Abstract:
The invention is a multiple-structure cathodic protective system that includes multiple voltage-controlled outputs to protect multiple structures with a common ground bed, or a single structure with multiple anode ground beds. Each output feeds an individual cable that is powered by independent pulse-width-modulated voltage power supplies. Current through the cables is monitored and the PWM control compensates to maintain the correct amount of current for maximum protection and long life.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    None. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    None. 
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    None. 
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
       [0004]    None. 
       BACKGROUND OF THE INVENTION 
       [0005]    1. Field of the Invention 
         [0006]    The invention pertains to cathodic protection of buried metal structures using continuous electric current from a single voltage source. 
         [0007]    2. Background Art 
         [0008]    Buried metal structures corrode due to voltage differences between the ground and the various different contact points of the structure. Because of the voltage difference, a continuous current develops in the structure that causes hydrogen to form in the cathodic points of contact, and iron corrosion in the anodic points of contact. 
         [0009]    Eventually, the electron flow will corrode enough iron to weaken a buried structure and develop leaks or failures. The term “cathodic protection” refers to the practice of using current through the structure to combat the corrosion by artificially forcing the structure to be at a negative voltage in comparison to the surrounding ground connecting the negative terminal of a direct current voltage source to the structure and connecting the positive terminal to a sacrificial anode buried near the structure. The term “sacrificial anode” comes from the use of the anode—it is slowly eaten away as it provides a steady stream of ions to provide the cathodic protection. 
         [0010]    When the applied voltage is correctly set, the electrons flow from the anode to the structure, and the structure does not corrode. The current flow must be set so that the structure remains cathodic (draws electrons to the structure from the surrounding ground), but not so cathodic that the electron flow is excessive, which causes the structure to become weak and brittle. 
         [0011]    The cathodic protection industry struggles with development of a system to protect multiple structures, as each structure exists in a slightly different resistive ground network. A single voltage source results in different current flow to each structure. The various approaches include the insertion of variable resistors along the cable network to even out the current (Al-Mahrous, U.S. Pat. No. 7,192,513), use of multiple buried anodes of different values of resistance that provide different amounts of protection, and use of multiple anodes that are switched off and on to provide more or less resistance (Husock U.S. Pat. No. 3,143,670). 
         [0012]    As these patents and the product offering of the cathodic industry demonstrates, the search for efficient protection of buried metal structures has been a fifty-plus year endeavor that continues to this day. Those in the industry must weigh many factors to select a proper product. Those factors include system hardware cost, system operational cost, system reliability, energy efficiency, and number and size of structures to be protected. There is no one-size-fits-all solution, as a pipeline with easy access to electric utilities will naturally call for a different solution than a single well in an isolated area. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    The invention is a multiple-structure cathodic protective system that includes multiple voltage-controlled outputs to protect multiple structures with a common ground bed, or a single structure with multiple ground beds. Each output feeds an individual cable that is powered by pulse-width-modulated voltage power supplies. Current through the cables is monitored and the PWM control compensates to maintain the correct amount of current for maximum protection and long life. 
         [0014]    In the current embodiment, a computerized control system monitors the current and voltage drop across each protected structure circuit, adjusting the current through each line corresponding to the drop by adjusting the voltage output. In this way, the system gives protection with a single bulk power supply and multiple power supply outlets, reducing cost while maintaining a flexible configuration that other single-source cathodic protection systems do not possess. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
         [0015]    FIG.  1 —A schematic diagram of the invention configured to protect multiple structures using a common anode bed. 
           [0016]    FIG.  2 —A schematic diagram of the invention configured to protect a single structure using multiple anode beds. 
           [0017]    FIG.  3 —An operational flowchart of the invention configured to protect multiple structures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  shows one embodiment of a computerized voltage-controlled multiple-output cathodic protection unit  11  that could be used to inhibit corrosion in buried metal structures. The cathodic protection unit  11  includes: a main computer  13 , said computer comprising an input power converter  17 , and one or more power supply output cards  31 ,  33 ,  35  (hereafter “ 31 ”) which fit into the main computer  13 , and each power supply card  31  providing a pulse-width-modulated voltage supply output at the card terminals. An optional user-interface  41  can be used to provide input to the system to make changes, read results, or report alarms. The positive terminals are tied to a sacrificial anode  19 , and the negative terminals of each output card  31  are tied through respective cables  21 ,  23 ,  25  to individual metal structures to be protected, shown as impedances Z 1 , Z 2  and Z 3 . In this embodiment, each card has its own output circuit that protects a single structure. 
         [0019]    In the embodiment shown in  FIG. 1 , the input power supply converter  17  accepts the available external power supply at input terminals  15  and converts it to an intermediate bus voltage. Depending upon the specific installation, the input power supply converter  17  is constructed to accept such typical supplies such as 120 Vac, 240 Vac, and 480 Vac. The input power supply converter  17  provides an intermediate bus or multiple buses of voltage levels convenient for the main computer  13  and as input power to the power supply cards  31 . 
         [0020]    Each power supply card  31  converts power from the input power supply  17  to a voltage level calculated by the cards to deliver optimal current flow through a particular protected device by traveling through a common buried anode  19 , through the soil to a particular protected structure, and then a cable back to the cathodic protection unit  11 . 
         [0021]    The system can be configured to operate in accordance with software programming permanently installed in the computer  13  such that no user interface is necessary, or be configured to accept user input during operation by an optional user interface  41 . The user interface  41  also represents the ability of the system to report operating conditions, including the voltage and current output of each output power supply card  31 , as well as any alarm conditions indicating a need for concern, such as an increase amount of current draw across a structure. Such reporting can be by digital output to a distant computer, a visual or audible alarm, or any other type of signal to operators using other currently available monitoring devices not discussed here. (In the current embodiment, output voltage of 2V or less causes an alarm.) 
         [0022]    In most electrical circuits, a power source is set to provide a set voltage for a set load. In cathodic protection systems, changing soil and moisture conditions can radically impact the load impedance, potentially causing a set voltage to deliver too little current to protect the structure, or too much current which embrittles the metal structure being protected. To counter this, the current through each return cable is monitored by an internal current meter contained within each power supply card  31  and each card independently adjusts its output voltage in real time to maintain an optimal constant current at an operator-preset level. Cathodic protection systems vary considerably on the optimal current, from a few milliamps to  15 A in some larger systems. The current embodiment can deliver  15 A to a multitude of outputs. 
         [0023]      FIG. 2  shows another possible embodiment, similar to the first described configuration, except that the system is employed to protect a single buried structure using multiple anode beds. To protect a single structure, the negative terminal of the output power cards  31  are connected to the structure to be protected, represented in  FIG. 2  as Z 4 , and the positive terminals are connected to different buried sacrificial anodes  51 ,  53  and  55 , typically spread out around the structure to be protected. This configuration efficiently protects a larger structure by providing multiple sources of current flow. 
         [0024]      FIG. 3  is a flowchart, showing the step-by-step operation of one embodiment of the invention. This flowchart assumes that a user has energized the system provides either AC or DC power to power input terminal  15 , and removal of power to the input terminals ceases operation of the invention. One rendition of the operational steps includes:
     101 ) A user begins operation of the cathodic protection by some action, such as pressing a button or turning a knob, or changing position of a switch from an “off” position to an “on” position. The switch or control can be located physically on the invention or at a hard-wired control panel located distant from the invention. At any time, an operator can cease cathodic protection by changing the position of the switch or other control back to its “off” position.     102 ) Once energized and in operation, the main computer  13  provides instructions and receives feedback from the output cards  31  regarding current and voltage conditions, including any alarm conditions, reporting those conditions to any optional user-interface  41 .     103 ) The main computer initially instructs the output cards  31  to provide an initially minimal voltage to its output terminals. The output cards determine the current through their respective terminals with an internal meter. The current through the terminals is compared to a target protective current set-point. If the terminal current is below the optimal current set-point, the card&#39;s output voltage is raised until the terminal current is within an operator-set acceptable range of the optimal set-point, OR a maximum allowable output voltage is reached.     104 ) If the terminal current is within the acceptable range of the optimal set-point, the voltage output card continues monitoring the voltage and current until the current through the terminals climbs above or below the acceptable range.     105 ) Optionally, if an unacceptably high level of current continues to flow through an output card without a current, or an acceptable current cannot be maintained for lack of sufficient input voltage, the computer  13  or card  31  sends an alarm to the user-interface.     106 ) Optionally, after sending indication of an alarm, shut down the system until the operator resets the system.   
 
         [0031]    While this invention has been described as it is currently built, the invention is not limited to the disclosed embodiments, but can be employed in various equivalent arrangements included within the spirit and scope of the claims. In particular, the change in configuration between protection of one multiple structures and one anode bed to a single structure protected using multiple anode beds is easily understood given the explanation provided to any person having ordinary skill in the art, as well as a configuration in which multiple structures are protected with multiple anode beds, though this configuration is not shown. 
         [0032]    Among the possible variants, the invention can be simplified such that the main computer need not exist. In this embodiment, the input power converter  17  provides an intermediate bus voltage that each output power supply converts to maintain a current setting that is hard-wired into the card, or set by a physical control on the card. Similarly, a more exotic embodiment might include computer control to create a rotating duty of output cards to reduce energy consumption or detect circuit flow interaction. 
         [0033]    The explanation contained throughout this specification discusses three-output units merely for discussion purposes. The invention is not limited to three outputs, but anticipates as many outputs as the input power can supply sufficient current to protect the target structures. In the current embodiment, four outputs and more are typical.