WO2013142553A2

WO2013142553A2 - System and method for islanding detection and protection

Info

Publication number: WO2013142553A2
Application number: PCT/US2013/033062
Authority: WO
Inventors: Maozhong Gong; Zhuohui Tan; Yunfeng Liu; Xueqin Wu; Huibin Zhu; David Smith
Original assignee: General Electric Company
Priority date: 2012-03-23
Filing date: 2013-03-20
Publication date: 2013-09-26
Also published as: WO2013142553A3; CN103326350B; CN103326350A

Abstract

An exemplary system and method for detecting an islanding condition are provided. The system includes a converter system for converting input power from a power source and providing output power and a converter controller for regulating active and reactive power components of the output power. The converter controller includes an anti-islanding protection module for performing the steps of generating an anti- islanding disturbance signal and a disturbance compensation signal based on at least one measured electrical value of the output value; separately applying the anti- islanding disturbance signal and the disturbance compensation signal to adjust either or both of the active and reactive power components to drive the at least one measured electrical value outside of a nominal range upon the occurrence of an islanding condition; and detecting the islanding condition.

Description

SYSTEM AND METHOD FOR ISLANDING DETECTION AND PROTECTION

BACKGROUND

[0001] Embodiments of the disclosure relate generally to systems and methods for islanding condition detection and protection.

[0002] Islanding is a situation in which distributed generation (DG) systems (e.g., solar power conversion systems, wind power conversion systems) continue supplying electric power to a portion of an electric system when the DG system is electrically separated from the main power system. Islanding raises some safety concerns. For example, when the grid is lost, workers who are sent to repair a portion of the grid may be injured if they are not aware of the portion of the grid that is receiving power from a DG system. Further, when the grid is tripped, the grid has no control over the voltage and frequency supplied to the islanded location. Hence, when the grid is restored, a large phase difference may exist between the grid voltage and the inverter output voltage. The large phase difference may cause a large surge current to flow from the grid to the DG system, which may damage the inverter devices residing in the DG system.

[0003] Certain industrial standards (e.g., IEEE 1547 and UL1741) have been developed to address the safety concerns. Typically, these industrial standards require the DG systems to provide an anti-islanding protection mechanism which is able to detect the islanding condition and disconnect a local load (or stop supplying electric power to the local load) within a certain period of time (e.g., two seconds). In general, two types of anti-islanding protection methods have been proposed to detect islanding conditions. One is a passive method which detects the voltage or frequency at the output of the DG systems. The DG systems are disconnected from the local load when the output voltage or frequency exceeds predefined thresholds or ranges for a specified period of time. The other is an active method which deliberately introduces a small disturbance (e.g., frequency disturbance, active power disturbance, or reactive power disturbance) to drive the voltage or frequency at the output of the DG systems outside of nominal ranges. Both of the passive and active methods have a non-detection zone (NDZ) which occurs when there is match between the DG systems and the local load. A NDZ refers to load condition which matches the DG system in some extent that, when the grid is lost, the DG system is not able to detect the islanding condition either through the passive or active method.

[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 comprises a converter system for converting input power from a power source and providing output power and a converter controller for regulating active and reactive power components of the output power. The converter controller comprises an anti-islanding protection module for performing the steps of generating an anti-islanding disturbance signal and a disturbance compensation signal based on at least one measured electrical value of the output value; separately applying the anti-islanding disturbance signal and the disturbance compensation signal to adjust either or both of the active and reactive power components to drive the at least one measured electrical value outside of a nominal range upon the occurrence of an islanding condition; and detecting the islanding condition.

[0006] In accordance with another embodiment disclosed herein, a method for anti-islanding protection of a power conversion system is provided. The method comprises receiving at least one measured electrical value at an output of the power conversion system; generating an anti-islanding disturbance signal and a disturbance compensation signal based on the at least one measured electrical value; applying the anti-islanding disturbance signal to drive the at least one electrical value outside of a nominal range upon an occurrence of an islanding event; and applying the disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

[0007] In accordance with another embodiment disclosed herein, a solar power conversion system is provided. The solar power conversion system comprises 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 configured to regulate the AC power at the output of the solar converter in a d-q reference frame. The solar controller comprises an anti-islanding protection module for performing the steps of generating an anti-islanding disturbance signal and a disturbance compensation signal based on at least one measured electrical value of the AC power, applying the anti-islanding disturbance signal for driving at least one electrical value outside of a nominal range upon an occurrence of an islanding condition, and applying the disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

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 block diagram of a solar power conversion system provided with an improved anti-islanding protection mechanism in accordance with an exemplary embodiment of the present disclosure;

[0011] FIG. 3 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with an exemplary embodiment of the present disclosure; [0012] FIG. 4 is a detailed block diagram of the current regulator and the dynamic over-compensation unit shown in FIG. 3 in accordance with an exemplary embodiment of the present disclosure;

[0013] FIG. 5 is a detailed block diagram of the current regulator and the dynamic compensation unit shown in FIG. 3 in accordance with another exemplary embodiment of the present disclosure;

[0014] FIG. 6 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0015] FIG. 7 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0016] FIG. 8 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0017] FIG. 9 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0018] FIG. 10 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0019] FIG. 11 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0020] FIG. 12 is a detailed block diagram of the current regulator and the dynamic compensation unit shown in FIG. 11 in accordance with one exemplary embodiment of the present disclosure; [0021] FIG. 13 is a detailed block diagram of the current regulator and the dynamic over-compensation unit shown in FIG. 11 in accordance with another exemplary embodiment of the present disclosure;

[0022] FIG. 14 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0023] FIG. 15 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0024] FIG. 16 is a detailed block diagram of the anti-islanding protection module shown in FIG. 2 in accordance with another exemplary embodiment of the present disclosure;

[0025] FIG. 17 is a block diagram of a solar power conversion system provided with an improved anti-islanding protection mechanism in accordance with another exemplary embodiment of the present disclosure; and

[0026] FIG. 18 is a flowchart which outlines an implementation of an anti- islanding protection method in accordance with an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

[0027] Embodiments disclosed herein generally relate to an active disturbance method and a power conversion system implemented with the active disturbance method for islanding detection and anti-islanding protection of the power conversion system. As used herein, "active disturbance method" refers to driving change of a target electrical parameter by deliberately introducing or injecting an anti-islanding disturbance signal for facilitating the detection of islanding conditions and providing anti-islanding protection for power conversion systems. More specifically, a "disturbance compensation" mechanism or method is proposed to eliminate or reduce a non-detection zone (NDZ) which is typically found in a distributed generation (DG) system with at least one local load coupled thereto. As used herein, "disturbance compensation" refers to a control mechanism or algorithm which is able to generate a compensation signal based on the anti-islanding disturbance signal and apply the compensation signal to eliminate, cancel, or reduce an undesirable change caused by applying the anti-islanding disturbance signal to a controller or processor of the power conversion system. For example, the undesirable change may comprise an undesirable active power change appearing in a d-axis control loop configured for active power regulation during or after the anti-islanding disturbance signal is applied to a q-axis control loop configured for reactive power regulation. It is observed that in the conventional power conversion systems or DG systems, the undesirable change, if not compensated, tends to pull back or slower the movement of a target variable in a direction opposite to what the anti-islanding disturbance signal intends to do, thus making the islanding detection less effective. As used herein, "disturbance compensation signal" may also be referenced as perturbance or perturbation signal and is described as a small signal introduced or injected to change one or more electrical parameters (e.g., voltage, frequency, phase, impedance, harmonics, active power, and/or reactive power) at the output of the power conversion system for facilitating easy detection of the islanding condition while not significantly affecting the output power quality in normal power generation processes.

[0028] In some embodiments, the disturbance compensation signal can be generated and applied to exactly cancel or remove the undesirable change. In some other embodiments, the disturbance compensation signals can be generated and applied in a manner not only canceling the undesirable change but also contributing to a positive feedback which can accelerate the movement or shift of a target variable such as frequency and voltage so as to make the target variable move more quickly outside of a nominal or threshold range, which can be referred to as "disturbance over-compensation." Further, in some embodiments, alternative or additional adaptive controls may be implemented to accelerate the process for detection of the islanding conditions upon a loss or trip of the grid. As used herein, "adaptive control" refers to applying a dynamic or adjustable gain during generation of the anti-islanding disturbance signal according to a target electrical parameter change rate. The "dynamic or adjustable gain" used herein could be a continuous variable (i.e., a gain value corresponding to a target electrical parameter change rate value) or in some embodiments could be non-continuous variable (i.e., a gain value corresponding to multiple target electrical parameter change rate values). For example, in an early stage of anti-islanding disturbance signal injection, a relatively large gain may be applied when the target electrical parameter change rate is relatively small to accelerate the drift of the target electrical parameter. While in a middle or later stage of anti-islanding disturbance signal injection, the gain can be adjusted to have a relatively small value to maintain the system stability when the target electrical parameter has a relatively large change rate due to a positive feedback.

[0029] One technical advantage or benefit of the disclosed systems and methods for islanding detection or anti-islanding protection is that the NDZ can be significantly reduced when the "disturbance compensation" control mechanism is implemented. The reduced NDZ allows accurate and reliable detection of the islanding condition, which may provide more safety to workers who may be sent to repair a portion of a tripped grid and may also provide protection of the inverter devices against damage risks when the grid is restored from a trip condition. Another technical advantage or benefit is that the proposed "disturbance compensation" method can introduce relatively smaller disturbance signal when an undesirable change is removed or reduced, resulting in smaller disturbance to the system. A further technical advantage or benefit is that the islanding condition detection time can be reduced when disturbance acceleration mechanism such as "disturbance overcompensation", "adaptive control", or a combination thereof are employed in the power conversion system.

[0030] One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. 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.

[0031] 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.

[0032] 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 generally comprises a converter system 14 which is configured for converting first electrical power 121 obtained from a power source 12 and providing second electrical power 141 to a local load 19 and/or a grid 16. The first electrical power 121 may either be a direct current (DC) power generated for example with a solar panel power source, or an alternating current (AC) power generated for example with a wind turbine power source. In other embodiments, the power source 12 may be any type of distributed generation (DG) power sources, including, but not limited to, fuel cell, battery, micro-turbine, hydrokinetic turbine etc. For DC-to-AC power conversion, the converter system 14 may be configured with a single-stage converter or two-stage converters (e.g., a DC- DC converter and a DC-AC converter) which functions to convert DC power to AC power with suitable voltage and frequency for grid 16 transmissions and distribution and/or for local load 19 consumption. For AC-to-AC power conversion, the converter system 14 may comprise a single-stage converter or two-stage converters (e.g., an AC-DC converter and a DC-AC converter) which functions to perform AC to AC power conversion and supply AC power with suitable voltage and frequency for grid 16 transmissions and distribution and/or for local load 19 consumption.

[0033] In the illustrated embodiments, the power conversion system 10 may further comprise a converter controller 18 which is coupled to obtain one or more electrical values (e.g., voltage, frequency, phase, impedance, harmonics, active power, and/or reactive power) at the output of the converter system 14 from one or more transducers or sensors (not shown in FIG. 1). In some embodiments, the location for electrical values sensing or measuring can be selected at any location along an electrical path between the converter system 14 and the grid 16. In a particular implementation, a location 143 referred to as point of common coupling (PCC) may be selected as a measurement point. With the measured electric signals 142 in combination with one or more command or reference signals, the converter controller 18 may be configured to implement one or more algorithms or control mechanisms and send control signals 182 (e.g., gating signals for switching semiconductor switches within the converter system 14) in response to the implementation to control the operation the converter system 14 such that one or more electrical values at the output of the converter system 14 can be regulated. More specifically, the converter controller 18 may be configured to implement a first control mechanism (such as, in one example, a d-axis control loop) to regulate an active power at the output of the converter system 14 or regulate a DC voltage at a DC bus of the converter system 14. The converter controller 18 can be further configured to implement a second control mechanism (such as, in one example, a q-axis control loop) to regulate the reactive power at the output of the converter system 14. In a particular embodiment, the first control mechanism and the second control mechanism may be decoupled to achieve independent control of active power and reactive power regulation. [0034] With continuing reference to FIG. 1, the converter controller 18 may comprise an islanding detection module or an anti-islanding protection module 180 (herein referred to as "AI protection module") which is implemented by the converter controller 18 to detect an islanding condition upon a grid being tripped or disconnected from the power conversion system 10 and to interrupt a power supply to the local load 19 and/or grid 16 in response to a detected condition that the power conversion system 10 is operating in islanded mode. In detailed implementation, the AI protection module 180 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 improved anti-islanding detection and protection functions. In the illustrated embodiment, the AI protection module 180 may be incorporated into the converter controller 18 in combination with the first and second control mechanisms for active and reactive power regulation as a single device. In some other embodiments, the AI protection module 180 may be alternatively configured as a separate stand-alone device.

[0035] Generally, in one implementation, the AI protection module 180 shown in FIG. 1 is configured to generate and provide an anti-islanding disturbance signal and a disturbance compensation signal. The anti-islanding disturbance signal may be generated in response to at least one electrical value change (such as a frequency change or a voltage change) of the output power and is applied to modify one or more command signals such that the at least one electrical value at the output of the converter system 14 can be varied for facilitating islanding condition detection. For example, the anti-islanding disturbance signal may be a q-axis reactive current disturbance signal intended for inducing a frequency change at the converter system 14 output by modifying a reactive current command to produce a reactive power change. However, due to a cross coupling between control mechanisms, for example, between a d-axis control loop and a q-axis control loop, the q-axis reactive current disturbance signal also may cause the active power at the output of the converter system 14 to be adversely changed. To address this problem, in one implementation, during the operation of anti-islanding disturbance signal injection, a disturbance compensation signal is generated and applied in a manner to prevent or minimize modification of the active power. In another implementation, the disturbance compensation signal can be particularly generated and applied in a manner which also has a positive effect on the electrical value to provide for earlier detection of the islanding condition.

[0036] FIG. 2 illustrates a block diagram of a solar power conversion system 20 in accordance with an exemplary embodiment of the present disclosure. Although the solar power conversion system 20 is illustrated for facilitating explanation of the detailed implementation of the improved islanding detection method or algorithm, a person having ordinary skills in the art can apply the islanding detection method disclosed herein to other types of power conversion systems such as, for example, fuel cell systems, wind power conversion systems, and hydrokinetic energy power conversion systems. As illustrated, the solar power conversion system 20 generally includes a solar power converter system 24 for converting DC power generated from a solar power source 22 and providing AC power with suitable voltage and frequency for grid 34 transmission and distribution and/or for local load 36 consumptions. In one embodiment, the solar power source 22 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.

[0037] In one implementation, the solar power converter system 24 shown in FIG. 2 is based on a two-stage structure which includes a PV side converter 26 and a line side converter 32. The PV side converter 26 may comprise a DC-DC converter, such as a DC-DC boost converter, which steps up a DC voltage received from the solar power source 22 and outputs a higher DC voltage onto a DC bus 28. The DC bus 28 may comprise one or more capacitors coupled either in series or parallel for maintaining the DC voltage of the DC bus 28 at a certain level, and thus the energy flow from the DC bus 28 to the grid 34 can be managed. The line side converter 32 may comprise a DC-AC inverter which converts the DC voltage on the DC bus 28 to AC voltage with suitable frequency, phase, and magnitude for feeding to the grid 34. For purpose of description, the solar power conversion system 20 shown in FIG. 2 illustrates a single line coupled between the line side converter 32 and the grid 34 for single-phase current signal and voltage signal measuring and sensing. It should be understood that the solar power conversion system 20 disclosed herein can be applicable to a variety of connections, including but not limited to, a three-phase connection or other multiple-phase connection.

[0038] In one implementation, the solar power conversion system 20 shown in FIG. 2 further comprises a solar converter control system 38 which functions to control operations of the line side converter 32 through implementation of control algorithms according to various feedback signals and command signals. More specifically, the solar converter control system 38 comprises a current transformation unit 164 which receives measured current signals 162 from a current sensor 152 placed between the output of the line side converter 32 and the load 36 or grid 34. In one implementation, the current transformation unit 164 performs a stationary-to- rotational coordinate transformation to convert the measured current signals 162 in a stationary reference frame to a d-axis current signal 216 and a q-axis current signal 218 in a rotational reference frame. The d-axis current signal 216 and the q-axis current signal 218 are supplied to a current regulator 208 for current regulation.

[0039] In one implementation, the solar converter control system 38 shown in FIG. 2 further comprises a voltage transformation unit 166 which is similar to the current transformation unit 164 in structure for coordinate transformation of voltage signals. In alternative embodiments, a single transformation unit may be used to perform both the current and voltage transformations. The voltage transformation unit 166 receives measured voltage signals 158 from a voltage sensor 156 placed between the output of the line side converter 32 and the load 36 or grid 34. The voltage transformation unit 166 transforms the measured voltage signals 158 to a d- axis voltage signal 175 and q-axis voltage signal 174 in a rotational reference frame. Detailed implementations stationary-to-rotational coordinate transformation are well known to those having ordinary skills in the solar field, thus more descriptions of such transformation will be omitted in the present disclosure. In one implementation, the solar power conversion system 20 shown in FIG. 2 may further comprise a line side filter 154 having one or more inductive elements or capacitive elements (not shown) for removing harmonic signals for AC voltage or AC current output from the line side converter 32. Although not illustrated, one or more other components including, but not limited to, transformers, contactors, and breakers could also be coupled between the line side converter 32 and the grid 34.

[0040] In the illustrated embodiment, the q-axis voltage signal 174 generated from the voltage transformation unit 166 is supplied to a phase locked loop (PLL) device 168 for generation of a frequency signal 176. The PLL device 168 for grid frequency tracking is a device which may be implemented by conventional techniques either in hardware or software, and the detailed descriptions of the PLL device 168 will thus be omitted here. The d-axis voltage signal 175 as well as the frequency signal 176 are supplied to an AI protection module 172 which may be similar to the AI protection module 180 described above with reference to FIG. 1. The d-axis voltage signal 175 and the frequency signal 176, in this embodiment, are two of the electrical values used to evaluate if an islanding condition is occurring with the solar power conversion system 20. In one example, an islanding rule is violated when an evaluated electrical value is either within or outside a predetermined nominal range of values such as, for example, exceeding an upper threshold and/or being less than a lower threshold. In other embodiments, the frequency signal, the voltage signal, and/or other types of signals such as phase, impedance, and harmonic signals can be used for islanding condition evaluation. In one implementation, if the frequency and/or voltage indication signals 176, 175 are determined to not be violating an islanding rule, the AI protection module 172 can be configured to generate a disturbance signal 178 and a disturbance compensation signal 184 according to the frequency signal 176 and the voltage signal 175 or a combination thereof. Once it is then determined that an anti-islanding event is actually occurring, an islanding protection function can be initiated to isolate the solar power conversion system 20 from the local load 36 and/or grid 34.

[0041] In one implementation, the anti-islanding disturbance signal 178 and the disturbance compensation signal 184 generated with the AI protection module 172 are separately applied to either a first control mechanism 185 or a second control mechanism 187. In one implementation, the first control mechanism 185 comprises a d-axis control loop and the second control mechanism 187 comprises a q-axis control loop. In one implementation, the d-axis control loop 185 comprises a DC voltage regulator 194 which is configured to receive DC voltage feedback signal 188 measured with a DC voltage sensor 186 and a DC command signal 192 representative of a desirable DC voltage to be achieved at the DC bus 28. The DC voltage regulator 194 generates a d-axis current command signal 196 according to the DC voltage feedback signal 188 and the DC voltage command signal 192. The q-axis control loop 187 comprises a Volt-Var regulator 198 which is configured to receive a reactive power command signal 204 indicating a desirable reactive power at the output of the line side converter 32 and a feedback reactive power signal 202 and perform regulations to generate a q-axis current command signal 206. The feedback reactive power signal 202 may be calculated according to the above-described measured current signal 162 and the measured voltage signal 158, for example.

[0042] In the embodiment of FIG. 2, the first control loop 185 and the second control loop 187 are coupled to a current regulator 208 which is configured to receive the d-axis current command signal 196 from the DC voltage regulator 194, the q-axis current command signal 206 from the Volt-Var regulator 198, and d-axis current feedback signal 216 and q-axis current feedback signal 218 from the current transformation unit 164 for current regulation. The current regulator 208 performs regulations according to the received d-axis and q-axis current feedback signals and command signals, and generates a d-axis voltage command signal 212 and a q-axis voltage command signal 214 accordingly. The d-axis voltage command signal 212 and the q-axis voltage command signal 214 are provided to a rotation and modulation unit 138 which functions to rotate the d-axis voltage command signal 212 and the q- axis voltage command signal 214 in the d-q reference frame back to signals in the stationary reference frame and modulate carrier wave signals with the converted stationary-reference-frame signals to generate control signals 182 for the line side converter 32. Hereinafter, detailed descriptions will be made to show various embodiments as to how the anti-islanding disturbance signal 178 and the disturbance compensation signal 184 are generated and separately applied to the two control loops 185, 187. [0043] FIGs. 3-10 illustrate several embodiments of the AI protection module 172 shown in FIG.2, in which the anti-islanding disturbance signal 178 is applied to the q-axis control loop or reactive power control loop 187 and the disturbance compensation signal 184 is applied to eliminate or remove a negative effect caused by the injection of anti-islanding disturbance signal 178.

[0044] FIG. 3 illustrates a detailed block diagram of the AI protection module 172 shown in FIG. 2 in accordance with an exemplary embodiment of the present disclosure. In the illustrated embodiment, the AI protection module 172 comprises a frequency variation detection unit 222 and a disturbance generation unit 226. The frequency variation detection unit 222 is coupled to the PLL device 168 to receive the frequency signal 176 generated therefrom. The frequency variation detection unit 222 is configured to determine whether the received frequency signal 176 is deviating from a nominal frequency value (e.g., 60Hz or 50 Hz). More specifically, the frequency variation detection unit 222 may compare the frequency signal 176 with the nominal frequency value, and determine a frequency difference between the frequency signal 176 and the nominal frequency value. In one implementation, the frequency difference signal 224 is provided to the disturbance generation unit 226 for generation of a current disturbance signal 228.

[0045] In one implementation, the current disturbance signal 228 comprises a q- axis current disturbance signal 228 that is generated by applying an appropriate gain to the frequency difference signal 224 provided from the frequency variation detection unit 222. The q-axis current disturbance signal 228 is introduced to vary a q-axis current command signal 238 applied in the q-axis control loop 187. In the illustrated embodiment of FIG. 3, the q-axis current disturbance signal 228 is limited by a first limiter unit 232. The limited q-axis current disturbance signal 234 then is supplied to a summation element 236 for modifying the q-axis current command signal 238. In another embodiment, the unlimited version of q-axis current disturbance signal 228 may be used for modifying the q-axis current command signal 238. The modified q-axis current command signal 242 may further be limited by a limiter unit 244 before being sent to the current regulator 208 for further regulation. In alternative embodiment, the modified q-axis current command signal 242 may be sent directly to the current regulator 208 for regulation and generation of the d-axis voltage command signal 212 and the q-axis voltage command signal 214.

[0046] Further referring to FIG. 3, in one implementation, the AI protection module 172 further comprises a disturbance compensation unit 248 for generation of a disturbance compensation signal 252. The disturbance compensation signal 252 is used to compensate an undesirable change appearing in the d-axis control loop 185 which is caused by the introduction of the q-axis current disturbance signal 228 or 234.

[0047] In absence of any disturbance signals or at steady state, the active power and the reactive power at the output of the line side converter 32 can be expressed with the following equations: p_inv = i.s(y_di_d + q) (i),

Q_inv = l.S (V_qI_d - V_dl_q) (2), where in equations (1) and (2), V_d and V_q are the d-axis and q-axis voltage components in a rotational reference frame, I_d and I_q are the d-axis and q-axis current components in the rotational reference frame, and Pj_nv and Qi_nv are the active power and reactive power at the output of the line side converter 32. According to equation (2), a small current disturbance ΔΙ on the q-axis current command signal 238 will cause the reactive power change at the output of the line side converter 32. Also, the reactive power can also be expressed according to the following equation:

where l^_nv is the output voltage of the line side converter 32, ω is the output frequency of the line side converter 32, L and C are the inductance and capacitance of the local load coupled to output of the line side converter 32. According to equation (3), the reactive power change will cause a frequency variation Δω at the output of the line side converter 32. The frequency variation Δω will further be used for generation of the current disturbance ΔΙ which has a same sign as that of the frequency variation Δω. This operates in a positive feedback manner that, with continuing accumulation of frequency variations, the frequency can be finally driven outside of the frequency nominal range. The nominal range is typically be a set range with a lower and upper limit. Alternatively, for some other electrical parameter analysis, the nominal range may have an upper limit or a lower limit but not both, for example. During normal operations, when the grid 34 is connected to the line side converter 32, the accumulation of frequency variation caused by anti-islanding disturbance signal injection is interrupted, as the grid frequency is stable enough to keep the output frequency substantially unchanged or with little fluctuations that would not trigger an islanding protection action. However, when the grid 34 is tripped or disconnected from the solar power conversion system 20, grid control over the output frequency is lost, hence frequency variation Δ ω caused by applying of the current disturbance ΔΙ will gradually accumulate, and ultimately the frequency can be driven away from the frequency nominal range. In this situation, an islanding condition (i.e., disconnection of the grid 34) can be detected and a trip action can be initiated (e.g., open a switch coupled between the converter system 14 and the local load 19) to isolate the local load 19 from the converter system 14 or simply shut down the converter system 14.

[0048] From equation (3), it is desirable to maintain the output voltage l^_nv unchanged to enable the frequency only be dependent on the reactive power. However, this is not the case when no compensation is done to the anti-islanding disturbance signal. More specifically, when the q-axis current anti-islanding disturbance signal is applied, the active power after disturbance can be expressed by the following equations:

Pinv_dis ^~ ^inv (4),

ΔΡ = 1.5¾ + Δ/_ί)Δ _ί (5), where AI_q is the q-axis current anti-islanding disturbance, ΔΙ^ is the q-axis voltage change caused by AI_q , Pj_{nv d}j_s is the active power at the output of the line side converter 32 after disturbance, and ΔΡ is the active power change caused by applying the q-axis current anti-islanding disturbance. From equations (4) and (5), it can be seen that the q-axis current disturbance also causes an active power change which is undesirable. The active power can also be expressed by the following equation: v i-nv ² (c

^"inv ^~ _R ν° where l^_nv is the output voltage of the line side converter 32, R is the resistance of the local load coupled to the output of the line side converter 32. To address the problem of the undesired active power change, in one implementation, the active power change caused by introduction of the q-axis current disturbance signal 228 can be reduced to zero by applying a voltage compensation signal which makes the final q-axis voltage change ΔΙ^ become zero. Thus, the output frequency can be more effectively changed by injection of the q-axis current disturbance signal without being affected by the undesirable active power change.

[0049] In one implementation, the disturbance compensation signal 252 generated with the disturbance compensation unit 248 comprises a q-axis voltage compensation signal 252. In one implementation, the q-axis voltage compensation signal 252 is limited by a limiter unit 254 and the limited voltage signal 256 is fed to a summation element 258. The summation element 258 subtracts the q-axis voltage compensation signal 256 from the q-axis voltage command signal 214 and produces a modified q-axis voltage command signal 262.

[0050] Further referring to FIG. 4, which illustrates a detailed block diagram of the dynamic compensation unit 248 and the current regulator 208 shown in FIG. 3 in accordance with an exemplary embodiment of the present disclosure. The current regulator 208 comprises a d-axis current regulator 227 which functions to regulate a d-axis current error signal 225 obtained by subtracting a d-axis current command signal 196 and a d-axis current feedback signal 216 in a summation element 223 and generate a d-axis voltage command signal 229. The d-axis voltage command signal 229 is fed to a summation element 230 which subtracts a voltage signal 231 calculated based in part on a q-axis current feedback signal 218 and an impedance 235 for purpose of decoupling from the d-axis voltage command signal 229 and produces a d- axis voltage command signal 233. The current regulator 208 further comprises a q- axis current regulator 241 which functions to regulate a q-axis current error signal 239 obtained by subtracting a q-axis current command signal 218 from a combination of a q-axis current command signal 206 and a q-axis current disturbance signal 228 or 234 in a summation element 237 and generate a q-axis voltage command signal 243. The q-axis voltage command signal 243 is fed to a summation element 258 which receives a voltage signal 249 calculated based in part on the d-axis current feedback signal 216 with an impedance 247 for purpose of decoupling. The summation element 258 also receives a q-axis voltage compensation signal 259 which may be same as the voltage compensation signal 252 or 256 generated from the dynamic compensation unit 248 shown in FIG. 3.

[0051] In the illustrated embodiment of FIG. 4, the dynamic compensation unit 248 comprises a q-axis compensation regulator 253 and an inverter unit 257. The q- axis compensation regulator 253 generates a q-axis voltage compensation signal 255 by applying a gain to the q-axis current disturbance signal 228 or 234. The q-axis voltage compensation signal 255 is negated by the inverter unit 257 and the negated voltage compensation signal 259 is used for compensating the undesirable voltage change caused by applying the q-axis current disturbance signal in the q-axis control loop 187. With voltage compensation, the active power at the output of the line side converter 32 is free from being affected by the injected q-axis current disturbance signal 228 or 234. As such, the target electrical value such as, for example, frequency can only be dependent on the q-axis current disturbance signal 228 or 234. When the grid is lost or disconnected from the solar power conversion system 20, the frequency can be quickly driven outside of the nominal frequency range within a required time, such as two seconds, for example. Thus, islanding conditions of the solar power conversion system 20 can be detected and appropriate action can be initiated. In one example, the line side converter 32 can be shut down to stop feeding power to the local load 36.

[0052] FIG. 5 illustrates another embodiment of the dynamic compensation unit 248 in connection with the current regulator shown in FIG. 3. In the illustrated embodiment, the dynamic compensation unit 248 is referred to as a dynamic overcompensation unit which comprises a q-axis compensation regulator 253 and a gain unit 261. The q-axis compensation regulator 253 generates a voltage compensation signal 255 according to the q-axis current disturbance signal 228 or 234. The gain unit 261 applies a gain coefficient K to produce a q-axis voltage over-compensation signal 263. In one implementation, the gain coefficient K can be adjusted to vary an over-compensation extent that the q-axis voltage over-compensation signal 263 impacts the change of the target electrical variable. Through the voltage over compensation, the target electrical variable such as, for example the output frequency can be more quickly driven outside of its nominal frequency range and thus, in this embodiment, the time of islanding condition detection can be reduced. In other embodiments, the dynamic compensation unit or the dynamic over-compensation unit 248 shown in FIG. 4 and FIG. 5 may comprise one or more function units, such as, for example, a filtering element, before the compensation signal 259 or overcompensation signal 263 are applied for voltage command compensation.

[0053] FIG. 6 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with another embodiment of the present disclosure wherein an adaptive control is incorporated for adjusting the magnitude of the anti- islanding disturbance signals. More specifically, in one implementation, the AI protection module 172 further comprises a frequency change rate detection unit 298 and an adaptive gain adjustment unit 304. The frequency change rate detection unit 298 is coupled to the frequency variation detection unit 222 for receiving the frequency difference signal 224 and generating a frequency change rate signal 302 based on the frequency difference signal 224. The adaptive gain adjustment unit 304 is coupled to the frequency change rate detection unit 298 for receiving the frequency change rate signal 302 and generating a gain signal 306 which is dynamically changed corresponding to the frequency change rate signal 302. In islanding detection processes, the frequency variation typically changes at a small rate in an early stage (i.e., at the beginning of a grid trip). In order to accelerate the movement of the frequency for facilitating fast detection of an islanding condition, it is desirable to quickly move the frequency outside of its nominal frequency range upon a grid connection being lost. In this early stage situation, the adaptive gain adjustment unit 304 can be configured to generate a gain signal 306 having a relatively large gain value corresponding to a relatively small frequency change rate. Thus, the gain signal 306 is applied to the current disturbance generation unit 226 for generation of the anti- islanding disturbance signal 228 with a large magnitude. In another situation, with positive feedback of the anti-islanding disturbance signal applied, the frequency at the output of the line side converter 32 will have a large change rate at a middle or later stage. To maintain system stability, the adaptive gain adjustment unit 304 can be configured to generate the gain signal 306 to have a relatively small gain value corresponding to the later state relatively large frequency change rate as detected by the frequency change rate detection unit 298.

[0054] FIG. 7 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with yet another embodiment of the present disclosure. In the illustrated embodiment, instead of generating a q-axis current anti-islanding disturbance signal for islanding detection, the AI protection module 172 comprises a reactive power disturbance generation unit 308 for generation a reactive power anti- islanding disturbance signal 312. The reactive power anti-islanding disturbance signal 312 or the resulting signal 314 limited by an limiter 232 can be supplied to a summation element 316 which generates a modified reactive power command signal 318 by combining the reactive power command signal 204 with the reactive power anti-islanding disturbance signal 312 or the limited reactive power anti-islanding disturbance signal 314. The modified reactive power command signal 318 is provided to the Volt-Var regulator 198 for generation of a q-axis current command signal 322 which is limited by a limiter unit 244 and the resulting command signal 324 is sent to the current regulator 208 for regulation. In alternative embodiments, the q-axis current command signal 322 can be sent directly to the current regulator 208 for regulation. Further, in the illustrated embodiment, instead of using the q-axis current anti-islanding disturbance signal as described above with reference to FIG. 3 and FIG. 6 for disturbance compensation signal generation, the reactive power anti- islanding disturbance signal 312 or the limited reactive power anti-islanding disturbance signal 314 may be used by the dynamic compensation unit 248 for generation of the disturbance compensation signal 252 which modifies the q-axis voltage command signal 214 provided from the current regulator 208 such that the undesirable change cause by applying the reactive power anti-islanding disturbance signal 312 or 314 in the reactive power control loop 187 will be compensated, canceled, or reduced. The structure of the dynamic compensation unit 248 shown in FIG. 7 may be similar to that shown in FIGs. 4 and 5, for example.

[0055] FIG. 8 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with yet another embodiment of the present disclosure wherein a frequency change rate detection unit 298 is used for detecting a frequency change rate. The AI protection module 172 further comprises an adaptive gain adjustment unit 304 for generation of a gain signal 306 according to the detected frequency change rate signal 302. Likewise, the gain signal 306 is applied to the reactive power disturbance generation unit 308 for dynamically changing the magnitude of the reactive power anti-islanding disturbance signal 312.

[0056] FIG. 9 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with yet another embodiment of the present disclosure wherein the AI protection module 172 comprises a composite disturbance signal generation unit 326 for generation of a q-axis current anti-islanding disturbance signal 328 as well as a reactive power anti-islanding disturbance signal 332. The q-axis current and reactive power anti-islanding disturbance signals 328 and 332 may be limited by a limiter unit 334, and the resulting q-axis current and reactive power anti- islanding disturbance signals 336 and 338 are supplied to a first summation element 236 and a second summation element 316 for modifying the q-axis current command signal 344 and the reactive power command signal 204 respectively. Further, the AI protection module 172 further comprises a dynamic compensation unit 248 for generation a q-axis disturbance compensation signal 252 according to both the q-axis current and reactive power anti-islanding disturbance signals 328 and 332 or 336 and 338. With the generated q-axis disturbance compensation signal 252, the undesirable change caused by applying the q-axis current and reactive power anti-islanding disturbance signals 328 and 332 or 336 and 338 can be eliminated, removed, or reduced, for facilitating fast and accurate detection of the islanding conditions.

[0057] FIG. 10 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with yet another embodiment of the present disclosure wherein adaptive control for managing the magnitude of the anti-islanding disturbance signals are also applied. More specifically, the gain signal 306 generated with the adaptive gain adjustment unit 304 can be modified to have a relatively large value during an early stage of disturbance signal injection. The current and power disturbance generation unit 326 can generate the q-axis current disturbance signal 328 and the q-axis reactive power disturbance signal 332 having relatively large magnitude. Thus, the target electrical variable, such as, for example, the output frequency, can be driven to move quickly outside of the nominal frequency range and the process of islanding detection can be shortened. Further, the gain signal 306 can also be changed to have a relatively small value during a middle or later stage of disturbance signal injection. The current and power disturbance generation unit 326 can provide the q-axis current disturbance signal 328 and the q-axis reactive power disturbance signal 332 having a relatively small magnitude according to the gain signal 306. In this situation, the system stability can be maintained.

[0058] FIGs. 11-16 illustrate several embodiments of the AI protection module 172 shown in FIG. 2 wherein the anti-islanding disturbance signal 178 is applied in the d-axis control loop or the active power regulation loop 185 and the disturbance compensation signal 184 is applied to eliminate or remove a negative effect caused by the injection of anti-islanding disturbance signal 178.

[0059] FIG. 11 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 for anti-islanding disturbance signal injection in the d-axis control loop 185 in accordance with one embodiment of the present disclosure. In the illustrated embodiment, the AI protection module 172 comprises a voltage variation detection unit 352 and a disturbance generation unit 356. The voltage variation detection unit 352 is coupled to the voltage transformation unit 166 to receive a d-axis voltage feedback signal 175 transformed therefrom. The voltage variation detection unit 352 is configured to determine whether the received d-axis voltage feedback signal 175 is deviating from a nominal voltage value. More specifically, the voltage variation detection unit 352 may compare the d-axis voltage feedback signal 175 with the nominal voltage value, and determine a voltage difference between the d-axis voltage feedback signal 175 and the nominal voltage value. In one implementation, the voltage difference signal 354 is provided to the disturbance generation unit 356 for generation of a current disturbance signal. In alternate embodiment, the disturbance signal to be applied in the d-axis control loop 185 may also be generated according to frequency difference signal 224 provided from frequency variation detection unit 222 (shown in FIG. 3 and FIGs. 6-10). In further embodiment, the disturbance signal may also be generated according to a combination of frequency difference signal 224 provided from frequency variation detection 222 and voltage difference signal 354 provided from voltage variation detection unit 352.

[0060] In one implementation, the current disturbance signal generated with the current disturbance generation unit 356 comprises a d-axis current disturbance signal 362. In one implementation, the d-axis current disturbance signal 362 is generated by applying an appropriate gain to the voltage difference signal 354 provided from the voltage variation detection unit 352. The d-axis current disturbance signal 362 is introduced to vary a d-axis command signal 196 applied in the d-axis control loop 185. According to equation (1) described above, a small current disturbance ΔΙ on the d-axis current command signal 196 will cause an active power change at the output of the line side converter 32. Further according to equation (6), the active power change forces a voltage variation AV at the output of the line side converter 32. The voltage variation AV will further be used for generation of the current disturbance A I which has a same sign as that of the voltage variation A V. This operates in a positive feedback manner that with continuing accumulation of voltage variations, the voltage can be finally driven outside of the nominal voltage range. During normal operations, when the grid 34 is connected to the line side converter 32, the accumulation of voltage variation caused by anti-islanding disturbance signal injection is interrupted, as the grid 34 is strong enough to hold the voltage substantially unchanged. However, when the grid 34 is tripped or disconnected from the solar power conversion system 20, grid control over the voltage is lost, hence voltage variation A V caused by applying of the current disturbance ΔΙ will gradually accumulate, and ultimately the voltage can be driven away from the voltage nominal range. In this situation, an islanding condition (i.e., disconnection of the grid 34) can be detected and a trip action can be initiated to isolate the local load 36 from the converter system 24 or simply shut down the converter system 24.

[0061] In the illustrated embodiment of FIG. 1 1 , the d-axis current disturbance signal 362 is limited by a first limiter unit 364. The limited d-axis current disturbance signal 366 then is supplied to a summation element 368 for modifying the d-axis current command signal 196. In another embodiment, the unlimited version of d-axis current disturbance signal 362 may be used for modifying the d-axis current command signal 196. The modified d-axis current command signal 372 may be further limited by a limiter unit 374 before being sent to the current regulator 208 for current regulation. In alternative embodiment, the modified d-axis current command signal 372 may be sent directly to the current regulator 208 for regulation and generation of the d-axis voltage command signal 212 and the q-axis voltage command signal 214.

[0062] Further referring to FIG. 1 1 , in one implementation, the AI protection module 172 further comprises a disturbance compensation unit 248 for generation of a disturbance compensation signal 382. The disturbance compensation signal 382 is used to compensate an undesirable change appearing in the reactive power loop which is caused by the introduction of the d-axis current disturbance signal.

[0063] In one implementation, the disturbance compensation signal 382 generated with the disturbance compensation unit 248 comprises a d-axis voltage compensation signal. In one implementation, the d-axis voltage compensation signal 382 is limited by a limiter unit 384 and the limited voltage signal 386 is fed to a summation element 388. The summation element 388 subtracts the d-axis voltage compensation signal 386 from the d-axis voltage command signal 212 and produces a modified d-axis voltage command signal 392. [0064] FIG. 12 illustrates a detailed block diagram of the dynamic compensation unit 248 and the current regulator 208 shown in FIG. 11 in accordance with an exemplary embodiment of the present disclosure. The embodiment shown in FIG. 12 has some features which are similar to those in FIG. 4, and detailed description of similar elements is omitted here. One difference in FIG. 12 is that the d-axis current disturbance signal 362 or 366 is applied to the summation element 223 which produces a d-axis current error signal 225. The d-axis current error signal 225 is regulated by the d-axis current regulator 227 for generation of a d-axis voltage command signal 227. Another difference is that the voltage compensation signal 271 generated with the dynamic compensation unit 248 is applied to the summation element 229. The voltage compensation signal 271 is used to compensate for the undesirable reactive power change caused by injection of the d-axis current disturbance signal 362 or 366. The dynamic compensation unit 248 shown in FIG. 12 comprises a d-axis compensation regulator 265 which generates a d-axis voltage compensation signal 267 based on the d-axis current disturbance signal 362 or 366. With voltage compensation, the reactive power change caused by the active current disturbance can be compensated, and thus the frequency will change towards the direction that keeps the reactive power constant. When the grid is lost or disconnected from the power conversion system 10, the voltage and/or frequency can be quickly driven outside of the nominal ranges within a required time such as two seconds for example. In this embodiment, when either voltage or frequency becomes outside of a nominal range, the solar power conversion system 20 will detect the islanding condition, and appropriate action can be initiated.

[0065] FIG. 13 illustrates another embodiment of the dynamic compensation unit 248 in connection with the current regulator shown in FIG. 11. In the illustrated embodiment, the dynamic compensation unit 248 is referred to as dynamic overcompensation unit which comprises a d-axis over-compensation regulator 267. The d-axis over-compensation regulator 267 generates an over-compensation d-axis voltage signal 275 according to the d-axis current disturbance signal 362 or 366. Through the voltage over compensation, the target electrical variable, such as, for example, the output voltage and/or frequency, can be more quickly driven outside of nominal range such that an islanding condition can be more quickly detected.

[0066] FIG. 14 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with another embodiment of the present disclosure. The AI protection module 172 shown in FIG. 14 comprises a voltage change rate detection unit 404 and an adaptive gain adjustment unit 408. The voltage change rate detection unit 404 is coupled to the voltage variation detection unit 352 for receiving a voltage difference signal 354 and generating a voltage change rate signal 406 according to the voltage difference signal 354. The adaptive gain adjustment unit 408 is coupled to the voltage change rate detection unit 404 for receiving the voltage change rate signal 406 and generating a gain signal 412 which is dynamically changed corresponding to the voltage change rate signal 406. For example, in early stage of disturbance signal injection, the gain signal 412 can be adjusted to have a relatively large value such that a large disturbance can be introduced for driving of the voltage and/or frequency to quickly move outside of the nominal voltage range. In a middle or later stage of disturbance signal injection, the gain signal 412 can be adjusted to have relatively small value such that a mild disturbance can be introduced for maintaining the system stability.

[0067] FIG. 15 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with another embodiment of the present disclosure wherein the AI protection module 172 comprises a voltage disturbance generation unit 414 which is configured to generate a DC voltage disturbance signal 416 for modifying a DC voltage command signal 192. Modifying the DC voltage command signal 192 has a similar effect of inducing an active power change as that of modifying a d-axis voltage command signal 196 (shown and described with reference to FIG. 11), and the active power change will force the output voltage and/or frequency to be changed such that the islanding condition can be detected in response to a disconnection of the grid 34. Further, in this embodiment, the DC voltage disturbance signal 416 or limited DC voltage disturbance signal 422 can be used by the dynamic compensation unit 248 for generation of a d-axis voltage compensation signal 382 which is applied to modify the d-axis voltage command signal 212 provided from the current regulator 208. The d-axis voltage compensation signal 382 or the limited version 386 is useful to remove the undesirable reactive power change caused by injection of the DC voltage disturbance signal 416 or 422, such that output voltage and/or frequency can be more effectively disturbed by the DC voltage disturbance signal 416 or 422.

[0068] FIG. 16 illustrates a block diagram of the AI protection module 172 shown in FIG. 2 in accordance with yet another embodiment of the present disclosure wherein the AI protection module 172 also employs an adaptive control of applying a dynamic gain signal 412 during generation of the DC voltage disturbance signal 416. The gain signal 412 is applied in a similar manner as described above with respect to FIG. 14 to allow the solar power conversion system 20 to quickly detect the islanding conditions and at the same time have good system stability during anti-islanding disturbance signal injection.

[0069] FIG. 17 illustrates a block diagram of a solar power conversion 40 in accordance with another embodiment of the present disclosure. In this embodiment, the solar power conversion system 40 includes a single-stage solar power converter system 14. More specifically, in one implementation, the solar power converter system 14 comprises a line side converter 148 which functions to convert DC voltage appearing at the DC bus 146 and generated from a solar power source 12 directly to AC voltage for feeding to the grid 16. The solar power conversion system 40 further comprises a solar power converter control system 18 which is coupled to the solar power converter system 14 for control operations of the solar power converter system 14. The solar power converter control system 18 is similar to the solar power converter control system 18 shown in FIG. 2, thus like elements, for example current transformation unit 164, voltage transformation unit 166, PLL device 168, current regulator 208, DC voltage regulator 194, Volt-Var regulator 198, and rotation and modulation unit 138 will not be described with more detail here.

[0070] In this embodiment, the solar converter control system 18 is also capable of being configured to implement an islanding detection mechanism or algorithm in a manner similar to that described with reference to FIGs. 2-16. For example, during anti-islanding disturbance signal injection, disturbance compensation is also introduced to remove or reduce the negative effect or undesirable change caused by the disturbance signal injection. In the single-stage situation, it is further useful to implement the islanding detection in combination with a maximum power point tracking (MPPT) control. As used herein, "MPPT" refers to a control scheme implemented by the solar power conversion system to ensure maximum solar power can always be extracted from the solar power source in view of varying solar irradiance and shading conditions for example.

[0071] Referring to FIG. 17, the solar power conversion system 40 comprises a MPPT module 26 which receives PV current signal 112 and a PV voltage signal 114 measured respectively with a current sensor 28 and a voltage sensor 32 placed between the output of the solar power source 12 and the line side converter 148. The MPPT module 26 is further configured to implement searching algorithm such as, for example, a perturbation and observation (P&O) algorithm, or an incremental conductance algorithm, to find an optimum operating point of the solar power source 12. In one implementation, the MPPT provides a DC voltage reference signal 192 according to the PV current signal 112 and the PV voltage signal 114. In the single stage situation, the DC bus voltage is substantially same as that of the PV output voltage, thus the maximum power operating point can be identified through the control of the DC bus voltage. The DC voltage reference signal 192 indicative of the desirable DC voltage to be achieved at the DC bus 146 is supplied to the DC voltage regulator 194 which also receives a DC voltage feedback signal 188 measured with a DC voltage sensor 186. The DC voltage regulator 194 generates a d-axis current command signal 196 according to the DC voltage reference signal 192 and the DC voltage feedback signal 188.

[0072] Further referring to FIG. 17, the AI protection module 172 may receive frequency signal 176 provided from PLL device 168 and d-axis voltage signal 175 from voltage transformation unit 166, detect a frequency change and a voltage change according to the frequency signal 176 and d-axis voltage signal 175 respectively, and generate anti-islanding disturbance signal 178 and disturbance compensation signal 184 according to the detected frequency change or voltage change or a combination thereof. In one implementation, the anti-islanding disturbance signal 178 is applied to modify the DC voltage reference signal 192 which is generated by the MPPT module 26. In this case, the DC voltage reference signal 192 of the MPPT module 26 can be modified by the anti-islanding disturbance signal 178 based on the detected voltage or frequency change or a combination thereof. Under this situation, changing the DC voltage reference will change the input power as well as the output power, and the balance between the line-side converter output and the local load can be quickly disrupted, allowing a more effective detection of the islanding event.

[0073] FIG. 18 illustrates a flowchart of a method for providing islanding condition detection and protection to the power conversion system 10 shown in FIG. 1 or solar power conversion systems 20 and 40 shown in FIGs. 2 and 17 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 nonremovable media implemented in any method or technology. The computer-readable 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.

[0074] In one implementation, the method 4000 may start at block 4002. At block 4002, one or more electrical values at the output of the power conversion system are obtained. For example, the frequency values and voltage values may be obtained with the use of voltage transformation unit and PLL devices.

[0075] At block 4004, the method continues to detect at least one electrical value variation. For example, the frequency variation can be detected by a frequency variation detection unit and the voltage variation can be detected by a voltage variation detection unit. [0076] At block 4006, as an optional block, adaptive gain control is implemented for anti-islanding disturbance signal generation. The block 4006 includes a sub-block 4003 and a sub-block 4005, at which electrical value change rate signals are determined and gain signals are generated corresponding to the determined electrical value change rate signals. For example, at sub-block 4003, a frequency change rate signal or a voltage change rate signal may be determined according to the frequency variation signal and the voltage variation signal detected at block 4004. At sub-block 4005, a gain signal is generated according to the frequency change rate signal or the voltage change rate signal. More specifically, the gain signal can be adjusted to have a relatively large value when the frequency or voltage change rate signals are small to allow the output frequency or voltage to be driven quickly outside of the nominal frequency or voltage range, while when the frequency or voltage change rate signals are large the gain signal can be adjusted to have a relatively small value to maintain system stability.

[0077] At block 4012, at least one anti-islanding disturbance signal is generated. In one implementation, a fixed gain signal is applied for generation of the anti- islanding disturbance signal according to the detected at least one electrical value variation at the output of the power conversion system. The anti-islanding disturbance signal may comprise a q-axis current disturbance signal, a reactive power disturbance signal, or a combination thereof. The q-axis current disturbance signal or the reactive power disturbance signal may be generated according to the frequency variation signal or the voltage variation signal. In another implementation, a dynamic gain signal generated at block 4006 can be applied for generation of the anti-islanding disturbance signal with adjustable magnitude.

[0078] At block 4014, a disturbance compensation signal is generated for removing undesirable change caused by injection of anti-islanding disturbance signals. In one implementation, the disturbance compensation signal may comprise a q-axis voltage compensation signal which is applied to cancel or reduce an undesirable active power change caused by injection of the q-axis current disturbance signal for example. In another implementation, the disturbance compensation signal may comprise an over-compensation component which can not only remove the undesirable change but also contribute a positive feedback for accelerating the movement of the target electrical value. For example, during compensation of the undesirable active power change caused by the injection of the q-axis current disturbance signal, the q-axis voltage compensation signal can be applied to cause the frequency to move in a same direction as the q-axis current disturbance signal is intended to do.

[0079] At block 4016, control signals are generated for a converter system based on at least in part on the anti-islanding disturbance signal and the disturbance compensation signal. The control signals may comprise switching signals for turning on or off semiconductor switches within the power converter system.

[0080] At block 4018, a determination is made to ascertain whether the at least one electrical value is outside of a nominal range. More specifically, the determination is made to ascertain whether the output frequency is outside of a nominal frequency range, or whether the output voltage is outside of a nominal voltage range. Once the at least one electrical value is outside of the nominal range, the process moves to block 4022. However, if the at least one electrical value is not outside of the nominal range, the process returns to block 4002 to continue the process of islanding condition detection.

[0081] At block 4022, islanding protection action is initiated in response to positive determination at block 4018 which confirms that an islanding condition is occurring at the power conversion system. In one implementation, the power flow between the islanded power conversion system and the local load and/or grid can be cut by opening a switch coupled between the power conversion system and the load and/or grid. In another implementation, the power conversion system can be shut down by disabling the converter system of the power conversion system so that no power will be supplied.

[0082] 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.

Claims

CLAIMS:

1. A power conversion system comprising: a converter system for converting input power from a power source and providing output power; and a converter controller for regulating active and reactive power components of the output power, wherein the converter controller comprises an anti-islanding protection module for performing the steps of generating an anti-islanding disturbance signal and a disturbance compensation signal based on at least one measured electrical value of the output value; separately applying the anti-islanding disturbance signal and the disturbance compensation signal to adjust either or both of the active and reactive power components to drive the at least one measured electrical value outside of a nominal range upon the occurrence of an islanding condition; and detecting the islanding condition.

2. The system of claim 1, wherein the anti-islanding protection module comprises: a frequency variation detection unit for receiving a frequency signal obtained from a phase locked loop device of the power conversion system and generating a frequency variation signal based on the frequency signal; a disturbance generation unit for receiving the frequency variation signal and generating the anti-islanding disturbance signal for driving the frequency of the output power outside of a nominal frequency range; and a compensation unit coupled for generating the disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

3. The system of claim 2, wherein the anti-islanding disturbance signal comprises a reactive current disturbance signal.

4. The system of claim 2, wherein the anti-islanding disturbance signal comprises a reactive power disturbance signal.

5. The system of claim 2, wherein the anti-islanding disturbance signal comprises a combination of a reactive current disturbance signal and a reactive power disturbance signal.

6. The system of claim 2, wherein the disturbance compensation signal comprises a q-axis voltage compensation signal.

7. The system of claim 2, wherein the compensation unit is further configured to generate an over-compensation signal which in combination with the anti-islanding disturbance signal causes the frequency at the output of the converter system to be shifted outside of the nominal frequency range in an accelerated manner upon the occurrence of the islanding condition.

8. The system of claim 2, wherein the anti-islanding protection module further comprises: a frequency change rate detection unit for generating a frequency change rate signal based on the frequency variation signal; and an adaptive gain adjustment unit for generating a dynamic gain signal based on the frequency change rate signal and for providing the dynamic gain signal to the disturbance generation unit for adjusting the magnitude of the anti-islanding disturbance signal.

9. The system of claim 1, wherein the anti-islanding protection module comprises: a voltage variation detection unit for receiving a voltage component signal from a voltage transformation unit of the power conversion system and generating a voltage variation signal based on the voltage component signal; a disturbance generation unit for receiving the voltage variation signal and generating the anti-islanding disturbance signal for driving the voltage of the output power outside of a nominal voltage range or driving the frequency of the output power outside of a nominal frequency range upon the occurrence of the islanding condition; and a compensation unit coupled to the disturbance generation unit for generating a disturbance compensation signal to compensate a change caused by the anti- islanding disturbance signal.

10. The system of claim 9, wherein the disturbance compensation signal comprises a d-axis voltage compensation signal.

11. The system of claim 9, wherein the compensation unit is further configured to generate an over-compensation signal which in combination with the anti-islanding disturbance signal cause the voltage at the output of the converter system to be driven outside of a nominal voltage range in an accelerated manner upon the occurrence of the islanding condition.

12. The system of claim 9, wherein the anti-islanding protection module further comprises: a voltage change rate detection unit for generating a voltage change rate signal based on the voltage variation signal; and an adaptive gain adjustment for generating a dynamic gain signal based on the voltage change rate signal and providing the dynamic gain signal to the disturbance generation unit for adjusting the magnitude of the anti-islanding disturbance signal.

13. A method for anti-islanding protection of a power conversion system, the method comprising: receiving at least one measured electrical value at an output of the power conversion system; generating an anti-islanding disturbance signal and a disturbance compensation signal based on the at least one measured electrical value; applying the anti-islanding disturbance signal to drive the at least one electrical value outside of a nominal range upon an occurrence of an islanding event; and applying the disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

14. The method of claim 13, wherein the disturbance compensation signal in combination with the anti-islanding disturbance signal contribute to an accelerated movement of the electrical value outside of the nominal range upon the occurrence of an islanding event.

15. The method of claim 13, wherein the anti-islanding disturbance signal and the disturbance compensation signal are generated according to a frequency variation signal, a voltage variation signal, or a combination thereof.

16. The method of claim 13, further comprising: generating an electrical value change rate signal; and generating a dynamic gain signal based on the electrical value change rate signal for adjusting a magnitude of the anti-islanding disturbance signal.

17. A solar power conversion system, comprising: 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 configured to regulate the AC power at the output of the solar converter in a d-q reference frame; wherein the solar controller comprises an anti-islanding protection module for performing the steps of generating an anti-islanding disturbance signal and a disturbance compensation signal based on at least one measured electrical value of the AC power, applying the anti-islanding disturbance signal for driving at least one electrical value outside of a nominal range upon an occurrence of an islanding condition, and applying the disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

18. The solar power conversion system of claim 17, further comprising a maximum power point tracking module configured to generate a DC voltage reference signal according to a PV voltage signal and a PV current signal at the solar power source; wherein the DC voltage reference signal is modified by the anti- islanding disturbance signal.

19. The solar power conversion system of claim 17, wherein the anti-islanding protection module comprises: a frequency variation detection unit for receiving a frequency signal obtained from a phase locked loop device of the solar power conversion system and generating a frequency variation signal based on the frequency signal; a disturbance generation unit for receiving the frequency variation signal and generating the anti-islanding disturbance signal for driving the frequency of the output power outside of a nominal frequency range upon the occurrence of the islanding event; and a compensation unit for generating a disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.

20. The solar power conversion system of claim 17, wherein the anti-islanding protection module comprises: a voltage variation detection unit for receiving a voltage component signal obtained from a voltage transformation unit of the solar power conversion system and generating a voltage variation signal based on the voltage component signal; a disturbance generation unit for receiving the voltage variation signal and generating the anti-islanding disturbance signal for driving the voltage of the output power outside of a nominal voltage range upon the occurrence of the islanding condition; and a compensation unit for generating a disturbance compensation signal for compensating a change caused by the anti-islanding disturbance signal.