WO2017028028A1

WO2017028028A1 - Systems and methods for open fault detection

Info

Publication number: WO2017028028A1
Application number: PCT/CN2015/087017
Authority: WO
Inventors: Jiao LIU; David Smith; Xueqin Wu; Wei Ren; Hao Yu
Original assignee: General Electric Company
Priority date: 2015-08-14
Filing date: 2015-08-14
Publication date: 2017-02-23

Abstract

A system is provided. The system includes an electrical machine, a target circuit and a controller. The electrical machine includes a rotor and a stator magnetically coupled to the rotor. The target circuit is coupled to the stator of the electrical machine. The controller includes an open-circuit detection module for receiving an electrical signal indicative of a target current flowing through supplied to the target circuit, separating an alternating-current component of the target current and a direct-current offset component of the target current, and determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component. A method for open fault detection is also provided. And a power generation system is also provided.

Description

SYSTEMS AND METHODS FOR OPEN FAULT DETECTION

BACKGROUND

Embodiments of the invention relate to a system and a method for open fault detection.

Wind power generation systems use wind energy to generate active and reactive power. When the power is to be delivered to a utility grid, it is desirable for the delivered power to be independent of system frequency. When fault conditions such as grid open faults, line-to-line low voltage ride-through (LVRT) faults, or line-to-line zero voltage ride-through (ZVRT) faults occur during wind turbine operation, wind power generation systems are typically expected to be able to respond. In order to respond effectively, it is useful to be able to accurately detect the different types of faults.

Grid open faults and line-to-line LVRT/ZVRT faults have similar characteristics that are reflected when monitoring grid current. In some embodiments, therefore, the type of fault is not distinguished, and the response is based merely on the grid current without distinguishing whether the fault is a grid open fault or a LVRT/ZVRT fault.

It is desirable to provide a system and a method to detect a grid open fault.

BRIEF DESCRIPTION

In accordance with one embodiment disclosed herein, a system is provided. The system includes an electrical machine, a target circuit and a controller. The electrical machine includes a rotor and a stator magnetically coupled to the rotor. The target circuit is coupled to the stator of the electrical machine. The controller includes an open-circuit detection module for receiving an electrical signal indicative of a target current flowing through supplied to the target circuit, separating an alternating-current component of the target current and a direct-current offset component of the target current, and determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component.

In accordance with another embodiment disclosed herein, a method for open fault detection in an electrical machine including a rotor and a stator magnetically coupled to the rotor is provided. The method includes receiving an electrical signal indicative of a target current flowing through a target circuit coupled to the stator of the electrical machine. The method further includes separating an alternating-current component of the target current and a direct-current offset component of the target current； and determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component.

In accordance with another embodiment disclosed herein, a power generation system is provided. The power generation system includes a power source, a converter system, a transformer, and a controller. The power source is for generating power. The converter system is for converting at least part of the generated power from the power source and providing conversion power. The transformer is for providing voltage or current transformation of the conversion power and isolation between the converter system and a grid and between the power source and the grid. The controller includes an open-circuit detection module for receiving an electrical signal indicative of a grid current supplied to the grid, separating an alternating-current component of the grid current and a direct-current offset component of the grid current, and determining whether an open fault occurs between the transformer and the grid based on at least one of the alternating-current component and the direct-current offset component.

DRAWINGS

These and other features and aspects of the present disclosure 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:

FIG. 1 is a schematic diagram of a wind power generation system in accordance with an embodiment；

FIG. 2 is a schematic diagram of a wind power generation system in accordance with another embodiment；

FIG. 3 is a schematic diagram of a wind power generation system in accordance with another embodiment；

FIG. 4 is a block diagram of an open-circuit detection module of the wind power generation system in accordance with an embodiment；

FIG. 5 illustrates simulation waveforms of grid currents in three phases during an open fault in accordance with one embodiment；

FIG. 6 illustrates simulation waveforms of grid currents at three phases during a line-to-line low voltage ride-through fault in accordance with one embodiment；

FIG. 7 is a block diagram of the open-circuit detection module in accordance with another embodiment；

FIG. 8 is a block diagram of the open-circuit detection module in accordance with another embodiment；

FIG. 9 is a block diagram of the open-circuit detection module in accordance with another embodiment；

FIG. 10 is a schematic diagram of a system utilizing the open-circuit detection module in accordance with an embodiment； and

FIG. 11 is a flow chart of a method for open fault detection in accordance with an embodiment.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The use of “including, ” “comprising” or “having” and variations thereof herein are meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect.

FIG. 1 illustrates a schematic diagram of a wind power generation system 10 for delivering electrical power to a power grid 12 in accordance with an embodiment. The wind power generation system 10 includes a power source 14, a converter system 16, a transformer 18, and a controller 20.

The power source 14 is configured to generate power and includes a wind turbine 22 and a generator 24. The wind turbine 22 is configured to rotate in response to wind contacting the wind turbine 22 to drive the generator 24 such that the generator 24 outputs AC power. In one embodiment, the generator 24 includes a doubly-fed induction generator (DFIG) . In the illustrated embodiment, the generator 24 is alternatively referenced as an electrical machine including a rotor 28 and a stator 26 magnetically coupled to the rotor 28. The stator 26 includes multiple windings. The stator 26 of the electrical machine 24 is coupled to the power grid 12 through the transformer 18. In the illustrated embodiment, the power grid 12 is alternatively referenced as a target circuit which is detected and monitored by the controller 20.

The converter system 16 is configured to regulate the generated power from the power source 14 and convert the power from the generator rotor 28. The converter system 16 includes a rotor-side converter 30, a line-side converter 32 and a direct current (DC) link 34. The DC link 34 connects the rotor-side converter 30 and the line-side converter 32. The rotor-side converter 30 is configured to convert the AC power from the generator rotor 28 of the generator 24 into DC power. The line-side converter 32 is configured to convert the DC power to AC power at a frequency compatible with the grid 12.

The rotor-side converter 30 may include an AC-DC converter which converts an AC voltage from the generator 24 to a DC voltage on the DC link 34. The DC link 34 may include one or more capacitors coupled either in series or parallel for maintaining the DC voltage of the DC link 34 at a certain level, and thus the energy flow from the DC link 34 to the grid 12 can be managed. The line-side converter 32 may include a DC-AC inverter which converts the DC voltage on the DC link 34 to AC voltage with suitable frequency, phase, and magnitude for feeding to the grid 12.

The transformer 18 is configured to provide voltage or current transformation of the power from the converter system 16 and the power from the generator stator 26, and provide isolation between the converter system 16 and the grid 12 and between the power source 14 and the grid 12. The transformer 18 is configured to step up the magnitude of the AC voltages output from the line-side converter 32 and the generator stator 26 to match the grid 12.

The controller 20 is configured to control operations of the rotor-side converter 30 and the line-side converter 32 through implementation of analog circuitry and/or digital control algorithms in response to various feedback signals and command signals. In one embodiment, the controller 20 includes a rotor-side controller (not shown) controlling the rotor-side converter 30 and a line-side controller (not shown) controlling the line-side converter 32. In the illustrated embodiment, the controller 20 includes an open-circuit detection module 36. The open-circuit detection module 36 is configured to detect an open-circuit fault at the target circuit. In the illustrated embodiment, the open-circuit detection module 36 is configured to detect a grid open-circuit fault at the grid 12, especially a single-phase grid open-circuit fault. The open-circuit detection module 36 is configured for receiving an electrical signal indicative of a target current flowing through the target circuit. In the illustrated embodiment, the open-circuit detection module 36 receives an electrical signal indicative of a grid current supplied to the grid 12. The grid current may include AC current flowing from the transformer 18 to the grid 12 in one embodiment. The open-circuit detection module 36 determines the grid open-circuit fault based on the grid current.

In one embodiment, the power generation system 10 includes a sensor unit 38 for measuring the electrical signal. The transformer 18 includes a primary side 40 coupled with the grid 12 and a secondary side 42 coupled with the converter system 16 and the generator 24. In one embodiment, the sensor unit 38 is configured to measure the electrical signal at the secondary side 42 of the transformer 18 and provide the electrical signal to the open-circuit detection module 36. The sensor unit 38, in one embodiment, may include current sensors detecting three phase currents at the secondary side 42 of the transformer 18. The open-circuit detection module 36 may be further configured for estimating the grid current at the primary side 40 based on the electrical signal and one or more parameters of the transformer 18, such as turn ratio.

In the embodiment of FIG. 1, the transformer 18 includes a double-winding transformer including a primary winding 17 and a secondary winding 19. The secondary side 42 includes a main circuit 44, a first branching circuit 46 and a second branching circuit 48. The first branching circuit 46 connects the stator 26 of the generator 24 to the main circuit 44 and then to the secondary winding 19. The second branching circuit 48 connects the line-side converter 32 to the main circuit 44 and then to the secondary winding 19. In this embodiment, the sensor unit 38 is positioned at the main circuit 44 to detect total current flowing to the secondary winding 19 of the transformer 18. Thus, by arranging the sensor unit 38 at the main circuit 44, it may be determined whether an open fault occurs at the main circuit 44, and it may be further determined whether an open fault occurs at the primary side 40 based on the parameters of the transformer 18.

In another embodiment of FIG. 1, the power generation system 10 may further include a sensor unit 52 positioned at the second branching circuit 48 for measuring an electrical signal, such as a current, from the line-side converter 32. In this embodiment, the open-circuit detection module 36 may estimate an electrical signal at the first branching circuit 46 based on the electrical signals at the main 44 and the second branching circuit 48 so as to determine whether an open fault occurs at the first branching circuit 46. Thus, by respectively arranging the

sensor units

38 and 52 at the main circuit 44 and the second branching circuit 48, it may be determined whether an open fault occurs at the main circuit 44 as well as at the first branching circuit 46.

In the embodiment of FIG. 2, two

sensor units

50 and 52 are employed to respectively detect electrical signals, such as currents, at the first branching circuit 46 and the second branching circuit 48 of the secondary side 42. The electrical signals from the

sensor units

50 and 52 are provided to the open-circuit detection module 36. The open-circuit detection module 36 may determine whether an open fault occurs at the first branching circuit 46 based on the electrical signal from the sensor unit 50. Furthermore, the open-circuit detection module 36 may further estimate the grid current based on the electrical signals at the first branching circuit 46 and the second branching circuit 48 and the parameters of the transformer 18. The total current is substantial a total amount of the currents at the first branching circuit 46 and the second branching circuit 48. The

sensor units

50 and 52 may be similar to the sensor unit 38 in FIG. 1. Thus, by respectively arranging the

sensor units

50 and 52 at the first branching circuit 46 and the second branching circuit 48, it may be also determined whether an open fault occurs at the main circuit 44, and it may be further determined whether an open fault occurs at the primary side 40 based on the parameters of the transformer 18. In addition, it may be also determined whether an open fault occurs at the first branching circuit 46.

In another embodiment of FIG. 2, the power generation system 10 may only include the sensor unit 50 without the sensor unit 52. The sensor unit 50 is configured to detect an electrical signal, such as a current, at the first branching circuit 46 the secondary side 42. In this embodiment, only the electrical signal from the sensor unit 50 is provided to the open-circuit detection module 36. The open-circuit detection module 36 is configured to only determine whether an open fault is occurring at the first branching circuit 46.

In the embodiment of FIG. 3, the transformer 18 includes a three-winding transformer including a primary winding 17 and two

secondary windings

21, 23. The first branching circuit 46 connects the stator 26 of the generator 24 to the first secondary winding 21 and the second branching circuit 48 connects the line-side converter 32 to the second secondary winding 23. The

sensor units

50 and 52 respectively detect the electrical signals at the first branching circuit 46 and the second branching circuit 48. The open-circuit detection module 36 may determine whether an open fault occurs at the first branching circuit 46 based on the electrical signal from the sensor unit 50. Furthermore, the open-circuit detection module 36 may further estimate the grid current based on the electrical signals at the first branching circuit 46 and the second branching circuit 48 and the parameters of the transformer 18. Thus, by arranging the

sensor units

50 and 52 at the first branching circuit 46 and the second branching circuit 48, it may be determined whether an open fault occurs at the first branching circuit 46 and it may be determined whether an open fault occurs at the primary side 40 in combination with the parameters of the transformer 18.

In another embodiment of FIG. 3, the power generation system 10 may only include the sensor unit 50 without the sensor unit 52. The sensor unit 50 is configured to detect an electrical signal, such as a current, at the first branching circuit 46 the secondary side 42. In this embodiment, only the electrical signal from the sensor unit 50 is provided to the open-circuit detection module 36. The open-circuit detection module 36 is configured to only determine whether an open fault is occurring at the first branching circuit 46.

The embodiments in FIGS. 1 to 3 are non-limiting examples. In another embodiment, the grid current may be detected at the primary side 40 of the transformer 18. The system 10 in FIGS. 1 to 3 may include one or more other elements which are not shown, such as a rectifier and an energy storage unit.

The power source 14 as shown in FIGS. 1 to 3 is taken the wind turbine 22 as an exemplary example. However, the power source 14 of the present disclosure shouldn’ t be limited to comprise the wind turbine 22. In another embodiment of the present disclosure, the power source 14 as shown in FIGS. 1 to 3 may also comprise a traditional generator having a rotor and a stator.

FIG. 4 illustrates a block diagram of the open-circuit detection module 36 of FIGS. 1-3 in accordance with an embodiment. The open-circuit detection module 36 receives the electrical signal indicative of the target current which is the grid current in the main example, and is further programmed for separating an alternating-current (AC) component of the target current and a direct-current (DC) offset component of the target current and determining whether an open fault is occurring at the target circuit based on at least one of the AC component and the DC offset component.

In the illustrated embodiment, the open-circuit detection module 36 includes a high-pass filter 54, an absolute value unit 56 and a determining unit 58. The high-pass filter 54 is configured to remove the DC offset component from the grid current and output the AC component. Three high-pass filters or filter modules are utilized to remove the DC offset components in three phases. The open-circuit detection module 36, in this embodiment, determines whether the open fault is occurring based on the AC components of the three phases. When the open fault is occurring at single phase between the transformer 18 and the grid 12, the real grid current in the open phase is zero, and, in embodiments wherein the sensors are on the secondary side 42 of the transformer 18, the estimated grid current may be close to zero, depending on the measurement accuracy of the sensor unit. As current is still flowing through the other two phases, at least temporarily, the total value of absolute values of currents in three phases or in the other two phases is still significant for a certain time, such as about 0.05 seconds.

FIG. 5 illustrates simulation waveforms of grid currents in three phases A, B and C during the open fault in accordance with one embodiment. When the grid phase A is open at about 4.81 second (s) , the current in the grid phase A shown as the waveform 60 is very small which equals to or approximates zero, and the current in the grid phase B shown as the waveform 62 and the current in the grid phase C shown as the waveform 64 are large before 4.85 s during the open fault. The total amount of the absolute values of the currents in three phases A, B and C is large before 4.85 s.

In some other conditions at the grid side, such as a line-to-line low voltage ride-through (LVRT) fault, a line-to-line zero voltage ride-through (ZVRT) or a line-line imbalance fault, current in one phase is small, and the total value of absolute values of currents in three phases or in the other two phases is large due to the DC offset component in the grid current. The line-line imbalance fault, for example, includes a condition that voltages in two phases may drop and/or a phase angle between two phases is changed. The grid current during these conditions has similar characters with the grid current during the single-phase grid open-circuit fault. Hereinafter, the line-to-line LVRT/ZVRT fault is illustrated as an example. The DC offset component is generated due to the generator 24 when the grid voltage drops during the line-to-line LVRT/ZVRT fault. The DC offset component is generated in a flux linkage of the generator 24 to maintain the flux linkage when the grid voltage drops, and the DC offset component is in the grid current during the line-to-line LVRT/ZVRT fault. The high-pass filter 54 of FIG. 4 removes the DC offset component from the grid current to isolate the AC component during the line-to-line LVRT/ZVRT fault. The AC component in the line-to-line LVRT/ZVRT fault is different from the AC component in the single-phase grid open-circuit fault such that the different types of faults can be determined by the determining unit 58.

FIG. 6 illustrates simulation waveforms of grid currents at three phases A, B, C during the line-to-line LVRT fault in accordance with one embodiment. In this embodiment, the line-to-line LVRT fault occurs between the phases A and C. The graph (a) illustrates simulation waveforms of the grid currents in three phases in an embodiment which does not include the high-pass filter 54. The graph (b) illustrates simulation waveforms of the grid currents in an embodiment which includes the high-pass filter 54. In the graph (a) , the current at one phase B shown as the waveform 66 is small, and the total value of the absolute values of the currents in the other two phases A and C shown as the

waveforms

68 and 70 is large when the line-to-line LVRT fault is occurring. In the graph (b) , the current in the phase B shown as the waveforms 72 is substantially the same with the current in the phase B shown as the waveforms 66 in the graph (a) , but the total value of the absolute values of the currents in the phases A and C shown as the

waveforms

74 and 76 is reduced more quickly since the DC offset component is removed.

Continuing to refer to FIG. 4, a bandwidth of the high-pass filter 54 is determined based at least in part on a power flowing through the target circuit or a power command for the target circuit. In an embodiment, the bandwidth of the high-pass filter 54 is determined based at least in part on the transformed power output from the transformer or a system power command. In one embodiment, the bandwidth of the high-pass filter 54 is changing monotonously with the power flowing through the target circuit or the power command. The bandwidth of the high-pass filter 54 is higher while the transformed power or the system power command is higher, and the bandwidth of the high-pass filter 54 is lower while the transformed power or the system power command is lower. In one embodiment, the bandwidth of the high-pass filter 54 is changing continuously as the transformed power or the system power command is changing in a linear manner or a non-linear manner. In another embodiment, the bandwidth of the high-pass filter 54 is changing in stepped variation. The bandwidth of the high-pass filter 54 may be set as a certain value for a certain range of the transformed power or the system power command. Accordingly, the bandwidth of the high-pass filter 54 is changing with the DC component.

In one embodiment, the bandwidth of the high-pass filter 54 is determined based on the transformed power. In another embodiment, the bandwidth of the high-pass filter 54 is determined based on the system power command before the fault or during the fault or after the fault. The system power command may be provided by a system controller (not shown) of the system 10. In another embodiment, the bandwidth of the high-pass filter 54 is fixed.

The absolute value unit 56 is configured to receive the AC component from the high-pass filter 54 and provide an absolute value of the AC component. The AC component includes three phase currents. The absolute value unit 56 may generate three absolute values of the three phase currents. The determining unit 58 is configured to determine whether the open fault is occurring between the transformer 18 and the grid 12 based on the absolute values of the three phase currents of the AC component. The determining unit 58 determines whether the absolute value of one phase current is relatively small and a sum of the absolute values of the other two phase currents or the three phase currents is relatively large.

In one embodiment, the determining unit 58 determines whether the absolute value of one phase current is less than a first threshold and a sum of the absolute values of the three phase currents or the other phase currents is larger than a second threshold. If the absolute value of one phase current is less than the first threshold and the sum of the absolute values of the other two phase currents or the three phase currents is larger than the second threshold, the determining unit 58 determines that the open fault is occurring. The determining unit 58 may send a signal indicating the open fault condition to the controller 20 and/or a system (or supervisory) controller (not shown) to control the

converters

30, 32 and other elements of the system 10 to stop operation or operate in a different manner according to particular applications. The first threshold and the second threshold may be set based at least in part on characters of the three phase currents in the open fault. The first threshold and the second threshold are set to distinguish the AC component in the open fault from the AC component in other conditions like the line-to-line LVRT/ZVRT fault.

In another embodiment, the determining unit 58 determines whether (1) the absolute value of one phase current is less than a predetermined percentage of the sum of the absolute values of the three phase currents or the other two phase currents and (2) the sum of the absolute values of the three phase currents or the other two phase currents is larger than a threshold. For example, the determining unit 58 determines whether the absolute value of the one phase current is less than one fourteenth of the sum. If the absolute value of the one phase current is less than the predetermined percentage of the sum and the sum is larger than the threshold, the determining unit 58 determines that the open fault is occurring. In another embodiment, the determining unit 58 may determine whether the open fault is occurring based on the AC component through another method. Accordingly, the open-circuit detection module 36 can distinguish the open fault from the other fault conditions like the line-to-line LVRT/ZVRT fault, so as to avoid the other fault conditions being detected as the open fault.

FIG. 7 illustrates a block diagram of the open-circuit detection module 36 in accordance with another embodiment. The open-circuit detection module 36 in FIG. 7 further includes a low-pass filter 78 for removing current values near zero from the AC component so as to avoid fake open faults being detected. In the illustrated embodiment, the low-pass filter 78 is positioned between the absolute value unit 56 and the determining unit 58 to remove the current values near zero from the absolute value of the AC component. The bandwidth of the low-pass filter 78 is determined based on grid frequency. For example, the grid frequency is 50Hz and the bandwidth of the low-pass filter 78 may typically be less than 100 Hz.

FIG. 8 illustrates a block diagram of the open-circuit detection module 36 in accordance with another embodiment. In the illustrated embodiment, the open-circuit detection module 36 is programmed for extracting the DC offset component from the grid current and determining whether the open fault is occurring based on the DC offset component. In one embodiment, the open-circuit detection module 36 includes a high-pass filter for removing the DC offset component from the grid current to provide the AC component, and the open-circuit detection module 36 is programmed for extracting the DC offset component based on the grid current and the AC component.

More specifically, in the illustrated embodiment, the open-circuit detection module 36 includes three high-pass filters 80-82 for respectively removing DC offset components in three phases from the three phase grid currents and remaining AC components in three phases. Three absolute value units 83-85 are provided for respectively calculating absolute values of the AC components in three phases from the high-pass filters 80-81. A first adder 86 is provided to obtain a sum of the absolute values of the AC components in three phases from the absolute value units 83-85. The open-circuit detection module 36 further includes another three absolute value units 87-89 for respectively calculating absolute values of the three phase grid currents, and a second adder 90 for generating a sum of the absolute values of the three phase grid currents. A third adder 92 is configured to calculate a difference between the sum of the absolute values of the three phase grid currents and the sum of the absolute values of the three phase AC components to obtain a sum of absolute values of the DC offset components in three phases.

The open-circuit detection module 36 further includes an absolute value unit 94, a low-pass filter 96 and a determining unit 98. The absolute value unit 94 is configured to generate an absolute value of the difference from the third adder 92. The absolute value unit 94 may be a similar type as the absolute value unit 56 in FIGS. 4 and 7. The low-pass filter 96 is for removing high frequency noise from the absolute value of the extracted DC offset component and making the absolute value of the extracted DC offset component smooth.

The determining unit 98 determines the open fault based at least in part on the DC offset component. In the illustrated embodiment, the determining unit 98 determines whether the sum of the absolute values of the DC offset components in three phases is less than the threshold. If the sum of the absolute values of the DC offset components in three phases is larger than a threshold, the determining unit 98 determines the open fault is not occurring. If the sum of the absolute values of the DC offset components in three phases is less than the threshold, the determining unit 98 further determines the open fault by using the AC component in a similar manner as discussed above. The threshold is set based at least in part on the DC offset component in the other fault conditions like the line-to-line LVRT/ZVRT fault.

In the embodiment of FIG. 8, the determining unit 98 determines the open fault based on the absolute values |I_a1|, |I_b1|, |I_c1| of the three phase currents of the AC components and the DC offset components from the absolute value units 87-89 when the sum of the absolute values of the DC offset components in three phases is less than the threshold. In this circumstance, the DC offset components are small, even equal to zero, and the determining unit 98 determines the open fault in a similar manner with respect to the determining unit 58 in FIGS. 4 and 7. For example, the determining unit 98 may determine whether the absolute value of one phase current is relatively small and a sum of the absolute values of the other two phase currents is relatively large in one embodiment, or may determine whether the absolute value of one phase current is less than a specified percentage of the sum of the absolute values of the other two phase currents and a sum of the absolute values of the three phase currents exceeds a threshold in another embodiment.

In an embodiment of FIG. 9, the determining unit 98 determines the open fault based on the absolute values |I_a2|, |I_b2|, |I_c2| of the three phase currents of the AC components without the DC offset component from the absolute value units 83-85 when the sum of the absolute values of the DC offset components in three phases is less than the threshold. In this circumstance, the determining unit 98 determines the open fault in a similar manner with respect to the determining unit 58 in FIGS. 4 and 7. For example, the determining unit 98 may determine whether the absolute value of one phase current is relatively small and a sum of the absolute values of the other two phase currents is relatively large in one embodiment, or may determine whether the absolute value of one phase current is less than a specified percentage of the sum of the absolute values of the other two phase currents and a sum of the absolute values of the three phase currents exceeds a threshold in another embodiment. The determining unit 98 in FIG. 9 determines the open fault in a similar manner with the determining unit 98 in FIG. 8.

FIG. 10 illustrates a schematic diagram of a system 100 in accordance with an embodiment. The system 100 includes a power source 110, a converter 120, a motor 130 coupled to a load 140, and a controller 150. The power source 110 is configured to provide power for the motor 130, which may be a DC power source or an AC power source 110. The converter 120 is configured to convert the power from the power source 110 to a power supplied to the motor 130. The motor 130 is driven by the power from the converter 120 and includes a motor stator 131 and a motor rotor 132. The motor stator 131 is coupled to the converter 120 to receive the power and the motor rotor 132 is coupled to a load to drive the load. The controller 150 is configured to control the converter 120 and includes an open-circuit detection module 36 similar to the open-circuit detection module 36 of FIGS. 1-3 and 10. The open- circuit detection module 36 in FIG. 10 is similar to the open-circuit detection module 36 in FIGS. 1-3 and 10. The open-circuit detection module 36 in FIG. 10 may have the structures in FIGS. 4 and 7-9 and operate in the manner described above.

In the embodiment, the open-circuit detection module 36 is configured to determine whether the open fault is occurring at a stator side 133 of the motor 130, i.e. a circuit from the motor stator to the converter 120. In the illustrated embodiment, the electrical machine is the motor 130, the target circuit the open-circuit detection module 36 monitoring is three phase lines of the stator side 133 and the target current is current flowing into the motor stator 131 of the motor 130.

FIG. 11 illustrates a flow chart of a method 200 for open fault detection in an electrical machine in accordance with an embodiment. The electrical machine including a rotor and a stator magnetically coupled to the rotor. The electrical machine may be a generator in a power generation system such as a wind power generation system, or a motor for receiving power to drive a load. In the power generating turbine embodiment, the generated power is output from the stator of the generator, and, in the motor embodiment, the power is supplied to the stator of the motor. The stator includes multiple windings.

The method 200 detects open faults at a target circuit coupled to the stator of the electrical machine. The target circuit may be connected to the stator directly or indirectly. In one embodiment, the target circuit is a grid coupled to the stator of the generator through a transformer, and the method 200 detects the open fault at the grid side. In another embodiment, the target circuit is a stator side of the generator or the motor, and the method 200 detects the open fault at the stator side.

In block 204, an electrical signal indicative of a target current flowing through the target circuit is received. In one embodiment, the target current is a grid current flowing from the transformer to the grid. In another embodiment, the target current is current flowing out from the stator of the generator or current flowing into the stator of the motor.

In block 206, an AC component of the target current and a DC offset component of the target current are separated. In block 208, the method 200 further includes determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component.

In one embodiment, the DC offset component is removed from the target current to obtain the AC component. The open fault is determined based on the AC component. In one embodiment, the DC offset component is filtered via a high-pass filter. In one embodiment, an absolute value of the AC component is further generated. The open fault is determined based on the absolute value of the AC component. In one embodiment, current values near zero are removed from the AC component. The current values near zero may be removed from the absolute value of the AC component in one embodiment.

In a more specific embodiment, the AC component includes three phase currents, and the open fault is determined based on the three phase currents of the AC component using the decision factors discussed above, for example.

In another embodiment, the DC offset component is extracted from the target current and the open fault is determined based at least in part on the DC offset component. The DC offset component is removed from the grid current to obtain the AC component, and the DC offset component is obtained based on the grid current and the AC component. If the sum of the absolute values of the DC offset components in three phases is larger than a threshold, the open fault is not occurring. If the sum of the absolute values of the DC offset components in three phases is less than the threshold, the determining unit 98 further determines the open fault based on the AC component.

While the actions of the method 200 are illustrated as functional blocks, the order of the blocks and the separation of the actions among the various blocks shown in FIG. 11 are not intended to be limiting. For example, the blocks may be performed in a different order and an action associated with one block may be combined with one or more other blocks or may be sub-divided into a number of blocks.

While embodiments of the invention have been described herein, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. The various features described, as well as other known equivalents for each feature, can be mixed and matched by one of ordinary skill in this art to construct additional systems and techniques in accordance with principles of this disclosure.

Claims

A system, comprising:

an electrical machine comprising a rotor and a stator magnetically coupled to the rotor；

a target circuit coupled to the stator of the electrical machine； and

a controller comprising an open-circuit detection module for receiving an electrical signal indicative of a target current flowing through the target circuit, separating an alternating-current component of the target current and a direct-current offset component of the target current, and determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component.
The system of claim 1, wherein the open-circuit detection module comprises a high-pass filter for removing the direct-current offset component from the target current and outputting the alternating-current component.
The system of claim 2, wherein the open-circuit detection module further comprises an absolute value unit for receiving the alternating-current component from the high-pass filter and providing an absolute value of the alternating-current component.
The system of claim 2, wherein the open-circuit detection module further comprises a low-pass filter for removing current values near zero from the alternating-current component.
The system of claim 2, wherein the open-circuit detection module is further programmed for determining a bandwidth of the high-pass filter based at least in part on a power flowing through the target circuit or a power command for the target circuit.
The system of claim 1, wherein the alternating-current component comprises three phase currents, and wherein the open-circuit detection module further comprises a determining unit for determining whether the open fault is occurring at the target circuit based on the three phase currents of the alternating-current component.
The system of claim 1, wherein the open-circuit detection module is programmed for extracting the direct-current offset component from the grid current and determining whether the open fault is occurring based at least in part on the direct-current offset component.
The system of claim 7, wherein the open-circuit detection module comprises a high-pass filter for removing the direct-current offset component from the target current to provide the alternating-current component, and the open-circuit detection module is programmed for extracting the direct-current offset component based on the target current and the alternating-current component.
The system of claim 1, wherein the electrical machine comprises a wind turbine or a generator which is coupled to a grid via a transformer, a primary winding of the transformer being connected to the grid, and a secondary winding of the transformer being connected to the stator.
The system of claim 9, wherein the target circuit comprises a first branching circuit at a side of the stator or a main circuit at a side of the second winding of the transformer.
The system of claim 10, wherein the system comprises a rotor-side converter coupled to the rotor and a line-side converter coupled to the secondary winding of the transformer, and the target circuit further comprises a second branching circuit at a side of the line-side converter.
The system of claim 9, wherein the system comprises a rotor-side converter coupled to the rotor and a line-side converter coupled to another secondary winding of the transformer, and the target circuit comprises a first branching circuit from the stator to the secondary winding of the transformer and a second branching circuit from the line-side converter to the another secondary winding of the transformer.
The system of claim 1, wherein the electrical machine comprises a motor.
The system of claim 13, wherein the target circuit comprises a circuit from the stator of the motor to a converter.
A method for open fault detection in an electrical machine comprising a rotor and a stator magnetically coupled to the rotor, the method comprising:

receiving an electrical signal indicative of a target current flowing through a target circuit coupled to the stator of the electrical machine；

separating an alternating-current component of the target current and a direct-current offset component of the target current； and

determining whether an open fault is occurring at the target circuit based on at least one of the alternating-current component and the direct-current offset component.
The method of claim 15, wherein the separating comprises removing the direct-current offset component from the target current to obtain the alternating-current component.
The method of claim 16, further comprising generating an absolute value of the alternating-current component.
The method of claim 16, further comprising removing current values near zero from the alternating-current component.
The method of claim 16, wherein removing the direct-current offset component comprises filtering the direct-current offset component via a high-pass filter.
The method of claim 15, wherein the alternating-current component comprises three phase currents, and wherein the determining comprises determining whether the open fault is occurring based on the three phase currents of the alternating-current component.
The method of claim 15, wherein the separating comprises extracting the direct-current offset component from the target current and the determining comprises determining whether the open fault is occurring based at least in part on the direct-current offset component.
The method of claim 21, wherein the extracting comprises removing the direct-current offset component from the target current to obtain the alternating-current component, and obtaining the direct-current offset component based on the target current and the alternating-current component.
A power generation system, comprising:

a power source for generating power；

a converter system for converting at least part of the generated power from the power source and providing conversion power；

a transformer for providing voltage or current transformation of the conversion power and isolation between the converter system and a grid and between the power source and the grid； and

a controller comprising an open-circuit detection module for receiving an electrical signal indicative of a grid current supplied to the grid, separating an alternating-current component of the grid current and a direct-current offset component of the grid current, and determining whether an open fault occurs between the transformer and the grid based on at least one of the alternating-current component and the direct-current offset component.
The power generation of claim 23, wherein the open-circuit detection module comprises

a high-pass filter for removing the direct-current offset component from the grid current and outputting the alternating-current component, and

an absolute value unit for receiving the alternating-current component from the high-pass filter and providing an absolute value of the alternating-current component.
The power generation system of claim 24, wherein the alternating current component comprises three phases, and wherein the open-circuit detection module further comprises a determining unit for determining whether the open fault is occurring between the transformer and the grid based on the three phases of the alternating-current component.
The power generation system of claim 25, wherein the power source comprises at least one wind turbine.