Why buy an isolation transformer when you don't need one? That's the philosophy behind the ground-breaking development of the first transformer-less medium-voltage (MV) drive in the industry, delivering improved efficiency, reduced size and weight and lower total cost of ownership.
The Allen-Bradley PowerFlex 7000 medium-voltage AC drive, with new Direct-to-Drive technology, was developed by Rockwell Automation to eliminate the disadvantages associated with using a drive isolation transformer.
The new technology connects the supply power directly to the MV drive.It is a strategic advantage for industries where control room space is at a premium or for example, at high altitudes in mining applications.
Direct-to-Drive technology can also benefit the oil and gas industry, sugar mills, water/wastewater, power plants and cement plants. The need for a better drive is universal. When the PowerFlex 7000 AC drives were first introduced in 1999, Rockwell Automation sold in countries around the world, including China, India, Australia and Korea. Today, in Latin America, there is immediate potential. In addition, Australia expects that 80% of its MV drive customers will use the new Direct-to-Drive technology.
In Europe, customers are requesting quotes and changing their specifications from transformer-fed drives to Direct-to-Drive technology, explains Bruce Ingram, Rockwell Automation's marketing manager in Switzerland.The PowerFlex 7000 Medium-Voltage AC drive with new Direct-to-Drive technology provides many benefits including common mode voltage (CMV) elimination, reduced harmonics, reduced drive system size and minimal drive complexity. Reduced capital cost, lower transportation and installation expenses, and increased operating efficiency all contribute to a lower total cost of ownership.
The PowerFlex 7000 Direct-to-Drive technology is the combination of three innovations: new CMV stress protection, the active front end (AFE) rectifier and the symmetrical gate commutated thyristor (SGCT).Until the development of Direct-to-Drive technology, most MV drive manufacturers used multi-winding isolation transformers to protect motors from CMV stress.
Transformers are used to isolate the supply system ground from the drive system earth, which allows the motor neutral point to be earthed.Although the motor is protected from CMV using this method, the high-level CMV stress that would have been imposed on the motor is now imposed on the transformer and the cable insulation. This requires extra transformer insulation and cable insulation to withstand the CMV stress, adding extra engineering requirements and extra costs.
Instead of using the transformer method, the PowerFlex 7000 Direct-to-Drive technology uses zero sequence impedance to virtually eliminate the CMV on the motor neutral, explains John Kwarta, product manager of Medium-Voltage Drives at the Rockwell Automation Medium-Voltage headquarters in Canada. The MV drive is now capable of using standard motor and cable designs with no isolation transformer.
Manufacturers use different methods to reduce harmonics, so there is a wide range in the complexity of drive designs. Isolation transformers phase-shift the transformer secondary windings to help reduce harmonics. The more secondary windings, says Kwarta, the higher the 'pulse number' and the better the harmonic elimination level. But with up to 15 sets of secondary windings, complexity and high component count are major disadvantages. In addition, this method requires a perfectly balanced three-phase distribution system for optimum harmonic elimination.
Instead of an isolation transformer, the PowerFlex 7000's Direct-to-Drive AFE rectifier uses semiconductor switching to reduce line current harmonics to levels that comply with the world's most accepted harmonic standards. The AFE rectifier, also known as the Pulse Width Modulated (PWM) rectifier, uses the SGCT to produce a PWM switching pattern that prevents the drive from producing high levels of line current harmonics while avoiding the use of a phase-shifting isolation transformer.
Reduced size and weightIndustries are always looking for ways to save control room space. An isolation transformer can represent 30-50% of a drive system's size and 50-70% of the system's weight. The PowerFlex 7000 with Direct-to-Drive technology is typically smaller and lighter than drive technologies using isolation transformers.
The typical volume of space required for a 950kW (1250hp) drive with isolation transformer is 13m3, and typical weight is 4200kg. The transformer-less PowerFlex 7000 of the same voltage and power is 60% smaller at 5.4m3 and 65% lighter at 1350kg.
A smaller and lighter drive system means there are no shipping splits to connect and no interwiring between drive and transformer. A transformer-less MV drive is perfect for retrofit projects with existing motors, switches and control rooms, where space is often limited or at a premium.
Control room space is very expensive on oil and gas platforms and in small European cities, says Ingram. Customers always ask for small frame solutions. The transformer-less option also improves system efficiency and reliability.
Reliability is one of the most important factors in the drive and motor system. The PowerFlex 7000 medium-voltage drive has already earned a reputation for reliability since its introduction in 1999, due to its high fault withstand capability and low component count.
It uses the DC link inductor and line reactor to limit fault currents to levels that are less likely to damage the drive system or connected equipment. Also, it eliminates the need for fuse protection in the drive. By using components with high voltage ratings, the PowerFlex 7000 medium-voltage drive reduces the component count to the lowest level in the industry.
In a typical 4160V, 750kW (1000 hp) motor application, an average MV drive can have up to 385 components in its power circuit. The PowerFlex 7000 with Direct-to-Drive technology has less than 30 components at 4160V.
Capital costs, operating costs and service costs can all be reduced using Direct-to-Drive technology. The simplicity of its design reduces initial capital investment because it not only eliminates the need to purchase an isolation transformer but also eliminates the need for an isolation transformer protection relay, a DV/DT filter, sine filter or motor terminator and special cables.
Installing an isolation transformer adds to total project costs because of extra cabling, air conditioning, civil engineering, concrete pad construction for outdoor transformers and overall installation. By purchasing a transformer-less drive, costs for isolation transformer crating, handling and transportation are also eliminated.
Shipping large transformers can add extra costs you don't see a return on, says Ingram. The transformers are too large for container storage and have to be specially crated and shipped on the deck, which can cost the customer thousands of dollars. This breakbulk shipment method requires pre-booking space on a vessel and longer shipment times due to limited availability of breakbulk vessels.
For example, a recent isolation transformer shipment to Argentina added costs of US$18,700 when no vessels were available to take breakbulk cargo. The transformers had to be sent to a New York port to be shipped, for a total of 25 days transit time. In a typical situation, shipping the transformer will add more than US$4,000. For example, an isolation transformer on a 950kW 1250 hp drive traveling from Los Angeles to Chile would cost about US$2,200 for crating and US$1,875 for shipping, plus additional handling and duty costs.
Typical transport time can be three or four weeks. If a transformer fails, this transportation time for the replacement equipment represents sig