This customer was so pleased with a previous project offshore he contacted us to provide a solution for this application.
In this complex they are adding several Elliott gas compressors. The prime movers range from 18,500HP to 6500HP. They are using 12,470V synchronous motors with three phase AC exciters. Our customer must start these motors with a restriction from the power company to limit the inrush power dip to 2.5%.
We looked at several different options for soft starting and settled in on a Toshiba MV VFD solution for this application. It was the only choice to address all the application challenges. Once our customer determined the maximum "un-loaded" starting requirements we selected the power rating of 10,000HP for the MV VFD. Normally, we would have used a smaller MV VFD for this application, but the customer wanted to partially load the compressor to avoid flaring gas during the starting operation.
The incoming power feeding the drive comes from the new switchgear line up at 12,470V. 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 the 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. We used an indoor dry type transformer with a two winding, delta-delta design suitable for VFD service.
This transformer feeds the customers' line-up of sync-tranfer switchgear. This switchgear will allow the customer to select the motor he wants to start and engage the MV VFD for starting. 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 mechanical 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 another motor.
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 are spared the impact of an across the line power up. For this customer, his last project had a dedicated power generation system. The MV VFD package that our competitor supplied did not have this feature included. When he went to power up, not run just power up, the inrush to the MV VFD tripped the power plant. The solution was to add this circuit in the field at quite an expense to physical space, cost and schedule.
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 with 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.
For this application we are powering up synchronous motors with three phase AC exciters. These motors did not have amortisseur windings, so the AC exciters had to be energized during starting. This was a problem. In this starting sequence, if not done properly, the MV VFD could allow the motor to turn backwards and damage the compressor. To avoid this condition, we needed the MV VFD to be direct connected to the motor so it could determine rotor position prior to power applied to he windings. We installed bypass breakers so that we could start the load with 4160V direct from the MV VFD. Once the load was started successfully we engage the step-up transformer and continue to accelerate the load. Once the frequency is matched to the utility, a bump-less transfer is initiated.
For his next application, we hope to supply high performance induction motors that can be optimized for higher efficiency when started on a MV VFD.
The dry type, two winding transformer has an electrostatic shield to ensure that during phase to ground and phase to phase fault conditions downstream, the MV VFD can detect the condition accurately and react properly. This transformer design also ensures that the motor voltage of 112,470V will not fault to the 4,160V winding and destroy the MV VFD.
We use an AB 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.
We also supplied Toshiba LV VFDs for the three phase AC exciters. They proved to be the best solution to power up the exciters.
Our complete package, including 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. Since this is a starting duty application, where the MV VFD is at full power for a very short amount of time, very little HVAC tonnage was required.
This starting system is fully operational, with a very satisfied client.