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Variable Frequency drives-Application, Limitations & Advancements

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Introduction to VFDs Variable frequency drives (VFDs) are energy efficient devices which are extensively

used for process productivity improvement and energy saving applications.

The process of speed variation requires the use of an input rectifier for AC-DC

conversion and an output inverter for converting DC to variable frequency AC.

This output is applied to a standard Squirrel Cage (SQC) motor for variable speed

operation.

| Confidential/Property of Danfoss Power Electronics A/S | APR HPD June 2012_R0 | 2

The input rectifier normally consists of a diode or diode/thyristor bridge and the

output stage of Insulated Gate Bipolar Transistor (IGBT) bridge circuit .

The generation of variable speed requires the standard motor to operate at variable

frequency whilst maintaining the flux in the machine constant.

Hence the motor voltage varies linearly with the frequency in order to maintain the

V/F ratio constant.

The voltage applied to the motor stator is a pulse width modulated (PWM) signal

which has a high frequency carrier modulated by the motor frequency signal.

Power Circuit of a VFD

Input 3


Input 3

Phase, 50Hz supply (415V normally)

Input rectifier (diode or

diode/ thyristorOutput Inverter (IGBT

based)

Drive Motor (usually SQC

type)

DC Link

capacitor

Pulse Width Modulation Control principle uses a sinusoidal reference voltage (Us) for each inverter

output.

The three reference voltages are superimposed by a delta voltage equal to the maximum switching frequency

(Carrier signal) of the inverter.


The period length of the sinusoidal voltage corresponds to the required basic frequency of the output voltage and

therefore represents the motor

frequency.

Sinusoidal PWM

The output voltage is changed by the ratio between the on and off time and

this ratio can be changed to generate the

required voltage.

Since the reference voltage is a sine wave


Since the reference voltage is a sine wave at the motor frequency, the technique is

called Sine Wave PWM.

The amplitude of the negative and positive voltage pulses thus always

corresponds to half the intermediate

circuit voltage (DC Bus Voltage).

Benefits of PWM Switching

Since the output voltage waveform

consists of the high frequency carrier, the motor inductance can easily filter this

signal.

The motor current waveform therefore

closely resembles a sine wave.

Carrier frequency- 3KHz.


closely resembles a sine wave.

The use IGBTs as output devices which

are capable of switching at high

frequencies, ensures a vastly reduced drive overall dimensions.

IGBTs are high efficiency devices which result in the drive having an overall

efficiency of 96-98.5% depending upon

the capacity.

Carrier frequency-

12KHz.

Benefits of using VFDs

Since the motor frequency increases with

a programmed linear ramp rate, the

inrush currents are limited to a maximum of typically 0.95 at full motor load.

LOAD TYPES Drive motor loads are generally classified

as constant torque (CT) and variable

torque (VT) type.

The CT load offers the same resistance

torque at the motor shaft at all speeds.

The VT load offers variable resistance

torque at the motor shaft at all speeds.

CT Load profile- Torque vs speed


CT loads normally require a high starting

torque, with the running torque being much less than at start.

VT loads typically require 110% torque at start, with the running torque being

decided by the load characteristic.

Typical applications for CT loads include

conveyers, mixers, hoist cranes among

others.

VT loads are typically centrifugal fans and

pumps.

CT Load profile- Torque vs speed

Dotted line represents power

variation with speed.

VT Load profile-Torque vs speed.

Energy Saving & VFDs

A typical VT load has a torque

requirement which varies with speed.

The flow rate Q in a typical centrifugal

pump application varies linearly with

speed & Q=KN, K being constant.

The pump head and power consumed are


The pump head and power consumed are

given by affinity laws shown alongside.

This means that if the pump motor is

operated at

Typical Example of Centrifugal Pump

The graph shown alongside represents

the variation of pump head m with flow

rate (m3/hr).

The variation in speed from 1480RPM to

1180RPM produces significant variation in the power consumption (from 150KW to


the power consumption (from 150KW to

60KW approximately).

Practically, the efficiency remains

constant over the speed range.

The affinity laws as described in the

previous slide give an idea of the pump

performance at various speeds.

Actual performance needs to take into

account the system curve where it becomes necessary to take into account

friction losses in the pipe.

Examples of Constant Torque Loads

A reciprocating compressor would be an example of a CT load.

The piston number of strokes/ minute will decide the air flow rate if the medium to be compressed is air, for example.

In many applications where the load to unload ratio is small (unloading time when the compressor runs at reduced load> the loading time), there is a potential for

energy saving.


energy saving.

This requires a trial on the actual unit in order to quantify the energy savings with a

VFD. The header pressure is normally the feedback signal for closed loop operation

with speed control. Energy savings of up to 25% can be realized in a real life situation.

Belt conveyers are CT loads where the benefit of improved process productivity, coupled with reduced mechanical stress on the equipment during starting and

stopping can result in a high efficiency of operation.

VFDs & Harmonics-The flip side Though the benefits of VFDs are many,

there is an issue arising out of their

operation which can adversely affect the supply network to which they are

connected.

The switching devices (input diodes and

thyristor) in the front end bridge rectifier Line current profile in a 6 Pulse VFD


thyristor) in the front end bridge rectifier

cause the input line current to deviate from its ideal sine wave profile.

This deviation gives rise to currents which are non-sinusoidal in nature and

therefore contain harmonics.

Typical input line current & line voltage

profiles with 690V motors running on VFD

supply are shown alongside.

This phenomenon is referred to as

distortion caused by harmonics.

Line current profile in a 6 Pulse VFD

operating at part load (690V Motor).

Line voltage profile in a 6 Pulse VFD

working on 690V supply.

Harmonics

The simplest definition of harmonics

would be-they are unwanted signals in

the power system.

They are defined as voltage and current

signals which are multiples of the power frequency.

The VFD for example is a nonlinear device

Ideal current plot (minimum distortion)


The VFD for example is a nonlinear device which introduces harmonics in the supply.

These harmonics occur at odd multiples of the power frequency (50Hz in our

case).

Hence the harmonics occur at 250Hz,

350Hz, 550Hz, 650Hz and so on.

These correspond to harmonic numbers

5,7,11 & 13 respectively.

Harmonics in the current waveform

Resultant distorted current waveform

Definition of Current & Voltage Distortion

Harmonic distortion is quantified by the

use of VTHD & ITHD for voltage and

current respectively.

The RMS current value is the vector sum

of the fundamental frequency current (50Hz in our case) and the harmonic

Mathematical definition of ITHD as

above.

This is expressed as a % of the


(50Hz in our case) and the harmonic

currents.

Harmonic current RMS value is again the

vector sum of the various harmonic

frequency currents (5th, 7th, 11th, 13th, 17th

and so on in the case of the VFD).

Similar definitions apply to the voltage signal as far as quantification of distortion

is concerned.

This is expressed as a % of the

fundamental frequency current.

In represents the harmonic current

value corresponding to the nth

harmonic.

I1 represents the fundamental

frequency (50Hz in our case) current

value.

Since the VFD produces only odd

harmonics which are not multiples of 3,

n will take the values 5,7,11,13,17 & so

on.

A similar definition can be applied to

VTHD.

Ill effects of harmonics in the supply

Voltage Harmonics

Malfunctioning of sensitive electronic

equipment.

Premature ageing of such equipment.

Increased EMI generation in the power system.

Current Harmonics Increase in resistive and hysteresis losses

and attendant temperature rise in the

winding and core of the supply transformer, is the first symptom of

excess current harmonics.

Nuisance tripping of electronic protection

relays and circuit breakers.


system.

Increase in core losses in motors which are directly operated from the utility

supply having a high percentage voltage

distortion.

Increased torque ripple in drive motors

directly operated from the utility supply having a high percentage voltage

distortion.

relays and circuit breakers.

Failure of power factor correction

capacitors (PFCC) due to series or parallel

resonance in the supply system.

Increased temperature rise in connecting

power cables.

Failure of the neutral current carrying

conductor in the supply transformer due to unbalanced single phase non-linear loads.

Measurement of Harmonics in the Power Supply

Measurement of the voltage and current distortion figures in any network requires

a good Power Analyzer which, in addition to measuring the circuit performance

parameters-Input Power, Input Voltage, Current, Power Factor & KVA consumed, is also able to measure the harmonics present in the supply voltage and current as well

as their magnitudes at various harmonic frequencies. A good power analyzer is able

to give the user harmonic frequency readouts up to n=50.

Additionally, the analyzer makes it possible to trend the circuit performance


Additionally, the analyzer makes it possible to trend the circuit performance

parameters over an extended period of time for the user to analyze the load and distortion variations with time. This information can then be used to determine the

predominant harmonics and devise an appropriate solution for harmonic mitigation.

Sophisticated computing tools, which give reasonably good estimates of the

distortion parameters, can also be used for the same purpose.

The measuring point for VTHD & ITHD needs to be defined properly in order to

determine the permissible limit for these parameters as per international standards.

The drive supply point (the point at which the drive gets connected to the power supply or Point of Common Coupling) is designated as PCC3; whilst the point at

which all equipments get connected to the power supply is designated as PCC2.

HT BUS Measuring

point PCC1

INTERMEDIATE

HT BUS

Typical Industrial Power System


Measuring

point PCC2

LT BUS

Measuring

point PCC3

NOTE

Blue arrows represent

the flow of harmonic

currents in the supply

network.

Typical data for VTHD & ITHD The data shown alongside represent the

performance of a VT load (ID Fan)

running with a 6 Pulse drive.

The measurement point is PCC3 for both

VTHD & ITHD.

The VTHD figure measured on all 3


The VTHD figure measured on all 3

phases is

Limits for VTHD & ITHD

The IEEE-519 1992 Standard is most commonly used as a reference

document for defining the limits of harmonics in an industrial supply

network.

It defines the limits for VTHD based on the type of installation. These are

classified as general industrial , dedicated & critical systems, with the limits being different in each case.

The Total Demand Distortion (TDD) refers to the current harmonics in a


The Total Demand Distortion (TDD) refers to the current harmonics in a supply network having a mix of harmonic (non-linear) & non-harmonic

(linear) loads.

The TDD figure will be < the ITHD figure measured at the device (VFD, for

example) terminals if linear loads are also connected to the supply

transformer.

The standard also defines the limits for the harmonic current magnitudes

along with the TDD figure.

TDD will vary as a function of Isc/Il where Isc is the Tx SC current and Il

the connected load demand.

IEEE 519-1992Critical

applicationGeneral system

Dedicated

system

VTHD 3 % 5 % 10 %

I /I h < 11 % 11h

Passive Devices

These are magnetic devices which are

cost effective & give moderate to good

performance.

Typical values of ITHD range from 10%-

40%, depending upon the type of mitigation device selected.

Typical Harmonic Mitigation Devices

DC ReactorAC Reactor


The devices can be graded in decreasing order of performance as Series Passive

Filter, 12 Pulse drive, and AC/DC

reactors.

Limiting the ITHD value will depend upon

the specification given by the end user/consultant. The device can then be

selected keeping in mind the cost and

performance.

12 Pulse Drive 18 Pulse Drive

Series Passive Filter

Active Devices

These mitigation devices use active

devices (IGBTs) for harmonic mitigation

(HM), as well as Power Factor (PF) improvement.

The shunt active filter is ideally suited for mitigation with all types of non-linear

loads (VFDs, DC Drives, Heating

Furnaces, for example).

Typical Harmonic Mitigation Devices

Shunt Active Filter connected in


Furnaces, for example).

Both devices generate harmonic currents

with the opposite polarity and magnitude to cancel the harmonic currents

generated by the connected load.

They can also supply reactive power to

the connected load in order to improve

PF.

The shunt filter can be sized for only the

harmonic currents, hence is a cost effective central solution.

Shunt Active Filter connected in

parallel with a non-linear load. The

schematic is identical for the low

harmonic drive (LHD).

PWM Drive with Series Active Filter

(Active Front end Rectifier)

Low Harmonic Drive

The hardware consists of a standard 6

Pulse rectifier with shunt active filter.

The filter design is optimized to give the

best performance with respect to

harmonic cancellation.

It has a high overall efficiency (96%)

and can be used as an energy efficient

Active Harmonic Mitigation Devices

Filtered Line

Current

Distorted Drive Input Current


and can be used as an energy efficient device.

Performance of the drive is unaffected by background VTHD.

Does not suffer from the disadvantage of having an output filter installed to limit

peak voltage and dv/dt at motor

terminals.

For Isc/Il >20, the ITHD figure is 5%Schematic of Low Harmonic Drive

(LHD)

Generated

harmonic

current profile

Optimized Shunt Active Filter

Input

Rectifier

Performance Details

The shunt active filter can be used as a

cost effective HM solution by virtue of its

design.

It can be easily installed at the supply

transformer secondary, since it only requires a 3 feeder, and 3 Nos CTs

mounted on the load side.

Shunt Active Filter Performance


It can be programmed for overall

compensation (cancelling all harmonics

up to n=25), or selective compensation (cancelling specific harmonics).

Since it can supply reactive power, PF improvement is an added benefit to the

user.

Typical performance plots are shown

alongside.

Danfoss Drives & Mitigation Solutions

Details

Danfoss has a wide range of offerings for

drives and HM solutions.

Drives can be engineered for 6 Pulse or

12 Pulse operation in both 400V as well

as 690V, with capacities ranging from 1MW.

They have DC reactor as a standard

Series Passive Filter for 5% and 10% ITHD


They have DC reactor as a standard feature.

Low Harmonic Drive (LHD) with inbuilt shunt active filter is available as standard

from 132KW-710KW in the voltage range

380V-480V AC.

Other HM solutions include series passive

filter & standalone hunt active filter.

Active Filters are available as standard in

capacities ranging from 190A-400A in 380V-480V supply voltage range.

132KW Low Harmonic

Drive (LHD)

190A Standalone

Active Filter

THANK YOU


Ganesh Iyer, the author, is presently

working as General Manager-

Applications Specialist with Danfoss Industries Private Limited for more than

7 years.

He has over 30 years of design and

application experience in the field of

drives, control systems & harmonic mitigation solutions.

About the Speaker


mitigation solutions.

He is an alumnus of Indian Institute of Technology-Mumbai, having completed

his graduation and post graduation with

specialization in Power Electronics and Control Systems.

He can be contacted at:[email protected]

Telephone # +919920373263 (M).

+912266817300-Ext #316

Literature References

Variable Speed Driven Pumps-Best Practice Guide- Brought out by British

Pump Manufacturers (BPMA) association & Gambica.

IEEE-519 1992 Standard-IEEE Recommended Practices and requirements

for harmonic control in electrical power systems.


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