Method and device for improving tolerance to frequency ramp fluctuations of islanding detection function in distributed power sources

Abstract

To provide a method and a device for improving the frequency ramp fluctuation tolerance of an islanding operation detection function in a distributed power source, which enables improvement of the frequency ramp fluctuation tolerance while maintaining the islanding operation detection function.SOLUTION: In parallel with islanding operation detection processing of an active islanding operation detection function unit 1, determination processing of an islanding operation possibility determination function unit 2 is executed that sets, as a determination threshold value, the minimum value of the average frequency change speed during islanding operation, which is obtained from the detection threshold value of the islanding operation detection function 1 and the detection time upper limit, and determines the possibility of islanding operation on the basis of whether the frequency change speed exceeds the determination threshold value in units of one cycle, and when both the result of islanding operation detection from the active islanding operation detection function unit 1, that is, the detection result signal, and the determination result of the possibility of islanding operation from the islanding operation possibility determination function 2, that is, the determination result signal are output, in other words, when input to an AND gate 3 is performed, a signal for paralleling off the distributed power source is output.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a technique for preventing unwanted operation of an islanding detection function in a distributed power generation system, and more particularly to a method and apparatus for improving the frequency ramp fluctuation tolerance of the islanding detection function. [Background technology]

[0002] When a large number of distributed power sources such as photovoltaic power generation facilities are connected to a grid, if these power generation facilities are all disconnected at once due to an instantaneous voltage drop or the like, there is a risk that the supply capacity will be insufficient, causing frequency and voltage instability and reducing grid stability, which could have a negative impact on power quality. Therefore, in accordance with the Fault-Resistance Test (FRT) requirements of the Grid Interconnection Regulations (Non-Patent Document 1), distributed power sources are designed to continue operation in the face of frequency ramp fluctuations with a frequency change rate of -2 to 2 Hz / second.

[0003] Fault-Resistance (FRT) requirements require that distributed generators avoid simultaneous tripping or sustained output reductions so as not to significantly affect the voltage and frequency maintenance of the entire system during various system transients, such as widespread instantaneous voltage drops and voltage phase jumps caused by transmission system faults, and instantaneous frequency increases and frequency ramp fluctuations caused by large-scale power source tripping or system isolation. Each distributed generator is designed to have the FRT performance to meet these requirements.

[0004] Distributed power sources connected to low-voltage distribution lines must be able to instantly detect and disconnect islanding in order to meet the required clearance time for high- and low-voltage contact faults. The Japan Electrical Safety and Environment Laboratory (JET)'s "Grid-Connected Protection Device Certification Test Method" requires that the detection and disconnection time be within 0.2 seconds. While various islanding detection methods are stipulated in the grid interconnection regulations, the current standard for islanding detection is the frequency shift method, which forcibly injects reactive power during islanding and detects the resulting frequency change. In particular, a new active method called the frequency feedback method with step injection, which reliably achieves this regardless of the number of connected units, is widely used as the standard islanding detection method for low-voltage distributed power sources (see, for example, Non-Patent Documents 2-4).

[0005] On the other hand, as the interconnection volume of inverter-type distributed power sources, primarily solar power generation, increases, the proportion of total power generation output from existing power sources using synchronous generators, which are responsible for maintaining frequency, decreases. This will result in a decrease in system inertia, and it is anticipated that the rate of frequency change in the power system, known as RoCoF (Rate of Change of Frequency), may increase during system fluctuations.

[0006] For this reason, it is desirable to improve the frequency ramp fluctuation tolerance of the islanding detection function, which can be a major cause of unnecessary parallel-off of distributed power sources, and in particular to improve the frequency ramp fluctuation tolerance of the new active islanding detection function, which is the standard method for low-voltage interconnection. [Prior art documents] [Patent documents]

[0007] [Non-Patent Document 1] Grid Interconnection Regulations (JEAC9701-2019). Japan Electric Association [Non-patent document 2] NEDO Commissioned Project Results Report for FY2008-2009. New Energy Technology Research and Development (Research and Development of Technology for Testing Multiple Islanding Operation Detection Devices) [Non-patent document 3] NEDO Commissioned Project Results Report for FY2003-2007. Demonstration Study of Centralized Grid-Connected Photovoltaic Power Generation System [Non-patent document 4] Japan Electrical Manufacturers' Association Standard (JEM1498). Japan Electrical Manufacturers' Association Summary of the Invention [Problem to be solved by the invention]

[0008] However, in active islanding detection devices using the frequency shift method and reactive power fluctuation method, which capture and detect frequency changes that occur during islanding operation by controlling reactive power, improving frequency ramp fluctuation tolerance and islanding detection performance are in a contradictory relationship, and it is difficult to achieve both. In particular, in frequency shift-based islanding detection devices, especially those using the frequency feedback method with step injection, it is difficult to achieve both improved frequency ramp fluctuation tolerance and islanding detection performance.

[0009] The present invention is devised to meet such demands, and aims to provide a method and apparatus for improving the frequency ramp fluctuation tolerance of the islanding operation detection function in a distributed power source, which enables improving the frequency ramp fluctuation tolerance while maintaining the islanding operation detection function. [Means for solving the problem]

[0010] In order to achieve this objective, the inventors conducted various experiments and research, and as a result, they performed computer simulations of the detection operating characteristics of the new active islanding detection function with respect to frequency ramp fluctuations, targeting the typical frequency change rate detection method used in the detection section of the islanding detection function. As a result, for example, in a 50 Hz system where the frequency ramp fluctuation tolerance is small, the detection threshold is not reached when the frequency change rate is in the range of -2 Hz / sec (when the frequency drops) to 2.4 Hz / sec (when the frequency rises), and it was confirmed that this range is the current tolerance for frequency ramp fluctuations. On the other hand, if we focus on the frequency change rate that reaches detection within the required islanding detection time of 0.2 seconds, we find that during islanding operation, there is always a time when this rate is below -4.2 Hz / sec (when the frequency drops) and above 4.4 Hz / sec (when the frequency rises). (In this case, the frequency change rates of -4.2 Hz / sec and 4.4 Hz / sec correspond to the average change rates for each of the optional frequency change patterns that reach the detection threshold within the upper limit of 0.2 seconds. As a result, as long as the detection threshold is reached within 0.2 seconds, there will always be a time when the frequency change rate reaches or exceeds the above values ​​for all frequency change patterns during islanding operation.) We have discovered that by adding these conditions to the detection elements, it is possible to significantly improve frequency ramp fluctuation tolerance.

[0011] The present invention is based on this finding, and the method for improving the tolerance to frequency ramp fluctuations of an islanding operation detection function in a distributed power source as set forth in claim 1 comprises the steps of: active The minimum value of the average frequency change rate during islanding operation, calculated from the detection threshold value of the islanding operation detection function and the upper detection time limit value, is used as the judgment threshold value, and the islanding operation possibility judgment function performs judgment processing to determine the possibility of islanding operation based on whether the frequency change rate exceeds the judgment threshold value on a cycle-by-cycle basis, and outputs a signal to parallel off the distributed power source when both the islanding operation detection result from the active islanding operation detection function and the judgment result that islanding operation is possible from the islanding operation possibility judgment function are output.

[0012] Here, the method for improving the tolerance of frequency ramp fluctuations of the islanding operation detection function in a distributed power supply of the present invention is to input the detection result from the active islanding operation detection function and the judgment result from the islanding operation possibility judgment function to an AND gate, respectively, and place the detection signal of the islanding operation detection function in a blocked state by the AND gate, and maintain the blocked state unless the judgment result of the islanding operation possibility judgment function judges that there is a possibility of islanding operation, and when it judges that there is a possibility of islanding operation, situation It is preferable to output a signal to disconnect the distributed generation by releasing the

[0013] Furthermore, the method of the present invention for improving the tolerance to frequency ramp fluctuations of the islanding operation detection function in a distributed power source preferably includes a detection result hold function that temporarily holds the detection signal from the active islanding operation detection function, and a judgment result hold function that temporarily holds the judgment signal from the islanding operation possibility judgment function, thereby absorbing discrepancies in the timing of signal transmission from the active islanding operation detection function and the islanding operation possibility judgment function.

[0014] Furthermore, in the method of improving the tolerance to frequency ramp fluctuations of the islanding operation detection function in a distributed power source of the present invention, when the islanding possibility determination function determines that there is a possibility of islanding operation, the cycle at which the frequency change rate exceeds the determination threshold is used as a reference cycle, and the difference between the frequency change rate of the third cycle including the reference cycle and the frequency change rate of the cycle immediately preceding the reference cycle is calculated after removing the frequency ramp fluctuation, and if this exceeds the error threshold, it is deemed to be different from a voltage phase step change and a determination is made that there is a possibility of islanding operation, and further if the difference between the frequency change rate of the fourth cycle and the frequency change rate of the cycle immediately preceding the reference cycle is equal to or less than the error threshold, it is deemed to be a frequency step fluctuation and the determination is returned to no possibility of islanding operation, but if it exceeds the error threshold, the determination that there is a possibility of islanding operation is maintained.

[0015] Furthermore, the device for improving the tolerance of frequency ramp fluctuations of the islanding detection function of a distributed power source of the present invention includes an active islanding operation detection function unit using a frequency shift type islanding operation detection method, and an islanding possibility determination function unit which determines the possibility of islanding operation on a cycle-by-cycle basis whether the frequency change rate exceeds a determination threshold value, using the minimum value of the average frequency change rate during islanding operation from the detection threshold value and the detection time upper limit value of the active islanding operation detection function unit as a determination threshold value, and an AND gate which receives as input a signal from the active islanding operation detection function unit and a signal from the islanding possibility determination function unit, and outputs a signal to parallel off the distributed power source when both the islanding operation detection signal and the islanding possibility determination signal are input, and the AND gate active The detection signal from the islanding operation detection function unit is placed in a blocked state, and in this state the frequency change rate is continuously monitored in cycle units. While the block is in effect, even if the active islanding operation detection function unit sends out a detection signal, the distributed power source will not be disconnected. If the frequency change rate exceeds the judgment threshold, it is determined that there is a possibility of islanding operation and the blocked state is released. If the active islanding operation detection function unit sends out a detection signal when the block is released, it is ultimately determined that islanding operation has occurred and the distributed power source is disconnected.

[0016] Here, the device for improving the tolerance to frequency ramp fluctuations of the islanding operation detection function in a distributed power source according to the present invention is as follows: active It is preferable to provide a detection result hold function unit that temporarily holds the detection results from the islanding operation detection function unit, and a judgment result hold function unit that temporarily holds the judgment results from the islanding operation possibility judgment function unit, thereby absorbing the timing discrepancy in signal transmission from the active islanding operation detection function unit and the islanding operation possibility judgment function unit.

[0017] Furthermore, in the device for improving the tolerance to frequency ramp fluctuations of the islanding operation detection function in a distributed power source of the present invention, when the islanding possibility determination function unit determines that there is a possibility of islanding operation because the rate of frequency change exceeds the determination threshold, it further sets the cycle at which the rate of frequency change exceeds the determination threshold as a reference cycle, calculates the difference between the rate of frequency change of the third cycle including the reference cycle and the rate of frequency change of the cycle immediately preceding the reference cycle after removing the frequency ramp fluctuation, and if this exceeds the error threshold, it determines that this is not a voltage phase step change and determines that there is a possibility of islanding operation, and if the difference between the rate of frequency change of the fourth cycle and the rate of frequency change of the cycle immediately preceding the reference cycle is equal to or less than the error threshold, it determines that it is a frequency step fluctuation and returns to the determination that there is no possibility of islanding operation, but it is preferable that the determination of there is a possibility of islanding operation be maintained if it exceeds the error threshold. [Effects of the Invention]

[0018] In the method and apparatus for improving the tolerance to frequency ramp fluctuations of the islanding detection function in a distributed power source of the present invention, active By blocking erroneous determinations of the islanding operation detection function, it is possible to improve the tolerance to frequency ramp fluctuations. As a result of various computer simulations by the inventors, it was confirmed that it is possible to improve the tolerance to frequency ramp fluctuations by approximately two times in a 50 Hz system where the tolerance to frequency ramp fluctuations is small. That is, active Islanding operation detection function Department By blocking the detection signal, unnecessary disconnection can be more effectively suppressed. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a functional block diagram showing an embodiment of an apparatus for implementing a method for improving the tolerance to frequency ramp fluctuations of an islanding operation detection function in a distributed power source according to the present invention; [Figure 2] FIG. 2 is an explanatory diagram showing the relationship between the frequency acquisition times for detecting a frequency deviation. [Figure 3]FIG. 10 is a waveform diagram illustrating the state of a voltage phase step change. [Figure 4] This shows an example of the detection method of the islanding detection unit in the new active method, where (a) is a detection flow diagram and (b) is an image diagram of the detection threshold and periodic changes. [Figure 5] This is a graph showing the results of simulation calculations for the frequency decrease and increase directions when connected to a 50 Hz system, where the vertical axis is periodic deviation, the horizontal axis is time, and the parameter is the rate of change of frequency. [Figure 6] This is an image diagram showing the frequency deviation change pattern during islanding operation detected in the current cycle using the new active method. [Figure 7] 1 is a graph showing the relationship between period deviation and injected reactive power in the new active method. [Figure 8] FIG. 10 is a flow chart showing an embodiment of a monitoring and determination process of an islanding possibility determination function. [Figure 9] Graphs showing various characteristics during frequency ramp fluctuations, where (a) shows frequency, (b) shows islanding possibility determination flag, and (c) shows islanding detection flag. [Figure 10] Graphs showing various characteristics when a voltage phase step change occurs during frequency ramp fluctuation, where (a) shows frequency, (b) shows islanding possibility determination flag, and (c) shows islanding detection flag. [Figure 11] FIG. 1 is a schematic diagram of a power distribution system model for analyzing islanding characteristics used in simulation verification. [Figure 12] FIG. 1 is a schematic diagram of a control system model of a power conditioner used in the verification. [Figure 13] The graphs show the characteristics during islanding operation obtained in the same simulation under the condition of 0% active power unevenness efficiency and 0% reactive power unevenness efficiency. (a) shows the frequency, (b) shows the islanding possibility determination flag, and (c) shows the islanding detection flag. [Figure 14] The graphs show the characteristics during islanding operation obtained in the same simulation under the condition of an active power unevenness efficiency of 5% and a reactive power unevenness efficiency of 0%, where (a) shows the frequency, (b) shows the islanding possibility determination flag, and (c) shows the islanding detection flag. [Figure 15] The graphs show the characteristics during islanding operation obtained in the same simulation under the condition of an active power unevenness efficiency of 0% and a reactive power unevenness efficiency of 5%, where (a) shows the frequency, (b) shows the islanding possibility determination flag, and (c) shows the islanding detection flag. DETAILED DESCRIPTION OF THE INVENTION

[0020] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The configuration of the present invention will be described in detail below based on the embodiments shown in the drawings.

[0021] [Islanding detection function] The active islanding detection device targeted by the present invention is not limited to a specific method, but as an example, it is preferable to use an active islanding detection function called a frequency feedback method with step injection (generally called a new active islanding detection function). This new active islanding detection function is currently standardized by the Japan Electrical Manufacturers' Association (JEM) standard and is called the "standard active islanding detection method." It is widely used as a standard islanding detection method for low-voltage grid-connected distributed power sources. Therefore, in this embodiment, the new active islanding detection function will be described as an example.

[0022] The new active islanding detection function belongs to the so-called frequency shift system and consists of a reactive power injection function that feeds back frequency deviation and injects reactive power during islanding operation to diverge the frequency, and an islanding detection function that detects sudden changes in frequency (i.e., frequency divergence) that occur during islanding operation.

[0023] The new active islanding detection function uses a method to detect islanding by capturing the rate of change in frequency and period caused by the frequency divergence function, but the specific specifications have not been standardized, and it is required to be designed on the condition that islanding detection requirements and FRT requirements are compatible. Here, the conditions required for the islanding detection function in relation to the islanding detection requirements and the FRT requirements in terms of frequency fluctuation and phase jump can be summarized as follows: (1) The number of interconnected units and the balance of active and reactive power between the loads Regardless of the load, islanding is detected within 0.2 seconds. (2) Three cycles in a step pattern (maximum 0.8 Hz when connected to a 50 Hz system) No unnecessary detection of frequency fluctuations that continue for a long time. (3) Ramp-like frequency fluctuation of -2 to 2 Hz / sec {Frequency upper limit: Frequency rise relay ( OFR setting, frequency lower limit: under frequency relay (UFR) setting Do not detect unnecessary (4) Step change in voltage phase (up to 41 degrees) that occurs when a two-phase short circuit occurs in a power transmission system Do not detect unnecessary The FRT requirements (2) to (4) are applicable to all inverter-connected distributed power sources, including those that use other islanding detection methods. Furthermore, the specifications for the frequency divergence function are specified in detail in the JEM standard, so a detailed explanation will be omitted here.

[0024] Regarding the requirement (1) of detecting islanding within 0.2 seconds, frequency changes are generally calculated by determining a base frequency value based on past frequencies and calculating the difference (deviation) between that value and the current frequency, as shown in Figure 2. Islanding is determined when the deviation exceeds a set threshold (called the detection threshold). The reference frequency value is generally calculated using a moving average over multiple cycles. Furthermore, to prevent the reference frequency from changing significantly during islanding, the reference frequency is generally calculated based on a frequency that is at least the required or targeted islanding detection time (within 0.2 seconds for the new active method). While the islanding detection time can be shortened by lowering the detection threshold, this may result in the failure to meet the requirements (2) and (3). Therefore, the detection threshold is generally set to the lowest or closest value that satisfies requirements (2) and (3). If islanding cannot be detected within the required time, measures such as increasing the reactive power injection gain of the active method are taken.

[0025] The frequency step fluctuations in (2) above can be dealt with by adjusting the detection threshold or by monitoring the continuity of deviations from the detection threshold. In the case of a 50 Hz system, the former can be avoided by setting the detection threshold to a value greater than 0.8 Hz (period deviation: 315 μsec). In the case of the latter, adding a continuous deviation of four cycles or more to the detection conditions can avoid unnecessary detection regardless of the detection threshold. However, both involve a trade-off with the islanding detection time in (1), so the choice must be made taking into account the compatibility of these two.

[0026] The frequency ramp fluctuations in (3) above are addressed by adjusting the detection threshold and the time difference between the current frequency acquisition cycle and the reference frequency acquisition cycle in Figure 2. The higher the detection threshold and the smaller the time difference, the more difficult it becomes to detect unwanted occurrences. However, these are also in a trade-off relationship with the islanding detection time, so they are determined taking compatibility into consideration.

[0027] Regarding (4) above, if a step change occurs in the voltage phase and the frequency is calculated from the time width between typical zero crossing points, the number of cycles in which the frequency change appears will be one or two cycles, depending on whether the change in voltage value due to the phase step change crosses the zero point, and then the frequency will return to its original value, as shown in Figure 3. As a result, a method is generally used in which the continuity of three or more cycles of deviation from the detection threshold is monitored, and if the detection threshold is exceeded in all of these cycles, islanding operation is determined to be occurring.

[0028] An example of a typical detection method for the detection function section of the new islanding detection method, which satisfies all four conditions required for the islanding detection function mentioned above, is shown in Figure 4. Note that the detection thresholds in Figure 4(b) are values ​​for a 50 Hz grid connection, and are set to 5 / 6 times the values ​​for a 60 Hz grid connection.

[0029] This method evaluates frequency based on the period. The reference period is set as the moving average of the period over 8 cycles, excluding the maximum and minimum values ​​of the 10 cycles 14 to 23 cycles before the current period. This prevents the reference period from changing significantly during the islanding period of 0.2 seconds or less until islanding is detected.

[0030] The frequency change rate is detected over a total of four cycles: the current cycle and three cycles prior to the current cycle. Note that the sign of the threshold value for the three cycles prior is reversed, so continuity is essentially monitored over three cycles. Deviation from the reference cycle is calculated for each cycle, and if any of the deviations exceed the set threshold, it is determined to be islanding. This continuous monitoring over multiple cycles avoids unnecessary detection of step changes in the voltage phase, satisfying the FRT requirements.

[0031] In addition, each threshold is not constant, but is increased sequentially from three cycles before. In particular, the detection threshold for the last current cycle (340 μs) is converted into a frequency change value of 0.84 Hz (when the frequency is decreasing) and 0.86 Hz (when the frequency is decreasing), which satisfies the FRT requirement for frequency step fluctuations (0.8 Hz, 3 cycles).

[0032] For frequency ramp fluctuations, the time interval between the reference period calculation cycle and the cycle that monitors period deviations from the current cycle to the three cycles prior, as well as the detection threshold for the current cycle (0.84 Hz for a 50 Hz system), are used to prevent unnecessary detection in the frequency change rate range of -2 Hz / sec to 2 Hz / sec, which is an FRT requirement.

[0033] In this way, islanding detection within a specified time is achieved while satisfying the FRT requirements for frequency step fluctuations, frequency ramp fluctuations, and voltage phase step changes.

[0034] [Detection characteristics of a new active method for frequency ramp fluctuations] Regarding the detection operation of this new active islanding detection function in response to frequency ramp fluctuations, the relationship between the time change in the periodic deviation detected in the current cycle of the detection function and the detection time when the frequency is ramping was examined, and the following was confirmed.

[0035] Figure 5 shows the results of simulation calculations for both the frequency decrease and increase directions when connected to a 50 Hz system. The parameter is the rate of change of frequency. Note that, in accordance with the grid connection regulations, the frequency is changed downward to 47.5 Hz, the detection level of the Under Frequency Relay (UFR), and upward to 51.5 Hz, the detection level of the Over Frequency Relay (OFR). In addition, a line for the detection threshold (±340 μsec) for the current cycle is also shown, and the time it takes for the period deviation to reach these detection thresholds is shown in parentheses as the detection time.

[0036] A characteristic of period deviation change is that from the start of the ramp change until 0.28 seconds, the period 14 to 23 cycles before, which determines the reference period, is in the region of the constant value (50 Hz) before the ramp change, so the reference period remains constant, and the period deviation increases or decreases linearly. That is, in this time region, the period deviation is the change from the standard frequency (50 Hz or 60 Hz). From that point on, the period 14 to 23 cycles before, which determines the reference period, enters the time region of the ramp change, and the reference period also increases or decreases accordingly, so the rate of change of period deviation over time decreases significantly. Note that, as shown in Figure 5, the time change characteristic of period deviation recognized in each cycle is an upward convex curve, and as shown in Figure 4(b), the detection threshold for each cycle increases exponentially. Therefore, if period deviation exceeds the detection threshold for the current cycle, it will also necessarily exceed the detection thresholds for the previous cycle and two cycles before.

[0037] When examining the detection status for each frequency change rate, we found that when the frequency change rate was between -2 Hz / sec and 2.4 Hz / sec, the period deviation did not reach the detection threshold within the frequency change range in either the decreasing or increasing direction, confirming that this range is the current tolerance for frequency ramp fluctuations. Next, when the frequency change rate exceeded the above values, in the case of frequency decrease, the threshold was exceeded at 0.28 seconds at a change rate of -3 Hz / sec and at 0.18 seconds at a change rate of -5 Hz / sec, resulting in detection. Furthermore, in the case of frequency increase, the threshold was exceeded at 0.3 seconds at a frequency change rate of 3 Hz / sec and at 0.18 seconds at a frequency change rate of 5 Hz / sec, resulting in detection.

[0038] Meanwhile, if we focus on the frequency change rate required to detect islanding within the required upper limit of 0.2 seconds, we find that it is -4.2 Hz / sec when the frequency drops and 4.4 Hz / sec when it rises, both of which are approximately twice the absolute value of the frequency ramp fluctuation tolerance mentioned above. Because these values ​​are for a ramp-like change in frequency, they correspond to the average frequency change rate for each frequency change pattern in which the periodic deviation reaches the detection threshold in 0.2 seconds. In other words, with the new active method, reactive power is controlled so that the average frequency change rate during islanding is equal to or exceeds the above values ​​in order to satisfy the required detection time.

[0039] From this, we found that one effective way to improve FRT performance against frequency ramp fluctuations is to add the frequency change rate that always occurs during such islanding operation as one of the detection elements.

[0040] Although not shown, computer simulation results for a system connected to a 60 Hz system also showed that, in the event of a frequency ramp fluctuation, the periodic deviation recognized in the current cycle of the islanding detection function did not reach the detection threshold when the frequency change rate was between -2.9 Hz / sec and 3.4 Hz / sec, confirming that this range is the tolerance for frequency ramp fluctuations in a 60 Hz system. Furthermore, the frequency change rate at which the periodic deviation reached the detection threshold in 0.2 seconds was -5.0 Hz / sec when the frequency dropped, and 5.3 Hz / sec when the frequency rose.

[0041] "Determining the possibility of isolated operation"

[0042] Furthermore, we examined the detection operation characteristics of a new active islanding detection function, which is an example of a frequency shift type active islanding detection function, and found that it is possible to determine the possibility of islanding.

[0043] In other words, the change pattern of frequency deviation (the periodic deviation obtained in Figure 4 converted into frequency deviation) during actual islanding operation in the new active method will vary depending on various conditions, such as the degree of balance between active and reactive power with the load immediately before islanding, the proportion of the motor load with inertia, and the capacity of the resonant circuit formed by the power factor improvement capacitor and the inductance of the load, but in the example of a frequency rise, it can be roughly divided into lines A, B, and C shown in Figure 6. Note that the frequency deviation at the start of islanding operation is all 0, and the time (= td) for the frequency deviation to reach the detection threshold for the current cycle is the same as the frequency deviation at that time.

[0044] For example, if both the active and reactive power with the load are balanced immediately before the transition to islanding, the rate of frequency change (corresponding to the slope of each line) will be small immediately after the transition to islanding, but then the rate of change will increase due to an increase in the amount of reactive power injected by the new active frequency feedback function, resulting in a pattern like line C. Also, if there is a large imbalance in reactive power with the load immediately before the transition to islanding, the inverter's constant power factor control function will act to increase the rate of frequency change immediately after the transition to islanding, and the injected reactive power will quickly reach its upper limit (see Figure 7), resulting in a pattern like line A. Furthermore, even if the reactive power is unbalanced, if a load with inertia, such as a motor, is present, the rate of frequency change will be suppressed immediately after the transition to islanding, and a ramp-like change like line B is expected.

[0045] Assuming that line B represents an ideal ramp change, the slope of line B indicates the average frequency change rate during the islanding period for the frequency change pattern, including lines A and C, that reaches the detection threshold at time td. To reach the detection threshold within the same time frame, there must be a period during the islanding period during which the frequency change rate reaches or exceeds this average value, including instantaneous changes. Therefore, if the new active method is guaranteed to detect islanding within 0.2 seconds, there must be a period during islanding where the frequency change rate exceeds the value of line B' in Figure 6 (where the frequency changes in a ramp pattern and reaches the detection threshold at 0.2 seconds, the upper detection limit). This value is the minimum average frequency change rate during islanding. In other words, when the frequency change rate is continuously monitored in cycle-by-cycle increments using the minimum average frequency change rate during islanding as the detection threshold, we found that the islanding possibility determination result is never output later than the islanding detection result.

[0046] In the case of the islanding detection function targeted in this embodiment, for example, when the upper detection time limit is set to 0.2 seconds, the judgment threshold value is -4.2 Hz / second when the frequency drops and 4.4 Hz / second when the frequency rises, in a 50 Hz system where the frequency ramp fluctuation tolerance is relatively small. If the frequency change rate is below this in absolute value, the output of the islanding detection function can be blocked, and a significant improvement in the frequency ramp fluctuation tolerance can be expected.

[0047] [Methods to avoid misjudgments of other system transients] Furthermore, as mentioned above, if the islanding possibility determination function were to make a determination based only on a deviation from the determination threshold for one cycle, there is a possibility that it would make an erroneous determination in the case of a "step change in voltage phase" and a "step fluctuation in frequency lasting three cycles," for which continued operation is required by the FRT requirements.

[0048] For example, in the case of a voltage phase step change, if the frequency remains constant before and after the step change, the new active method itself will not detect it as unnecessary, according to the FRT requirements. However, when a system abnormality causes a frequency ramp change, emergency load shedding or system switching may be performed to suppress the change, which may also cause a voltage phase step change. If the islanding possibility determination function makes an incorrect determination, it could lead to the disconnection of the distributed power source if the rate of frequency ramp change exceeds the tolerance of the new active method. For this reason, the islanding possibility determination function must avoid incorrect determination of voltage phase step changes, including frequency ramp changes.

[0049] Furthermore, a three-cycle frequency step fluctuation is a hypothetical phenomenon that occurs when a transmission line fault occurs. In accordance with the FRT requirements, the new active method itself does not detect this as unnecessary, but the frequency may subsequently transition to a ramp fluctuation due to rapid fault clearance, etc. This could lead to the parallel-off of distributed power sources during the subsequent frequency ramp fluctuation if the islanding possibility determination function erroneously determines the frequency step fluctuation. For this reason, it is necessary to avoid erroneous determinations for the same frequency step fluctuation as well.

[0050] [1. How to avoid erroneous judgments regarding voltage phase step changes] Because voltage phase step changes affect only one or two cycles, incorporating some kind of three-cycle continuity into the judgment criteria can avoid erroneous judgments. For example, using a method that determines the possibility of islanding when the frequency change rate, including positive and negative continuity, exceeds the threshold three times in a row can avoid erroneous judgments regarding voltage phase step changes. However, during islanding operation, the frequency may fluctuate, fluctuating to some extent, due to factors such as the inertia of the motor load and interactions with the control systems of other distributed power sources, especially during the period immediately after transition to islanding when the injected reactive power from the new active system is small. In such cases, the frequency change rate may change positively or negatively in one-cycle increments, potentially leading to a delay or failure to release the islanding detection signal block. Therefore, we focused on the fact that the frequency change value based on the frequency immediately before the event occurs basically diverges during islanding operation, but returns to 0 within three cycles when there is a voltage phase step change, and came up with the idea of ​​incorporating the difference between the divergence of this frequency change value and its return to 0 as a judgment factor for identification.

[0051] [When the frequency is constant (steady state) before and after the voltage phase step change] For example, if the frequency before and after the voltage phase step change is constant, (a) If the frequency change rate exceeds the threshold, the frequency change rate is calculated for three consecutive cycles, including the cycle in which the threshold was exceeded. (b) Next, the integral value of the frequency change rate over the same three-cycle period (the frequency change value in the third cycle) is calculated, and if this value is 0 or close to 0, it is deemed that there is no possibility of islanding. This procedure can avoid erroneous judgments.

[0052] [When a voltage phase step change occurs during a frequency ramp] Next, if the frequency before and after the voltage phase step change is ramp-fluctuation at a constant rate, erroneous determination can be avoided by the following procedure. (c) If the frequency change rate exceeds a threshold, the cycles that exceed this threshold are included. Therefore, the second-order time derivative of the frequency with respect to time is calculated for three consecutive cycles. (d) Next, for each cycle, from the cycle in which the threshold value is exceeded to the cycle in question The integral value of the second derivative of the frequency in the interval is calculated, and further, The integral value of the second derivative of the frequency obtained by the calculation is integrated over the same three-cycle interval to obtain a three-cycle The frequency change value of the next step is calculated. If this value is 0 or close to 0, It is considered that there is no possibility of solo operation.

[0053] As mentioned above, we have proposed methods for avoiding erroneous judgments by dividing the cases into those in which the frequency before and after a voltage phase step change is constant and those in which it fluctuates ramp-up. However, since both cases can be handled by the method of integrating the second-order frequency differential, it is preferable to incorporate this method of integrating the second-order frequency differential into the islanding possibility judgment function.

[0054] [2. How to avoid false positives due to frequency step fluctuations] On the other hand, for a frequency step fluctuation that continues for three cycles, the frequency step fluctuation becomes 0 in the fourth cycle after the occurrence of the fluctuation, so a method that makes a judgment based on the frequency change value in the third cycle as described above will lead to an erroneous judgment. One possible solution to this problem would be to increase the number of cycles required for judgment to four, but this would be more than the actual number of detection cycles of three for the new active islanding detection function targeted in this embodiment, and would affect the islanding detection time.

[0055] Therefore, taking into consideration that the new active method itself does not perform unnecessary detection of frequency step fluctuations, and as a way to avoid both the impact on islanding detection time and the subsequent disconnection of distributed power sources due to frequency ramp fluctuations etc. caused by an erroneous judgment, the possibility of islanding operation is deliberately determined in two stages (a) and (b) as follows. (a) The frequency change value in the third cycle after the frequency differential value exceeds the threshold is 0 (or is less than or equal to a threshold close to 0), the detection block of the islanding detection function is Even if the block is released, the islanding detection function itself will not be By satisfying the requirements, unnecessary detection for frequency ramp fluctuations is not performed at this point. It won't come out. (b) Next, check the frequency change value in the fourth cycle and see if it is 0 or close to 0. If it is not below the threshold value (referred to as the false positive threshold value in this specification), This prevents the failure to detect the problem during isolated operation. On the other hand, if the error is below the threshold value of 0 or close to 0, it is not islanding. It is considered to be a blocked state, and the state is returned to the blocked state to avoid future unnecessary detections.

[0056] Based on this knowledge, we came up with the idea of ​​using an "islanding possibility determination function" that determines the possibility of islanding operation by monitoring and evaluating the frequency change rate on a cycle-by-cycle basis, using the frequency change rate that always occurs during islanding operation as a threshold value, in conjunction with the islanding operation detection function, as a method to improve frequency ramp fluctuation tolerance while maintaining the current islanding operation detection performance as much as possible, and by blocking the detection signal of the islanding operation detection function depending on the situation, we can strengthen the suppression of unnecessary parallel-off. That is, the method and device for improving the tolerance of frequency ramp fluctuations of the islanding detection function in a distributed power supply of the present invention is configured to incorporate an islanding possibility determination function in parallel with the islanding detection function, and the basic method for determining the possibility of islanding operation and the final method for paralleling off the distributed power supply are as follows: (i) Detection threshold and detection time upper limit of the new active islanding detection function (e.g. The minimum speed of average frequency change during islanding operation is now 0.2 seconds (current standard). This is determined as the decision threshold value. (ii) As an initial condition, the detection signal of the islanding detection function is set to the blocked state. In this state, the frequency change rate is continuously monitored in cycle units. In other words, while the islanding detection function is blocking, the distributed This will prevent the power supply from being disconnected. (iii) If the frequency change rate exceeds the threshold value set in (i), the power Determine whether it is due to other system transient fluctuations such as pressure phase step changes, and if not, If this happens, it is determined that there is a possibility of islanding and the block is released. is also good. (iv) If the islanding detection function sends a detection signal when the block is released, It is determined that islanding has occurred and the distributed power source is disconnected. Among these, at least the conditions (i), (ii) and (iv) are satisfied, and preferably the condition (iii) is also satisfied.

[0057] FIG. 1 shows an embodiment of an apparatus for implementing a method for improving the tolerance to frequency ramp fluctuations of an islanding operation detection function in a distributed power source according to the present invention.

[0058] The device for improving the tolerance to frequency ramp fluctuations of an islanding operation detection function according to this embodiment is configured to combine an active islanding operation detection function unit 1 and an islanding operation possibility determination function unit 2 in parallel, and to output a distributed power source parallel-off signal only when the detection result of the active islanding operation detection function unit 1 and the determination result of the islanding operation possibility determination function unit 2 are consistent. Note that the reference numeral 6 in the figure denotes a period calculation unit.

[0059] In this embodiment, the detection signal from the islanding detection function unit 1 and the determination signal from the islanding possibility determination function unit 2 are input to the AND gate 3, and a distributed power source disconnection signal is output only when both the islanding detection signal and the islanding possibility determination signal are input. The AND function of the AND gate 3 blocks the detection signal from the islanding detection function as an initial condition, and in this state, the frequency change rate is continuously monitored in cycle-by-cycle units. In other words, by inputting the detection signal from the active islanding detection function 1 and the determination result signal from the islanding possibility determination function 2, which are connected in parallel, to the AND gate 3, the AND gate 3 blocks the detection signal from the islanding detection function 1, and the blocked state is maintained unless the determination result from the islanding possibility determination function 2 determines that islanding operation is possible. When it determines that islanding operation is possible, the AND gate 3 releases the block and outputs a signal to disconnect the distributed power source.

[0060] In addition, in order to accommodate the timing discrepancy in signal transmission from each function unit 1, 2, a detection result hold function unit 4 and a judgment result hold function unit 5 are provided downstream of each of the active type islanding detection function unit 1 and islanding possibility determination function unit 2, respectively, which hold the output value for a certain period of time, and are configured to absorb the timing discrepancy in signal transmission from the active islanding detection function unit 1 and islanding possibility determination function unit 2.

[0061] Here, the active islanding detection device is not limited to a specific method, but as mentioned above, it is preferable to use an active islanding detection function called a frequency feedback method with step injection, as an example.

[0062] On the other hand, the islanding possibility determination function unit 2 determines the possibility of islanding operation based on whether the frequency change rate exceeds the determination threshold value on a cycle-by-cycle basis, using the minimum value of the average frequency change rate during islanding operation, which is calculated from the detection threshold value of the islanding detection function and the upper detection time limit value, as the determination threshold value.

[0063] Here, for example, in a 50 Hz system where the frequency ramp fluctuation tolerance is small, the judgment threshold when the detection time upper limit is 0.2 seconds is -4.2 Hz / s or less (when the frequency drops) and 4.4 Hz / s or more (when the frequency rises) during islanding operation. In this case, the frequency change rates of -4.2 Hz / s and 4.4 Hz / s correspond to the average change rates of each of the arbitrary frequency change patterns that reach the detection threshold within the upper limit of 0.2 seconds.

[0064] In the process of determining the possibility of islanding operation in the islanding operation possibility determination function unit 2, the possibility of islanding operation is determined based on whether or not the frequency change rate exceeds the determination threshold value on a cycle-by-cycle basis. However, if the rate exceeds the determination threshold value A, it is preferable to further determine whether or not this is an erroneous determination of other system transient fluctuations to avoid erroneous determination.

[0065] An example of the monitoring and judgment process in the above-mentioned islanding possibility judgment function unit 2 is shown in Figure 8. Note that each output in the flow indicates the output to the judgment result hold function 5 in Figure 1. The overall procedure, including the final output control method of the judgment result hold function 5, is as follows:

[0066] In islanding possibility determination function 2, if the rate of frequency change in the i-th cycle from the start of monitoring exceeds determination threshold value A, the frequency ramp fluctuation is removed as described above, and the frequency change value for the i-1th cycle in the i+2th cycle is found, and if this value exceeds erroneous determination threshold value B, it is deemed to be different from a voltage phase step change, and a "possibility possible" signal is output, thereby avoiding a delay in islanding operation detection.

[0067] However, in the case of a frequency step change, this will result in an incorrect judgment, so the frequency change value for the i-1 cycle from the i+3 cycle is calculated using the same method, and if these are below the incorrect judgment threshold B, it is considered to be a frequency step change and a "not possible" signal is output. If the incorrect judgment threshold B is still exceeded, the "possible" signal continues to be output.

[0068] Here, the judgment threshold A is the minimum value of the average frequency change rate (absolute value) during islanding operation, calculated by the required islanding detection time upper limit and the detection threshold of the islanding operation detection function. In the case of the islanding operation detection function targeted in this embodiment, the frequency increase rate is 4.4 Hz / s and the frequency decrease rate is 4.2 Hz / s. On the other hand, the false judgment threshold B is ideally 0, but in practice, it is considered appropriate to have a certain degree of flexibility due to fluctuations in the reference frequency itself during system transient fluctuations. The specific value must be determined taking into consideration the distinction from islanding operation. Here, as a tentative proposal, the threshold value is set to 1 / 5 of the maximum frequency change value (absolute value) within three or four cycles of the relevant frequency. If this value is exceeded, a "possible" signal is output or the output of this signal is continued. If the frequency change value is equal to or less than the false judgment threshold B, a "not possible" signal is output or the output of this signal is continued.

[0069] If the determination result hold function unit 5 receives a "possible" signal while the function is outputting a "no possibility" signal for islanding, it immediately switches its output to a "possible" signal. However, if the "no possibility" signal is received in the next cycle, it immediately switches its output back to the original "no possibility" signal and maintains that output until the next "possible" signal is received. On the other hand, if the "possible" signal is received in the next cycle, it continues to output the "possible" signal for a certain period (e.g., 0.5 seconds). If the islanding detection result hold function unit 4 separately outputs a "detection" signal during this continuation period, it continues to output the "possible" signal until the output of the "detection" signal ceases. On the other hand, if the islanding detection result hold function unit 4 does not output a "detection" signal during the maintenance period, it switches its output back to the "no possibility" signal and maintains that output until the next "possible" signal is received.

[0070] In this manner, only when both the islanding operation detection signal and the islanding operation possibility determination signal are input to the AND gate 3, a distributed generation disconnection signal is output.

[0071] Furthermore, the method and device for improving the tolerance to frequency ramp fluctuations of the islanding operation detection function according to this embodiment can be considered to be able to be implemented essentially by simply improving the software of the islanding operation detection function of the PCS, and is thought to involve almost no cost burden. Furthermore, this technology is considered to contribute not only to improving the FRT performance of distributed power sources, but also to improving system resilience, such as by enabling both islanding operation detection when a distribution line fault occurs and an outage of the upper system, and achieving independent operation of the distribution line when a fault occurs and the upper system fault occurs.

[0072] The above-described embodiment is one example of a preferred embodiment of the present invention, but is not limited to this and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, a new active type islanding operation detection unit is mainly used as an example, but the present invention is not particularly limited to this and can be applied to any type of islanding operation detection function as long as it is a frequency shift type islanding operation detection method.

[0073] Furthermore, the isolated operation possibility determination function unit 2 in the above-described embodiment incorporates a method of integrating the second-order frequency differential value as a method of avoiding erroneous determination for voltage phase step changes, thereby dealing with the situation without distinguishing between cases where the frequency is constant before and after the step change and cases where there is ramp fluctuation. However, it is also possible to distinguish between cases where the frequency is constant before and after the step change and cases where there is ramp fluctuation, and avoid erroneous determination for each case.

[0074] In addition, in some cases, such as when the number of cycles required for detection by the target islanding detection function is four or more, the number of cycles required for determination may be increased to four as a method of avoiding erroneous determination of frequency step fluctuations. [Example]

[0075] The validity of the islanding possibility determination function according to the present invention was confirmed by computer simulation.

[0076] [Avoidance of unnecessary disconnections due to frequency ramp fluctuations and sudden phase changes] First, we will confirm the ability of distributed power sources to avoid unnecessary parallel-off during frequency ramp fluctuations. Here, we will focus on a representative case of frequency drop, and set judgment threshold A in the flow diagram of Figure 8 to -4.2 Hz / sec. Furthermore, when the frequency change rate exceeds judgment threshold A, we set erroneous judgment threshold B to 1 / 5 of the maximum frequency change value (absolute value) in the following three or four cycles.

[0077] Based on the above judgment threshold A, the frequency change rate of the frequency ramp fluctuation was set to -4.0 Hz / second, which is close to judgment threshold A, and the time trends of the output of the judgment result hold function of the islanding possibility judgment function (flag value 1: possibility, 0: not possibility) and the output of the detection result hold function of the new active islanding detection function (flag value 1: detected, 0: not detected) were confirmed by calculation.

[0078] The results are shown in Fig. 9. The islanding detection function determined islanding in approximately 0.22 seconds and output a detection signal, but the islanding possibility determination function continued to determine that islanding was not possible until the end, and unnecessary parallel-off was avoided as designed.

[0079] Next, a voltage phase step change occurs in the sixth cycle of the frequency ramp fluctuation period at a frequency change rate of −4.0 Hz / s, which results in a transient change in the frequency in the sixth and seventh cycles (approximately 0.12 to 0.14 seconds later). Figure 10 shows the results.

[0080] Due to the voltage phase step change, the islanding detection function outputs a detection signal in the sixth cycle, but the islanding possibility determination function continues to determine that islanding is not possible until the end, as in Fig. 9, and it was confirmed that unnecessary parallel-off can be avoided as designed.

[0081] [Islanding detection time maintenance characteristics] In the event of islanding operation, the islanding possibility determination function is required to determine that there is a possibility of islanding operation before the islanding detection function detects it, or at the latest simultaneously with the detection, and release the detection block, so as not to affect the islanding detection time.To confirm this, a simulation evaluation of the dynamic characteristics during islanding operation was carried out using EMTP, using an instantaneous value analysis model of a low-voltage grid-connected power conditioner (PCS).

[0082] [Simulation model] Each simulation model is shown below.

[0083] a. Power distribution system As shown in Figure 11, a model was used in which a single-phase 200V low-voltage distribution line was connected between each line of a basic three-phase 6.6kV high-voltage distribution line via a pole-mounted transformer. A 5kW-class PCS and a static load with R, L, and C of the same specifications were connected to each low-voltage distribution line. The PCS was given as a current source whose instantaneous output current was controlled by the control system model shown in Figure 12. In addition, to simulate islanding operation, a circuit breaker was installed at the send-out point of each high-voltage distribution line.

[0084] b.PCS control system model A typical current control model, shown in Figure 12, is used, with constant power control (AVR control) and unity power factor control. In the case of photovoltaic power generation, maximum power point tracking (MPPT) control is typically used, which is determined by the solar irradiance at that time. However, for the events of less than one second considered here, the solar irradiance is constant, and constant power control is sufficient to simulate them. Specifically, the instantaneous value of the interconnection point voltage (Vsys) is first monitored, and a sine wave whose phase matches Vsys is generated using a PLL circuit. This is then multiplied by a coefficient determined by the AVR control to determine the current source value at that time. The active islanding detection function is implemented by applying the new active reactive power control model shown in Figure 7, which changes the phase of the PLL circuit output Vin according to the periodic deviation. The control constants are the AVR integrator time constant T1: 0.06 seconds, and the PLL integrator time constant T2: 0.01 seconds. In addition, the frequency feedback gain in the new active method was set to the standard 0.45 pu / Hz.

[0085] The model of the islanding detection function section, which includes the islanding possibility determination function shown in Figure 1, was incorporated into the grid-connection protection device section. Determination threshold A was set to 4.4 Hz / s when the frequency increases and -4.2 Hz / s when the frequency decreases. False determination threshold B was set to 1 / 5 of the maximum frequency change value (absolute value) in the three or four cycles after the frequency change rate exceeded determination threshold A.

[0086] c. Load Model The load was simulated using static R, L, and C loads. In addition, based on the islanding detection test method in the IEC standard, a resonant circuit with the same kVA capacity (5 kVA per low-voltage distribution line) as the rated capacity of the test PCS was connected in parallel to the load.

[0087] [Variable parameters] The islanding detection time depends heavily on the balance between the power generation and the load in the section of the distribution line that will be isolated before the transition to islanding. Therefore, we varied the active power P and reactive power Q passing through the circuit breaker in Figure 11 before the transition to islanding as parameters, and performed simulations for each case. Specifically, the ratios of P and Q to the total PCS output (15 kVA) were defined as the P unbalance rate and the Q unbalance rate, and were varied in 5% increments between -10% and 10%. Regarding the definition of each sign, the P unbalance rate is positive when the load power consumption is greater than the generated power and flows from the grid to the distribution line at the circuit breaker. The Q unbalance rate is positive when the entire distribution line is inductive when viewed from the grid.

[0088] [Simulation Results] Figures 13 to 15 show typical examples of the frequency before and after the transition to islanding operation, as well as the time transition of the output flags of the islanding operation possibility determination function and islanding operation detection function. Note that a "1" in each flag indicates that the possibility determination function has output a "possible" signal in the determination result hold function, and that the islanding detection function has output a "detected" signal in the detection result hold function. Furthermore, the frequency was calculated in cycle units, and in order to confirm in detail the relationship between the change in frequency and the operation of the determination and detection functions, here the PCS continues operating even when the outputs of both the determination and detection functions are 1.

[0089] Figure 13 shows the results for a case where both the P and Q unbalance rates are 0%, and where both P and Q are balanced immediately before switching to islanding. It can be seen that the frequency barely changes in the first cycle after switching to islanding, but from the second cycle onwards, it begins to drop due to the effects of frequency feedback control. From the seventh cycle onwards, the frequency drop saturates, but this is due to the limit set on injected reactive power. The islanding possibility determination function outputs a "possible" determination signal in the fourth cycle after switching to islanding, and the islanding detection function outputs a detection signal in the fifth cycle, and it can be seen that in this case the islanding transition possibility determination function does not affect the islanding detection time.

[0090] Figure 14 shows the results for the setting case in this study where the P unbalance rate was 5% and the Q unbalance rate was 0%, which was the case for which the islanding detection time was longest. After the transition to islanding operation, the frequency repeatedly increased and decreased slightly until the fourth cycle, after which it began to decrease. This increase and decrease is thought to be due to interference with other control systems such as the PLL. Even in this case, the islanding possibility determination function outputs a "possible" determination signal in the fourth cycle, before the islanding detection function, and it can be seen that this does not affect the islanding detection time either.

[0091] Figure 15 shows the results when the P unbalance rate is 0% and the Q unbalance rate is 5%. Because Q was in an inductive unbalanced state, the frequency immediately began to rise after the transition to islanding operation. As a result, the islanding detection signal was output as early as the third cycle, but the "possible" judgment signal was also output in the same cycle, which shows that this also did not affect the islanding detection time. The output time of the "possible" judgment signal and the output time of the islanding detection signal for each case are summarized in Table 1. The hatched areas indicate cases where the output time of the islanding detection signal exceeded 0.1 seconds.

[0092] [Table 1]

[0093] From this table, it was confirmed that there were no cases in which the islanding detection signal was not judged as "possible" and no cases in which the "possible" judgment was output later than the islanding detection signal output time. It was also confirmed that in cases in which the islanding detection signal output time was particularly long, the "possible" judgment tended to be made earlier.

[0094] From the above, it was confirmed that there is no impact on the islanding detection time. In other words, it was confirmed that it is possible to improve the tolerance to frequency ramp fluctuations while maintaining the islanding detection function. [Explanation of symbols]

[0095] 1 Islanding operation detection function section 2. Islanding operation possibility determination function 3. AND Gate 4 Detection result hold function section 5. Judgment result hold function section

Claims

1. A method for improving the frequency ramp fluctuation tolerance of an islanding operation detection function in a distributed power source, characterized in that, in parallel with the islanding operation detection processing of the active islanding operation detection function, a judgment processing of an islanding operation possibility judgment function is performed, which uses the minimum value of the average frequency change rate during islanding operation, calculated from the detection threshold value of the active islanding operation detection function and the detection time upper limit value, as a judgment threshold, and judges the possibility of islanding operation based on whether the frequency change rate exceeds the judgment threshold on a cycle-by-cycle basis, and outputs a signal to parallel off the distributed power source when both the islanding operation detection result from the active islanding operation detection function and the judgment result of the possibility of islanding operation from the islanding operation possibility judgment function are output.

2. 2. A method for improving the frequency ramp fluctuation tolerance of an islanding operation detection function in a distributed power supply as claimed in claim 1, wherein the detection result from the active islanding operation detection function and the judgment result from the islanding operation possibility judgment function are each input to an AND gate, the detection signal from the islanding operation detection function is put into a block state by the AND gate, the block state is maintained unless the judgment result from the islanding operation possibility judgment function judges that there is a possibility of islanding operation, and when it judges that there is a possibility of islanding operation, the block state is released and a signal is output to parallel off the distributed power supply.

3. 3. A method for improving the frequency ramp fluctuation tolerance of an islanding operation detection function in a distributed power source as described in claim 1 or 2, characterized in that it is provided with a detection result hold function that temporarily holds a detection signal from the active islanding operation detection function, and a judgment result hold function that temporarily holds a judgment signal from the islanding operation possibility judgment function, and absorbs the timing difference between the signal transmission from the active islanding operation detection function and the islanding operation possibility judgment function.

4. 4. The method for improving frequency ramp fluctuation tolerance of an islanding detection function in a distributed power source according to claim 1, wherein, when the determination result of the islanding possibility determination function determines that there is a possibility of islanding operation, the cycle at which the rate of change of frequency exceeded the determination threshold is used as a reference cycle, and a difference between the rate of change of the frequency of three cycles including the reference cycle and the rate of change of frequency of the cycle immediately preceding the reference cycle is calculated after removing a frequency ramp fluctuation, and if this difference exceeds an error determination threshold, the difference is deemed to be different from a voltage phase step change, and it is determined that there is a possibility of islanding operation; and if the difference between the rate of change of the frequency of the fourth cycle and the rate of change of the frequency of the cycle immediately preceding the reference cycle becomes equal to or less than the error determination threshold, it is deemed to be a frequency step fluctuation, and the determination is returned to that there is no possibility of islanding operation, whereas if the difference exceeds the error determination threshold, the determination that there is a possibility of islanding operation is maintained.

5. an active islanding operation detection function unit of a frequency shift type islanding operation detection method and an islanding operation possibility determination function unit which determines the possibility of islanding operation based on whether the frequency change rate exceeds the determination threshold value in one cycle, using the minimum value of the average frequency change rate during islanding operation from the detection threshold value and the detection time upper limit value of the active islanding operation detection function unit as the determination threshold value; an AND gate that receives a signal from the active islanding operation detection function unit and a signal from the islanding possibility determination function unit, and outputs a signal to parallel off the distributed generation when both the islanding operation detection signal and the islanding possibility determination signal are input, The AND gate blocks the detection signal from the active islanding operation detection function unit, and in this state the frequency change rate is continuously monitored in cycle units. While the block is active, the active islanding operation detection function unit does not send out a detection signal, which does not result in the disconnection of the distributed power source. If the frequency change rate exceeds the judgment threshold, it is determined that there is a possibility of islanding operation, and the block state is released. If the active islanding operation detection function unit sends out a detection signal when the block state is released, it is ultimately determined that islanding operation has occurred, and the distributed power source is disconnected.

1. A device for improving the tolerance to frequency ramp fluctuations of an islanding detection function in a distributed power source.

6. 6. A device for improving the tolerance to frequency ramp fluctuations of the islanding operation detection function in a distributed power source as described in claim 5, characterized in that it is equipped with a detection result hold function unit that temporarily holds the detection result from the active islanding operation detection function unit, and a judgment result hold function unit that temporarily holds the judgment result from the islanding operation possibility judgment function unit, and absorbs the timing difference between the signal transmission from the active islanding operation detection function unit and the islanding operation possibility judgment function unit.

7. 7. The device for improving frequency ramp fluctuation tolerance of an islanding detection function in a distributed power source according to claim 5 or 6, wherein when the islanding possibility determination function unit determines that there is a possibility of islanding operation because the rate of change of frequency exceeds the determination threshold, it further sets the cycle at which the rate of change of frequency exceeded the determination threshold as a reference cycle, calculates a difference between the rate of change of the frequency in three cycles including the reference cycle and the rate of change of the frequency in the cycle immediately preceding the reference cycle after removing a frequency ramp fluctuation, and if this difference exceeds the erroneous determination threshold, it deems it to be different from a voltage phase step change and determines that there is a possibility of islanding; and if the difference between the rate of change of the frequency in the fourth cycle and the rate of change of the frequency in the cycle immediately preceding the reference cycle is equal to or less than the erroneous determination threshold, it deems it to be a frequency step fluctuation and returns to a determination that there is no possibility of islanding, but maintains the determination that there is a possibility of islanding if the erroneous determination threshold is exceeded.

Images