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Motor Thermal Capacity Used
How Does the Relay Know When I’ve Reached 100%?
Ken Farison - ADM
Tom Ernst– GE Grid Solutions
2018 Texas A&MProtective Relay Conference
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• Introduction
• Review of motor thermal capability curves
• How the relay uses thermal overload curves
• Coordinating thermal overload curves with upstream devices
• Case Studies
• Conclusions
Agenda
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Applying high technology to an old problem……
Micro-Processor Motor Protection Relays……
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Introduction• How does the relay thermal OL element know when the
insulation is at the limiting temp?• Use sta tor current to calculate sta tor temperature rise
• Stator heating proportional to I2*t*R• Do not know R
• Biased with RTDs, cur unbalance and harmonics• When do we need to worry about coordination with
upstream relays?• Thermal element is a time-current-temp curve
• Published curves are for a cold motor• Upstream devices are time-current curves• Do they belong together on a TCC plot?• When is the time between curves fixed and when is it
variable?
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Review of motor thermal capability curves
• Motor thermal cap curves• Time to reach limiting
temp• Cold (40° C)• Hot (Op at SF)
• Starting (LR) curves• Running (OL) curves• Acceleration time
• 100% voltage• 80% or 90% voltage• Spec or NEMA std
load
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Review of motor thermal capability curves
• Starts per hour• 2 cold = 1 cold + 1 hot
• Stator is hot after first cold start• Assumes starts are successful• Only 1 start possible for LRtrip
• Stator is a t limiting temp• Assumes acceleration time ≤ acceleration curves
• Accel time > accel curves: 2nd start might not be possible
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Review of motor thermal capability curves
• Running overload curves• Cold overload curve has limited applicability
• Stator is not cold when running• Immediately after starting• During steady sta te loading• Gradually increasing overloading
• Often only hot curve is provided• Assume hot if only 1 curve
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Sometimes we use things beyond their intended design
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How the relay uses thermal overload curves
• Relay calculates normalized sta tor temp (TCU)• Continuously – independent of load level• Uses sta tor current [heating f(I, t , R)]
• Biased with RTDs, current unbalance and harmonics
• Normalizes sta tor temp as a percentage• Thermal Capacity available/used• 0% TCU = 40° C• 100% TCU = insulation limiting temp
• Trip when reaches 100% TCU
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How the relay uses thermal overload curves
• Relay uses the selected OLcurve to define 100% TCU• Curve is f(t , I and TCused(t-1) )
• For a cold motor (40°C):
Where:ttrip = time to trip for a cold statorTDM= time dial multiplierImotor/FLA = normalized motor stator current
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How the relay uses thermal overload curves
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How the relay uses thermal overload curves
For a hot motor:
Where:TCused(t) = current power cycle thermal capacity usedTCused(t-1) = previous power cycle thermal capacity used Ƭsystem = period of one power system cycleTtrip = time to trip for a cold stator
As the stator heats up the time to trip gets shorter• Curves move down
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What is obvious to me might not be to you
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Coordinating thermal overload curves with upstream devices• Modern coordination software will draw thermal OLcurve on
the TCC• Upstream device may plot below the OLcurve
• Apparent mis-coordination• OLcurves are time-current-temperature devices
• Other devices are time-current devices• Must be room for a successful start• Mis-coordination is deceiving, especially for a hot
motor• Running and hot LRstart typically not a concern
• Mis-coordination is real for a cold LRstart
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Coordinating thermal overload curves with upstream devices
Options1. Re-coordinate2. Use definite time
acceleration timer to trip motor before the upstream device
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Coordinating thermal overload curves with upstream devices
Options3. Use inverse-time
acceleration timer to trip motor before the upstream device for variable starting voltages
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Case Study 1: 3250 HP compressor with fast upstream device
• Motor incorrectly uses 80-95% TCU to start• TCC shows a mis-coordination with upstream device
• Cannot re-coordinate upstream devices • Selected a low OLcurve to fix the coordination
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Case Study 1: 3250 HP compressor with fast upstream device
• OLcurve 8 fits motor curves well
• Selected curve 1 is too fast and causes high TCU calculations• No hot restart• Occasional trip on
cold start
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Case Study 1: 3250 HP
compressor with fast upstream
device• Select OLcurve 8• Use 15 second
acceleration timer to assure coordination
• Cold LRwill trip by acceleration timer before upstream device
• What about hot LR?
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Case Study 2: 2500 HP CO2 compressor
• Motor curves do not match a single curve well• Curve 5 matches
starting region• Curve 9 matches
running region
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Case Study 2: 2500 HP CO2 compressor
Option 1: Switch curves• Curve 5 when
stopped and starting• Curve 9 when
running Curve 5
Curve 9Enabled when running
Curve 5Enabled when stopped or starting
Use setting groups to switch between curve 5 and 9.
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Case Study 2: 2500 HP CO2 compressor
Customer choose option 2: Custom curve• Curve 5 data points
for starting region• Curve 9 data points
for running region• Custom data points
in the accelerating region between
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Conclusions• Motor thermal capability curves describe the loading points
where the insulation will reach the limiting temperature• Relay OLcurves define the loading points corresponding to
100% TCU• Selecting an OLcurve that matches the motor curve gives the
relay the correct definition of TCU• Too fast a curve causes excessive TCU calculation
• Prevents hot restarts• Nuisance cold starting trips
• Too slow a curve will a llow insulation thermal damage without tripping
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Conclusions• Relay OLcurves are 3 dimensional (time, current and starting
TCU)• Cannot directly coordinate with upstream time-current
devices on a TCC when motor is hot• OLcurve drops as the motor temperature increases
• Use acceleration timer to reduce cold LR trip time - not a faster OLcurve
• Custom OLcurves and curve switching can improve OLcurve match with motor curves
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Thank You
Questions?