Abstract:
An approach for reduction in generator-sourced fault current contribution is disclosed. In one aspect, automatic excitation control of a generator is coordinated with a generator step-up transformer operating on maximized tap selection to reduce generator fault current contribution to an electrical power distribution network.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to electrical power distribution networks and more particularly to reducing generator-sourced fault current contribution in an electrical power distribution network. 
     An electrical power distribution network typically includes transmission lines and other connection components that connect a number of electric power producers such as generators to electrical loads. When a fault occurs on one of the transmission lines, the generators that are connected at the time of the fault create a short current fault contribution. Typically, in the event of a fault, each generator will tend to increase its output current in an attempt to maintain the output voltage at a rated value. This results in increased current flowing over the electrical power distribution network, which is referred to as fault current. Switchgear provided at various locations of the electrical power distribution network is typically used to interrupt the fault current. In order for the switchgear to function properly the fault current should not be above the rated capacity of the switchgear, which is referred to as fault level. As more generators are added to the electrical power distribution network to serve electrical loads, fault levels required on the network increase by increasing fault currents. When the required fault level exceeds the rated levels of the switchgear, the switchgear can be upgraded or replaced to allow a higher fault level. This may be an expensive option for established electrical power distribution networks and may be an impediment when it comes time for power producers to decide whether to add additional generators. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one aspect of the present invention, a system is provided. The system comprises a generator including field winding that is configured to generate a magnetic field in response to receiving a field excitation current and a generator terminal that is configured to generate an output voltage that is a function of the field excitation current applied to the field winding. A generator step-up transformer, coupled to the generator terminal of the generator, is configured to adjust the output voltage generated from the generator terminal for interconnection with an electrical power distribution network. The generator step-up transformer operates on a maximum tap selection. The system further comprises an excitation system that is configured to supply field excitation current to the field winding of the generator. An excitation system compensator is configured to regulate the field excitation current supplied by the excitation system to the generator. The regulated field excitation current effectuates a change in the output voltage at the generator terminal that is a function of reactive power generated from the generator to reduce generator-sourced fault current contribution to the electrical power distribution network. 
     In a second aspect of the present invention, a system for reducing fault current contribution from a power plant to a point of interconnection with an electrical power distribution network is disclosed. In this aspect of the present invention, the system comprises a generator including field winding that is configured to generate a magnetic field in response to receiving a field excitation current and a generator terminal that is configured to generate an output voltage that is a function of the field excitation current applied to the field winding. A generator step-up transformer, coupled to the generator terminal of the generator, is configured to adjust the output voltage generated from the generator terminal for interconnection with an electrical power distribution network. The generator step-up transformer operates on a maximum tap selection. An excitation system is configured to supply the field excitation current to the field winding of the generator. An excitation system compensator is configured to determine a compensation voltage for use by the excitation system to regulate the supply of the field excitation current to the generator that effectuates voltage control of the generator to reduce generator-sourced fault current contribution to the electrical power distribution network during a normal operating mode of the generator before occurrence of a fault. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a system for reducing generator-sourced fault current contribution from a power plant to a point of interconnection with an electrical power distribution network according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments of the present invention are directed to reducing generator-sourced fault current contribution to a point of interconnection with an electrical power distribution network. In one embodiment, a high impedance generator that receives an excitation supply from an excitation system generates an output voltage that is supplied to the electrical power distribution network via a high impedance transformer. An excitation system compensator is used to regulate the supply of field excitation current to the generator that causes voltage control of the generator in a manner that reduces generator-sourced fault current contribution to the electrical power distribution network before occurrence of a fault. Even if a fault were to occur, embodiments of the present invention would enable the generator to produce a fault at a lower fault current than it otherwise would. 
     Technical effects of the various embodiments of the present invention include enabling a fault-constrained grid (i.e., electrical power distribution network) to accommodate additional power plants without requiring major expenditures on higher voltage transmission lines due to the safety limitation of switchgear fault current ratings. Another technical effect associated with the various embodiments of the present invention include enabling power plants to provide volt-ampere-reactive (VAR) support over a wide range of grid code mandated system conditions of voltage level and system strength. Other technical effects with the various embodiments of the present invention include providing enhanced critical clearing times of faults generated from under-excited operation of a generator, which helps with grid code compliance of fault ride-through criteria. 
     Referring to the drawings,  FIG. 1  is a schematic diagram illustrating a system  100  for reducing generator-sourced fault current contribution from a power plant to a point of interconnection with an electrical power distribution network according to one embodiment of the present invention. As shown in  FIG. 1 , system  100  includes a generator  105  having field winding  110  that is configured to generate a magnetic field in response to receiving a field excitation current and a generator terminal  115  that is configured to generate an output voltage that is a function of the field excitation current applied to field winding  110 . For ease of illustration of the various embodiments of the present invention, other components that are associated with a generator are not shown in  FIG. 1 . Those skilled in the art will recognize that generator  105  would have a rotor wrapped with field winding  110  and a rotor shaft mounted within a stator wrapped in armature winding. In operation, the rotor shaft would be driven by a turbine such as a steam turbine or gas turbine, so that field winding  110  of the rotor produces a constant magnetic field in response to receiving a supply of field excitation current. The magnetic field interacts with the armature winding of the stator to generate an output voltage at generator terminal  115 . 
     Those skilled in the art will also recognize that not all auxiliary systems associated with generator  105  are illustrated in  FIG. 1 . For example, those skilled in the art will appreciate that generator  105  can have auxiliary systems that typically include a supply of water or other coolants provided to the generator coolers (heat exchangers), a stator winding cooling system, a hydrogen supply and control system for generators using hydrogen as the primary coolant, and bearing lubrication systems. 
     Also, for ease of illustration of the various embodiments of the present invention, other parts of the power plant that would work in conjunction with generator  105  are not shown in  FIG. 1 . Those skilled in the art will appreciate that the power plant could include, for example, the use of steam turbines, gas turbines, heat recovery steam generators. 
     Referring back to  FIG. 1 , system  100  further includes a generator step-up transformer  120  coupled to generator terminal  115  of generator  105  via a current sensor  130  (e.g., current transformer, Hall Effect sensor, shunt, Rogowski coil, fiber optic current sensor, etc.). Generator step-up transformer  120  is configured to adjust the output voltage generated from generator terminal  115  for interconnection with the electrical power distribution network. In particular, generator step-up transformer  120  raises the voltage provided from generator terminal  115  to a level that is compatible with the electrical power distribution network. In operation, generator step-up transformer  120  operates on a maximum tap selection over a wide range of operating conditions to provide a high impedance. As used herein, a high impedance generator step-up transformer is a transformer that has increased self reactance not mutually coupled between the windings that can have a range of about 15% to about 35% impedance on the generator volt-ampere base. Generator step-up transformer  120  can utilize a no-load tap changer or an on-load tap changer to obtain a maximum tap selection over a wide range of operation conditions. As used herein, a maximum tap selection over a wide range of operation conditions comprises a range of about 1.05 to about 1.20 per unit on the high voltage side of the transformer. 
     The use of the maximum tap selection of generator step-up transformer  120  results in an increased turns ratio of the transformer. An increased turns ratio enables generator step-up transformer  120  to reduce the fault current contribution on the high voltage side of the transformer. In one embodiment, the turns ratio of generator step-up transformer  120  is defined as:
 
 Vt: V grid* n tap,  (1)
 
wherein Vt is the voltage at the output terminal of generator  105 , Vgrid is the nominal voltage supplied to the electrical power distribution network and ntap is an off-nominal tap range defining an open circuit voltage of the electrical power distribution network. The use of the maximum tap and turns ratio to facilitate a reduction in generator-sourced fault current contribution is discussed below in more detail.
 
       FIG. 1  shows that system  100  further includes an excitation system  135  that is configured to generate an excitation supply used for generating direct current (DC) power to generator  105 . In particular, a field excitation supply  140  uses the excitation supply generated by excitation system  135  to inject direct current into field winding  110  of generator  105 . As mentioned above, injection of the direct current or field excitation current to the field winding of the generator facilitates the generator&#39;s ability to generate an output voltage at generator terminal  115 . Excitation system  135  may be any commercially available exciter that can provide an excitation supply used for generating DC power. In one embodiment, excitation system  135  may be an EX2100 excitation system provided by the General Electric Company. In one embodiment, field excitation supply  140  may be a silicon-controlled rectifier (SCR) bridge. Those skilled in the art will recognize that other devices such as rotating or brushless AC to DC rectifiers, batteries, or other static power frequency conversion equipment can be used to inject direct current in generator  105 . 
     System  100  further includes an excitation system compensator  140  that is coupled to current sensor  130  and excitation system  135 . Excitation system compensator  140  is configured to regulate the field excitation current supplied by excitation system  135  to generator  105  via field excitation supply  140 . In one embodiment, the regulated field excitation current effectuates a change in the output voltage at generator terminal  115  as a function of the reactive power generated from generator  105 . As explained below, this assists in reducing generator-sourced fault current contribution to the electrical power distribution network. Although,  FIG. 1  shows that excitation system compensator  140  is a separate component apart from excitation system  135 , those skilled in the art will recognize that it can reside as a functionality within excitation system  135  or as part of a controller used to control operation of generator  105  and excitation system  135 . A MARK VIe controller provided by the General Electric Company is one example of a controller that can be used to control operation of generator  105  and excitation system  135 , and that could be used to implement the control strategy associated with the compensatory functionalities provided by excitation system compensator  140 . 
     Regardless of its location within system  100 , excitation system compensator  140  can be implemented in the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment containing both hardware and software elements. In one embodiment, the processing functions performed by excitation system compensator  140  may be implemented in software, which includes but is not limited to firmware, resident software, microcode, etc. 
     Furthermore, the processing functions performed by excitation system compensator  140  can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system (e.g., processing units). For the purposes of this description, a computer-usable or computer readable medium can be any computer readable medium that can contain or store the program for use by or in connection with the computer or instruction execution system. 
     The computer readable medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus or device). Examples of a computer-readable medium include a semiconductor or solid state memory, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include a compact disk-read only memory (CD-ROM), a compact disk-read/write (CD-R/W) and a digital video disc (DVD). 
     In operation, excitation system compensator  145  is configured to determine a compensation voltage for use by excitation system  135  to regulate the supply of the field excitation current to generator  105 . This effectuates voltage control of generator  105  to reduce generator-sourced fault current contribution to the electrical power distribution network. In particular, the compensation voltage is derived as a function of the output voltage at generator terminal  115 , current produced by the generator that is measured by current sensor  130 , and a predetermined impedance compensation that is proportional to the sub-transient reactance  125  of generator  105 . As is well-known, sub-transient reactance is the inherent impedance associated with generator  105 . 
     Although  FIG. 1  shows a processing block for sub-transient reactance  125 , those skilled in the art will recognize that this parameter has already been determined and that its depiction in the FIGURE is to illustrate that there is inherent impedance associated with generator  105  that is subsequently utilized by excitation system compensator  145 . As is known in the art, the sub-transient reactance of a generator can be used to calculate the flow of short circuit current. When a short circuit fault occurs in an electrical power distribution network the fault current is a function of the internal voltage of the connected machines (e.g., generators), the impedance of the machine and the impedance to the point of the fault. Consequently, the internal voltage of the generator and generator impedance determines the current that flows when the terminals of the generator are shorted. 
     The above-noted compensation voltage determined by excitation system compensator  145  is derived in accordance with the following equation:
 
 Vc=Vt+Z*Ig,   (2)
 
wherein Vc is the compensation voltage, Vt is the voltage at generator terminal  115 , Z is the predetermined impedance compensation associated with generator  105  and Ig is the current produced by generator  105  as measured by current sensor  130 .
 
     In one embodiment, excitation system  135  uses the compensation voltage Vc to adjust the excitation supply provided to generator  105  via field excitation supply  140  in order to effectuate a change in the output voltage at generator terminal  115 . In particular, the change in the output voltage effectuated by the combination of the tap selection of generator step-up transformer  120  and the use of excitation system compensator  145  will be a function of the reactive power generated from generator  105 . In one embodiment, the output voltage at generator terminal  115  will decrease as the reactive power generated from generator  105  increases. This kind of compensation is commonly used to allow parallel generators at the same bus to share VAR loading while maintaining stable voltage control of individual generators to respond automatically to power system demands. For purposes of the various embodiments of the present invention, this compensation allows a single generator to have stable voltage control and minimize internal voltage of the generator to reduce the generator-sourced fault current contribution and open-circuit voltage. Essentially, this allows generator step-up transformer  120  operate on the maximum tap over a wide range of operating conditions, while providing grid friendly voltage control and reactive power support. Furthermore, by determining the compensation in proportion to the sub-transient reactance of generator  105 , system  100  can regulate the voltage at a location that is part-way internal to generator  105 . 
     The combination of the compensation provided by excitation system compensator  145  and utilizing a high impedance generator step-up transformer  120  operating with a maximum tap selection enables system  100  to effectively increase the impedance of the power plant as viewed from the point of interconnection with the electrical power distribution network. As a result, in one embodiment, the compensation can reduce an internal voltage of the generator in response to a lagging power factor condition. In this embodiment, the reduced internal voltage compensates for the maximum tap selection of the generator step-up transformer to reduce generator-sourced fault current contribution to the electrical power distribution network. In another embodiment, the compensation can boost an internal voltage of the generator in response to a leading power factor condition. In this embodiment, the boosted internal voltage compensates for the maximum tap selection of the generator step-up transformer. Boosting internal voltage at a leading power factor condition facilitates an increased critical clearing time of faults generated from an under-excited operation of a generator, which helps with grid code compliance of fault ride-through criteria. In either embodiment, the combination of the compensation provided by excitation system compensator  145  and the maximum tap selection of generator step-up transformer  120  facilitate reduced generator-sourced fault current contribution at the point of interconnection with the electrical power distribution network, which would be on the high voltage side of the generator step-up transformer that connects with the network. In addition, the configuration of excitation system compensator  145  and the maximum tap selection of generator step-up transformer  120  enables system  100  to automatically reduce generator-sourced fault current contribution to the electrical power distribution network before occurrence of a fault. If a fault were to occur, the combination of the compensation provided by excitation system compensator  145  and the maximum tap selection of generator step-up transformer  120  would cause system  100  to generate a fault at a lower fault current. 
     While the disclosure has been particularly shown and described in conjunction with a preferred embodiment thereof, it will be appreciated that variations and modifications will occur to those skilled in the art. Therefore, it is to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the disclosure.