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Suggestion Required- 3 Phase 2 Leg OR 3 Phase 3 Leg Control for TX Load

Hello Sir,

We are in need of technical suggestion from your end for thyristor power controller in SQF application with Transformer coupled load.

Below are some of our queries and request you to please advise.

1. Is 3 phase 2 leg control with BURST firing shall be suitable for Transformer coupled load.
Are there any chances of flickering/load unbalance/load failure etc. while shifting from Mains Supply to DG and vice versa.

2. Is 3 phase 3 leg control with Phase Angle firing model is the only suitable configuration for Transformer coupled load?

3. Please let us know the merits of Phase Angle over Burst fixed/variable modes of firing.

Please do the needful.

Regards,
Sujit Jejurkar
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  • Bob Anderson (Technical Support Engineer) January 10, 2019 13:31
    Hello Sujit,

    I will attempt to answer your questions.

    1. With a mobile generator the important thing here is to ensure the generator is as big as necessary.
    With burst mode, you apply full power at each firing cycle, so you must oversize the generator to accept starts without losing too much voltage .
    When the generator starts, 50Hz frequency and nominal voltage are established after some seconds, so "frequency alarm" and "voltage fault alarm" need disabling.

    In Burst Fix mode, you have to tune the modulation period in order to identify the length of modulation perios with less impact on the mobile source, including flickering.

    In Burst Var mode, the length of the modulation period depends on the setpoint, But in this case too, you can adjust "the min on time".

    Link to benifits using 2phase control below.
    https://www.eurotherm.com/products/po...

    2. No, you can control a transformer load with 2 phase control using burst firing.
    To cope with this we use delayed first cycle firing.
    Optimum delay angle can be adjusted to minimise transient over current, dependent on the transformer, load and Q factors, typically around 60deg.
    This inrush current can be minised whilst the benifits of burst firing realised.
    To add to this we also have soft start and soft stop.
    So if necessary we can add a little phase angle to soften the start and/or stop of the cycles.

    3.Using a generator with Phase angle does not present any problems at all with a mobile source.
    This is a very soft start which prevents inrush current to the transformer, but gives poor power factor and creates harmonics.

    Whereas Burst firing turns on at full power so inrush can be a problem, albeit we have delayed technology to deal with this, but to our favour we have no harmonics and good power factor, so very little wasted energy.

    Hope this helps.
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  • Hello Bob,

    Thank you very much for your valuable reply.

    Thanks,
    Sujit
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