|May10-08, 11:40 PM||#1|
Should DC operated oil rigs convert to AC?
Todayís world would seem to dictate that a changeover is needed from DC powered operations to AC for drilling rigs. Is this actually true? Is AC a better deal or a sleeping dog best left alone? DC rig operations have been in existence for many years and have proven reliable so, why the interest in AC?
It is often stated by those suppliers of AC equipment that their AC drives and motors are more efficient than DC (why not, if I sold blue cars they would absolutely be better than red ones). Letís think about all this for a moment. The AC variable speed drive is a component that requires two stages of operation. First, the incoming AC main is converted into DC in order for the drive electronics to manipulate the signal. This is accomplished through the use of rectifiers for conversion and capacitors for filtering. These components are susceptible to failure and can be very costly to replace, not to mention the amount of energy used at this first stage. Now that DC is present in the drive, giant transistors (IGBT) are used to reproduce a simulated AC for the motor (hence the term inverter). The IGBTsí (Insulated Gate Bipolar Transistor) or giant transistors are another component which can fail, and are expensive, not to mention the cost of replacement for the AC drive electronics. Though this signal is not truly AC it is close enough that the motor can use it. This process alone introduces another factor, the motors must be able to handle this PWM (Pulse Width Modulation) produced signal or early failure will occur; the motor will burn up (they generally do anyway; it is the lamination that poses the problem, oh, and the bearings as well). The DC drive has only one stage, the AC is converted to DC performed by SCRís (Silicon Controlled Rectifier) for a pure DC output; manipulated by firing the gate for control the signal is sent directly to the DC motor, how more simple can that be? DC drive components are far fewer and less expensive to replace. Failure on an AC system can be catastrophic. So much for efficiency of the drive and motor; what about power, speed control, and braking? Glad you asked.
Can more power, or torque, be achieved by changing to an AC system? Yes and no. The paradox of which gives more power is dependent on differing factors. These factors are speed and horsepower. At lower speeds the DC motor far excels the AC. In a lower HP range up to 100, the AC seems to have better characteristics. In the higher HP ranges the DC motor overcomes its inefficiencies. In order for the AC to deliver equivalent power throughout, Vector control would be required (here again efficiency is lost).
Ok, so now we have a rig with centrifugal mud pumps and we want to control speed, well not really. We want to control strokes and pressure; we donít care what the speed is. Power is needed and DC is the answer. All that is applicable is a simple drive, inexpensive components, easy replacement, basic operations and troubleshooting so, why the AC here? Draw works? Same principle, here you want power. DC delivers that power dependably. Speed control is not that precise on a draw works that I have experienced. Since we mentioned precise speed control, use an encoder with any system you have, problem solved. True, if we only used armature feedback to calculate speed on a DC drive we could not have accurate control so, put on an encoder and use tachometer feedback. Good practice for the rotary table here. So, where do we want accurate speed control on a rig? The rotary table or top drive and thatís all. In the addition of a top drive it may be beneficial to be AC with its own power generating station, this is still debatable.
Braking, a non-regenerative DC bridge is less expensive than an AC braking system. The braking system of a DC motor is simplistic compared to that of its AC neighbor. Braking in an AC drive system has a tendency to destroy itself if not implemented correctly. The energy during braking must be absorbed somewhere, either back through the circuit possibly destroying expensive parts or through a Dynamic Braking system with resistors and a chopper network. Overall DC wins here.
So, do we change the DC rig over to AC? If the money is there to spend then go for it. Oh, did we mention the fact that most rig electricians would be totally lost with the AC drive systems? And by the way, on a DC rig any motor may be operated by either SCR drive, they are all the same; not so with the AC rig, if one drive fails thatís it unless you have a technician that can make the necessary changes on site, the draw works drive parameters are far different than the mud pump. Donít forget about the harmonics introduced with the AC system. Did we mention that the AC motors should be underrated? In other words a 20 HP AC motor should be used where a 15 HP is needed. Ok, noted the DC motor has brushes. So, keep the commutator clean and replace the brushes on a regular schedule.
|May11-08, 02:05 AM||#2|
The driving force behind the choice to convert to AC (back in the days of Tesla and Westinghouse) was the simple fact that line-losses over long distances could be limited by the change, and the lines did not have to be as large as would be required by a DC system. DC can work very well, can in fact be more efficient than AC when we don't have to factor in transmission line-loss. Since nobody is going to start running transmission lines from the shore to oil rigs, what's the problem with them running their systems on DC?
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