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Power Quality

Basics

Presented by: Jason Huneycutt

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Power Quality

What is Power Quality?

The concept of powering and grounding sensitive equipment

in a manner that is suitable to the operation of that equipment.

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Introduction

The modern power grid is changing.

The addition of green energy sources

• Solar

• Wind energy

The reduction of coal bulk generation plants

As loads increase coupled with the intermittent nature of solar

and wind energy the voltage stability will suffer.

New technologies such as, electric vehicles are adding new

loads to the grid which, can lead to altered peak hours.

The implementation of the smart grid technology which is

designed to makes the grid more efficient.

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Smart Grid

Will smart grid reduce power quality issues, making power

quality investigations rare?

No!

• The smart grid will increase grid reliability not the quality of the

power being delivered.

As technology advances there will always be new types of loads

and sources added to the grid. This will always create new power

quality challenges.

The most common power quality issues faced today include sags

and swells, transients, unbalance as well as harmonics.

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Types of Power Quality Phenomenon

Under-Voltage

Over-Voltage

Dips (Sags) and Swells

Transients

Unbalance

Flicker

Harmonics (THD/TDD)

RVC

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Under-voltage

An under-voltage is a decrease in RMS voltage less than 0.9

pu (percentile unit) for a duration longer than 1 min.

• Typical values are between 0.8 pu and 0.9 pu.

Under-voltages are caused by loads switching on, or capacitor

banks switching off.

The under-voltage can continue until voltage regulation

equipment on the system can bring the voltage back within

tolerances.

Overloaded circuits can also result in under-voltages.

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Over-voltage

An over-voltage is an RMS increase in ac voltage greater than

1.1 pu for a duration longer than 1 min.

• Typical values are 1.1 pu to 1.2 pu.

Over-voltages can be the result of the following:

• Load switching (switching off a large loads such as motors)

• Variations in the reactive compensation (switching of cap

banks).

• Solar Panels

• Poor system voltage regulation capabilities or controls.

• Incorrect tap settings on transformers

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Voltage Dips (Sags) and Swells

Voltage sags and swells are two of the most common power

quality events. Voltage Sags and Swells cannot be prevented

on the power system. As impedances change during the

course of a day the voltage will momentarily change as well.

Even instantaneous short duration sags can cause process

shutdowns requiring hours to re-start.

Voltage swells are one of the most common cause of tripping

breakers.

The malfunction or failure of this equipment can cause large

financial losses to various manufacturers.

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Voltage Dips (Sags) and Swells

Common Causes of voltage sags include source voltage

changes, inrush currents as well as inadequate wiring.

Common Causes of voltage swells can include load switching,

utility faults, and damaged or loose neutral connections.

Another Common Cause includes the wrong voltage for

equipment in use coming into the building. These wrong

voltages can include 230volt equipment being fed from 208 volts

or vice versa, 460 volt equipment being fed from 480 volts.

The goal of power quality is to limit the number of sags and

swells as well as the magnitude of these events such that they

do not cause equipment malfunction or failure.

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Voltage Dips (Sags) and Swells

On single-phase systems a voltage dip

begins when the Urms(1/2) voltage

falls below the dip / sag threshold.

The event ends when the Urms(1/2)

voltage is equal to or above the dip

threshold plus the hysteresis voltage.

Note This value is used only for

voltage dip (sags), swells, interruption,

and RVC detection and evaluation.

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Voltage Dips (Sags) and Swells

Class A – Dip (Sag) Detection

(Poly-phase)

The dip / sag begins when the

Urms(1/2) voltage of one or more

channels is below the dip threshold.

The dip / sag ends when the

Urms(1/2) voltage on all measured

channels is equal to or above the dip

threshold plus the hysteresis voltage.

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Voltage Dips (Sags) and Swells

Class A – Swell Detection

On single-phase systems a swell begins when the Urms(1/2)

voltage rises above the swell threshold,

The swell ends when the Urms(1/2) voltage is equal to or below

the swell threshold minus the hysteresis voltage.

On poly-phase systems a swell begins when the Urms(1/2)

voltage of one or more channel rises above the swell threshold.

The swell ends when the Urms(1/2) voltage on all measured

channels is equal to or below the swell threshold minus the

hysteresis voltage

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Transients

Generally there are two different types

of transient over voltages: low frequency

transients with frequency components in

the few-hundred-hertz region typically

caused by capacitor switching,

(Oscillatory transients) and high-

frequency transients with frequency

components in the few-hundred-

kilohertz region typically caused by

lighting and inductive loads. (Impulsive

Transients)

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Transients

Transient voltages can result in degradation or immediate

dielectric failure in all classes of equipment.

High magnitude and fast rise time contribute to insulation

breakdown in electrical equipment like switchgear,

transformers and motors.

Repeated lower magnitude application of transients to

equipment can cause slow degradation and eventual insulation

failure, decreasing equipment mean time between failures.

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Transients

Transients can damage insulation

because insulation, like that in

wires has capacitive properties.

Both capacitors and wires have

two conductors separated by an

insulator.

The capacitance provides a path

for a transient pulse.

If the transient pulse has enough

energy it will damage that section

of insulation.

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Transients

This can be understood by

examining the basic formula

for Capacitive Reactance.

It can now be seen that as

the value of the frequency

increases, the lower the

reactive capacitance and

therefore the lower the

impedance path.

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Transients

Lightning is a major cause of

transients. • A bolt of lightning can be over 5

miles long, reach temperatures in

excess of 20,000 degrees

Celsius.

Lightning strikes or high

electromagnetic fields

produced by lighting can

induce voltage & current

transients in power lines &

signal carrying lines.

These are typically seen as

unidirectional transients.

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Transients When capacitor banks are

switched on there is an initial

inrush of current.

This will lead to a low-

frequency transient that will

have a characteristic ringing.

These types of transients are

referred to as oscillatory

transients.

Oscillatory transients can

cause equipment to trip out

and cause UPS systems to

turn on and off erroneously.

08/12/2007 14:22:30.600 SUBCYCLE on X3-1

Time (ms)

35.48 45.12 54.76 64.40 74.04-905.00

-354.50

196.00

746.50

1297.00

X2-3

(V

olts)

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Transients

Extremely fast transients, or EFT's, have rise and fall times in

the nanosecond region. They are caused by arcing faults, such

as bad brushes in motors, and are rapidly damped out by even

a few meters of distribution wiring. Standard line filters,

included on almost all electronic equipment, remove EFT's.

These typically will cause issues in areas with short cable

runs, such as off shore platforms

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Unbalance

Unbalance is a condition in a poly-phase system in which the

RMS values of the line voltages (fundamental component), or

the phase angles between consecutive line voltages, are not

all equal per IEEE 1159 and IEC 61000-4-27.

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Unbalance

Voltage unbalance more

commonly emerges in

individual customer loads due

to phase load imbalances,

especially where large, single

phase power loads are used,

such as single phase arc

furnaces. • A small unbalance in the phase

voltages can cause a large

unbalance in the phase currents.

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Unbalance

Unbalanced voltages can

effect equipment on the

power system, such as

induction motors and

adjustable speed drives. In

addition unbalance voltages

can cause heating effects in

transformers and neutral

lines.

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Unbalance

Voltage unbalance can be

described as a set of

symmetrical components. • In a balanced three phase system

the three line-neutral voltages are

equal in magnitude and phase

and are displaced from each

other by 120 degrees.

Any change in voltage

magnitudes and/or a shift in

the phase will cause an

unbalanced

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Flicker

Flicker is a very specific

problem related to human

perception and

incandescent light bulbs. It

is not a general term for

voltage variations.

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Flicker

Humans can be very sensitive to light flicker that is caused by voltage fluctuations.

Human perception of light flicker is almost always the limiting criteria for controlling small voltage fluctuations.

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Flicker

The figure illustrates the

level of perception of light

flicker from a 60 watt

incandescent bulb for

rectangular variations. The

sensitivity is a function of

the frequency of the

fluctuations and it is also

dependent on the voltage

level of the lighting.

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Flicker

In general today, flicker is measured using the IEC method.

• (IEC61000-4-15)

In this method we take the instantaneous voltage and compare it to a

rolling average voltage.

The deviation between these two is multiplied by a value in a

weighted curve.

This curve is based on the sensitivity of the human eye at 120V 60Hz

or 230V 50Hz.

The end value is called a percentile unit. The percentile units go

through a statistical analysis in order to calculate 2 values.

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Flicker

Short Term flicker or Pst; is

calculated based on the

Flicker percentile unit.

Pst is based on a 10 minute

interval.

Long Term flicker or Plt; is

calculated based on the

Pst.

Plt is based on a 2 hour

interval.

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Flicker

The basic criteria is simple. If the

Pst is less than 1.0 then flicker

levels are good. If Pst is greater

than 1.0 then the flicker levels

could be causing irritation.

This applies to incandescent

lighting ONLY. Other types of

lighting cannot be tested using

this curve.

Since it uses a weighting curve it

applies only to 120V 60Hz and

230V 50Hz.

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Harmonics

Harmonics are a sinusoidal component of periodic waves that have frequencies that are multiples of the fundamental frequency

Harmonics can cause many problems, such as:

• Neutral wires to over heat

• Motors to overheat

• Transformers to overheat

• Electronic Failures

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Harmonics

IEEE 519 Defines a harmonic:

• A component of order greater than one of the Fourier series of a periodic quantity.

– For example, in a 60 Hz system, the harmonic order 3, also known as the “third harmonic,” is 180 Hz.

IEC 61000-4-30 Defines a harmonic frequency as a frequency which is an integer multiple of the fundamental frequency

IEC 61000-4-30 Defines a harmonic component as any of the components having a harmonic frequency

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Harmonics

Linear Loads such as incandescent light and motors draw current equally throughout the waveform.

Non-Linear loads such as switching power supplies draw current only at the peaks of the wave.

It is these non linear loads that cause harmonics.

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Harmonics

Typically current harmonics

will not propagate through a

system.

Voltage harmonics will

propagate through a

system, as they will pass

through transformers.

When non-linear loads get

high enough they can

cause harmonics in the

voltage.

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Harmonics

Harmonics can be characterized based on

their order.

Odd Harmonics are harmonics with odd order

numbers.

Even Harmonics are harmonics with even

order numbers.

• Non-symmetrical due to faulty rectifiers.

Triplens are odd harmonics that are multiples

of 3.

• These will not cancel out and will add and cause high

neutral currents.

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Harmonics

Harmonics can characterized in different sequences, based on the

rotation of their magnetic field.

Positive sequence harmonics create a magnetic field in the

direction of rotation. The fundamental frequency is considered to be

a positive sequence harmonic.

Negative sequence harmonics develop magnetic fields in the

opposite direction of rotation. This reduces torque and increases the

current required for motor loads.

Zero sequence harmonics create a single-phase signal that does

not produce a rotating magnetic field of any kind. These harmonics

can increase overall current demand and generate heat.

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Harmonics

In three-phase systems, the fundamental currents will cancel

each other out, add up to zero amps in the neutral line.

Zero sequence harmonic (such as the third harmonic) will be in

phase with the other currents of the three-phase system.

Since they are in phase they will sum together and can lead to

high neutral currents.

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Harmonics

The positive, negative, and zero

sequence harmonics run in

sequential order (positive,

negative, and then zero).Since

the fundamental frequency is

positive, this means that the

second order harmonic is a

negative sequence harmonic.

The third harmonic is a zero

sequence harmonic.

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THD

Total harmonic distortion (THD) is the measure of the sum of the

harmonic components of a distorted waveform.

THD can be calculated for either current or voltage.

THD is the RMS (root-mean-square) sum of the harmonics,

divided by one of two values: either the fundamental value, or the

RMS value of the total waveform.

THD is typically, represented as a percentage of fundamental

amplitude

THD = 𝑆𝑢𝑚 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑞𝑢𝑎𝑟𝑒𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑎𝑙𝑙 𝑡ℎ𝑒 ℎ𝑎𝑟𝑚𝑜𝑛𝑖𝑐 𝑜𝑟𝑑𝑒𝑟𝑠

𝑆𝑞𝑢𝑎𝑟𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑎𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑢𝑛𝑑𝑎𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒x 100%

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THD

THD can be misleading when analyzing current harmonics.

THD can be referenced to the amplitude of the fundamental.

The voltage fundamental value is always present in non-faulted

conditions.

Not necessarily true for current.

The current amplitude will fluctuate with the loads impedance.

• As loads turn off, the fundamental current amplitude decreases.

If the current being drawn by the load is low (near zero) then the THD

value will appear to be very high.

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THD

If the total harmonic current is 0.2A and the fundamental current

being drawn by the load is 200A then the THD will equal 3.16%

THD = 0.2

200x 100 = 3.16%

If the fundamental current being drawn by the load then drops to

200mA then the THD will equal 100%

THD = 0.2

0.200x 100 = 100%

This is deceiving because the current THD level appears to be

high, but this is only because there is little to no current being

drawn.

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TDD

Total demand distortion (TDD) measurements should be used for

total current harmonic measurements.

The total demand distortion references the total root-sum-square

harmonic current distortion, to the maximum average demand

current recorded during the test interval.

Therefore, the reference value is the same throughout the test

interval and it is a valid value.

Total Demand Distortion should be calculated in accordance with

the IEEE 519 document: “Recommended Practices and

Requirements for Harmonic Control in Electrical Power

Systems”), published by the IEEE Standards Association.

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THD & TDD

The power quality industry has developed certain index values to

assess the distortion caused by the presence of harmonics.

The two values most frequently indexed are total harmonic

distortion and total demand distortion.

Individual harmonic values are also indexed in different

specifications, such as the North American IEEE 519 document

and the European Standard EN50160 on power quality; issue by

the European Committee for Electrotechnical Standardization

(CENELEC).

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RVC

A rapid voltage change (RVC) is a fast rise

or fall of the RMS voltage.

RVC events cause mainly changes in

lighting and will normally not bring damage

to electrical equipment.

• A reduction in the voltage by 10% can

result in a 34% reduction in the light

intensity from a 60 W incandescent lamp.

Residential households are most

commonly affected, especially in weak

networks.

This is seen as a lighting continuously

changing in intensity.

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RVC

RVC Events can be caused

by the switching on of a

specific load or by a sudden

change in source voltage.

Sudden source voltage

changes can occur in solar

grids when the sun is

obscured by clouds.

Source voltage changes can

also occur in wind farms

when then wind speed

decreases.

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RVC

An RVC event starts with a steep down-

going step followed by an up-going ramp

ending at a voltage value less than that

existing before the switching.

• The fall time can be as short as 10 ms,

while the recovery ramp can last several

cycles.

NOTE: for an event to be classified as a

rapid voltage change the voltage must

not fall below the lower voltage limit.

• If the voltage did fall below the lower

tolerance limit then the event would be

classified as a voltage dip (sag).

Questions or Comments?

Email Nicole VanWert-Quinzi nicole.vanwert@Transcat.com

Transcat: 800-800-5001

www.Transcat.com

For related product information, go to:

www.Transcat.com/Megger

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Questions?

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