Medium-voltage transformerless drives save on weight and space

Transformers are typically between 98 and 99percent efficient. A 1000kVA transformer generates up to 2000watts of heat energy (or 3400 to 6800BTU/hour) and will require .28 to .56tons of air conditioning. Utilising active air creates additional challenges as the volume of stale and moisture laden air is introduced into the control room.

Eliminating the transformer translates to immediate spaceweightassociated lossesand cost savings. Fewer transformers also reduce issues with platform balance. For onshore applicationssuch as a pipeline pumping stationinstalling drives with transformers requires more robust concrete pads to prevent spillsoften in environmentally sensitive locations. Take away the transformer and savings will be quickly realised. New medium voltage power control technology now helps oil and gas engineers shed transformer space and weight while improving motor protection.

Traditionallymotors for use with medium-voltage variable-frequency drives had to be specified as ‘inverter duty’which included upgrading groundwall insulation to withstand command mode voltages and upgrading
turn-to-turn insulation to withstand repetitive dv/dt and the consequent reflected wave issues. Direct-to-drive technology mitigates common-mode voltage (CMV) and produces essentially sinusoidal output waveform with limited dv/dt (10volts per microsecond) and has been factory and field tested with long cables typical of ESP applications.

These drives are crucialbut they have historically required transformerswhich increase costspace and weight. Power converters in medium voltage motor ac drives (2.4kV to 7.2kV) generate common mode voltages that can cause premature failure of the motor winding insulation. Medium-voltage drive engineers have traditionally used transformers to reduce CMV stress and mitigate electrical harmonics with multi-pulse configurations.

Direct-to-drive technology gives engineers an alternative to bigheavy and costly transformers. An isolation transformer can represent 30–50percent of a drive system’s size and 50–70percent of the system’s weight. According to pricing indexes at National Material Companypricing of grain-oriented steelused in producing transformersrose 258percent from 2004 to 2006. In addition to soaring raw material costsan isolation transformer adds to total project costs with extra cablingair conditioning to cool the transformerengineering timeconcrete pad construction for outdoor transformers and overall installation.

Complexmulti-winding transformers can mitigate harmonics through phase shifting. Today’s proven Symmetric Gate Commutated Thyristors (SGCTs) based Active Front End drivesfeature a seven-step PWM modulation which provides 42 switching operations per ac cycle to suppress harmonics electronically through pulse timing using Selective Harmonic Elimination switching algorithms.

n additionSGCT within transformerless drives allow high switching frequency operation. The close proximity of the gate driver and power semiconductor give the transformerless drive a superior switching pattern that prevents it from producing high levels of line current harmonics and minimises snubber-electrical circuits used to suppress electrical
transients-requirements. As a resultthe Total Harmonic Distortion (THD) of the input current falls within IEEE-519 harmonic guidelines. Both current and voltage waveforms are near sinusoidalresulting in minimal added voltage stress on the motor windingeven if connected through long cables. With this technologya transformer for the purpose of harmonic mitigation is redundant.

To mitigate CMVengineers had no other option than to use isolation transformerseither by grounding the neutral point of the dc link or by grounding the neutral of the motor or wye-point of the output filter capacitors through a grounding network. Although transformers protect the motor from CMVthe high level CMV stress that would have been imposed on the motor is imposed on the transformer and cable insulation.

This requires extra transformer and cable insulation to withstand the CMV stressadding extra engineering requirements and extra costs. In a tranformerless drivethe integrated common-mode
dc choke is used to block the common-mode voltage and mitigate motor neutral to ground voltage. The drive uses existing components by integrating
common-mode impedanceresulting in a drive capable of using standard motor and cable insulation designs without an isolation transformer.

ombining innovations in common-mode impedanceactive front end rectifiers and SGCTs resulted in the patented Direct-to-Drive technology from Rockwell Automation. Collaborating with The Ryerson University Power Research Team in Torontocompany engineers developed a transformerless option for its
Allen-Bradley PowerFlex7000 medium voltage drives. Direct-to-Drive technology connects the power supply directly to the medium voltage drive without an isolation transformer. The inverter and the rectifier use SGCTsand both operate at optimal switching frequency to minimise the switching losses. Oil and gas industry engineers can use standard motors and eliminate the need for extra motor or cable insulation.

Time to first oil is a major focus in bringing a new platform online. Eliminating the need for transformers can increase time to first oil by eliminating the need for designinstallation and maintenance of transformers. Installation adds extra cablingnetworking and room for error.

Simply shipping the transformer overseas can take three to four weeks in transport time and add thousands of dollars in cost.

fficiency improvements result in substantial long term results. Capital cost is the usual focus for most end users when in fact this is often less than 10percent of the long term cost. More than 50percent of the 20 year cost for equipment relates to the energy used by the equipment in operation.

Transformers have significant losses – typically 1percent – which not only serve to reduce efficiencybut the resultant heat must be removed from the location as well.