1. Technical Field
The present invention relates to a dispersed power source device with isolated operation detection function used between a power generating device used in a photovoltaic power generation system, fuel cell power generation system, wind power generation system, or the like, and a utility grid.
2. Related Art
FIG. 5 shows a grid interactive inverter system configuration and a heretofore known example of an isolated operation detection device. As shown in FIG. 5, a very small fluctuation is cyclically applied to the setting value of a reactive power setter included in a grid interactive inverter during grid interaction, the reactive power of a grid interaction point is constantly cyclically changed, and the frequency of the grid interaction point or grid interactive inverter is monitored.
When a very small signal of a phase advanced as seen from the grid interactive inverter is generated when a very small rise in this frequency is detected, the frequency rises further in the event of an isolated operation (also called islanding) condition. Also, when a very small signal of a phase delayed as seen from the grid interactive inverter is generated when a very small fall in the monitored frequency is detected, the frequency falls further in the event of an isolated operation condition. Focusing on this phenomenon, an isolated operation phenomenon is reliably and swiftly detected.
In FIG. 5, a fuel cell power generation system 1 is formed by or configured to include a fuel cell main body 2, a grid interactive inverter 3, and a distribution transformer 4. Reference numeral 5 refers to a load, and 6 refers to a utility grid. Also, reference numeral 7 refers to a load distribution circuit breaker, 8 refers to a dispersed power source side circuit breaker of the fuel cell power generation system 1 that acts as a dispersed power source, and 9 refers to a grid connection point circuit breaker for receiving power from the utility grid 6.
Furthermore, an isolated operation detection device 10 is formed by or configured to include a frequency detection circuit 11 formed from a frequency to voltage converter or the like, and an isolated operation detection section 20. The isolated operation detection section 20 is formed by or configured to include a fluctuation signal generator circuit 21 that generates a very small, cyclical, sinusoidal signal, a frequency rise monitor circuit 22 and frequency fall monitor circuit 23 formed from a comparator and setter that monitor a very small fluctuation in the output value of the frequency detection circuit 11, an OR circuit 24 that detects that either the frequency rise monitor circuit 22 or frequency fall monitor circuit 23 has acted, a Schmidt circuit 25 that converts the output of the fluctuation signal generator circuit 21 into a pulse waveform, a counter circuit 26 that measures the pulse waveform, a negative polarity peak hold circuit 27 as an advanced phase signal generator circuit that holds the negative polarity peak value of the output of the fluctuation signal generator circuit 21, a positive polarity peak hold circuit 28 as a delayed phase signal generator circuit that holds the positive polarity peak value of the output of the fluctuation signal generator circuit 21, a switching circuit 29 formed from an AND-OR gate circuit, three-state buffer, or the like, that applies a very small fluctuation to the setting value of reactive power setter (not shown) of the grid interactive inverter 3 by switching the signals of each of the fluctuation signal generator circuit 21, negative polarity peak hold circuit 27, and positive polarity peak hold circuit 28, and computes the signal to be selected by the output of each of the frequency rise monitor circuit 22, frequency fall monitor circuit 23, and counter circuit 26, a frequency upper limit monitor circuit 30 and frequency lower limit monitor circuit 31 formed from a comparator and setter that constantly slightly vary the reactive power of a grid connection point A, and monitor the deviation between the output of the frequency detection circuit 11 and a predetermined reference frequency, and an isolated operation signal generator circuit 32 formed from an OR gate that generates an isolated operation signal when either the frequency upper limit monitor circuit 30 or frequency lower limit monitor circuit 31 acts.
In FIGS. 6A to 6C, when the utility grid 6 is shut off by the grid connection point circuit breaker 9 due to an accident or the like on the utility grid 6 side at a time t0 shown in FIG. 6B, in a condition in which the amount of power supplied from the grid interactive inverter 3 of the fuel cell power generation system 1 and the amount of power consumed by the load 5 are balanced, an isolated operation phenomenon occurs between the fuel cell power generation system 1 and load 5.
When this kind of condition occurs, a reactive power fluctuation is applied by the fluctuation signal generator circuit 21 and switching circuit 29, and when setting in such a way that the fuel cell power generation system 1 applies a reactive power fluctuation with a phase more delayed than the utility grid 6 when the fluctuation has positive polarity, and the fuel cell power generation system 1 applies a reactive power fluctuation with a phase more advanced than the utility grid 6 when the fluctuation has negative polarity, as shown in FIG. 6A, the kind of frequency fluctuation between the times t0 and t1 shown in FIG. 6B occurs, the frequency rise monitor circuit 22 acts at the time t1, and the waveform of the reactive power fluctuation of the grid connection point A is made the output of the negative polarity peak hold circuit 27 by the switching circuit 29 (refer to FIG. 6A).
As the frequency fluctuation at this time increases as between the times t1 and t2 shown in FIG. 6B, the frequency upper limit monitor circuit 30 acts at the time t2 shown in FIG. 6B, and an isolated operation phenomenon is detected by the isolated operation signal generator circuit 32 (refer to FIG. 6C).
As described in Japanese Patent No. 3,353,549, reactive power is changed from a fluctuating output to a constant output by a frequency rise or fall monitor circuit stage level being exceeded at a time t1. It is necessary to set this stage level at a level such that is not exceeded other than at a time of an isolated operation, even when a frequency fluctuation occurs due to a grid disturbance, but as the frequency is liable to fluctuate due to a load fluctuation with a small scale grid formed from a diesel engine utilized on a remote island, a problem occurs in that there is liable to be a false detection of an isolated operation.