JP2962967B2 - Method of detecting islanding operation of interconnection inverter - Google Patents

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 of an inverter used in connection with a commercial power system.

[0002]

2. Description of the Related Art In recent years, attention has been paid to clean energy free from environmental pollution, particularly to solar power generation using solar cells, due to the increasing awareness of global environmental protection. In the case of photovoltaic power generation, the generated power fluctuates greatly according to the amount of solar radiation, so in order to ensure stable power supply and effective use of surplus generated power, a photovoltaic power generation system (such as a photovoltaic And an inverter)
Is preferably connected to a commercial power system. In other words, usually, power is supplied to the load by parallel operation of the inverter and the commercial power system, and part or all of the power required for the house is covered by the photovoltaic power and the power generated by the photovoltaic cells remains. , A so-called reverse power flow to the commercial power system is performed.

In this type of photovoltaic power generation system,
Monitor the output voltage and frequency, and when at least one of these values is out of the specified range, that is, when power supply abnormality occurs, immediately stop the operation of the inverter or operate the circuit breaker to Is provided.

At the time of a power failure in the commercial power system, the inverter may enter an independent operation state. At this time, if power is supplied solely by the photovoltaic power generation system, the power transmission line of the commercial power system may include a pole transformer or the like. Since the high voltage boosted by the above is applied, safety such as restoration work of a power failure is impaired. That is, in order to ensure the safety of the maintenance work of the commercial power system, it is necessary to stop the power supply immediately after the state of the islanding operation.

[0005] Here, if there is a difference between the output power of the inverter and the power consumption of the load equal to or more than a predetermined value, the potential of the distribution line greatly increases or decreases at the time of the isolated operation state. The power supply is stopped by the interconnection protection device.
However, in a load equilibrium state in which the output power of the inverter and the power consumption of the load are substantially equal, no power supply abnormality occurs even in the stand-alone operation state, so that the power supply stoppage by the interconnection protection device cannot be expected.

In view of the above, various islanding detection methods for stopping power supply regardless of whether the load is in a balanced state have been proposed. As the islanding detection method, there are a passive detection method and an active detection method.

[0007]

The conventional passive and active islanding detection method has a problem that the configuration of the control unit of the inverter is complicated.

The present invention has been made in view of the above problems, and has as its object to provide an active islanding detection method capable of realizing desired detection by adding a simple electric circuit.

[0009]

According to a first aspect of the present invention, there is provided a method for driving a voltage-controlled current-controlled inverter connected to a commercial power system, the method comprising the steps of: as, to intermittently connect the capacitor by conduction control of the switching element
The power factor of the load fluctuates more
The present invention detects the presence or absence of a change in the output frequency of the inverter.

According to a second aspect of the present invention, there is provided the method of
It detects the presence or absence of a change in the output frequency in synchronization with a change in the power factor .

According to a third aspect of the present invention, there is provided a method for phase shifting.
Circuit to periodically fluctuate the phase of the current command value signal.
Output power factor from the inverter
Then, the presence or absence of a change in the output frequency of the inverter due to the change in the output power factor is detected.

[0012]

When the capacitor is connected intermittently, the power factor of the load on the inverter and the commercial power system fluctuates. However, during parallel operation, reactive power is supplied from the commercial power system to compensate for the load fluctuation, and the inverter is not affected. .

When the power supply from the commercial power system is stopped, feedback control is performed in the inverter to adjust the output frequency so that the output power factor and the load power factor match. This shifts the frequency from the commercial frequency,
The feedback control positively causes the frequency abnormality.

In a voltage-source current control type inverter, when the phase of a current command value signal is periodically changed, the output power factor fluctuates, causing a difference in power factor between the output power factor and the load. Then, also in this case, when power supply by the commercial power system is stopped, feedback control for adjusting the output frequency is performed.

[0015]

FIG. 1 is a block diagram showing a state in which a variable load circuit 3 according to a first embodiment of the present invention is connected to an electric power system.
FIG. 2 is a block diagram showing a configuration of the inverter 20 of FIG.

In FIG. 1, a solar power generation system 1 includes:
For example, it is composed of a solar cell 10 having a maximum output voltage of about 200 V and an inverter 20 of a voltage source current control system, and is interconnected with a commercial power system (frequency: 50/60 Hz) 5 via a transformer or a switch (not shown). Have been. Distribution line 6
Is connected to a load Z such as various electric appliances.

Further, between the photovoltaic power generation system 1 and the commercial power system 5, a variable load circuit 3 for actively detecting the isolated operation of the inverter 20 is provided. The variable load circuit 3 includes a switching element 31 having a self-turn-off function, for example, a capacitor 32 having a capacity of about 1000 to 2000 μF, and an oscillation circuit 33. The oscillation circuit 33 generates a switching pulse signal Ssw having a cycle of about 0.5 to 2 seconds and
Apply to 1. The application of the switching pulse signal Ssw is asynchronous with the control of the inverter 20.

The switching element 31 is periodically turned on in response to the switching pulse signal Ssw, whereby the capacitor 32 is intermittently connected to the inverter 20 as its load.

As shown in FIG. 2, the inverter 20 includes a bridge-type inverter circuit 21 including a plurality of switching elements, a PWM control unit 22 for performing switching control of the inverter circuit 21, an inverted L-type output filter 23, and a serial inverter. It comprises a system protection device 24.

The PWM control unit 22 includes a differential amplifier 220,
Multiplier 221, operational amplifier (error amplifier) 222, P
A feedback control system including a WM pulse generation unit 223 and a band-pass filter 224 for extracting a reference frequency component of the output, and a plurality of sets of which the pulse width is adjusted so that the output state of the inverter 20 is appropriate. PW
The M pulse is output to the inverter circuit 21 as a conduction control signal for each switching element.

The PWM control unit 22 generates an input error signal Sa indicating a difference between the DC input voltage Vi that fluctuates according to the amount of solar radiation and the voltage command value Vref.
By multiplying a by a signal Sb corresponding to the fundamental frequency component of the output voltage (that is, the commercial AC voltage), a current command value signal Si indicating a control target value is generated. That is, the magnitude (amplitude) of the output current is set by the input error signal Sa, and the phase of the output current is set by the system voltage.
The voltage command value Vref is set to a value near the optimal operation voltage of the solar cell 10.

The current command value signal Si is corrected and appropriately amplified in accordance with the signal Sc indicating the phase of the actual output current, and thereafter the current command value signal Si Is compared with a triangular wave signal for modulation of about 20 kHz, and a predetermined PW
M pulses are generated.

The inverter 20 of such a control type is
The phase-matched voltage and current are output. That is, it has the ability to output only AC power with a power factor of one. Therefore, during the parallel operation, the commercial power system 5 supplies the reactive power required by the load Z (normally, the power factor is smaller than 1) and the capacitor 32 of the variable load circuit 3.

On the other hand, the interconnection protection device 24 comprises a circuit breaker (MCCB) 241 and an abnormality detection circuit 242. The abnormality detection circuit 242 monitors the voltage and the frequency on the distribution line 6, and when at least one of them deviates from the proper range of the operation regulation of the commercial power system 5, immediately puts the wiring breaker 241 into a cutoff state. I do. The appropriate voltage range is 101 ± 6 V when the reference voltage is 101 V, and the appropriate frequency range is ± 1% of the reference frequency (59.4 to 60.6 when the reference frequency is 60 Hz).
Hz).

Next, the operation of the variable load circuit 3 will be described. By providing the variable load circuit 3, the load state (power factor) periodically fluctuates. In the case of the parallel operation state, the commercial power system 5 has a reactive power supply capability and its power capacity is sufficiently large, so that the load fluctuation is absorbed by the commercial power system 5 and power supply abnormality occurs. Absent.
In this case, the frequency of the output of inverter 20 is defined by commercial power system 5 and is constant.

On the other hand, when the power transmission of the commercial power system 5 is stopped and the inverter 20 enters the isolated operation state, the load balance state in which the power consumption of the load Z and the output power of the inverter 20 substantially match. Even in a state where the voltage does not fluctuate, the influence of the load fluctuation appears on the distribution line 6.

That is, in the inverter 20, for example, the power factor cos θ of the load is 1 as expressed by the equation (1).
If not, output control is performed to adjust the output frequency f so that the power factor cos θ approaches the same value 1 as the power factor of the output power.

Cos θ = R / [R ² + (1 / 2πfC) ² ] ^1/2 (1) R: resistance value C: capacity Therefore, when the power factor cos θ of the load periodically fluctuates, The output frequency f also fluctuates, and the fluctuation is detected as a power supply abnormality by the interconnection protection device 24, and power supply is stopped. At this time, as a result, the interconnection protection device 24 has detected the occurrence of the islanding operation.

FIG. 3 is a block diagram showing the configuration of the protection device 4 according to the second embodiment of the present invention. In the figure, components having the same functions as those in FIG. 1 are denoted by the same reference numerals, and descriptions thereof will be omitted or simplified. The same applies to the following drawings.

The protection device 4 is, for example, 500 to 1000 μm.
A variable load circuit 3A having a capacitor 32A having a capacity of about F, a frequency fluctuation detection circuit 41, and a circuit breaker 42
It is composed of

The frequency fluctuation detection circuit 41 includes a current transformer 41
1, high-pass filter 412, low-pass filter 41
3, an OR circuit 414, an AND circuit 415, and a low-frequency resonance unit 416. When the output current of the inverter 20 fluctuates in synchronization with the switching pulse signal Ssw for controlling the connection of the capacitor 32A, disconnection for wiring is performed. The vessel 42 is turned off.

That is, the protection device 4 intentionally constantly varies the power factor of the load of the inverter 20, and performs the isolated operation depending on whether the output frequency of the inverter 20 varies or not. And the function of immediately canceling the connection between the inverter power system 1 and the commercial power system 5 when the islanding is detected.

In the protection device 4, the isolated operation is detected based on the presence or absence of the fluctuation of the output frequency in synchronization with the fluctuation of the power factor of the load, whereby the frequency fluctuation due to the disturbance in the parallel operation and the frequency in the single operation are detected. Since it is distinguished from fluctuation, no erroneous detection occurs. The capacitor 32
Even when the capacity of A is reduced to reduce the fluctuation width of the output frequency, the isolated operation can be reliably detected.

FIG. 4 is a block diagram showing a state in which a photovoltaic power generation system 1B according to a third embodiment of the present invention is connected to a commercial power system 5, and FIG. 5 is a block diagram showing a configuration of an inverter 20B in FIG. .

In FIG. 4, the solar power generation system 1B
Is a solar cell 10 and a voltage-source current control type inverter 2
0B, and is always connected to the commercial power system 5. The load Z is connected to the distribution line 6.

As shown in FIG. 5, the basic configuration of the inverter 20B is the same as that of the inverter 20 of FIG. However, in the inverter 20B, a phase shift circuit 225 is provided between the bandpass filter 224 and the multiplier 221 of the PWM control unit 22B.

The phase shift circuit 225 changes the phase of the signal Sb output from the bandpass filter 224 to, for example, 0
Periodically fluctuates in the range of ５5 °. As a result, the phase of the current command value signal Si fluctuates.
The output power of B is AC power in which the phase of the current is periodically shifted with respect to the voltage. That is, the output power factor is not always 1, but fluctuates periodically.

In the parallel operation state, power is supplied from the commercial power system 5 so as to compensate for the difference between the output power factor and the power factor of the load Z. Therefore, the frequency of the output voltage and output current of the inverter 20B is The reference frequency of the commercial power system 5 is used.

On the other hand, when the inverter power system 1B is in the isolated operation state, the inverter 20B performs output control for adjusting the frequency so that the power factor of the load Z approaches the output power factor. For this reason, the frequency of the output also fluctuates with the periodic fluctuation of the output power factor, and the frequency fluctuation is detected by the interconnection protection device 24 as a power supply abnormality, and the power supply is stopped. At this time, as a result, the interconnection protection device 24 has detected the occurrence of the islanding operation.

According to the embodiment shown in FIGS. 1 and 3, the variable load circuit 3 or the protection device 4 is provided as an external component of the inverter 20 and the active content of the inverter 20 is not changed at all. A simple islanding detection can be realized. In particular, according to the embodiment of FIG. 3, even when the fluctuation range of the output frequency is reduced, the isolated operation can be reliably detected, so that the capacity of the capacitor 32A connected as the load of the inverter 20 can be reduced. Can.
Further, according to the embodiment shown in FIG. 3, double monitoring is performed by the interconnection protection device 24 built in the inverter 20 and the external protection device 4, so that safety can be improved.

According to the embodiment of FIG. 5, the inverter 20B
By simply providing the phase shift circuit 225, the reactive power can be easily output, and a frequency abnormality can be caused in the isolated operation state. If this circuit is applied, for example, the phase to be shifted is controlled by a microprocessor unit or the like, and when the load is reduced and the voltage of the distribution line 6 approaches the upper limit value of the appropriate range, the circuit breaker of the interconnection protection device 24 As a method of suppressing the voltage rise so that the H.241 does not operate, a method of increasing the reactive power while maintaining the apparent power can be used instead of the conventional method of narrowing the active power. Does not need to be moved from the maximum power point to the low output side, so that the power generated by the solar cell 10 can be effectively used.

In the above-described embodiment, another power generation device such as a fuel cell may be used instead of or together with the solar cell PV. In the above embodiment, the capacity of the capacitors 32 and 32A is
May be appropriately selected according to the rated output capacity of the power supply. The generation and correction of the current command value signal Si may be performed by a microprocessor unit. In addition, the configurations of the PWM control units 22 and 22B, the interconnection protection device 24, the variable load circuits 3 and 3A, and the protection device 4 can be appropriately changed within the scope of the present invention.

[0043]

According to the present invention, islanding operation can be reliably detected without using a complicated control circuit.

According to the second aspect of the present invention, erroneous detection due to disturbance can be prevented, and the size and weight of an electric circuit for detection can be reduced. According to the invention of claim 3, when a solar cell is used as the DC power supply, it is possible to effectively use the generated power during the parallel operation.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a state where a variable load circuit according to a first embodiment of the present invention is connected to a power system.

FIG. 2 is a block diagram showing a configuration of the inverter of FIG.

FIG. 3 is a block diagram illustrating a configuration of a protection device according to a second embodiment of the present invention.

FIG. 4 is a block diagram showing a state where an inverter according to a third embodiment of the present invention is connected to a commercial power system.

FIG. 5 is a block diagram showing a configuration of the inverter of FIG.

[Explanation of symbols]

 5 Commercial power system 20, 20B Inverter 31 Switching element 32, 32A Capacitor Ssw Switching pulse signal (conduction control signal) Si current command value signal

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yasuhiro Makino 2-18-18 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-62-71428 (JP, A) 4-229024 (JP, A) Japanese Utility Model Showa 61-162259 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) H02J 3/00-5/00

Claims (3)

(57) [Claims] 1. A method for detecting an isolated operation of an inverter of a voltage type current control system linked to a commercial power system, comprising: intermittently connecting a capacitor as a load of the inverter by controlling conduction of a switching element.
Fluctuates the power factor of the load, and
Isolated operation detecting method of interconnection inverter, characterized by detecting the presence or absence of variation in the output frequency of Jill said inverter. 2. The islanding operation detection method according to claim 1, wherein the presence / absence of the fluctuation of the output frequency in synchronization with the fluctuation of the power factor of the load is detected. Detection method. 3. A inverter islanding detection method of the commercial power interconnection voltage type current control scheme to the system, the by varying the phase of the current command value signal periodically using a phase shift circuit Output power factor from inverter
Characterized by constantly changing the output power factor and detecting the presence or absence of a change in the output frequency of the inverter due to the change in the output power factor .