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Petrochemical Process Optimization

For this project, our customer is a major chemical complex on the Mississippi River in south Louisiana.

This customer supplies a wide range of chemical products for industrial, pharmaceutical, automotive, oil, gas and chemical industries.

In this complex there are two Acetylene compressors. The prime movers are 10,000HP/13,200V induction motors that are started across the line. Our customer wants to use a MV VFD to start each motor to eliminate the damage they are seeing during the across the line starting sequence. They also want to control the speed of the motor to optimize their process and improve efficiency.

We are providing a Toshiba MV VFD solution for this application.

The incoming power feeding the drive comes from the existing switchgear line up at 13,200V. The MV VFD package has a primary section rated 15kV with an input disconnect, arresters, soft charge circuit and primary fuses. This section feeds two multi-pulse transformers that power the diode rectifiers on the front end of he drive. These transformers also provide harmonic cancellation for the power electronics. A tertiary winding on each transformer provides power for cooling fans and control power.

The diode rectifiers feed multiple power cells in the MV VFD. These power cells use IGBTs and power capacitors to create a five level, 4160V output. This output of the power cells feeds a sine-wave filter ahead of the step-up transformer. This Niagara transformer is a two winding, delta-delta design suitable for VFD service.

This transformer feeds a line-up of PowerCon sync-tranfer switchgear. This switchgear will allow the customer to select the motor he wants to start and engage the MV VFD for starting or slow speed operation. Once the motor is selected for sync-transfer, the MV VFD will match the utility frequency and transfer the motor across the line. This is a bump-less transfer to minimize any mechanical impact on the motor/gearbox/compressor/couplings drive train. The MV VFD stays engaged until it gets a positive indication the the breaker closed properly and power is flowing.

The MV VFD is then ready to start the second motor. It can also re-engage either running motor to change motor speed to match process conditions. This also can improve efficiency and allow the compressor to react quickly to process changes.

The MV VFD package has several special features to optimize performance.

The soft charge circuit reduces the inrush power normally required when you energize the two input transformers. This condition is complicated by the uncharged power capacitors on the secondaries of each transformer. The reduction in inrush power also extends the life of the transformers and capacitors, as they see are spared the impact of an across the line power up.

The output of the MV VFD is a balanced, five level, output waveform where the neutral is tied to ground. This eliminates the need for special VFD cables with special insulation ratings and customized VFD rated motors. This output waveform is ideal for application on existing motors.

We have applied a sine-wave filter to the output of the MV VFD to provide the step-up transformer sees a clean, smooth waveform. Voltage spikes and quick waveform transitions can shorten the life of a close coupled transformer, power cables, connections and the motor.

Luckily, this time we are powering up induction motors thru the step up transformer. The MV VFD can see the motor directly and monitor the performance during initial starting, acceleration and deceleration.

We use a two winding transformer with an electrostatic shield to ensure that during phase to ground and phase to phase fault conditions the MV VFD can detect the condition accurately and react properly. This transformer design also ensures that the motor voltage of 13,200V will not fault to the 4,160V winding and destroy the MV VFD.

We use a PLC based control system to coordinate the sync-transfer function between the MV VFD and the switchgear. The PLC also provides the interface between field commands, compressor control commands and the MV VFD.

Our complete package, excluding the step-up transformer, is housed in a Lectrus equipment center. This pre-fabricated metal enclosure includes an HVAC system to properly cool the drive under all conditions.

 

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