review the power drive system from application towards the grid-focus on the transformer

Why feed the MV-drive with a transformer?

In fact, I haven’t found any Medium Voltage source inverter without a transformer either separately located or fully integrated into the drive cabinet, yet. So, what makes the transformer so important?

Converter transformers  offering great advantages for MV-drives

In our chapter “topologies of  drives” you noticed the presence of transformers feeding medium voltage source inverters.

3 Level 12 pulse VSI with transformer utility-Circuit Breaker-converter transformer

3 Level 12 pulse VSI with utility transformer, circuit breaker and converter transformer

By using the transformer, the power drive system is no longer directly connected to the network and is electrically isolated.
This offers following advantages:

  • Provides flexibility of selection of supply voltage of drive and motor voltage.
  • The impacts of transients such as lightning and other equipment switching in the grid voltage occurring on the primary side of the transformer are strongly attenuated by the transformer.
  • Limits the short-circuit power to the maximum permissible values of the drive at the secondary of the transformer.
  • Allows for ungrounded operation of converter and motor. (the ground-fault monitor integrated in the converter monitors the insulation resistance against ground.)
  • Limits line harmonic distortions and voltage ripple 

What is basically a converter transformer?
What are the advantages of the two types?

The converter transformers need to fore fill specific requirements because of the converter duty:

  • Supplementary losses due to current harmonics up to 15% of the normal sinus load.
  • Need to withstand DC-components of at least 1% of the fundamental current.
  • The accuracy of the phase shift of the secondaries must be better than 0.5 °el.
  • Voltage increases due to commutation up to twice the nominal voltage.
  • Higher mechanical forces in case of surge loads.
  • Transformer will be powered-up/powered-down under no-load conditions using a circuit-breaker.

Oil transformers are preferably supplied for drives with higher power rating.

  • The oil filling for cooling purpose should of course be pollutant-free oil.
  • Location outdoor which lowers operation costs for heat dissipation.
  • Preferred type in case of high primary voltages.
  • Need to consider fire and water protection and monitoring.

Dry-type transformers are designed as cast-resin transformers. The material for the
transformer winding may either be copper or aluminium.

  • Preferred type for indoor use.
  • Close placement to drive can reduce cable costs.
  • Practically maintenance free.
  • No special requirements with regards to water protection or fire protection.

It is safe to say that the selection of the transformer should be in close collaboration with the MV-drive manufacturer. In fact, it make sense to include the converter transformer into the requirements of the complete power drive system.  This way, the manufacturer or EPC could take full responsibility for the right match.

If we lost you along the line, do not hesitate to contact us for a free video call

In all cases above we are ready to assist you. All it takes is that you contact us for a free consultation, no strings attached.
Further down the road, we can facilitate informed decision-making by offering you our independent advice and support for any of following tasks:

  • Setup of the project plan presentation.
  • Definition and management of the requirements of the power drive system.
  • Evaluation of the composition of the power drive system.
  • Internal and external pre-bid meetings.
  • Set-up of invitation to bid.
  • Screening of preferred vendors.
  • Verification quotation and exceptions VS specs.
  • Selection of the vendor.
  • Order clarification.
  • Factory acceptance tests requirements.
  • Witnessed factory acceptance tests.
  • Site acceptance tests requirements.
  • Witnessed factory acceptance tests.

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Do not let uncertainty hold you back. Reach out for practical solutions and expert advice tailored to your power drive system needs. Contact us today to ensure your critical machines are driven reliably, efficiently and safe.