variable speed drives harmonics

© Confederation of Indian Industry

Variable Speed Drives

&

Harmonics


Variable Speed Drives

� VSD

� Device used for varying the speed of

a driven equipment

� Applications

� Pumps, Fans & Blowers,

Compressors, conveyors etc.


Why VSD’s ?

�Many Applications subject to

varying loads

� Due to Process Variations

� Changes in capacity Utilisation

� Conventional controls - High energy

loss


� VSD – Advantages

� Accurate Speed Control – Exact

match to Process Requirement

� Energy Savings

� Productivity increase

� Low maintenance cost

� Smooth Starting

� Easy operation

Why VSD’s ?


Types of VSD’S

� DC Drives

� Eddy Current Drives

� Rotor Resistance Control (GRR)

� Slip Power Recovery System (SPRS)

� Variable Fluid Coupling (VFC)

� AC Drives (VFD/VVVF)


DC Drives

� Most Common VSD – Till AC VFD’s

� Advantage of DC Motors

� Good Speed Torque Characteristics

� Easy method of control - Armature voltage

control

� Wide speed range

� Low loss

� Smooth start

� Good dynamic response

� Applications – Steel, Paper, Cement, Sugar,

Tyre & Alloy Industries


AC Drives

� AC motors are most common in industry

� AC Drives – Becoming popular

� Advantages over DC Motors

� No Carbon brushes, Commutator

� No regular Maintenance compared to DC

motors

� Can be fitted in Hazardous & adverse

conditions


Eddy Current Drive


Rotor Resistance Control

� Applicable only for Slip Ring

Induction Motors





� Higher rotor resistance

� Higher slip and lower speed

� Advantages

� Large speed control below rated

� No harmonics

� No electronic components

� Can be installed in any environment



� Disadvantages

� Power loss in the rotor resistance

� Cooling fans required – extra power

� Speed variation in steps

� Has lot of contactors

�Maintenance is required

� GRR loss = s(1 – s2) x Actual kW I/P


SPRS ControlSlip- ring induction motor

Y

R

B

3-phaseinput

3-Phase Inverter Rectifier

Microcontroller

Firing Circuit

Actual speed

Speed feedback

Actual speed

Set speed


SPRS Control

� Speed is controlled by drawing

power from rotor

� Advantages

� Slip power is recovered

� Disadvantage

� Limited speed control


Speed Control in AC Motors

� Changing Supply Frequency

N = 120 f

P

� Variation in frequency – Speed Variation

� Keeping V/f Ratio constant

� Two different Poles – Winding

� 2 Pole / 4 Pole

� 4 Pole / 6 Pole


AC Drives

� VFD

� Power electronic equipment

� Converts fixed AC supply into variable

AC supply

� Frequency & Voltage


AC Drives

� AC Drives – Frequency Controller(V/f)

� Controlling Variable – Voltage ,

Frequency

� Torque Can not be controlled & only

speed can be varied

MModulator V/f ratioFreq reference


AC Drives

� AC Drives – Flux Vector Control

� Indirect Method of torque control

� Closed loop Control

� Good torque response

� Accurate Speed Controls

� Full Torque from Zero Speed

MModulator Torque Control

Speed Control

Tacho


Block Diagram of AC Drive

Rectifier

Bridge

FILTER

InverterWith

VoltageControl

LOAD

AC DRIVESYSTEM

3 φφφφSUPPLY


.

What is a frequency converter ?VFD

Control

V & FSpeed

Reference

RectifierDC Link

Choke

415V,50Hz

Input

IGBT

Output

Stage

Motor

Output0-415V,0-50Hz

Fixed AC - DC- Filter DC -Variable AC


AC Drives

� Advantages

� Soft Starting

� Precise speed & Torque control

� Wide Speed range

� High reliability

� Low noise

� Capability for speed reversal /

regenerative braking

� Energy saving


Technical Specifications - VSDs

System Layout

Conditions - Equipment Operate

System Requirements


� Input supply range

� Motor rating

� Application

� Type of load

� Speed range

� Environmental conditions

� I/O requirements

� Motor cable length

VFD Selection Criteria


� VFDs are available in the voltage range – 220V, 380-500V and 525-690V

� All VFDs can take care of supply frequency range

� Note down motor nameplate details� Voltage, kW & Current

� Frequency

� RPM

� Current

VFD Selection Criteria – Input Supply


� Select VFD that can give output suitable to motor - This is the minimum size of VFD required

� Application to be studied� Constant torque or variable torque

� VFDs with 110% overload are suitable for variable torque � 150% overload for constant torque applications

VFD selection criteria – Motor rating


� Speed-torque characteristics to know the initial torque requirement

� Select VFD accordingly

� Does the application demand open loop OR closed loop operation ?

� Most VFDs will work in open loop

� Performance with closed loop device connected to motor ?

VFD selection criteria –Application


� If the application needs process

feedback to be taken in VFD

� It should have minimum 2 analog

inputs…..one for reference and

second for feedback

� VFD should have PID loop

� E.g. pressure control

VFD selection criteria –Application


� Depending on where the VFD will be mounted, select VFD with proper IP class of protection� VFDs are available with IP00, IP20, IP21, IP54 and IP66 protection class

� VFDs are available with 40oC, 45oC and 50oC operating ambient temperature� Select VFD appropriate to the ambient

VFD selection criteria –Environment


� VFD with 40oC ambient temp needs to be derated while operating at 45 or 50oC

� VFDs pollute the electrical environment by injecting harmonics in the electrical system

� Select VFD which has inbuilt DC or AC choke� This reduces % of harmonics injected

VFD selection criteria –Environment


� VFD manual specifies maximum cable length between VFD and motor� Select VFD which can support the installation to avoid additional hardware in the circuit

� Automatic Energy Optimization feature will reduce the consumption of the power at light loads

VFD selection criteria – Cable length


Maximum Cable Length w/o O/P Chokes

300 m120 kw

200 m45-110 kw

150 m15-37 kw

100 m5.5-11 kw

60 m4.0 kw

50 m2.2 kw

40 m1.5 kw

30 m1.1 kw

25 m0.75kw


� Select VFD which has innovative cooling concept to reduce the air conditioner load

VFD selection criteria – Others


Precautionary Measures &

Trouble Free Operation ofVSDs


Physical Installation Procedure -Electronic VSDs

Drives - Not to be installed

� Direct Sunlight� High Temperature� High Humidity� Excessive Vibration


Ambient Conditions - Drives

Dust Free

Corrosion Free

Temperature < 40° C


Operation of VSDs :

Reliable

Trouble-free

Substantial Energy Savings


Impact of Harmonics


= ++

What are Harmonics ?

Resultant Wave Fundamental 3rd Harmonic 5th Harmonic


HARMONICS

� Normal Supply system operates at 50 Hz

� The Harmonics are sinusoidal waves that

are integral multiples of fundamental

frequency (150 Hz, 250Hz, 350Hz ...)

� The overall impact of Voltage/ current

harmonic distortion on a power system

wave form is called as Total Harmonic

Distortion(THD)

� Pollution in Electrical System


Non Linear Non Linear Non Linear Non Linear

LoadLoadLoadLoad

HarmonicI

Voltage to other

circuits and the

pollution

circulates

Harmonics

Generation

Non Linear Loads

draw non linear

Current From a

linear source

When Linear Voltage is

applied to

Distorts voltage


Sources of Harmonics

� Non-linear Loads

� Static Switches - Diodes, SCR’s,

GTO’s, Transisters, IGBT’s etc.

� Devices / Equipment

� Variable Speed AC & DC Drives

� Arc Furnaces

� Induction Furnaces

� Welding sets

� UPS


Effect of Harmonics

� Power Distribution System

� Additional Losses in wires & cables

� Extra heating/noise of Transformers

� Circuit breaker & Protective Relays malfunction

� Other Equipment

� Erratic operation of computers, Telecommunication, Video Monitors & Electronic test equipment

� Failure of Capacitors


Effect of Harmonics

� Other Equipment

� Overheating of Motors

� De-rating of Generators

� Malfunction of Measuring instruments


Problems created by harmonics

� Excessive harmonic currentmay lead to overheating (or even burning) of network components


Problems created by harmonics

�Capacitor

problems

Due to its lower

impedance, capacitors

are even more

susceptible to higher

order harmonics


Harmonics Distortion

� Voltage Harmonic

� Distortion in Voltage Waveform

� More harmful than Current

Harmonics

� Electrical equipment sensitive to

Voltage harmonics

� Current Harmonics

� Distortion in Current waveform

� Load Dependent


Total Harmonic Distortion

%100)(

)(1

2

xV

VVTHD

n∑=

Where

n = 2 …. 31Vn = Harmonic Voltage of nth orderV1 = Fundamental voltage


Acceptable Limit -International Guidelines

Supply SystemVoltage (kV) atpoint of common

coupling

TotalHarmonicVoltage

Distortion VT(%)

IndividualHarmonicVoltage

Distortion (%)

Odd Even0.415 5 4 2

6.6 and 11 4 3 1.75

33 and 66 3 2 1

132 1.5 1 0.5

Harmonic Voltage Distortion limits at any point on the system

IEEE G.5/3 Sept. 1976 : Limits for Harmonics.


Harmonic Distortion - IEEE Norm

� Voltage Distortion

� 415 Volts : < 5% THD

� 33 KV : < 3% THD

� Current Distortion

� Depends on Isc/IL ratio of bus

� Generally less than 15%

�Voltage Harmonics - More harmful to electrical equipment

�Motors, Capacitors etc.

� Current Harmonics – Magnetic interference and increase in distribution losses


Options for Reducing Harmonic Distortion

� Use of PWM AC Drives

� Use of drive with effective DC link

filtering

� Use of 12 - pulse rectifier drive

� Installation of Harmonic filters


Harmonic Filters

� Two types of filters

� Active filters

� Passive filters

� Active filters

� Introduces signals in the opposite

direction to compensate harmonic

distortion

� Costly


Harmonic Filters

� Passive filters

� Built with passive components

� Capacitors & Reactors

� Economical

� Do not eliminate harmonics

completely


Harmonic Filters

� Benefits of filters

� Reduced system losses

� Improved system protection

� Reduced neutral to ground voltage

� Improved PF of non linear loads


Harmonic Study

� Observe any malfunction

� If yes, List & Quantify non-linear

Loads

� Conduct Preliminary Harmonic

Study

� Compare with Standards

� Detailed study & Cost economics

� Install Harmonic filters

variable speed drives harmonics

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