Mining Electrical Maintenance & Safety Association
2015 Technical Conference
Clearwater Beach
June 10-12, 2015
Maximizing Motor Life with
Optimal Protection and Source Transfer
6190-118th Avenue · Largo, Florida 33773-3724 U.S.A.
PHONE (727) 544-2326 · FAX (727) 546-0121
whartmann@beckwithelectric.com
www.beckwithelectric.com
Presenter Contact Info
Wayne Hartmann
VP, Protection and Smart Grid Solutions
Beckwith Electric Company
whartmann@beckwithelectric.com
904-238-3844
Wayne Hartmann is VP, Protection and Smart Grid Solutions for
Beckwith Electric. He provides Customer and Industry linkage to
Beckwith Electric’s solutions, as well as contributing expertise for
application engineering, training and product development.
Before joining Beckwith Electric, Wayne performed in application, sales and marketing management
capacities with PowerSecure, General Electric, Siemens Power T&D and Alstom T&D. During the
course of Wayne's participation in the industry, his focus has been on the application of protection and
control systems for electrical generation, transmission, distribution, and distributed energy resources.
Wayne is very active in IEEE as a Senior Member serving as a Main Committee Member of the IEEE
Power System Relaying Committee for 25 years. His IEEE tenure includes having chaired the Rotating
Machinery Protection Subcommittee (’07-’10), contributing to numerous standards, guides,
transactions, reports and tutorials, and teaching at the T&D Conference and various local PES and
IAS chapters. He has authored and presented numerous technical papers and contributed to McGrawHill's “Standard Handbook of Power Plant Engineering, 2nd Ed.”
2
Our Session Today
•
•
•
•
•
Explore motor failure modes and causes
Discuss motor protective relay applications
Detail thermal issues and modeling
Information needed to set a motor protection relay
Special Considerations/Applications
– Variable Frequency Drives
– Motor Bus Transfer (Motor Source Transfer)
3
A Motor Life Depends On…..
Ambient
Temperature
and Cooling
Power
 What you feed it
 Quality of power
• Voltage unbalance
• Voltage level
• Harmonics
 Staying cool
 High ambient, compromised
Load
ventilation
 How onerous of a load
 Pulsating load
 Too great a load
4
What Fails?
Per IEEE 2007 Survey
Summary:
•Bearings (40 - 50%)
•Stator (25 - 35%)
•Rotor (<5%)
•Other Failures
5
Motor Failure Modes
IEEE Recommended Practice for the Design of Reliable Industrial and
Commercial Power Systems: IEEE Std. 493-2007, Table 10-19
6
Thermal Stress Causes Motor Failure
 Many of the motor failure contributors (IEEE Survey) and failed motor
components (EPRI Survey) are related to motor overheating.
 Thermal stress potentially can cause the failure of all the major motor parts:
Stator, Rotor, Bearings, Shaft and Frame.
ROTOR
STATOR
BEARINGS
SHAFT
FRAME
7
Motor Electrical Protection
 Phase Fault
 Ground Fault
 Abnormal Operating Conditions
 Voltage (Over/Under)
 Frequency (Under)
 Voltage and current imbalance
 Load loss
 Jamming
 Jogging
 Thermal Overload
 Process caused (too much load)
 High ambient conditions (Hot, Blocked ventilation)
 Power supply issues (Vbal, Harmonics)
8
Short Circuit Development Within Motor
 Short circuits in a motor of typically caused by
insulation breakdown in stator
 Insulation breakdown from heating caused by
issues with
 Load
 Motor power supply quality
• Voltage level, unbalance
• Waveform distortion
Environment
9
Short Circuit: Phase Fault
 Phase Overcurrent (50)
 Stator winding φ-φ faults
 Used with breaker rated for fault
interruption
3Y
50
52
MPR
• Do not use with fused starters as the
contactors are not rated for phase fault
current interruption
M
10
Short Circuit: Phase Fault
 Phase Differential (87)
 Typically applied only on very large
motors ($$$)
• Needs extra CTs and cable routing
• One window CT per phase
• Neutral must be made once neutral end
cabling is passed through window CT
 Covers stator winding φ-φ faults
 May cover certain ground faults
depending on source system grounding
 Used with breaker rated for fault
interruption
• Do not use with fused starters as the
contactors are not rated for phase fault
current interruption
11
Short Circuit: Ground Fault
 Ground Overcurrent (50G)
 Used on medium/high impedance grounded
systems
51
G
1
52
MPR
• If maximum ground fault current is lower
than rating, this scheme may be applied with
fused contactor starters
 Window CT employed with low ratio for
increased sensitivity
 Residual Overcurrent (50N)
 Sum of phase CT currents
•
M
*
3Y
52
51
N
MPR
Summing may be made by residual connection
or mathematically calculated by relay
 Used on low impedance (solidly) grounded
systems
M
12
Abnormal Operating Condition Protection [1]
 Load-Loss (37)
 Protection against pumps running dry,
deadheaded, belt/linkage breakage
 Under-power or undercurrent
2
or 3Y
 Load-Jam or Stall (39)
 Faster then waiting for thermal overload
 May lessen damage to drive train
 Starts/Hour, Time Between Starts (66)
 Anti-jogging protection
 Current Unbalance Element (46)
3Y
37
39
66
46
MPR
52
M
 Negative sequence currents rapidly heat
stator when running at rated speed
 Caused by voltage unbalance in supply
13
Abnormal Operating Condition Protection [2]
 Phase Reversal Protection (46 or 47)

 Anti-Backspin Protection


2
Supply phases reversed after event
“Down hole” pumps
Leaky check valves cause backflow
3Y
46
or 3Y
ABS
47
MPR
52
 Voltage Unbalance (47)


Caused by unbalanced load on supply
bus or loss of phase (single phasing)
Adjusts thermal model (decreases
capacity) based on NEMA derating
schedule
M
14
Abnormal Operating Condition Protection [3]
 Undervoltage (27)
 Longer starts (less torque on start)
 May not allow a start as torque is too low
 Higher current draw once running
2
or 3Y
 Overvoltage (59)
 Less current draw
 May violate dielectric constraints
 Power Factor Element (55)
3Y
55
59
27
81
U
MPR
52
 Is PF correction connected / effective?
 Under-frequency Element (81-U)
 Decreases ventilation
 V/Hz Issues
M
• low f makes for higher V/Hz at same voltage
15
Slide #15
Motor Mechanical – Possible Bearing Problems
 Lubricant issues
 Grade, contaminants, availability
 Mechanical
 Excessive radial loading, axial loading
 Rough surfaces
 Fatigue, cracks, shaft currents
 Vibration
 Unbalanced phase currents and harmonics
16
Resistance Temperature Detectors (RTDs)
 Detect Bearing Temperature (38)
 Detect mechanical issues
• Friction causes hear
• Misalignment cause heat
 Detect Winding Temperature (49)
 Electrical or non-electrical heating
• Overrides the relay thermal model
• Shifts the relay thermal model
 Detects Loss of Cooling Efficiency
• Cooling system failure
• High ambient temperature
17
Protection Offered by Thermal Modeling
T
I
M
E
 Best way to prevent
short in motor is not to
overheat and degrade
the insulation
 Repeated overheating
of motor insulation
causes cumulative
degradation
 Protects both the stator
and the rotor from
overheating
Ref: ANSI C37.96-2012
18
How Heat Is Made
Efficiency:
An indication of how much electrical energy is converted
to output shaft mechanical energy expressed as a
percentage.
Losses
Core loss
Stator loss
Input:
Electrical
Energy
Rotor Loss
Output
Mechanical
Energy
Friction
and
Windage
Stray loss
Electrical Energy in = Mechanical Energy out + Losses (mostly heat)
19
Thermal Model - Start
The sources of thermal energy that will fill the vessel or heating the motor are:
• Ambient temperature
• Motor losses due to current unbalances and I squared T
• Motor heating due to a start – model protects from too many starts / hour
20
Thermal Model - Run
• The fan is representative of the additional cooling
effect of the motor’s cooling system which is
commonly a fan mounted on the motor shaft.
21
Effect of Voltage
 Starting time and
current are voltage
dependent
 Lower voltage causes
lower current and lower
torque, therefore longer
start times
Ref: ANSI C37.96-2012
22
Voltage Unbalance Derates Thermal Capacity
 Standing voltage unbalance causes current unbalance which
creates negative sequence current in motor
 Negative sequence current causes heating in both the stator
and rotor
23
Standard
O/L Curves
24
Effect of
High Inertia
Time (secs)
 High inertial starts tend to
use a lot of the thermal
capacity available in a
motor
 Energy absorbed in the
rotor during a start ~ energy
in the load at running RPM
 Difficult to coordinate with
single OC curve
Ref: ANSI C37.96-2012
% Current
25
INTERTIAL START MOTOR AT 100%
Effect of
High Inertia
 High inertial starts tend to use
a lot of the thermal capacity
available in a motor
 Difficult to coordinate with
single overcurrent curve (OC)
Ref: ANSI C37.96-2012
26
Rotor Heating
on Start Up
 The thermal capacity of the rotor cannot be measured directly
 Inferred from curves supplied by the motor manufacturer and
monitoring of operation
 On start, rotor resistance is up and large current is drawn
 Large negative sequence current at start as rotor is at
standstill
27
Motor Specifications
Motor Data Sheet
Hot/Cold Safe
Stall Time Ratio
HCR =
LRTHOT
LRTCOLD
Overload Curve
Method
LRT cold=8sec
LRT hot=6sec
LRC=5.5FLA
28
Motor Thermal Parameters
E
F
Motor Data Sheet Parameters
E. Temperature Rise, Insulation Class
F. Locked Rotor Time; Cold/Hot
G. Number of Starts; Cold/Hot
G
29
Motor Thermal Limit Curves
A.
B.
C.
D.
Cold Running Overload
Hot Running Overload
Cold Locked Rotor Curve
Hot Locked Rotor Curve
A
C
B
D
30
Slide #
Start Inhibit Example
Thermal Capacity required to start
40%
Assume that a motor requires 40%
of it’s thermal capacity to start.
Thermal Capacity used due to Overload
80%
20%
80%
60%
If the motor had been running in an overload
condition prior to stopping, the thermal
capacity would be some value; say 80%.
Thermal Capacity must decay by
20% (from 80% to 60% Used) in
order to start the motor.
31
Thermal Capacity Used
Starts per Hour & Cooling Time Constant
100%
66.6%
2 S tarts per H our
50%
33.3%
0%
Thermal Capacity Used
T im e
100%
66.6%
3 S tarts per H our
50%
33.3%
0%
T im e
Motor heating due to a start – model protects
from too many starts / hour
32
Motor Protection
33
Synchronous Motor Protection & Control
34
SPECIAL MOTOR
PROTECTION TOPICS
35
Special Application #1
Variable Speed Drives
36
Why Drives Are
Great from a
Motor Thermal
Perspective
 VFD is Ultimate soft start
 Never go near locked rotor amp draw
 Much less mechanical stress versus locked rotor across
the line starting
 Much more thermal reserve available after a VFD start if
motor is stopped and then restart requested
37
Protection
Zones
1
Diode
Rectifier
IEGT of IGBT PWM
Voltage Source
Inverter with Diode
Converter
Inverter
(IGBT
or IEGT)
 Blown Fuse
 Over / Under Volts
 Phase loss
 Short Circuit
DC BusOver /
Under Volts
2
 Inverter Overtemp
 Overload
 Short circuit
 Sensor Integrity
 Ground fault
 Current balance
3
38
VFD Application Protection Zones
 ZONE 1: Drive Source
 Protects for short circuit in cabling/bus to drive and
a failure of the interrupter
 May include transformer supplying drive
 Suitable transformer protection should be provided
 ZONE 2: The Drive Itself
 Protections covered by others in this seminar
•
•
Zone 1 uses protective relays operating at nominal frequency
and voltage (in non-fault state)
Zone 1 protective relays are not affected by off-nominal drive
produced frequency and voltage
39
VFD Application Protection Zones
• Zone 3: The Motor
Senses motor input current
 Off-nominal frequency and voltage
 Harmonics
Protects for short circuits, negative sequence
overcurrent, thermal overload
 46 and 49 should not be applied at drive input to
sense these issues at the motor terminals
Phase differential may also be applied
40
Zone 1 – Source & Input Transformer
 Primary phase and ground overcurrent protection
 Phase O/C has to allow for motor acceleration
 Ground O/C may require zero sequence CT depending
on expected ground fault current
 If Transformer is in zone
 Differential protection may be provided on transformer
• This may have limited effectiveness for ground faults
where the supply system is resistance grounded
 Phase O/C is applied for phase faults
• Phase O/C has to allow for motor acceleration
 Ground O/C
• On transformer secondary, Ground O/C may require
zero sequence CT depending on expected ground
fault current
41
Zone 2 – Drive Itself
 Typically monitors the input and output voltages and will alarm
and/ or trip for over or under voltages and voltage unbalances
 Some drives may also include protection of overvoltage on the
DC link
 Overcurrent protection is provided to protect the converter
electronics and interconnected bus or wiring
 Current levels are limited to acceptable levels by control
action and the drive is tripped if current is above these levels
for a preset time.
 Volts/Hz limiters and protection to avoid overfluxing at lower
frequencies
 Additional protection may be supplied by monitoring the
temperature of the drive and cooling medium
 If a link reactor is used it may also have temperature
monitoring and trip settings
42
Zone 3 Relay Protection of Motors on VFDs
 Primary protection for the drive is contained within the drive
itself
 Additional motor protection outside of the drive should be
applied if the motor is started across the line or is transferred
from the drive to line (multiplexed drive application)
 Secondary protection may be added in addition to imbedded
drive control/protection
– Philosophical decision
– Relay and sensors must be checked to ensure reliable operation
(accurate, secure and dependable)
– Entire relay or certain relay elements may not be able to reliably
function at all expected off-nominal frequency and voltage and
therefore not be applied or selectively blocked/disabled when frequency
and voltage are outside of reliable operation limits
43
Cautions for a Motor Relay
Applied at VFD Output
 Motor Protective Relay (MPR) must operate properly at all
expected frequencies and voltages
 Properly = accurately, securely and dependably
 Instrument transformers (ITs) must operate properly at all
expected frequencies and voltages
 Properly = accurately, securely and dependably
 If MPR/ITs cannot react properly at any expected
frequency, the MPR must be blocked from operation
– This may be accomplished by:
• External signal from drive control system to block relay
• Depowering the relay
• On-board frequency/voltage sensing and blocking logic built into
the relay
44
Zone 3 – Motor on VFD
 If relay and relay elements operate reliably at all expected
operating conditions, certain elements may be applied




Differential (87)
Thermal model (49)
Phase overcurrent (50/51)
Negative sequence overcurrent (46)
 Not advised due to lower off nominal conditions:




Load loss
Under frequency
Under voltage
Starts per hour (anti-jogging)
 Over limit protections, such as over frequency, overvoltage
and over excitation protection are not affected by lower off
nominal conditions
45
Bypass Contactor
Diode
Rectifier
IEGT or IGBT PWM
Voltage Source
Inverter with Diode
Converter
Inverter
(IGBT
or IEGT)
2 Blown Fuse
2 Over / Under Volts
2 Phase loss
2 Short Circuit
DC BusOver /
Under Volts
2 Inverter Overtemp
2 Overload
2 Short circuit
2 Sensor Integrity
2 Ground fault
2 Current balance
• MPR [Motor
Protective Relay] is
applied when motor
is supplied directly
from the line
• Drive Protects motor
while on VFD
• Three contactor
application
46
Special Application #2
Motor Source Re-energization (Motor Bus Transfer)
Incoming 1
Utility
Supply System
Bus 1
Supply Source
BUS 1 VT
Bus 2
Supply Source
INCOMING 1 VT
(Bus 2 Backup Source)
(Bus 1 Backup Source)
INCOMING 2 VT
N.C.
Bus Tie
Bus 1
Incoming 2
N.C.
BUS 2 VT
Bus 2
N.O.
M
M
M
M
M
M
47
Why Apply Motor Bus Transfer Systems?
• Present source to motor bus is deenergized or challenged
• Proactive plant isolation
• Unplanned utility outage
• Planned utility outage
• Fault on utility source
• Fault within plant system supplying motor bus
• If challenge to supply occurs, you want to switch to a new
source, very quickly if possible, to avoid restart of motors
• Process upset
• Process interruption
• Locked rotor starting
• Transient torques on motor
• Transient torques to driven load
• The transfer has to be correct
• Very dynamic situation while motors are still spinning




Phase angle rapidly moves
Slip frequency between motors and new source increases
Voltage on stranded motor bus decays
Coast down period for deenergized motor bus can range cycles to seconds
48
Motor Bus Transfer Terminology
• Power supply transfer of on a single motor or motor bus
–
–
–
–
Old source: Source motors are connected to before transfer
New source: Source for motors after transfer
Parallel transfer: Old source and new sources are paralleled
Sequential transfer: Old source and new sources are not paralleled
Incoming 1
Utility
Supply System
Bus 1
Supply Source
BUS 1 VT
Bus 2
Supply Source
INCOMING 1 VT
(Bus 2 Backup Source)
(Bus 1 Backup Source)
INCOMING 2 VT
NC
Bus Tie
Bus 1
Incoming 2
NC
BUS 2 VT
Bus 2
NO
M
M
M
M
M
M
49
Motor Bus Transfer
Methods
Type
•
Closed Transition
•
Hot Paralleled
•
Sequential
•
•
•
•
Fast
In-Phase
Residual
Time Delayed
(Open Transition)
Considerations
•
•
Hot Paralleled, Fast and In-Phase Transfers do not cause
process interruption, motor starting, load shedding
Residual and Time Delayed Transfers cause process
interruption, motor starting, and may necessitate load shedding
50
Closed Transition – Hot Parallel Transfer
Source 1
Source 2
(Old Source)
(New Source)
Motor Bus
M
M
M
Old Source Closed
51
Closed Transition – Hot Parallel Transfer
Source 1
Source 2
(Old Source)
(New Source)
Motor Bus
M
M
M
Both Sources Closed For Transfer
52
Closed Transition – Hot Parallel Transfer
Source 1
Source 2
(New Source)
(Old Source)
Motor Bus
M
•
•
M
M
New Source Remains Closed
Old Source Opened
53
Open Transition
Source 1
Source 2
(Old Source)
(New Source)
Motor Bus
M
M
M
Old Source Closed
54
Open Transition
Source 1
Source 2
(Old Source)
(New Source)
Motor Bus
M
M
M
Old Source Opened
55
Open Transition
Source 1
Source 2
(New Source)
(Old Source)
Motor Bus
M
•
•
M
M
New Source Closes
Old Source Remains Opened
56
Two-Breaker Configuration
MAIN SOURCE
STARTUP SOURCE
UNIT AUXILIARY
TRANSFORMER
CT-M
VT-M
52
M
STATION SERVICE
TRANSFORMER
VT-SU
MBTS
VT-B
N.C.
CT-SU
N.O.
52
SU
STATION BUS SYSTEM
M
M
57
Three-Breaker Configuration
(Main-Tie-Main)
NORMAL SOURCE (Main 1 )
ALTERNATE SOURCE (Main 2)
NORMAL SOURCE
TRANSFORMER
ALTERNATE SOURCE
TRANSFORMER
CT-M2
CT-M1
MBTS
MBTS
VT-M1
52
M1
VT-M2
N.C.
N.O.
VT-B1
BUS 1
CT-B1
M
M
52
SU
VT-B2
52
Tie
N.O.
BUS 2
CT-B2
M
M
58
Effect of Motor/Load Inertia
•
High inertial loads tend to hold up motor buses
•
Motors on a bus create a composite decay characteristic
59
Closed Transition - Hot Parallel Transfer
Advantages
 No disruption of plant process
 Simple to implement with sync-check relay supervision across
new source breaker
 No transient torque on motors during the transfer
Disadvantages
 Cannot use during fault conditions
 Can use only for planned transfers
 The two sources must be in sync or within an acceptable small
static phase angle difference of each other
 Design must ensure that a parallel condition is temporary
 If fault occurs when sources are paralleled, circuit breaker and
through-fault withstand ratings may be violated
 The two sources may not be derived from the same primary
source
60
Open Transition Methods:
Fast Transfer
In-Phase Transfer
Residual Voltage Transfer
-20°
Bus
Slow
0°
40
+20°
Bus
Fast
Bus Transfer Zones
61
MBT Oscillography; Fast Transfer
62
Fast Transfer Method
 Presently, the majority of fast transfer systems are NOT
supervised by high-speed sync-check relays !
 In many cases, Fast Transfer cannot be correctly performed
without a high-speed sync check relay
 Some modern solid-state or microprocessor-based sync check
elements have a minimum time delay of 0.1 second or 100
milliseconds
 By the time they respond to the phase angle of a decaying motor
bus, the possibility of a successful transfer is long gone
 Worse yet, the contacts may be still closed and permit transfers
at excessive angles and damage critical motors
63
Fast Transfer Method
0°
-20°
Bus
Slow
+20°
Bus
Fast
40
 New source circuit breaker is
closed if the phase angle
between the motor bus and the
new source is within or moves
into the Phase Angle Limit
 This method requires high-speed
sync-check supervision
-20°
Bus
Slow
0°
40
• Must be able to close high
speed
• Must be able to block high
speed
+20°
Bus
Fast
 Circuit breaker closing is also
supervised by:
• Upper and Lower Voltage Limit
check on the new source
• Slip Frequency Limit (∆F)
64
In-Phase
Transfer Method
65
In-Phase Transfer Method
 The new source breaker will be closed by predicting movement
through phase coincidence between the motor bus and the new
source during the In-Phase Transfer Enable Window
 Due to the decaying motor bus frequency, slip frequency and
rate-of-change of frequency between the motor bus and the new
source must be calculated to correctly compensate for the
breaker closing time
 High speed (quarter-cycle or less) response is recommended.
 Predicted phase coincidence is used with breaker closing time of
the new source breaker to achieve a breaker close at phase
coincidence
 Additional supervision:
 Upper and Lower Voltage Limit check on the new source
 Slip (∆F) Frequency Limit between the motor bus and the new
source
66
MBT Oscillography; In-Phase Transfer
Phase Angle
67
Fast and In-Phase Transfers
 Fast Transfer




Requires ultra high speed sync check
Must be able to determine phase angle near instantaneously
Must be able to block for unfavorable phase angle in ½ cycle
Conventional sync check relays have 100mS minimum drop out time –
too long
 In-Phase Transfer
• Requires ultra high speed autosync
• Must be able to determine frequency, rate of change of frequency
(df/dT) and use breaker closing time to effect proper closure
• Must be able to measure high slip frequencies, and decaying
frequency accompanied by decaying voltage
68
Fast and In-Phase Sequential Transfers
Advantages
 No disruption of plant process
 Minimizes or eliminates transient torque on motors during the
transfer
 Can be used during fault conditions
 Can be used for planned transfers
 Applicable when two sources are not in sync or within an
acceptable small static phase angle difference of each other
 No concerns of exceeding fault ratings of circuit breakers or
through fault rating od transformers due to paralleling sources
 Applicable for use where two sources may not be derived from the
same primary source, or on a single source
Concerns
 These transfers must be performed correctly
69
Residual Voltage Transfer Method
Disadvantages
 Slow and cannot be used for planned transfers during plant startup
 Undervoltage relay must be accurate and reliable at low voltages and low
frequencies
 If motors are held in with contactors, latching or dc-operated contactors
must be used to ensure that the contactors do not drop out.
 Transfers must be completed before the bus voltage drops so low that the
motor protection’s undervoltage elements time out and trip
 During the time necessary to wait for sufficient voltage decay, the
frequency may have decayed past the stall point of motors, and load
shedding may be necessary.
 Restarting of motors subjects them to high starting currents/torques
 Properly sequenced motor may be required to prevent excessive
voltage dip
 Load shedding may also be necessary in the case where the new source
cannot reaccelerate all bus motors simultaneously. Process is interrupted.
70
Conditions Across New Source Breaker?
 Immediately prior to Transfer Initiate and on trip of Old
Source Breaker
 Instantaneous Phase Angle Shift as cut-loose motors change power
profile
 Effects of a Fault - System faults can temporarily depress the New Source
Voltage and can cause a Phase Angle difference between the Motor Bus
and the New Source.
 Load Angle or System Separation between Incoming Supply Sources
 Supply Source Transformer Winding Phase Shift
 Out-of-Step (OOS) Generator Trip – The angular difference between the
HV Bus & the Generator Terminals at the point of an OOS Trip will be the
Motor Bus Transfer initial angle relative to the New Source.
 ANSI STANDARD C50.41-2012, clause 14.3 states,
“calculations should account for any phase angle difference
between the incoming and running power supplies.”
71
ANSI STANDARD
C50.41-2012
 ANSI STANDARD C50.41-2012, clause 14.3 states,
“calculations should account for any phase angle difference
between the incoming and running power supplies.”
72
Industry Guidance
•
NEMA MG 1-2006 and NEMA/ANSI C50.41-2000, 1.33 V/Hz
vector difference to define a safe transfer of an induction
motor bus and its connected loads from one source to an
alternate power supply
•
This is where the 0.25pu voltage for residual transfers
originates
•
Goal of MBT System is to keep resultant V/Hz below
1.33, and minimize motor reacceleration current and
torques
73
Dynamic Test of Motor Bus Transfer System:
Initial Static Phase Angles
74
Standard Decay MBT Test Results
75
Standard Decay MBT Test Results
• Test voltage and frequency decay characteristics of High,
Medium, and Low Inertia Motor Buses
• Tests with Multiple Initial Static Phase Angles
• All 15 tests closed under 0.26 pu V/Hz.
• All 15 tests closed well below the 1.33 pu V/Hz and 90 degree
limits*
* ANSI C50.41 Polyphase Induction Motors for Power Generating Stations
• All 15 tests were performed with NO changes to settings.
 Fast Transfer Method Phase Angle Limit = 20°
 Fast Transfer Method Slip Frequency Limit = 2.0 Hz **
 In-Phase Transfer Method Slip Frequency Limit = 10.0 Hz
** Used to coordinate the actions of the Fast Transfer and the In-Phase Transfer
Methods to achieve an optimal close with the In-Phase Transfer Method.
76
Standard Decay MBT Test Results
• The ANSI C50.41 “10 cycles or less” criteria would reject
perfectly good transfers by the In-Phase Transfer Method:
 A High Inertia close at 0.24 pu V/Hz took 27 cycles
 A Medium Inertia close at 0.15 pu V/Hz took 16.7 cycles
 A Low Inertia close at 0.15 pu V/Hz took 13.3 cycles
• The arbitrary 10-cycle limit should be ignored as it may take
more than 10 cycles for the motors to rotate back into
synchronism.
• How fast can the motors transfer?

When the motors allow it by rotating back into sync ! ! !
• In the fast-moving world of motor bus transfer:
 10 cycles (167 ms) is an eternity
 10 cycles never was a safe limit for fast transfer*
* Even at a medium frequency decay of 20 Hz/sec (RS), with zero initial slip
frequency (SINIT), the angle movement (ΔØ) in 10 cycles (T) is a dangerous 100°.
ΔØ = 360(SINIT+0.5RST)T
77
Standard Decay MBT Test Results
 All transfers used the Sequential Transfer Mode
 This inherent breaker failure scheme adds a little time to the transfer,
still yielding excellent transfer results
 Avoids the possibly catastrophic result where the two breakers are
closed at the same time
 Simultaneous Transfer Mode initiates both trip and
supervised close breaker operations simultaneously
 It does not prevent the new breaker from closing if the old breaker
fails to trip
 Except in cases of extremely low inertia, the need for speed
could become a vestige of the past
 With modern technology, we now have the luxury to wait for the old
breaker to trip
78
Standard Decay MBT Test Results
• Synchronous Fast and In-Phase Transfers occur well before
the 0.33 pu voltage level of the Residual Voltage Slow Transfer
would operate.
• Synchronous Transfers vs. blind Residual Voltage Transfers:
 Much higher voltages
 Much lower slip frequencies
 With synchronous closure
• Residual Voltage Transfers subject motors and loads to:
 The jarring effect of a large phase angle at breaker closure
 High reacceleration current and associated torque
• Results demonstrate that the Fast and In-Phase Methods, can
also be applied to Low Voltage Motor Buses, rather than
having to resort to Residual Voltage Slow Transfers.
79
Actual MBT from Loaded Facilities
Live Open Transition Transfers Under Normal Operating Load Conditions
MBT FIELD RESULTS
Advance
Ø Angle
VS =
Close
Ø Angle Close ΔF Close Volts
ANSI
C50.41
pu V/Hz
120
FS =
60
Open
Max
Max
Transfer Transfer Transfer Torque
Time
Amps /
pu
Ratio
cycles
FLA
Power T PK/TL
LOCATION
Transfer Mode
Transfer
Method
FACILITY 1
Simultaneous
FAST
-0.1
-20.0
-2.83
93.8
0.3622
1.3
4.6
21.5
4.12
FACILITY 2
Sequential
FAST
-10.8
-16.3
-0.19
100.4
0.3054
5.0
2.4
5.9
2.38
FACILITY 3
Simultaneous
FAST
-3.0
-18.5
-0.81
103.4
0.3260
3.3
3.1
9.3
2.48
FACILITY 4
Sequential
FAST
-0.8
-6.8
-0.23
107.9
0.1489
2.9
2.7
7.2
1.97
FACILITY 5
Simultaneous
FAST
-1.2
-12.6
-1.76
103.2
0.2360
1.3
2.2
4.9
1.87
FACILITY 6
Simultaneous
FAST
-1.1
-16.5
-2.25
102.0
0.2939
1.4
1.8
3.3
1.62
FACILITY 7
Sequential
FAST
-2.8
-17.1
-0.49
98.7
0.3201
2.9
2.9
8.4
2.08
FACILITY 8
Sequential
-2.2
-12.7
-0.38
99.0
0.2635
2.9
1.8
3.3
1.50
FACILITY 9
Sequential
FAST
Residual
Voltage
152.4
128.4
-1.66
34.7
1.2074
48.7
4.8
23.0
21.74
FACILITY 10
Sequential
IN-PHASE
ØINIT =115°
55.0
-7.7
-2.77
44.4
0.6178
9.4
2.4
6.0
2.39
FACILITY 11
Sequential
IN-PHASE
78.9
7.1
-4.48
37.7
0.6644
17.7
2.3
5.2
1.89
FACILITY 12
Simultaneous
FAST
-0.1
-20.3
-2.23
89.4
0.3838
1.7
1.8
3.4
1.79
Actual MBT from Loaded Facilities
• Fast Transfers occurred in 9 instances
• In-Phase Transfers occurred in 2 instances.
• All Synchronous Transfers were completed at between 0.15
and 0.66 pu V/Hz
• All Synchronous Transfer breaker close commands occurred
at voltages above which the Residual Voltage Transfer
undervoltage element would have operated.
• A Residual Voltage Transfer occurred in 1 test when the
Synchronous Transfer Methods were purposely disabled, so
the results for a Residual Voltage Transfer could be observed.
• The Residual Voltage Transfer closed at 1.21 pu V/Hz.
81
Actual MBT from Loaded Facilities
•
ANSI 50.41 Refers to Ratio: Peak Inrush Current at Transfer, Max
Transfer Amps (MTA) ÷ Subsequent Steady State Full Load Amps (FLA).
•
Common Requirement: Motor Starting Current ≤ Specified Multiple of Full
Load Current at rated voltage for across-the-line full voltage starting.
•
Correlation: As the pu V/Hz rises, then the MTA/FLA would also rise?
NO CORRELATION
*
•
Range of MTA/FLA from 1.8 to 4.8 is reasonable compared to a normal
motor start. MTA/FLA may be overstated as load amps may be < FLA.
*2 In-Phase Transfers vs. 6 Fast Transfers: Higher pu V/Hz (0.62 and
0.66 pu V/Hz) but middle of the range MTA/FLA (2.4 and 2.3)
82
Actual MBT from Loaded Facilities
 The pu V/Hz calculation depends on only three values at closure
compared to the new source: the bus voltage difference, the bus
frequency difference, and the phase angle difference.
 One could imagine two vastly different sets of motors with two vastly
different sets of loads, but transferring with the same three values at closure


The calculated pu V/Hz would be exactly the same, but one wonders if the
motors and loads think so.
Therefore, the use of the 1.33 pu V/Hz limit across the open breaker as a
criterion for the safe transfer of motor buses leaves room for possible
improvement.
 The above FACILITY 1 through 12 oscillographic records of live motor bus
transfers will now be analyzed to derive a new transfer metric, based on the
voltage and current during inrush at the close of the new source breaker.

These values will be measured in the time domain and employed to
calculate the resultant peak torque at transfer as a multiple of load torque
prior to transfer as if the aggregate bus were a single induction motor
drawing the same current and power.
83
Actual MBT from Loaded Facilities
Motor Torque Calculation
The torque produced is equal to the electromagnetic power transferred
through the air gap (PAG) divided by the synchronous speed (ωS):
T = PAG/ωS
Assumes all losses (copper losses, iron losses, friction and windage losses) are neglected
The Air Gap Power is calculated for two different conditions:
•
Steady state Motor Torque prior to the Transfer (TL)
(uses current signal taken from the existing source along with the motorbus voltage signal)
•
Peak Motor torque (TPK) after the transfer has taken place
(uses current signal taken from the new source along with motorbus voltage signal)
•
The ratio TPK /TL is calculated for each facility
The Torque Ratio provides a normalized way of looking at transient torque during motorbus
transfer
84
Actual MBT from Loaded Facilities
Facility
1
2
3
4
5
6
7
8
9
10
11
12
Torque
Ratio
(TPK/TL)
4.12
2.38
2.48
1.97
1.87
1.62
2.08
1.50
21.74
2.39
1.89
1.79
Pu
V/Hz
0.3622 0.3054 0.3260 0.1489 0.2360 0.2939 0.3201 0.2635 1.2074 0.6178 0.6644 0.3838
Residual Voltage
Fast
In-Phase
85
Actual MBT from Loaded Facilities
Motor Torque Ratio TPK /TL Observations
 There is low correlation between pu V/Hz and Torque Ratio
 In-Phase Transfer cases (Facilities 10 and 11) have higher pu V/Hz but
lower inrush current ratios (Max Transfer Amps/FLA) and Torque Ratios
(TPK/TL).
 Torque Ratios for the two In-Phase Transfers fall right in the middle of
the Torque Ratios for all the Fast Transfers
 ANSI C50.41 states that transient torques during improper transfers can
reach 20 pu. Facility 9 results demonstrate this with a Torque Ratio of
21.74 for a Residual Voltage Transfer close at 128.4 degrees.
 Yet the ANSI C50.41 pu V/Hz limit of 1.33 would give this Residual
Voltage Transfer a passing grade at 1.2074 pu V/Hz.
 Max Transfer pu Power is almost the same (21.5 for Facility 1 and 23 for
Facility 9) whereas the motor Torque Ratios are vastly different (4.12 for
Facility 1 and 21.74 for Facility 9).
86
Actual MBT from Loaded Facilities
Motor Torque Ratio TPK /TL Conclusions
 ANSI 50.41 pu V/Hz is not a good measure of motor torque
 Max Transfer pu Power is not a good measure of motor torque
 Motor Torque Ratio (TPK /TL) can be calculated using the voltage and
current waveforms recorded at transfer and can indicate if a transfer is
performed within safe motor torque design limits
 Residual Voltage Transfer can produce dangerously high torques.

Phase angle and slip frequency are ignored
 In-Phase Transfer keeps motor torque well within safe limits

A good choice when Fast Transfer is not possible due to a:
• Large initial angle
• Too fast an initial slip frequency
87
Summary
 Proper MBT offers a way to provide process continuity and
motor and driven load asset life
 The Fast and In-Phase Transfers are methods to use:


Where Hot Parallel transfer cannot be done
Where Residual Transfer takes too long and causes process upset,
and high transient torques
 Specialized relays and systems are required to
successfully implement MBT
 MBT System are commercially available

You do not have to cobble together systems out of non-purpose
designed hardware and software
88
Questions?
Thank You
Remember, only you can prevent motor damage
89
References:

C37.96-2012, “IEEE Guide for AC Motor
Protection”

“Adjustable Speed Drive Motor
Protection Applications and Issues,”
IEEE PSRC Report, 10/08

GE Digital Energy 469 Motor Relay
Instruction Book

“Motor Bus Transfer Applications Issues
and Considerations,” IEEE PSRC
Report, 05/12

“Motor Bus Transfer System
Performance Testing and the Search for
a New Transfer Success Criterion”; T.
Beckwith, Dr. Murty Yalla; Beckwith
Electric; 2015 Georgia Tech Protective
Relay Conference

Beckwith Electric M-4272 Motor Bus
Transfer System Instruction Book
90