Modelling Workshop

8/12/2019 Modelling Workshop

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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


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


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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Modelling Workshop

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