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KORADI TRAINING CENTRE, KORADI

A

SEMINAR REPORT

ON

Variable Frequency Drive .

Submitted By :

Mr. Samir B.Ahmad

Junior Engineer,

PARAS (2x250 MW), K-57 Batch KTC

Under the guidance of:

Course Director Course Co- ordinator

Shri. M.G.Ganoje,Shri. P.P.Gawande,

Executive Engineer, KTC Deputy Executive Engineer, KTC


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ACKNOWLEDGEMENT

Apart from my efforts, the success of any project depends

largely on the encouragement and guidelines of many others. I

take this opportunity to express my gratitude to the people whohave been instrumental in the successful completion of this

project.

It is with deep sense of respect that, I would like to show

my greatest appreciation to Shri. S.G.Gattewar, C.E.

(Training) for his valuable guidance, constant motivation, often

useful suggestions and kind words of encouragement throughoutthis project.

I would also like to thank our Course Director Shri.

M.G.Ganoje ,E.E. & Course Co-ordinator Shri.

P.P.Gawande, Dy.E.E. for their valuable and inspirational

thoughts and comforting support throughout our project.

I am grateful to all the Engineers who have delivered their

lectures to us and thus enhanced our knowledge about the

power plant by sharing their valuable experiences throughout

our training.

Sincere thanks are also due to our training library

assistants for being kind enough and going that extra step

ahead in allowing us to work after office hours and helping us in

many ways.

Last but not the least I am thankful to all those who were

directly or indirectly involved with the completion of my project.

Mr.S.B.Ahmad

Jr. Engg. (Trainee)


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Batch K-57

CERTIFICATE

This is to certify that Mr. Samir B. Ahmad ,

Junior Engineer,

Paras Thermal Power Station (2x250 MW), K-

57 Batch KTC has completed the Project on

Variable Frequency Drives.

This Project is submitted in partial fulfillment

towards the completion of Induction level training

program & the project has been assessed & it

appears satisfactory up to the standard envisaged

for the level of the course.

Shri. S.G.Gattewar,

C.E.,Training, KTC

Shri. B. U. Waghmare,

S.E., KTC

Course Director

CourseCordinator


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Shri. M.G. Thool ,

Shri. P.P. Gawande,

E.E., KTC

Dy. E.E., KTC

ABSTRACTWith ever increasing cost of generation of electricity and the

demand for electricity outstripping the availability, energy conservation hasbecome the key factor of efficiency in all area. Power consumption by

auxiliaries in large power plants is of the order of 810%.To reduce

auxiliary power consumption, losses in Air/Flue gases and Water/Steam

paths because of mechanical devices used for flow control shall be

reduced.Techno-economic analysis carried out recommends that

installation of energy saving devices like variable frequency devices.

Installation of site measurements has proved saving power visavis

theoretical calculation. Installation of VFDs for ID fan have proven their

worth and given confidence to the utility engineers to use VFDs for boiler

feed pumps in Indian power plants.


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1. INTRODUCTION

The synchronous speed of an AC motor is determined by the frequencyof the AC supply and the number of poles in the stator winding, according

to the relation: RPM = 120 *f / p

WhereRPM = Revolutions per minute,

f = AC power frequency (hertz),

p = Number of poles (an even number)By varying the frequency of the voltage applied to the motor, its speed can

be changed. Synchronous motors operate at the synchronous speeddetermined by the above equation.

1.1 What is a LCI (LOAD COMMUTATED INVERTER) TYPE VFD ? LCI type VFD is a variable speed mechanism, which

adjusts the fan speed so as to get the desired output. It is static, adjustablefrequency drive system that controls a synchronous machine from near zero to rated speed. In this type of drive system, flow demand signal isreceived from the control system due to change in load conditions and thesystem adjusts the speed of the motor to the desired level resulting inoptimum power consumption.Drives can be classified as:

Constant voltage

Constant current

Cycloconverter
http://en.wikipedia.org/wiki/Cycloconverterhttp://en.wikipedia.org/wiki/Cycloconverter


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In a constant voltage converter, the intermediate DC link voltage remainsapproximately constant during each output cycle. In constant currentdrives, a large inductor is placed between the input rectifier and the outputbridge, so the current delivered is nearly constant. A Cycloconverter has noinput rectifier or DC link and instead connects each output terminal to theappropriate input phase.The most common type of packaged VF drive is the constant-voltage type,using pulse width modulation to control both the frequency and effectivevoltage applied to the motor load.

1. TYPES AND SYSTEM DESCRIPTION

2.1 Available VFD Configurations:

LCI type VFD can be either (6 pulse) single channel type (Fig2.1)or (12 pulse) dual channel type (Fig 2.2). Each channel consists of an

isolating transformer, source converter, DC link inductor and load

converter. In a single channel type VFD, synchronous motor will have one

winding whereas in dual channel type VFD, the synchronous motor will

have two windings, one for each channel. The source side converter

operates in rectifier mode whereas the load side converter operates in

inverter mode. Commutation VARs for source side converter is taken from

the source whereas leading VARs produced by the synchronous motor is

used for commutation of load side converter. The DC link inductor

effectively isolates load side frequency and source side frequency and

smoothens DC Link Current.

The demand signal received from the control system

prompts the source side converter to provide the required current to the DClink inductor at the DC voltage level set by the load side converter.

Thus the source side converter plus the DC link inductor become a current

source controller to the motor. The motor torque, frequency (hence speed),

and voltage level get adjusted to the load requirements.

The choice of (6 pulse) single channel or (12 pulse) dual channel VFD

depends on harmonics that are allowed to be injected into the grid and

whether a redundant fan/pump is available.
http://en.wikipedia.org/wiki/Pulse_width_modulationhttp://en.wikipedia.org/wiki/Pulse_width_modulation


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A 12 pulse system is recommended where lower harmonic

injection is desired (weak grids) and fan/pump redundancy is not available.

The components in each channel can be rated for desired capacity and

hence in case of dual channel, redundancy can be built into the VFD

system. A 6 pulse system, because of lower cost, is recommended where

redundant fan/pump is available and the grid is strong.


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Fig.2.1: 6 Pulse LCI fig.2.2: 12 Pulse LCI

2.2 Basic LCI System Construction:

The Load Commutated Inverter (LCI) is static, adjustablefrequency drive system that controls a synchronous machine from near zero to rated speed. The basic system consists of line Commutated, phase


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controlled thyristor converter that feeds a load Commutated thyristor converter through a dc link reactor. Fig 2.1 is a simplified one-line diagramof a single channel

Fig. 2.2 Simplified LCI One Line DiagramThe transformer provides isolation from the ac system bus and

provides the correct voltage at the terminals of the rectifier. Also, theinternal impedance of the transformer limits the magnitude of anydownstream bus faults.

The rectifier is a thyristor bridge whose gating is controlled toproduce a variable dc. Voltage at its output. The output of the rectifier is fedthrough the dc link reactor, whose function is to smooth the current andkeep it continuous over the operating range of the system.The dc link reactor output is then fed into the inverter bridge, whichprovides variable frequency ac at the stator terminals of the synchronousmachine. The Inverter Bridge and the Rectifier Bridge use the same power hardware and are controlled by microprocessor-based electronics.Although the bridges are labeled rectifier and inverter. It is possible for their roles and hence power flow to reverse. In this case, the synchronousmachine would be braked by pumping its energy back into the ac line. Amore general nomenclature is to call the line Side Bridge the source

converter, and to call the machine Side Bridge the load converter.The synchronous motor field is usually excited by a brushless exciter coupled to the motor shaft. The brushless exciter is a wound rotor inductionmachine whose rotor voltage is rectified to supply field current to thesynchronous machine.

The stator voltage for the brushless exciter is supplied from astatic excitation voltage controller included as part of the LCI control panel.This controller is gated/controlled entirely by the LCI electronic controlmodule. The LCI controls the excitation to produce the required machineflux and provides field over / under current protection.


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2.3 Available Configuration and power modules of LCI System:

1) Frequently used ratings are:a. 1.2 kV modulesb. 2.3 kV modulesc. 4.2 kV modules

Generally 1.2 kV modules are used in 210 MW, 2.3 kV for 500 MW units.

2) Configuration: Source side converter consists of 6 power modules in 3 phase

bridge configuration to get DC.

Load side converter consists of 6 power modules in 3 phase bridge

configuration to get Variable Frequency Variable Voltage

3) Generally a Power module consists of the following:a. Thyristors as switching device

b. Heatsinks to dissipate heat generated in device

c. Snubber resistor and capacitor to protect device against overvoltages

d. Sharing resistors for equalizing the voltage across series connected

thyristors.

e. Pulse transformer to supply gate pulses to thyristor and to provide

isolation between control and power circuit.

4) For a 2.3kV Power module construction consists of: Three thyristors are used in series.

Snubber provides protection to thyristor against over voltages.

HPTK module gives firing pulses and indicates healthiness of

thyristors. NTDA module indicates healthiness of thyristors.

XPTN module gives attenuated voltage for control.

HMPG - 80286 microprocessor which is the main controller.

NSFE - Field Exitation Control Board.


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Some of the pictures for 2.3kV Power module components are shown here:

SCR mounting with Heat sink HPTK firing card module


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Tyristor (SCR) Snubber circuit

3. LCI BASED VFD WORKING /OPERATIONBefore looking deep into working of LCI module let us see the basic

structure and properties of VFD:


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3.1 Basic operation:A more detailed diagram of an LCI is shown in Fig3.1


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Fig3.1 LCI DriveAs shown in this figure, the electronic control receives the following signalinputs:1. Attenuated line and load bus voltage signals.2. Attenuated line and load current signals from current transformers.3. Speed reference signal.4. Process commands such as stop, start, etc.The attenuated bus voltage signals are used by the control to synchronizethyristor firing with the line and load voltage, to provide voltage feedback,and over/under voltage detection. The attenuated current signals are usedfor regulator current feedback, electronic over-current detection, andsoftware implemented fault detection. The electronic control evaluates theprocess commands and internal status signals to determine whether the

drive should be in a stopped, started, alarmed, or faulted conditionThe source converter current is successfully transferred from one leg to thenext by the synchronous machine stator voltages.The principles illustratedhere apply to a rectifier bridge as well as to an inverter bridge. This phase-controlled switching is accomplished by using the following two thyristor characteristics:(1) when the voltage across the thyristor is positive, it can be triggered intoconduction, and(2) it will not permit current flow in the reverse direction.

Thus in an alternating voltage circuit, thyristor conduction will

cease and reverse voltage will begin to appear when the current becomeszero. Current transfer must be completed before voltage cross over with a


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positive margin angle. The angle must be long enough to allow thepreviously conducting leg thyristors to recover to their blocking state beforeforward voltage is applied. This is why the fundamental component of current must lead the voltage, from an inverter / machine viewpoint. Fromthe rectifier / source viewpoint, the fundamental component of current willalways lag the voltage.

When operating in any mode, the electronic control mustsynchronize firing of the source and load converter to the ac line andsynchronous machine bus voltages, respectively. The primary feedbackfor accomplishing this is the attenuated bus-to-ground signals for bothconverters. Attenuated bus-to-ground voltages are brought into theelectronic control and combined to produce line-to-line analogs for bothconverters. These line-to-line voltages are then integrated to obtain fluxsignals. The zero crossings of these signals are used in thesynchronization of the phase locked loop for the firing control of bothsource and load converters. At low speed, before the phase locked loop iseffective on load side, the zero crossing marks are used as a timingreference for firing in force-Commutated operation .

3.2: Commutation techniques:The source side converter for the LCI always operates line

Commutated; i.e. the ac line provides the means for transferring conductionfrom one thyristor to the next. The load side converter may operate either load (self) Commutated or force Commutated, depending on motor speedand flux level. As the synchronous machine rotor (field) rotates, the near sinusoidally shaped field flux cuts the stator windings, producing a set of

three sinusoidal voltages in the stator. These sinusoidal voltages areangularly displaced by 120 electrical degrees. The magnitude of thecounter emf is proportional to speed and field strength. At low speeds, theinduced emf is insufficient to commutate thyristors in the load sideconverter. In this mode, the load converter must operate forcedcommutated.

Force commutated operation is used when starting thesynchronous machine from zero or low speed and continues until themachine under counter emf is sufficient for self-commutation. There areseveral modes of forced commutation involved in the starting of the

synchronous motor. In tachless LCI application, the starting current is madelarge enough to accelerate the machine to 0.5 Hz in one or two inverter firings. This is approximately the minimum frequency at which the LCI cansense machine flux and begin to control torque and speed. In this mode,inverter firing is synchronized to crossovers of the machine flux and themachine is operated near unity power factor to obtain maximum torque.Forced commutated operation continues until the synchronous machinereaches a frequency where its emf is sufficient to commutate the load sideconverter. At this point, the control transitions to self commutated operation

In the self-commutated mode , the machine must be operated

at a leading power factor in order to be able to insure commutation of theload converter. The electronic control acts to keep the machine power


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factor, and therefore torque per ampere, as high as possible For a givenload current and machine inductance, a corresponding amount of voltseconds is required for a successful commutation. The control reads thepeak volt-seconds of the integrated line-to-line machine voltage andmachine current. The machine commutating inductance is a constantstored in the microprocessor system memory. Using the current andinductance the amount of commutation volt-seconds required is calculatedby the microprocessor. Using this value volt-seconds and the peak volt-seconds of the previous flux wave, the latest possible time to fire iscalculated to give a specific margin after commutation is completed.

3.3 Twelve Pulse Operation:Twelve-pulse operation is like two identical, separate six pulse

drives operating from common source at the same current and firing angle,with the firing reference angles shifted 30 degrees by the isolatingtransformers. The two motors are combined into one frame. This reducesthe motor cost, including isolation, and reduces the torque pulsationamplitude while raising the torque pulsation frequency. This is achieved byseparating the stator winding into two identical windings, but isolated andphase shifted 30 degrees.

This construction, utilizing a common magnetic frame, including acommon field, cause the load-side converter voltage to be equal inamplitude and frequency between the two drive channels. The transformer design makes the source-side converter voltage equal, but also 30 degreesapart between the two channels. It is not necessary that source-side and

load-side voltages be phase-shifted. The motor windings are shifted toobtain smoother torque for equal current. The transformer windings areshifted to reduce harmonic distortion on the power system and raise theharmonic frequencies. Inter-channel communication allows one channel tobe master and the other the slave, which takes its torque reference fromthe master. This communication allows the two motor winding currents tobe balanced. Thus the channel drivers equal power, take equal current,and fire at the same relative firing angle.

A twelve-pulse system also presents the capability of shuttingdown one channel for maintenance while the motor continues to run on the

other channel (with reduced torque and usually reduced speed range).When the out of service channel is ready for operation, it may be returnedto service without interrupting the drive system .

4. COMPARISON WITH EXISTING TECHNIQUESIt has been common practice to use constant speed

induction motor for fans with inlet guide vane/ outlet damper/ hydraulic

coupling for control of air/flue gas. In case of inlet guide vane/outlet

damper, due to increase in the system resistance, lot of energy is wasted


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resulting in low system efficiency. In case of a hydraulic coupling, the

efficiency of the coupling itself is very low, approximately equal to the ratio

of output speed to input speed. In the case of a VFD, speed of the motor is

varied rather than varying the system resistance for changing the operating

point. Since the power is proportional to cube of speed there will be an

appreciable saving in power consumption, especially when the fans/ pumps

are required to operate at low loads. Efficiency of hydraulic coupling is very

poor at reduced speeds (at 50% speed efficiency will be less than 50 %).

Because of design margins, the ID fans normally operate at around 70% of

rated loads even when the power plant is operating at its rated capacity,

thus making the available mechanical means for flow control highly

inefficient.Typical efficiency curves of VFD, hydraulic coupling, inlet guide vane and

outlet damper or throttling are shown in Fig 4.1.

Figure 4.1Efficiency Curves of Various

Flow Control Devices


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SL.NO GEN.MW

ID

A(K

1 200 3 750

8003 8504 260 4250 9005 280 4450 9506 300 4600 10507 320 4800 1150

8 340 1900 12309 360 5050 132010 380 5200 150011 400 5350 175012 420 5450 210013 440 5600 240014 500 5900 2900

TA

SL.NO GEN.MW

UNIT 3TOTAL

POWER

(KW)

ID (K

Example: ID

Example: ID fa


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5. OTHER APPLICATIONS

5.1 In Power Plants:Other than for ID Fan VFDs can also be used:

In Boiler Feed Pumps (BFP) for the flow control of feed water in place

of hydraulic coupling and scoop control.

In Condensate Extraction Pumps (CEP) for condensate flow control.

It can also be used in PA Fans and CW pumps.

5.2 In other industries: Can be used for Lathe machine to optimize feed rate into cutting tool.

In printing Press speed can vary on weight & coating of paper &

characteristics ink to be used.

VFD equipment can also used for Gas-Turbine starting.

6. ADVANTAGES AND DISADVANTAGES

6.1 Advantages of VFDs: Low motor staring current.

Reduction of thermal & mechanical stresses on motor low motor

starting currents.

Reduce maintenance.

Simple installation.

High power factor.

VFD can run 10-20% higher in speed & make up for lost capacity in

flow & demand type system.

Energy saving and improved efficiency.


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6.2 Disadvantages of VFD,s: VFDs put a lot of stress on motors. They increase total harmonic

distortion and generate voltage spikes that pit motor bearings,, and

shorten motor life. Facilities with VFDs often must use more

expensive inverter-duty motors, which include extra insulation tohandle the poorer power quality.

VFDs are big and expensive theyre a complicated piece of

electronic equipment. They take up a lot of floor space and may

require air conditioners (an additional source of energy consumption

that affects efficiency).

CONCLUSION :The use of Variable frequency drives for ID fans (in place of

modulating damper or hydraulic coupling) and for Boiler feed pumps (in

place of gear box and hydraulic coupling) in a power plant reduces the

auxiliary power consumption approximately by 10% at peak generation and

by 15 % at 60 % generation. The payback periods for the additional

investment are quite attractive. However, provision of space is an additionalrequirement.

Harmonic analysis and torsional analysis must be carried out and

corrective measures taken, if required.

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