motor thermal capacity used how does the relay know when i...

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

• 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

Applying high technology to an old problem……

Micro-Processor Motor Protection Relays……

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?

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


• 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


• 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

Sometimes we use things beyond their intended design

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


• 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


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

What is obvious to me might not be to you

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

Coordinating thermal overload curves with upstream devices

Options1. Re-coordinate2. Use definite time

acceleration timer to trip motor before the upstream device

Coordinating thermal overload curves with upstream devices

Options3. Use inverse-time

acceleration timer to trip motor before the upstream device for variable starting voltages

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

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

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?

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


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.


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

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

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

Thank You

Questions?

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