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An Introduction to Power Quality

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Moderator

n  Ron Spataro AVO Training Institute Marketing Manager

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Q&A

n  Send us your questions and comments during the presentation

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Today’s Presenter

n  Andy Sagl Megger® Product Manager

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Power Quality

n  What is Power Quality?

n  The concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment.

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Types of Power Quality Phenomenon

n  Under-Voltage n  Over-Voltage n  Dips (Sags) and Swells n  Transients n  Unbalance n  Flicker n  Harmonics (THD/TDD) n  RVC

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Under-voltage n  An under-voltage is a decrease in rms voltage

less than 0.9 pu for a duration longer than 1 min.

•  Typical values are between 0.8 pu and 0.9 pu.

n  Under-voltages can be caused by faults or overloaded lines.

n  Under voltage conditions can cause motors to overheat.

n  Motors require constant power. When the voltage drops the current needs to rise.

n  The rise in current causes a rise in heating.

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Over-voltage n  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.

n  Over-voltages can be the result of the following: •  Variations in the reactive compensation (switching of cap

banks). •  Solar Panels •  Poor system voltage regulation capabilities or controls.

n  Over-voltage conditions can cause motors to go into saturation, breakers to trip, excessive energy use, reduced life in lighting systems to name a few.

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Voltage Dips (Sags) and Swells n  A voltage dip or sag is a decrease in rms voltage less than 0.9 pu for a

duration less than 1 min. n  An voltage swell is an rms increase in ac voltage greater than 1.1 pu

for a duration less than 1 min. n  The most common power quality events. n  Cannot be prevented on the power system. As impedances

change during the day the voltage will fluctuate. n  Sags can cause process shutdowns requiring hours to re-

start. n  Swells are a common cause of tripping breakers.

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Voltage Dips (Sags) and Swells n  Sag and Swells: n  Voltage sags are caused by abrupt

increases in loads such as short circuits or faults, motors starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance, typically caused by a loose connection.

n  Voltage swells are almost always caused by an abrupt reduction in load on a circuit with a poor or damaged voltage regulator, although they can also be caused by a damaged or loose neutral connection.

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Voltage Dips (Sags) and Swells n  Class A – Dip (Sag) Detection

(Single Phase) n  On single-phase systems a voltage dip begins

when the Urms(1/2) voltage falls below the dip / sag threshold.

n  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 n  Class A – Dip (Sag) Detection

(Poly-phase) n  The dip / sag begins when the Urms(1/2)

voltage of one or more channels is below the dip threshold.

n  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 n  Class A – Swell Detection n  On single-phase systems a swell begins when the Urms(1/2) voltage rises

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

threshold minus the hysteresis voltage.

n  On poly-phase systems a swell begins when the Urms(1/2) voltage of one or more channel rises above the swell threshold.

n  The swell ends when the Urms(1/2) voltage on all measured channels is n  equal to or below the swell threshold minus the hysteresis voltage.

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Transients n  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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500

1000

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Transients n  Transient voltages can result in degradation or immediate

dielectric failure in all classes of equipment. n  High magnitude and fast rise time contribute to insulation

breakdown in electrical equipment like switchgear, transformers and motors.

n  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 n  Transients can damage insulation

because insulation, like that in wires has capacitive properties.

n  Both capacitors and wires have two conductors separated by an insulator.

n  The capacitance provides a path for a transient pulse.

n  If the transient pulse has enough energy it will damage that section of insulation.

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Transients n  This can be understood by

examining the basic formula for Capacitive Reactance.

n  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 n  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.

n  Lightning strikes or high electromagnetic fields produced by lighting can induce voltage & current transients in power lines & signal carrying lines.

n  These are typically seen as unidirectional transients.

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Transients n  When capacitor banks are switched

on there is an initial inrush of current.

n  This will lead to a low-frequency transient that will have a characteristic ringing.

n  These types of transients are referred to as oscillatory transients.

n  Oscillatory transients can cause equipment to trip out and cause UPS systems to turn on and off erroneously.

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Transients n  (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.)

n  These typically will cause issues in areas with short cable runs, such as off shore platforms

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Unbalance n  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.

-200

-100

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RMSVoltage

PhaseAVoltage PhaseBVoltage PhaseCVoltage

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Unbalance n  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 n  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 n  Voltage unbalanced 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.

n  Any change in voltage magnitudes and/or a shift in the phase will cause an unbalanced

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Flicker n  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 n  Humans can be very sensitive

to light flicker that is caused by voltage fluctuations.

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

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

•  (IEC61000-4-15) n  In this method we take the instantaneous voltage and compare it to a

rolling average voltage. n  The deviation between these two is multiplied by a value in a weighted

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

230V 50Hz. n  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 n  Short Term flicker or Pst; is

calculated based on the Flicker percentile unit.

n  Pst is based on a 10 minute interval.

n  Long Term flicker or Plt; is calculated based on the Pst.

n  Plt is based on a 2 hour interval.

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Flicker n  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.

n  This applies to incandescent lighting ONLY. Other types of lighting cannot be tested using this curve.

n  Since it uses a weighting curve it applies only to 120V 60Hz and 230V 50Hz.

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Harmonics n  Harmonics are a sinusoidal

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

n  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 n  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.

n  IEC 61000-4-30 Defines a harmonic frequency as a frequency

which is an integer multiple of the fundamental frequency

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

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Harmonics n  Linear Loads such as

incandescent light and motors draw current equally throughout the waveform.

n  Non-Linear loads such as

switching power supplies draw current only at the peaks of the wave.

n  It is these non linear loads

that cause harmonics.

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Harmonics n  Typically current harmonics

will not propagate through a system.

n  Voltage harmonics will propagate through a system, as they will pass through transformers.

n  When non-linear loads get high enough they can cause harmonics in the voltage.

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Harmonics n  Harmonics can be characterized based on their order. n  Odd Harmonics are harmonics with odd order numbers. n  Even Harmonics are harmonics with even order numbers.

•  Non-symmetrical due to faulty rectifiers.

n  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 n  Harmonics can characterized in different sequences, based on the

rotation of their magnetic field. Positive sequence harmonics creates a magnetic field in the direction of rotation. The fundamental frequency is considered to be a positive sequence harmonic.

n  Negative sequence harmonics develop magnetic fields in the

opposite direction of rotation. This reduces torque and increases the current required for motor loads.

n  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 n  In three-phase systems, the fundamental currents will cancel each

other out, add up to zero amps in the neutral line. n  Zero sequence harmonic (such as the third harmonic) will be in phase

with the other currents of the three-phase system. n  Since they are in phase they will sum together and can lead to high

neutral currents.

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

harmonic components of a distorted waveform. n  THD can be calculated for either current or voltage. n  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.

n  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 𝑣𝑎𝑙𝑢𝑒   x 100%

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THD n  THD can be misleading when analyzing current harmonics. n  THD can be referenced to the amplitude of the fundamental. n  The voltage fundamental value is always present in non-faulted

conditions. n  Not necessarily true for current. n  The current amplitude will fluctuate with the loads impedance.

•  As loads turn off, the fundamental current amplitude decreases. n  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 n  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% n  THD = √� 0.2/200   x 100 = 3.16% n  If the fundamental current being drawn by the load then drops to

200mA then the THD will equal 100% n  THD = √� 0.2/0.200   x 100 = 100% n  This is deceiving because the current THD level appears to be high,

but this is only because there is little to no current being draw.

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TDD n  Total demand distortion (TDD) measurements should be used for total

current harmonic measurements. n  The total demand distortion references the total root-sum-square

harmonic current distortion, to the maximum average demand current recorded during the test interval.

n  Therefore, the reference value is the same throughout the test interval and it is a valid value.

n  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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RVC n  A rapid voltage change is a fast rise or fall of the

RMS voltage. n  RVC events cause mainly changes in lighting

and will normally not bring damage to electrical equipment.

n  Residential households are most commonly affected, especially in weak networks.

n  This is seen as a lighting continuously changing in intensity.

n  If the RMS drops below 3% of the steady state average then a RVC event is triggered.

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RVC n  RVC Events can be caused by the

switching on of a specific load or by a sudden change in source voltage.

n  Sudden source voltage changes can occur in solar grids when the sun is obscured by clouds.

n  Source voltage changes can also occur in wind farms when then wind speed decreases.

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AVO Hands-On Power Quality Training

n  Power Quality & Harmonics: 4 Days – 3.2 CEUs

n  Protective Device Coordination for Utilities: 3 Days – 2.4 CEUs

n  Protective Device Coordination for Industry: 3 Days – 2.4 CEUs

n  Power Factor Testing: 3 Days – 2.4 CEUs

n  Short Circuit Analysis: 4 Days – 3.2 CEUs

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