Electric power system

An electric power system includes: an uninterruptible power supply including an AC switch provided between a commercial power supply and an output part, a secondary battery, and an inverter provided between the AC switch and secondary battery; an important load connected to the output part; a distributed power supply connected to the output part; total load power consumption detection section detecting the power consumption of all loads including the important load; charge/discharge power detection section detecting charge/discharge power of the secondary battery; output power detection section detecting the output power of the distributed power supply; important load power consumption detection section detecting the power consumption of the important load; and a controller that inputs thereto detection values from the total load power consumption detection section, charge/discharge detection section, output power detection section, and important load power consumption detection section and outputs a control command value for controlling the secondary battery.

TECHNICAL FIELD

The present invention relates to a microgrid-based electric power system including a small-sized distributed power supply.

BACKGROUND ART

In a conventional electric power system, energy supply has been conducted from a power plant such as an atomic power plant, a thermal power plant, a hydraulic power plant, etc, by means of a large-scale power grid. Meanwhile, in recent years, a concept called “microgrid” in which a small-sized distributed power supply (photovoltaic, wind turbine, biomass, etc.) is connected to constitute a power network for energy supply to a predetermined area has been proposed and is now becoming widespread. Such a microgrid-based energy supply system realized by the small-sized distributed power supply is required to perform connected operation that controls power generation based on system connection such that purchasing electric power from a commercial system becomes constant at normal operation time, while in the event of emergency such as power failure, to perform load following operation which is islanded operation that supplies high quality (fluctuation in voltage and frequency are small) power to a microgrid system.

In constructing the microgrid, how the amount of power supply, which varies momentarily, is balanced is the most important issue. As factors causing the power supply amount to vary, a variation in a load, a variation in the power generation of a small-sized distributed power supply such as wind turbine power generation or photovoltaic power generation (hereinafter, variations in the wind turbine power generation and photovoltaic power generation are collectively referred to as “electric power variation”), and the like can be exemplified.

The electric power variation has various frequency components from quite abrupt variation to comparatively gentle variation depending on the load or power generation state of the small-sized distributed power supply. By combining distributed power supplies having various load following characteristics, it is possible to suppress the variation of all the frequency components of the electric power variation. Specifically, storage facility such as a secondary battery or an electric storage facility is used to cope with a variation (quite abrupt variation) in high-frequency components and a power generation facility such as a gas engine is used to cope with a variation (comparatively gentle variation) in low-frequency components, whereby the electric power variation can be suppressed.

Further, there is known a system that realizes the load following operation by managing power demand of a building by linkage of power receiving/transforming facility of a commercial system and a distributed power supply (refer to, e.g., Patent Document 1) and a method that utilizes a storage facility when an original operation plan has been significantly modified, to realize stable system operation (refer to, e.g., Patent Document 2).Patent Document 1: JP-A-2005-160286Patent Document 2: JP-A-2007-215290

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

With reference toFIG. 7, a configuration example of a conventional microgrid will be described.FIG. 7illustrates an example of a system configuration in which, at power failure time, power supply is performed by combining an emergency power generator190and photovoltaic power generation using a photovoltaic power supply140. A part surrounded by a chain line inFIG. 7corresponds to a configuration of an uninterruptible power supply100.

In an electric power system as illustrated inFIG. 7, during connected operation with a commercial system, a first circuit breaker191and an ACSW (AC semiconductor switch) are in a closed state, while a second circuit breaker192is in an open state and, in this state, output of a secondary battery130is controlled in accordance with a variation in photovoltaic power generation or electric power load to thereby perform peak-cut operation.

Meanwhile, at power failure time, the first circuit breaker191is opened, and the ACSW is opened in accordance with a state signal of the first circuit breaker191. At the same time, the emergency power generator190is started up. After start-up of the emergency power generator190, the second circuit breaker192is closed to perform islanded operation while utilizing photovoltaic power generation output.

Assuming that in the electric power system illustrated inFIG. 7, the maximum charge/discharge power of the secondary battery130is −90 kW to 90 kW, the maximum generating power of the photovoltaic power supply140is 90 kW, and the maximum power consumption of an important load150is 50 kW, the forward maximum power flow and backward maximum power flow in the ACSW120ofFIG. 7are as follows. The definitions of the forward direction (+) and backward direction (−) are as indicated inFIG. 7.

The backward maximum power flow (at fine weather) is calculated as “maximum discharge power (−90 kW) of secondary battery130+maximum output (−90 kW) of photovoltaic power supply140−minimum value (0 kW) of important load150=−180 (kW)”.

The forward maximum power flow (at cloudy/rainy weather) is calculated as “maximum discharge power (90 kW) of secondary battery130+maximum output (0 kW) of photovoltaic power supply140−minimum value (50 kW) of important load150=140 (kW)”.

For example, the power flow in the ACSW120can be in a range of −180 kW to 140 kW depending on the output of the secondary battery130or photovoltaic power supply140or magnitude of the important load150, so that assuming that the withstand capacity of the ACSW120is in a range of −90 kW to 90 kW, the power flow in the ACSW120become excessive, causing the ACSW120to be damaged.

In order to cope with this problem, it can be considered that the ACSW120in the uninterruptible power supply100is replaced with one with higher withstand capacity. However, the ACSW120with high withstand capacity is expensive and replacement of the ACSW120with one with higher withstand capacity increases cost for constructing the electric power system.

Means for Solving the Problems

The present invention has been made to solve the above problem, and the invention according to claim1is an electric power system including: an uninterruptible power supply including an AC switch provided between a commercial power supply and an output part, a secondary battery, and an inverter provided between the AC switch and secondary battery; an important load connected to the output part; a distributed power supply connected to the output part; a total load power consumption detection means for detecting the power consumption of all loads including the important load; a charge/discharge power detection means for detecting charge/discharge power of the secondary battery; an output power detection means for detecting the output power of the distributed power supply; an important load power consumption detection means for detecting the power consumption of the important load; and a controller that inputs thereto detection values from the total load power consumption detection means, charge/discharge power detection means, output power detection means, and important load power consumption detection means and outputs a control command value for controlling the secondary battery. The controller determines the control command value for operating the secondary battery based on the detection values from the total load power consumption detection means, charge/discharge power detection means, output power detection means, and important load power consumption detection means.

Advantages of the Invention

According to the electric power system of the present invention, performing output control of the secondary battery allows the power flow in the AC switch (ACSW) constituting the uninterruptible power supply to be controlled adequately, preventing the AC switch from being damaged. This eliminates the need to increase the withstand capacity of the AC switch to be used to thereby suppress cost increase. Further, in the case where the output of the distributed power supply such as the photovoltaic power supply is large while the power consumption of the important load is small, the secondary battery is charged. This allows a reduction in the installed capacity of the secondary battery.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described below with reference to the accompanying drawings.FIG. 1is a view illustrating the outline of an electric power system according to the embodiment of the present invention. InFIG. 1, reference numeral100denotes an uninterruptible power supply,101denotes an input section,111denotes an electric power detection section,102denotes an output part,110denotes a controller,120denotes an AC switch (ACSW),130denotes a secondary battery,131denotes an electric power detection section,135denotes an inverter,140denotes a photovoltaic power supply,145denotes a power conditioner,150denotes an important load,160denotes a security load,170denotes a disaster preventive load,180denotes a general load,181denotes an electric power detection section,190denotes an emergency power generator,191denotes a first circuit breaker,192denotes a second circuit breaker, and200denotes a commercial power supply. Note thatFIG. 1illustrates only a wire connection and a control system for a single phase, i.e., one phase. Further, in the electric power system according to the present embodiment, a range (A) including the most important load (150) and being made islanded even immediately after power failure and a range (B) being made islanded during operation of the emergency power generator190at power failure time are defined.

For example, it is assumed that in the present embodiment, the maximum charge/discharge power of the secondary battery130is −90 kW to 90 kW, the maximum generating power of the photovoltaic power supply140is 90 kW, the maximum power consumption of an important load150is 50 kW, and the withstand capacity of the ACSW120is in a range of −90 kW to 90 kW. The definitions of the forward direction (+) and backward direction (−) in the ACSW120are as indicated inFIG. 1.

InFIG. 1, the general load180is connected to a feed line extending from the commercial power supply200of a commercial system, and when the feed line falls into an abnormal state such as power failure or the like, power supply thereto is cut off. The security load160serving as a load for security and disaster preventive load170serving as a load for disaster prevention are loads of high importance and are connected to the feed line via the first circuit breaker191. To the connection line of the security load160and disaster preventive load170, the emergency power generator190and important load150are connected via the second circuit breaker192and AC switch (ACSW)120, respectively. When the feed line loses power, power is supplied from the emergency power generator190to the respective loads. The important load150, which is, e.g., a server, is a load having higher importance than the security load160and disaster preventive load170. To the connection line of the important load150, the secondary battery130and photovoltaic power supply140are connected via the inverter (INV)135and power conditioner (PCS)145, respectively so as to enable islanded operation even if the feed line loses power and emergency power generator190is stopped.

The emergency power generator190is activated using heavy oil or other fuel as a power source when the feed line of the commercial power supply200falls into an abnormal state (power failure). During the abnormal state, the emergency power generator190continues operating so as to supply power to the security load160, disaster preventive load170, and important load150in place of the commercial power supply200. However, the abnormal state of the feed line of the commercial power supply200continues for a long time and accordingly the emergency power generator190continues operating for a long time, fuel is exhausted (fuel shortage) with the result that the operation of the emergency power generator190is stopped. Even after the exhaustion of the fuel of the emergency power generator190, power supply to the important load150continues as long as power generation using the photovoltaic power supply140is maintained and secondary battery130is charged.

The secondary battery130is a repeatedly chargeable/dischargeable capacitor or secondary battery. The secondary battery130is connected to the connection line of the important load150via the inverter135so as to be changed by the commercial power supply200, photovoltaic power supply140, and emergency power generator190according to need and to discharge power to the general load180, security load160, disaster preventive load170, and important load150. The secondary battery130incorporates a control circuit (charge/discharge control circuit) for controlling charge/discharge of the secondary battery. To the control circuit, a control command value from the controller to be described later is input, and charge/discharge of the secondary battery constituting the secondary battery130is controlled in accordance with the input control command value.

The inverter135is an electric power converter that can convert bidirectionally between AC and DC power. The inverter135converts AC power into DC power in an operation mode in which the secondary battery130is charged from the commercial power supply200, photovoltaic power supply140, or emergency power generator190and converts DC power into AC power in an operation mode in which power is discharged from the secondary battery130to the important load150.

The photovoltaic power supply140is connected to the connection line of the important load150via the power conditioner145so as to supply power output to the general load180, security load160, disaster preventive load170, and important load150in an independent manner. The power conditioner145converts the DC output of the photovoltaic power supply140that is not adapted to a predetermined frequency or voltage of the connection line of the important load150into predetermined AC power for adaptation of the frequency or voltage to that of the electric power supplied along the feed line. The power conditioner145includes, at its output part, a current control type inverter so as to supply the maximum possible power.

The first circuit breaker191is closed at general connected operation time where the feed line of the commercial power supply200to which the general load180is connected is in a feeding state and is opened (turned off) when the feed line of the commercial power supply200loses power. The second circuit breaker192is opened when the first circuit breaker191is closed to put the feed line of the commercial power supply200into a feeding state and is closed when the feed line of the commercial power supply200loses power. When this second circuit breaker192is closed, the emergency power generator190is activated, and the generated output of the emergency power generator190is fed to the security load160, disaster preventive load170and important load150. When the emergency power generator190is stopped, the second circuit breaker192is opened.

The electric power detection section181detects power failure occurring in the feed line of the commercial power supply200and controls close/open of the first and second circuit breakers191and192and activation/stop of the emergency power generator190. When the feed line of the commercial power supply200loses power, a power failure detection controller11opens the first circuit breaker191, closes the second circuit breaker192, and activates the emergency power generator190. When the power failure of the feed line of the commercial power supply200is recovered, the power failure detection controller11closes the first circuit breaker191, opens the second circuit breaker192, and stops the emergency power generator190.

An electric power system in the islanded range immediately after power failure includes the AC switch (ACSW)120, inverter135, secondary battery130, and important load150. The AC switch (ACSW)120provided between the commercial power supply AC6(200?) and inverter135corresponds to a signal phase and includes two thyristors Thl and Th2(not illustrated) connected in inverse parallel.

When the commercial power supply200is in a normal condition (including recovery state) and power needs to be supplied to the range (A), power flows in the forward direction in the AC switch (ACSW)120, and AC power is supplied from the commercial power supply200to the secondary battery130or important load150via the AC switch (ACSW)120.

Meanwhile, when the commercial power supply200falls into a power failure state, the AC switch (ACSW)120is put into a cut-off state to stop supply of AC power from the commercial power supply200. When an abnormality occurs in the commercial power supply200, the AC switch (ACSW)120is put into a cut-off state as in the case where it falls into a power failure state to stop supply of AC power from the commercial power supply200.

Further, in the case where surplus power is generated in the range (A) by the discharge of the secondary battery130, output of the photovoltaic power supply140, and the like, power flows in the backward direction in the AC switch (ACSW)120, whereby power can be supplied from the secondary battery130or photovoltaic power supply140to the security load160, disaster preventive load170, or general load180.

The uninterruptible power supply100includes at least the AC switch (ACSW)120provided between the commercial power supply200and an output part102, secondary battery130, and inverter135provided between the AC switch (ACSW)120and secondary battery130. Further, the uninterruptible power supply100includes the controller110, which may be subjected to control by a higher-level controller.

The controller110is a main controller for performing various control tasks for the electric power system according to the present invention. The control tasks of the controller110may be realized by using a general-purpose information processor provided with a CPU, a RAM, a ROM, and the like and by previously storing in the ROM a program allowing the CPU to execute operation of outputting a command to a predetermined block based on an input predetermined information.

The electric power detection section131is provided between the inverter (INV)135and secondary battery130, and charge/discharge of the secondary battery130can be detected by the electric power detection section131. A value detected by the electric power detection section131is transmitted to the controller110.

The electric power detection section111is provided between the power conditioner (PCS)145and AC switch (ACSW)120, and the sum of the electric energy of the photovoltaic power supply140and important load150can be detected by the electric power detection section111. A value detected by the electric power detection section111is transmitted to the controller110.

The electric power detection section181is provided in the middle of the feed line extending from the commercial power supply200and functions as a total load power consumption detection means for detecting the power consumption of all the loads including the important load150(however, power supplies from the secondary battery130, photovoltaic power supply140, and emergency power generator190are excluded). A value detected by the electric power detection section181is transmitted to the controller110.

In the electric power system according to the present invention, the controller110outputs a control signal at least to the AC switch (ACSW)120, inverter135in the secondary battery130, power conditioner145, secondary battery130(charge/discharge circuit) so as to control the abovementioned components.

Next, the control tasks performed in the electric power system according to the present invention configured as described above will be described.FIG. 2is a control block diagram of the electric power system according to the embodiment of the present invention. The processing based on the control block diagram is executed in the controller110.

In the control performed in the controller110, a detection value WLoadacquired from the detection section181and a detection value WBATacquired from the detection section131are input to the controller110and added to each other. Then, the resultant value is made to pass through a band-pass filter. The band-pass filter is a filter for removing a negligible variation in the electric power within a predetermined time period.

The signal passed through the band-pass filter is then input to a limiter. In this limiter, the input value is limited by the upper limit value (WACSWmax) and lower limit value (WACSWmin) of the power flow in the AC switch (ACSW)120. A signal WACSWrefpassed through the limiter is used as a command value for the AC switch (ACSW)120. Further, an important load power consumption WImpotant Loadwhich is a detection value of the electric power detection section151and a photovoltaic power supply output voltage WPVwhich is a detection value of the electric power detection section141are added to and subtracted from the WACSWref, respectively, and the resultant value is output to the secondary battery130(charge/discharge circuit) as a control signal WBATreffor the secondary battery130.

That is, an output command value for the secondary battery130can be calculated using the following equation:
(WBATref)=(command valueWACSWref)−(output of photovoltaic power supply 140)+(power consumption of important load 150).

The (output of photovoltaic power supply140) is a value acquired as the detection value from the electric power detection section141, and (power consumption of important load150) is a value acquired as the detection value from the electric power detection section151.

According to the electric power system of the present invention, performing output control of the secondary battery130allows the power flow in the AC switch (ACSW)120constituting the uninterruptible power supply100to be controlled adequately, preventing the AC switch (ACSW)120from being damaged. This eliminates the need to increase the withstand capacity of the AC switch to be used to thereby suppress the cost. Further, in the case where the output of the distributed power supply such as the photovoltaic power supply is large while the power consumption of the important load is small, the secondary battery is charged. This allows a reduction in the installed capacity of the secondary battery.

A control example based on the electric power system according to the present invention will be described.FIG. 3is a view illustrating an example of the power flow at fine weather based on the control according to a conventional approach,FIG. 4is a view illustrating an example of the power flow at cloudy/rainy weather based on the control according to a conventional technique,FIG. 5is a view illustrating an example of the power flow at fine weather based on the control according to the electric power system of the present invention, andFIG. 6is a view illustrating an example of the power flow at cloudy/rainy weather based on the control according to the electric power system of the present invention.

Throughout the drawings, a thin chain line represents the power consumption of all the loads, a thin solid line represents the power flow in the AC switch (ACSW)120, a thin dotted line represents electric power charged/discharged by the secondary battery130, a thin two-dot chain line represents the output of the photovoltaic power supply140, and a thick chain line represents the power consumption of the important load150.

It can be seen from a comparison betweenFIGS. 3 and 5and a comparison betweenFIGS. 4 and 6that the secondary battery130is adequately controlled by the output of the photovoltaic power supply140and load electric power of the important load150in the electric power system according to the present invention to make the power flow in the AC switch (ACSW)120always fall within a range of −90 kW to 90 kW corresponding to its withstand capacity.

Next, another embodiment of the present invention will be described. While the photovoltaic power supply140is used as the distributed power supply for constituting the microgrid in the above embodiment, a wind turbine power generation, a secondary battery, a rotary electric power generator, a fuel battery, a waste power generation, cogeneration, and the like are used as the distributed power supply in this another embodiment. The abovementioned distributed power supplies may be used independently or in combination.

As in the case of the abovementioned embodiment, in the present embodiment using the distributed power supply other than the photovoltaic power supply140, the control signal WBATreffor the secondary battery130(charge/discharge circuit) is calculated based on the upper limit value and lower limit value in the limiter ofFIG. 2calculated using the following equations:
Upper limit value;WBATmax=(upper limit value of AC switch 120)−(output of distributed power supply)+(power consumption of important load 150)
Lower limit value;WBATmin=(lower limit value of AC switch 120)−(output of distributed power supply)+(power consumption of important load 150).

Also according to the electric power system of this another embodiment, performing output control of the secondary battery130allows the power flow in the AC switch (ACSW)120constituting the uninterruptible power supply100to be controlled adequately, preventing the AC switch (ACSW)120from being damaged. This eliminates the need to increase the withstand capacity of the AC switch to be used to thereby suppress cost increase. Further, in the case where the output of the distributed power supply such as the photovoltaic power supply is large while the power consumption of the important load is small, the secondary battery is charged. This allows a reduction in the installed capacity of the secondary battery.

Industrial Applicability

According to the electric power system of the present invention, it is possible to adequately control the power flow in the AC switch (120) constituting the uninterruptible power supply, making it possible to construct an electric power system at low cost without using an expensive AC switch (120) having a high withstand capacity, which provides a better industrial applicability.