modelling workshop
TRANSCRIPT
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WECC Modeling Workshop
Load ModelMotor Protection, Drives and Lighting
John Kueck
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A Range of Applicable Standards
Most Small Motors Only Have Over-current
and Overload Protection
Newer Standards Are Requiring Under-voltage
The Ice Cube Relay Is the real culprit
Point of Wave Has More Effect than Voltage
Dip Magnitude
Motor Protection and Control
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Many Well Known Standards: Buff Book IEEE Std 242-2001 Protection and Coordination of Industrial and
Commercial Power Systems
Gray Book IEEE Std 241-1990 Electric Power Systems in CommercialBuildings
Red Book IEEE Std 141-1993 Electric Power Distribution for Industrial Plants
Gold Book IEEE Std 493-2007 Design of Reliable Industrial and CommercialPower Systems
Blue Book IEEE Std 1015-2006 Applying Low Voltage Circuit Breakers Usedin Industrial and Commercial Power Systems
The National Electrical Code (NEC, 2011)
The above standards provide little guidance on the suggested
setpoint for undervoltage trip for various types of equipment,especially for equipment under 600 volts. There is significantlatitude allowed to the designer, and really no requirement that thedesigner rigorously follow the standard.
Biggest problem may be the common ice cube relay.
A Range of Standards for Equipment
Protection and Voltage Tolerance
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Thermal capability(overload)
Locked Rotor
Short Circuit
(Instantaneous) Single Phasing
Undervoltage
Unbalance
Protection settingwith new motorstarters is mucheasier than it usedto be.
Basic Motor Protection
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Voltage Unbalance
Has a Major Impact on Motor Efficiency
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Motor Torque
Induction motor electrical torque is a functionof the terminal voltage squared.
During a rapid dip, the motor goes into
regeneration and will be slowed.A stiff system or dc offset slows the motor faster
Low inertia in the driven equipment means evenfaster deceleration
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NEMA Torque Designs
B Most common by far. Normal starting torque forfans, pumps, miscellaneous machinery. Mediumstarting torque, high breakdown torque.
C High starting torque, medium breakdown torquefor high inertia loads such as conveyors, positivedisplacement pumps.
D Extra high starting torque, low breakdown torquefor very high inertia loads, may have high slip. Used for
motor operated valves.
E No longer included, but was for high efficiencymotors, similar to shape of B, but lower.
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NEMA Design Motor
Speed Torque Curves
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For this Hypothetical Motor, the Torque Available
at 75% Voltage May Fail to Accelerate the Load
Percent
Full
Load
Torque
Percent Full Load Speed
600
40
80
120
160
200
20 40 80 100
Motor Torque Rated V
Motor Torque 75% V
Load Torque
Motor May Fail to
Accelerate Load
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Synchronous motors run without slip and do not have torque curves like
induction motors; if the "pull out" torque is exceeded,
the motor will pull out of step
Armature
Amps
% Power
Factor
Field Amps
15
0
20
40
60
80
10
0
5 10 20
Full Load Power Factor
Full Load Current
Lagging
PF Leading PF
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Representative Speed Torque Curves
for Various Types of Loads
Percent
Full
Load
Torque
Percent Full Load Speed
600
40
80
120
160
200
20 40 80 100
Centrifugal Pump
or Fan (Open
Discharge)
Axial Pump or Fan
Centrifugal Pump or
Fan (Closed Discharge)
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The IEEE Buff Book (242, 2001) Section
10.3.2.1 Purposes of Under-voltage Protection
To prevent a motor from automatically restarting whenvoltage returns.
To avoid excessive inrush to the total motor load on thepower system.
To avoid reaccelerating motors before their fieldscollapse.
Time delay undervoltage protection will often not besatisfactory because magnetically held contactors maydrop out before the undervoltage protection.
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Very Few Other Standards Mention
Undervoltage Protection for Motors
For large motors, the IEEE Red Book (IEEE_Standard_141,1993) states in Section 5.6.3.1 motor protection may include:
Internal fault protection - either overcurrent relays or percentage
differential relays; sometimes ground fault protection is provided
using a zero sequence approach.
Sustained overloads and locked rotor- Conventional over current
relays may provide too much margin between the motor thermal
capability curve and the relay operating time characteristic.
Overcurrent relays do, however, provide excellent locked rotor and
short circuit protection. Thermal relays will give adequate protection
for light and medium overloads.
Under voltage - Large motors and medium voltage motors should have
separate undervoltage protection.
For small motors, the Red Book and the NEC do not require
undervoltage protection.
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Emerson Secure Start and ComfortAlert
for Air Conditioning Compressors
ComfortAlert will flash an alert if the voltage is below71%
Secure Start
Monitors supply voltage in air conditioning compressorsand protects against low voltage or locked rotor.
Also provides a reduced voltage soft start.
Can be used in areas with problems in voltage variation.
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Programmable Logic Controllers
Adjustable Speed Drives
Personal Computers
Fluorescent Lighting
Power Electronic Loads
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0 200 400 600 800
Duration of Sag
(milliseconds)
Upper Range Average Lower Range
%V
8060
40
PLC Power Supply Voltage Sag Tolerance (CIGRE 412)
20
10
0
Voltage Tolerance of Programmable
Logic Controllers, A Wide Range
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480 VCircuit Breaker
VFD
ControlsVoltage and
Frequency
Motor
VFD to Control Fan Speed Instead of Dampers
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Useful
Work
Controller losses
10%
Coupling device losses10% for large speed
reduction
Driven load losses
30 to 50%
for pumps and fans
Load modulation
devices
0 to >50%
Typical Motor System Losses
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Controlling Speed and Torque with a VFD
Is More Efficient than Valves or Dampers
Power Required to
Achieve Needed Flow
Percent of Full Flow
Vane Adjustment
VSD
Control
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Variable Frequency Drive (VFD)
Tolerance and Protection
0 200 400 600 800
Duration of Sag
(milliseconds)
Upper Range Average Lower Range
%
V
80
60
40
ASD Voltage Sag Tolerance (Djokic, 2005)
20
10
0
Overcurrent Protection
Undervoltage
Protection
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VFD Voltage Sag Test
Our data comes from a test at which the vfd frequency
reference was set to 30 Hz.
vp
vdc 20% voltage drop, held for
10 seconds, and then
ramped back over 10seconds.
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Voltage Sag Test Contd.
Power is roughly constant,except at the point of voltage
drop.
Speed is almost
constant
Power is not strongly dependenton voltage in this data.
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Lighting
Incandescent filament lamps are quite tolerant to voltagesags, but the light output and lifetime are dramaticallyimpacted by sustained voltage deviations.
Fluorescent lamps:
Electronic ballast ( vast majority today)o tend to drop out at lower voltages 10 to 20%,
o have nearly constant current characteristic at the fundamentalfrequency, dim at lower voltages
o Current contains significant harmonics
o New dimmable fluorescent lamps with electronic ballasts are
controlled with either a 0 to 10 volt dc signal or a digital signal, not byvarying voltage
Magnetic ballasto Usually drop out between 70 and 80% in as little as 10 milliseconds
o Highly non-linear reactive power characteristic
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Electronic Ballast Fluorescent Lights
4 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.1
-200
-100
0
100
200
Time
Voltage
Fluorescent Voltage and Current
4 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.1
-4
-2
0
2
4
Current
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Electronic Ballast Fluorescent Lights
0 20 40 60 80 100 120 140-10
0
10
20
30
40
50
60
Voltage [V]
RealPower[W]
0 20 40 60 80 100 120 140-30
-25
-20
-15
-10
-5
0
5
ReactivePower[VAR
]
Fluorescent Power
Reactive Power
Active Power
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Compact Fluorescent Lights
0 20 40 60 80 100 120
-10
0
10
20
30
Voltage [V]
RealPower[W]
0 20 40 60 80 100 120
-15
-10
-5
0
5
Compact Fluorescent Power
ReactivePower
Reactive Power
Active Power
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Magnetic Ballast Fluorescent Lights
4 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.1
-200
-100
0
100
200
Time [sec]
Voltage
Fluorescent Old-Ballast Voltage and Current
4 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.1
-2
-1
0
1
2
Time [sec]
Current
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Magnetic Ballast Fluorescent Lights
0 20 40 60 80 100 120 140-50
0
50
100
Voltage [V]
RealPower[W]
Fluorescent Old-Ballast Power
0 20 40 60 80 100 120 140-20
0
20
40
Voltage [V]
ReactivePower[VA
R]
Active Power
Reactive Power
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Ice Cube Relay, the Achilles Heel
These relays are commonly used in 120 volt controlcircuits, for example:
Emergency stop circuits for pumps and fans
Door interlock circuitsAir compressor starter controls
Chiller controls
Conveyor controls
Oven controlsPLC - Motor interface circuit
ASD start circuit
Vending machines
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0 200 400 600 800
Duration of Sag (milliseconds)
Upper Range Lower Range
%V8060
40
20
10
0
Ice Cube Relay Voltage Sag Tolerance
(EPRI)
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Contactors
Large Motor Starter ContactorsMotor starter contactors may open at 65 to 75% voltage
in the case of 2300 or 4600 Volt motors and 55 to 65% inthe case of 460 Volts and below.
The contactor dropping out or control relays droppingout is often the only fast undervoltage protection thatmotors under 600 volts typically have.
Large Air Conditioners
Large three phase air conditioning in industrial orcommercial applications typically have undervoltagerelays which trip in perhaps six cycles after the voltagedrops below 0.6 pu.
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0 200 400 600 800
Duration of Sag (milliseconds)
Upper RangeDC
Lower Range
%
V
80
60
40
20
10
0 AC Upper
Range
AC Lower
Range
DC
Contactors Voltage Sag Tolerance
(CIGRE 412)
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Other Parameters Are Probably More
Important than the Dip Magnitude
More recent studies have shown that there are anumber of parameters which have a major impact thecapability of a device to ride through an interruptionthan just the dip magnitude and duration.
(CIGRE_Voltage_Dip_Immunity_of_Equipment_and_Installations 2010Voltage Dip Immunity of Equipment and Installations C4.110)
These parameters include:Pre dip voltage magnitude and distortion of sine wave.
Unbalance during dip for three phase devices, dip shape,and point on the sine wave where the dip starts.
Speed of recovery of dip.
Source impedance (distribution transformer).
Other equipment connected close by.
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Point on Wave
The point on the sine wave is important because of theenergy stored in the magnetic circuit.
The stronger the field when the dip begins, the morelikely the contactor or relay will ride through the dip.
Because of the lagging current drawn by the contactorcoil, contactors are most sensitive to dips that begin at90 degrees and least sensitive to dips that begin at zerocrossing.
Dips initiated at 90 degrees may drop out a contactor
in less than 10 msec. Dips initiated at 0 degrees maynot drop out the same contactor for 80 msec even atzero volts.
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0 20 40 60 80
Duration of Sag (milliseconds)
90 Degree Point on
Wave of Sag Initiation0 Degree Point on
Wave of Sag Initiation
%
V
40
20
60
Pass
Fail
Contactor Point on Wave Influence,
CIGRE Data
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