSL (COLD)
TD1+TD2 TD1 tST
Hot Stall tSL (HOT)
start time
TD2 Full load Current
TD1 > tST
(TD1 + TD2) < tSL(COLD) TD2 < tSL(HOT)
33
> Motor Protection
Io/c
Current
Locked Rotor Protection Start Time > Cold Stall Withstand Time
Motors with high inertia loads may often take longer to start than the stall withstand time However, the rotor is not being damaged because, as the rotor turns the “skin effect” reduces, allowing the current to occupy more of the rotor winding This reduces the heat generated and dissipates the existing heat over a greater area
Detect start using tachometer input
34
> Motor Protection
Stall Protection Tstart > Tstall Use of tachoswitch and definite time overcurrent relay.
-
+ TACHO
O/C
Time
Tacho opens at 10% speed
TD
TD < Tstall > Tacho opening
TD 86
Start Time TRIP
Stall - Tstall
TD
Full load Current
35
> Motor Protection
Io/c
Current
Unbalanced Supply Protection
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Motor Currents NEGATIVE SEQUENCE CURRENT
fr
Relative frequency of stator field = f + fr But Therefore
37
> Motor Protection
fr = (1-s)f f + fr = (2-s)f
Stator Field f
Operation on Supply Unbalance
At normal running speed POSITIVE SEQ IMP NEGATIVE SEQ IMP
STARTING CURRENT NORMAL RUNNING CURRENT
Negative sequence impedance is much less than positive sequence impedance. Small unbalance = relatively large negative sequence current. Heating effect of negative sequence is greater than equivalent positive sequence current because they are HIGHER FREQUENCY.
39
> Motor Protection
Equivalent Motor Current
Heating from negative sequence current greater than positive sequence
take this into in thermal calculation
Ieq = (I12 + nI22)½
where : n = typically 6
40
small amount of I2 gives large increase in Ieq and hence calculated motor thermal state.
> Motor Protection
Loss of 1 Phase While Starting STAR A
Normal starting current VAN z With 1 phase open ΙA
C
B
Ι' A
3VAN VAB 2z 2z 0.866 x Ι A
1 1 (Ι' A aΙ'B ) (1- a)Ι' A 3 3 1 Ι1 Ι A 2 1 1 Ι 2 (Ι' A a2 Ι'B ) (1- a2 )Ι' A 3 3 1 Ι2 Ι A 2 Ι1
41
> Motor Protection
DELTA A
z
z
C z B
Normal
3VAB z
1 Phase open 3 VAB x 2z 0.866 x normal 1 winding carries twice the current in the other 2.
Single Phase Stalling Protection
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 current i.e. Rated Current
42
> Motor Protection
Single Phasing While Running
Difficult to analyse in simple
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.
43
> Motor Protection
Insulation Failure
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Insulation Failure
Results of prolonged or cyclic overheating
Instantaneous Earth Fault Protection Instantaneous Overcurrent Protection Differential Protection on some large machines
45
> Motor Protection
Stator Earth Fault Protection
Rstab
50
(A) Residually connected CT‟s
M
50
M
Note:
(B) Core Balance (Toroidal)CT
* In (A) CT‟s can also drive thermal protection * In (B) protection can be more sensitive and is stable
46
> Motor Protection
Typical Core-Balance CT Application
47
> Motor Protection
50 Short Circuit
Due to the machine construction internal phase-phase faults are almost impossible Most phase-phase faults occur at the machine terminals or occasionally in the cabling Ideally the S/C protection should be set just above the max Istart (I>>=1.25Istart), however, there is an initial start current of up to 2.5Istart which rapidly reduces over 3 cycles
Increase I>> or delay tI>> in small increments according to start conditions
Use special I>> characteristic
48
> Motor Protection
Differential Protection
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
High-Impedance Winding Differential Protection A
B
C
87 A
Note: 50
87 B
87 C
Protection must be stable with starting current.
> Motor Protection
Self-Balance Winding Differential Protection A
87 A
B
87 B
C
87 C 51
> Motor Protection
Instantaneous Earth Fault or Neg. Seq. Tripping is not Permitted with ors TRIP
TIME MPR FUSE M
MPR ELEMENT
Ts
Is
Icont
CURRENT
Ts > Tfuse at Icont. 52
> Motor Protection
Bearings
GRID Technical Institute
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Copyright © of AREVA T&D UK Limited
Bearing Failure
Electrical Interference Induced voltage Results in circulating currents May fuse the bearings to take precautions - earthing
Mechanical Failure
Increased Friction Loss or Low Lubricant Heating
54
> Motor Protection
Bearing Failure
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
55
> Motor Protection
Use of RTDs
RTD sensors at known stator hotspots
Absolute temperature measurements to bias the relay thermal characteristic Monitoring of motor / load bearing temperatures Ambient air temperature measurement
56
> Motor Protection
49 Thermal protection with ambient compensation
125%
Ith (%setting)
100% 75% 50% 25% 0% 0°C
20°C
40°C
60°C
Temperature 57
> Motor Protection
80°C
100°C
26 Stator RTDs
RTDs cannot provide protection to the rotor The protection provided during overloads is dependant upon their positioning in the Stator windings and the insulation around them.
In general current measurement is considered superior under large overload conditions They do, however, respond to other conditions not present in the current measurement (blocked cooling etc)
58
> Motor Protection
Undercurrent Protection
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
60
> Motor Protection
37 Loss of load
In most applications it is desirable to stop the motor if the mechanical coupling is lost. In addition a pump can be damaged if it becomes unprimed
No load current is normally about 50-60% of Ifl On lightly loaded machines underpower provides better discrimination between low load and load loss
No load power about 10%
May need to inhibit during start
61
> Motor Protection
Normal Shutdowns
On machines where the thermal element is limited during start, it is critical to ensure that restarts do not damage the machine Selectable thermal start inhibit Jogging
Selectable number of hot starts, cold starts, period and inhibit time
Selectable time between 2 starts
62
> Motor Protection
49 Thermal start inhibit
Motor halt Prohibit START Threshold
PROHIBIT START
63
> Motor Protection
New restart
66 Limited starts/period
2 hot starts Hot start
Cold start
Hot start
Treference Tprohibit Information on « Start Prohibited »
64
> Motor Protection
66 Time between 2 starts
Tbetween 2 starts
Tbetween 2 starts
Tbetween 2 starts
Signal « Minimum time between 2 restarts »
65
> Motor Protection
86 Lockout
Some trips require maintenance before the machine can be restarted. A latched trip may be applied for the following conditions
Short circuit Earth faults Loss of Phase
66
> Motor Protection
Emergency Restart
In certain applications, such as mine exhaust and ship pumps, a machine restart is required knowing that it will result in reduced life or even permanent damage.
All start up restrictions are inhibited Thermal state limited to 90%
67
> Motor Protection
Synchronous Motors
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Synchronous Machines OUT OF STEP PROTECTION
Inadequate field or excessive load can cause the machine to fall out of step. This subjects the machine to overcurrent and pulsating torque leading to stalling
Field Current Method
Detect AC Current Induced In Field Circuit.
Power Factor Method Detect Heavy Current At Low Power Factor.
69
> Motor Protection
Synchronous Machines
LOSS OF SUPPLY On loss of supply motor should be disconnected if supply could be restored automatically. Avoids supply being restored out of phase.
70
Overvoltage & Underfrequency Underpower & Reverse Power
> Motor Protection
Synchronous Machines
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
71
> Motor Protection
Synchronous Machines
Underfrequency
> Motor loaded: Decelerate fairly quickly & frequency of terminal voltage will fall.
72
> Motor Protection
Synchronous Machines
Underpower Only applicable when power reversals do not occur under normal operating conditions Arranged to look into the machine; applicable when there is A possibility of no load connected on loss of supply.
Time delay required to overcome momentary power reversals due to faults
73
> Motor Protection
Synchronous Machines
Reverse Power Only Applicable When Power Reversals Do Not Occur Under Normal Operating Conditions
Arranged To Look Away From The Machine; Applicable Where There Is Aways Load Connected. Time Delay Required To Overcome Momentary Power Reversals Due To Faults
74
> Motor Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Introduction
Many different applications
Different motor characteristics
Difficult to standardise protection
Protection applied ranges from FUSES
2
> Motor Protection
to
RELAYS
Introduction
COST & EXTENT OF PROTECTION
=
POTENTIAL HAZARDS
SIZE OF MOTOR, TYPE & IMPORTANCE OF THE LOAD
3
> Motor Protection
Motor Protection
4
SYSTEM
MOTOR CIRCUIT
LOAD
Voltage Dips
Insulation failure
Overload
Voltage Unbalance
Open circuits
Locked rotor
Loss of supply
Short circuits
Coupling faults
Faults
Overheating
Bearing faults
> Motor Protection
For Induction motors
The main types to be considered :
Overtemperature
Thermal overload Stall / Locked rotor
Phase unbalance and single phasing
5
Short Circuit
Earth Fault
Undercurrent
> Motor Protection
Motor Protection Application Voltage
< 600V
Rating
< 11kW
Switching Device
or
Protection
(i) Fuses (ii) Fuses + direct acting thermal O/L + U/V releases
< 600V
11 - 300kW
or
Fuses + Electronic O/L
3.3kV
100kW - 1.5MW
or
+ Time delayed E/F Options :- Stalling
6.6kV
1MW - 3MW
or
6.6kV
> 1MW
Circuit Breaker
Undercurrent
As above + Instantaneous O/C
11kV
6
> Motor Protection
> 1MW
Circuit Breaker
+ Differential
Introduction
Protection must be able to :-
Operate for abnormal conditions
Protection must not :Affect normal motor operation
Considerations :-
7
> Motor Protection
Starting current Starting time Full load current Stall withstand time (hot & cold) Thermal withstand
Motor Currents
INDUCTION MOTOR
Stator
fr
Field f
Define Slip, S, as the per unit difference in speed between the stator and rotor fields
Slip “S” = f - fr f Speed of stator field relative to rotor f - fr = sf 8
> Motor Protection
Motor Currents INDUCTION MOTOR
Magnitude of induced volts : Proportional to sf
Frequency of induced rotor current :
Equal to sf
R2 kVS
9
> Motor Protection
X2 = 2fL (STAND STILL)
Rotor Equivalent Circuits STANDSTILL :
R2
kVS
RUNNING : X2 = 2fL ROTOR REACTANCE AT STANDSTILL
R2 2sfL = sX2
SkVS
Rotor Current
skVs
R 2 S X 2 2
2 2
kVs
R2 2 2 X 2 S2
1/2
1/2
R2 kVS
10
> Motor Protection
S
X2
Rotor Equivalent Circuits
R2
2sfL = sX2
SkVS
Rotor Current
skVs
R2 2 S2 X22
R2 2 S2
1/2
kVs
1/2 2 X2
R2 kVS
11
> Motor Protection
S
X2
Motor Starting Characteristics Rotor Current per phase Time
Start Time
R22 S2 X22
R 2 2 S2
> Motor Protection
kVs
1/2
1/2
X 22
X2 >> R2 Therefore R2 >> X2 when s is small
Full Load Current 12
skVs
Current
About to Start
Phase Loss Low Volts
13
> Motor Protection
Reverse Phase Sequence Starting
Protection required for lift motors, conveyors
Instantaneous I2 unit Time delayed thermal trip
Separate phase sequence detector for low load current machines
14
> Motor Protection
Undervoltage Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Undervoltage
Causes low output torque
machine cannot reach rated speed draws high stator current
Use time delayed undervoltage protection
16
> Motor Protection
47 Three Phase Voltage Check
V2
Prevents reverse operation of machine Vabc>Start Low V Setting
Avoids excessive start times on DOL machines caused by inadequate voltage
17
> Motor Protection
Undervoltage 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 :-
18
Disconnects motor from failed supply
Disconnects motor after dip long enough to prevent successful re-acceleration
> Motor Protection
Undervoltage Tripping Means of undervoltage tripping :
AC holding coil for fused or
Undervoltage release
Undervoltage relay for shunt trip
Definite time Inverse time Considerations:-
19
U/V tripping should be delayed for essential motors so that they may be given a chance to re-accelerate following a short voltage dip (< 0.5s)
Delayed drop-out of fused or could be arranged by using a capacitor in parallel with the AC holding coil
> Motor Protection
Mechanical Overload
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Mechanical Overload OVERLOAD
HEATING
INSULATION DETERIORATION
OVERLOAD PROTECTION
FUSES 21
> Motor Protection
THERMAL REPLICA
Motor Heating
HEAT STORED INCREASES THE MOTOR TEMPERATURE
HEAT DEVELOPED AT A CONSTANT RATE DUE TO CURRENT FLOW
HEAT DISSIPATED AT A RATE PROPORTIONAL TO MOTOR TEMPERATURE 22
> Motor Protection
Motor Heating MOTOR TEMPERATURE
T = Tmax (1 - e-t/) TMAX
Time or as temp rise
Rate of rise depend on motor thermal time constant (current)2
T = KI2max (1 - e-t/) 23
> Motor Protection
Motor Heating I2 I22
T2 T1
I12 IR2
TMAX
t2 t1
Time
Time
t1
Thermal Withstand
t2
IR I1 I2 24
> Motor Protection
Current
Motor Heating Current2 I2eq I 2 I 2m
tTRIP I2 - I2m = (I2eq - I2m) (1 - e-t/) Rearranging this expression in of time t
or alternatively t 25
> Motor Protection
Ι2eq - Ι2m 1n Ι2eq - Ι2
K2 - a2 1n K2 - 12
Time
Motor Cooling COOLING EQUATION :
I2m' = I2m e-t/r Current2 Im
I m' 0
t
Time
After time „t‟ equivalent motor current is reduced from Im to Im‟.
26
> Motor Protection
Motor Heating Temp
Trip Tmax T
Cooling time constant r
t1
t1 = Motor restart not possible t2 = Motor restart possible 27
> Motor Protection
t2
Time
Start / Stall Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Stalling Protection
Required for : Stalling on start-up (locked rotor) Stalling during running
With normal 3Ø supply :ISTALL = ILOCKED ROTOR @ ISTART
Cannot distinguish between „STALL‟ and „START‟ by current alone.
29
Most cases :-
tSTART < tSTALL WITHSTAND
Sometimes :-
tSTART > tSTALL WITHSTAND
> Motor Protection
Locked Rotor Protection Start Time < Stall Withstand Time
Where Starting Time is less than Stall Withstand Time :
Use thermal protection characteristic Use dedicated locked rotor protection
30
> Motor Protection
Stall Protection :tSTART < tSTALL Thermal relay provides protection against 3Ø stall.
t
Thermal Stall Withstand
tS tLS T
Start
IFL 31
> Motor Protection
IST ISL
I
If Stall Withstand Is Below Thermal Curve Separate stalling relay required :- Definite time O/C.
Definite Time Thermal
tS t LS
O/C (IS)
(tS) T
Stall Withstand
tSTART
Trip tSL > tS > tSTART
IS 32
> Motor Protection
IST ISL
Stall Protection Tstart < Tstall Use of motor start and 2 stage definite time overcurrent relay. Time
-
+ MSD
TD1
TD1
O/C
TD2
TD2 86
TRIP
Cold Stall tSL (COLD)
TD1+TD2 TD1 tST
Hot Stall tSL (HOT)
start time
TD2 Full load Current
TD1 > tST
(TD1 + TD2) < tSL(COLD) TD2 < tSL(HOT)
33
> Motor Protection
Io/c
Current
Locked Rotor Protection Start Time > Cold Stall Withstand Time
Motors with high inertia loads may often take longer to start than the stall withstand time However, the rotor is not being damaged because, as the rotor turns the “skin effect” reduces, allowing the current to occupy more of the rotor winding This reduces the heat generated and dissipates the existing heat over a greater area
Detect start using tachometer input
34
> Motor Protection
Stall Protection Tstart > Tstall Use of tachoswitch and definite time overcurrent relay.
-
+ TACHO
O/C
Time
Tacho opens at 10% speed
TD
TD < Tstall > Tacho opening
TD 86
Start Time TRIP
Stall - Tstall
TD
Full load Current
35
> Motor Protection
Io/c
Current
Unbalanced Supply Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Motor Currents NEGATIVE SEQUENCE CURRENT
fr
Relative frequency of stator field = f + fr But Therefore
37
> Motor Protection
fr = (1-s)f f + fr = (2-s)f
Stator Field f
Operation on Supply Unbalance
At normal running speed POSITIVE SEQ IMP NEGATIVE SEQ IMP
STARTING CURRENT NORMAL RUNNING CURRENT
Negative sequence impedance is much less than positive sequence impedance. Small unbalance = relatively large negative sequence current. Heating effect of negative sequence is greater than equivalent positive sequence current because they are HIGHER FREQUENCY.
39
> Motor Protection
Equivalent Motor Current
Heating from negative sequence current greater than positive sequence
take this into in thermal calculation
Ieq = (I12 + nI22)½
where : n = typically 6
40
small amount of I2 gives large increase in Ieq and hence calculated motor thermal state.
> Motor Protection
Loss of 1 Phase While Starting STAR A
Normal starting current VAN z With 1 phase open ΙA
C
B
Ι' A
3VAN VAB 2z 2z 0.866 x Ι A
1 1 (Ι' A aΙ'B ) (1- a)Ι' A 3 3 1 Ι1 Ι A 2 1 1 Ι 2 (Ι' A a2 Ι'B ) (1- a2 )Ι' A 3 3 1 Ι2 Ι A 2 Ι1
41
> Motor Protection
DELTA A
z
z
C z B
Normal
3VAB z
1 Phase open 3 VAB x 2z 0.866 x normal 1 winding carries twice the current in the other 2.
Single Phase Stalling Protection
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 current i.e. Rated Current
42
> Motor Protection
Single Phasing While Running
Difficult to analyse in simple
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.
43
> Motor Protection
Insulation Failure
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Insulation Failure
Results of prolonged or cyclic overheating
Instantaneous Earth Fault Protection Instantaneous Overcurrent Protection Differential Protection on some large machines
45
> Motor Protection
Stator Earth Fault Protection
Rstab
50
(A) Residually connected CT‟s
M
50
M
Note:
(B) Core Balance (Toroidal)CT
* In (A) CT‟s can also drive thermal protection * In (B) protection can be more sensitive and is stable
46
> Motor Protection
Typical Core-Balance CT Application
47
> Motor Protection
50 Short Circuit
Due to the machine construction internal phase-phase faults are almost impossible Most phase-phase faults occur at the machine terminals or occasionally in the cabling Ideally the S/C protection should be set just above the max Istart (I>>=1.25Istart), however, there is an initial start current of up to 2.5Istart which rapidly reduces over 3 cycles
Increase I>> or delay tI>> in small increments according to start conditions
Use special I>> characteristic
48
> Motor Protection
Differential Protection
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
High-Impedance Winding Differential Protection A
B
C
87 A
Note: 50
87 B
87 C
Protection must be stable with starting current.
> Motor Protection
Self-Balance Winding Differential Protection A
87 A
B
87 B
C
87 C 51
> Motor Protection
Instantaneous Earth Fault or Neg. Seq. Tripping is not Permitted with ors TRIP
TIME MPR FUSE M
MPR ELEMENT
Ts
Is
Icont
CURRENT
Ts > Tfuse at Icont. 52
> Motor Protection
Bearings
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Bearing Failure
Electrical Interference Induced voltage Results in circulating currents May fuse the bearings to take precautions - earthing
Mechanical Failure
Increased Friction Loss or Low Lubricant Heating
54
> Motor Protection
Bearing Failure
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
55
> Motor Protection
Use of RTDs
RTD sensors at known stator hotspots
Absolute temperature measurements to bias the relay thermal characteristic Monitoring of motor / load bearing temperatures Ambient air temperature measurement
56
> Motor Protection
49 Thermal protection with ambient compensation
125%
Ith (%setting)
100% 75% 50% 25% 0% 0°C
20°C
40°C
60°C
Temperature 57
> Motor Protection
80°C
100°C
26 Stator RTDs
RTDs cannot provide protection to the rotor The protection provided during overloads is dependant upon their positioning in the Stator windings and the insulation around them.
In general current measurement is considered superior under large overload conditions They do, however, respond to other conditions not present in the current measurement (blocked cooling etc)
58
> Motor Protection
Undercurrent Protection
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
60
> Motor Protection
37 Loss of load
In most applications it is desirable to stop the motor if the mechanical coupling is lost. In addition a pump can be damaged if it becomes unprimed
No load current is normally about 50-60% of Ifl On lightly loaded machines underpower provides better discrimination between low load and load loss
No load power about 10%
May need to inhibit during start
61
> Motor Protection
Normal Shutdowns
On machines where the thermal element is limited during start, it is critical to ensure that restarts do not damage the machine Selectable thermal start inhibit Jogging
Selectable number of hot starts, cold starts, period and inhibit time
Selectable time between 2 starts
62
> Motor Protection
49 Thermal start inhibit
Motor halt Prohibit START Threshold
PROHIBIT START
63
> Motor Protection
New restart
66 Limited starts/period
2 hot starts Hot start
Cold start
Hot start
Treference Tprohibit Information on « Start Prohibited »
64
> Motor Protection
66 Time between 2 starts
Tbetween 2 starts
Tbetween 2 starts
Tbetween 2 starts
Signal « Minimum time between 2 restarts »
65
> Motor Protection
86 Lockout
Some trips require maintenance before the machine can be restarted. A latched trip may be applied for the following conditions
Short circuit Earth faults Loss of Phase
66
> Motor Protection
Emergency Restart
In certain applications, such as mine exhaust and ship pumps, a machine restart is required knowing that it will result in reduced life or even permanent damage.
All start up restrictions are inhibited Thermal state limited to 90%
67
> Motor Protection
Synchronous Motors
GRID Technical Institute
This document is the exclusive property of Alstom Grid and shall not be transmitted by any means, copied, reproduced or modified without the prior written consent of Alstom Grid Technical Institute. All rights reserved.
Copyright © of AREVA T&D UK Limited
Synchronous Machines OUT OF STEP PROTECTION
Inadequate field or excessive load can cause the machine to fall out of step. This subjects the machine to overcurrent and pulsating torque leading to stalling
Field Current Method
Detect AC Current Induced In Field Circuit.
Power Factor Method Detect Heavy Current At Low Power Factor.
69
> Motor Protection
Synchronous Machines
LOSS OF SUPPLY On loss of supply motor should be disconnected if supply could be restored automatically. Avoids supply being restored out of phase.
70
Overvoltage & Underfrequency Underpower & Reverse Power
> Motor Protection
Synchronous Machines
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
71
> Motor Protection
Synchronous Machines
Underfrequency
> Motor loaded: Decelerate fairly quickly & frequency of terminal voltage will fall.
72
> Motor Protection
Synchronous Machines
Underpower Only applicable when power reversals do not occur under normal operating conditions Arranged to look into the machine; applicable when there is A possibility of no load connected on loss of supply.
Time delay required to overcome momentary power reversals due to faults
73
> Motor Protection
Synchronous Machines
Reverse Power Only Applicable When Power Reversals Do Not Occur Under Normal Operating Conditions
Arranged To Look Away From The Machine; Applicable Where There Is Aways Load Connected. Time Delay Required To Overcome Momentary Power Reversals Due To Faults
74
> Motor Protection
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