METHOD FOR CONTROLLING DISTRJBUTED POWER SOURCES
BACKGROUND OF THE INVENTION Field of the Invention
The present invention relates to a control method for compensating for load disturbances by systematically controlling a plurality of types of distributed power sources having different load-following capabilities for the load disturbances.
Priority is claimed on Japanese Patent Application No. 2009-204071, filed September 3,2009, the content of which is incorporated herein by reference.
Description of Related Art
Recently, power sources using solar photovoltaic or wind-power generation are being introduced and promoted as a national strategy. Since power sources using natural energy such as solar photovoltaic or wind-power generation depend significantly on weather, there is concern that supply reliability may not be guaranteed, and supply-and-demand balance may be diificult to obtain in a commercial electricity system.
To address this problem, a "microgrid" has been introduced to alleviate the load on a commercial electricity system and provide an interworking relationship by generating electricity to respond to demand within a certain area using distributed power sources capable of adjusting outputs (this is called a "load-following operation"). The energy supply system (hereinafter, referred to as a "microgrid") using distributed power sources constructed by employing the microgrid concept to perform a load-following operation is advantageous in that: (1) it is possible to prevent the natural energy power sources having an unstable output from harming the commercial electricity system; and (2) in the event of abnormality in the commercial electricity system, such as interruption
of an electricity service, it is possible to keep an autonomous operation with respect to
the load within a corresponding area with a stable electricity service quality (in terms of
frequencies or voltages) by disconnecting the microgrid from the commercial electricity
system.
For example, J^anese Unexamined Patent Application, First Publication No. 2006-246584 (hereinafter refened to as Patent Document 1) discloses a method for performmg the load-following operation of the distributed power source in the microgrid. hi this control method, using a plurality of types of distributed power sources having different load-following capabilities, the load disturbance is compensated for by setting, in advance, a frequency bandwidth of the load disturbance to be compensated for depending on the ioad-following capability of the distributed power source and controlling the output of the distributed power source to follow the load disturbance preferentially from the power source having a lower load-following capability.
The load disturbance compensation operation described in Patent Document 1 will be described with reference to FIG 5. FIG. 5 is a control block diagram when outputs of three power sources are controlled. In this control method, the disturbance component of measured load electric power PLOAD to be compensated for in a gas engme generator 1, amplitude of which is limited by an amplitude limiter 41, and further a predetermined frequency component of which is extracted by a low pass filter (LPF) 42, is set as an output specification value PSG to the gas engine generator 1. The gas engine generator 1 outputs electric power Pi based on this output specification value PSQ. A load disturbance that could not be followed using the gas engine generator 1, is obtained by subtracting the output electric power Pi of the gas engine generator I from the load electric power PUJAD- This load disturbance, frequency of which is limited by an amplitude limiter 43, and further a predetermined frequency component of which is
extracted by a LPF 44, is set as an output specification value PSBES of a nickel metal
hydride battery 2. The nickel metal hydride battery 2 outputs an output electric power
P2 based on this output specification value PSBES- A load disturbance that could not be
followed using the gas engine generator 1 and the nickel metal hydride battery 2, is
obtained by subtracting the output powers Pi and P2 of the gas engine generator I and the
nickel metal hydride battery 2 from the load electric power PLOAD- This load
disturbance, frequency of which is limited by an amplitude limiter 45, and further a
predetermined frequency component is extracted by a LPF 46, is set as an output
specification value PSEDLC of an electric double-layer capacitor 3. The electric
double-layer capacitor 3 outputs an electric power P3 based on this output specification
value PSEDLC. Through these control processes, the load disturbance can be
compensated for.
According to this control method disclosed in Patent Document 1, considering
the load-following capabilities of the distributed power sources, an electricity storage
device such as the secondary battery or the electric double-layer capacitor is usually used
as a power source which compensates for the disturbance in the fastest frequency
bandwidth. The chargeable/dischargeable capacity of the electricity storage device
depends on the remaining capacity in the capacitor body (hereinafter, refened to as a
state of charge (SOC)). Therefore, if SOC management is not performed, it may be
impossible to obtain a satisfactory output for the demanded output specification as the
SOC fluctuates. In this regard, the SOC management control is necessary to effectively
implement the load-following operation in the electricity storage device having a storage
capacity as small as possible. As a method for implementing the SOC management
control, for example, using the electric double-layer capacitor, Japanese Unexamined
Patent Application, First Publication No. 2007-020361(hereinaiter referred to as Patent
Document 2) discloses a method for subtracting the difference between a voltage setup
value and a detection value of a direct-current (DC) from an output specification using a
voltage controller configured by such as a PI controller or a PID controller.
However, even in the method disclosed in Patent Document 2, since the
electricity storage device autonomously adjusts the output thereof to perform the SOC
management, the precision of the load-following operation may be degraded if means for
compensating for the output fluctuation associated with the SOC management is not
provided.
SUMMARY OF THE INVENTION
The present invention has been conceived in view of such circumstances. An object of the present invention is to provide a method for controlling the distributed power sources capable of addressing the aforementioned problems in that it may be impossible to obtain an output corresponding to the demanded output specification as the SOC of the electricity storage device fluctuates, and the precision of the load-following operation may be degraded if the SOC management is performed by the electricity storage device itself.
According to a present invention, there is provided a method for systematically controlling a plurality of distributed power sources having different responsive capabilities for a load disturbance in which the distributed power sources include an electricity storage device. The method includes: obtaining a component to be compensated for using a power source having a responsive capability equal to or lower than that of the electricity storage device based on a difference value between a remaining capacity of the electricity storage device and a target remaining capacity; and compensating for the component to be compensated for using the power source having a
responsive capability equal to or lower than that of the electricity storage device.
It is preferable that the electricity storage device include an electric double-layer capacitor or a secondary battery, and the power source having a responsive capability equal to or lower than that of the electricity storage device includes a secondary battery or a synchronous generator.
According to the present invention, when the load-following operation is performed using a plurality of types of distributed power sources, the SOC management of the electricity storage device is performed using a power source having a responsive capability lower than that of the electricity storage device. As a result, it is possible to manage the SOC of the electricity storage device without degrading the control precision of the load-following operation. It is also possible to address problems in that the output power corresponding to the output specification may not be obtained as the SOC of the electricity storage device fluctuates, and the precision of the load-following operation may be degraded,
BRIEF DESCRIPTION OF THE DRAWINGS
FIG 1 is a block diagram illustrating a configuration according to an embodiment of the invention.
FIG 2 is a control block diagram illustratmg a control system shown in FIG 1.
FIG 3 illustrates output behaviors of three power sources shown m FIG 1.
FIG 4A illustrates a result of an operation when the SOC management control is not performed.
FIG 4B illustrates a result of an operation when the SOC management control is performed.
FIG 5 is a control block diagram illustrating a control operation of distributed
power sources of the related art
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a method for controlling distributed power sources according to an embodiment of the mvention will be described with reference to the accompanying drawings. FIG 1 is a block diagram illustrating a configuration of an energy supply system using distributed power sources according to an embodiment of the invention. The energy supply system includes a gas engine generator 1, a nickel metal hydride battery 2, an electric double-layer capacitor 3, and a control system 4 for controlling output electric power. The gas engine generator 1 is provided with an electric power meter 11. The electric power meter 11 measures an active power (PG) and outputs it to the control system 4. The nickel metal hydride battery 2 is provided with an electric power meter 12. The electric power meter 12 measures an active power (PBES) and outputs it to the control system 4. The electric double-layer capacitor 3 is provided with an electric power meter 13 and a voltage meter 14. The electric power meter. 13 measures an active power (PBDLC) and outputs it to the control system 4. The voltage meter 14 measures a direct-current voltage (VEDLC) and outputs it to the control system 4. An electric power meter 15 measures a load electric power (PLOAD) of a load 5, and the output of the electric power meter 15 is input to the control system 4.
The control system 4 receives the measurement values PG, PBES, PEDLC, and PLOAD from the electric power meter U, 12,13, and 15, and the measurement value VEDLC from the voltage meter 14, obtains active power output specifications Pso, PSBBS, and PSEDLCI and outputs them to the gas engine generator 1, the nickel metal hydride battery 2, and the electric double-layer capacitor 3 to perform a control operation so that stable electric power can be supplied.
Next, a process of performing the load-following operation of each power
source while performmg the SOC management in the control system 4 shown in FIG. 1
will be described with reference to FIG. 2. FIG 2 is a control block diagram illustrating
a configuration of a control system 4. Out of three power sources shown in FIG 1, both
the nickel metal hydride battery 2 and electric double-layer capacitor 3 function as
electricity storage devices demanding the SOC management. In this case, particularly,
it is assumed that only the electric double-layer capacitor 3 having a small capacity is set
as an SOC management target, and the SOC of the electric double-layer capacitor 3 is
managed using the nickel metal hydride battery 2.
If the electric double-layer capacitor 3 is substituted by a simple model as a condenser, a proportional relationship (Q = CVEDLC) can be established between the direct current (DC) voltage VEDLC aiid the charge amount Q. Therefore, it is possible to obtain the SOC value by measuring the DC voltage VBDLC. In the system configuration shown in FIG 1, the DC voltage VEDLC is measured using the voltage meter 14, and the measurement value is set as the SOC value.
In FIG. 2, like reference symbols denote like elements as in the control block of the related art shown in FIG 5, and descriptions thereof will be omitted. The control block shown in FIG 2 is different from the control block of the related art shown in FIG 5 in the following points. That is, the SOC (i.e., the DC voltage VEDLC) of the electric double-layer capacitor 3, which is the SOC management target, is measured. The difference between the measurement value and a predetermined SOC management reference value SOCref is calculated. The difference is multiplied by a proportional gain K using a multiplier 47. Then, a component that can be compensated for in the nickel metal hydride battery 2 is extracted using a LPF 48, and that value is subtracted frxjm the output specification of the nickel metal hydride battery 2. In this configuration.
it is possible to control output powers of each power source while managing the SOC
value.
In FIG 2, the difference between the SOC and the SOCref is multiplied by the proportional gain K using the multiplier 47, and the result is input as the specification value of the nickel metal hydride battery 2 via the LPF 48. However, a proportional-plus-integral controller may be used instead of the multiplier 47 for multiplying the proportional gain K and the LPF 48. The SOC management control may be performed using the gas engine generator 1 which has a lower load-following capability than that of the electricity storage device.
Next, a process of controlling the output of each power source by managing the SOC will be described with reference to FIG 3. A section of FIG 3 illustrates a control process according to the control method for the related art and B section of FIG 3 a process of the SOC management control of electric double-layer capacitor 3 according to an embodiment of the invention. In the event that a load disturbance (reference symbol Al) having a step shape is added at a certain time point, the output powers of each power source change as indicated by the reference symbols A2 and A3 of FIG 3 according to the control method disclosed in Patent Document 1 (refer to FIG 5). As a result, the SOC of the electric double-layer capacitor 3 is lower than in the initial state (reference symbol A4).
On the other hand, according to the control method for the embodiment of the present invention (B section in FIG 3), as the SOC of the electric double-layer capacitor 3 is lowered (reference symbol 31), the output shown as the reference symbol B2 is added to the output specification value of the gas engine generator 1 or the nickel metal hydride battery 2 which has a lower responsive capability than that of the electric double-layer capacitor 3. As a result, the total output of the gas engine generator 1 and
the nickel metal hydride battery 2 has an output waveform shown as the reference
symbol B3. Since the output of the electric double-layer capacitor 3 (reference symbol
34) changes to perform the charging operation after the discharging operation so that the
total output of all power sources matches with the load electric power, the SOC of the
electric double-layer capacitor 3 can be recovered. In other words, it is possible to
rapidly recover the remaining capacity of the electricity storage device (i.e., the electric
double-layer capacitor 3) by mcreasing the output electric power of the power source (i.e.,
the nickel metal hydride battery 2) except for the electricity storage device after
discharging the electronic power fitjm the electricity storage device (i.e., the electric
double-layer capacitor 3).
Next, results of the operations when the SOC management control is performed
and when the SOC management control is not performed will be described with
reference to FIGS. 4A and 4B. FIG 4A illustrates a result of an operation when the
SOC management control is not performed. FIG 43 illustrates a result of an operation
when the SOC management control is performed. The operation voltage range of the
electric double-layer capacitor 3 is set to 240 to 400 V, which corresponds to the range of
the SOC value 0 to 100%. A standard voltage of the electric double-layer capacitor 3 is
336 V (which corresponds to 60% of the SOC value). It is recognized that, when the
SOC management control is not performed as shown in FIG 4A, the SOC value is
lowered to approximately 25% at the time point of 300 sec. However, when the SOC
management control is performed as shown in FIG 4B, the SOC value can be normally
maintained at approximately 60%. By monitoring the system frequency variation width
of the microgrid, it is possible to recognize how precisely the load-following operation is
performed. Even by comparing both operations when the SOC management control is
performed (see, FIG 43) and when the SOC management control is not performed (see,
FIG 4A), it is recognized that the system frequency variation width is not quite different,
and the precision of the load-following operation is not degraded.
While, in the aforementioned descriptions, the electric double-layer capacitor, the nickel metal hydride battery, and the gas engine generator are employed, any secondary battery or any synchronous generator may be employed regardless of the types thereof The secondary battery refers to a chargeable/dischargeable battery and includes a nickel metal hydride battery, a lead-acid storage battery, an NAS battery, a lithium ion battery, and the like. The gas engine generator may be substituted with any other type of synchronous generator such as a gas turbine generator or a diesel engine generator in addition to the gas engine synchronous generator.
As described above, when the load-following operation is perfonned using a plurality of types of distributed power sources, it is possible to manage the SOC of the electricity storage device (e.g., the electric double-layer capacitor 3) without degrading the control precision of the load-following operation by performing the SOC management using a power source (e.g., the nickel metal hydride battery 2) having a lower responsive capability than that of the corresponding electricity storage device. Therefore, it is possible to prevent the performance degradation of the load-following operation that may be caused by the fluctuation in the SOC of the electricity storage device. In addition, it is possible to prevent degradation in the load-following operation performance of the electricity storage device when the SCX) management is performed by the electricity storage device itself. Since the capacity of the electricity storage device included in the distributed power source can be reduced by managing the SOC, it is possible to reduce the cost of the system.
While preferred embodiments of the invention have been described and illustrated above, it should be imderstood that these are exemplary of the invention and
are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made vWthout departing from the scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.

What is claimed is:
1. A method for systematically controlling a plurality of distributed power sources
having different responsive capabilities for a load disturbance, the distributed power sources including an electricity storage device, the method comprising:
obtaining a component to be compensated for using a power source having a responsive capability equal to or lower than that of the electricity storage device based on a difference value between a remaining capacity of the electricity storage device and a target remaining capacity; and
compensating for the component to be compensated for using the power source having a responsive capability equal to or lower than that of the electricity storage device.
2. The method according to claim 1, wherein the electricity storage device includes
an electric double-layer capacitor or a secondary battery, and the power source having a responsive capability equal to or lower than that of the electricity storage device includes a secondary battery or a synchronous generator.