Three-phase Semi-converter with inductive load (With source side inductance)

Semi-controlled Converters

The ac to dc converter circuit with suitable examples and sketches are discuss here advanced. ac to dc converter circuit is needed to read.

Semi-controlled converter is a hybrid between uncontrolled and fullycontrolled converters.  It is also known as β€œsemi-converter”.

Semi-converter offers variable positive DC output voltage only. Regeneration or inverter mode operation is not available

Single-phase Semi-converter with inductive load

In a semi-converter, thyristor conduction is delayed by angle Ξ± but diode conduction begins at their default points (i.e. zero delay-angle).  For continuous load current (which we assumed), at any time, a thyristor from the upper-half and a diode from the lower-half must conduct the load current πΌπ‘œ, i.e. last two active devices conduct at any point of time.

By varying 𝛼 over 0Β° to 180Β°, we can adjust  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› between  8𝑉𝑝𝑕 πœ‹  and 0.  

In  𝐼𝑠waveform, front-end of current pulse (in each half-cycle) is moved to the right by 𝛼 but rear-end is fixed.  For a given delay angle 𝛼, the width of current pulse is  πœ‹ βˆ’π›Ό .

Note: We assumed continuous load current for all delay angles.  In reality, at higher delay-angles, load current becomes discontinuous.  Then the input current will be different.

There are some other implementations of the single-phase semi-converter, all offering the same input-output performances.

Standard Implementation

Alternative Implementation – 1

Alternative Implementation – 2

Alternative Implementation – 3

Which implementation we want to select is partly our own preference and partly technical /operational requirements.

Thyristors (also diodes) are available as individual devices or multiple devices.  Twin devices are available in common-cathode, common-anode or bridgedleg options.  Selection of the converter configuration may depend on the type of available devices. 

Technically, common-cathode configuration of standard implementation eases the assembly of two gate drive circuits sharing the cathodes.   Threeleg implementation  gives the choice between  semi-converter and fullconverter during operation by making diode D5 in or out.

Self Attempt:

Draw the output voltage and input current waveforms for the other three configurations of 1-phase semi-converter, and indicate the devices conducting in different intervals of the cycle.

Single-phase Semi-converter with inductive load (With Source Inductance)

Inductance Ls does not allow current Is to make step changes at T-to-T and D-to-D commutation points.  Current changes taking a brief time.  During these brief intervals both the incoming and the outgoing devices conduct simultaneously. This is a conduction overlap.  Conduction overlap angle for thyristor commutation and diode commutation are different, because they occur at different points of Vs.

ac to dc converter schematic

𝑒𝑇 = Thyristor conduction overlap angle 𝑒𝐷 = Diode conduction overlap angle Diode conduction overlap does not influence output voltage, because it occurs during 𝑉 π‘œ is zero.  However it affects the voltage waveform at the PCC. 

Let consider the conduction overlap after triggering 𝑇1. During this interval  𝑇1,𝐷2,𝑇3 all conduct simultaneously, short-circuiting the load, as well as the source. 

ac to dc converter schematic

Reduction of volt-radian area (A) from 𝑉 π‘œ due to the conduction overlap,

ac to dc converter schematic

Reduction in  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› due to conduction overlap,

ac to dc converter schematic

Similarly, let consider conduction overlap at  𝐷4 taking up conduction.  During this interval 𝐷4,𝐷2,𝑇1 conduct, short circuits the load, as well as the source.  Load voltage during this interval is zero, anyway. 

ac to dc converter schematic

Three-phase Semi-converter with inductive load

Diodes in the lower-half conduct at default forward-bias points.  Thyristor conduction is delayed by delay angle Ξ±.   It is assumed that load current is approximately constant due to load inductance.

The assumption that load current is continuous and constant is quite valid when the delay angle is not large.  For larger delay angles in semiconverters, load current can become discontinuous, in which case the operating waveforms will be somewhat different.

Output voltage, input current and thyristor-1 voltage waveforms

At any point of time, the two previously active devices conduct and pass the corresponding line-voltage  to the output.

𝑉 π‘œ π‘šπ‘’π‘Žπ‘› is variable but positive only for the 0Β° to 180Β° range of Ξ±.

With respect to the case of three-phase diode converter, the input current waveform’s positive half-cycle is shifted to the right by angle Ξ± but the negative halfcycle remains un-shifted.

Mathematically,

ac to dc converter diagram

Order of harmonics in 𝐼𝑠 is = 5, 7, 11, 13, …..=  6π‘˜ Β±1 ,π‘˜ = 1,2,3…

From a qualitative view point too, we can deduce input displacement angle.  If positive half-cycle only shifts to the right by angle 𝛼, the fundamental component will be shifted to the right by angle  𝛼 2. 

Once 𝐷𝑖𝑠𝐹 is known,  𝐼𝑠,𝐹𝑒𝑛𝑑 π‘Ÿπ‘šπ‘  can be found by equating input and output power too.

ac to dc converter diagram

Operating waveforms of three-phase semi-converter will be different depending on whether 𝛼 is below or above 60Β°.  For example, below 60Β°, 𝑉 π‘œ has 6 ripples per AC cycle but above 60Β°, 𝑉 π‘œ has only 3 ripples per AC cycle.   However, the expressions of the  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› , 𝐷𝑖𝑠𝐹Operating waveforms of three-phase semi-converter will be different depending on whether 𝛼 is below or above 60Β°.  For example, below 60Β°, 𝑉 π‘œ has 6 ripples per AC cycle but above 60Β°, 𝑉 π‘œ has only 3 ripples per AC cycle.   However, the expressions of the  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› , 𝐷𝑖𝑠𝐹 and (Is,Fund)rms are valid for all values of Ξ±.

ac to dc converter diagram

Operating waveforms for Ξ± = 60Β°

ac to dc converter diagram

Operating waveforms for Ξ± >Β  60Β°

Three-phase Semi-converter with inductive load (With source side inductance)

Each thyristor-to-thyristor commutation involves with a conduction overlap angle 𝑒𝑇, and each diode-to-diode commutation involves with a conduction overlap angle 𝑒𝐷.  Here too, during overlap, the three recently active devices conduct simultaneously and outside overlaps the two recently active devices conduct simultaneously.

Waveforms of output voltage, input current, thyristor-1 voltage and voltage at PCC

Let consider the thyristor-overlap after triggering 𝑇1, during which  𝑇1,𝐷6,𝑇5 conduct simultaneously.  Following the same steps followed in the case of full-bridge before, we can show that,

Similarly, let consider the diode-overlap after  𝐷2  becoming forward biased, during which 𝐷2,𝑇1,𝐷6 conduct simultaneously. Again, following the same steps as in thyristor-overlap, we can show,

Net reduction in  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› due to conduction overlaps is,

As in the case without source-inductance, operating waveforms will be different depending on whether 𝛼 is below or above 60Β°.   For 𝛼 > 60Β°, ο‚· 𝑉 π‘œ has only 3 ripples per AC cycle. ο‚· During every overlap  𝑉 π‘œ = 0 ο‚· Line voltage at the PCC contains 2 notches only, both are down to zero. However, expressions for  𝑉 π‘œ π‘šπ‘’π‘Žπ‘› , βˆ†π‘‰ π‘œ, 𝐷𝑖𝑠𝐹, 𝑒𝑇, 𝑒𝐷 all remain unchanged.

Waveforms of output voltage, input current, thyristor-1 voltage and voltage at PCC for Ξ± >  60Β°

Example ac to dc converter circuit

 Derive expressions for βˆ†π‘‰ π‘œ,𝑒𝑇 and 𝑒𝐷 for a three-phase semi-converter operating at a delay angle above 60Β°.  Hence verify that they are same as for delay angles below 60Β°.

Ans

Let consider the overlap after triggering 𝑇1.  With respect to operating waveforms given before, 𝑇1,𝐷2,𝑇5 all conduct simultaneously during this interval.

conversion of ac to dc
conversion of ac to dc

Net reduction of   𝑉 π‘œ π‘šπ‘’π‘Žπ‘› occurs due to thyristor-overlaps only, because diode overlaps are occurring while  𝑉 π‘œ is already zero.

conversion of ac to dc

Considering diode overlap after  𝐷4 becoming forward biased, during which  𝐷4, 𝑇1, 𝐷2 all conduct simultaneously,

conversion of ac to dc

Integrating,

conversion of ac to dc

Thus, the expressions of βˆ†π‘‰ π‘œ,𝑒𝑇 and 𝑒𝐷 for 𝛼 > 60Β° are same as those for 𝛼 < 60Β°.

The ac to dc converter circuit with suitable examples and sketches are discuss here advanced. ac to dc converter circuit is needed to read.

Reference ac to dc converter circuit

  1. Page ac to dc converter circuit
  2. Page ac to dc converter circuit

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