6 Pulse Converter

7/29/2019 6 Pulse Converter

1/19

Introduction 6-Pulse Converter

Operation of a 6-Pulse Converter

Voltage and Current waveforms of an ideal 6-Pulse Converter

Effect of source reactance

Voltage and Current waveforms of an 6-PulseConverter with overlap angle

Equations

Sub: APS Topic: 6 Pulse Converter overlap angle


2/19

The demand for large amount of electricpower, today necessitates transmission atincreasingly high voltages

Power generation by large size generators is

feasible in AC only. However various problems are faced in AC

transmission during transport of bulk powerover large distance hence HVDC systems came

into existence. In a HVDC system sources and generation of

power is usually in AC mode.



3/19

So a HVDC system requires a conversion of ACto DC at one end and inversion back to AC atother end.

All modern HVDC systems use either 6-pulse

or 12-pulse converters for this purpose

HVDC Transmission Principle



4/19

6-PulseConverter



5/19

This circuit arrangement is known as Graetz

circuit and it uses six thyristor valves.

If the firing angle of thyristor is less than 90,

the converter operates as a rectifier.

If the firing angle > 90, the converter

becomes an inverter.



6/19

Each thyristor conducts for 120

Every 60 one Thyristor from +ve limb and one

Thyristor fromve limb is triggered

Each thyristor will be triggered when voltage across

it becomes positive

Thyristor commutates the current automatically

when the voltage across it becomes

ve. Hence,this process is called natural commutation and the

converters are called Line Commutated converters



7/19

Triggering can be delayed from this point and this iscalled firing angle

Output voltage of the converter is controlled bycontrolling the Rectifier action

If > 90 negative voltage is available across thebridge Inverter action

Due to finite transformer inductance, current transferfrom one thyristor valve to the other cannot take

place instantly This delay is called over lap angle and the

reactance called commutating reactance. This alsocauses additional drop in the voltage



8/19

= 0Overlap = 0

Voltage and Current waveforms with = 0



9/19

The source reactance introduces overlap ofcurrents due to simultaneous conduction ofSCRs to be commutated during commutation

period. Due to source inductance , the phase currents cannot

change instantly, therefore the transfer of current

from one phase to other takes time called overlap

or commutation time



10/19

6pulse converter with Lc0



11/19

Voltage Waveform with overlap angle



12/19

< 600 : Normal Operation (200 250)During commutation, three

valves conduct

Between commutation, twovalves conduct

Since a new commutation beginsevery 60

and last for angle , the angularinterval when two valves conduct is 60

.The sequence is12, 123, 23, 234, 34, 345, 45, 456, 56, 561,

61, 612

= 600: Always a set of three valves

conduct.123, 234, 345, 456, 561, 612.

600 < < 1200 (Abnormal operation)Alternatelythree and four valvesconduct

= 0


13/19

During commutation,

or


14/19


15/19

Thus due to overlap, the voltage of point p rises to instead of( )

2

a be e

be


16/19


17/19

Without overlap, the direct voltage was , therefore with overlap

-

0 22

d d

d s

V I

V I

0

2

cos

2

dd d

s

IV V

I


18/19

Substituting from and from

6c

fL

Equivalent circuit of bridge rectifier

Rectifier Equation:


19/19

Documents

6 Pulse Converter