Claims:1. A method for enhancing a fault current in a power generation system comprising a doubly-fed induction generator (DFIG) having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a point of common coupling (PCC), wherein the rotor-side converter and the line-side converter are coupled to each other via a direct current (DC) link, and wherein the stator winding is configured to be coupled to the PCC, the method comprising:
detecting an electrical fault in the power generation system;
controlling, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding via the rotor-side converter; and
generating at least a portion of the fault current by the DFIG at the stator winding based at least partially on the electrical current supplied to the rotor winding.

2. The method as claimed in claim 1, wherein the power generation system further comprises a prime mover operatively coupled to the DFIG, wherein the prime mover comprises at least one of an engine, a wind turbine, a hydro turbine, a gas turbine, or a compressor.

3. The method as claimed in claim 1, further comprising, in response to the detection of the electrical fault, detecting whether the DFIG is operational.

4. The method as claimed in claim 3, further comprising, if the DFIG is operational, disabling an operation of the line-side converter, wherein disabling the operation of the line-side converter comprises discontinuing one or more control signals to the line-side converter.

5. The method as claimed in claim 3, further comprising, if the DFIG is operational, disconnecting the line-side converter from the PCC via a switching unit.

6. The method as claimed in claim 3, wherein, if the DFIG is operational, controlling the supply of the electrical current comprises controlling a frequency of the electrical current supplied to the rotor winding via the rotor-side converter, wherein the frequency of the electrical current supplied to the rotor winding is determined based on an operating speed of the DFIG.

7. The method as claimed in claim 6, wherein at least the portion of the fault current is generated by the DFIG based at least partially on the frequency of the electrical current supplied to the rotor winding.

8. The method as claimed in claim 3, further comprising, if the DFIG is operational, supplying an additional portion of the fault current via the line-side converter.

9. The method as claimed in claim 3, further comprising, if the DFIG is not operational, connecting the stator winding to the PCC via a switching unit.

10. The method as claimed in claim 3, further comprising, if the DFIG is not operational, operating the DFIG as a transformer.

11. The method as claimed in claim 3, further comprising, if the DFIG is not operational, supplying an additional portion of the fault current via the line-side converter.

12. The method as claimed in claim 1, further comprising supplying a DC power to the DC-link from at least one power source electrically coupled to the DC-link, wherein the at least one power source comprises an energy storage device and an auxiliary power source, and wherein the auxiliary power source comprises a photovoltaic (PV) power source, a fuel cell, a renewable energy based power generator, a non-renewable energy based power generator, or combinations thereof.

13. The method as claimed in claim 12, further comprising determining if a state of charge of the energy storage device is less than a predetermined charged level.

14. The method as claimed in claim 13, further comprising charging the energy storage device using an electrical power generated by the auxiliary power source if the state of charge of the energy storage device is less than the predetermined charge level.

15. The method as claimed in claim 13, further comprising disconnecting the auxiliary power source from the DC-link if the state of charge of the energy storage device is not less than the predetermined charge level.

16. The method as claimed in claim 1, wherein controlling the supply of the electrical current supplied to the rotor winding via the rotor-side converter comprises varying at least one of a magnitude and a frequency of the electrical current by the rotor-side converter.

17. The method as claimed in claim 16, further comprising determining the frequency of the electrical current supplied to the rotor winding based on an operating speed of the DFIG.

18. A control sub-system for enhancing a fault current in a power generation system comprising a doubly-fed induction generator (DFIG) having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a point of common coupling (PCC), wherein the rotor-side converter and the line-side converter are coupled to each other via a direct current (DC) link, and wherein the stator winding is configured to be coupled to the PCC, the control sub-system comprising:
one or more sensors disposed in the power generation system and configured to generate one or more electrical signals;
a controller operatively coupled to one or more of the rotor-side converter, the line-side converter, and the one or more sensors, wherein the controller is configured to:
detect an electrical fault in the power generation system based on the one or more electrical signals received from the one or more sensors;
control, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding via the rotor-side converter; and
generate at least a portion of the fault current by the DFIG at the stator winding based at least partially on the electrical current supplied to the rotor winding.

19. The control sub-system as claimed in claim 18, further comprising a switching unit disposed between the stator winding and the PCC.

20. The control sub-system as claimed in claim 19, wherein if the DFIG is not operational, the controller is further configured to:
connect the stator winding to the PCC via the switching unit;
operate the DFIG as a transformer; and
facilitate a supply of an additional portion of the fault current to the PCC via the line-side converter.
, Description:BACKGROUND
[0001] Embodiments of the present specification generally relate to a power generation system and in particular, to method and system for enhancing fault current in a power generation system.
[0002] Some currently available hybrid power generation systems employ a doubly-fed induction generator (DFIG), power sources such as a prime mover and an auxiliary power source (e.g., photovoltaic (PV) power source). In some configurations of a power generation system, the auxiliary power source is coupled to the DFIG via one or more power converter(s). During operation of the power generation system, electrical power may be generated by one or both of the DFIG and the auxiliary power source. The electrical power thus generated may be supplied to electrical loads and/or an electric grid coupled to the power generation system.
[0003] During operation of such DFIG based power generation system, an electrical fault may occur within the power generation system or at the electrical load. Typically, the electrical fault causes a short-circuit that demands a fault current which is significantly higher than the current which flows during a normal operation of the power generation system. It may be desirable to supply more current to ensure appropriate management of downstream protective devices.
[0004] In conventional power generation systems that employ the DFIG and partial power converters coupled to a rotor winding of the DFIG, electrical currents within these power generation systems and/or electrical current supplied to electrical loads is limited due to parameters such as a slip value of the DFIG, an operating speed of the DFIG, rated power of the partial power converters, or combinations thereof. Consequently, if an electrical fault occurs in the conventional power generation systems, these conventional power generation systems may not be able to supply more fault current. Consequently, management of the electrical fault becomes a challenging task in the conventional power generation system. Moreover, increased flow of a fault current through the partial power converters may damage the partial power converters used in the conventional power generation systems.
BRIEF DESCRIPTION
[0005] In accordance with one embodiment of the present specification, a method for enhancing a fault current in a power generation system is presented. The power generation system includes power generation system having a doubly-fed induction generator (DFIG) having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a point of common coupling (PCC). The rotor-side converter and the line-side converter are coupled to each other via a direct current (DC) link and the stator winding is coupled to the PCC. The method includes detecting an electrical fault in the power generation system. The method further includes controlling, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding via the rotor-side converter. Moreover, the method includes generating at least a portion of the fault current by the DFIG at the stator winding based at least partially on the electrical current supplied to the rotor winding.
[0006] In accordance with one embodiment of the present specification, a method for enhancing a fault current in a power generation system is presented. The power generation system includes power generation system having a DFIG having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a PCC. The rotor-side converter and the line-side converter are coupled to each other via a DC-link and the stator winding is coupled to the PCC. The method includes detecting an electrical fault in the power generation system. Further, the method includes controlling, in response to the detection of the electrical fault, a frequency of an electrical current supplied to the rotor winding via the rotor-side converter, wherein the frequency of the electrical current supplied to the rotor winding is determined based on an operating speed of the DFIG. Moreover, the method includes generating at least a portion of the fault current by the DFIG at the stator winding based at least partially on the frequency of the electrical current supplied to the rotor winding.
[0007] In accordance with one embodiment of the present specification, a control system for enhancing a fault current in a power generation system is presented. The power generation system includes power generation system having a DFIG having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a PCC. The rotor-side converter and the line-side converter are coupled to each other via a DC-link and the stator winding is coupled to the PCC. The control sub-system includes one or more sensors disposed in the power generation system and configured to generate one or more electrical signals. The control sub-system further includes a controller operatively coupled to one or more of the rotor-side converter, the line-side converter, and the one or more sensors. The controller is configured to detect an electrical fault in the power generation system based on the one or more electrical signals received from the one or more sensors. The controller is further configured to control, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding via the rotor-side converter. Moreover, the controller is configured to generate at least a portion of the fault current by the DFIG at the stator winding based at least partially on the electrical current supplied to the rotor winding.
DRAWINGS
[0008] These and other features, aspects, and advantages of the present specification will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
[0009] FIG. 1 is a block diagram representation of a power generation system, in accordance with one embodiment of the present specification; and
[0010] FIG. 2 is a flow diagram of a method for enhancing a fault current in the power generation system of FIG. 1, in accordance with one embodiment of the present specification.
DETAILED DESCRIPTION
[0011] As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances, the modified term may sometimes not be appropriate, capable, or suitable.
[0012] In accordance with some embodiments of the present specification, a method for enhancing a fault current in a power generation system is presented. The power generation system includes a doubly-fed induction generator (DFIG) having a stator winding and a rotor winding, a rotor-side converter coupled to the rotor winding, a line-side converter coupled to a point of common coupling (PCC). The rotor-side converter and the line-side converter are coupled to each other via a direct current (DC) link and the stator winding is coupled to the PCC. The method includes detecting an electrical fault in the power generation system. The method further includes controlling, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding via the rotor-side converter. Moreover, the method includes generating at least a portion of the fault current using the DFIG, at the stator winding based at least partially based on the electrical current supplied to the rotor winding.
[0013] FIG. 1 is a block diagram representation of a power generation system (100) in accordance with one embodiment of the present specification. In some embodiments, the power generation system (100) includes one or more of a prime mover (102), a doubly-fed induction generator (DFIG) (104), a rotor-side converter (106), a line-side converter (108), a power source (110), and a control sub-system (112). The DFIG (104) is operatively coupled to the prime mover (102). The rotor-side converter (106) is disposed between the line-side converter (108) and the DFIG (104). The line-side converter (108) and the DFIG (104) are also coupled to a point of common coupling (PCC) (114) as shown in FIG. 1. The line-side converter (108) is coupled to the PCC (114) via a link (116). A stator winding (described later in FIG. 1) of the DFIG (104) is configured to be coupled to the PCC (114) via a link (118). Each of the links (116, 118) may be a multi-phase link, for example, a three-phase electrical link as shown in FIG. 1. In some embodiments, the power generation system (100) may also include a transformer (120) coupled to the PCC (114).
[0014] In some embodiments, the power generation system (100) is an islanded power generation system, sometimes also referred to as an isolated power generation system which not connected to an electric grid (not shown). By way of example, a power generation system such as the power generation system (100) may be deployed where connection to the electric grid is not desired or the electric grid is not available. The power generation system (100) may be coupled to an electrical load (122). The electrical load 122 may include one or more devices/equipment that consumes electricity when operated. In certain embodiments, the power generation system (100) may be connected to the electric grid. The power generation system (100) may be coupled to the electrical load (122) and/or the electric grid via the transformer (120).
[0015] The power generation system (100) may be configured to generate an alternating current (AC) electrical power and supply the AC electrical power from an output power port (124) of the power generation system (100). The AC electrical power at the output power port (124) may be a single phase or multi-phase, such as a three-phase electrical power. In some embodiments, the control sub-system (112) may be a part of the power generation system (100) to enhance electrical power generation by the power generation system (100). In certain embodiments, the control sub-system (112) may control the electrical power generation by the power generation system (100) such that predefined Balance of Plant (BoP) limits of the transformer (120) are not violated. The BoP limits include at least one of a maximum active power limit of the transformer (120), a maximum apparent power limit of the transformer (120), a maximum apparent current limit of the transformer (120), a maximum temperature limit of the transformer (120).
[0016] The prime mover (102) is configured to aid in imparting a rotational motion to rotary element (e.g., a rotor) of the DFIG (104). Non-limiting examples of the prime mover (102) may include a wind turbine (see FIG. 4), a tidal turbine, a hydro turbine, an engine that may be operable at variable speeds, a gas turbine, a compressor, or combinations thereof. Hereinafter, in the description, the prime mover (102) is described as an engine operable at variable speeds, alternatively referred to as a variable speed engine.
[0017] The DFIG (104) includes a stator (126), a stator winding (128) wound on the stator (126), a rotor (130), and a rotor winding (132) wound on the rotor (130). In some embodiments, both the stator winding (128) and the rotor winding (132) may be multi-phase winding such as a three-phase winding. The DFIG (104) is mechanically coupled to the prime mover (102). For example, the rotor (130) of the DFIG (104) is mechanically coupled to a rotary element of the prime mover (102) via a shaft (123) such that rotations of the rotary element of the prime mover (102) cause rotations of the rotor (130) of the DFIG (104).
[0018] The rotor (130) of the DFIG (104) is operated at a rotational speed which may be a synchronous speed, a sub-synchronous speed, or a super-synchronous speed depending on the rotational speed of the rotary element of the prime mover (102). In one example, the synchronous speed of the rotor (130) may be defined using equation (1).
Equation (1)
[0019] In equation (1), represents the synchronous speed of the rotor (130), represents poles in the rotor (130), and represents a frequency of a stator voltage. Accordingly, a sub-synchronous speed of the rotor (130) may be defined as any speed that is lower than the synchronous speed of the rotor (130). Similarly, a super-synchronous speed of the rotor (130) may be defined as any speed that is higher than the synchronous speed of the rotor (130).
[0020] During operation, the DFIG (104) is configured to generate the electrical power at the stator winding (128) depending on the rotational speed of the rotor (130). The electrical power that is generated at the stator winding (128) is hereinafter alternatively referred to as a “stator power.” Further, the DFIG (104) is configured to generate or absorb electrical power at the rotor winding (132) depending on the rotational speed of the rotor (130). For example, the DFIG (104) is configured to generate electrical power at the rotor winding (132) when the rotor (130) is operated at a super-synchronous speed. The DFIG (104) is configured to absorb the electrical power at the rotor winding (132) when the rotor (130) is operated at a sub-synchronous speed. The electrical power that is generated or absorbed at the rotor winding (132) is hereinafter alternatively referred to as a “slip power.” The magnitude of the slip power is dependent on a slip value of the DFIG (104). In one embodiment, the slip value S may be determined using equation (2).
Equation (2)
where represents the synchronous speed of the rotor (130) and represents revolutions per minute (rpm) of the rotor (130).
[0021] The rotor-side converter (106) is electrically coupled to the rotor winding (132) of the DFIG (104) via a link (135). The link (135) may be a multi-phase link, for example, a three-phase electrical link as shown in FIG. 1. The line-side converter (108) is electrically coupled to the PCC (114), either directly or via a transformer (not shown). The rotor-side converter (106) and the line-side converter (108) are electrically coupled to each other via a DC-link (134). The DC-link (134) may include at least two conductors (not shown) – one maintained at a positive potential and another maintained at a negative potential. The DC-link (134) may also include a DC-link capacitor (not shown) electrically coupled between two conductors of the DC-link (134). The rotor-side converter (106) may be an AC-DC converter and configured to convert an AC power into a DC power and vice-versa. The line-side converter (108) may be a DC-AC converter and configured to convert the DC power into an AC power and vice-versa. In some embodiments, one or both of the rotor-side converter (106) and the line-side converter (108) are designed such that the rotor-side converter (106) and/or the line-side converter (108) are capable of withstanding fault currents.
[0022] Each of the rotor-side converter (106) and the line-side converter (108) may include one or more switches, for example, semiconductor switches, configured to facilitate power conversion from AC to DC and vice-versa. The semiconductor switches may be controlled by supplying control signals to respective control terminal (e.g., gate terminal) of the semiconductor switches.
[0023] Further, the stator winding (128) is configured to be coupled to the PCC (114) to supply the stator power. In some embodiments, coupling of the stator winding (128) to the PCC (114) may be controlled via the control sub-system (112). In certain embodiments, the stator winding (128) is coupled to the PCC (114) via a transformer (not shown). When the rotor (130) is operated at a super-synchronous speed, the slip power is supplied to the PCC (114) via the rotor-side converter (106) and the line-side converter (108).
[0024] Moreover, the power generation system (100) also includes the power source (110) that is coupled to the DC-link (134). The power source (110) is capable of generating and/or supplying a secondary power such as a DC power to the DC-link (134). The power source (110) may include an energy storage device (136). The energy storage device (136) may include one or more batteries, capacitors, or a combination thereof. In some embodiments, the power source (110) may also additionally include an auxiliary power source (138). Non-limiting examples of auxiliary power source (138) may include a photovoltaic (PV) power source, a fuel cell, a renewable energy based power generator, a non-renewable energy based power generator, or combinations thereof. In the description, hereinafter, the auxiliary power source (138) is described as the PV power source without limiting the scope of the present specification. The PV power source (138) may include one or more PV arrays, where each PV array may include at least one PV module. A PV module may include a suitable arrangement of a plurality of PV cells. The PV power source (138) may generate a DC voltage constituting a secondary electrical power that depends on solar insolation, weather conditions, and/or time of the day. Accordingly, the PV power source (138) may be configured to supply at least a portion of the secondary electrical power to the DC-link (134).
[0025] In certain embodiments, the energy storage device (136) and the PV power source (138) in the power source (110) may be coupled to the DC-link (134) via respective DC-DC converters (146, 148) to control supply of the secondary electrical power to the DC-link (134). The DC-DC converter (146, 148) may be operated as a buck converter, a boost converter, or a buck-boost converter, and may be controlled by the control sub-system (112).
[0026] In certain instances, during an operation of the power generation system (100), an electrical fault may occur within the power generation system (100) or at the electrical load (122). Such an instance of electrical fault occurrence is hereafter referred to as a fault condition. Typically, the electrical fault causes a short-circuit that demands a fault current which is significantly higher than the current which flows during a normal operation of the power generation system (100). Advantageously, if the fault current is supplied by the power generation system (100), an additional fault management sub-system (not shown) may be able to manage the fault condition efficiently.
[0027] In conventional power generation systems that employs a DFIG and partial power converters coupled to rotor winding of the DFIG, electrical currents within such conventional generation systems and/or electrical current supplied to electrical loads is limited due to parameters such as a slip value of the DFIG, an operating speed of the DFIG, rated power of the partial power converters, or combinations thereof. Consequently, if an electrical fault occurs in such conventional power generation system, the conventional power generation system may not be able to supply a desired surge in the current. Disadvantageously, management of the electrical fault becomes a challenging task. Moreover, increased flow of a fault current through the partial power converters may damage the partial power converters used in the conventional power generation systems.
[0028] In accordance with aspects of the present specification, the power generation system (100) includes a control sub-system (112) (shown using a dashed region) for enhancing/increasing the fault current in the power generation system (100) during the fault condition. Advantageously, such an increase/enhancement of the fault current aids in an efficient management of the fault condition by the additional fault management sub-system (not shown). In one embodiment, in case of the fault condition, the control sub-system (112) may be configured to increase a current generated by the stator winding (128) of the DFIG (104) to supply at least a portion of the fault current. In some embodiments, in case of a fault condition, the control sub-system (112) may be configured to increase a current generated by the line-side converter (108) to supply an additional portion of the fault current. To facilitate enhancement of the fault current, the control sub-system (112) includes one or more of sensors (140), a controller (142), and switching unit (144, 145).
[0029] The operations of the switching unit (144, 145) may be controlled by the controller (142). The switching unit (144) is coupled between the line-side converter (108) and the PCC (114). When the switching unit (144) is turned-on, the switching unit (144) connects the line-side converter (108) to the PCC (114). When the switching unit (144) is turned-off, the switching unit (144) disconnects the line-side converter (108) from the PCC (114). The switching unit (145) is coupled between the stator winding (128) and the PCC (114). When the switching unit (145) is turned-on, the switching unit (145) connects the stator winding (128) to the PCC (114). When the switching unit (145) is turned-off, the switching unit (145) disconnects the stator winding (128) from the PCC (114). The switching units (144, 145) may include one or more circuit breakers, one or more static switches, three-phase switches, or combinations thereof.
[0030] In some embodiments, the sensors (140) may include current sensors. In some other embodiments, the sensors (140) may include voltage sensors. As depicted in FIG. 1, the sensors (140) are shown as coupled to the stator winding (128). More particularly, in FIG. 1, the sensors (140) are coupled to the phase lines (150, 152, and 154) of the stator winding (128) by way of electrical or electro-magnetic coupling. It is to be noted that the sensors (140) are shown coupled to the stator winding (128) for illustrative purpose. Additionally or alternatively, the sensors (140) may also be coupled to one or more of the rotor winding (132) on the link (135), the link (118), the PCC (114), the transformer (120), the electrical load (122), and the DC-link (134) without limiting the scope of the present specification. Furthermore, although three sensors (140) are shown in FIG. 1, fewer or greater number of sensors may also be employed. By way of example, two sensors (140) may be employed, where one sensor (140) may be disposed on any two of the three phase lines (150, 152, and 154).
[0031] During operation, the sensors (140) are configured to generate electrical signals indicative of respective three-phase stator current signal. For example, each sensor (140) is configured to generate an AC signal indicative of a current flowing through the respective stator phase line coupled thereto. In certain embodiments, each sensor (140) is configured to generate an AC signal indicative of a phase voltage of the phase lines (150, 152, 154).
[0032] In some embodiments, the control sub-system (112) also includes an RPM sensor (155) disposed at the prime mover (102). The RPM sensor (155) generates an electrical signal indicative of the operating speed (in rpm) of the prime mover (102). The RPM sensor is connected to the controller (142) and configured to send the electrical signal indicative of the operating speed to the controller (142).
[0033] The controller (142) may be operatively coupled to the sensors (140) and/or the RPM sensor (155). In some embodiments, the controller (142) may also be operatively coupled to one or more of the rotor-side converter (106), the line-side converter (108), and the switching units (144, 145) for controlling flow of electrical power therethrough. The controller (142) may send one or more control signals to the control terminals of the switches in the rotor-side converter (106) and the line-side converter (108) to control operation of the rotor-side converter (106) and the line-side converter (108). The controller (142) may also be coupled to another supervisory controller (not shown) and configured to control operations of one or more of the sensors (140), the rotor-side converter (106), the line-side converter (108), the switching units (144, 145), and the RPM sensor (155) based on control commands/set-points received from the supervisory controller.
[0034] In some embodiments, the controller (142) may include a specially programmed general purpose computer, an electronic processor such as a microprocessor, a digital signal processor, and/or a microcontroller. Further, the controller (142) may include input/output ports, and a storage medium, such as an electronic memory. Various examples of the microprocessor include, but are not limited to, a reduced instruction set computing (RISC) architecture type microprocessor or a complex instruction set computing (CISC) architecture type microprocessor. Further, the microprocessor may be a single-core type or multi-core type. Alternatively, the controller (142) may be implemented as hardware elements such as circuit boards with processors or as software running on a processor such as a personal computer (PC), or a microcontroller.
[0035] In some embodiments, the controller (142) may be configured to detect the electrical fault based on the electrical signals received from one or more of the sensors (140). The controller (142) is further configured to control, in response to the detection of the electrical fault, a supply of an electrical current to the rotor winding (132) via the rotor-side converter (106). In some embodiments, controller (142) is further configured to increase an amount of the electrical current flowing through the rotor winding (132). Moreover, the controller (142) is configured to generate at least a portion of the fault current by the DFIG (104) at the stator winding (128) based at least partially on the electrical current supplied to the rotor winding (132). The increased amount of the electrical current flowing through the rotor winding (132) causes an increase in the electromagnetic flux in the DFIG (104). The increase in the electromagnetic flux in the DFIG (104) in turn results in an increase in the stator phase currents in comparison to the stator phase currents during no fault condition. Such increased stator phase current contributes to the fault current. Accordingly, the fault current is enhanced/increased. Additional details of the operations performed by the controller (142) are described in conjunction with a method of FIG. 2.
[0036] Referring now to FIG. 2, a flow diagram (200) of a method for enhancing a fault current in the power generation system (100) of FIG. 1 is presented. During normal operation, at step (202) the power generation system (100) is operated in a partial power conversion mode. In the partial power conversion mode, the DFIG (104) may be operated at an operating speed which is synchronous, super- synchronous, or sub- synchronous speed. The DFIG (104) may generate the stator power at the stator winding (128). Moreover, the DFIG (104) may either generate or absorb the slip power at the rotor winding (132). Accordingly, at least a portion of the electrical power available at the PCC (114) may be processed via one or more of the rotor-side converter (106) and line-side converter (108).
[0037] During operation of the power generation system (100) in the partial power conversion mode, at step (204), the controller (142) is configured to receive electrical signals from the sensors (140). In some embodiments, the controller (142) may receive the electrical signals from the sensors (140) periodically at a predetermined time intervals. In certain embodiments, the controller (142) may receive the electrical signals continuously or randomly from the sensors (140). In the configuration of FIG. 1, the electrical signals may be indicative of phase voltages and/or phase currents of the stator winding (128).
[0038] Further, at step (206), the controller (142) may be configured to analyze the electrical signals received from the sensors (140). The analysis at step (206) may include a time domain and/or frequency domain analysis of the electrical signals received from the sensors (140). In some embodiments, the electrical signals received from the sensors (140) may be analyzed to determine a rate of change of one or more phase voltage and/or a rate of change of one or more phase currents. The rate of change of the phase voltage and/or the rate of change of the phase current may be determined based on a rate of change (i.e., a slew rate) of the electrical signal received from the sensors (140). Moreover, at step (208), the controller (142) is configured to perform a check to determine if an electrical fault has occurred. In some embodiments, the controller (142) may determine an occurrence of the electrical fault based on the rate of change of one or more phase voltages and/or the rate of change of one or more phases. By way of example, if a phase current on one or more phase lines (150, 152, 154) increases at a rate beyond a first threshold rate, the controller (142) may determine that the electrical fault has occurred. In some embodiments, if a phase voltage on one or more phase lines (150, 152, 154) increases at a rate beyond a second threshold rate, the controller (142) may determine that the electrical fault has occurred. In certain embodiments, values of the first threshold rate and the second threshold rate may be stored in a memory device (not shown) associated with the controller (142). At step (208), although the electrical fault is described herein as being detected based on the rate of change of the phase voltages and/or phase currents and/or phase angles or any combination, the controller (142) may be configured to determine the electrical fault using other known techniques and other measurements such as phase angles of the voltages, currents, power, or combinations thereof, without limiting the scope of the present specification.
[0039] At step (208), if the controller (142) determines that no electrical fault has occurred, the controller (142) may continue to operate the power generation system (100) in the partial power conversion mode. At step (208), if the controller (142) determines that the electrical fault has occurred, the controller (142) may be configured to operate the power generation system (100) in a fault current enhancement (FCE) mode, as indicated by step (210). In the FCE mode, the controller (142) is configured to enhance/increase the fault current in the power generation system (100). The method of operating the power generation system (100) in the FCE mode to enhance/increase the fault current includes sub-steps (212-228).
[0040] At step (212), the controller (142) is configured to perform a check whether the DFIG (104) is operating. In some embodiments, the controller (142) may be configured to determine whether the DFIG (104) is operating based on an electrical signal received from the RPM sensor (155). In certain embodiments, the controller (142) may be configured to determine whether the DFIG (104) is operating based on the phase voltages and/or phase currents of the stator winding (128). In certain other embodiments, the controller (142) may be configured to determine whether the DFIG (104) is operating based on phase voltages and/or phase currents of the rotor winding (132). By way of example, if the operating speed of the prime mover (102) is non-zero, the controller (142) may determine that the DFIG (104) is operating. if the operating speed of the prime mover (102) is zero, the controller (142) may determine that the DFIG (104) is not operational.
[0041] At step (212), if it is determined that the DFIG (104) is operating, in some embodiments, the controller (142), at step (214) is configured to control supply of an electrical current to the rotor winding (132) via the rotor-side converter (106). Controlling of the supply of the electrical current at step (214) may include varying at least one of a magnitude and a frequency of the electrical current supplied to the rotor winding (132). The controller (142) is configured to control the rotor-side converter (106) such that the electrical current having an increased magnitude is supplied the rotor winding (132) in comparison to the operation of the DFIG (104) prior to the detection of the electrical fault at step (208). In some embodiments, to control the supply of the electrical current to the rotor winding (132), the controller (142) is configured to control a positive sequence of the electrical current supplied to the rotor winding (132), a negative sequence of the electrical current supplied to the rotor winding (132), or both. In certain embodiments, the controller (142) may select at least one of the positive sequence of the electrical current or the negative sequence of the electrical current based on the type of the electrical fault.
[0042] In some embodiments, the controller (142) may additionally be configured to vary the frequency of the electrical current supplied to the rotor winding (132). In particular, the controller (142) may be configured to determine the frequency of the electrical current supplied to the rotor winding (132) based on an operating speed of the DFIG (104) and the synchronous speed of the DFIG (104). In some embodiments, the frequency of the electrical current supplied to the rotor winding (132) may be determined by the controller (142) based on the slip value of the DFIG (104) and a type (i.e., positive or negative) of the sequence of the electrical current supplied to the rotor winding (132). For example, if the positive sequence of the electrical current is supplied to the rotor winding (132), the controller (142) may determine the frequency ( ) of the electrical current using equation (3).
Equation (3)
[0043] By way of another example, if the negative sequence of the electrical current is supplied to the rotor winding (132), the controller (142) may determine the frequency ( ) of the electrical current using equation (4).
Equation (4)
[0044] Moreover, at step (216), at least a portion of the fault current may be generated at the stator winding (128) of the DFIG (104) due to the electrical current (i.e., electrical excitation) supplied to the rotor winding (132). Increased amount of the electrical current flowing through the rotor winding (132) causes an increase in the electromagnetic flux in the DFIG (104). The increase in the electromagnetic flux in the DFIG (104) in turn results in an increase in the stator phase currents in comparison to the stator phase currents during no fault condition. Such increased stator phase current contributes to the fault current. Accordingly, the fault current is enhanced/increased. In some embodiments, a major portion of the fault current is generated and supplied from the stator winding (128) of the DFIG (104). Furthermore, at step (218), an additional portion of the fault current may be supplied via the line-side converter (108). Such an additional current via the line-side converter (108) further increases the fault current.
[0045] In certain embodiments, step (212), if it is determined that the DFIG (104) is operating, the controller (142) is configured to disable an operation of the line-side converter (108). To disable the operation of the line-side converter (108), the controller (142) may be configured to discontinue/stop control signals to the line-side converter (108). For example, the controller (142) may stop sending control signals to one or more switches of the line-side converter (108). Additionally or alternatively, in some embodiments, the controller (142) is configured to disconnect the line-side converter (108) from the PCC (114) via the switching unit (144). The controller (142) may send a control signal to the switching unit (144) to discontinue an electrical path between the line-side converter (108) and the PCC (114). Advantageously, such disabling or disconnecting of the line-side converter (108) from the PCC (114) aids in protecting the line-side converter (108) from any possible overflow of electrical current through the line-side converter (108). Moreover, since the line-side converter (108) is disabled and/or disconnected from the PCC (114), the DC-link (134) may be maintained by the power source (110). In an embodiment, when the line-side converter (108) is disabled or disconnected from PCC (114), the step (218) may not be performed.
[0046] Referring again to step (212), if it is determined that the DFIG (104) is not operating, in some embodiments, the controller (142) is configured to turn-on the switching unit (145), as indicated by step (220). The controller (142) may send one or more control signals to the switching unit (145) to turn-on the switching unit (145). The switching unit (145), when turned-on, connects the stator winding (128) of the DFIG (104) with the PCC (114). Further, at step (222), the DFIG (104) is operated as a transformer. In such an operation of the DFIG (104), the rotor (130) remains stationary, and the rotor winding (132) and the stator winding (128) respectively function as a primary winding and a secondary winding of the transformer.
[0047] Moreover, at step (224), the controller (142) is configured to control a supply of an electrical current to the rotor winding (132) via the rotor-side converter (106). The control of the supply of the electrical current at step (224) may include varying at least one of a magnitude and a frequency of the electrical current supplied to the rotor winding (132). In some embodiments, at step (224), the magnitude of the electrical current supplied to the rotor winding (132) is independent of a slip value of the DFIG (104). The controller (142) is configured to control the rotor-side converter (106) such that the electrical current having an increased magnitude is supplied the rotor winding (132) in comparison to the operation of the DFIG (104) prior to the detection of the electrical fault the step (208).
[0048] Moreover, at step (226), due to the electrical current (i.e., electrical excitation) supplied to the rotor winding (132), at least a portion of the fault current is generated at the stator winding (128) of the DFIG (104). The electrical current (i.e., electrical excitation) supplied to the rotor winding (132) is transferred to the stator winding (128) due to a transformer effect between the rotor winding (132) and the stator winding (128). Consequently, a current is supplied on one or more phase lines (150, 152, 154). Such increased phase current contributes to the fault current. Accordingly, the fault current is enhanced/increased.
[0049] In addition, at step (228), an additional portion of the fault current may be supplied via the line-side converter (108). The controller (142) is configured to operate the line-side converter (108) as an inverter (i.e., DC to AC converter) to supply the additional portion of the fault current. Such an additional current via the line-side converter (108) further increases the fault current.
[0050] Additionally, irrespective of an operating condition (operational or non- operational) of the DFIG (104), it may be desirable to maintain a state of charge of the energy storage device (136) to a predetermined charged level. Accordingly, the controller (142), in some embodiments, is configured to determine if the state of charge of the energy storage device (136) is less than a predetermined charged level. In some embodiments, the predetermined charge level is equivalent to a maximum charge that can be stored in the energy storage device (136). In certain embodiments, the maximum charge may also be referred to as a rated capacity of the energy storage device (136). In certain embodiments, the predetermined charge level of the energy storage device (136) may be set at a value lower than the rated capacity of the energy storage device (136).
[0051] The controller (142) may determine that the energy storage device (136) is not charged to the predetermined charged level, if the state of charge of the energy storage device (136) is less than the predetermined charged level. Further, the controller (142) may be configured to charge the energy storage device (136) using an electrical power generated by the auxiliary power source (138) if the state of charge of the energy storage device (136) is less than the predetermined value. If the state of charge of the energy storage device (136) is not less than (i.e., greater than or equal to) the predetermined value, the controller (142) may disconnect the auxiliary power source (138) from the DC-link (134). In one embodiment, the controller (142) may disconnect the auxiliary power source (138) from the DC-link (134) by discontinuing control signals to the DC-DC power converter (148). Consequently, a voltage level of the DC-link (134) is maintained by the energy storage device (136).
[0052] Any of the foregoing steps may be suitably replaced, reordered, or removed, and additional steps may be inserted, depending on the needs of an application.
[0053] In accordance with the embodiments described herein, the control sub-system (112) and the method (200) facilitate enhancement of fault current when an electrical fault is detected. Advantageously, such an increase/enhancement of the fault current aids in an efficient management of the fault condition by the additional fault management sub-system (not shown). Moreover, in certain situations such as a fault condition when the DFIG (104) is operating, the controller (142) disables and/or disconnects the line-side converter (108) from the PCC (114). Consequently, the line-side converter (108) may be protected from overcurrent. Thus, reliability of the power generation system (100) may be improved.
[0054] It will be appreciated that variants of the above disclosed and other features and functions, or alternatives thereof, may be combined to create many other different applications. Various unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art and are also intended to be encompassed by the following claims.