EE-3202: Power Electronics design LabTopic: Design of PWM Rectifier

Submitted BY-Vikas Kumar Pandey(07000428) Sarika Chauhan(07000427)Vaibhav Sharma(07000428)Abdul Lathief (07000430)Rahul Reddy (07000431)Mohit Kumar Gupta (07000432)

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INDEX

S.No. Topic Page No.

1. Introduction 3

2. PWM rectifier- Operation 4

3. Phasor Diagram 5

4. Control loop Diagram 6

5. Circuit Analysis 7

6. Calculations 9

7. Calculated Results 10

8. Comparison of calculated and simulated results

10

9. Simulated Circuit and different output curves.

11

10. Conclusion and its further application 17

11. References 18

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INTRODUCTION

The electric energy conversion made by semiconducting converters is being used more and more. This had led to the growth of negative phenomenon that appeared negligible, when only a few converters are being used. However, the development of semiconductor structures has enabled higher power to be transmitted and has also led to wide spread of converters. In this way; converters have a negative effect on the supply network. The regressive effects of overloads with harmonics and reactive power consumption are becoming major disadvantages of phase controlled (mostly thyristor) rectifiers. These side effects need to be compensated by additional filtering circuits with capacitors or inductances. However, such circuits raise the costs and also increase material and space requirements for the converter. Phase control and commutation of semiconducting devices impact the phase displacement between the first harmonics of the consumed current and the first harmonics of the supply voltage. This displacement leads to power factordegradation and to reactive power consumption. The consumed current harmonics cause non-sinusoidal voltage drops on the supply network impedances and lead to supply voltage deformation. This may cause malfunctions of other devices that are sensible to the sinusoidal shape of the supply voltage (e.g. measurement apparatuses, communication and control systems). The reactive power rises with longer control angle delays, so the rectifier acts as a time variable impedance that is nonlinear and causes deformed current consumption.

In order to suppress these negative phenomena caused by the power rectifiers, use is made of rectifiers with a more sophisticated control algorithm. Such rectifiers are realized by semiconductors that can be switched off i.e. IGBT transistors. The rectifier is controlled by pulse width modulation. A rectifier controlled in this way consumes current of required shape, which is mostly sinusoidal. It works with a given phase displacement between the consumed current and the supply voltage. The power factor can also be controlled and there are minimal effects on the supply network. The function of the rectifier depends on the supply type of network. There are two types of supply network – “hard” and “soft”. Ordinary rectifiers, which work on a relatively “hard” supply network, do not affect the shape of the supply voltage waveform. Harmonics produce electromagnetic distortion, and the network will be loaded with reactive power.The PWM rectifier aims to consume sinusoidal current and to work with given power factor. A PWM rectifier connected to the “soft” supply network has more potential to affect the shape of the supply voltage network. It can be controlled, so the current consumed by the PWM rectifier will partly compensate the non-harmonic consumption of other devices connected to the supply network.

The PWM rectifier is a very popular topic nowadays. With the expansion of electronics, conversion of electric parameters is also needed. For this purpose, the side effects of passive rectifiers, e.g. production of harmonics and reactive power, must be taken into account. All these side effects fall away with the application of PWMrectifiers.

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PWM RECTIFIERS

PWM rectifier is realized by semiconductor devices that can be switched off (IGBTs). The rectifier is controlled by pulse width modulation. Rectifier controlled in this way consumes current with demanded waveform that is mostly sinusoidal. It works with given phase displacement between consumed current and supply voltage, enables control of power factor, and has minimal effects on the supply network.

Main features of PWM rectifiers are: Bi-directional power flow. Nearly sinusoidal input current. Regulation of input power factor to unity. Low harmonic distortion of line current (THD below 5 %). Adjustment and stabilization of DC link voltage (or current). Reduced capacitor (or inductor) size due to the continuous current. It can be properly operated under line voltage distortion and line frequency

variations.

Load

iO

E

iS

VS CH-Bridge

VB

VL

L

Figure 1: PWM rectifier

OPERATIONWe assume that C is big enough so that E is smooth. In addition, there is a control loop (see later) which regulates the power flow so that E is constant regardless of variations in the load. Power can flow in either direction so that the load can transfer energy to the supply if required (for example a motor that needs to slow down without a mechanical brake).VB is a PWM voltage (2 or 3 level) which is modulated so that its fundamental component is at the same frequency as the supply voltage VS. An equivalent circuit for the AC side (see Figure 2) can be drawn by splitting VB into two sources, one representing the fundamental component (VB1) and the other representing the harmonics (VBH) which are the PWM distortion components.

iS

VS

VB1

VL

L

Figure 2: Equivalent circuit for the AC side

VBH

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Clearly only VS and VB1 affect the fundamental component of the current iS. and,of course, this is the only component that gives rise to real power flow (assuming that VS is not distorted). The harmonics VBH simply cause distortion in the current iS and do not affect the real power flow. By using a suitable inductor and PWM switching frequency, the harmonics can be made small and the current iS is then virtually sinusoidal ( unity distortion factor). Furthermore, by adjusting the magnitude and phase of VB1 with respect to VS, it is possible to force iS to be in phase with VS (unity displacement factor). Power flow can then be regulated by controlling the amplitude of iS again by varying VB1.

PHASOR DIAGRAMS:

Figure 3 shows phasor diagrams which explain the how the circuit operates for either direction of power flow. Note that the magnitude of the current (iS) is proportional to the

magnitude of the voltage VL since LVi LS / . This means that the magnitude of the

power flow can be varied by varying the length of VL by adjusting VB1. Unity displacement factor is ensured by making sure that VL is 90o ahead of VS. In practice, some sort of closed loop control scheme is employed to do this automatically and to ensure that the power flow is controlled to keep E constant.

VL

VS

VB1

VL

iS

VL

VS

VB1

VL

iS

POWER FLOW AC-DC

POWER FLOW DC-AC

Figure 3: Phasor diagrams for both directions of power flow

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CLOSED LOOP CONTROL

There are many ways in which the control can be arranged. Figure 4 shows a block diagram of one simple way. In practice a more sophisticated approach may be used but this simple example embodies most of the ideas.

A slow acting voltage control loop regulates the peak magnitude of the supply currrent. If power flow is reversed (ie DC to AC) then the output of this controller will be negative. The output from this controller must vary slowly in comparison to the supply frequency (for this type of control loop) and ensures that the correct AC current is drawn to balance the power requirements of the load and keep the DC voltage constant. By scaling and phase shifting the measured supply voltage, a template for the desired shape of the inductor voltage is produced. Multiplying this by the peak current and the inductor reactance produces the desired inductor voltage waveform. This is subtracted from the measured supply voltage to produce the desired H-bridge voltage on the AC side. This then forms the input to the PWM modulation process which (taking into account the DC link voltage magnitude) ensures that the correct voltage is produced. A separate handout shows typical waveforms for each direction of power flow.

VOLTAGECONTROLLER(Slow Acting)

90O PHASE SHIFT ANDAMPLITUDE SCALING

L

DC VOLTAGEDEMAND

MEASURED DCVOLTAGE

MEASUREDSUPPLY VOLTAGE

+

+

++

-

-

VS Sin t

Cos t

IS IS Cos t

VL Cos t

VB1(t)

Modulating wavefor PWM modulation

Figure 4: Example of control loop for PWM rectifier

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SINGLE PHASE PWM RECTIFIER - ANALYSIS

Load(Power =PO)

id

E

iS

vS CH-Bridge

vB

vL

L

PWM rectifier

iO

iC

We assume that there is no energy stored or lost in the inverter and that E and iO are smooth. Since we are looking at the low frequency power flow, we negelect the harmonics in VB.

Let:

)sin(ˆ)()sin(ˆ)( tItiandtVtv SSSS (Unity displacement factor assumed)

)sin(ˆ)( tVtv BB

Clearly:

2

ˆˆSS

OO

Od

IVEiP

ii

AC Side

VS

VLVB

Phasor diagram

22

11

)(ˆˆ

ˆ

ˆtan

ˆ

ˆtan

XIVV

LXwhereV

XI

V

V

SSB

S

S

S

L

The PWM modulation index at which the converter has to operate at is given by:

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E

VM B

I

ˆ

Note that MI must generally be < 1 for the PWM to work correctly. Hence the voltage on the DC side must be greater than the peak of the supply voltage.

The conditions are normally arranged so the the normal value of MI is between say 0.8 and 0.9.

DC Side:Ignoring the high frequency harmonic components, id has 2 components:

A DC component which represents the real power flow – this flows to the load. A component at twice the supply frequency which flows in the capacitor and causes

ripple in the DC side voltage. The capacitor must be sized to keep this ripple below whatever value is acceptable for the application.

Since there is no energy lost of stored in the H-bridge it follows that:

)2cos(2

ˆˆ)()cos(

2

ˆˆ

)2cos(2

ˆˆ)cos(

2

ˆˆ

)sin(ˆ)sin(ˆ

tE

VIiand

E

VIi

tE

VI

E

VIi

tVtIviEi

BSC

BSO

BSBSd

BSBSd

The ripple in the DC side voltage at frequency 2 can be calculated from the current magnitude (the phase is not important) and the reactance of the capacitor (at frequency 2). Hence the peak ripple is given by:

CE

rmsVrmsI

CE

VIpeakE BSBS

2

1)()(

2

1

2

ˆˆ)(

The peak to peak ripple (the quantity normally quoted) is twice this value.

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CALCULATIONS:

Suppose,

Ac supply (input) to the rectifier is 240V, 50Hz and has negligible reactance. DC voltage is to be maintained at 500 V by a control loop which also maintain

displacement factor at unity. At AC side, a series inductor of 5 mH is used. A 2000F capacitor is connected on the dc side in parallel with a load which maintains

the output voltage constant. Input current is 20A (rms).

If harmonics can be neglected, equivalent scheme is—

iS

VS

VB1

VL

L

Equivalent circuit for the AC side

As is is in phase with Vs ,

Pin = Vs × is × cosΦ=Pout (cosΦ)

Pout = 240×20=4800 W

VL= w×L×is=100 ×π ×5×10-3×20=31.42 V

VB1 = (Vs2+ VL

2)1\2

VB1 = (2402+ 31.422)1\2

VB1 = 242 V

Modulation Index = (√2× VB1)/E= (√2×242)/500 = 0.6846

Idc=(VB1×is)/E=(242×20)/500=9.68 A (peak)

Impedance of DC link capacitor, Zc=1/(2 ×π×100×2000×10-6) = 0.796 ohm

ΔΕ=Idc×Zc=9.68×0.796=7.7V (peak)

The peak to peak DC link voltage ripple=15.4 V (peak to peak)

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RESULTS:

Output voltage = 499.71 V

Average Output current = 9.99 A

Peak output current= 10.16 V

Peak to peak ripple in output voltage

Modulation index = 0.68

These are the results we obtained from numerical analysis of our circuit

COMPARISON OF CALCULATED AND SIMULATE

Calculated Values

Peak Output current = 9.68A

Peak to peak ripple voltage =15.4V

Modulation Index = 0.6846

= 9.99 A

= 10.16 V

Peak to peak ripple in output voltage= 16.8 V

These are the results we obtained from numerical analysis of our circuit

COMPARISON OF CALCULATED AND SIMULATED RESULTS

Calculated Values

Peak Output current =

Peak to peak ripple voltage =15.4V

Modulation Index =

Simulated values

Peak Output current = 10.16A

Peak to peak ripple voltage =16.8V

Modulation Index = 0.68

Simulated values

Peak Output current = 10.16A

Peak to peak ripple voltage =16.8V

Modulation Index = 0.68

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CONCLUSION AND ITS FURTHER APPLICATION:

The PWM rectifier overcomes the two main problems of the diode bridge rectifier in that the supply current is virtually sinusoidal and that the power flow can be reversed. The disadvantage is of coursethe extra cost. Using a circuit like that described above to provide the DC supply to another H-bridge gives a bidirectional AC-AC converter. The 3-phase version of this (Figure shown below) finds application in motor drives, particularly where regeneration is required, since it removes the need for resistive braking on the DC link. This would be required if a diode bridge was used since any power returned through the inverter from the motor cannot return through the diode bridge and must be dissipated in a resistor.

3-PhaseMotor

PWM RECTIFIER DC LINK PWM INVERTER

3-phase PWM inverter drive with PWM rectifier

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REFERENCES :

1. Handouts from “ http://hermes.eee.nott.ac.uk/teaching/h5cpe2/”.

2. Power Electronics- Circuits, Devices And Applications by Muhammad H. Rashid.

3. J. Bauer: “Single-Phase Pulse Width Modulated Rectifier”