clearwater beach june 10-12, 2015 maximizing motor life with optimal ... · maximizing motor life...

Maximizing Motor Life withOptimal Protection and Source Transfer

Mining Electrical Maintenance & Safety Association2015 Technical Conference

6190-118th Avenue · Largo, Florida 33773-3724 U.S.A.PHONE (727) 544-2326 · FAX (727) 546-0121

[email protected]

Clearwater BeachJune 10-12, 2015

Presenter Contact Info

Wayne Hartmann is VP, Protection and Smart Grid Solutions forBeckwith Electric. He provides Customer and Industry linkage toBeckwith Electric’s solutions, as well as contributing expertise forapplication engineering, training and product development.

Wayne HartmannVP, Protection and Smart Grid Solutions

Beckwith Electric Company

[email protected]

904-238-3844

Before joining Beckwith Electric, Wayne performed in application, sales and marketing managementcapacities with PowerSecure, General Electric, Siemens Power T&D and Alstom T&D. During thecourse of Wayne's participation in the industry, his focus has been on the application of protection andcontrol 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 IEEEPower System Relaying Committee for 25 years. His IEEE tenure includes having chaired the RotatingMachinery Protection Subcommittee (’07-’10), contributing to numerous standards, guides,transactions, reports and tutorials, and teaching at the T&D Conference and various local PES andIAS chapters. He has authored and presented numerous technical papers and contributed to McGraw-Hill's “Standard Handbook of Power Plant Engineering, 2nd Ed.” 2

• 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)

Our Session Today

3

A Motor Life Depends On…..

What you feed it

Quality of power• Voltage unbalance• Voltage level• Harmonics

Staying cool

High ambient, compromisedventilation

How onerous of a load

Pulsating load

Too great a load

Power Load

AmbientTemperatureand Cooling

4

What Fails?Per IEEE 2007 Survey

•Bearings (40 - 50%)

•Stator (25 - 35%)

•Rotor (<5%)

•Other Failures

Summary:

5

Motor Failure Modes

IEEE Recommended Practice for the Design of Reliable Industrial andCommercial 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 motorcomponents (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.

STATORROTOR

BEARINGS

FRAME

SHAFT

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 byinsulation breakdown in stator

Insulation breakdown from heating caused byissues 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 faultinterruption• Do not use with fused starters as the

contactors are not rated for phase faultcurrent interruption

52

M

50

MPR

3Y

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 faultinterruption• Do not use with fused starters as the

contactors are not rated for phase faultcurrent interruption

11

Ground Overcurrent (50G) Used on medium/high impedance grounded

systems• If maximum ground fault current is lower

than rating, this scheme may be applied withfused contactor starters

Window CT employed with low ratio forincreased sensitivity

Residual Overcurrent (50N) Sum of phase CT currents

• Summing may be made by residual connectionor mathematically calculated by relay

Used on low impedance (solidly) groundedsystems

Short Circuit: Ground Fault

52

M

51N

MPR

3Y*

52

M

51G

MPR

1

12

Abnormal Operating Condition Protection [1]

Load-Loss (37) Protection against pumps running dry,

deadheaded, belt/linkage breakage Under-power or undercurrent

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) Negative sequence currents rapidly heat

stator when running at rated speed Caused by voltage unbalance in supply

M

MPR

3Y 39 66 46

2 or 3Y

52

37

13

Phase Reversal Protection (46 or 47) Supply phases reversed after event

Anti-Backspin Protection “Down hole” pumps Leaky check valves cause backflow

Voltage Unbalance (47) Caused by unbalanced load on supply

bus or loss of phase (single phasing) Adjusts thermal model (decreases

capacity) based on NEMA deratingschedule


M

MPR

3Y 47

2 or 3Y

52

46 ABS

14

Slide #15

Undervoltage (27) Longer starts (less torque on start) May not allow a start as torque is too low Higher current draw once running

Overvoltage (59) Less current draw May violate dielectric constraints

Power Factor Element (55) Is PF correction connected / effective?

Under-frequency Element (81-U) Decreases ventilation V/Hz Issues

• low f makes for higher V/Hz at same voltage


M

MPR

3Y 27

2 or 3Y

52

55 59 81U

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

Best way to preventshort in motor is not tooverheat and degradethe insulation

Repeated overheatingof motor insulationcauses cumulativedegradation

Protects both the statorand the rotor fromoverheating

Ref: ANSI C37.96-2012

T

I

M

E

18

Efficiency:

An indication of how much electrical energy is convertedto output shaft mechanical energy expressed as apercentage.

Core loss

Stator loss

Rotor Loss

FrictionandWindage

Stray loss

Losses

Electrical Energy in = Mechanical Energy out + Losses (mostly heat)

OutputMechanicalEnergy

Input:Electrical

Energy

How Heat Is Made

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 coolingeffect of the motor’s cooling system which iscommonly a fan mounted on the motor shaft.

21

Effect of Voltage

Starting time andcurrent are voltagedependent

Lower voltage causeslower current and lowertorque, therefore longerstart times

Ref: ANSI C37.96-2012

22

Voltage Unbalance Derates Thermal Capacity

Standing voltage unbalance causes current unbalance whichcreates negative sequence current in motor

Negative sequence current causes heating in both the statorand rotor

23

StandardO/L Curves

24

Effect ofHigh Inertia

High inertial starts tend touse a lot of the thermalcapacity available in amotor

Energy absorbed in therotor during a start ~ energyin the load at running RPM

Difficult to coordinate withsingle OC curve

Ref: ANSI C37.96-2012% Current

Tim

e(s

ec

s)

25

Effect ofHigh Inertia

High inertial starts tend to usea lot of the thermal capacityavailable in a motor

Difficult to coordinate withsingle overcurrent curve (OC)

Ref: ANSI C37.96-2012

INTERTIAL START MOTOR AT 100%

26

Rotor Heatingon Start Up

The thermal capacity of the rotor cannot be measured directly

Inferred from curves supplied by the motor manufacturer andmonitoring of operation

On start, rotor resistance is up and large current is drawn

Large negative sequence current at start as rotor is atstandstill 27

Hot/Cold SafeStall Time Ratio

COLD

HOT

LRT

LRTHCR =Motor Data Sheet

Overload CurveMethod

LRC=5.5FLA

LRT cold=8sec

LRT hot=6sec

Motor Specifications

28

Motor Data Sheet ParametersE. Temperature Rise, Insulation Class

E

G. Number of Starts; Cold/Hot

G

F. Locked Rotor Time; Cold/Hot

F

Motor Thermal Parameters

29

Slide #

Motor Thermal Limit Curves

B

A

D

C

B. Hot Running Overload

A. Cold Running Overload

D. Hot Locked Rotor Curve

C. Cold Locked Rotor Curve

30

Start Inhibit Example

40%

80%

Thermal Capacity required to start

Thermal Capacity used due to Overload

Thermal Capacity must decay by20% (from 80% to 60% Used) inorder to start the motor.

80%

60%20%

If the motor had been running in an overloadcondition prior to stopping, the thermalcapacity would be some value; say 80%.

Assume that a motor requires 40%of it’s thermal capacity to start.

31

Starts per Hour & Cooling Time Constant

Tim e

Th

erm

al

Cap

acit

yU

sed 100%

50%

33.3%

66.6%

0%

Tim e

Th

erm

alC

ap

acit

yU

sed 100%

50%

33.3%

66.6%

0%

2 Starts per Hour

3 Starts per Hour

Motor heating due to a start – model protectsfrom too many starts / hour

32

Motor Protection

33

Synchronous Motor Protection & Control

34

SPECIAL MOTORPROTECTION TOPICS

35

Special Application #1

Variable Speed Drives

36

Why Drives AreGreat from a

Motor ThermalPerspective

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 requested37

ProtectionZones

1

2

3

Inverter(IGBT

or IEGT)

DiodeRectifier

IEGT of IGBT PWMVoltage SourceInverter with DiodeConverter

Blown Fuse Over / Under Volts Phase loss Short Circuit

Inverter Overtemp Overload Short circuit Sensor Integrity Ground fault Current balance

DC BusOver /Under Volts

38

VFD Application Protection Zones

ZONE 1: Drive Source

Protects for short circuit in cabling/bus to drive anda 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 frequencyand voltage (in non-fault state)

• Zone 1 protective relays are not affected by off-nominal driveproduced 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 sequenceovercurrent, 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 faultswhere 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 groundfault current

41

Typically monitors the input and output voltages and will alarmand/ or trip for over or under voltages and voltage unbalances

Some drives may also include protection of overvoltage on theDC link

Overcurrent protection is provided to protect the converterelectronics 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 levelsfor a preset time.

Volts/Hz limiters and protection to avoid overfluxing at lowerfrequencies

Additional protection may be supplied by monitoring thetemperature of the drive and cooling medium If a link reactor is used it may also have temperature

monitoring and trip settings

Zone 2 – Drive Itself

42

Zone 3 Relay Protection of Motors on VFDs

Primary protection for the drive is contained within the driveitself

Additional motor protection outside of the drive should beapplied if the motor is started across the line or is transferredfrom the drive to line (multiplexed drive application)

Secondary protection may be added in addition to imbeddeddrive 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 andtherefore not be applied or selectively blocked/disabled when frequencyand voltage are outside of reliable operation limits

43

Cautions for a Motor RelayApplied at VFD Output

Motor Protective Relay (MPR) must operate properly at allexpected frequencies and voltages Properly = accurately, securely and dependably

Instrument transformers (ITs) must operate properly at allexpected frequencies and voltages Properly = accurately, securely and dependably

If MPR/ITs cannot react properly at any expectedfrequency, 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 expectedoperating 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, overvoltageand over excitation protection are not affected by lower offnominal conditions

45

Bypass Contactor

• MPR [MotorProtective Relay] isapplied when motoris supplied directlyfrom the line

• Drive Protects motorwhile on VFD

• Three contactorapplication

Inverter(IGBT

or IEGT)

DiodeRectifier

IEGT or IGBT PWMVoltage SourceInverter with DiodeConverter

2 Blown Fuse2 Over / Under Volts2 Phase loss2 Short Circuit

2 Inverter Overtemp2 Overload2 Short circuit2 Sensor Integrity2 Ground fault2 Current balance

DC BusOver /Under Volts

46

Special Application #2

Motor Source Re-energization (Motor Bus Transfer)

47MMM

Bus 1Supply Source

(Bus 2 Backup Source)

Bus 2Supply Source


M M M

UtilitySupply SystemIncoming 1 Incoming 2

N.C.

N.O.

Bus TieBus 1 Bus 2

BUS 1 VT BUS 2 VT

INCOMING 1 VT

INCOMING 2 VTN.C.

• Present source to motor bus is deenergized or challenged• Unplanned utility outage• Fault on utility source• Fault within plant system supplying motor bus

• If challenge to supply occurs, you want to switch to a newsource, very quickly if possible, to avoid restart of motors• Process upset• Process interruption• Locked rotor starting

• 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

Why Apply Motor Bus Transfer Systems?

• Proactive plant isolation• Planned utility outage

• Transient torques on motor• Transient torques to driven load

48

• 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

Motor Bus Transfer Terminology

MMM

Bus 1Supply Source


Bus 2Supply Source


M M M

UtilitySupply SystemIncoming 1 Incoming 2

NC NC

NO

Bus TieBus 1 Bus 2

BUS 1 VT BUS 2 VT

INCOMING 1 VT

INCOMING 2 VT

49

Motor Bus Transfer

Type

• Closed Transition

• Sequential(Open Transition)

Methods

• Hot Paralleled

• Fast• In-Phase• Residual• Time Delayed

Considerations

• Hot Paralleled, Fast and In-Phase Transfers do not causeprocess interruption, motor starting, load shedding

• Residual and Time Delayed Transfers cause processinterruption, motor starting, and may necessitate load shedding

50

MMM

Motor Bus

Source 1(Old Source)

Source 2(New Source)

Closed Transition – Hot Parallel Transfer

Old Source Closed

51

MMM

Motor Bus




Both Sources Closed For Transfer

52


MMM

Motor Bus



• New Source Remains Closed• Old Source Opened

53

MMM

Motor Bus



Open Transition

Old Source Closed

54

MMM

Motor Bus



Open Transition

Old Source Opened55

MMM

Motor Bus



Open Transition

• New Source Closes• Old Source Remains Opened

56

VT-SU

VT-B

STATION BUS SYSTEM

STARTUP SOURCE

N.C.

CT-M

N.O.

CT-SU

UNIT AUXILIARYTRANSFORMER

STATION SERVICETRANSFORMER

MAIN SOURCE

MBTS

52M

52SU

M M

VT-M

Two-Breaker Configuration

57

VT-B1

NORMAL SOURCE (Main 1 )

VT-M2

ALTERNATE SOURCE (Main 2)

BUS 1 BUS 2

VT-B2

VT-M1

CT-M1 CT-M2

CT-B1 CT-B2

TRANSFORMERNORMAL SOURCE ALTERNATE SOURCE

TRANSFORMER

MBTS MBTS

N.C.52M1 N.O.

52SU

N.O.

52Tie

MMMM

Three-Breaker Configuration(Main-Tie-Main)

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 acrossnew 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 smallstatic phase angle difference of each other

Design must ensure that a parallel condition is temporary

If fault occurs when sources are paralleled, circuit breaker andthrough-fault withstand ratings may be violated

The two sources may not be derived from the same primarysource 60

Open Transition Methods:Fast TransferIn-Phase TransferResidual Voltage Transfer

Bus Transfer Zones

40

BusFast

BusSlow

0°-20° +20°

61

MBT Oscillography; Fast Transfer

62

Fast Transfer Method

Presently, the majority of fast transfer systems are NOTsupervised by high-speed sync-check relays !

In many cases, Fast Transfer cannot be correctly performedwithout a high-speed sync check relay

Some modern solid-state or microprocessor-based sync checkelements have a minimum time delay of 0.1 second or 100milliseconds 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

New source circuit breaker isclosed if the phase anglebetween the motor bus and thenew source is within or movesinto the Phase Angle Limit

This method requires high-speedsync-check supervision

• Must be able to close highspeed

• Must be able to block highspeed

Circuit breaker closing is alsosupervised by:

• Upper and Lower Voltage Limitcheck on the new source

• Slip Frequency Limit (∆F)

40

BusFast

BusSlow

0°-20° +20°

40

BusFast

BusSlow

0°-20° +20°

64

In-PhaseTransfer Method

65

In-Phase Transfer Method

The new source breaker will be closed by predicting movementthrough phase coincidence between the motor bus and the newsource during the In-Phase Transfer Enable Window

Due to the decaying motor bus frequency, slip frequency andrate-of-change of frequency between the motor bus and the newsource must be calculated to correctly compensate for thebreaker closing time High speed (quarter-cycle or less) response is recommended.

Predicted phase coincidence is used with breaker closing time ofthe new source breaker to achieve a breaker close at phasecoincidence

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 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

Fast and In-Phase Transfers

68

Fast and In-Phase Sequential Transfers

Advantages No disruption of plant process

Minimizes or eliminates transient torque on motors during thetransfer

Can be used during fault conditions

Can be used for planned transfers

Applicable when two sources are not in sync or within anacceptable small static phase angle difference of each other

No concerns of exceeding fault ratings of circuit breakers orthrough fault rating od transformers due to paralleling sources

Applicable for use where two sources may not be derived from thesame primary source, or on a single source

Concerns These transfers must be performed correctly

69

Disadvantages Slow and cannot be used for planned transfers during plant startup

Undervoltage relay must be accurate and reliable at low voltages and lowfrequencies

If motors are held in with contactors, latching or dc-operated contactorsmust be used to ensure that the contactors do not drop out.

Transfers must be completed before the bus voltage drops so low that themotor protection’s undervoltage elements time out and trip

During the time necessary to wait for sufficient voltage decay, thefrequency may have decayed past the stall point of motors, and loadshedding may be necessary.

Restarting of motors subjects them to high starting currents/torques

Properly sequenced motor may be required to prevent excessivevoltage dip

Load shedding may also be necessary in the case where the new sourcecannot reaccelerate all bus motors simultaneously. Process is interrupted.

Residual Voltage Transfer Method

70

Conditions Across New Source Breaker?

Immediately prior to Transfer Initiate and on trip of OldSource Breaker Instantaneous Phase Angle Shift as cut-loose motors change power

profile

Effects of a Fault - System faults can temporarily depress the New SourceVoltage and can cause a Phase Angle difference between the Motor Busand 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 theHV Bus & the Generator Terminals at the point of an OOS Trip will be theMotor 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 differencebetween the incoming and running power supplies.” 71

ANSI STANDARDC50.41-2012

72

ANSI STANDARD C50.41-2012, clause 14.3 states,“calculations should account for any phase angle differencebetween the incoming and running power supplies.”

Industry Guidance

• NEMA MG 1-2006 and NEMA/ANSI C50.41-2000, 1.33 V/Hzvector difference to define a safe transfer of an inductionmotor bus and its connected loads from one source to analternate power supply

• This is where the 0.25pu voltage for residual transfersoriginates

• Goal of MBT System is to keep resultant V/Hz below1.33, and minimize motor reacceleration current andtorques

73

Dynamic Test of Motor Bus Transfer System:Initial Static Phase Angles

74

Standard Decay MBT Test Results

75

• 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 degreelimits*

* 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 TransferMethods to achieve an optimal close with the In-Phase Transfer Method.


76

• The ANSI C50.41 “10 cycles or less” criteria would rejectperfectly 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 takemore than 10 cycles for the motors to rotate back intosynchronism.

• 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 slipfrequency (SINIT), the angle movement (ΔØ) in 10 cycles (T) is a dangerous 100°.

ΔØ = 360(SINIT+0.5RST)T


77

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 andsupervised 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 speedcould become a vestige of the past With modern technology, we now have the luxury to wait for the old

breaker to trip


78

• Synchronous Fast and In-Phase Transfers occur well beforethe 0.33 pu voltage level of the Residual Voltage Slow Transferwould 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, canalso be applied to Low Voltage Motor Buses, rather thanhaving to resort to Residual Voltage Slow Transfers.


79

Live Open Transition Transfers Under Normal Operating Load ConditionsMBT FIELD RESULTS VS = 120 FS = 60

LOCATION Transfer Mode

Transfer

Method

Advance

Ø Angle

Close

Ø Angle Close ΔF Close Volts

ANSI

C50.41

pu V/Hz

Open

Transfer

Time

cycles

Max

Transfer

Amps /

FLA

Max

Transfer

pu

Power

Torque

Ratio

TPK/TL

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 9 Sequential

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


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 0.15and 0.66 pu V/Hz

• All Synchronous Transfer breaker close commands occurredat voltages above which the Residual Voltage Transferundervoltage element would have operated.

• A Residual Voltage Transfer occurred in 1 test when theSynchronous Transfer Methods were purposely disabled, sothe results for a Residual Voltage Transfer could be observed.

• The Residual Voltage Transfer closed at 1.21 pu V/Hz.


81

• 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 normalmotor 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

The pu V/Hz calculation depends on only three values at closurecompared to the new source: the bus voltage difference, the busfrequency difference, and the phase angle difference.

One could imagine two vastly different sets of motors with two vastlydifferent 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 possibleimprovement.

The above FACILITY 1 through 12 oscillographic records of live motor bustransfers will now be analyzed to derive a new transfer metric, based on thevoltage 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 torqueprior to transfer as if the aggregate bus were a single induction motordrawing the same current and power.


83

Motor Torque Calculation

The torque produced is equal to the electromagnetic power transferredthrough 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 motorbustransfer


84

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/Hz0.3622 0.3054 0.3260 0.1489 0.2360 0.2939 0.3201 0.2635 1.2074 0.6178 0.6644 0.3838

Residual Voltage

In-Phase

Fast


85

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 butlower 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 canreach 20 pu. Facility 9 results demonstrate this with a Torque Ratio of21.74 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 forFacility 9) whereas the motor Torque Ratios are vastly different (4.12 forFacility 1 and 21.74 for Facility 9).


86

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 andcurrent waveforms recorded at transfer and can indicate if a transfer isperformed 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 andmotor 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 tosuccessfully 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 MotorProtection”

“Adjustable Speed Drive MotorProtection Applications and Issues,”IEEE PSRC Report, 10/08

GE Digital Energy 469 Motor RelayInstruction Book

“Motor Bus Transfer Applications Issuesand Considerations,” IEEE PSRCReport, 05/12

“Motor Bus Transfer SystemPerformance Testing and the Search fora New Transfer Success Criterion”; T.Beckwith, Dr. Murty Yalla; BeckwithElectric; 2015 Georgia Tech ProtectiveRelay Conference

Beckwith Electric M-4272 Motor BusTransfer System Instruction Book

90

clearwater beach june 10-12, 2015 maximizing motor life with optimal ... · maximizing motor life...

Documents