DETECTION AND METHOD THEREOF

BACKGROUND

[0001] Embodiments of the disclosure relate generally to systems and methods for open-circuit fault detection and protection of power generation systems against open-circuit fault.

[0002] With the rising cost and scarcity of conventional energy sources such as fossil fuels and concerns about the environment, there is a growing interest in replacing the conventional energy sources with alternative energy sources such as solar power generation systems, fuel cells, wind turbine generators, and marine hydrokinetic generators. In the past, since the power generated with these alternative energy sources accounted for only a small part of the overall power supplied to the grid, the alternative energy sources had been allowed to trip offline during a grid fault such as, for example, a low voltage event.

[0003] Currently, in order to more reliably feed power to the power grid, grid operators typically require the alternative energy sources to meet certain power grid interconnection standards. One of the interconnection standards is fault-ride through capability which requires that the power generation system must remain connected to the power grid when one or more grid faults are occurring with the power grid. However, during fault conditions, keeping the power generation system online may result in some problems. One of the problems may be how to quickly and accurately detect or identify an open-circuit fault condition, as a failure to timely detect the open- circuit fault may lead to wrong response of the controller so that one or more electrical devices within the power generation system or one or more loads coupled with the power generation system may be damaged by the over-voltage or over- current caused from the open-circuit fault.

[0004] Therefore, it is desirable to provide systems and methods to address the above-mentioned problems. BRIEF DESCRIPTION

[0005] In accordance with one embodiment disclosed herein, a power conversion system is provided. The power conversion system includes a converter system for converting input power from a power source and providing output power; a transformer for providing voltage or current transformation of the output power and isolation between the converter system and a load; and a converter controller. The converter controller includes an open-circuit detection module for receiving a first electrical signal and a second electrical signal measured at one side of the transformer; determining whether a difference value between the measured first and second electrical signals is smaller than a predetermined threshold value; and sending a control signal to cause the converter system to reduce the magnitude of the output power upon determination that the difference value is smaller than the predetermined threshold value. The first and second electrical signals include current or voltage signals.

[0006] In accordance with another embodiment disclosed herein, a method for detection of an open-circuit fault occurring with a load that a power conversion system is coupled thereto is provided. The method includes receiving a first phase signal and a second phase signal at the output of the power conversion system measured in a stationary reference frame; determining at least one open-circuit fault condition that is occurring at the load based at least in part on the measured first and second phase signals in the stationary reference frame; and sending a control signal for protection of the power conversion system against the open-circuit fault condition in response to at least one determined open-circuit fault condition.

[0007] In accordance with another embodiment disclosed herein, a solar power conversion system is provided. The solar power conversion system includes a direct current (DC) bus for receiving DC power from a solar power source; a solar converter for converting the DC power at the DC bus to AC power; and a solar controller for regulating the active power component of the AC power according to an active power command signal and an active power feedback signal. The solar controller comprises an open-circuit detection module for receiving electrical signals measured at the output of the solar converter in a three-phase reference frame and detecting at least one open-circuit fault condition occurring with a load that the solar converter is supplying AC power thereto based at least in part on the measured electrical signals in the three-phase reference frame.

DRAWINGS

[0008] These and other features, aspects, and advantages 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:

[0009] FIG. 1 is an overall block diagram of a power conversion system in accordance with an exemplary embodiment of the present disclosure;

[0010] FIG. 2 is a detailed structure of a transformer shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure;

[0011] FIG. 3 is a block diagram showing one aspect of the detailed implementation of the open-circuit detection module shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure;

[0012] FIG. 4 is a block diagram showing one aspect of the detailed implementation of the open-circuit detection module shown in FIG. 1 in accordance with another exemplary embodiment of the present disclosure;

[0013] FIG. 5 is a block diagram showing one aspect of the detailed implementation of the open-circuit detection module shown in FIG. 1 in accordance with yet another exemplary embodiment of the present disclosure;

[0014] FIG. 6 is a block diagram showing another aspect of the detailed implementation of the open-circuit detection module shown in FIG. 1 in accordance with one exemplary embodiment of the present disclosure; [0015] FIG. 7 is a block diagram showing another aspect of the detailed implementation of the open-circuit fault module shown in FIG. 1 in accordance with another exemplary embodiment of the present disclosure;

[0016] FIG. 8 is a graph illustrating various waveforms of the power conversion system prior to, during, and subsequent to an open-circuit fault in accordance with an exemplary embodiment of the present disclosure;

[0017] FIG. 9 is a flowchart which outlines an implementation of an open- circuit fault detection method in accordance with an exemplary embodiment of the present disclosure; and

[0018] FIG. 10 is a flowchart which outlines an implementation of an open- circuit fault detection method in accordance with another exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

[0019] Embodiments disclosed herein generally relate to fault ride-through capabilities of power conversion systems or power generation systems such as, for example solar power generation systems, wind turbines, fuel cells, and marine hydrokinetic power generation systems (herein referred to as power conversion systems), and a combination of these systems. In particular, the power conversion system is provided with an "open-circuit ride through" capability which refers to keeping the power conversion system online or remaining connected to at least a portion of a power grid during one or more open-circuit fault conditions, such that, when the power grid is restored from the open-circuit fault, the power conversion system can continue to supply power to the power grid. The phrase "open-circuit fault condition" as used herein refers to an abnormal circuit condition wherein part of a circuit is opened, interrupted, or broken to discontinue a current flow or a circuit has an extremely high impedance value that causes the current to cease. More specifically, embodiments of the disclosure propose a "stationary reference frame open-circuit detection algorithm" which can be implemented by the power conversion system to allow faster detection of the open-circuit fault conditions. As used herein, "stationary reference frame open-circuit detection algorithm" may also be referred to as "three-phase open-circuit detection algorithm" which is particularly designed to allow open-circuit fault detection based at least in part on one or more electrical parameters measured in the stationary reference frame or three-phase reference frame instead of relying on the transformed components of the measured electrical parameters in a rotational reference frame for open-circuit fault detection.

[0020] In some embodiments, the "stationary reference frame open-circuit detection algorithm" proposed herein can be implemented based on a voltage source control (VSC) structure or scheme. As used herein, "voltage source control structure or scheme" refers a control embodiment wherein one of the primary control parameters is AC voltage including a voltage magnitude and a phase angle at the output of the power conversion system. The voltage source control differs from current source control by generating an internal frequency reference instead of depending on a phase angle provided by a phase locked loop (PLL) device. In some embodiments, upon detection of one or more open-circuit fault conditions by implementation of the "stationary reference frame open-circuit detection algorithm," one or more actions may be taken to protect one or more components of the power conversion system that otherwise may be damaged. One action may be reducing a power reference to limit the current at the output of the power conversion system in response to a detected open-circuit fault condition. Another action may be disabling a negative sequence current control module implemented within the power conversion system to stop the regulation of the negative sequence current in response to the detected open-circuit fault condition. In some specific embodiments, a current parameter at a secondary side of a transformer is measured for determining whether one or more open-circuit fault conditions are occurring. Further, to enable more reliable open-circuit fault condition detection, an anti-misjudgment mechanism may be employed. The "anti-misjudgment mechanism" used herein refers to the reporting of what appears to be an open-circuit fault only after the existence of the apparent open-circuit fault is determined to be continuing for a predetermined amount of time. Additionally, in some embodiments, voltage parameters at the secondary side of the transformer or the output of the power conversion system are also measured for facilitating open-circuit fault detection.

[0021] One technical advantage or benefit of the disclosed systems and methods for open-circuit fault detection is that the open-circuit fault can be detected more quickly by the implementation of the proposed "stationary reference frame open- circuit fault detection algorithm". With faster detection of the open-circuit fault, one or more protection actions can be taken in a more timely manner to protect one or more components of the power conversion system, such that the power conversion system can more safely ride-through the open-circuit fault conditions. Another technical advantage or benefit is that misjudgment of the open-circuit fault condition can be avoided by incorporation of the time triggering mechanism as well as by taking into consideration both the current and voltage measurement at the output of the power conversion system. Other technical advantages or benefits will become apparent to those skilled in the art by referring to the following detailed descriptions of one or more embodiments and accompanying drawings of the present disclosure.

[0022] In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the one or more specific embodiments. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation- specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

[0023] 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 "first", "second", and the like, as used herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. Also, 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 term "or" is meant to be inclusive and mean either any, several, or all of the listed 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. Furthermore, the terms "circuit" and "circuitry" and "controller" may include either a single component or a plurality of components, which are either active and/or passive and may be optionally be connected or otherwise coupled together to provide the described function.

[0024] FIG. 1 illustrates an overall block diagram of a power conversion system 10 in accordance with an exemplary embodiment of the present disclosure. In the illustrated embodiment, the power conversion system 10 is particularly shown as a solar power conversion system for facilitating explanation of open-circuit fault detection algorithms. However, a person having ordinary skills in the art can apply the control embodiments disclosed herein to other types of power conversion systems such as, for example, fuel cell systems, wind power conversion systems, and marine hydrokinetic energy power conversion systems. As illustrated, the solar power conversion system 10 generally includes a solar power converter system 14 for converting DC power generated from a solar power source 12 and providing AC power with suitable voltage and frequency for grid 18 transmission and distribution. In one embodiment, the solar power source 12 may include one or more photovoltaic arrays (PV arrays) having multiple interconnected solar cells that can convert solar radiation energy into DC power through the photovoltaic effect.

[0025] In one implementation, the solar power converter system 14 shown in FIG. 1 is based on a two-stage structure which includes a PV side converter 142 and a line side converter 144. The PV side converter 142 may include a DC-DC converter, such as a DC-DC boost converter, which steps up a DC voltage received from the solar power source 12 and outputs a higher DC voltage onto a DC bus 146. In some embodiments, a PV side filter 22 may be coupled between the solar power source 12 and the PV side converter 142. The PV side filter 22 may include one or more capacitive and inductive elements for removing ripple components of the DC power output from the solar power source 12 and blocking ripple signals transmitted from the PV side converter 142 to the solar power source 12. The DC bus 146 may include one or more capacitors coupled either in series or parallel for maintaining the DC voltage of the DC bus 146 at a certain level, and thus the energy flow from the DC bus 146 to the grid 18 can be managed. The line side converter 144 may include a DC-AC inverter which converts the DC voltage on the DC bus 146 to AC voltage with suitable frequency, phase, and magnitude for feeding to the grid 18. In some embodiments, a transformer 24 may be coupled between the line side converter 144 and the grid 18. The transformer 24 functions to isolate the line side converter 144 from the grid 18 and step up the magnitude of the output AC voltage to match the grid 18.

[0026] In one implementation, the solar power conversion system 10 shown in FIG. 1 further includes a solar converter control system 16 which functions to control operations of the PV side converter 142 and the line side converter 144 through implementation of control algorithms according to various feedback signals and command signals. More specifically, the solar converter control system 16 includes a PV side controller 164 and line side controller 156. The PV side controller 164 may be configured to send PV side control signals 168 to the PV side converter 142 based on DC voltage command signal 166 and DC voltage feedback signal 154 measured with a DC voltage sensor 145. The line side controller 156 may be configured to send line side control signals 172 to the line side converter 144 based on active power command signal 158, reactive power command signal 162, voltage feedback signal 148 measured with a voltage sensor 36, and current feedback signal 152 measured with a current sensor 34.

[0027] More specifically, in one embodiment, the line side controller 156 is constructed based at least in part on a voltage source control (VSC) structure. For example, the line side controller 156 may be designed with a voltage magnitude control loop which is configured to regulate the voltage magnitude of the output voltage of the line side converter 144 based at least in part on the reactive power command signal 162 and a feedback reactive power signal calculated from the voltage feedback signal 148 and current feedback signal 152. The line side controller 156 may be further designed with a phase angle control loop which is configured to regulate the phase angle of the output voltage of the line side converter 144 based at least in part on the active power command signal 158 and active power feedback signal calculated from the voltage feedback signal 148 and the current feedback signal 152.

[0028] For purposes of description, the PV side controller 164 and line side controller 156 are shown as separate components; however, in some embodiments, the PV side controller 164 and the line side controller 156 may be embodied as a single controller. In practical applications, the controller may be implemented as a micro-controller, a digital signal processor (DSP), or any other appropriate programmable device. In some embodiments, among other elements, the PV side controller 164 and the PV converter 142 can be packed together in a single housing. Similarly, among other elements, the line side converter 144 and the line side controller 156 can be packed together in a single housing.

[0029] Further referring to FIG. 1, the solar converter control system 16 may further include an open-circuit detection module 210. In general, the open-circuit detection module 210 is implemented to detect one or more open-circuit faults occurring at the grid 18 or any other system components such as, for example, the transformer 24 coupled between the grid 18 and the line side converter 144 and provide one or more status signals indicative of the existence of open-circuit fault. In response to the detected open-circuit fault conditions, the open-circuit detection module 210 is further implemented to transition the solar power conversion system 10 from a normal mode to an open-circuit protection mode. As used herein, "normal mode" refers to one operating state of the solar power conversion system 10 in which the solar power conversion system 10 is synchronized to the grid 18 and AC power with nominal voltage magnitude and frequency can be fed from the solar power conversion system 10 to the grid 18. The "open-circuit protection mode" refers to another operating state of the solar power conversion system 10 in which one or more actions are initiated in response to the detected open-circuit fault condition for protection of one or more components of the solar power conversion system 10 against one or more problems such as over-current problems caused from the open- circuit fault. In addition, after the open-circuit fault is ended or cleared, the open- circuit detection module 210 may be further implemented to transition the solar power conversion system 10 back to the normal mode to continue supplying AC power with nominal voltage magnitude and frequency to the grid 18. In the illustrated embodiment, the open-circuit detection module 210 is incorporated into the line side controller 156 to form a single device. In some other embodiments, the open-circuit detection module 210 may be alternatively configured as a separate stand-alone device. In detailed implementation, the open-circuit detection module 210 could be embodied as a piece of software program having multiple executable instructions stored in a non-transitory memory device, or embodied as a hardware circuit with multiple interconnected electronic elements which are capable of being operated to provide the open-circuit detection functions, or a combination thereof.

[0030] FIG. 2 illustrates a detailed configuration of the transformer 24 shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure. As shown, the transformer 24 includes three primary side windings 188, 192, 194 located proximate to the grid 18 and three secondary side windings 187, 191, 193 located proximate to the line side converter 144. Each of the primary side windings 188, 192, 194 may share a common core leg (not shown) with each of the secondary side windings 187, 191, 193 correspondingly. For example, a first primary side winding 188 and a first secondary side winding 187 may be commonly wrapped around a core leg of the transformer 24. At the primary side 171, the three primary windings 188, 192, 194 are connected head-to-tail to form a delta configuration. Also, the three primary windings 188, 192, 194 are coupled to the grid 18 via three AC lines 174, 176, 178 for supplying AC power typically with a high voltage to the grid 18. At the secondary side 173, the three secondary windings 187, 191, 193 are commonly connected to a neutral point n to form a star configuration. Also, the three secondary windings 187, 191, 193 are coupled to the line side converter 144 via three AC lines 182, 184, 186 for receiving AC power typically with a low voltage converted from the line side converter 144. [0031] During normal operations, when there is no open-circuit fault occurring, the transformer 24 operates to step up the AC voltage output from the line side converter 144 for grid 18 transmissions. The three-phase AC voltages and currents transmitted along AC lines 174, 176, 178 are equal in magnitude but differ in phase angle by 120°. Applying Kirchhoff s current law at node a would yield the following equation: where Ii _oad a * ^s C current flowing through AC line 174, / _α ¾ is the AC current flowing through primary side winding 188, and I _ca is the AC current flowing through primary side winding 192. Also, the AC current I _ca and l _a j _j can be described according to the following transformer equations

where I _ca and l _a j _j are two phase currents flowing through primary side windings

192, 188 respectively, I _c and I _a are two phase currents flowing through the secondary side windings 191 and 187 respectively, and p is the turns ratio between the primary side winding 192 and the secondary side winding 191 or between the primary side winding 188 and the secondary side winding 187. Substituting the two equations (2) and (3) into equation (1) yields the following equation:

From equation (4), it can be seen that the two phase currents I _c and I _a are not equal to each other when the solar power conversion system 10 is operating in the normal mode. However, this is not the case when one or more open-circuit fault is occurring. In one embodiment, assuming there is an open-circuit fault occurring at the grid 18 at location 175 of the AC line 174, in this condition, the current li _o d a flowing through the AC line 174 would be zero or be a value near zero due to the open-circuit fault. According to equation (4), the AC current I _a is equal to or approximately equal to the AC current I _c . Similarly, when there is an open-circuit fault occurring at the AC line 176, the AC current I _a is equal to or approximately equal to the AC current . Also, when there is an open-circuit fault occurring at the AC line 178, the

AC current I _c is equal to or approximately equal to the AC current . A summarize of the open-circuit fault and the corresponding current relationship can be shown as below in table- 1.

Table- 1 Open-circuit fault and corresponding current relationship

[0032] Thus the solar power conversion system 10 will have different current relationships at the secondary side of the transformer 24 with and without the existence of one or more open-circuit fault conditions. It is therefore possible to use the current relationship summarized in table- 1 as one of the criteria for determining if one or more open-circuit faults are occurring at the grid 18. Detailed descriptions will be given below as to how to determine the current relationship for open-circuit fault condition detection.

[0033] FIG. 3 illustrates a detailed block diagram of one aspect of the open- circuit detection module 210 shown in FIG. 1 in accordance with an exemplary embodiment of the present disclosure. In general, the open-circuit detection module 210 includes a current relationship detection module 245 which is configured to determine if any two-phase currents of the three-phase currents are equal or close to each other according to a predefined standard. More specifically, in the illustrated embodiment, the current relationship detection module 245 includes a first summation element 216, a second summation element 238, and a third summation element 262. The first summation element 216 receives a measured first phase current signal 214 and a measured second phase current signal 212 and performs subtraction with respect to the received two phase current signals 212, 214 to generate a first current difference signal 218. The second summation element 238 receives the measured second phase current signal 212 and a measured third phase current signal 236 and performs subtraction with respect to the received two phase current signals 212, 236 to generate a second current difference signal 242. The third summation element 262 receives the measured third phase current signal 236 and the measured first phase current signal 214 and perform subtraction with respect to the received two phase current signals 236, 214 to generate a third current difference signal 264. In one embodiment, the measured first, second, and third phase current signals 214, 212, 236 may be obtained from current sensors placed for measuring the AC currents flowing through secondary side windings 187, 193, and 191 respectively. In another embodiment, the measured first, second, and third phase current signals 214, 212, 236 may be obtained from current sensors placed for measuring the AC currents flowing through the AC lines 182, 186, and 184 respectively.

[0034] Further referring to FIG. 3, the current relationship detection module 245 further includes a first calculation unit 222, a second calculation unit 244, and a third calculation unit 266. The three calculation units 222, 244, 266 are respectively coupled to the three summation element 216, 238, 262 for calculating absolute current difference signals 224, 246, 268 according to the current difference signals 218, 242, 264. In some embodiments, the absolute current difference signals 224, 246, 268 are optionally processed by three filter units 226, 248, 272 to provide filtered signals 228, 252, 274 by removing noise signals contained in the current difference signals 224, 246, 268. The absolute current difference signals 224, 246, 268 or the filtered versions 228, 252, 274 are supplied to three current comparators 232, 254, 276 which compare the absolute current difference signals 224, 246, 268 or 228, 252, 274 with a predetermined threshold value 275 and provide indication signals 234, 256, 278 indicating if any two phase measured current signals are equal or approximately equal to each other. In one exemplary embodiment, the indication signals 234, 256, 278 include logic signals. For example, the indication signals 234, 256, 278 may have logic value "1" when the absolute current difference signals 224, 246, 268 is determined to be smaller than the predetermined threshold value 275 and have logic value "0" when the absolute current difference signals 224, 246, 268 is determined to be larger than the predetermined threshold value 275.

[0035] In one embodiment, the three current comparators 232, 254, 276 shown in FIG. 3 are set with the same threshold value 275 for comparison. In other embodiments, the three current comparators 232, 254, 276 may be set with different threshold values for comparison. In one embodiment, the predetermined threshold value 275 may comprise a fixed value which may be set by an operator according to practical requirements. In another embodiment, the predetermined threshold value 275 may be a changeable one. For example, the predetermined threshold value 275 may be associated with a maximum allowable power command and is changeable with the maximum allowable power command. More specifically, in the illustrated embodiment, the predetermined threshold value 275 is provided from a selection unit 299 which selects one of a first threshold value 295 and a second threshold value 298 whichever is smaller. The first threshold value 295 is a fixed value which defines a minimum value. The second threshold value 298 is associated with a maximum power command value 293 which defines a maximum power output of the solar power conversion system 10. In different applications, the solar power conversion system 10 may be required to provide output power with different levels. Thus, it is beneficial to set different threshold values for open-circuit fault detection to allow more reliable detection of the open-circuit faults. In one embodiment, a calculation unit 296 is used to multiply the maximum power command value 293 with a small coefficient for generation of the second threshold value 298.

[0036] Further referring to FIG. 3, in one embodiment, the current relationship detection module 245 may further include a logic module 279 for reporting current relationship determination result. In the illustrated embodiment, three current relationships are determined. In alternative embodiment, when only one current relationship is determined, the logic module 279 could be omitted from the current relationship detection module 245. That is, any one of the three indication signals 234, 256, 278 can be sent directly to an open-circuit judgment unit 286 which will be described below with more detail for open-circuit fault detection. In the illustrated embodiment, the logic module 279 includes a first logic unit 258 and a second logic unit 282 which function together to provide a current status signal 284. More specifically, the first logic unit 258 is coupled to receive the first indication signal 234 from the first comparator 232 and the second indication signal 256 from the second comparator 254. In one embodiment, the first logic unit 258 provides a logic signal 274 having a logic value "1" for example, when at least one of the first indication signal 234 and the second indication signal 256 is logic "1". The second logic unit 282 is coupled to receive the first logic signal 274 and the third indication signal 278 from the third comparator 276. The second logic unit 282 provides the current status 284 with a status value representing an open circuit when at least one of the logic signal 274 and the third indication signal 278 is logic "1". It is beneficial to use the first and second logic units 258, 282 because only one current status signal 284 is provided to indicate the determination result of multiple sets of current relationships.

[0037] With continuing reference to FIG. 3, in one embodiment, the open- circuit detection module 210 further includes an open-circuit judgment unit 286 for reporting an open-circuit fault condition based at least in part on the current status signal 284 provided from the current relationship detection module 245. In one embodiment, to ensure reliable detection of the open-circuit fault condition, the open- circuit judgment unit 286 is further configured for calculating a length of time that the current status signal remains unchanged and determining whether the length of time exceeds a predetermined period of time. The open-circuit judgment unit 286 provides a control signal 288 indicating at least one open-circuit fault is occurring upon determination that the length of time that the current status signal 284 remaining unchanged exceeds the predetermined period of time. On the other hand, if the length of time that the current status signal 284 remains unchanged does not exceed the predetermined period of the time, the open-circuit judgment unit 286 either does not provide control signal 288 for open-circuit fault report or does provide a signal indicating no open-circuit fault is occurring. In one embodiment, the control signal 288 comprises a logic signal. When the open-circuit fault condition is confirmed, the control signal 288 may have a logic value "0" for example.

[0038] Further referring to FIG. 3, in one embodiment, the open-circuit detection module 210 may optionally or additionally include a signal generating unit 292. The signal generating unit 292 provides a signal 294 to enable or disable the open-circuit judgment unit 286 according to various operating conditions of the solar power conversion system 10. For example, when the solar power conversion system 10 is known to be experiencing a short-circuit fault condition, the signal generating unit 292 may provide a disable signal 294 to stop operation of the open-circuit judgment unit 286. .

[0039] In another embodiment, to enable more reliable open-circuit fault detection, in addition to using the current relationship as described above for open- circuit fault detection, the open-circuit detection module 210 may further determine a voltage relationship. More specifically, a further evaluation can be made to ascertain whether the output voltage of the line side converter 144 is deviated from a nominal voltage. It is observed that, during open-circuit fault conditions, the output voltage of the line side converter 144 substantially remains unchanged with respect to the nominal voltage. This is different from the occasion when a short-circuit fault condition occurs, in which the output voltage may be dropped to a low value due to short-circuit fault. In this alternative embodiment, it is beneficial to consider the voltage relationship to avoid misjudgment of the open-circuit fault.

[0040] In one exemplary embodiment, as shown in FIG. 4, the open-circuit detection module 210 further includes a voltage status detection module 243 which is configured to determine whether the output voltage of the line side converter 144 is deviated from a nominal voltage. More specifically, the voltage status detection module 243 determines whether the output voltage is smaller than the nominal voltage according to a predetermined standard. In one embodiment, the voltage status detection module 243 includes a summation element 235 which receives a measured output voltage signal 231 and a nominal voltage signal 233 and subtracts the received voltage signal to generate a voltage difference signal 237. In one implementation, the output voltage signal 231 may be a single-phase voltage signal or a three-phase voltage signal. Also, the output voltage signal 231 could be a maximum magnitude value or a root-mean-square value. In a particular embodiment, the output voltage signal 231 is measured at a point of terminal connection (POTC) or point of common coupling (POCC). The voltage difference signal 237 is supplied to a voltage comparator 239 which compares the voltage difference signal 237 with a predetermined voltage threshold value and outputs a voltage status signal 241 according to the comparison result.

[0041] In one embodiment, the voltage status signal 241 is logic signal which has logic value "0" for example, when the voltage difference signal 237 is determined to be smaller than the predetermined voltage threshold value. In this case, the solar power conversion system 10 may undergo a short-circuit fault which causes the output voltage to be much smaller than the nominal voltage, and the open-circuit detection module 210 should not report an open-circuit fault even if the current relationship as described with reference to FIG. 3 is satisfied. In the illustrated embodiment, the open-circuit judgment unit 286 does not provide the control signal 288 or provides a status signal indicating that no open-circuit fault is occurring based at least in part on the voltage status signal 241. On the other hand, in one embodiment, the voltage status signal 241 has a logic value "1" for example, when the voltage difference signal 237 is determined to be not smaller than the predetermined voltage threshold value. In this case, the open-circuit judgment unit 286 provides the control signal 288 based at least in part on the current status signal 284, the voltage status signal 241, and the time enable signal 294 indicating that at least one open- circuit fault condition is occurring.

[0042] In a further embodiment, one or more command signals provided to solar power conversion system 10 for setting a target value of one or more system variables can be taken into account for open-circuit fault detection. In an exemplary embodiment, as shown in FIG. 5, the open-circuit judgment unit 286 further receives an active power command signal 158 which defines a desired active power to be achieved at the output of the solar power conversion system 10. The open-circuit judgment unit 286 determines if the active power command signal 158 is zero or has a value near zero. As used herein with respect to the active power command signal, zero or near zero means that the value is either zero or a slightly positive or negative value that is small enough in magnitude for open-circuit condition determination. In one embodiment, if the active power command signal 158 is zero or near zero, the open-circuit judgment unit 286 will not provide control signal 288 or will provide a status signal indicating that no open-circuit fault condition should be reported. On the other hand, if the active power command signal 158 is not zero or not near zero, the open-circuit judgment unit 286 then follows the same rules as described above with reference to FIG. 4 to determine whether a control signal 288 should be issued or not based at least in part on the current status signal 284, the voltage status signal 241, and the time enable signal 294.

[0043] FIG. 6 and FIG. 7 show exemplary protection actions that the solar power conversion system 10 shown in FIG. 1 may take upon one or more open-circuit fault conditions being confirmed by the open-circuit detection module 210. In particular, FIG. 6 illustrates one of the exemplary actions of reducing a power command signal upon detection of one or more open-circuit fault conditions. In general, the open-circuit detection module 210 further includes a power command limit signal generating unit 320 coupled to the open-circuit judgment unit 286 for receiving the control signal 288 provided from the open-circuit judgment unit 286. The power command limit signal generating unit 320 is configured to generate normal power command limit signals for limiting the power command signal 158 in the event that the control signal 288 indicates no open-circuit fault condition is occurring such that the solar power conversion system 10 can operate in the normal mode. The power command limit signal generating unit 320 is further configured to generate open-circuit power command limit signals for limiting the power command signal 158 in the event that the control signal 288 indicates that at least one open-circuit fault condition is occurring such that the solar power conversion system 10 can operate in open-circuit protection mode or ride through the open-circuit fault conditions. [0044] More specifically, as shown in FIG. 6, in one embodiment, the open- circuit judgment unit 286 is coupled to a switch 316 which is responsive to the control signal 288. In one embodiment, the control signal 288 is a logic signal having a logic value "0" when at least one open-circuit fault is detected and a logic value "1" when no open-circuit fault is detected. In response to the control signal 288 having logic value "1", the switch 316 couples to the first power command limits generator 312 to allow normal power limit signals 314 generated therefrom to be transmitted to a command limiter unit 344. The normal power limit signals 314 may include an upper limit and a lower limit which define a maximum allowable power and a minimum allowable power for the solar power conversion system 10. The command limiter unit 344 limits the power command signal 158 according to the normal power limit signals 314 and provides limited power command signal 346 to an active power regulator 348. The active power regulator 348 further regulates a power feedback signal 356 according to the limited power command signal 346 and provides a phase angle command signal 352 which is used by the line side controller 156 shown in FIG. 1 for generation of the line side control signals 172.

[0045] With continuing reference to FIG. 6, when the control signal 288 has a logic value "0" indicative of at least one open-circuit fault condition, the solar power conversion system 10 should be transitioned to operate in the open-circuit protection mode. More specifically, the switch 316 is operated to couple a second power command limits generator 324 to the command limiter unit 344. The second power command limits generator 324 transmits open-circuit power limit signals 322 to the command limiter unit 344. In an exemplary embodiment, the open-circuit power limit signals 322 may be zero or near zero. In this case, the command limiter unit 344 reduces the power command signal 158 to zero or a near zero and the active power regulator 348 regulates the power feedback signal 356 according to the reduced power command signal 158, such that power at the output of the line side converter 144 is reduced to zero or near zero. Reducing the output power to zero or near zero can protect one or more electrical components such as, for example the line side converter 144 of the solar power conversion system 10 from being damaged by the over-current problems caused by one or more open-circuit faults. As used herein with respect to the output of the line side converter, zero or near zero means that the value is either zero or a slightly positive or negative value that is small enough in magnitude to protect equipment from being damaged during the fault conditions.

[0046] Further referring to FIG. 6, the protection action initiated to reduce the output power to zero or near zero should be withdrawn to allow the solar power conversion system 10 to transition back to the normal mode once the gird 18 is restored or the one or more open-circuit fault conditions are cleared. In the case of one or more open-circuit fault conditions being ended or cleared, the open-circuit judgment unit 286 updates the control signal 288 which is transmitted to the switch 316. When returning back to the normal mode, the switch unit 316 recouples the first power command limits generator 312 with the command limiter unit 344, such that the limit value for limiting the power command signal 158 is set by the normal power limit signals 314 provided from the first power command limits generator 312.

[0047] FIG. 7 illustrates another action that the solar power conversion system 10 may take in response to one or more detected open-circuit fault conditions. In general, FIG. 7 shows an exemplary embodiment of disabling a negative sequence current control module 360 when one or more open-circuit fault conditions are detected and confirmed. The negative sequence current control module 360 can be implemented in software or hardware or a combination thereof for performing regulations of the negative sequence current component which may result from a nonsymmetrical short-circuit fault. In the illustrated embodiment, the negative sequence current control module 360 includes a d-axis negative sequence current regulator 358 which generates a d-axis negative voltage command signal 359 based at least in part on a d-axis negative sequence current signal 353 and a q-axis negative sequence current signal 355. The negative sequence current control module 360 further includes a q-axis negative sequence current regulator 362 which generates a q-axis negative voltage command signal 361 based at least in part on the d-axis negative sequence current signal 353 and the q-axis negative sequence current signal 355. In one implementation, the negative sequence current control module 360 further includes a negative sequence limiter 350 which is generally configured to limit the d- axis negative voltage command signal 359 and the q-axis negative voltage command signal 361 based at least in part on the control signal 288 received from the open- circuit judgment unit 286. For example, the negative sequence limiter 350 reduces the d-axis negative voltage command signal 359 and the q-axis negative voltage command signal 361 to zero or near zero when the control signal 288 indicates at least one open-circuit fault condition is occurring. Further, the negative sequence limiter 350 allows the d-axis negative voltage command signal 359 and the q-axis negative voltage command signal 361 to pass through when the control signal 288 indicates that no open-circuit fault condition is occurring.

[0048] Further referring to FIG. 7, more specifically, in one embodiment, the negative sequence limiter 350 includes a first multiplication unit 364 and a second multiplication unit 366. The d-axis negative sequence voltage command signal 359 and the q-axis negative sequence voltage command signal 361 are supplied to the first multiplication unit 364 and the second multiplication unit 366 respectively. In normal operation, for this embodiment, the open-circuit judgment unit 286 sends the control signal 288 having a logic value of "1" indicating that no open-circuit fault is occurring or no open-circuit fault should be reported. The first and second multiplication units 364, 366 multiply the control signal 288 having logic value "1" with the voltage signals 359, 361 and provide d-axis and q-axis negative sequence voltage command signals 363 and 365 for current regulation. On the other hand, when one or more open-circuit fault conditions have been detected, the open-circuit judgment unit 286 provides a control signal 288 having a logic value "0" which is used by the first and second multiplication units 364, 366 to do multiplication operations. In this case, the d-axis and q-axis negative sequence voltage command signals 363 and 365 are zero, such that the negative sequence control module 360 is disabled.

[0049] In a similar way as described above with reference to FIG. 6, in one embodiment, upon one or more open-circuit fault conditions having ended or cleared, the negative sequence current control module 360 should be enabled to regulate the negative sequence current at the output of the line side converter 144 after the solar power conversion system 10 is transitioned back to normal mode. In this condition, the open-circuit judgment unit 286 updates the control signal 288 transmitted to the first and second multiplication units 364, 366. In one embodiment, the control signal 288 having a logic value "1" is multiplied by the first and second multiplication units 364, 366 to make the negative sequence current control module 360 to have normal d- axis and q-axis negative sequence voltage signals 363, 365. In one embodiment, when the solar power conversion system 10 is transitioning from an open-circuit protection mode to a normal mode, the control signal 288 may be switched from "0" to "1". In another embodiment, a time dependent coefficient may be applied to make the control signal 288 provided from the open-circuit judgment unit 286 ramp up from "0" to "1" in a given time interval. In this case, the negative sequence current control module 360 can restore to normal operation slowly to avoid large transients.

[0050] Further referring to FIG. 7, in alternative embodiment, the control signal 288 provided from the open-circuit judgment unit 286 can be optionally or additionally provided to the d-axis negative sequence current regulator 358 and the q- axis negative sequence current regulator 362. More specifically, in one embodiment, in response to the control signal 288 having a logic value "0" indicative of an open- circuit fault condition for example, one or more integrators within the d-axis negative sequence current regulator 358 and the q-axis negative sequence current regulator 362 may stop working to avoid integrator saturation during open-circuit fault. On the other hand, in response to the control signal 288 having logic value "1" indicative no open-circuit condition for example, the one or more integrators within the d-axis negative sequence current regulator 358 and the q-axis negative sequence current regulator 362 can be enabled for normal integration operations.

[0051] FIG. 8 illustrates a simulation result which comprises various waveforms of the solar power conversion system 10 prior to, during, and subsequent to the implementation of open-circuit protection mechanism in accordance with an exemplary embodiment of the present disclosure. More specifically, as shown in FIG. 8, the upper graph 371 shows the three-phase current waveforms 375, 377, 379 at the secondary side 173 of the transformer 24, and the lower graph 373 shows the active power command waveform 381 and measured active power waveform 383. During time period tg to tj, there is no open-circuit fault occurring, and the solar power conversion system 10 operates in the normal mode to provide AC power output according to the active power command 381. It can be seen the measured or actual active power 383 exactly follows the active power command 381. Also, during this time period tQ-ti, the solar power conversion system 10 provides three-phase currents

375, 377, 379 that are equal in magnitude and are spaced apart in phase angle by 120°.

Starting from time point tj, an open-circuit fault is occurring, and the two-phase currents 375, 377 are equal both in magnitude and phase angle which can be used as one of the criteria for open-circuit fault detection. Starting from time point t2, the active power command 381 is reduced to zero as an open-circuit fault condition was confirmed. Following the zero active power command 381, the actual reactive power 383 changes gradually to zero, and the three-phase currents 375, 377, 379 reduce their magnitudes to near zero, such that the various electrical components within the solar power conversion system 10 can be protected from over-current problems. Starting from time point tj, when the open-circuit fault is ended or is cleared, the active power command 381 is changed from zero to nominal power command value from 13 to 14.

After time point 14, in response to the nominal active power command 381, the solar power conversion system 10 returns to operate in the normal mode to supply the three-phase currents 375, 377, 379 having equal magnitude phase angle spacing apart by 120°.

[0052] FIG. 9 illustrates a flowchart of a method for open-circuit fault detection and protection of the power conversion system 10 shown in FIG. 1 in accordance with an exemplary embodiment. The method 4000 may be programmed with software instructions stored in a computer-readable storage medium which, when executed by a processor, perform various steps of the method 4000. The computer-readable storage medium may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology. The computer-readable storage medium includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other non-transitory medium which can be used to store the desired information and which can accessed by an instruction execution system.

[0053] In one implementation, the method 4000 may start at block 4002. At block 4002, measured current signals are received. In one implementation, the current signals include three-phase current signals which may be measured at a secondary side of the transformer 24 as shown in FIG. 1 and FIG. 2 by measuring current flowing through the secondary side windings 187, 193, 191 for example. In alternative embodiment, the three-phase current signals can also be measured at the output of the line side converter 144 by measuring current flowing through the AC lines 182, 186, 184 coupled between the line side converter 144 and the secondary side 173 of the transformer 24 for example.

[0054] At block 4004, the method 4000 continues to calculate at least one current difference value between any two phases of the measured three-phase current signals. More specifically, in one embodiment, an absolute value between any two phases of the measured three-phase current signals is calculated. In an optional embodiment, the calculated absolute value of the current difference may be further processed by filtering operations.

[0055] At block 4006, a determination is made to ascertain whether the calculated current difference value meet certain current criteria. In one implementation, the determination is made to ascertain whether the absolute current difference value is smaller than a predetermined threshold value. If the determination is positive, the process goes to block 4008. Otherwise, the process goes back to block 4002 for continuing receiving measured current signals.

[0056] At block 4008, at least one current status signal is provided in response to the positive determination that at least two phase currents are equal or approximately equal which indicates that at least one open-circuit fault is possibly occurring with the grid 18. In a particular embodiment, when three-phase open- circuit fault conditions are to be detected, a first logic unit 258 and a second logic unit 282 shown in FIGS. 3-5 for example can be used to report current status signals indicative of at least two phase of the three-phase currents being equal or approximately equal to each other. As can be understood, when only one phase open- circuit fault is to be detected, the first logic unit 258 and the second logic unit 282 can be omitted in some alternative embodiments.

[0057] At block 4012, a determination is made to ascertain whether the current status signal has been kept unchanged for a predetermined period of time. Because some transient events happening at the solar power conversion system 10 or the grid 18 may cause two-phase currents to be equal, the determination at block 4012 can make sure true open-circuit fault conditions are detected. In one particular embodiment, the current status signal is a logic signal having a logic values. If the determination at 4012 is positive, for example, the logic signal with a logic value "0" has been kept unchanged for the predetermined period of time, the process goes to block 4014. Otherwise, the process returns to block 4002 for continuing receiving measured current signals.

[0058] At block 4014, the method 4000 continues to provide a control signal indicating at least one open circuit fault is occurring in response to the positive determination at block 4012. The control signal is provided for initiating one or more protection actions that the solar power conversion system 10 should take for open- circuit fault ride-through.

[0059] At block 4016, in response to the open-circuit signal, one protection action is to reduce a power command signal to make the solar power conversion system 10 provide reduced power. In one embodiment, a switch unit 316 as shown in FIG. 6 is employed which can respond to the control signal 288 provided from the open-circuit judgment unit 286 by selectively transmitting command limit signals 322 to a command limiter unit 344 for limiting the power command signal 158. In a particular embodiment, the command limit signals 322 is set to be zero or near zero which helps to reduce the power command signal 158 to be zero or near zero such that the solar power conversion system 10 substantially has no power output. Reducing the power output allows various electrical components within the solar power conversion system 10 and one or more loads coupled to the solar power conversion system 10 to be protected against one or more problems such as over-current problems caused from the open-circuit fault. Therefore, the solar power conversion system 10 can ride-through open-circuit fault conditions.

[0060] Block 4018 is an optional block performed to disable a negative sequence current control module in response to the open-circuit signal provided from block 4014. In one implementation, as shown in FIG. 7, the negative sequence current control module is disabled by employing two multiplication units 364, 366 which, in one embodiment, multiply the open-circuit signal 288 having a logic value "0" with voltage command signals generated from the internal regulators to cause the negative sequence current control module to have no command signal outputs.

[0061] The method 4000 described above may be modified in various ways in accordance with certain embodiments of the present disclosure. For example, one or more operations of the method 4000 may be eliminated (e.g., the block 4018 may be removed in alternative embodiment) or executed out of order. Additionally, one or more operations may be added to method 4000. For example, following the block 4016 and block 4018, the method 4000 may further include a determination block for determining whether the open-circuit fault is ended or cleared. Upon determining that the open-circuit fault has been ended or cleared, the method 4000 may further include blocks to withdraw the protection actions initiated at block 4016 and 4018 for example. In this condition, the solar power conversion system 10 is able to resume normal mode operation.

[0062] FIG. 10 illustrates another embodiment of the method 4000 for open- circuit fault detection and protection of the solar power conversion system 10. As shown in FIG. 10, following the positive determination at block 4012, the method 4000 may additionally include a determination step 4013 to ascertain whether a voltage relationship of the solar power conversion system 10 is satisfied. More specifically, at block 4013, a voltage at the output of the line side converter 144 or more specifically a voltage at the point of terminal connection (POTC) or point of common coupling (POCC) is measured. The determination is made to ascertain whether a voltage difference between the measured voltage and a nominal voltage is smaller than a predetermined voltage threshold value. If the determination is negative, the process goes to block 4014 which is the same as that described above with reference to FIG. 9. Otherwise, the process returns to block 4002 for continuing receiving current signals.

[0063] While the invention has been described with reference to exemplary embodiments, 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. Furthermore, the skilled artisan will recognize the interchangeability of various features from different embodiments. Similarly, the various method steps and features described, as well as other known equivalents for each such methods and feature, can be mixed and matched by one of ordinary skill in this art to construct additional assemblies and techniques in accordance with principles of this disclosure. 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.