Power Quality Issues

Basic Requirements for Critical Loads

Continuous, No Break in Power

Voltage Regulation Sine wave Supply Isolation Constant Frequency

LoadVs

Is

Understanding Reactive Power and

Power Factor

Vs

Is

R

Is

Vs XL

Reactive P

ower(Q

) in V

AR

Appare

nt

Power(

S) in

VA

Imp. Phase

Angle

True Power(P)in Watts

XL

Vs

Is

R

Displacement and True Power Factor

Displacement Power Factor: “Ratio of the active power of the fundamental, in watts, to the apparent Power of the fundamental wave, in volt-amperes”

True Power Factor: “Ratio of the total power, in watts, to the total volt-amperes. This includes fundamental and all harmonic components ”

Understanding Crest Factor

Non Linear Load

Vs

Is

Current drawn by single Phase diode rectifier

Crest Factor = --------------------------Peak Amplitude

RMS Value

Understanding Harmonics

Non Linear Load

Vs

Is

Deviation from a perfect sine wave can be represented by Harmonics.

Sinusoidal Component having a frequency that is an integral multiple of the Fundamental frequency.

Total Harmonic Distortion Defines the total harmonic content of current or voltage

Ratio of the RMS of the harmonic content to the RMS of the Fundamental, as % of Fundamental

THD = sum of squares of amplitudes of all harm onics

square of amplitude of fundamental x 100

Mathematically, THD of a voltage wave form ca n be defined as,

THD = V

V

100 x h

h

h 2

122

Power Quality

Supply Voltage at load must

The Voltage at any point in the distribution system is uniquely described by

Where v,f are constants (declared Value) for all t

)2(2 V(t) ftVSin

have fundamental component only

be balanced

have declared magnitude under all conditions

PQ affected by Polluting Load

Polluting Load

A

B

PCC

Distorted voltage

Pure Sinusoidal

Line Impedance ZL

Common PQ Disturbances

Reactive Power Demand Harmonic Distortion Voltage sags and swells Undervoltages and overvoltages Voltage Unbalance Voltage Flicker Voltage Notching Voltage Interruption Transient Disturbances Frequency variations

Harmonics-Polluting Loads

Rectifiers

Arc furnaces

Adjustable Speed drives

Power Electronic converters

Harmonics-FFT Analysis Current drawn by PC

Harmonic Spectrum of PC Current Waveform

6 Pulse Converter/Battery Charger

Harmonic Spectrum of 6 Pulse Converter/Battery Charger

Implication of Reactive and

Harmonics currentsOversize of all installation equipments to transmit Reactive and Harmonic currents namely

Transformer

Cables

Circuit breakers & distribution switch boards

Neutral overloads

Increase in Transmission & Distribution loss

Reduction in voltage stability margin

Overheating and loss of life & equipments

Major Increase in cost

Voltage sags and swells

• Reduction in the ac voltage, at the power frequency, for durations from a half-cycle to a few seconds.

• Voltage Sag is Characterized by two parameters – Magnitude and Duration

• Power Electronics Loads are Sensitive to Voltage Sags

Causes of Voltage sags and swells

Causes for Sag Motor Starting Transformer Energization Transmission Faults

Causes for Swell Single line to ground fault Removing a large load /

adding a large capacitor bank

0 1 2 3 4 5 6 7 8 9 10 11190

195

200

205

210

215

220

RM

S v

olta

ge in

V

Time in Cycles

0 5 10 15 20 251010.110.210.310.410.510.610.710.810.911

Time in Cycles

RM

S v

olta

ge in

kV

Voltage sag due to Motor Starting

Voltage sag due to Transformer Energization

Voltage UnbalanceDefinition In a balanced sinusoidal supply system the three line-neutral voltages

are equal in magnitude and are phase displaced from each other by 120 degrees

Causes for Unbalance Unequal system impedances Unequal distribution of

single-phase loads

Phase to Phase loads Unbalanced Three phase loads

Va

VbVc

1200

1200

1200

Va

VbVc

1200

1210

1190

Balanced System Unbalanced System

Effect of Voltage Unbalance

Induction Motor drive• Overheating and loss of Insulation life• Reduced Motor Efficiency• Noisy in their operation due Torque and speed pulsation • Motor derating factor

NEMA Induction motor derating curve

Effect of Voltage Unbalance AC Variable Speed drive

M

3Ø Diode Rectifier

Line Current of 3Ø diode Rectifier for Balanced Input

Line Current of 3Ø diode Rectifier for 5%unbalanced Input

Line Current of 3Ø diode Rectifier for 15%unbalanced Input

• Draws uncharacteristic triplen harmonics• Triplen harmonic current can lead to undesirable

harmonic problems• Excessive thermal stress on diodes

Voltage FlickerDefinition

Repetitive or random variations of the voltage envelope modulated at frequencies less than 25 Hz, which the human eye can detect as a variation in the lamp intensity of a standard bulb due to sudden changes in the real and reactive Power drawn by a load

Voltage waveform showing flicker created by an arc furnace

Voltage Flicker

Effect lamp flicker Human eye is most sensitive to voltage waveform modulation around a frequency of 6-8Hz.

Causes Induction Motor drive

• Arc furnaces• Arc welders• Frequent motor starts

Voltage Notching

Causes Adjustable Speed Drives

Solid State rectifiers

Voltage Interruption

Complete loss of electrical supply

Cause Transmission Fault clearing time

• Opening / Recloser of circuit breaker

Transient DisturbancesTransient disturbances are caused by the injection of energy by switching or by lightningCauses

Lightning Capacitor Switching Load switching

Oscillatory transient waveform caused by capacitor energizing

Need for Power QualityBusiness Problems:

• Momentary disturbance can cause scrambled data, interrupted communications, system crashes and equipment failure

• Lost productivity and idle people and equipment

• Overtime required to make up for lost work time

• Revenue and accounting problems such as invoices not prepared, payments held up etc

According to Electric Light and Power Magazine, 30 to 40 percent of all business downtime is related to Power Quality Problems

IEEE 519 Harmonic Standard

IEEE 519 “Recommended Practices and Requirements for Harmonic Control in Electric Power Systems”

Specifies load current harmonic limits at PCC

Specifies supply voltage harmonic limits at PCC

HARMONIC CURRENT DISTORTION LIMITS IN % OF IL

V 69 kV

h 11< 11 h 17 17 h 23 23 h 35 35 h TDD4.0 2.0 1.5 0.6 0.3 5.07.0 3.5 2.5 1.0 0.5 8.010.0 4.5 4.0 1.5 0.7 12.012.0 5.5 5.0 2.0 1.0 15.0

ISC / IL 20

20-5050-100100-1000 1000 15.0 7.0 6.0 2.5 1.4 20.0

PQ Mitigation

Reactive and Harmonic Demand Active

Filter Passive Filter

Voltage sag and Swell

Dynamic Voltage Restorer Tap changing transformer

PQ Mitigation Voltage Unbalance Static Power balancer Redistribution of single-phase loads equally to all phases (Utility level).

Load Balancing (Plant level) Voltage Flicker Distribution static VAR compensator

PQ Mitigation

Transient Disturbances Surge Arrester

Isolation transformer Active/Passive Filter