Reduction of the fault current contribution from the generator

Abstract

System (100), comprising: a generator (105) having a field winding (110) configured to generate a magnetic field in response to receiving a field excitation current, and a generator terminal (115) configured to generate an output voltage that is a function of the field excitation current applied to the field winding (110); a generator step-up transformer (120) connected to the generator terminal (115) of the generator (105) and configured to adapt the output voltage generated by the generator terminal (115) for connection to an electricity distribution network, the generator step-up transformer (120) operating at a maximum selection of taps; an excitation system (135) configured to supply the field excitation current to the field winding (110) of the generator (105); and an excitation system compensator (145) configured to regulate the field excitation current supplied by the excitation system (135) to the generator (105), wherein the regulated field excitation current causes a change in the output voltage at the generator terminal (115) as a function of the reactive power generated by the generator (105) to reduce the fault current contribution from the generator to the electricity distribution network.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to electricity distribution networks and, more particularly, to the reduction of the generator-derived fault current contribution in an electricity distribution network.

[0002] An electricity distribution network typically includes transmission lines and other interconnecting components that connect a number of electricity producers, such as generators, to electrical consumers. When a fault occurs on one of the transmission lines, the generators connected at the time of the fault generate a short-circuit 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 nominal value. This results in an increased current flowing through the electricity distribution network, known as a fault current. Switchgear, provided at various locations within the electricity distribution network, is typically used to interrupt the fault current.For the switchgear to function properly, the fault current should not exceed the switchgear's rated capacity, which is known as the fault level. As more generators are added to the electricity distribution network to supply electrical loads, the fault levels required on the network increase with increasing fault currents. If the required fault level exceeds the switchgear's rated levels, the switchgear can be upgraded or replaced to accommodate a higher fault level. This can be an expensive option for existing electricity distribution networks and can be a hindrance when it comes time for power producers to decide whether to add additional generators.

[0003] DE 199 28 711 A1 discloses a system comprising: a generator having a field winding configured to generate a magnetic field in response to receiving a field excitation current, and a generator terminal 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 connected to the generator terminal of the generator and configured to adapt the output voltage generated by the generator terminal for connection to an electricity distribution network, and an excitation system configured to supply the field excitation current to the field winding of the generator.

[0004] SPÄTH, Helmut: Electrical Machines - An Introduction to the Theory of Operating Behavior; Berlin: Springer, 1973, pp. 96-139, describes in the context of a system analysis of a three-phase synchronous machine that, when the three-phase synchronous machine is operated as a generator on a symmetrical rigid network, the electrical power delivered to the network is controlled by the internal torque of the drive machine and the power factor or reactive power is controlled by the excitation current.

[0005] From US 2009 / 0 292 488 A1 it is known to use a transformer with multiple taps on the secondary coil in an electrical power distribution network with several generators and several electrical load elements in order to change a generator output voltage. BRIEF DESCRIPTION OF THE INVENTION

[0006] According to the present invention, a system is provided. The system comprises a generator having a field winding configured to generate a magnetic field in response to receiving a field excitation current, and a generator terminal 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 connected to the generator terminal is configured to adjust the output voltage generated by the generator terminal for connection to an electricity distribution network. The generator step-up transformer operates with a maximum tap selection. The system further comprises an excitation system 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 to the generator by the excitation system.The controlled field excitation current causes a change in the output voltage at the generator terminal, which is a function of the reactive power generated by the generator, in order to reduce a fault current contribution originating from the generator to the electricity distribution network. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram illustrating a system for reducing the generator-derived fault current contribution from a power plant to a connection point with an electricity distribution network according to embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0007] Various embodiments of the present invention are directed to reducing the generator-derived fault current contribution to a connection point with an electricity distribution network. In one embodiment, a high-impedance generator receiving an excitation supply from an excitation system produces an output voltage that is supplied to the electricity distribution network via a high-impedance transformer. An excitation system compensator is used to regulate the supply of field excitation current to the generator, providing voltage control of the generator in a manner that reduces the generator-derived fault current contribution to the electricity distribution network prior to the occurrence of a fault. Even if a fault were to occur, embodiments of the present invention would allow the generator to produce a fault at a lower fault current than otherwise.

[0008] Technical effects of the various embodiments of the present invention include enabling a fault-limited grid (i.e., an electricity distribution grid) to accommodate additional power plants without requiring major expenditures on high-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 includes enabling power plants to provide reactive current-voltage (VAR) support over a wide range of basic grid system conditions of voltage level and grid strength.Other technical effects associated with the various embodiments of the present invention include providing improved critical clearance times of faults generated by underexcited operation of a generator, which aids grid code compatibility of fault ride-through criteria.

[0009] With reference to the drawings, Fig. 1 is a schematic diagram of a system 100 for reducing the generator-derived fault current contribution from a power plant to a connection point with an electricity distribution network according to an embodiment of the present invention. As shown in Fig. 1, the system 100 includes a generator 105 having a field winding 110 configured to generate a magnetic field in response to receiving a field excitation current, and a generator terminal 115 configured to generate an output voltage that is a function of the field excitation current applied to the field winding 110. To simplify the illustration of the various embodiments of the present invention, other components associated with a generator are not shown in Fig. 1. Those skilled in the art will recognize that generator 105 would include a rotor wound by field winding 110 and a rotor shaft mounted within a stator wound into an armature winding. During operation, the rotor shaft would be driven by a turbine, such as a steam turbine or a gas turbine, so that field winding 110 of the rotor generates a constant magnetic field in response to receiving an input of field excitation current. The magnetic field interacts with the stator armature winding to produce an output voltage at generator terminal 115.

[0010] Experts also recognize that in Fig. 1 not all auxiliary systems associated with generator 105 are shown. For example, those skilled in the art will recognize that generator 105 may include auxiliary systems, typically including a supply of water or other coolants supplied to generator radiators (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.

[0011] In addition, to simplify the illustration of the various embodiments of the present invention, other parts of the power plant that would operate in conjunction with the generator 105 are not shown in Fig. 1. Those skilled in the art will recognize that the power plant could, for example, include the use of steam turbines, gas turbines, and heat recovery steam generators.

[0012] With reference to Fig. 1, the system 100 further includes a generator step-up transformer 120 connected to the generator port 115 of the generator 105 via a current sensor 130 (e.g., a current transformer, a Hall-effect sensor, a shunt, a Rogowski coil, a fiber optic current sensor, etc.). The generator step-up transformer 120 serves to adapt the output voltage present at the generator port 115 to the electrical distribution network to be connected. In particular, the generator step-up transformer 120 boosts the voltage supplied by the generator port 115 to a level compatible with the electrical distribution network. During operation, the generator step-up transformer 120 operates over a wide range of operating conditions with a maximum selection of taps to provide high impedance.As used herein, a high-impedance generator step-up transformer is a transformer that has increased self-reactance (self-inductance) that is not mutually effective between windings, which may have an impedance in the range of about 15% to about 35% based on the generator current and voltage. The generator step-up transformer 120 may utilize a no-load tap changer or an on-load tap changer to obtain maximum tap selection over a wide range of operating conditions. As used herein, maximum tap selection over a wide range of operating conditions encompasses a range of about 1.05 to about 1.20 per unit on the high-voltage side of the transformer.

[0013] Utilizing the maximum tap selection of the generator step-up transformer 120 results in an increased turns ratio of the transformer. An increased turns ratio enables the 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 the generator step-up transformer 120 is defined as follows: Vt:Vgrid*ntap, where Vt is the voltage at the output terminal of generator 105, Vgrid is the nominal voltage supplied to the electricity distribution grid, and ntap is an off-rated tap range that defines an open-circuit voltage of the electricity distribution grid. The use of the maximum tap and turns ratio to enable a reduction in the fault current contribution from the generator is explained in more detail below.

[0014] Fig. 1 shows that the system 100 further includes an excitation system 135 that serves to generate an excitation supply used to generate direct current (DC) to the generator 105. In particular, a field excitation supply 140 utilizes the excitation supply generated by the excitation system 135 to inject DC current into the field winding 110 of the generator 105. As mentioned above, injecting the DC or field excitation current into the field winding of the generator enables the generator to generate an output voltage at the generator terminal 115. The excitation system 135 may be any commercially available exciter capable of providing an excitation supply used to generate DC current. In one embodiment, the excitation system 135 may be an EX2100 excitation system supplied by the General Electric Company.In one embodiment, the 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 frequency conversion devices, may be used to supply DC power to the generator 105.

[0015] The system 100 further includes an excitation system compensator 140 connected to the current sensor 130 and the excitation system 135. The excitation system compensator 140 is configured to regulate the field excitation current supplied from the excitation system 135 to the generator 105 via the field excitation supply 140. In one embodiment, the regulated field excitation current causes a change in the output voltage at the generator terminal 115 as a function of the reactive power generated by the generator 105. As explained below, this helps reduce the fault current contribution from the generator to the electricity distribution grid. Although Fig. While Figure 1 shows that the excitation system compensator 140 is a separate component from the excitation system 135, those skilled in the art will recognize that it may be located as functionality within the excitation system 135 or as part of a controller used to control the operation of the generator 105 and the excitation system 135. A MARK VIe controller supplied by General Electric Company is an example of a controller that may be used to control the operation of the generator 105 and the excitation system 135, and that could be used to implement the control strategy associated with the balancing functionality provided by the excitation system compensator 140.

[0016] Regardless of its location within system 100, excitation system compensator 140 may be implemented as a fully hardware embodiment, a fully software embodiment, or an embodiment that includes both hardware and software elements. In one embodiment, the processing functions performed by excitation system compensator 140 may be implemented in software, including, but not limited to, firmware, resident software, microcode, etc.

[0017] Additionally, the processing functions performed by the excitation system compensator 140 may take the form of a computer program product accessible from a computer-usable or computer-readable medium that provides program code for use by or in connection with a computer or instruction execution system (e.g., processing units). For the purposes of this description, a computer-usable or computer-readable medium may 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.

[0018] The computer-readable medium may be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, device, or apparatus. Examples of a computer-readable medium include semiconductor or solid-state memory, random access memory (RAM), read-only memory (ROM), a hard disk, and an optical disk. Current examples of optical disks include compact disc read-only memory (CD-ROM), compact disc read / write memory (CD-R / W), and digital video discs (DVDs).

[0019] During operation, the excitation system compensator 145 is configured to determine a compensation voltage for use by the excitation system 135 to regulate the supply of field excitation current to the generator 105. This effects voltage control of the generator 105 to reduce the fault current contribution from the generator to the electricity distribution grid. In particular, the compensation voltage is derived as a function of the output voltage at the generator terminal 115, the current generated by the generator as measured by the current sensor 130, and a predetermined impedance compensation proportional to the sub-transient reactance 125 of the generator 105. As is well known, the sub-transient reactance is the inherent impedance associated with the generator 105.

[0020] Although the Fig.1 shows a process block for sub-transient reactance 125, those skilled in the art will recognize that this parameter has already been determined and that its representation in the figure is intended to illustrate that there is an inherent impedance associated with generator 105, which is subsequently used 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 electricity 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 at the point of fault. Thus, the generator's internal voltage and the generator impedance determine the current that flows when the generator's terminals are short-circuited.

[0021] The above compensation voltage determined by the excitation system compensator 145 is derived according to the following equation: Vc=Vt+Z*Ig, where Vc is the compensation voltage, Vt is the voltage at the generator terminal 115, Z is the predetermined impedance compensation associated with the generator 105, and Ig is the current generated by the generator 105 as measured by the current sensor 130.

[0022] In one embodiment, the excitation system 135 uses the compensation voltage Vc to adjust the excitation supply provided by the generator 105 via the field excitation supply 140 to cause a change in the output voltage at the generator terminal 115. In particular, the change in output voltage caused by the combination of the tap selection of the generator step-up transformer 120 and the use of the excitation system compensator 145 will be a function of the reactive power generated by the generator 105. In one embodiment, the output voltage at the generator terminal 115 will decrease as the reactive power generated by the generator 105 increases.This type of compensation is commonly used to allow parallel generators on the same bus to share a VAR load while maintaining stable voltage control of the individual generators to automatically respond to power grid demands. For the purposes of the various embodiments of the present invention, this compensation enables a single generator to have stable voltage control and minimize the generator's internal voltage to reduce the fault current contribution and open-circuit voltage from the generator. Essentially, this allows the generator step-up transformer 120 to operate at maximum tap over a wide range of operating conditions while providing grid-friendly voltage control and reactive power support.Furthermore, by setting the compensation relative to the sub-transient reactance of the generator 105, the system 100 can regulate the voltage at a location halfway inside the generator 105.

[0023] The combination of the compensation provided by the excitation system compensator 145 and a high-impedance generator step-up transformer 120 operating with a maximum tap selection enables the system 100 to effectively increase the impedance of the power plant from the perspective of the connection point to the electricity distribution grid. As a result, in one embodiment, the compensation may reduce an internal voltage of the generator in response to an inductive load factor condition. In this embodiment, the reduced internal voltage compensates for the maximum tap selection of the generator step-up transformer to reduce the fault current contribution from the generator to the electricity distribution grid. In another embodiment, the compensation may boost an internal voltage of the generator in response to a capacitive load factor condition.In this embodiment, the boosted internal voltage compensates for the maximum tap selection of the generator step-up transformer. Increasing the internal voltage at a capacitive load factor condition enables increased critical clearance time for faults generated from under-excited operation of a generator, which aids in grid code compliance with fault survival criteria. In either embodiment, the combination of the compensation provided by the excitation system compensator 145 and the maximum tap selection of the generator step-up transformer 120 allows the generator-derived fault current contribution at the connection point to the electricity distribution grid to be reduced, which would be on the high-voltage side of the generator step-up transformer connecting to the grid.Furthermore, the configuration of the excitation system compensator 145 and the maximum tap selection of the generator step-up transformer 120 enables the system 100 to automatically reduce a generator-derived fault current contribution to the electricity distribution grid prior to the occurrence of a fault. Should a fault occur, the combination of the compensation provided by the excitation system compensator 145 and the maximum tap selection of the generator step-up transformer 120 would allow the system 100 to generate a fault at a lower fault current.

[0024] An approach to reducing the fault current contribution from a generator is disclosed. In one aspect, the automatic excitation control of a generator 105 is coordinated with a generator step-up transformer 120 operating on a maximized tap selection to reduce the generator fault current contribution to an electricity distribution network. LIST OF REFERENCE SYMBOLS: 100 System for reducing the fault current contribution from the generator 105 Generator 110 Field winding 115 Generator connection 120 Generator step-up transformer 125 sub-transient reactance 130 Current sensor 135 Excitation system 140 field excitation supply 145 Excitation system compensator

Claims

[1] System (100), comprising: a generator (105) having a field winding (110) configured to generate a magnetic field in response to receiving a field excitation current, and a generator terminal (115) configured to generate an output voltage that is a function of the field excitation current applied to the field winding (110); a generator step-up transformer (120) connected to the generator terminal (115) of the generator (105) and configured to adapt the output voltage generated by the generator terminal (115) for connection to an electricity distribution network, the generator step-up transformer (120) operating at a maximum selection of taps; an excitation system (135) configured to supply the field excitation current to the field winding (110) of the generator (105); and an excitation system compensator (145) configured to regulate the field excitation current supplied by the excitation system (135) to the generator (105), wherein the regulated field excitation current causes a change in the output voltage at the generator terminal (115) as a function of the reactive power generated by the generator (105) to reduce the fault current contribution from the generator to the electricity distribution network. [2] The system (100) of claim 1, wherein the excitation system compensator (145) is configured to determine a compensation voltage for use by the excitation system (135) to regulate the supply of field excitation current to the generator (105) that provides voltage control of the generator (105) to reduce the fault current contribution from the generator to the electricity distribution network. [3] The system (100) of claim 2, wherein the compensation voltage is derived as a function of the output voltage at the generator terminal (115), the current generated by the generator (105), and a predetermined impedance compensation proportional to a sub-transient reactance of the generator (105). [4] The system (100) of claim 3, wherein the compensation voltage is derived according to the following equation: Vc=Vt+Z*Ig, where Vc is the compensation voltage, Vt is the voltage at the generator terminal, Z is the predetermined impedance compensation and Ig is the current generated by the generator (105). [5] The system (100) of claim 2, wherein the compensation voltage-induced voltage control of the generator (105) comprises reducing an internal voltage of the generator (105) in response to an inductive load factor condition, the reduced internal voltage compensating for the maximum tap selection of the generator step-up transformer (120) to reduce the fault current contribution from the generator to the electricity distribution network. [6] The system (100) of claim 2, wherein the compensation voltage-induced voltage control of the generator (105) comprises boosting an internal voltage of the generator (105) in response to a capacitive load factor condition, wherein boosting the internal voltage of the generator (105) during a capacitive load factor condition enables increased critical clearance time of faults generated by under-excited operation of the generator. [7] The system (100) of claim 6, wherein the boosted internal voltage compensates for the maximum tap selection of the generator step-up transformer (120) to reduce the fault current contribution from the generator to the electricity distribution network.

Images