Abstract:
A hybrid power plant is characterized by a substantially constant load on generators regardless of momentary swings in power load. Short changes in power load are accommodated by DC components such as capacitors, batteries, resistors, or a combination thereof. Resistors are used to consume power when loads in the power plant are generating excess power. Capacitors are used to store and deliver power when the loads in the power plant demand additional power. Reducing rapid changes in power load as seen by the generators allows the generators to operate at higher efficiencies and with reduced emissions. Additionally, power plants employing combinations of generators, loads, and energy storage devices have increased dynamic performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/734,761 filed Jan. 4, 2013, which is a continuation of U.S. patent application Ser. No. 12/816,576 filed Jun. 16, 2010 and now U.S. Pat. No. 8,373,949 granted Feb. 12, 2013, both of which are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure generally relates to power transmission networks. More specifically, this disclosure relates to operating a DC power system from one or more AC or DC power generators. Even more specifically, this disclosure relates to improving efficiency of an AC generators when connected to a DC bus by providing a nearly constant load to the generators. 
       BACKGROUND OF THE INVENTION 
       [0003]    Power transmissions networks can be made of AC systems, DC systems, or a combination of the two. AC power networks have conventionally been used throughout the world. However, DC power networks have certain advantages. DC power networks are easier to design and implement because they introduce no reactance into the power system. Higher efficiencies from generators can be achieved in DC systems because only real power is transmitted. Additionally, parallelization of power supplies is simple because no synchronization is required when additional supplies or loads are brought onto the network. 
         [0004]    Therefore, in power networks that experience large swings in load on the generators and require reliable operation, a combination of DC systems and AC systems is beneficial. One example of such a power network is found on drilling platforms or vessels to operate onboard thrusters. Drilling vessels are not anchored in the ocean but are dynamically controlled to maintain a desired position in the ocean. Thrusters are propeller drives that can have variable rotation speed and azimuthal angle of the blades. They are used to maintain a position within specified tolerances of a drilling apparatus. These thrusters are operated by a power supply onboard the drilling vessel. Any failure of the power supply can lead to displacement of the vessel out of the tolerances of the drilling apparatus. In such a case, the drilling apparatus would need to be mechanically decoupled and recoupled after the power supply is restored and the position of the drilling vessel is corrected. 
         [0005]    One method of facilitating a reliable power supply is to utilize a DC bus for powering thrusters and other components. Such a power transmission system is demonstrated in  FIG. 1 . In such a system, the power supply is generally made of AC generators coupled to an AC-to-DC converter, such as AC-to-DC converter  112 . The AC-to-DC converter places power from the AC generators on an intermediate DC bus. Each motor or thruster, as well as other devices utilizing the intermediate DC bus, on board the drilling vessel is coupled to the intermediate DC bus through a DC-to-AC converter. 
         [0006]      FIG. 1  is a block diagram illustrating a conventional DC voltage bus coupling multiple AC voltage generation systems to various loads. Power system  100  includes generators  102 . The generators  102  are coupled to an AC bus  104  through isolators  106 . The isolators  106  allow the generators  102  to be removed from the AC bus  104  when they are not used or are malfunctioning. The AC bus  104  is coupled to a transformer  108  to condition power for transmission to a line  110 . An AC-to-DC converter  112  is coupled to the line  110  and converts AC power on the line  110  to DC power for output onto an intermediate DC bus  120 . Coupled to the DC bus  120  are DC-to-AC converters  130 . The DC-to-AC converters  130  convert DC power on the DC bus  120  to AC power that most devices are designed to use. Coupled to the DC-to-AC converters  130  is a line  132  to which loads may be connected. A power dissipating device  134  is coupled to the line  132 , and the power dissipating device  134  may be, for example, a thruster. Additionally, a transformer  135  is coupled to the line  132  to condition power for a load  136 . The load  136  may be, for example, a light bulb. 
         [0007]    Another example of the motor  134  may be the draw works onboard a drilling platform. The draw works is a machine that reels out and reels in the drilling line and conventionally includes a large-diameter steel spool, brakes, and a power source. Operation of the draw works to reel in drilling line may require the full capacity of the ship-board generators. However, there are operations conditions where the draw works may consume zero power. In reverse operation, the draw works may generate power that is placed back on the line  132  while gravity assists reeling out of the drilling line. The power load changes may occur nearly instantaneously. 
         [0008]    Rapid changes in the load on the generator require the generator to increase power output to generate the power demanded by the load. Diesel generators are designed to consume fuel at an optimized rate in a small range of the available power output. Diesel fuel costs are the highest expense incurred by operating a diesel generator over its lifetime. Therefore, an operator desires to keep the generator operating in the power output range optimized for fuel consumption. 
         [0009]    Turning now to  FIG. 2 , a power output curve for a diesel generator are examined.  FIG. 2  is graph illustrating the operation of a diesel generator. A curve  220  represents fuel consumption in kilograms per kilowatt-hour of the diesel generator at various engine loads (power output). A range between 0 and 100 percent of rated output demonstrates a variation in the kg/(kw/hour) ratio, or efficiency of fuel consumption In order to operate efficiently a range  230  of power load on the diesel generator should be maintained. If the load increases or decreases, the engine fuel consumption and efficiency changes. 
         [0010]    In addition to fuel consumption issues, scrubbers on diesel generators that reduce the dangerous exhaust are sensitive to the volume of exhaust. Rapidly varying engine power changes the rate of flow of exhaust and chemical components of the exhaust. Because the scrubber is designed to operate optimally on a continuous and stable flow of exhaust, emissions output may not be minimized if the power load varies rapidly. 
         [0011]    Further, dynamic performance of diesel generators is limited. That is, diesel generators may not increase power output rapidly enough to match an increasing power load on the diesel generator. Conventionally, additional diesel generators would be brought online if the rate of increase of power load exceeds the rate of increase of diesel generator power output. Neither diesel generator is operating efficiently and results in increased fuel consumption and express capacity when the power load peaks. 
         [0012]    Referring now to  FIG. 3 , generators and power loads will be examined in a conventional power plant.  FIG. 3  is a block diagram illustrating power distribution on a conventional power plant  300 . The power plant  300  includes an AC generator  302  coupled to a switchboard  308  through an AC line  306 . The switchboard  308  is coupled to multiple loads. For example, typical shipboard and drilling loads are represented by a power dissipating device  312  coupled to the switchboard  308  by an AC line  310 . Additionally, the switchboard  308  is coupled to an AC-to-DC converter  318 . The AC-to-DC converter  318  is coupled to an AC line  316  and a DC line  320 . Additional loads may be coupled to the DC line  320 . For example, a light  322  may be coupled to the DC line  320  or a DC-to-AC converter  324 . The DC-to-AC converter  324  couples to additional AC loads such as a power dissipating device  326 . The power dissipating device  326  may be a draw works as described above or a motor. Each of the loads  312 ,  322 ,  326  produces different power loads on the AC generator  302 . The effect on the AC generator  302  will now be examined. 
         [0013]      FIGS. 4A to 4E  are graphs illustrating power consumption in a conventional power plant such as  FIG. 3 . A line  402  in  FIG. 4A  indicates power consumption at the power dissipating device  312 . Shipboard loads such as the power dissipating device  312  operate as a constant load over long periods of time such as hours on the AC generator  302 . The line  402  is positive indicating consumption of power. A line  404  in  FIG. 4B  indicates power consumption at the power dissipating device  326 . Draw works such as the power dissipating device  326  operate as a varying load, which may change rapidly such as in milliseconds, on the AC generator  302 . The line  404  varies between positive and negative values indicating the load consumes power at some times and produces power at other times. A line  406  in  FIG. 4C  indicates power consumption at the light  322 . The light  322  operates as a constant load over long periods of time such as hours on the AC generator  302 . 
         [0014]    Total power transferred through the AC-to-DC converter  318  is represented by adding the line  404  to the line  406  and is shown in a line  408  in  FIG. 4D . The line  408  is total power consumption with respect to time of the DC line  320 . Total power delivered by the AC generator  302  is shown in a line  410  in  FIG. 4E  and is a sum of lines  408 ,  402 . In the conventional power plant  300  power delivered by the AC generator  302  varies in time. This leads to undesirable qualities exhibited by the AC generator  302  as indicated above including inefficient fuel consumption and poor exhaust scrubbing. 
         [0015]    Thus, there is a need for a power plant design that produces a substantially constant load on the AC generators and increases dynamic performance. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    A power plant includes an AC generator, an AC-to-DC converter coupled to the AC generator and a DC bus, and a switch coupled to the DC bus. The power plant further includes an active power compensation system coupled to the switch. The active power compensation system reduces power load variations in the power plant. The switch may include a DC-to-DC converter. The active power compensation system may include power consumption devices. The power consumption devices may be resistors. The power plant may also include power storage devices. The power storage devices comprise ultracapacitors. The ultracapacitors may be coupled to one or more microcontrollers. The one or more microcontrollers may regulate the ultracapacitors. The power storage devices may include batteries or rotating machines. 
         [0017]    A method of reducing variations in a power load on a generator includes routing power between the generator and a power consuming device during a time when the power load on the generator is lower than a first level. The power consuming device may include a resistive element. The first level may be based, in part, on a fuel efficiency of the generator. 
         [0018]    A method of reducing variations in a power load on a power plant having a generator includes routing power between the generator and a energy storage device during a time when the power load on the power plant is lower than a first level. The energy storage device stores energy provided by the generator. The energy storage device may include at least one ultracapacitor. The energy storage device may include at least one battery. The first level may be based, in part, on a fuel efficiency of the generator. The method may also include routing power between the generator and the power storage device during a time when the power load on the power plant is higher than a second level. The second level may be higher than the first level. The energy storage device may deliver power to the power plant. The second level may be chosen, in part, based on a fuel efficiency of the generator. The method further includes routing power between the generator and a power consuming device during a time when the power load on the power plant is lower than a third level. The third level may be lower than the first level. The third level may be chosen based, in part, on a capacity of the energy storage device. 
         [0019]    A power plant includes means for generating power to meet a power load of the power plant. The power plant also includes means for reducing variation in the power load of the power plant. The means for reducing variation may include means for consuming energy. The variation reducing means may include means for storing energy. 
         [0020]    The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter which form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the technology of the disclosure as set forth in the appended claims. The novel features which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings. 
           [0022]      FIG. 1  is a block diagram illustrating a conventional DC voltage bus coupling multiple AC voltage generation systems to various loads. 
           [0023]      FIG. 2  is a graph illustrating the operation of a diesel power generator. 
           [0024]      FIG. 3  is a block diagram illustrating power distribution on a conventional power plant. 
           [0025]      FIGS. 4A to 4E  are graphs illustrating power consumption in a conventional power plant such as  FIG. 3 . 
           [0026]      FIG. 5  is a block diagram illustrating power distribution on an exemplary power plant with power dissipating devices to consume regenerated energy according to one embodiment. 
           [0027]      FIGS. 6A to 6F  are graphs illustrating power consumption in an exemplary power plant with resistors to consume regenerated energy according to one embodiment. 
           [0028]      FIG. 7  is a block diagram illustrating power distribution on an exemplary power plant with active power compensation according to one embodiment. 
           [0029]      FIGS. 8A to 8G  are graphs illustrating power consumption in an exemplary power plant with active power compensation according to one embodiment. 
           [0030]      FIGS. 9A to 9G  are graphs illustrating power consumption in an exemplary power plant with active power compensation and a capacity limited energy storage device according to one embodiment. 
           [0031]      FIG. 10  is a block diagram illustrating an exemplary active power compensation system according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    Reducing variation of the load on a generator in a power plant may be accomplished by adding devices that dissipate power during short times when power loads are volatile. In this arrangement, the generator may be able to continue operation at a higher output while the power dissipating devices remove power generated by some loads. Without the power dissipating devices to remove energy generated by the loads, the generators would reduce power output and allow other loads to absorb the regenerated power. 
         [0033]      FIG. 5  is a block diagram illustrating power distribution on an exemplary power plant with power dissipating devices to consume regenerated energy according to one embodiment. A hybrid power plant  500  includes an AC generator  502  coupled to a switchboard  508  through an AC line  506 . The switchboard  508  is coupled to the AC line  506  and an AC line  510 . A power dissipating device  512  is coupled to the AC line  510 . The power dissipating device  512  may represent, for example, shipboard loads. The switchboard  508  is also coupled to an AC-to-DC converter  518  through an AC line  516 . The AC-to-DC converter  518  provides power to a DC line  520 . A light  522  couples to the DC line  520 . Additionally, a DC-to-AC converter  524  is coupled to a power dissipating device  526  and the DC line  520 . The power dissipating device  526  may be a draw works as described above. Additionally, a DC-to-DC converter  532  couples a power dissipating device  534  to the DC line  520 . The power dissipating device  534  may be any device capable of consuming energy. For example, the power dissipating device  534  may be a resistor, variable resistor, water brake, or a combination of the aforementioned devices. The power demand on the AC generator  502  from the loads  512 ,  522 ,  526 ,  534  will now be examined. 
         [0034]    Referring to  FIG. 6  the loads at various locations on the hybrid power plant  500  are examined.  FIGS. 6A to 6F  are graphs illustrating power consumption in an exemplary power plant with resistors to consume regenerated energy according to one embodiment. A line  602  in  FIG. 6A  indicates power consumption at the power dissipating device  512 . Shipboard loads such as the power dissipating device  512  operate as a constant load over extended periods of time on the power plant. A line  606  in  FIG. 6C  indicates power consumption at the light  522 . The light  522  operates as a constant load over extended periods of time on the hybrid power plant  500 . A line  604  in  FIG. 6B  indicates power consumption at the power dissipating device  526 . Draw works such as the power dissipating device  526  have a power load that varies rapidly with time in as small as millisecond intervals. In the case of power dissipating device  526 , the power load is positive at some times and negative at other times. During the positive portion of the line  604  the power dissipating device  526  consumes power; during the negative portion of the line  604  the power dissipating device  526  delivers power to the power plant. 
         [0035]    During a time when the power dissipating device  526  is delivering power to the hybrid power plant  500  the AC generator  502  will reduce power output to accommodate the regenerated power. As described above, the AC generator  502  loses efficiency when its power output is reduced or changes rapidly. Therefore, the power dissipating device  534  may be switched on by the DC-to-DC converter  532  to consume excess power on the DC line  520 . This allows the AC generator  502  to continue operating at a nearly constant power output. A line  608  in  FIG. 6D  indicates power consumption by the power dissipating device  534 . The line  608  is positive because the power dissipating device  534  is only capable of consuming power. The DC-to-DC converter  532  is switched on at times that it would be advantageous to add additional power consumption to the hybrid power plant  500 . According to one embodiment, the line  608  represents power consumption substantially equal in magnitude to the line  604  during the period of time that the line  604  is negative. Therefore, the power dissipating device  534  consumes power generated by the power dissipating device  526 . The DC-to-DC converter  532  may be switched on for a longer time or shorter time depending on the condition of other loads on the hybrid power plant  500 . 
         [0036]    Total power transferred through the AC-to-DC converter  518  is indicated by a line  610  in  FIG. 6E . The line  610  is a summation of the lines  604 ,  606 ,  608 . Total power delivered by the AC generator  502  is indicated by a line  612  in  FIG. 6F . The line  612  is a summation of the lines  610 ,  602 . The line  612  indicates the load on the hybrid power plant  500  is confined to a more narrow range than that of the line  410  in  FIG. 4E  in which no power dissipating device is implemented. For example, the line  612  has a minimum of 1 MW whereas the line  410  has a minimum of 0 MW The addition of the power dissipating device  534  and the DC-to-DC converter  532  limits power output reduction of the AC generator  502  when one of the loads in the hybrid power plant  500  generates power. The most inefficient operating range of the AC generator  502  is at low power output, therefore, efficiency of the AC generator  502  in the hybrid power plant  500  is improved by not operating the AC generator  502  at low power loads. 
         [0037]    The power plant may be further adapted to increase efficiency if the energy generated by loads may, instead of being dissipated, be stored and used at a later time when power demand increases. As a result, an increase in load on the power plant would result in a discharge of the stored energy allowing the AC generator to continue operating at a nearly constant engine power load. A system for storing energy and delivering energy depending on conditions in the power plant is referred to as an active power compensation system. 
         [0038]      FIG. 7  is a block diagram illustrating power distribution on an exemplary power plant with active power compensation according to one embodiment. A hybrid power plant  700  includes a energy storage device  744  coupled to the DC line  520  through a DC-to-DC converter  742 . The energy storage device  744  may be switched on by the DC-to-DC converter  742  when additional power should be delivered to the DC line  520 . The energy storage device  744  may also be switched on at times when excess power is delivered to the DC line  520  such that the energy may be stored by the energy storage device  744 . The energy storage device  744  may be any energy storing device including, but not limited to, spring tension, fuel cells, flywheels, capacitors, variable capacitor, ultracapacitors, batteries, or a combination of the aforementioned devices. In addition to energy storage device  744 , the hybrid power plant  700  may, in one embodiment, also include the power dissipating device  534  coupled to the DC-to-DC converter  532 . 
         [0039]    Turning now to  FIG. 8 , the load on the hybrid power plant  700  at various locations will be examined.  FIGS. 8A to 8G  are graphs illustrating power consumption in an exemplary power plant with active power compensation according to one embodiment. The lines  602 ,  604 ,  606  of  FIGS. 8A, 8B, and 8C , respectively, are identical to those in  FIG. 6 . A line  809  in  FIG. 8E  indicates power load of the energy storage device  744 . The line  809  has substantially the same magnitude as the line  604 , but of opposite polarity. The line  809  is a mirror image of the line  604 . The energy storage device  744  stores energy during periods of excess power generation and delivers energy during periods of power generation shortage. As a result, variations in power load on the AC generator  502  are reduced. The reduction is a result of the energy storage device  744  consuming power during time that the power dissipating device  526  and delivering that power back to the hybrid power plant  700 . A line  808  in  FIG. 8D  indicates the power load on the power dissipating device  534 . Power load at the AC-to-DC converter  518  in the hybrid power plant  700  is indicated by a line  810  in  FIG. 8F . The line  810  is a summation of the lines  808 ,  809 ,  606 ,  604  and is a substantially constant value. A line  812  in  FIG. 8G  indicates total power load on the AC generator  502  and is a summation of lines  810 ,  602  and is also a nearly constant value. 
         [0040]    Thus, the use of the energy storage device  744  reduces the effects of a varying power load on the AC generator  502 . The energy storage device  744  may adapt to changes in the power load of the power dissipating device  526  and other loads in the hybrid power plant  700 . The nearly constant power load on the AC generator  502  allows for continuous operation in the most efficient operating region of the AC generator  502 . Additionally, the energy storage device  744  increases dynamic performance of the hybrid power plant  700 . The AC generator  502  in response to an increasing power load may not be capable of increasing output quickly enough to match the increasing power load. The energy storage device  744  may have a shorter response time to the increasing power load and deliver additional power while the AC generator increases output to match the power load on hybrid power plant  700 . According to one embodiment, the improved dynamic performance of the hybrid power plant  700  having the energy storage device  744  allows the AC generator to remain at a substantially constant power output. 
         [0041]    The power dissipating device  534 , in one embodiment, is used to consume power when power generation by the power dissipating device  526  exceeds a capacity of the energy storage device  744 .  FIGS. 9A to 9G  are graphs illustrating power consumption in an exemplary power plant with active power compensation and a capacity limited energy storage device according to one embodiment. The line  909  in  FIG. 9E  represents power at the energy storage device  744 . According to one embodiment, the energy storage device  744  has an energy capacity of 1 megaJoule. During power consumption of line  604 , the line  909  is negative indicating the energy storage device  744  is providing power. During power generation of the line  604 , the line  909  is positive indicating the energy storage device  944  is storing power. As the energy storage device  744  reaches a maximum energy capacity at time t 2 , the power dissipating device  534  will engage to absorb regenerated power from the load  526  in order to maintain a substantially constant load on the AC generator  502 . The actual energy capacity of the energy storage device  744  may vary from the embodiment demonstrated. The line  908  in  FIG. 9D  illustrates that during the portion of time that the energy storage device  744  is near capacity, the power dissipating device  534  consumes power. As a result, the summation of the switchboard  508  yields the same power load as in  FIG. 8 . 
         [0042]      FIG. 10  is a block diagram illustrating an exemplary active power compensation system according to one embodiment. An active power compensation system  1000  may be employed to store and deliver energy to the hybrid power plant  700 . An input line  1012  is used to connect the active power compensation system to a power plant. The active power compensation system  1000  includes several columns  1034  of power storage devices. Each column  1034  includes energy storage devices  1042 . The energy storage devices  1042  may be, for example, ultracapacitors, capacitors, batteries, or fly wheels. The energy storage devices  1042  are stacked in series to obtain a desired voltage and in columns  1034  to obtain a desired current or optimal energy density. The energy storage devices  1042  are controlled by microcontrollers  1044  to regulate charging and discharging activities. For example, the microcontrollers  1044  may disconnect defective or damaged power storage devices  1042  from the columns  1034 . 
         [0043]    Examples of hybrid power plants for drilling vessels including shipboard loads have been shown in the above embodiments. However, the power plants as disclosed may be adapted for use in a number of other applications. Additionally, the power plants may include AC or DC generators and loads. AC-to-DC, DC-to-AC, and DC-to-DC converters as shown in the figures above may be unidirectional or bidirectional. One of ordinary skill in the art would be capable of substitution, e.g., an AC-to-DC for a DC-to-AC converter, depending upon load configuration and characteristics (i.e., DC load or AC load) of a particular power plant. 
         [0044]    Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present invention, disclosure, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.