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.

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.
  • Reduces the maximum value of short-circuit power in case of short-circuit fault in the drive.
  • Allows for ungrounded operation of converter and motor. A first ground fault does not interrupt the IT-system and therefore increases the availability of the system. The ground-fault monitor integrated in the medium voltage converter monitors the insulation resistance against ground.
  • Electrostatic shield to minimize high-frequency disturbances. 
  • Limits line harmonic distortions and voltage ripple  by using multi-phase transformers with well defined relative phase shift.

Some VFD designs omit the isolation transformer for cost reasons. However, this may make the system more prone to misoperation and damage from lightning and line voltage surge. Transformerless designs may also be more susceptible to ground faults.

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.
  • The vfd transformer should have a static screen between the primary and secondary windings.

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:

• Author the project plan presentation.
• Define and manage the requirements of the power drive system PDS.
• Evaluate of the composition of the power drive system.
• Set-up and coordinate internal and external pre-bid meetings.
• Set-up of invitation to bid with Engineering, Procurement and Contracting companies and/or vendors.
• Assess preferred manufacturers of transformer, inverter and motor.
• Verify the offers and exceptions in regard to your specs of your PDS.
• Help to select the vendor of transformer, variable frequency drive and motor.
• Order clarification of your PDS.
• Witnessed and non-witnessed factory acceptance tests (FAT) requirements and test plan.
• Site acceptance tests (SAT) requirements.
• Validate the site acceptance tests for the comonents of your power drive system PDS on-site.