Abstract:
A power system for an electrical system with highly fluctuating loads is powered by one or more power sources that are slow to react to load changes. The power sources are connected to electrical equipment used on the drill rig which provide active load to the generators. One or more load banks may be positioned to provide passive load to the generators to maintain generally constant generator load, while allowing for instant access to power as active load increases. Generators may be run at 100% capacity, a maximum efficient capacity, or at a high enough level to allow for a sufficiently rapid increase in power output. At least one parameter of a drilling operation may be utilized to anticipate load demand changes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional application which claims priority from U.S. provisional application No. 61/935,472 filed Feb. 4, 2014 and U.S. provisional application No. 62/010,652 filed Jun. 11, 2014. 
    
    
     TECHNICAL FIELD/FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to electric power transmission from a power source to a time-variant load, and specifically to powering electrical systems with highly fluctuating loads from one or more power sources that are slow to react to load changes. 
     BACKGROUND OF THE DISCLOSURE 
     In a modern drilling rig, much of the associated equipment is driven electrically. For some drilling rigs, generators are used to supply electricity to the drilling rig. In general, generators are most efficient when producing power within a certain range of power output. During drilling operations, electric loads may vary greatly depending on what is happening at the rig at any given time. Electrical equipment, including drawworks, mud pumps, top drives, rotary tables, etc. may consume large amounts of power when in use. Because each piece of equipment is used intermittently, the power drawn by the drilling rig may vary greatly at different times, at times going from very high to very low in short intervals. At other times, very little power is consumed by the drilling rig equipment. Additionally, a rapid decrease in electric load may cause a power spike which may cause the rig and generator to automatically shut down. 
     SUMMARY 
     The present disclosure provides for a power system for running an electrically driven device using a power load defining an active load when in operation. The power system may include a generator having a minimum efficient load rating. The power system may also include a load bank electrically coupled to the generator and positioned to provide a power load defining a passive load on the generator when engaged. The power system may also include a controller positioned to engage the load bank and activate the passive load. 
     The present disclosure also provides for a method for controlling a load bank. The method may include providing a power system for running one or more electrically driven devices using a power load defining an active load when in operation. The power system may include one or more generators. Each generator may have a minimum efficient load rating. Each generator may be electrically coupled to the electrically driven device. The power system may further include the load bank. The load bank may be electrically coupled to the generator and positioned to provide a power load defining a passive load on the generator when engaged. The power system may also include a controller positioned to engage the load bank and activate the passive load. The method may also include calculating the minimum total load of the one or more generators; calculating a total power demand of the one or more electrically driven devices; calculating, from the minimum total load and the total power demand, a load bank power demand; and engaging the load bank with the controller to provide passive load to the generators generally equal to the load bank power demand. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a block diagram of a drilling rig electrical system consistent with embodiments of the present disclosure. 
         FIG. 2  is a power flow diagram of the drilling rig electrical system of  FIG. 1 . 
         FIG. 3  is a block diagram for a control system for a resistor bank consistent with embodiments of the present disclosure. 
         FIG. 4  is a graph of power consumption in a typical tripping operation for a drilling rig electrical system consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of various embodiments. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     In some embodiments, a drilling rig power system is powered by one or more electric generators. The electric generators power electrical equipment on the drilling rig, as well as other electrical systems. Electrical equipment may include, for example and without limitation, drawworks, mud pumps, top drives, rotary tables, power tongs, pipe spinners, hydraulic pumps for hydraulic systems, etc. Auxiliary electrical systems may include without limitation, lights, computer systems, control systems, HVAC units, one or more LNG skids, etc. As would be understood by one having ordinary skill in the art with the benefit of this disclosure, these auxiliary electrical systems, unlike the electrical equipment, may generally draw a relatively constant and time-invariant amount of electric power. 
       FIG. 1  depicts drilling rig electrical system  100  consistent with embodiments of the present disclosure. Generators  101  may be driven by engines  103 . In some embodiments, engines  103  may be driven by liquefied natural gas. Generators  101  may supply power through supply lines  105  to supply electrical power to drilling rig electrical system  100 . In some embodiments, auxiliary electrical systems  106  may be coupled directly to supply lines  105  as their power demand may remain relatively constant. In some embodiments, the power supplied by generators  101  is rectified by one or more rectifiers  108 . In  FIG. 1 , rectifiers  108  are depicted as single diodes, but one having ordinary skill in the art with the benefit of this disclosure will understand that any suitable rectifier arrangement may be used, including without limitation, half bridge, full bridge, single or multiphase, etc. The output electricity, coupled to DC power bus  110 , may then be used to power the electrical equipment. The electrical equipment electrically loads generators  101 . The load on the generators caused by the electrical equipment is referred to herein as “active load”. 
     In some embodiments, as depicted in  FIGS. 1, 2 , the electrical equipment may include mud pumps  107 , drawworks  109 , and top drive  111 . In some embodiments, each piece of electrical equipment may be powered by a corresponding inverter  113  capable of being controlled by one or more variable frequency drive (VFD) controllers  115   a ,  115   b . In  FIG. 1 , two VFD controllers  115   a ,  115   b  are depicted, separated between power house  117  and driller&#39;s cabin  119 . One having ordinary skill in the art with the benefit of this disclosure will understand that a drilling rig need not include power house  117  or driller&#39;s cabin  119 . Furthermore, although two VFD controllers  115   a ,  115   b  are depicted, one having ordinary skill in the art with the benefit of this disclosure will understand that one or more VFD controllers  115  may be used to control the plurality of inverters  113 . Inverters  113  are depicted as choppers, but one having ordinary skill in the art with the benefit of this disclosure will understand that any other suitable electronic component or circuit may be substituted within the scope of this disclosure. For example, for a three-phase AC motor, the corresponding inverter  113  would be a three phase inverter and may be controlled by a pulse width modulated (PWM) signal supplied by a corresponding VFD controller  115  as understood in the art. Alternatively, for a DC motor, inverter  113  may be driven by a silicon controlled rectifier (SCR) drive to supply variable voltage controlled by the SCR to provide DC power to the motor. 
     In some embodiments system programmable logic controller (PLC)  121  may be utilized to control one or more elements of drilling rig electrical system  100 . As depicted in  FIG. 1 , PLC  121  is positioned to control VFD controller  115   a  and generator controller  123 . Generator controller  123  may control the power output of generators  101  by, for example, varying the power output of engines  103  to maintain the proper speed. 
     As understood in the art, electrical equipment such as mud pumps  107 , drawworks  109 , and top drive  111  may use large amounts of power when in operation. During a drilling operation, however, each piece of electrical equipment is used in a discontinuous manner causing the active load on generators  101  to vary in time. For example, during a normal (simplified) tripping-out operation, drawworks  109  may be used to lift a pipe string using a pipe elevator, thus consuming a large amount of power supplied through its corresponding inverter  113  as controlled by VFD controller  115   b . Drawworks  109  then stops, consuming little or no power, as the upper pipe stand is removed from the pipe string. Drawworks  109  then lowers the elevator to engage the top of the pipe string and repeat the process. While lowering, if regenerative or dynamic braking is used, drawworks  109  may return power to drilling rig electrical system  100 . Thus, the active load on generators  101  may vary greatly during the course of drilling operations. Additionally, when the active load is varied rapidly, generators  101  may not be able to supply enough power, causing a potential blackout as electrical equipment may shut off when insufficient power is available. Likewise, voltage spikes may be damaging for electrical equipment or generators  101  themselves. 
     To regulate the power level of drilling rig electrical system  100 , in some embodiments, generator controller  123  may lower the output power of generators  101  by reducing the fuel supplied to engines  103  or reducing excitation to generator  101 . In some embodiments, generator controller  123  may shut down one or more of generators  101  depending on current rig conditions. 
     Generators  101  may operate most efficiently when producing a certain range of electrical power. Likewise, generators  101  may operate most efficiently when electrically loaded. Thus, there may be a lower limit to the power output capable of being produced efficiently by generators  101  and a lower limit on electrical loading to allow generators  101  to operate efficiently or safely. Additionally, because starting up and shutting down generators  101  may require a large amount of time and/or fuel, it may be inefficient to entirely power down one or more generators  101  during normal drilling operations. Furthermore, because the active load may rapidly increase due to, for example, the use of drawworks  109  in the different steps of the tripping operation described above, the time required to change the power output of engines  103  to vary the power output of generators  101  may result in insufficient power availability to drawworks  109 . 
     The power output of engines  103  may be controlled by varying the amount of fuel supplied to the engine to maintain the speed of generators  101 . However, engines  103 , including engines  103  powered by LNG or pipeline gas, may not be able to respond quickly enough to maintain generator speed with rapid changes in active load. Changes in the amount of fuel provided to the engine may be slowed by, for example, fuel travelling through fuel lines, compressing larger amounts of fuel, and revaporization of the fuel for the engine. Engines  103  may, as understood in the art, be able to more rapidly change in power output if already running over a certain load level. In some embodiments of the present disclosure, generators  101  may be operated at a relatively constant power output, at or near the minimal efficient power output level. In such embodiments, the engine may thus be more able to cope with rapid increases in active. 
     In some embodiments, the generators may be operated at a power output level corresponding with maximum power output efficiency as dictated by the design and specifications of the generators. In some embodiments, the generators may be operated at maximum power output to, for example, maximize the instantaneous power available to the drilling rig. 
     In some embodiments, to maintain generally even power load to generators  101  or to reduce load fluctuations, one or more load banks  125  may be connected to generators  101 . In some embodiments, load banks  125  may be electrically coupled to generators  101  through load bank inverters  127 . In some embodiments, inverters  127  may be choppers as understood in the art and may be connected to DC power bus  110 . In some embodiments, inverters  127  may be AC converters coupled to an AC power bus. Load banks  125  may, as understood in the art, be positioned to dissipate electric power produced by generators  101 , by adding so called “passive load” to the generator power supply. Although not directly used by drilling rig electrical system  100  during a drilling operation as is the other electrical equipment, the passive load may be utilized to balance changes in active load. Thus, generators  101  may operate under generally constant loading and load fluctuations may be minimized. In some embodiments, load banks  125  may include resistive elements as shown in  FIG. 1 , positioned to provide passive load by converting electric power to heat. In other embodiments, load banks  125  may be any other load bank, including, for example and without limitation, load banks  125  adapted to apply one or more of resistive load, inductive load, liquid load (provided by, for example, a pump and a choke), wind resistance load, regenerative load (which may supply power to a separate grid such as the utility grid), capacitive load, or inertial load (such as a flywheel), used to power a motor/generator set, or used to charge a battery. VFD controller  115   a , controlled by PLC  121 , controls load bank inverters  127  to provide passive load to generators  101  by supplying electrical power to load banks  125 , thus allowing generators  101  to operate at an efficient power output regardless of active load from other electrical equipment on the drilling rig by adding passive load. Additionally, any negative active load, such as power generated by dynamic braking of drawworks  109 , may likewise be dissipated by load banks  125 . 
     In some embodiments which utilize a resistive element in load banks  125 , total power dissipated by load banks  125  may be given by the following equation: 
                       P   d     =     3   ·     N   B     ·   M   ·       V   dc   2     R         ,           (   1   )               
where P d  is the power dissipated as passive load, N B  is the number of three phase load banks, R is the per-phase resistance, and M is the duty cycle which varies between 0 and 1. As understood in the art, duty cycle refers to the fraction of time the load banks are on in a PWM control system. The PWM control system thus allows load banks  125  to proportionately dissipate any power level between 0-100% of their full power dissipation capability. In an exemplary drilling rig electrical system  100 , each load bank  125  may include three 2Ω resistors each with a power rating of 300 kW and 400 kW peak. The continuous rating for each load bank  125  is thus 900 kW, and peak of 1200 kW. If three load banks  125  are included in drilling rig electrical system  100 , the total continuous and peak dissipation ratings are thus 2.7 MW and 3.6 MW respectively. One having ordinary skill in the art with the benefit of this disclosure will understand that a similar equation may be formulated for any other type of load bank, and the power dissipated as passive load will likewise depend on the duty cycle.
 
     As an example, during operation, the voltage on DC bus  110  may be, for example, 780V. According to equation (1), load banks  125  may thus provide between zero and 2737 kW of power dissipation. Thus, for drilling rig electrical system  100  including three generators  101 , the minimum efficient load rating for each generator  101  may be up to approximately 900 kW. 
     The additional power generated by dynamic braking of drawworks  109  as previously described, however, may also be dissipated through load banks  125 . The total generator load may thus be calculated by the following equation:
 
 L   G,Total   −P   d   +L   aux   −P   DW ,  (2)
 
where L G, Total  is the total generator load, L aux  is the load of auxiliary electrical systems  106 , and P DW  is the power generated by drawworks  109  during dynamic braking Equation (2) may be used to determine the maximum power that may be regenerated by drawworks  109  while maintaining the minimum efficient load rating for generators  101 , maintaining a generally constant load on generators  101 . Depending on the number of active generators  101 , it may be necessary to operate drawworks  109  at a lower ramp rate on deceleration to ensure the maximum regenerated power is not exceeded.
 
     In some embodiments, PLC  121  or a separate controller may determine the amount of passive load to apply with load banks  125 . As depicted in  FIG. 3 , total minimum generator load  201  may be calculated by multiplying the minimum generator load  203  by the number of generators online  205 . Actual generator loads  207 , as supplied by the generator controller, are subtracted from total minimum generator load  201  to create a differential power error signal to be used by controller  209  to calculate minimum DC link power  211 . Controller  209  may be part of PLC  121  or a separate controller. Minimum DC link power  211  may be limited by limiter  213  between a value of zero and total minimum generator load  201 . Additionally, minimum DC link power  211  may be fed into controller  209  to, for example, prevent windup as understood in the art. In some embodiments, the maximum load change able to be handled by drilling rig electrical signal  100  may be fed into controller  209  as well. 
     Total power demand  214  which corresponds to the active load may be calculated as the sum of the power demands for each piece of electrical equipment. The power demands include mud pump power demand  215   a - b , top drive power demand  217 , and drawworks power demand  219   a - b . As previously discussed, drawworks power demand  219   a - b  may be negative during dynamic braking. 
     In some embodiments, controller  209  may be a proportional integral derivative (PID) controller. One having ordinary skill in the art with the benefit of this disclosure will understand that controller  209  may be any controller capable of operating as described including, without limitation, a step change controller, a state controller, a proportional controller (P), a proportional integral controller (PI), a PID controller, a proportional derivative controller (PD), an adaptive controller, or a predictive controller. In certain embodiments, anticipated load change may be based on process variables in addition to actual generator load to form a multi-variable control system. In some embodiments, additional process variables may include operational parameters, including, for example and without limitation, depth of wellbore, hook load, pump pressure, pump rate, length of drill string, and weight on bit, as well as any changes or requested changes thereto. In some embodiments, additional process variables may include power generation and distribution parameters, including, for example and without limitation, increase or change in current, increase or change in power, and number of engines online, as well as any requested changes thereto. As a non-limiting example, it may be anticipatable that a drilling bit at a greater depth may result in a larger top drive power demand  217 . As another example, a longer drill string may result in a larger drawworks power demand  219   a - b  during, for example, a tripping operation. By incorporating an anticipated load into total power demand  214 , the response time for controller  209  may, for example, be improved. 
     Total power demand  214  may be subtracted from minimum DC link power  211  to determine load bank power demand  221  or the amount of passive load to add to the system. Additionally, any auxiliary load may also be subtracted as well. Load bank power demand  221  may then be used to calculate (at  223 ) load bank duty cycle  225  according to the following equation, derived from Equation 1 above: 
     
       
         
           
             
               
                 
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       FIG. 4  depicts power flow during an exemplary tripping cycle as previously discussed. Any values depicted are shown for exemplary purposes only and are not intended to be limiting in any way. Depicted is generator power  301  (dotted line), load bank power  303  (solid line), and drawworks power  305  (dashed line) over time. Auxiliary load is assumed to be a constant 300 kW, and no power is going to any other electrical equipment. Additionally, drawworks dynamic braking power is limited to 1.5 MW. 
     From time 0 to time 22, the drawworks is lifting the drill string. The drawworks is utilizing 1900 kW, while the generators provide 2200 kW. The 300 kW difference is consumed by the auxiliary load, and thus the load banks are dissipating no power. At time 22, the drawworks are stopped, causing a large negative inductive power spike and a negative (regenerative) load during the slow-down of the drawworks. The load banks are activated, in some embodiments at a 100% duty cycle, to dissipate the power spike. In some embodiments, generator power output may be reduced to the minimum efficient power output, here 1500 kW. If the load banks were not activated, negative power from the drawworks may over speed the generators. Such an event may trigger a generator safety circuit which would shut the generators off, thus causing a “black-out”. 
     Once the inductive spike is dissipated and the drawworks has stopped, the load banks are used to dissipate excess power from the generators. The load bank duty cycle is calculated such that the load banks dissipate 1200 kW. 
     At time 70, the drawworks are beginning to lower the elevator, causing a positive inductive power spike followed by a period of negative power lasting until the drawworks is stopped. The load banks are deactivated, and the output of generators is increased to supply sufficient power to absorb the inductive spike. After the spike, as the drawworks lower, dynamic braking thereof generates 300 kW of power. The load bank duty cycle is modified so that the load banks dissipate 1500 kW of power. The drawworks are then stopped, again causing a large negative inductive spike, again dissipated by the load banks. Thus after time 90, the drawworks are drawing no power, the generators generating 1500 kW, and the load banks dissipating 1200 kW, again the difference between the generator output and the auxiliary load. 
     The foregoing outlines features of several embodiments so that a person of ordinary skill in the art may better understand the aspects of the present disclosure. Such features may be replaced by any one of numerous equivalent alternatives, only some of which are disclosed herein. One of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. One of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.