|
Medium-voltage transformerless drives save on weight and space
Historically, transformers played a significant role in platform functionality, especially with motor-driven applications. Offshore platforms, for example, typically require 20 medium-voltage drives (sometimes up to 40 or more) to control pump motors on downhole wells, and each drive requires a multi-ton transformer. Associated weight, energy and maintenance costs can quickly 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 space, weight, associated losses, and cost savings. Fewer transformers also reduce issues with platform balance. For onshore applications, such as a pipeline pumping station, installing drives with transformers requires more robust concrete pads to prevent spills, often 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.
Traditionally, motors 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 These drives are crucial, but they have historically required transformers, which increase cost, space 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 big, heavy 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 Company, pricing of grain-oriented steel, used in producing transformers, rose 258percent from 2004 to 2006. In addition to soaring raw material costs, an isolation transformer adds to total project costs with extra cabling, air conditioning to cool the transformer, engineering time, concrete pad construction for outdoor transformers and overall installation.
Complex, multi-winding transformers can mitigate harmonics through phase shifting. Today’s proven Symmetric Gate Commutated Thyristors (SGCTs) based Active Front End drives, feature 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 addition, SGCT 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 To mitigate CMV, engineers had no other option than to use isolation transformers, either 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 CMV, the 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 stress, adding extra engineering requirements and extra costs. In a tranformerless drive, the integrated common-mode
ombining innovations in common-mode impedance, active 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 Toronto, company engineers developed a transformerless option for its 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 design, installation and maintenance of transformers. Installation adds extra cabling, networking 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 efficiency, but the resultant heat must be removed from the location as well.
The transformerless design configuration utilises a more efficient integral line reactor which is one reason for lower overall drive system losses which, when considered in the long term, result in substantial energy savings.
Luis Gamboa is Manager of Industry Solutions, Rockwell Automation, For more information, visit www.rockwellautomation.com/industries/oilgas |
