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Electric Power Quality

TutorialPart II: Sources and Mitigation

Schemes  

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Sources of Power Quality

Problems

•  Nonlinear Loads•  Sources of Harmonics

•  Sources of Flicker

•  Sources of Sag•  Different Converter Schemes

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Proliferation of Nonlinear Loads 

• Impact customer loads, distribution feeders

and substation equipment.

• Impacts individual customers’ neighbors andultimately the source.

• Lack of National standards complicate the

issue.

• Utilities, customers and suppliers need to work

together.

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Sources of Harmonics

• Power Electronic Devices

 – Phase-angle regulators in lighting/heating

controllers

 – Rectifiers/Inverters

 – Adjustable speed motor drives

 –  Interface of wind /solar power converterswith the utility

 – HVDC systems

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Sources of Harmonics & Flicker

• Ferromagnetic devices

 – Transformers (saturation non-linearity)

• Arcing Devices

 – Fluorescent lamps

 – Arc welders

 – Arc furnaces

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• Starting of Heavy Loads – Large motors

• Brown Outs

 – Large loads

• Fault clearing times on distribution

feeders: 5 to 15 cycles.

• Sags range from 20 to 50%.

Sources of Sags

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Effects of Sags

• Voltage sags not perceptible to human eye.

• Sensitive electronic equipment affected byvoltage sags.

• Sags interrupt service to loads such asautomated processes for many hours.

• Results in loss of revenue of millions ofdollars.

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Sags

• Need compensation devices to avoid interruption.

• Need to evaluate the role of protective devices.

 –  Reclosings after a fault

 –  Operating speed of circuit breakers, fuses and reclosers.

• Need to analyze causes for faults. 

 –  e.g.: Tree falling.

 –  Multiple reclosings maybe questionable from powerquality perspective.

• Adopt optimum tree trimming policies.

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Phase-angle Regulators

• Example: Lamp dimmer

• Comments:

 –  Current distortion can be reduced by proper sizing of the choke.

 –  THD and radiated EMI is low for triggering angles close to 0 and 180  (i.e., full or zero brightness)

 –  THD and radiated EMI is highest for angles to 90  (half-bright).

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Lamp dimmer circuit waveforms

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Spectrum of current drawn by lamp dimmer circuit

• High frequency components which lead to EMI are reduced by the choke.

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Single-phase Rectifiers

• Examples: Computer power supplies, Battery chargers

• The rectifier conducts only when the line voltage magnitude exceedsthe capacitor voltage.

• The capacitor gets charged by drawing current at the peak of thevoltage cycle and gets discharges slowly into the switching regulatorbetween the voltage peaks.

• Thus the circuit draws short pulses of current during line voltagepeaks.

Typical computer power supply front-end

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Current Drawn by a Computer Power Supply

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Sequence Classification of Harmonics

• In AC systems, the current

and voltage waveforms have

rotational symmetry. –  even harmonics will not be

 present.

• Power system harmonics are

hence predominantly theodd, i.e 3rd, 5th, 7th, etc.

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Three-phase RectifiersSix-pulse Rectifier

• Used in motor drives, traction, electrochemical plants, etc.

• The high inductance in the dc side causes the dc current,

Id to be essentially constant.

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Three-phase Rectifiers (cont.)

Six-pulse Rectifier

• The Fourier series for the line current for a diode

rectifier is:

• For symmetrical ideal triggering, only harmonics of the

order 6n 

1 are present in the AC side currents.

• The presence of source reactance and commutation

effects lead to smoother current waveforms.

...t13sin13

1

 t11sin11

1

 t7sin7

1

 tsin55

1

 tsin I

32

 t)(  dai         

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Supply voltage and current waveforms for three-phase bridge

with highly inductive load

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Twelve-pulse Rectifier (cont.)

• Used in high power motor drives, traction, hvdc converters, etc.

• The Fourier series for the line current for a twelve-pulse dioderectifier is:

• For symmetrical ideal triggering, only harmonics of the order

12n 1 are present in the AC side currents.

...13sin

13

111sin

11

1tsin

32)(   t t  I t i

d a     

 Supply voltage and current waveforms for twelve pulse bridge with highly inductive load

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Cycloconverters

• Used in large mill drives in cement and mining industries.

• The characteristic harmonics generated are:

  061   nf    f   pm  f  h  

f o=output frequency of the cycloconverter;

m=1,2,3,… ; n=0,1,2,… 

Typical input current harmonics of a six-pules cycloconverter with 5-Hz output frequency

• The harmonic spectrum

varies as the output

frequency is varied.

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Integral-cycle Controllers or Pulse Burst Modulation

(PBM)

• This technique is used in applications such as heating, ovens,furnaces, etc.

• Subharmonics are predominant. DC component can also bepresent.

• High frequency harmonics above 200 Hz are practically

absent.

Pulse-burst-modulation power conditioning .Current wave: n=6; g=4/6

Harmonic spectrum for g6/8.

Currents generated by a typical PBM system.   

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A Demonstration That a Balanced 3-Phase Load Can

Result In Neutral Current

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Types of Filters

• Passive filters

 –  provide low impedance path to ground at resonance

frequency,

 –  use tuned RLC components,

 –  economical.

• Active filters

 –  inject harmonic currents (or voltages) out of phase

with the ambient harmonics, –  use components such as switches and amplifiers,

 –  expensive.

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Passive Filters

• Series tuned circuit offers very low impedance atresonance frequency

• Parallel tuned circuit offers very high impedance

at resonance frequency

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Capacitor as a Filter

• A shunt capacitor is the simplest form ofpassive filter –  economical,

 –  also provides reactive power (Q) compensation.

• Guidelines for sizing capacitive filters –  resonance between capacitor and circuit inductive reactance

should not occur exactly at an integer multiple of fundamental

frequency.

 –  sensitivity of resonant point to drift in capacitor value shouldbe investigated,

 –  voltage and var support provided should not be excessive,

 –  IEEE Standard 18 should be consulted for sizing and

placement of capacitor.

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Active Filters - Voltage and Current Type

• Voltage type active filter

 –  capacitor (dc source),

 –  voltage source inverter (VSI).

• Current type active filter

 –  inductor (current source),

 –  current source inverter (CSI).

Voltage type (left) and current-type active filters.

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Active Filters - Technology Overview

• Active filters are also referred to as active power

line conditioners.

• Reference [3] gives an extensive literature survey of

existing and proposed line conditioning

methodologies.• Active filters may also be classified depending on

their correction method.

 –  correction in time-domain,

 –  correction in frequency domain.

• Table 1 in [3] gives a summary of the various

publications in the area of active filters - a few

entries of which are shown here in Table 1.

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Active Filters - Technology Overview

• In Table 1,

 –  ‘Type’ indicates voltage type (V) or current type (I), 

 –  ‘P.E. Device’ indicates the power electronic switch used. 

Table 1: Summary of publications on active power filters [3].

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Static Var Compensator

• Consists of electronically switched capacitor and/or

inductor.

• Some SVC technologies

 –  Thyristor Controlled Reactor (TCR) with fixed capacitor (FC) –  TCR with thyristor switched capacitor (TSC).

• The Adaptive Var Compensator (AVC), developed

at the University of Washington, is essentially a

bank of TSCs.

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Static Condenser (STATCOM)

• FACTS and Custom Power Device

 –  reactive power compensation,

 –  voltage regulation (by reactive power compensation), –  harmonic current compensation.

• Behaves as a voltage source connected in shunt to

the power system through an inductor.

Figure 22: Functional

block diagram of a

STATCON.

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Dynamic Voltage Restorer (DVR)

• FACTS and Custom Power Device

 –  voltage regulation (by series compensation),

 –  harmonic line voltage compensation.

• Behaves as a voltage source connected with the power

line.

Figure 23: Functional

block diagram of a DVR.

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Solid State Breaker (SSB)

•Custom Power Device –  for instantaneous fault clearing, –  subcycle reclosing,

 –  zero current/voltage closing.

Figure 24: Functional schematic of a solid state breaker.

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Adaptive Power Quality Compensator

• Decouple reactive and harmonic

compensation

• Arrange Converters so that –  slow switching reactive converter bears most of

the stress

 –  fast switching harmonic converter handleslower voltages and currents.

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ShuntShunt CompensatorCompensator ImplementationImplementation

Sensing&

Control

HPFilter 

PWM inverter - phase A

Stepped-wave inverter -reactive compensator 

Gate s ignals

AB

C

CH

LH

LQ

IH

IQ

LoadSource

ILI S

CQ

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Shunt Compensator Simulation Results

Compensation of Rectifier Load

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Operation of reactive converter

Shunt Compensator Simulation Results

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Current sharing between reactive and PWM converters

Shunt Compensator Simulation Results

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The Adaptive Var Compensator (AVC)

• No harmonics or transients are introduced by the AVC.

• Compensates on cycle to cycle basis

 –  power factor control,

 –  voltage control.

 –  Voltage flicker control

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The Adaptive Var Compensator (AVC)

• Essentially a bank of switched capacitors.

• Capacitors are in a binary ratio (1:2:4).

• SCRs switched at zero current and zero voltagecrossing.

AVC is a bank of electronically switched capacitors in a binary ratio.

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