JPH07322506A - Detection operation of single operation - Google Patents

Abstract

PURPOSE:To obtain a detection method which ensures the reliability of a detection, which reduces the change with of an inverter output and which increases the utilization factor of a DC generation capacity. CONSTITUTION:The output of electric power of an inverter is changed periodically. Then, an active single-operation detection which detects a link-point state change, at a definite cycle, appearing in a single operation and a passive single-operation detection which detects a change in the specific state amount DELTAVhv of a link point such as a third harmonic component or the like appearing in a shift to the single operation are performed in parallel. At this time, when the specific state amount DELTAVhv exceeds a preset pseudo single-operation criterion value Vth2, the change width (w) of the output electric power is made large as compared with a case in which the specific state amount does not exceed the pseudo single-operation criterion value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting an isolated operation in a grid-connected system of distributed power sources.

[0002]

2. Description of the Related Art In recent years, several kW for solar cells, fuel cells, etc.
A system that interconnects (connects) a distributed power supply facility that includes a DC power supply of a certain degree and an inverter that converts the output of the power supply to an AC, and reverses the flow of power to the grid and supplies power to loads such as home appliances. The interconnection system has been put to practical use.

In the grid interconnection system, in order to ensure the safety of maintenance work of the commercial power system, no matter what state the commercial power system is subjected to unexpected power failure or work power failure, especially the DC power source and the system Even if the load on the power consumer side is balanced from the interconnection point and it is the most difficult to detect a power outage, immediately stop the operation of the inverter or immediately activate the switch to cancel the interconnection. Therefore, the function of disconnecting the inverter from the commercial power system, that is, the function of preventing independent operation of the inverter is essential.

Therefore, various methods classified into a passive method and an active method have been proposed as the isolated operation detecting method. However, in practice, detection by the passive method and detection by the active method are carried out in parallel in order to reliably detect the islanding operation, and when the islanding operation is detected by at least one of the methods, the inverter solution is immediately detected. The columns are done.

Generally, in a solar power generation system equipped with a current control type inverter, a passive detection method is as follows.
A third harmonic voltage distortion rapid increase detection method is used which detects a rapid increase in the third harmonic component due to the excitation characteristics of the pole transformer. Further, as an active detection method, the output power (active power) of the inverter is fluctuated in a cycle of a frequency of about several Hz and with an appropriate fluctuation width, and the interconnection point state (voltage, current, or The active power fluctuation method that detects the periodic fluctuation of frequency) is used.

By the way, in the active detection method, it is necessary to positively fluctuate the inverter output within a fluctuation width of a predetermined value or more depending on the detection sensitivity. For example, in a conventional photovoltaic power generation system, inverter control is performed so that the output active power fluctuates within a constant fluctuation range of about 10% of the power generated by the solar cell.

[0007]

Therefore, in the prior art,
The operating point of the solar cell periodically deviates relatively greatly from the optimum operating point due to the output fluctuation for detecting the islanding operation, which is disadvantageous in terms of effective utilization of the power generation capacity of the solar cell.

In the active detection, there is known a method for facilitating the detection by emphasizing the change in the interconnection point state, which is remarkable in the isolated operation, by the positive feedback amplification. However, even when that method is used, the fluctuation range of the inverter output cannot be extremely reduced in order to secure the detection reliability.

The present invention has been made in view of the above problems, and it is an object of the present invention to reduce the fluctuation range of the inverter output and increase the utilization rate of the DC power generation capacity while ensuring the reliability of detection. .

[0010]

In order to solve the above-mentioned problems, the method according to the invention of claim 1 is such that the output power of the inverter is periodically changed, and thereby the continuous cycle of a constant cycle that appears during islanding operation. With the islanding detection method that performs active islanding detection that detects system point fluctuations and passive islanding detection that detects changes in the specific state quantity of the interconnection point that appears when switching to islanding operation in parallel. Therefore, when the specific state quantity exceeds a preset pseudo isolated operation reference value, the fluctuation range of the output power is made larger than when the specific state quantity does not exceed the pseudo isolated operation reference value. is there.

According to the second aspect of the present invention, the active isolated operation detection for detecting a fixed period change in the connected point state occurring during the isolated operation and the third order of the connected point voltage appearing during the transition to the isolated operation. A method for detecting an islanding operation in parallel with a passive islanding detection for detecting a sudden increase in harmonic components, wherein the amount of change in the third harmonic component exceeds a preset pseudo islanding reference value. The fluctuation range of the output power is made larger than when the change amount does not exceed the pseudo isolated operation reference value.

[0012]

The fluctuation range of the output power of the inverter in the active islanding detection is determined by the specific state quantity of the interconnection point that is the monitoring target in the passive islanding detection (for example, the third harmonic component included in the interconnection point voltage. Change amount or absolute amount).

That is, as schematically shown in FIG. 6, when the specific state amount ΔVhv exceeds the pseudo isolated operation reference value Vth2 in an abnormal state, it can be reliably detected at the time of the isolated operation. A relatively large value is set as the fluctuation range w of the output power so that a noticeable fluctuation in the interconnection point state occurs. On the other hand, when the specific state amount ΔVhv does not exceed the pseudo independent operation reference value Vth2 in the normal state (when it is clearly not in the independent operation), a smaller value (including zero) is changed as compared with the abnormal state. It is set as the width w.

As a result, it is possible to minimize the reduction in the inverter output during the normal period, which occupies most of the operation period, and to increase the utilization rate of the DC power generation.

[0015]

1 is a block diagram showing a circuit configuration of a photovoltaic power generation system 1 to which the present invention is applied, and FIG. 2 is a PWM control unit 2
2 is a block diagram functionally showing the configuration of FIG. 2, and FIG. 3 is a block diagram showing the configuration of the third-order harmonic extraction unit 40.

In FIG. 1, the solar power generation system 1 is
It is composed of a solar cell 10 having a rated voltage of about 200 V and an inverter 20 of a voltage-type current control system, and is connected to a commercial power system 2 via a switchboard, a power meter, etc. not shown. A load Z such as various home appliances is connected to the distribution line 3. During parallel operation, the interconnection point state (for example, voltage amplitude) is defined by the commercial power system 2 and is substantially constant regardless of the output power fluctuation of the inverter 20.
However, in the isolated operation, the influence of the output power fluctuation of the inverter 20 remarkably appears in the interconnection point state.

The inverter 20 includes an inverter main circuit 21 composed of a plurality of switching elements, a PWM control section 22,
From the current transformer CT which detects the output current Io, the transformer PT which detects the output voltage (voltage at the interconnection point) Vo, the microcomputer 24, the third harmonic extraction unit 40, the circuit breaker CB, etc. It is configured. The PWM control unit 22 and the microcomputer 24 are supplied with the output voltage Vo stepped down by, for example, 1/30 by the transformer PT.

The PWM control unit 22 includes the inverter main circuit 2
1 for controlling the switching so that the output voltage (DC input voltage) Vi of the solar cell 10 matches the voltage command value Vref given by the microcomputer 24.
Adjust the output current Io.

That is, in the PWM control unit 22, as shown in FIG. 2, the comparison unit 221 controls the voltage Vi and the voltage command value V.
An input error signal Sa indicating the difference from ref is generated. At this time, for example, the isolation amplifier 220 is used to detect the voltage Vi. By the multiplication processing unit 223,
The input error signal Sa is multiplied by the fundamental frequency component Sb of the output voltage Vo extracted by the bandpass filter unit 222, and the current command value signal Si indicating the control target value is generated. The current command value signal Si is corrected and appropriately amplified by the feedback signal Sc of the output current Io in the error amplifier 224. Current command value signal S after correction
Based on i, the pulse generator 225 generates a PWM pulse Pwm that defines the conduction period of the switching element. The PWM pulse Pwm is the gate circuit 226.
To the driver circuit 227 via the driver circuit 2
A switching pulse Psw, which is a control signal for the inverter main circuit 21, is generated by 27.

The microcomputer 24 is responsible for overall control of the inverter 20, appropriately sets the above-mentioned voltage command value Vref, and supplies it to the PWM control section 22, and also interconnects with the output signal Shv of the third harmonic extraction section 40. The isolated operation state is detected by monitoring the output voltage Vo which is the point state quantity.
In addition, as the voltage command value Vref, the solar cell 10 is normally used.
A value in the vicinity of the optimum operating voltage is set, but this voltage command value Vref fluctuates in a cycle of a frequency of about 2 Hz which is sufficiently lower than the commercial frequency due to the modulation processing for active islanding detection. . Then, the fluctuation range is changed according to the interconnection point state by a passive detection process described later.

The third harmonic extraction unit 40, as shown in FIG.
An input level adjuster 41, a bandpass filter 42 for extracting a third harmonic component from the interconnection point voltage Vo whose level is adjusted appropriately, and an absolute value circuit for performing full-wave rectification of the extracted third harmonic component. 43 and an integrating circuit 44 that outputs the harmonic extraction signal Shv of a voltage level corresponding to the amplitude of the third harmonic component.

Next, the contents of the islanding operation detection process executed by the microcomputer 24 will be described. FIG. 4 is a flowchart of the islanding operation detection process, and FIG. 5 is a flowchart of the passive detection process.

The islanding detection process is a passive detection process (#
1) and active detection processing (# 2).
In the active detection process, a 2 Hz fluctuation (fluctuation) in the interconnection point voltage Vo due to the modulation of the voltage command value Vref is detected by frequency analysis or the like.

In the passive detection process of FIG. 5, the microcomputer 24 samples the harmonic extraction signal Shv input to the analog port as a specific state quantity at the interconnection point at a constant cycle (for example, 10 ms), and in parallel with it. Then, the average value of the sampling values for a plurality of times (for example, five times) is obtained, and the average value is used as the measured value V
Take in as hv. That is, 50 (= 10 × 5) ms
The third harmonic component is measured in the cycle of (# 11).

Every time the measured value Vhv is fetched, it is judged whether there is a sudden increase in the third harmonic component (difference judgment) (# 1).
2). The difference determination here is the change amount Δ per unit time.
As Vhv, the difference between the newly taken measured value Vhv and the difference reference value Vbase is obtained, and it is a process of checking whether or not the variation ΔVhv exceeds a predetermined first threshold value Vth1.
As the threshold value Vth1, for example, when the reference voltage of the commercial power system 2 is 101V (effective value), a value of about 0.7 to 1V is selected in conversion to the voltage level at the interconnection point.

When the amount of change ΔVhv exceeds the first threshold value Vth1, first, in order to make active islanding detection reliable and quick, first, in order to ensure active islanding, the interconnection point voltage V is set during true islanding.
A value that is sufficiently larger than that in normal times is set as the fluctuation range of the voltage command value Vref so that o significantly changes (#
13). For example, the output active power at that time is 20 to 30
Change within the range of about%.

Next, the time counter for distinguishing the momentary system power failure which does not need to be disconnected from the others is updated, and the fixed time T is counted (# 14). The fixed time T is, for example, 0.2 to 0.3 s, which is sufficiently longer than the measurement cycle of the third harmonic component of 50 ms, and is a passive power failure detection defined in the operation of the commercial power system 2. The reaction time is shorter than the reaction time (for example, 0.5 s).

After updating the time limit counter, if the fixed time T is being measured, the process returns to the main routine of the isolated operation detection process (# 15). As a result, the measurement of the third harmonic component and the difference determination are consequently repeatedly executed.
At that time, unless the change amount ΔVhv becomes equal to or less than a second threshold value Vth2 described later, the difference reference value Vbase at the time when the sudden increase of the third harmonic component is first detected is used as it is.

When the time counting by the time limit counter ends, the high level state of the third harmonic component after the rapid increase has continued for a certain period of time T or more. In this case, the microcomputer 24 determines that it is in the isolated operation state, and immediately executes the operation stop processing (# 15, 16).
In the operation stop processing, the control signals Sf1 and Sf2 for turning off the gate circuit 226 and the circuit breaker CB are output, the switching elements of the inverter main circuit 21 are gated blocked, and the inverter 20 is disconnected from the commercial power system 2. . This double safety measure prevents the inverter 20 from operating alone.

On the other hand, if the amount of change ΔVhv does not exceed the first threshold value Vth1 in step 12 above, then it is checked whether or not the amount of change ΔVhv exceeds the second threshold value Vth2 (# 17). The second threshold value Vth2 is smaller than the first threshold value Vth1 (Vth2 <Vth
1), it is set as a pseudo independent operation reference value for distinguishing a normal state in which parallel operation is apparently performed and a non-normal state in which there is a possibility of independent operation.

When the change amount ΔVhv exceeds the second threshold value Vth2, the voltage command value Vref is obtained as in step # 13.
A larger value is set as the fluctuation range of the value than in normal times, and the process returns (# 18). That is, also in this case, the output active power is largely changed to enhance the active islanding detection.

On the other hand, when the variation ΔVhv does not exceed the second threshold value Vth2 in a normal state, the active fluctuation of the output active power is suppressed and the utilization rate of the power generation capacity of the solar cell 10 is increased. As the fluctuation range of the voltage command value Vref, a value smaller than that in the non-normal state is set (# 19). Here, in principle, the fluctuation range of the voltage command value Vref may be zero. However, it is desirable to set the fluctuation range of the voltage command value Vref so that the minimum fluctuation of the interconnection point state that can be detected by the active detection process occurs for safety.

Thereafter, the difference reference value Vbase is updated in preparation for the next difference determination, and the time counter is initialized (# 20, 21). In the update process of the difference reference value Vbase, the measured value Vhv to be determined this time is not set as it is as the difference reference value Vbase, but
As the next difference reference value Vbase, the weighted average value of the measured value Vhv and the current difference reference value Vbase is set. At that time, the difference reference value Vbase is close to the value of the third harmonic component in an ideal state without noise by setting the weighting of the difference reference value Vbase at the present time to a value (for example, about several times) larger than the measured value Vhv. The value becomes the value, and an appropriate difference determination can be performed for the measured value Vhv obtained in the next detection.

As is clear from the above description, in the solar power generation system 1 of this embodiment, the fixed time T for determining whether or not the high level state of the third harmonic component is temporary is measured. Including the inside, the change amount ΔVhv is the second threshold value Vth.
The period exceeding 2 is regarded as the non-normal period, and the fluctuation of the output active power (that is, the fluctuation of the operating point of the solar cell 10) is suppressed during the other normal periods.

According to the above-described embodiment, when the high level state of the third harmonic component continues for a certain time T,
Since the operation stop control is performed, it is possible to avoid an unnecessary operation stop in response to a sudden increase in the third harmonic component, and it is possible to minimize the loss of power generated by the solar cell 10.

In the above-mentioned embodiment, a function such as a frequency analysis for detecting a change in the specific frequency of the interconnection point voltage Vo may be realized by using a digital signal processor (DSP). Further, various changes can be made to the circuit configuration for changing the output active power, the configuration of the inverter 20, and the content of the islanding operation detection process.

In the above-described embodiment, the judgment by comparing the absolute value of the third harmonic component with the reference value is performed instead of or in combination with the difference judgment, thereby distinguishing between normal and non-normal. Good.

In the above-mentioned embodiment, the example of detecting the islanding operation by the presence or absence of the rapid increase of the third harmonic component is shown. However, other passive islanding detection for detecting the phase or frequency of the interconnection point voltage is shown. The present invention can be applied even when the method is adopted. It can also be applied to islanding operation detection of distributed power equipment equipped with DC power supplies other than solar cells such as fuel cells.

[0039]

According to the present invention, it is possible to reduce the fluctuation range of the inverter output during normal times and increase the utilization rate of the DC power generation capacity while ensuring the reliability of active islanding detection.

[Brief description of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a photovoltaic power generation system to which the present invention has been applied.

FIG. 2 is a block diagram functionally showing a configuration of a PWM control unit.

FIG. 3 is a block diagram showing a configuration of a third harmonic extraction unit.

FIG. 4 is a flowchart of islanding operation detection processing.

FIG. 5 is a flowchart of a passive detection process.

FIG. 6 is a schematic diagram showing the content of islanding detection processing.

[Explanation of symbols]

 2 Commercial power system 20 Inverter ΔVhv Change amount of third harmonic component (specific state amount) Vth2 Second threshold value (pseudo-isolation operation reference value) w Output power fluctuation range

Claims (2)

[Claims] Claim: What is claimed is: 1. An output power of an inverter is fluctuated periodically, whereby active islanding detection is performed to detect a fixed cycle variation in the interconnection point that appears during islanding. A passive islanding detection for detecting a change in a specific state quantity of a system point, which is an islanding detection method performed in parallel, wherein when the specific state quantity exceeds a preset pseudo islanding reference value, A method for detecting an isolated operation, characterized in that the fluctuation range of the output power is made larger than when the state quantity does not exceed the pseudo independent operation reference value. 2. An active isolated operation detection for periodically changing the output power of the inverter, thereby detecting a change in the interconnection point state with a constant cycle that appears during isolated operation, and a connection that appears when shifting to the isolated operation. What is claimed is: 1. An isolated operation detection method for performing a passive isolated operation detection for detecting a sudden increase in the third harmonic component of a system point voltage in parallel, wherein the variation amount of the third harmonic component is preset A method of detecting an isolated operation, wherein when the value exceeds the isolated operation reference value, the fluctuation range of the output power is made larger than when the change amount does not exceed the pseudo isolated operation reference value.