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OVERVIEW OF THERMAL POWER PLANT & VARIABLE FREQUENCY DRIVE

Sharad Akbari ( B.E. Electrical )

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220 KV Switchyard

220 KV IPBD or Cable

Generator X’mer220 / 10.5 KV

Unit X’mer10.5 / 6.9 KV

Unit Service X’mer6.6 / .433 KV

Station X’mer220 / 6.6 KV

Station Service X’mer6.6 / .433 KV

6.6 KV6.6 KV

• BFP Motor• ID Fan• FD Fan• PA Fan• Pulverizer

Electrostatic Preceptor

• HFO Pump• Cooling Water

Pump

• HVAC• Cooling Tower

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STG10.5 KV, 50 Hz3000 rpm

BoilerTurbine

Motors Used in Power Plant

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• Boiler Feed Pump• P.A Fan• I.D Fan• F.D Fan• Coal Processing• Pulverizer• Various Pumps

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Starters

1.DOL Starter2.Star-Delta Starter3.Auto Transformer Starter4.Soft Starter5.Forward & Reverse Starter

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Variable Frequency Drive

A Variable Frequency Drive is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Motor Speed N=120*f / P Example

At 50 Hz N=120*50/2 = 3000 rpm

60 Hz N=120*60/2 = 3600 rpm

What is VFD?

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Block Diagram of VFD

AC to DCConverter

DC to ACInverter

DC Bus Motor

AC Input

Detail Diagram of VFD

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1. AC to DC Converter• Comprised of six diodes• Allow current to flow in only one

direction• Whenever A-phase voltage is more

positive than B or C phase voltages, then that diode will open and allow current to flow.

• When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close.

• The same is true for the 3 diodes on the negative side of the bus.

• We get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”

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2. DC BUS

• We can get rid of the AC ripple on the DC bus by adding a capacitor

• Capacitor absorbs the ac ripple and delivers a smooth dc voltage.

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3. DC to AC InverterSwitching Sequence for converting DC to AC

When we close one of the top switches in the inverter, that phase of the motor is connected to the positive dc bus and the voltage on that phase becomes positive.

1. 2.

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Switching Sequence

When we close one of the bottom switches in the converter, that phase is connected to the negative dc bus and becomes negative. Thus, we can make any phase on the motor become positive or negative at will and can thus generate any frequency that we want. So, we can make any phase be positive, negative, or zero. 

3. 4. 5.

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Pulse Width Modulation (PWM)

• Waveforms obtained earlier are nowhere sinusoidal and therefore are not acceptable. They contains harmonics.

• The Solution is to use Pulse Width Modulation Technique to eliminates Harmonics.

• Pulse Width Modulation (PWM) VFDs provide a more sinusoidal current output to control frequency and voltage supplied to an AC motor. PWM VFDs are more efficient and typically provide higher levels of performance.

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• It is desirable to provide the maximum torque at any operating frequency, a VFD is designed so that the magnetizing flux density is the same at every operating frequency.

• The flux density is determined by the equation

V = 4.44*f*N*A*B V is the RMS voltage, f is the frequency, N is the number of turns, A is the core area and B is the peak flux density.• From that equation it can be seen that if B is constant, V/f must be constant. 

• Torque reduces at the square of the voltage reduction when the frequency remains the same. • In a VFD, you are changing not only the voltage but the frequency at the same time; maintaining that same

V/Hz ratio, so torque remains the same throughout the speed range. That is the fundamental criteria for using a VFD.

• As the speed reduces and torque remains the same, the total shaft power is of course reduced, being that the power is a function of torque and speed. So at 20% speed (assuming you have 50Hz primary power), the motor power will be 20% of what it would be at full speed.

V/f Control

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Controlling of VFD 1. Scalar Mode

• A standard VFD (lets call it a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent upon the load conditions. The Scalar drive knows nothing about that is happening in Motor, it only tells the motor what to do.

• A 400V scalar drive is told to run a 400V, 50 Hz motor at 50% speed Following V/F pattern

Drive Motor

Speed Setting = 50%

Motor Speed = 50%

No Load

Drive Motor

Speed Setting = 50%

Motor Speed = 40%

Increased Load

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2. Vector Control (Field Oriented Control)

• More Accurate than V/f Control. Two types • Field Oriented Control with sensor• Sensor less Field Oriented Control

• No feedback through speed sensor• Feedback is derived through motor

terminals• Drive need to go through “Auto tuning”

• Feedback through encoder• Better speed regulations up to 0.01%• Faster response to load variations

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Acceleration and Deceleration Time

• The acceleration time is the time required of output frequency from 0 Hertz to the maximum frequency

• The deceleration time is from maximum frequency decrease to 0 Hz.

• Acceleration time setting requirement Limit the acceleration current under the over-current capacity of the variable frequency ac drive, to avoid VFD drive tripped in speed loss of over current.

• Deceleration time set point Avoid excessive voltage of the smoothing circuit, prevent ac drive tripped from regeneration over voltage.

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Stop Methods

Frequency

Run Command

Command removed

DC Break

DC Brake

Frequency

Command removed

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Motor Speed

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Motor Speed

Ramp to Stop Coast to Stop

1. Ramp to stop

1. Coast to stop

Stop According to deceleration time

Free Running to stop

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Benefits

1. Efficient Energy Saving

• In the United States, an estimated 60-65% of electrical energy is used to supply motors, 75% of which are variable-

torque fan, pump, and compressor loads. Eighteen percent of the energy used in the 40 million motors in the U.S. could be saved by efficient energy improvement technologies such as VFDs.

Horsepower α (Speed)³

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2. Low Inrush Motor Starting Current

• AC induction motors to draw 6 to 8 times their full load amps when they are started across the line.

• Starting Current is never exceeding Full Load Current of the Motor in VFD

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3. High Power Factor

• KVA = Volt * Amp * 1.732 • KW = Volt * Amp * P.F * 1.732

• Decreasing Reactive current will increase Power Factor

• The VFDs include capacitors in the DC Bus that perform the same function and maintain high power factor on the line side of the VFD.

• Eliminates the need to add power factor correction equipment to the motor or use expensive capacitor banks.

Power Factor = KW / KVA

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4. Reduced Mechanical and Thermal stress at starting

5. Easy Installation

6. Tighter Process Control with Variable Speed Drives

7.Extended Equipment Life and Reduced Maintenance

Other Benefits

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Applications

•  Machinery • Hoists• Conveyors• Printing presses• Positive displacement pumps• Mixers and extruders• Compressors

• Pumps • Fans

Constant Torque

Variable Torque

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Thank You