(i) Fuses (ii) Fuses + direct acting thermal O/L + U/V releases
< 600V
11 - 300kW
Contactor
3.3kV
100kW - 1.5MW
Contactor
6.6kV
1MW - 3MW
Contactor
6.6kV
> 1MW
Circuit Breaker
11kV
> 1MW
Circuit Breaker
Fuses + Electronic O/L + Time delayed E/F Options :- Stalling Undercurrent As above + Instantaneous O/C + Differential
Introduction 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
Mechanical Overload
Mechanical Overload OVERLOAD
HEATING
INSULATION DETERIORATION
OVERLOAD PROTECTION
FUSES
THERMAL REPLICA
Motor Heating MOTOR TEMPERATURE T = Tmax (1 - e-t/τ) TMAX
Time Rate of rise depend on motor thermal time constant τ
or as temp rise ∝ (current)2 T = KI2max (1 - e-t/τ)
Motor Heating I2 I22
T2 T1
I12 IR2
TMAX
t2 t1
Time
Time
t1
Thermal Withstand
t2
IR I1 I2
Current
Motor Cooling COOLING EQUATION : I2m' = I2m e-t/τr Current2 Im
Im' 0
t
Time
After time ‘t’ equivalent motor current is reduced from Im to Im’.
Motor Heating Temp
Trip Tmax T
Cooling time constant τr
t1
t1 = Motor restart not possible t2 = Motor restart possible
t2
Time
Emergency Restart
z
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%
Start / Stall Protection
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. Most cases :-
tSTART < tSTALL WITHSTAND
Sometimes :-
tSTART > tSTALL WITHSTAND
Locked Rotor Protection Start Time < Stall Withstand Time
Where Starting Time is less than Stall Withstand Time : z Use thermal protection characteristic z Use dedicated locked rotor protection
Locked Rotor Protection :- tSTART < tSTALL Thermal relay also provides protection against 3Ø stall. t
Thermal Cold Curve Cold Stall Withstand
tSL tST Start
IFL
Thermal Hot Curve IST ISL
I
Dedicated Locked Rotor Protection
Definite Time Thermal Cold tSL tS
Cold Stall Withstand
tSTART
O/C (IS)
(tS) T
Trip
tSL > tS > tSTART IS
IST ISL
Hot Stall Protection Tstart < Tstall Use of motor start contact to distinguish between starting and hot stall Time
Hot Stall Withstand start time
tSL (HOT) Full load Current
Io/c
Current
Locked Rotor Protection Start Time > Cold Stall Withstand z z
z
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 z Detect start using tachometer input
Stall Protection Tstart > Tstall Use of tachoswitch and definite time overcurrent relay. Time
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.
Operation on Supply Unbalance At normal running speed POSITIVE SEQ IMP ≈ NEGATIVE SEQ IMP CURRENT
STARTING CURRENT NORMAL RUNNING
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.
Equivalent Motor Current Heating from negative sequence current greater than positive sequence →
take this into account in thermal calculation
Ieq = (I12 + nI22)½ where : n = typically 6 →
small amount of I2 gives large increase in Ieq and hence calculated motor thermal state.
Loss of 1 Phase While Starting STAR A
C
Normal starting current V Ι A = AN z With 1 phase open
DELTA
B
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 2 Ι 2 = (Ι' A + a Ι'B ) = (1- a2 )Ι' A 3 3 1 Ι2 = Ι A 2 Ι1 =
A
z
z
C z
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
z z z
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
Single Phasing While Running
Difficult to analyse in simple terms z Slip calculation complex z Additional I2 fed from parallel equipment Results in :z I2 causes high rotor losses. Heating considerably increased. z Motor output reduced. May stall depending on load. z Motor current increases.
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
Undervoltage Protection
Undervoltage Considerations z z z z
Reduced torque Increased stator current Reduced speed Failure to run-up
Form of undervoltage condition :z Slight but prolonged (regulation) z Large transient dip (fault clearance) Undervoltage protection :z Disconnects motor from failed supply z Disconnects motor after dip long enough to prevent successful re-acceleration
Undervoltage Considerations z
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)
z
Delayed drop-out of fused contactor could be arranged by using a capacitor in parallel with the AC holding coil
Insulation Failure
Insulation Failure
Results of prolonged or cyclic overheating z Instantaneous Earth Fault Protection z Instantaneous Overcurrent Protection z Differential Protection on some large machines
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
50 Short Circuit z z z
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
Instantaneous Earth Fault or Neg. Seq. Tripping is not Permitted with Contactors
TRIP
TIME MPR FUSE M MPR ELEMENT
Ts
Is
Icont
CURRENT
Ts > Tfuse at Icont.
Differential Protection
High-Impedance Winding Differential Protection A
B
C
87 A
87 B
87 C
Note: Protection must be stable with starting current.
Self-Balance Winding Differential Protection A
87 A
B
87 B
C
87 C
Bearings
Bearing Failure
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
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
Synchronous Motors
Synchronous Machines z
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.
Synchronous Machines
z
LOSS OF SUPPLY On Loss Of Supply Motor Should Be Disconnected If Supply Could Be Restored Automatically. Avoids Supply Being Restored Out Of Phase. >Over voltage & Under frequency >Under power & Reverse Power
