impact of power electronics on electrical system design

8/7/2019 Impact of Power Electronics on Electrical System Design

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RELIANCE PATALA GANGA

IN 1981

PHASE - I

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SINGLE LINE DIAGRAM

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400 KW INDUCTION MOTOR HAS TO BE INSTALLED FOR

SPECIAL PURPOSE.

DESIGN POINTS


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For400 KW Motorwith K=7.2, Running P.F.=0.9, Efficiency=0.92

Starting Capacity of Motor (Ps ) = KVA

Motor With 415V, Starting current = A

Motor with 6.6 KV, Starting Current = A

3MVA, 22/0.433KV Transformer Rated Secondary Current is A

Transformer Sizing for400 KW MotorPs = Starting Capacity of Large Motor

L = Load efficiency

PM = Rated output of Largest Motor (KW)

Cos US = Starting Power factor of largest Motor

%R = Percent resistance for transformer

%X = Percent reactance for transformer

I = Voltage Regulation of transformer

PT = Transformer rated capacity (KVA)


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MOTOR IS CONNECTED TO 6.6KVSWITCH BOARD


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AS PER IS 2026 std-1977

1600 KVA TransformerWith in tolerance %R = 0.5

With in tolerance %X= 5

MOTOR Starting P.F. = 0.25

During starting or re acceleration of large

Motor under normal load running,

The Voltage Regulation at the TransformerSecondary Terminals shall be 15% or less


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Cost AnalysisConsidering the costs for following

22 KV Switch Board to Transformer 22 KV HT Cable of 3c x 400Sqmm

1600 KVA, 22/6.9 KV Transformer

Transformer secondary to 6.6 KV Switch Board 6.6KV HT Cable of 3C x

400Sqmm

6.6 KV Switch Board.

Approximately

In addition to equipment , allotted area and routing of the cable etc are the most

expensive.

Due to the high starting current , cost is effectively increased.


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In this case, we have chosen Slip Ring IM to limitIn this case, we have chosen Slip Ring IM to limit

starting currentstarting current

Slip Ring

Induction Motor

Squirrel CageInduction Motor

Speed can be controlled easily

Excellent starting torque for highinertia loads.

Low starting current

Speed is resistance variable over50% to 100% full speed.

Efficiency is low.

Resistive loss during acceleration.

Low cost, Long life

High efficiency

Large ratings available

large number of standardizedtypes

Starting inrush current is highcauses additional voltage dip from

the source to the motorSpeed control requires variablefrequency source

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SLIP RING (WOUND ROTOR) INDUCTION MOTOR

Typical Circuit diagram

Increasing the rotor resistance

Applied to wound-rotor (slip-ring) motors.

The rotor resistor(s) is (are) bypassed as the motor accelerates.

Starting current is reduced.Starting torque is increased.

However, the projected speed reduces by raising

the rotor resistance.


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SLIP RING (WOUND ROTOR) INDUCTION MOTORTORQUE CURRENT-SPEED CURVES


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As per the optimum design point of view,

NOTE:IN 1981, SOFT STARTER IS UNAVAILABLE

SLIP RING INDUCTION MOTOR OF 400 KW IS INSTALLED.


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IN 1985

PHASE - II

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SINGLE LINE DIAGRAM


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Where

Actual Calculation of TVD =

WhereXid Transient Reactance of Selected DG

Xe Reciprocal of vectorial magnitude of Motor Start KVA

Transient Reactance of DG is 0.23 ( Approximately)

For 800 KW Motor Required DG Capacity = 9268.68 KVA

For 1150 KW Motor Required DG Capacity= 13322.96 KVA

Selected Total DG Capacity for 800 KW is 10000 KVA

1150 KW is 15000 KVA

Actual Transient Voltage Dip for 800 KW is 14.5%

Actual Transient Voltage Dip for 1150 KW is 13.5%

DG Capacity = Starting KVA of Biggest Motor x Transient Reactance ofDG x ( )


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1150 KW and 800 KW motors are connected to 6.6 KV Bus fed

from DGs as shown below


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All DGs cannot start simultaneously.

Motor has to start immediately, for continuous operations.

Solution for this to start the motor with Auto Transformer Starter

Auto Transformer Starter

The motor terminal voltage is not a function of load current andremains constant during the acceleration time

Due to the turn ratio advantages the primary line current is lessthan the secondary motor currents. A three-coil autotransformer isconnected in a wye configuration and connected to the motor insuch a way as to supply reduced voltage to the motor when the linevoltage is applied to Autotransformer.

Taps are provided different values of reduced voltage( NEMA standards are 80%,65% and 50% of the full line voltage)


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Advantages:

For hard to start loads, adjustable starting torque, can be usedwith any standard motors and less strain on the motor.

Auto Transformer Starter Circuit Diagram


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IN 1993

PHASE - III

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LINE DIAGRAMSeries reactor

The reactor is connected inseries

with the stator windings.

The starting current is limited

by increasing the motorimpedance.

The torque is reduced, due to the

voltage drop across the reactor.

The reactor has to sustain the

starting current.

The reactor is bypassed after the

acceleration period is over.


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Depending on the motor kW rating & application the driveselections should be done

Before Selecting any motor for application the motor torque curve

& load torque requirement should be matched

Soft Starter VFD.


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Soft starterReduced voltage is applied through

controlled ac chopper.

Provides improved controllability during

starting.

The voltage is ramped up by controlling

of the solid-state switches.

The switches are controlled such thatthe current is limited to 3-5p.u.

The solid-state switches are bypassed

after the motor reaches the rated speed

to avoid switching losses.

U

M3~

V W


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Soft Starter Features:A) Control Methods

Torque Control - Start / Stop RampTorque control, quadratic

Pump Application Control (Start & Stop)

Jog Forward & Reverse

Dynamic vector braking (Dynamic DC-Brake)

Reverse current braking (Soft brake)

Zero speed shut off without sensorB) Protection

Over and Under voltage protection

Voltage imbalance protection

Phase reversal protection

Phase loss protectionThermal overload protection

Shaft power protection Max and Min

Limit number of starts per hour

C) Keypad Display

D) I/O Interface


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VFD CIRCUIT DIAGRAM


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VFD Working Principle

Adjustable-speed drives for ac motors, control the frequency andvoltage of the motor to adjust its speed.

Disadvantages:

High cost: Not recommended when speed adjustment is not needed.

Harmonics are injected into the motor. Special motors are required (e.g.

with forced ventilation, bigger frame sizes, higher insulation levels, ).

VFDs are nonlinear loads and draw non-sinusoidal currents from the grid.


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Soft starters are usually current or voltage controlled, not

frequency

Soft starters do not have the ability to control speed

Soft starters will have significant price benefits

Soft starters will have a smaller foot print on larger HP

application

Comparison between Soft starter and VFD


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Power Electronics optimize the system performance and

reliability and Cost

Reduce the need for large area and equipments.

Conclusion


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impact of power electronics on electrical system design

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