Abstract:
A control system for cogeneration system provides operation combination calculating means which calculates the combination of operations capable of keeping the voltage of the distribution line within a predetermined tolerance within voltage tolerance based on following (1)-(3). (1) the predicted value of the electric power demand of each customer calculated by electric power demand predicted value calculating means; (2) the impedance of the distribution line recorded by distribution line information recording means; and (3) the combination of cogeneration systems to be operated, as calculated by operation combination calculating means.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application serial no. 2005-283231, filed on Sep. 29, 2005, the content of which is hereby incorporated by reference into this application.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to a control system and a control method for a cogeneration system.  
       BACKGROUND OF THE INVENTION  
       [0003]     In recent years, problems of global warming and depletion of natural resources have begun surfacing. There has been a growing concern over the effective use of energy. Particularly in the field of global warming, the Kyoto Protocol providing for a CO 2 -reduction goal took effect. During the period from 2008 through 2012, Japan is required to reduce the CO 2  emission volume by 6% with reference to the 1990 level. Against this background, the distributed power generation provided inside the customer, can be utilized in the form of a cogeneration system that allows the customers to use not only the generated electric power, but also the waste heat having been unused heretofore. From the viewpoint of effective use of energy, it is expected to come into widespread use.  
         [0004]     When the distributed power generation has come into widespread use, it is expected to meet the following requirements:  
         [0005]     (1) Operate the distributed power generation at a high efficiency according to the actual load.  
         [0006]     (2) Maintain the electric power quality of the distribution system in case there is a reverse flow into the distribution system.  
         [0007]     In the “cogeneration network system for household use” described in Japanese Patent Laid-open No. 2003-134674 (Patent Document 1), the distributed power source installed in each customer in a electric power network formed by a plurality of customers is operated on a selective sources in response to the electric power demand and heat demand of each customer, and the electric power generated by the distributed power generation is interchanged among the customers. This arrangement provides a system for ensuring an effective supply of electric power and heat to the customers within the electric power network.  
         [0008]     Moreover, in the “electric power quality maintenance support method and system in the distribution system” described in Japanese Patent Laid-open No. 2004-274812 (Patent Document 2), when the voltage of the distribution system is not kept within tolerance, the specific customer distributed power generation and system control device are controlled by the command value based on the computation result of simulation, whereby the system voltage can be kept within the tolerance.  
       SUMMARY OF THE INVENTION  
       [0009]     The Patent Document 1 discloses the technique wherein some of the cogeneration systems for household use installed at customers of the electric power network are operated at high power, and the electric power that cannot be consumed by some customers is supplied to other customers. This ensures an efficient supply of electric power and heat energy to the customers in the network.  
         [0010]     Electric power from a high voltage system is usually supplied to a low voltage electric power distribution system through a pole transformer. In this case, the voltage of the electric power distribution system is stipulated to remain within 101±6 volts by the Electricity Enterprises Law. In case no distributed power generation is available in each household, or distributed power source is installed in each household but the reverse flow is not tolerance, the voltage of the electric power distribution system is gradually reduced from the pole transformer to the terminal costumer, as shown in  FIG. 10 . In this case, the voltage of the electric power distribution system can be kept within the tolerance by adequately setting the voltage value just at the back of the transformer, the number of customers under the charge of one transformer, and the length and size of the distribution line (or the number of distribution lines).  
         [0011]     However, in the Patent Document 1 wherein there is a reverse flow from the distributed power generation to the electric power distribution system, the voltage of the electric power distribution system may get out of the range of tolerance, depending on the selection of the cogeneration system, as shown in  FIG. 11  (operation of the distributed power generation system marked by a slant line). This may deteriorate the electric power quality. In  FIG. 11 , the reference numeral  1101  denotes a control apparatus,  1102  a communication network line,  1103  a distribution line,  1104  a customer,  1105  a distributed electric power generation system,  1106  a hot water pipe and  1107  a electric power line.  
         [0012]     According to the Patent Document 2, if the voltage of the electric power distribution system is not kept within the tolerance, the distributed power generation and system control devices of the customer are controlled in response to the command value based on the computation result of simulation, whereby the voltage of the electric power distribution system is kept within the tolerance.  
         [0013]     In this case, however, to maintain the electric power quality of the electric power distribution system and to control the distributed power generation or system control devices on a priority basis, devices are operated without due consideration given to the energy demand of the customer, particularly to the heat demand. From the viewpoint of energy supply, this will lead to an increase in energy consumption, without effective use of the waste heat, or this will lead to a cost increase. In addition to this problem, the following problem also arises: After making sure that the measured value of voltage in the electric power distribution system is out of the tolerance, the distributed power generation and system control devices are controlled in real time, and the voltage quality is maintained. Accordingly, the on-off operations of the distributed power generation and system control devices may be performed too frequently or a time lag may occur in handling.  
         [0014]     According to the aforementioned conventional art, an efficient supply of the electric power and heat from the electric power supply of each household connected to the low-voltage electric power distribution system is not compatible with the maintenance of electric power quality of the electric power distribution system. The conventional art fails to meet these two requirements simultaneously. In order to meet these two requirements simultaneously, it is necessary to measure the electric power of each household required to ensure efficient supply of electric power and heat from the distributed power generation, and the voltage of each household required to maintain the electric power quality of the electric power distribution system.  
         [0015]     The object of the present invention is to provide a control system and control method for a cogeneration system which can ensure an efficient supply of electric power and heat while maintaining the high electric power quality of a electric power distribution system.  
         [0016]     One of the features of the present invention is found in that the control system for the cogeneration system of the present invention is designed to supply customers with electric power and heat, and to control a plurality of cogeneration systems connected to a distribution line, and this control system comprises:  
         [0017]     electric power demand predicted value calculating means for calculating the predicted value of the electric power demand for each of the costumers;  
         [0018]     distribution line information recording means for recording the impedance of the distribution line;  
         [0019]     operation priority calculating means for calculating the priority of operation based on the amount of the hot water stored by a customer or the demand for hot water used by a customer;  
         [0020]     operation combination calculating means for calculating the combinations of cogeneration systems to be operated according to the operation priority calculating means; and  
         [0021]     operation combination calculating means within voltage tolerance that calculates the combination of operations capable of keeping the voltage of the distribution line within a predetermined tolerance, based on the predicted value of the electric power demand of each customer calculated by the electric power demand predicted value calculating means, the impedance of the distribution line recorded by the distribution line information recording means, and the combination of cogeneration systems to be operated, as calculated by the operation combination calculating means.  
         [0022]     Another feature of the present invention is found in the description of Best Mode for Carrying Out the Invention.  
         [0023]     The present invention provides a control system and a control method for cogeneration system which ensures an efficient supply of electric power and heat while maintaining the high electric power quality of a electric power distribution system.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a system configuration diagram representing the first embodiment of the present invention;  
         [0025]      FIG. 2  is a system configuration diagram representing the detailed configuration of the distributed power generation system of the present invention;  
         [0026]      FIG. 3  is a system configuration diagram representing the detailed configuration of the control system of the present invention;  
         [0027]      FIG. 4  is a system control flow representing the first embodiment of the present invention;  
         [0028]      FIG. 5  is a conceptual view representing the method of determining the operation status of the distributed power generation;  
         [0029]      FIG. 6  is a system configuration diagram representing the second embodiment of the present invention;  
         [0030]      FIG. 7  is a system control flow representing the second embodiment of the present invention;  
         [0031]      FIG. 8  is a system control flow representing the third embodiment of the present invention;  
         [0032]      FIG. 9  is a system control flow representing the fourth embodiment of the present invention;  
         [0033]      FIG. 10  is a conceptual view (Part  1 ) representing the voltage distribution of a low voltage distribution system according to the conventional art;  
         [0034]      FIG. 11  is a conceptual view (Part  2 ) representing the voltage distribution of a low voltage distribution system according to the conventional art;  
         [0035]      FIG. 12  is a conceptual view representing the voltage distribution of a low voltage distribution system according to the present invention; and  
         [0036]      FIG. 13  is a diagram showing the method of calculating the voltage in the embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     Referring to the embodiments illustrated in drawings, the following describes the cogeneration system for household use according to the present invention.  
       Embodiment 1  
       [0038]      FIG. 1  is a system configuration diagram representing the first embodiment of the present invention. This system includes a plurality of households  1  as customers, a distributed power generation system  2  installed in each household to generate electric power and heat, a distribution line  3 , a control system for controlling the operation of each distributed power generation system, a control system  4  for controlling the operation of each distributed power generation system, a electric power sensor  5  for measuring the electric power flowing into or out of each household, and a voltage sensor  6  for measuring the delivered voltage just at the back of the pole transformer. The distributed power generation system and control system are connected by a communication network line  7  to permit exchange of data and control commands. Electric power and hot water generated by the distributed power generation system are supplied to each household through a electric power line  8  and a hot water tube  9 . The remainder of the electric power generated by the distributed power generation system, apart from the electric power having been consumed by the household, is supplied to the surrounding households through the distribution line  3 .  
         [0039]     As shown in  FIG. 2 , the distributed power generation system  2  contains: a electric power generation section  10  for generating electric power; a hot water storage tank  11  for producing hot water by heat exchange with the waste heat generated simultaneously with electric power, and storing this hot water; a control section  12  for controlling the operation of the distributed power generation system in response to the command from an operation command section (to be described later) of the control system; a display section  13  for displaying the distributed power generation operation status and notifying the costumer; an input section  14  for inputting the customer information and instructions; and a recording section  15  for storing the data measured by the distributed power generation system. The waste heat generated by the electric power generation section  10  is sent to the hot water storage tank  11  through a waste heat tube  16 . In this case, a prime mover such as an engine, or a fuel cell is used as the electric power generation section  10 .  
         [0040]     The distributed power generation system measures the data on operation time, the amount of electric power, the amount of hot water in the storage tank and others. This data is transmitted to the monitoring section (to be described later) of the control apparatus through the communication network line  7  whenever necessary. All apparatuses inside the distributed power generation system are connected with each other by the distributed power generation system communication line  17 . Information such as various forms of measurement data and commands is exchanged as required. The input section  14  inputs information in the case of an abnormal situation different from the normal life such as absence of the resident or visit by a guest. Based on this input value and past data, the control section  12  predicts daily amount of electric power and hot water to be consumed. For example, according to the basic information on a weekday, holiday or season, reference is made to the past data based on the basic information. If the resident is absent or is visited by a guest on a particular date, correction is made based on the input information from the display section, and the amount of electric power and hot water to be consumed are predicted.  
         [0041]     The following method, for example, can be considered to calculate the predicted value for amount of electric power and hot water to be consumed. The past data on the amount of electric power and hot water consumed in the past, and temperature data are organized by working out the average according to the season, weekday or holiday. If that date falls on a weekday in summer, the average data on the amount of electric power and hot water consumed on a weekday in summer in the past is used as a basic predicted value. Further, if the temperature on that date is higher than the average, electric power consumption is estimated to increase due to more frequent use of an air conditioner. Thus, the basic predicted value for the electric power consumption is corrected to a higher value. If that date falls on a weekday in winter, the average data on amount of electric power and hot water consumed is used as the basic predicted value. Further, if the temperature on that date is lower than the average level, there will be an increase in the amount of hot water to be used. Thus, the basic predicted value for the amount of hot water to be consumed is corrected to a higher level. If a guest is coming on that date, there will be an increase in the amount of electric power used for air conditioners and the amount of hot water to be consumed. This information is inputted into the input section, whereby the basic predicted values for the amount of electric power and hot water to be consumed are corrected to higher levels.  
         [0042]     As shown in  FIG. 3 , the control system  4  contains a monitoring section  18  for monitoring various measurement data, a system status estimating section  19  for estimating the voltage of each household based on the calculation of the system status, a priority calculating section  20  for determining the priority of operating the distributed power generation system from the measurement data of all the distributed power generation systems, an operation command section  21  for issuing commands for the operation status of each distributed power generation system based on the information of the system status estimating section and priority calculating section, and an overall recording section  22  for storing the data on the impedance of the distribution line, in addition to the data sent to the monitoring section. The electric power measurement value by the electric power sensor and voltage measurement value by the voltage sensor, in addition to the measurement data of the aforementioned distributed power generation system, are sent to the monitoring section  18  through the communication network line  7 . The apparatuses inside the control system are connected with each other by a control system communication line  23 , so that various forms of measurement data and commands are exchanged as required.  
         [0043]     The distribution line  3  is connected with the high voltage system through a pole transformer. This arrangement ensures that the electric power in the amount equivalent to the difference between the overall electric power consumption of each household and the overall the amount of electric power produced in the distributed power generation is supplied from the high voltage pressure.  
         [0044]     Based on the electric power value measured by the electric power sensor, the voltage value measured by the voltage sensor and the database of the overall recording section  22 , the system status estimating section  19  of the control system  4  performs estimation and calculation of the distribution system status, thereby estimating the combination of the operation statuses of the distributed power generation systems wherein the voltage of the distribution system is kept within the tolerance.  
         [0045]     The following describes an example of measuring the voltage of the distribution system with reference to  FIG. 13 . The voltage of the distribution system is calculated, for example, by the following equation (wherein an approximate expression is used on the assumption that “V” does not change much from 1 p.u.).  
         [0046]     To start with, the following calculation is performed: 
 
 P ( n )= P   g ( n )− P   1 ( n ) 
 
 Q ( n )= Q   g ( n )− Q   1 ( n ) 
 
         [0047]     Then the following is calculated sequentially from i=n-1 through 1: 
 
 P ( i )= P ( i+ 1)− R ( i+ 1)· P ( i+ 1)+ P   g ( i )− P   1 ( i ) 
 
 Q ( i )= Q ( i+ 1)− X ( i+ 1)· Q ( i+ 1)+ Q   g ( i )− Q   1 ( i ) 
 
         [0048]     Based on the results of the aforementioned equations, the following is calculated sequentially from i=1 through n: 
 
 V ( i )= V ( i− 1)− R ( i )· P ( i )+ X ( i )− Q ( i ) 
 
         [0049]     Based on the data sent from the monitoring section  18 , the priority calculating section  20  of the control system  4  determines the priority of operating each distributed power generation system.  
         [0050]     The data used in this case is exemplified by the amount of hot water to be stored in each distributed power generation system, and the predicted values for the amount of electric power and hot water to be consumed by the customer. For example, the priority of the distributed power generation systems can be determined as follows: In the first place, selection is carried out to determine the distributed power generation systems that meet the requirements that the amount of hot water generated by the distributed power generation system does not exceed the predicted value for the hot water to be consumed by each costumer and the daily operation time does not exceed a predetermined period of time. These systems are assigned with priority in such a way that higher priority is given to the system having a smaller amount of hot water in the hot water storage tank. To put it more specifically, the following procedure is taken.  
         [0051]     The amount of hot water to be consumed subsequently in each household (e.g. ten hours) is subtracted from the current amount of stored hot water of each distributed power generation system. Hot water must be supplied earlier into a hot water storage tank wherein the value obtained from this subtraction is smaller. Thus, the priority in the operation of the distributed power generation system must be determined in the ascending order of this value. Further, when there are a plurality of distributed power generation systems without much difference in the amount of stored hot water (e.g. ±10%), higher priority is given to the distributed power generation system of less operation time.  
         [0052]     The difference from the Patent Document 1 is found in the fact that the system voltage may get out of the tolerance, as shown in  FIG. 11 , according to the method of operating the distributed power generation system disclosed in the Patent Document 1, whereas the present invention keeps the system voltage within the tolerance.  
         [0053]     The operation command section  21  of the control system  4  finally determines the operation status of the distributed power generation system, based on two factors—the combination of the operation statuses of the distributed power generation system wherein the voltage of the distribution system calculated by the system status estimating section  19  is kept within the tolerance, and the priority of operation calculated by the priority calculating section  20 .  
         [0054]      FIG. 4  is a system control flow representing the first embodiment of the present invention. The delivery voltage just at the back of the pole transformer is measured by the electric power sensor, and the electric energy running in and out of each household is measured by the electric power sensor (S 1 ). In the system status estimating section of the control system, the estimated value for the voltage of the distribution system is obtained by estimation and calculation of the distribution system status based on the measured value (S 3 ).  
         [0055]     In addition to this estimated voltage value, the measured value (S 2 ) for each distributed power generation system—e.g. predicted values for the amount of stored hot water in the distributed power generation system, the operation time, the amount of storage and the amount of hot water consumed by each customer—are normally sent to the monitoring section  18  of the control system. Such data is monitored by the monitoring section  18  (S 4 ), and at the same time, is stored by the overall recording section  22 . Here when the monitored data has exceeded the specified condition,—for example, when the estimated voltage value has exceeded the voltage tolerance, when the operation time of the distributed power generation system has exceeded the set value, or when the amount of generated hot water of the distributed power generation system has exceeded the estimated value of the amount of consumed hot water—the monitoring section  18  reviews the operation status of each distributed power generation system (S 5 ). Further, it also reviews the operation status of the distributed power generation system for each of the specified times set in advance (S 5 ).  
         [0056]     The following procedure is taken to review the operation status of each distributed power generation system: The delivery voltage just at the back of the pole transformer is measured by the electric power sensor, and the electric energy flowing in and out of each household is measured by the electric power sensor (S 6 ). Based on the measured value, the operation command section  21  of the control system determines the overall the amount of produced electric power of the distributed power generation system (the number of the systems to be operated) (S 8 ).  
         [0057]     From an idealistic viewpoint, the overall amount of electric power produced should be determined by the “rated output of the distributed power generation system”×“the number of systems to be operated”, whereby the distributed power generation system is operated at the rated output wherein the distributed power generation system has a high efficiency. Here, the electric energy purchased from the system electric power is the value obtained by subtracting the overall amount of electric power produced by the distributed power generation system, from the overall electric power consumption of each household. Based on the number of systems to be operated, the system status estimating section  19  of the control system takes the step of estimation and calculation of the electric power distribution system to find out what kind of combinations of the distributed power generation systems will be appropriate to ensure that the voltage of the electric power distribution system is kept within the tolerance, when operating the aforementioned number of distributed power generation systems (S 10 ).  
         [0058]     It is also possible to predict the voltage for the specific time set in advance using the predicted value for the electric power consumption, and to estimate the combination of the distributed power generation systems wherein the voltage value is kept within the tolerance. This arrangement eliminates the need of reviewing the operation status within the specified period of time, and removes the possibility of increasing the frequency of starting and stopping the distributed power generation system and causing a delay in handling.  
         [0059]     In the meantime, the priority calculating section  20  of the control system determines the priority of the operation of distributed power generation systems, based on the measurement data (S 7 ) of each distributed power generation system (S 9 ).  
         [0060]     In the final phase, the operation command section  21  of the control system determines the operation status of each distributed power generation system, based on the priority in the operation of each distributed power generation system and the result of estimating and calculating the electric power distribution system status (S 11 ).  
         [0061]     To determine the operation status of each distributed power generation system, it is necessary to ensure that the distributed power generation systems having a higher priority of operation should be operated. To achieve this, the following method can be considered, for example. To put it more specifically, consider the case shown in  FIG. 5  wherein three distributed power generation systems out of six are operated.  FIG. 5  shows the priority in the operation of the distributed power generation systems and the combination of the distributed power generation systems wherein the voltage value is kept within the tolerance. In the first place, combinations (A) and (B) remain as possible candidates, according to whether or not the distributed power generation system assigned with the top operation priority is operated. Then the combination (A) is selected according to whether or not the distributed power generation system assigned with the second operation priority is operated. In this case, this is the final decision. If the final decision cannot be reached, this procedure is repeated in the lower order of priority.  
         [0062]     In the aforementioned example, the electric power generated by the distributed power generation system during the operation is supplied to each household through a distribution line. As a result, the distributed power generation system is operated at the rated output wherein the highly efficient operation is performed. This arrangement ensures a highly efficient energy supply as a total system, as compared to the cases where the distributed power generation system is separately operated in each household only for the required electric power.  
         [0063]     The following describes the effects of the present invention. The average amount of electric power produced for each household is generally 0.3 through 0.5 kW. Thus, when the distributed power generation system having a electric power capacity of 1 kW is independently operated in each household, operation is performed at an approximate output of about 0.3 through 0.5 kW (30 through 50% of the rated output). This reduces the operation efficiency. In the meantime, for example, when a distributed power generation system is used in ten households, the total electric power consumption is approximately 3 through 5 kW. Three through five distributed power generation systems are selected for operation, the distributed power generation systems can be operated at the rated output at all times.  
         [0064]     At the same time, the voltage of the low voltage distribution system, for example, can be kept within the tolerance, as shown in  FIG. 11 . Thus, the voltage in the form illustrated in  FIG. 12  can be kept within the tolerance. In this case, only the electric energy flowing into and out of each customer should be measured. This eliminates the need of measuring the voltage for the purpose of maintaining the electric power quality of the distribution system.  
         [0065]     When there are a great number of the customers under the charge of one pole transformer or the distribution line is longer or thinner (or there are a smaller number of distribution lines), the voltage fluctuation of the distribution system is easily affected by the electric power demand of the customer or the amount of electric power produced by each distributed power generation system. This requires the normal systems to be reinforced. Since electric power quality can be maintained even in this case, the equipment investment cost for the distribution system can be placed under control. Further, even there is some change in the delivery voltage just at the back of the pole transformer, the electric power quality of the distribution system can be maintained. This feature eliminates the need of an electric utility making efforts to ensure that the delivery voltage just at the back of the pole transformer is kept under strict control. This allows the electric utility to cut down the management cost and equipment investment.  
         [0066]     Comparison will be made between the Patent Document 2 and the present embodiment. In Patent Document 2, the operating conditions of the distributed power generation systems are determined only from the viewpoint of maintaining the electric power quality. No consideration is given to the possibility of using the waste heat from the distributed power generation. In the Patent Document 2, the distributed power generation systems are operated independently of the calorific value to be consumed. This leads to a greater possibility of resulting in excessive or insufficient amount of waste heat from the distributed power generation systems. To be more specific, the waste heat from the distributed power generation system in the Patent Document 2 is less effectively used than that in the present invention.  
       Embodiment 2  
       [0067]      FIG. 6  is a system configuration diagram representing the second embodiment of the present invention. In the example in  FIG. 6 , the installation positions of the electric power sensor and voltage sensor are different from those in the first embodiment ( FIG. 1 ). In this example, a electric power sensor for measuring the electric energy just at the back of the pole transformer and a voltage sensor for measuring the voltage at the measurement point installed in each household are provided.  
         [0068]      FIG. 7  is a system control flow representing the second embodiment of the present invention. In the second embodiment, the voltage of the distribution system is measured by the voltage sensor installed in each household (S 1 ). Accordingly, the measured voltage value measured by this voltage sensor is transferred to the monitoring section of the control system through the communication network, and is monitored by the monitoring section in real time (S 3 ).  
         [0069]     The system status estimating section  19  of the control system estimates the electric energy at each household, based on the electric energy just at the back of the pole transformer measured by the electric power sensor and the voltage measured by the voltage sensor installed at each household (S 5 ).  
         [0070]     Then the total electric energy consumed by each household is calculated to determine the number of the distributed power generation systems to be operated (S 7 ). To put it more specifically, the total electric energy and the number of the distributed power generation systems to be operated are determined by the following steps: Assume, for example, that each of the ten households is equipped with a distributed power generation system having a electric power capacity of 1 kW, and the total electric energy consumed by ten households is 4.5 kW. In this case, four distributed power generation systems should be operated. Then the amount of electric power produced by the distributed power generation systems is 4 kW, and the remaining 0.5 kW is supplied from the system electric power.  
         [0071]     The number of the distributed power generation systems to be operated depends on the heat value consumed in each household. When only a small amount of heat is consumed, it would be possible to make arrangements that the amount of electric power produced by the distributed power generation system should be reduced to 3 kW, and 1.5 kW should be supplied from the system electric power.  
         [0072]     After that, similarly to the case of the first embodiment, when the aforementioned number of distributed power generation systems are to be operated, estimation is made to determine the type of a combination of the distributed power generation systems that will ensure that the voltage of the electric power distribution system can be kept within the tolerance (S 9 ).  
         [0073]     The priority calculating section  20  determines the operation priority of each distributed power generation system, according to the measurement data (S 6 ) of each distributed power generation system (S 8 ).  
         [0074]     In the final step, the operation command section  21  of the control system determines the operation status of each distributed power generation system according to the operation priority of each distributed power generation system and the result of the distribution system status estimation and calculation (S 10 ).  
         [0075]     In the present embodiment, the accuracy of the distribution system status estimation and calculation can be improved by comparison between the measured voltage of the distribution system and the estimated voltage and by correction of such data as inductance of the distribution line stored in the overall recording section  22 , although this is not illustrated in the control flow of  FIG. 7 .  
         [0076]     In the second embodiment, the same advantages as those in the first embodiment can be provided. Since only the voltage needs to be measured by each customer, it is not necessary to measure the electric energy intended to ensure an effective supply of electric power and heat from the distributed power generation system.  
       Embodiment 3  
       [0077]      FIG. 8  is a system control flow representing the third embodiment of the present invention. The system schematic diagram in this case is the same as that of  FIG. 1 .  
         [0078]     The procedures up to the steps S 7  and S 8  in the third embodiment are the same as those in the first embodiment. After that, based on the operation priority of the distributed power generation systems determined by the priority calculation section of the control system, the system status estimating section of the control system performs the step of estimation according to the order wherein the operation priority of the distributed power generation system is higher, to see whether or not the distribution system voltage is kept within the tolerance. This procedure is repeated until the distribution system voltage is kept within the tolerance. Then it detects the combination of the distributed power generation systems wherein the distribution system voltage is kept within the tolerance, and determines the operation status of each distributed power generation system in the final phase (S 11 ).  
         [0079]     In the third embodiment, the same advantages as those in the other embodiments can be provided. Unlike the case of the first embodiment, in the distribution system estimation and calculation of the control system, it is not necessary to calculate in advance all the combinations wherein the voltage of the distribution system is kept within the tolerance. This arrangement saves the calculation time.  
         [0080]     In the third embodiment, the steps S 10  and S 11  in the control flow of  FIG. 8  are combined after steps S 7  and S 8  also in the case of the system configuration of  FIG. 6  for measuring the electric energy just at the back of the pole transformer and the voltage at the measuring point of each household.  
       Embodiment 4  
       [0081]      FIG. 9  is a system control flow representing the fourth embodiment of the present invention. The system configuration diagram in this case is the same as that of  FIG. 6 .  
         [0082]     The procedures up to step S 5  in the fourth embodiment are the same as those of the second embodiment. After that, based on the-operation priority (S 6 ) of each distributed power generation system determined in the priority calculating section  20  of the control system, the operation command section  21  of the control system temporarily determines the operation status of each distributed power generation system, and operates the distributed power generation system (S 7 ). As a result, if the value measured by the voltage sensor is kept within the tolerance (S 8 ), the operation continues. If the value measured by the voltage sensor is not kept within the tolerance (S 8 ), a combination of the second highest priority is selected according to the result of operation priority of each distributed power generation system. Then the operation status is determined temporarily and the distributed power generation system is operated (S 7 ). This procedure is repeated until a decision step is taken to determine the operation status of the distributed power generation system wherein the voltage of the low-voltage distribution system is kept within the tolerance.  
         [0083]     In the fourth embodiment, the same advantages as those in the other embodiments can be provided. Unlike the case of the second embodiment, there is no need of estimating and calculating the distribution system status. This eliminates the need of providing a system status estimating section of the control system.  
         [0084]     The aforementioned embodiments of the present invention takes into account the demand for electric power and heat by the customers within the low-voltage distribution system, thereby ensuring both an effective supply of electric power and heat to each customer and maintenance of the electric power quality of the distribution system.  
         [0085]     When there are a great number of the customers under the charge of one pole transformer or the distribution line is longer or thinner (or there are a smaller number of distribution lines), the voltage fluctuation of the distribution system is easily affected by the electric power demand of the customer or the amount of electric power produced by each distributed power generation system. This requires the normal systems to be reinforced. Since electric power quality can be maintained even in this case, the equipment investment cost for the distribution system can be placed under control. Further, even there is some change in the delivery voltage just at the back of the pole transformer, the electric power quality of the distribution system can be maintained. This feature eliminates the need of an electric utility making efforts to ensure that the delivery voltage just at the back of the pole transformer is kept under strict control. This allows the electric utility to cut down the management cost and equipment investment.