FEATURE
OFFSHORE
Comparison of Solid State Device Characteristics
Relative Power Semiconductor Characteristics
Type GTO IGBT IGCT Speed Medium Fast Fastest Gain Medium High Efficiency Low High Control Method Current Voltage High Maximum Rating, A 10,000 (snubbered) 2,000 (unsubbered) Current 4,000 (unsnubbered)
Turn-off, low; Conduction, high
(Table from "The Next Stage in Power Semiconductors", by Leslie Langau)
frequency square wave output from the variable speed drive, typically 2000 Hz to 5000 Hz, and then modulating the width of each square pulse of that higher frequency to create a torque current in the motor, causing it to rotate it at any speed desired, including zero rpm. The modulation signal can be controlled to cause an AC motor to behave virtually identically to a DC motor. Modulating the width of a square wave (effectively what is occurring in the control circuitry) is fairly simple, making a PWM drive reliable and comparatively inexpensive. The other really nice thing about PWM is it works well with induction motors. AC induction motors are extremely reliable, inexpensive compared to DC motors of the same horsepower rating, and very low maintenance. To work well this technique requires a device that can quickly turn on and off. That is one of the features of a transistor. But transistors have their shortcomings, like higher internal losses, limited current carrying capability, and limited voltage level. These limitations that have been largely overcome in the last few years. However, this has not stopped development of competing technologies. ABB and Siemens have both been developing a device called an IGCT (Integrated Gate Commutated Thyristor), which they claim combines
the best features of an SCR and an IGBT. It has fast speed of operation, very low losses in conduction, and is usable at voltages typically higher than those at which an IBGT can be used.
Merits of AC vs DC Drives
If the characteristics of DC are so desirable, why change? DC has a good track record and is very reliable. But it isn't perfect, and there are other forces at work driving manufacturers away from large DC applications. A single DC motor is limited to about 8MW maximum output. This mostly is due to limitations of the commutator and brushes in a DC motor. The brushes have current density limitations, while the commutator segments have voltage limitations. DC motors are also complex devices, expensive to build in large sizes, and heavy. Although electrically simple overall, it is still something of an art to get every element of DC motor installation, like brush alignment, brush chemistry, field alignment, and commutator film development, to come together just right and all at the same time for optimum operation of the motor. DC motors also have much higher maintenance requirements than either AC synchronous or induction motors. There are fewer manufacturers of large DC motors, fewer service centers, and fewer
operating engineers with any real experience or understanding of the nuances of maintaining a DC motor in good condition. Add to this that economic forces are driving manufacturers of DC drives away from the marine market, and it starts to become clear why most manufacturers of marine propulsion drives are pursuing Variable Frequency Drives (VFD) so intensively over DC. It is expensive to build both motors and drives to the special rules and standards that the maritime authorities require. Original Equipment Manufacturers (OEM's) are less inclined to produce and stock as standard products anything that doesn't sell in large quantities. This argument is not so strong in regards to the motors, particularly if it is a choice between a large synchronous motor or a large DC motor. Both are special construction. But in drives it is a leading factor in the decision of such companies as Siemens to no longer produce an Offthe-Shelf marine rated DC propulsion drive. These top tier OEM manufacturers are putting their effort into VFD development and slowly letting go of DC technology. There are still third party integrators operating in niche markets where DC can fill the bill both technically and economically. Companies like EPD
(Electronic Power Design, Inc.) of Houston, Texas, are supplying significant quantities of marine propulsion and platform motors and controls in a DC format. Using the ubiquitous GE 752 motor and marinizing OEM DC drives for the purpose, they are supplying propulsion systems for PSV's, jack-up rigs, and other oil field applications that are familiar with and still demand this type of system.
Benefits of Diesel Electric
Diesel Electric propulsion has significant advantages in many areas when compared to other propulsion systems. Since diesel has replaced just about all other propulsion systems except nuclear/steam on some Navy aircraft carriers and submarines, this comparison will limit itself to equivalent diesel direct mechanical drive propulsion systems. A diesel-only system requires some kind of mechanical linkage from the diesel output coupling to the propeller. This necessitates putting the diesel inline with some set of components making up that linkage, typically a reduction gear and shaft. The reduction gear is used to match the diesel speed to the required propeller speed, which typically is lower than the engine rpm for best efficiency of operation. The reduc-
Typical 12 and 24 Pulse Configurations. (Diagram Courtesy of Siemens E&A)
Typical Medium Voltage High Power DE Installation. (Diagram Courtesy of Siemens E&A)
Integrated Gate Commutated Thyristor. (Photo Courtesy of ABB)
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