Voltage Influence on Typical Protection and Controls for Motors,

Power Electronics, and Other Common Loads

Presentation to:WECC Modeling and Validation Work Group

March 3, 2011John Kueck

The Study Examines a Range of Standards for Both Equipment Protection and

Voltage Tolerance• 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 Commercial Buildings– Red Book IEEE Std 141-1993 Electric Power Distribution for Industrial Plants– Gold Book IEEE Std 493-2007 Design of Reliable Industrial and Commercial Power

Systems– Blue Book IEEE Std 1015-2006 Applying Low Voltage Circuit Breakers Used in

Industrial and Commercial Power Systems– The National Electrical Code (NEC, 2008)

• This report covers all load from 120 volts to 13.8 kV, but 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 significant latitude allowed to the designer, and really no requirement that the designer rigorously follow the standard.

• Biggest problem may be the ice cube relay.

Voltage Tolerance of Programmable Logic Controllers, A Wide Range

0 200 400 600 800Duration of Sag (milliseconds)

Upper Range Average Lower Range

%V

80

60

40

PLC Power Supply Voltage Sag Tolerance (CIGRE 412)

20

100

Adjustable Speed Drive Toleranceand Protection

0 200 400 600 800Duration of Sag (milliseconds)

Upper Range Average Lower Range

%V

80

60

40

ASD Voltage Sag Tolerance (Djokic Paper)

20

100

Overcurrent Protection

Undervoltage Protection

Motor Torque

• Induction motor torque is a function of the terminal voltage squared.

• During a rapid dip, the motor goes into regeneration and will be slowed. – A stiff system slows the motor faster– Low inertia in the driven equipment means even

faster deceleration– Tests by SCE show air conditioning compressors

can stall in 1 to 18 cycles

NEMA Design Motor Speed Torque Curves

For this Hypothetical Motor, the Torque Available at 75% Voltage May Fail to Accelerate the Load

PercentFull

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

NEMA Torque Designs

• B – Most common by far. Normal starting torque for fans, pumps, miscellaneous machinery. Medium starting torque, high breakdown torque.

• C – High starting torque, medium breakdown torque for high inertia loads such as conveyors, positive displacement pumps.

• D – Extra high starting torque, low breakdown torque for very high inertia loads, may have high slip. Used for motor operated valves.

Sometimes Starting Procedures Are Used to Ensure that the Motor Will Start the Load: For example, starting a large

(20,000Hp) vertical, centrifugal pump:

• First, the pump discharge valve is closed.• Second, compressed air is used to blow the water out of

the pump so the pump can be started dry (The pump itself may be 100 feet tall.)

• The dry pump is started.• After the pump is started and has come up to speed,

water is allowed into the pump and the pump is filled and the water is spinning in the pump.

• Finally, the discharge valve is slowly opened and water begins to pump through the system.

• The entire starting process may take 5 minutes or more.

Representative Speed Torque Curves for Various Types of Loads

PercentFull

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)

Synchronous motors run without slip and thus do not have torque curves like induction motors; if the "pull out" torque is exceeded, the motor will pull out of step

Armature Amps

% PF

Field Amps

150

20

40

60

80

100

5 10 20

Full Load Power Factor

Full Load Current

Lagging PF Leading PF

The IEEE Buff Book (242, 2001) Section 10.3.2.1 Motor Protection, Undervoltage

• To prevent a motor from automatically restarting when voltage returns.

• To avoid excessive inrush to the total motor load on the power system.

• To avoid reaccelerating motors before their fields collapse.

• Time delay undervoltage protection will often not be satisfactory because magnetically held contactors may drop out before the undervoltage protection.

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.

What Percentage Are Large Motors?

• Industrial motor systems account for approximately twenty three per cent of all electricity consumed.

• Large motors, i.e. those over 200 horsepower, account for only one percent of the motors in the entire manufacturing inventory, but use 45 percent of the energy use.

• Approximately 70% of motors which are >200 horsepower, fall into the 250-500 hp size

• How many of these are equipped with undervoltage protection? A growing number.

Emerson Secure Start and ComfortAlert for Air Conditioning Compressors

• ComfortAlert will flash an alert if the voltage is below 71%

• Secure Start– Monitors supply voltage in air conditioning

compressors and protects against low voltage or locked rotor.

– Also provides a reduced voltage soft start.

– Can be used in areas with problems in voltage variation.

Ice Cube Relay, the Achilles Heel

• These relays are ubiquitous in 120 volt control circuitry, for example:

– Emergency stop circuits– Door interlock circuits– Air compressor starter controls– Chiller controls– Conveyor controls– Oven controls– PLC - Motor interface circuit– ASD start circuit– Vending machines

0 200 400 600 800

Duration of Sag (milliseconds)

Upper Range Lower Range

%V

80

60

40

Ice Cube Relay Voltage Sag Tolerance (EPRI)

20

100

Contactors• Large Motor Starter Contactors • Motor starter contactors may open at 65 to 75% voltage

in the case of 2300 or 4600 Volt motors and 55 to 65% in the case of 460 Volts and below.

• The contactor dropping out or control relays dropping out is really the only fast undervoltage protection that motors under 600 volts typically have. Motors over 600 volts represent a very small percentage of the population.

• Large Air ConditionersLarge three phase air conditioning in industrial or commercial applications typically have undervoltage relays which trip in perhaps six cycles after the voltage drops below 0.6 pu.

0 200 400 600 800

Duration of Sag (milliseconds)

Upper Range DC Lower Range

%V

80

60

40

Contactors Voltage Sag Tolerance (CIGRE 412)

20

100

AC Upper Range

AC Lower Range

DC

Lamps

• Incandescent filament lamps are quite tolerant to voltage sags, but the light output and lifetime are dramatically impacted by sustained voltage deviations.

• Fluorescent lamps may stop working anywhere between 80 and 40% voltage and in as little as 10 milliseconds. Some electronic ballasts may keep the light working at 40% indefinitely.

ANSI Voltage Tolerance Limits for Low Voltage Regulated Power Distribution System, 120 V Base

Range A Range BMaximum allowable voltage

126 127

Voltage drop allowance for primary distribution line

9 13

Minimum primary service voltage

117 114

Voltage drop allowance for distribution transformer

3 4

Minimum secondary service voltage

114 110

Voltage drop allowance for plant wiring

6 (See Concern Below) 6

Minimum Utilization Voltage

108 104

Possible Concern• NEC FPN4 states that conductors for branch circuits

… should be … sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating and lighting loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5%, yielding 108 at the motor.

• But, the code requirement is only for ampacity, not for voltage. As houses get larger, is the FPN note met?

• Also, what is the voltage drop across the meter? It is assumed by all of the above to be zero.

Japanese Study

• The load drop does not occur if the lowest voltage is higher than 0.85 pu. The load drop occurs if the lowest voltage becomes lower than 0.85 pu, and rapidly increases when the lowest voltage is around 0.6 pu. The load drop, however, saturates after that and does not increase over 30% ..."

Other Parameters Are Probably More Important than the Dip Magnitude • More recent studies have shown that there are a

number of parameters which have a major impact the capability of a device to ride through an interruption than just the dip magnitude and duration. (CIGRE, 2010) 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.

Point on Wave

• The point on the sine wave is important because of the energy stored in the magnetic circuit.

• The stronger the field when the dip begins, the more likely the contactor or relay will ride through the dip. Because of the lagging current drawn by the contactor coil, contactors are most sensitive to dips that begin at 90 degrees and least sensitive to dips that begin at zero crossing.

• Dips initiated at 90 degrees may drop out a contactor in less than10 msec. Dips initiated at 0 degrees may not drop out the same contactor for 80 msec even at zero volts.

0 20 40 60 80

Duration of Sag (milliseconds)

90 Degree Point on Wave of Sag Initiation

0 Degree Point on Wave of Sag Initiation

%V

40

20

Contactor Point on Wave Influence, CIGRE Data

60

Pass

Fail

Conclusions• Point on wave may have a greater impact than

magnitude and duration.• High efficiency motors are more prone to stall.• Ice cube relays are used everywhere and will drop

at 70% V in one cycle.• Voltage drop in branch circuits and other parasitic

drop in low voltage systems may be a growing problem.

• Motors at 600 volts and below usually do not have undervoltage protection unless supplied by manufacturer (air conditioning compressor).

• Motor contactors will typically provide the undervoltage protection by dropping out.