Patent Publication Number: US-2017353033-A1

Title: Power system stabilization device and method

Description:
TECHNICAL FIELD 
     This invention relates to a power system stabilization device configured to control a generator in order to prevent, when a failure has occurred in a power system due to lightning or other reasons, the failure from influencing the power system. 
     BACKGROUND ART 
     When a failure has occurred due to lightning or other reasons in a bus or a transmission line, which is a component in a power system, a voltage of the power system is decreased and an electric output transmitted from a generator falls below input energy to the generator, so that the generator is accelerated. A plurality of generators coupled to the power system operate in synchronization with one another. Once the acceleration is generated in a part of the generators, a synchronization deviation occurs among the generators. When the synchronization deviation increases, the number of disturbed generators increases, and the synchronization among the generators cannot be maintained, resulting in loss of synchronization. The occurrence of the loss of synchronization may lead to a massive blackout in the worst case. 
     For measures against such a phenomenon, development has been made on various kinds of power system stabilization devices configured to specify a generator having a large acceleration and disconnect the generator from a power system (herein after referred to as “generator shedding”), thereby suppressing the synchronization deviation to stabilize the system. A calculation method in the power system stabilization device is roughly classified into “pre-calculation” and “post-calculation”. 
     The pre-calculation power system stabilization device is configured such that, before the occurrence of a failure, periodically measured data in a power system before the occurrence of the failure is used to periodically determine stabilization measures (such as generator shedding) by stability calculation for a failure that is assumed to occur in the power system set in advance (hereinafter referred to as “assumed failure”) (hereinafter referred to as “pre-calculation”), and the stabilization measures determined in advance are taken when a failure occurs, thereby maintaining the stability of the system. Control effects become higher as the stabilization measures are taken much earlier after the occurrence of a failure. Thus, the pre-calculation capable of determining stabilization measures in advance and immediately executing control when a failure occurs is advantageous in that the control effects are high. 
     The post-calculation power system stabilization device is configured such that, after the occurrence of a failure, stabilization measures are determined by stability calculation using one or both of data in a power system constantly measured before the occurrence of a failure and data in the power system constantly measured after the occurrence of the failure (hereinafter referred to as “post-calculation”, and the stabilization measures are immediately executed, thereby maintaining the stability of the system. The pre-calculation involves stability calculation using measured data in the power system before the occurrence of a failure, but the post-calculation involves stability calculation using one or both of data in the power system constantly measured before the occurrence of a failure and data in the power system constantly measured after the occurrence of the failure. Thus, the post-calculation is advantageous in that control that is more adapted to an actual system state than that by the pre-calculation can be executed. 
     One background art in this technical field is PTL 1. PTL 1 describes “a system stabilization control system configured to be applied to a power system including a plurality of electric power plants formed from a plurality of generators and configured to stabilize the power system by executing generator control suited for an accident condition, the system stabilization control system being configured to: execute main control by a pre-calculation method in which stability determination based on the equal-area method is executed for each accident case assumed in advance to calculate the amount of control; and subsequently execute, when the amount of control by the main control is insufficient, correction control by a post-calculation method in which stability determination based on the equal-area method is executed on the basis of measurement information after the occurrence of an accident to calculate the amount of control” (see abstract). 
     Another background art in this technical field is PTL 2. PTL 2 indicates that “an unbalance amount (DP value) among generators for deceleration force, which indicates a difference in stability in a system configuration after failure clearance, is used as a stability index, the value of the stability index is compared with a threshold set in advance, and when the DP value is larger than the threshold, it is provisionally determined that the power system is unstable against an assumed disturbance (screening), and detailed stability calculation is executed to determine the stability of the power system in detail” (see abstract). 
     Another background art in this technical field is PTL 3. PTL 3 describes “a power system prevention and control apparatus including: system information collection means for collecting power system connection states and power supply and demand states as system information; power flow state calculation means for calculating a system power flow state on the basis of the system information collected by the system information collection means and system facility data; determination means for determining whether the power system is stable for each assumed disturbance on the basis of a relation between a value of unbalance of acceleration energy among generators and a reference value set in advance, which is determined on the basis of the output of each generator at a plurality of assumed disturbance occurrence time points in the current power flow state determined by the power flow state calculation means; output adjustment amount calculation means for determining, for an assumed disturbance with which the power system is determined to be unstable by the determination means, a generator output at the corresponding assumed disturbance occurrent time point and calculating an output adjustment amount of the generator necessary for maintaining transient stability by nonlinear programming; and control means for adjusting the generator output on the basis of the output adjustment amount of the generator determined by the output adjustment calculation means, thereby improving the transient stability of the system” (see abstract). 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         Japanese Patent Application Publication No. 2013-66262 
         [PTL 2] 
         Japanese Patent Application Publication No. H7-135738 
         [PTL 3] 
         Japanese Patent No. 2603929 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the future, a power supply (output fluctuating power supply) whose output fluctuates depending on weather conditions, such as renewable energy (solar power generation, wind-power generation, and the like), is planned to be widely introduced in a power system. As a result of the recent advancement of deregulation of electric utilities around the world, facility investment for power systems is suppressed, and the volume of power flow flowing through existing transmission lines is increasing (heavy power flow). If the power flow greatly fluctuates in the heavy power flow state, the stability of the power system (system stability) may deteriorate, which makes it difficult to supply electric power stably when a failure occurs in the power system. In the worst case, the failure may be cascaded to cause a massive blackout. Power system stabilization device that can support such an unstable phenomenon are sought after. 
     The conventional pre-calculation power system stabilization device does not assume an output fluctuation of the output fluctuating power supply, and hence an error may occur in periodically measured data in a power system before the occurrence of a failure, and an error may occur in the amount of control for pre-calculation stabilization measures. When the output fluctuates in the direction in which the system stability deteriorates, there is a problem in that the amount of control is insufficient, and a massive blackout occurs in the worst case. 
     As described in PTL 1, the post-calculation power system stabilization device is configured to accumulate data on generator output or transmission line active power before and after the occurrence of a failure, create a P-δ curve during an accident and after the clearance of the accident by using a generator phase angle calculated from the data and information on the accumulated generator output, calculate the value of acceleration energy VA and the value of deceleration energy VD, and compare the magnitudes of both the energies, thereby determining the stability and executing the control. Thus, the stability of a system can be maintained even when an output fluctuation of the output fluctuating power supply is not assumed. 
     However, the stability is determined on the basis of the equal-area method, and the control is executed, and hence an infinite bus needs to be prepared for a power system to which a generator or an electric power plant to be subjected to stabilization control (stabilization subject) is coupled via a transmission line. It is therefore difficult to take into consideration the influence of other generators in the power system on the generator or the electric power plant serving as the stabilization subject. For the application as a power system stabilization device for a power system in practice, there is a problem in that labor is required for much parameter tuning (parameter settling). 
     It is an object of this invention to provide a technology capable of maintaining stability of a power system by stabilization control even when a power flow of the power system has increased to the degree that is not assumed by pre-calculation. 
     Solution to Problem 
     A power system stabilization device according to one aspect of this invention performs stabilization control of a power system, and includes: an indicator calculation unit configured to calculate, by using a generator output that is an output of a generator configured to supply electric power to the power system and a generator phase difference that indicates a change of a phase angle of the generator output with respect to time, an acceleration index that is an index representing an acceleration of the generator; a threshold value determining unit configured to determine whether the acceleration index exceeds a threshold set in advance; and a control command unit configured to issue, when the acceleration index exceeds the threshold, a control command for control details for correcting the stabilization control, which are set for the threshold in advance. 
     Advantageous Effects of Invention 
     This invention can execute stabilization control capable of maintaining stability of power system against a power flow fluctuation that is not assumed by pre-calculation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating an example of an overall configuration of a power system stabilization device. 
         FIG. 2  is a diagram illustrating an example of a hardware configuration of the power system stabilization device and an overall configuration of a power system. 
         FIG. 3  is a diagram illustrating the outline of information to be transmitted and received between the power system stabilization device and a central stabilizer, a failure detection apparatus, a measurement apparatus, and a generator control apparatus. 
         FIG. 4  is a diagram illustrating contents of program data in the power system stabilization device. 
         FIG. 5  is a diagram illustrating system data about generator phase difference data. 
         FIG. 6  is a diagram illustrating system data about threshold and control data. 
         FIG. 7  is a diagram illustrating system data about determination control result data. 
         FIG. 8  is a flowchart illustrating the whole processing in the power system stabilization device. 
         FIG. 9  is a flowchart illustrating processing in a generator phase difference calculation unit. 
         FIG. 10  is a diagram for describing the calculation of a generator phase difference immediately after a failure. 
         FIG. 11  is a diagram for describing the calculation of the generator phase difference. 
         FIG. 12  is a flowchart illustrating processing in a generator energy calculation unit. 
         FIG. 13  is a diagram for describing the calculation of generator energy. 
         FIG. 14  is a diagram for describing processing in a threshold value determining unit. 
         FIG. 15  is a diagram for describing processing in a threshold value determining unit. 
         FIG. 16  is a diagram illustrating an overall configuration of the central stabilizer. 
         FIG. 17  is a diagram illustrating a hardware configuration of the central stabilizer and an overall configuration of the power system. 
         FIG. 18  is a diagram illustrating contents of program data in the central stabilizer. 
         FIG. 19  is a flowchart illustrating the whole processing in the central stabilizer. 
         FIG. 20  is a flowchart illustrating processing in a stability limit search unit. 
         FIG. 21  is a flowchart illustrating processing in a transient stability direction search flow. 
         FIG. 22  is a configuration diagram of the power system for describing the processing in the transient stability direction search flow. 
         FIG. 23  is a diagram illustrating a power flow fluctuation in each area for describing the processing in the transient stability direction search flow. 
         FIG. 24  is a flowchart illustrating processing in a threshold and correction control detail calculation unit. 
         FIG. 25  is a diagram illustrating how a stability limit search unit searches for a stability limit. 
         FIG. 26  is a diagram for describing processing in a generator energy calculation unit. 
         FIG. 27  is a diagram for describing processing in a threshold and correction control detail calculation unit. 
         FIG. 28  is a time chart illustrating timings from the occurrence of a failure to each control in the power system stabilization device. 
         FIG. 29  is a diagram illustrating an example of search range data. 
         FIG. 30  is a diagram illustrating an example of assumed failure data. 
         FIG. 31  is a diagram illustrating how the stability is improved by threshold determination and generator control. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of this invention are described with reference to the accompanying drawings. 
     First, in regard to an example of a power system stabilization device according to this embodiment, an example of an overall configuration of input, output, and processing is described with reference to  FIG. 1 . Next, hardware configurations of a power system  100 , a partial power system  101 , a central stabilizer  210 , the power system stabilization device  10 , a failure detection apparatus  150 , a measurement apparatus  44   a , and a generator control apparatus  160  are described with reference to  FIG. 2 . 
       FIG. 1  is a block diagram illustrating an example of the overall configuration of the power system stabilization device  10  according to this embodiment. Referring to  FIG. 1 , the power system stabilization device  10  includes a generator phase difference calculation unit  30   a , a generator energy calculation unit  31   a , a threshold value determining unit  32 , and a control command unit  33 . 
     The power system stabilization device  10  holds generator phase data D 20 , generator phase difference data D 2  (not shown), generator output data D 1 , failure data D 6 , threshold and control data D 3 , and determination result data D 7 , and transmits control command data D 5  to the generator control apparatus  160  configured to control a generator  110   a.    
     Input data to the power system stabilization device  10  are the generator phase data D 20 , the generator phase difference data D 2 , the generator output data D 1 , the failure data D 6 , and the threshold and control data D 3 . 
     The power system stabilization device  10  calculates the generator phase difference data D 2  from the generator phase data D 20  before and after the occurrence of a failure, and calculates generator energy by using the generator output data D 1  and the calculated generator phase difference data D 2 . The threshold and control data D 3  is data calculated by the central stabilizer  210  and notified to the power system stabilization device  10 , and includes a threshold used for failure threshold determination and information on what control is to be executed for a failure at each location. How to determine the threshold is described later in the description of the central stabilizer  210 . The power system stabilization device  10  executes threshold determination with use of the threshold and control data D 3  and the calculated generator energy, and calculates and transmits the control command data D 5  to the generator control apparatus  160  coupled to the generator  110   a  and an electric power plant including the generator  110   a  on the basis of the result of the determination. 
     The generator phase difference calculation unit  30   a  in the power system stabilization device  10  calculates the generator phase difference D 2  by using the generator phase data D 20  before and after the occurrence of a failure. The generator energy calculation unit  31   a  in the power system stabilization device  10  calculates generator energy by using the failure data D 6 , the generator phase difference D 2 , and the generator output data D 1 . The threshold value determining unit  32  in the power system stabilization device  10  determines whether the generator energy exceeds a threshold by using the threshold and control data D 3  and the generator energy. The generator energy is energy for accelerating or decelerating a generator, and can be regarded as an index representing acceleration (acceleration index). The control command unit  33  in the power system stabilization device  10  selects an appropriate control detail on the basis of the result of the threshold determination and the threshold and control data D 3 , transmits the control command data D 5  having the selected control detail to the generator control apparatus  160 , and generates the determination result data D 7 . For example, when the control detail is generator tripping control (generator shedding), the generator control apparatus  160  that has received the control command is shut down from the power system  100  in accordance with the control command. 
     The above-mentioned generator phase difference may be calculated by an external apparatus instead of the power system stabilization device  10 . 
     Each of the generator phase data D 20 , the generator output data D 1 , the failure data D 6 , and the threshold and control data D 3  may be acquired as necessary or may be stored in a predetermined database in advance. 
     The generator tripping control (generator shedding) has been exemplified as a control command. Other examples of the control command include load shutdown control (load control) and phase modifying equipment control. 
       FIG. 2  is a diagram illustrating an example of a hardware configuration of the power system stabilization device  10  and an overall configuration of the power system.  FIG. 2  illustrates the power system  100 , its partial power system  101 , the central stabilizer  210 , the power system stabilization device  10 , and the failure detection apparatus  150 . The measurement apparatus  44   a  and the generator control apparatus  160  are illustrated inside the partial power system  101 . 
     The power system  100  includes any one or more of the generator  110   a , a transformer  130   a , the measurement apparatus  44   a , the failure detection apparatus  150 , a load (not shown), and other measurement apparatus and control apparatus, which are each coupled to the power system  100  via a branch (line)  140   a  and a node (bus)  120   a.    
     The power system  100  includes one or more partial power systems  101 . The partial power system  101  includes anyone or more of the generator  110   a , the branch  140   a , the transformer  130   a , a node  121   a , the measurement apparatus  44   a , and the generator control apparatus  160 , which are each coupled to the partial power system  101  via the node  120   a.    
     Examples of the generator  110   a  include a generator that can be shut down from the power system  100  in case of emergency, such as a thermal power generator. The generator control apparatus  160  controlled by the power system stabilization device  10  is assumed to control the generator  110   a  as a control subject. However, when the power system stabilization device  10  executes control for maintaining transient stability as well as other voltage stability and frequency stability, the power system stabilization device  10  may directly or indirectly control a power supply, a load, a battery, and other control devices as control subjects. 
     The load includes a home electric appliance in a consumer which is not assumed to be controlled but only consumes electric power, such as an air conditioner, a refrigerator, and a washing machine, and a controllable load which is assumed to be controlled, such as a heat pump. Even an apparatus which is not assumed to be controlled may be controlled via a home server configured to communicate with devices using electric power, such as a Home Energy Management System (HEMS). When the power system stabilization device  10  controls loads, the power system stabilization device  10  may perform control for each device serving as individual loads, may control the load for each individual consumer, or may perform control on a set of a plurality of loads. The power system stabilization device  10  may control a load via an aggregator who implements energy management for cluster housing or buildings on consignment. 
     Examples of the battery include a rechargeable secondary battery, an EV storage battery, and a flywheel. 
     Examples of the measurement apparatus  44   a  include an apparatus (such as a VT (Voltage Transformer), a PT (Potential Transformer), and a CT (Current Transformer)) configured to measure any one or more of a node voltage V, a branch current I, a power factor Φ, active power P, and reactive power Q. The measurement apparatus  44   a  is a telemeter (TM) having a function of transmitting data including data measurement location identification ID and built-in timestamps of the measurement apparatus. 
     The measurement apparatus  44   a  may include an apparatus configured to measure absolute time-added power information (voltage phasor information) using GPS, a Phasor Measurement Units (PMU), and other measurement apparatuses. 
     In  FIG. 2 , the measurement apparatus  44   a  is illustrated as being located outside the power system stabilization device  10 . However, the measurement apparatus  44   a  may be included inside the generator control apparatus  160  or the power system stabilization device  10 . 
     Examples of the failure detection apparatus  150  include a failure detection relay, such as an undervoltage relay. Examples of the generator control apparatus  160  include a control board installed in an electric power plant capable of controlling one (uniaxial) or more (polyaxial) generators. The control board is a terminal apparatus configured to receive control commands from, for instance, the power system stabilization device  10 , and is also called “terminal equipment”. 
     Referring to  FIG. 3 , various kinds of data to be transmitted and received among the central stabilizer  210 , the power system stabilization device  10 , the failure detection apparatus  150 , the measurement apparatus  44   a , and the generator control apparatus  160  via a communication network  300  are described.  FIG. 3  is a diagram showing an example of a schematic flow of information to be transmitted and received among the power system stabilization device  10 , the central stabilizer  210 , the failure detection apparatus  150 , the measurement apparatus  44   a , and the generator control apparatus  160 . The central stabilizer  210  is coupled to a communication unit  13   a  in the power system stabilization device  10  via the communication network  300 . The central stabilizer  210  transmits information  53  (threshold and control data D 3 ) to the power system stabilization device  10 , and receives information  57  (determination control result data D 7 ) from the power system stabilization device  10 . 
     The failure detection apparatus  150  coupled to the power system  100  is similarly coupled to the power system stabilization device  10  via the communication network  300 , and transmits information  56  (failure data D 6 ) to the power system stabilization device  10 . The failure data D 6  to be transmitted by the failure detection apparatus  150  includes the location and condition of a failure. The condition is information indicating the state of a failure, and includes a value that can be used for threshold determination. The power system stabilization device  10  collates the failure data D 6  with the threshold and control data D 3  to perform threshold determination, and executes control on the basis of the result of the determination. 
     The measurement apparatus  44   a  is similarly coupled via the communication network  300  to the power system stabilization device  10  that is coupled to the generator  110   a , the bus  121   a , the transformer  130   a , and the bus  120   a  in the partial power system  101  via a branch  140   b . The measurement apparatus  44   a  transmits information  51  (generator output data D 1 ) and information  52  (generator phase data D 20 ) to the power system stabilization device  10 . 
     The generator control apparatus  160  configured to transmit a control command to the generator  110   a  in the partial power system  101  is similarly coupled to the power system stabilization device  10  via the communication network  300 , and receives information  58  (control command data D 8 ) from the power system stabilization device  10 . 
     Various kinds of data illustrated in  FIG. 3  may be communicated in the form including a specific number for identifying data and a timestamp in addition of the original data. 
     Returning to  FIG. 2 , the configuration of the power system stabilization device  10  is described. 
     The power system stabilization device  10  includes a display unit  11   a , an input unit  12   a  such as a keyboard and a mouse, the communication unit  13   a , a computer or computer server (CPU: Central Processing Unit)  14   a , a memory  15   a , and various kinds of databases (generator phase database  20 , generator phase difference database  22 , generator output database  21 , failure database  26   a , threshold and control database  23   a , determination result database  27   a , control command database  25 , and program database  28   a ). These components are coupled to a bus line  43   a.    
     The display unit  11   a  is configured as, for example, a display apparatus. Alternatively, for example, the display unit  11   a  may use a printer apparatus or a voice output apparatus in place of, or together with, the display apparatus. 
     The input unit  12   a  includes at least one of a pointing apparatus such as a keyboard switch and a mouse, a touch panel, and a voice instruction apparatus. 
     The display unit  11   a  and/or the input unit  12   a  is not necessarily required. 
     The communication unit  13   a  includes a circuit and a communication protocol used for connection to the communication network  300 . 
     The CPU  14   a  reads a predetermined computer program from a program database  24   a  and executes the read computer program. The CPU  14   a  may be configured as one or more semiconductor chips, or may be configured as a computer apparatus such as a calculation server. 
     The memory  15   a  is configured as, for example, a RAM (Random Access Memory). The memory  15   a  stores therein a computer program read from the program database  28   a , and stores therein calculation result data and image data necessary for each processing. Screen data stored in the memory  15   a  is transmitted to the display unit  11   a  and displayed. An example of the screen displayed on the display unit  11   a  is described later. 
     Referring to  FIG. 4 , stored contents in the program database  28   a  are described.  FIG. 4  is a diagram illustrating an example of program data in the power system stabilization device  10 . In this example, a generator phase difference calculation program P 10 , a generator energy calculation program P 20 , a threshold determination program P 30 , and a control command transmission program P 40  are stored in the program database  28   a.    
     Returning to  FIG. 2 , the CPU  14   a  executes calculation programs (generator phase difference calculation program P 10 , generator energy calculation program P 20 , threshold determination program P 30 , and control command transmission program P 40 ) read from the program database  28   a  into the memory  15   a , thereby calculating a generator voltage phase difference, calculating generator energy, calculating threshold determination, calculating a control command value, instructing image data to be displayed, and searching for data in various kinds of databases. 
     The memory  15   a  is a memory configured to temporarily store calculation temporary data and calculation result data, such as display image data, control data, and control result data. The CPU  14   a  generates and displays necessary image data on the display unit  11   a  (for example, display screen). The display unit  11   a  in the power system stabilization device  10  may be only a simple screen used to rewrite each control program and database. 
     As understood from  FIG. 2 , roughly divided eight databases are stored in the power system stabilization device  10 . The generator phase database  20 , the generator output database  21 , the generator phase difference database  22 , the threshold and control database  23   a , the control command database  25 , the failure database  26   a , and the determination result database  27   a  other than the program database  28   a  are described below. 
     In the generator phase database  20 , a voltage phase angle at the node  120   a  that couples the power system  100  and the partial power system  101  to each other is stored as the generator phase data D 20 . The voltage phase angle may be measured with a measurement apparatus using PMU or GPS. 
     In the generator output database  21 , the output of a generator or an electric power plant, which is a line power flow at the branch  140   a  coupled to the node  120   a  that couples the power system  100  and the partial power system  101  to each other is stored as the generator output data D 1 . A line power flow P is calculated from a current I and a voltage V measured by VT or PT, thereby measuring the output of the generator or the electric power plant. The output of the generator or the electric power plant may be the output of a generator for each axis or may be the total output of an electric power plant. 
     In the generator phase difference database  22 , the generator phase difference data D 2  at the node  120   a , which is calculated by the generator phase difference calculation unit  30   a  by using the voltage phase angle at the node  120   a  that couples the power system  100  and the partial power system  101  to each other, the voltage phase angle being stored in the generator phase database  20 . 
     Reference is now made to data in  FIG. 5 .  FIG. 5  is an example of the generator phase difference D 2  at the node  120   a . In this example, the generator phase difference data D 2  is stored for each location and time section. A method of calculating the generator phase difference data D 2  is described later. 
     In the threshold and control database  23   a , the threshold and control data D 3  is stored. Reference is now made to data in  FIG. 6 . In the data in  FIG. 6 , a failure condition, a control subject, and a threshold corresponding to each failure location are stored. Although not illustrated in  FIG. 6 , one or more thresholds may be present for one failure while divided on a time axis. The period of control is set in advance, and hence although not illustrated in  FIG. 6 , control is executed in a period determined in advance. 
     The control subject is basically one (uniaxial) generator, but may be a plurality of generators. 
     Although not illustrated in  FIG. 6 , first-stage control data D 11  described later is also stored in the control data. 
     In the control command database  25 , for example, a CB (Circuit Breaker) release signal to be transmitted from the power system stabilization device  10  to the generator control apparatus  160  is stored as the control command data D 5  to be issued when a threshold is exceeded. 
     In the failure database  26   a , the failure data D 6  to be transmitted from the failure detection apparatus  150  to the power system stabilization device  10  is stored. The location and condition of a failure are stored in the failure data D 6 . The power system stabilization device  10  collates the failure data D 6  with the threshold and control data D 3  to perform threshold determination, thereby determining a control detail to be executed. 
     In the determination result database  27   a , the determination result data D 7  is stored. Reference is now made to data in  FIG. 7 . In the data in  FIG. 7 , what kind operation has occurred at each time and a specific content of the operation are stored. For example, what kind of failure has occurred at a time point, what kind of data causes the threshold excess in the operation of its threshold determination, and what kind of control has been executed at a time point. Although not illustrated in  FIG. 7 , the value of generator energy that has used for determination is also stored. When stability has not exceeded a threshold or when control has failed, this fact is recorded in the determination result data D 7 . The power system stabilization device  10  notifies the central stabilizer  210  of the determination result data D 7 . 
     Next, calculation processing contents in the power system stabilization device  10  are described with reference to  FIG. 8 .  FIG. 8  is a flowchart illustrating an example of the whole processing in the power system stabilization device  10 . 
     First, the flow of the processing is briefly described. 
     The power system stabilization device  10  calculates a generator phase difference by using the generator output data D 1  and the generator phase data D 20  received from the measurement apparatus  44   a , and stores the generator phase difference data D 2  as the result of the calculation. The power system stabilization device  10  further calculates generator energy by using the generator output data D 1  received from the measurement apparatus  44   a  and the calculated generator phase difference data D 2 , and accumulates the generator energy in the memory  15   a . The power system stabilization device  10  further compares the calculated generator energy with a threshold in the threshold and control data D 3  received from the central stabilizer  210 , thereby determining whether the generator energy has exceeded the threshold. 
     When the generator energy has exceeded the threshold, the power system stabilization device  10  selects a control command by using the threshold and control data D 3  and the failure data D 6  received from the failure detection apparatus  150 , and transmits the control command data D 8  to the generator control apparatus  160 . The power system stabilization device  10  then transmits the determination result data D 7  to the central stabilizer  210 , and finishes the calculation. In this case, various kinds of calculation results and the data accumulated in the memory in the course of calculation may be transmitted to the central stabilizer  210  and sequentially displayed on a screen of the central stabilizer  210 . This configuration enables an operator to easily grasp operation states of the power system stabilization device  10 . The control command data D 8  is data on a control command such as a CB release signal, and is transmitted to the control board at the terminal equipment. 
     The power system stabilization device  10  may display, on the basis of the data described above, operating states on the screen, such as the states in which the power system is under monitoring, the threshold has been exceeded, and the control is being executed. This configuration enables an operator to easily grasp the operation states of the power system stabilization device  10 . The power system stabilization device  10  may display the generator output, or may display generator energy and/or the threshold determination result. 
     Until the control is executed, screen display for the states from the reception of various kinds of data to the transmission of the control command and determination result is repeated. 
     Details of the above-mentioned processing are described with reference to  FIG. 8 . 
     Reference is made to  FIG. 8 . First, in Step S 1 , the power system stabilization device  10  receives data necessary for the calculation of a generator phase difference, the calculation of generator energy, the threshold determination, and the selection of a control command. In this case, the power system stabilization device  10  automatically receives the failure data D 6  from the failure detection apparatus  150 . The power system stabilization device  10  automatically receives the generator output data D 1  and the generator phase data D 20  from the measurement apparatus  44   a  at a constant cycle, and automatically stores the generator output data D 1  and the generator phase data D 20 . The power system stabilization device  10  automatically receives the threshold and control data D 3  from the central stabilizer  210  at a constant cycle, and automatically stores the threshold and control data D 3 . 
     Next, in Step S 2 , the power system stabilization device  10  calculates a generator phase difference by using the generator phase data D 20  received in Step S 1 , and calculates and stores the generator phase difference data D 2 . 
     Referring to  FIG. 9 , the flow of calculating the generator phase difference is described.  FIG. 9  is a flowchart for describing an example of processing in the generator phase difference calculation unit.  FIG. 9  illustrates a method in which the generator phase data D 20  is read and when a failure has occurred, the generator phase difference data D 2  is calculated from the generator phase data D 20  through Steps S 11  to S 19 . The flow of the above-mentioned processing is described in detail below. 
     Reference is made to  FIG. 9 . First, in Step S 11 , the power system stabilization device  10  reads the generator phase data D 20  received in Step S 1  into the memory  15   a . Next, in Step S 12 , the power system stabilization device  10  continuously calculates a phase average value in a predetermined period of time and examines a temporal change of the phase average value, thereby determining whether a failure has occurred on the basis of the result of the examination. In this example, the temporal change of the phase average value is the generator phase difference data D 2 . 
     The failure determination may be based on one or more of a temporal change of the generator phase data D 20  and change amounts (voltage drops) and the like of other received data, such as the generator output data D 1 , the node voltage V, and the current I. For example, it may be determined that a failure has occurred when the amplitude of the voltage decreases and the phase of the voltage increases to be larger than a prescribed value. 
     When it is determined in the failure determination in Step S 12  that no failure has occurred, the flow returns to Step S 11 . 
     When a failure has occurred, in Step S 13 , in order to exclude a region where the voltage has transiently decreased due to the failure and the phase is not accurately calculated from the calculation, the power system stabilization device  10  calculates a calculation exclusion time in the region on the basis of a time point at which the phase starts changing and a time point at which the change of the phase ends. For example, when there is a period during which a change rate of the phase with respect to time exceeds a certain threshold for a predetermined period of time, this period may be set as the period from the start of the phase change to the end of the phase change. 
     In Step S 14 , the power system stabilization device  10  calculates an average of generator phases from one increment of sampling before the calculation exclusion time calculated in Step S 13  to a predetermined number of previous increments, thereby calculating a generator phase before the occurrence of the failure. Next, in Step S 15 , the power system stabilization device  10  calculates an average of generator phases from one increment after the calculation exclusion time calculated Step S 13  to a predetermined number of subsequent increments, thereby calculating a generator phase after the occurrence of the failure. 
     Now, an example of the calculation from Step S 12  to Step S 15  is illustrated in  FIG. 10 .  FIG. 10  is a diagram illustrating an example of calculating a generator phase difference immediately after a failure. 
     In the failure determination in Step S 12 , the power system stabilization device  10  determines that a failure has occurred in a period during which a phase average value in a predetermined period of time T set in advance has abruptly changed as illustrated in  FIG. 10 . The calculation exclusion time in Step S 13  is calculated by adding a margin set in advance to the period from the start of phase change to the end of phase change before and after the failure occurrent time. As illustrated in  FIG. 10 , T 0  is a calculation exclusion time, which is a continuous time region where a change equal to or more than a certain threshold has not occurred. The calculation of the generator phase before the occurrence of the failure in Step S 14  is executed in the manner that, as illustrated in  FIG. 10 , generator voltage phase angles δv in a predetermined period of time T 1  are averaged to determine an average value δv 1 , and the obtained average value δv 1  in the predetermined period of time T 1  is set as the generator phase before the occurrence of the failure. The calculation of the generator phase after the occurrence of the failure in Step S 15  is executed in the manner that, as illustrated in  FIG. 10 , generator voltage phase angles δv in a predetermined period of time T 2  are averaged to determine an average value δv 2 , and the obtained average value δv 2  in the predetermined period of time T 2  is set as the generator phase after the occurrence of the failure. 
     Returning to  FIG. 9 , in Step S 16 , on the basis of the generator phase before the occurrence of the failure and the generator phase after the occurrence of the failure determined in Steps S 14  and S 15 , the power system stabilization device  10  determines and stores an initial step amount Δδv 1  of the generator phase difference data D 2  by Expression (1). 
       [Math. 1] 
       Δδ V1 = δ V2   − δ V1     (1)
 
     Next, in Step S 17 , the power system stabilization device  10  calculates an average of a plurality of generator phases included in a cycle next to the cycle of the generator phase after the occurrence of the failure, that is, an average of generator phases from one increment after the generator phase used for the calculation in Step S 14  to a predetermined number of subsequent increments, thereby calculating a generator phase in the next cycle. 
     In Step S 18 , the power system stabilization device  10  calculates the generator phase difference data D 2  on the basis of a difference (time deviation) between the generator phase in the next cycle calculated in Step S 17  and the generator phase after the occurrence of the failure calculated in Step S 14 , and stores the calculated generator phase difference data D 2  in the memory. 
     Now, an example of the calculation from Step S 17  to Step S 18  is illustrated in  FIG. 11 .  FIG. 11  is a diagram illustrating an example of calculating a generator phase difference. 
     The calculation of the generator phase in the next cycle in Step S 17  is executed in the manner that, as illustrated in  FIG. 11 , generator voltage phase angles δv in a predetermined period of time T 3  are averaged to determine an average value δv 3 , and the obtained average value δv 3  in the predetermined period of time T 3  is set as the generator phase in the next cycle. The calculation of a generator phase after the next cycle is similarly executed in the manner that generator voltage phase angles δv in a predetermined period of time T 4  are averaged to determine an average value δv 4 , and the obtained average value δv 4  in the predetermined period of time T 4  is set as the generator phase after the next cycle. 
     In Step S 18 , on the basis of a pair of the generator phase after the occurrence of the failure and the generator phase in the next cycle and a pair of the generator phase in the next cycle and the generator phase after the next cycle, which are determined in Step S 17 , the power system stabilization device  10  determines the next step amount Δδv 2  and the second next step amount Δδv 3  of the generator phase difference data D 2  by Expression (2) and Expression (3), respectively, and stores the determined step amounts in the memory. 
       [Math. 2] 
       Δδ V3 = δ V4   − δ V3     (2)
 
       [Math. 3] 
       Δδ V2 = δ V3   − δ V2     (3)
 
     The generator voltage phase angle δv in  FIG. 10  and  FIG. 11  is calculated in consideration of a time delay by filtering, because the generator voltage phase angle δv is subjected to filtering in order to exclude harmonics from a transient change of the generator voltage phase angle δv. 
     Returning to  FIG. 9 , in Step S 19 , when a predetermined period of time has not elapsed from the start of the calculation of the generator phase difference data D 2 , the power system stabilization device  10  returns to Step S 17 , and when a predetermined period of time has elapsed, the power system stabilization device  10  finishes the flow and returns to Step S 11 . Even when a failure is not determined to have occurred, the generator phase difference data D 2  is calculated and held in the memory for a predetermined cycle, and updated. The calculation of the generator phase difference data D 2  may be executed by the measurement apparatus  44   a  or may be executed by the power system stabilization device  10 . While the failure determination is executed on the basis of the phase of the generator in this example, the failure determination may be executed on the basis of other indices such as the voltage and current of the generator and signals issued from the failure relay. 
     Returning to  FIG. 8 , in Step S 3 , the power system stabilization device  10  calculates generator energy by using the generator phase difference data D 2  calculated in Step S 2 , the generator output data D 1 , and the threshold and control data D 3 , and stores the calculated generator energy in the memory. 
     Referring to  FIG. 12 , the flow of the generator energy calculation is now described.  FIG. 12  is a flowchart illustrating an example of processing in the generator energy calculation unit. 
       FIG. 12  illustrates a method in which the generator output data D 1  and the generator phase difference data D 2  are used integrate the generator output with respect to a time deviation of the generator voltage phase angle and calculate the generator energy through Steps S 20  to S 22 . The flow of the above-mentioned processing is described in detail below. The generator phase difference data D 2  is hereinafter referred to also as “voltage phase angle time deviation Δδv”. 
     First, in Step S 20 , the power system stabilization device  10  reads the generator output data D 1  received in Step S 1  and the generator phase difference data D 2  calculated in Step S 2  into the memory  15   a . Next, in Step S 21 , the power system stabilization device  10  executes the integral calculation in the manner that rectangular areas formed by the generator voltage phase angle time deviation of the generator output for each predetermined time increment are integrated, thereby calculating the generator energy. 
     Referring to  FIG. 13 , an example of the calculation from Step S 20  to Step S 21  is now described.  FIG. 13  is a diagram illustrating an example of the generator energy calculation. 
     As illustrated in  FIG. 13 , the reading of each data in Step S 20  is started at a point at which a generator output Pg is a generator initial output Pg 0  and the voltage phase angle time deviation Δδv is 0, and is continued until a predetermined monitoring cancellation time has elapsed. The monitoring cancellation time is set in advance. 
     In Step S 21  in which the generator output is integrated with respect to the generator voltage phase angle time deviation to calculate generator energy, as illustrated in  FIG. 13 , the integral calculation is executed in the manner that, for each predetermined time increment, rectangular areas formed by the predetermined time and the generator voltage phase angle time deviation of the generator output are integrated. In a region where the generator output Pg is lower than the generator initial output Pg 0 , the area is calculated as acceleration energy. In a region where the generator output Pg is higher than the generator initial output Pg 0 , the area is calculated as deceleration energy. The generator energy may be calculated by integration of trapezoidal areas instead of integration of rectangular areas. 
     The acceleration energy and the deceleration energy based on the generator output Pg and the voltage phase angle time deviation Δδv as illustrated in  FIG. 13  can be determined by Expression (4) and Expression (5). The generator output Pg may be an electric power plant output, which is the sum of outputs of a plurality of generators included in an electric power plant. In this example, a time deviation of the voltage phase angle at the bus of the generator is used as the voltage phase angle time deviation, but the voltage phase angle time deviation may be a time deviation of a voltage phase angle at an electric power plant bus. In consideration of the relation between the distance and transmission delay time, a difference between the voltage phase at the bus of the generator or the electric power plant and a voltage phase at a bus at a predetermined distance from the generator may be used as the voltage phase angle time deviation. 
     
       
         
           
             
               
                 
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     where EA′ represents the acceleration energy and ED′ represents the deceleration energy. 
     Returning to  FIG. 12 , in Step S 22 , when a predetermined period of time has not elapsed from the start of the processing of the generator energy calculation, the power system stabilization device  10  returns to Step S 20 , and when a predetermined period of time has elapsed, the power system stabilization device  10  finishes the flow and proceeds to Step S 4 . 
     Returning to  FIG. 8 , in Step S 4 , the power system stabilization device  10  uses the generator energy calculated in Step S 3  and the threshold and control data D 3  to perform threshold determination for determining whether the generator energy exceeds a threshold. Generator energy Elimit′ is calculated by Expression (6). When the generator energy Elimit′ is smaller than a threshold Elimit as expressed by Expression (7), the generator energy is determined to be stable. 
       [Math. 6] 
         E′   A   +E′   D   =E′   limit   (6)
 
     where Elimit′ represents the generator energy. 
       [Math. 7] 
         E   limit   &gt;E′   limit  (where  E′   A   +E′   D &lt;0)  (7)
 
     where Elimit represents the threshold. 
     Referring to  FIG. 14  and  FIG. 15 , the flow of the threshold determination is now described.  FIG. 14  and  FIG. 15  are diagrams for describing an example of processing in the threshold value determining unit  32 .  FIG. 14  is an example where the generator energy exceeds the threshold.  FIG. 15  is an example where the generator energy does not exceed the threshold. In  FIGS. 14 and 15 , the solid lines represent the threshold, and the broken lines represent the calculated generator energy E. 
       FIG. 14  is an example where three periods are set for a threshold that changes with time. The first period (period (1)) starts from a time point at which, after the clearance of a failure, first-stage control is executed immediately when the failure has occurred by using the conventional control function included in the central stabilizer  210  and the power system stabilization device  10  to a threshold excess determination timing 1 set in advance. In the period (1), threshold excess determination is executed once at a timing at which Δt has elapsed from the execution of the first-stage control in order to confirm the effect of the first-stage control. Δt is set in advance to a value that takes a time period necessary for the calculation of measured data into consideration. 
     The second period (period (2)) is a period from the threshold excess determination timing 1 to the next threshold excess determination timing 2. The period width of the period (2) is set in advance. Also in the period (2), threshold excess determination is executed once at a timing at which Δt has elapsed from the threshold excess determination timing 1. Δt in the period (2) is not necessarily required to be the same as Δt in the period (1). 
     The third period (period (3)) is a period from the threshold excess determination timing 2 to a monitoring cancellation time. The period width of the period (3) is also set in advance. Also in the period (3), threshold excess determination is executed once at a timing at which Δt has elapsed from the threshold excess determination timing 2. Δt in the period (1) and the period (2) and Δt in the period (3) are not necessarily required to be the same. 
       FIG. 14  illustrates how the excess determination is executed on the above-mentioned threshold and periods when the generator energy changes as indicated by the dotted line. 
     In the threshold determination in the period (1), it is determined that the generator energy is less than the threshold and is stable. In the threshold determination in the period (2), however, it is determined that the generator energy exceeds the threshold and is unstable, and control is executed. This control decreases the generator energy, and in the threshold determination in the period (3), it is determined that the generator energy is less than the threshold and is stable. The control is executed immediately when the generator energy is determined to be unstable in the threshold determination in the period (2), and hence the threshold determination in the period (3) may be omitted. 
     Next,  FIG. 15  is an example where generator energy is determined to be less than a threshold and be stable in any of the period (1) to the period (3) and the control is unnecessary unlike  FIG. 14 . The number of periods and the monitoring cancellation time are determined in advance on the basis of one or both of a limit time necessary for the generator energy to be stable by the control and a first wave end time. 
     Returning to  FIG. 8 , when it is determined in Step S 4  that the generator energy has exceeded a threshold, in Step S 5 , the power system stabilization device  10  uses the failure data D 6  and the threshold and control data D 3  to select a control command associated with the condition that the failure has occurred, and stores the determination result indicating that the failure has occurred as well as the content of the selected control command. In this example, the determination result together with the control command is stored, but the control command is not necessarily required to be stored. 
     In Step S 6 , the power system stabilization device  10  transmits the control command selected in Step S 5  and the determination result stored in Step S 5  to the generator control apparatus  160  and the central stabilizer  210 , respectively, and finishes the processing. Then, the power system stabilization device  10  returns to Step S 1 . 
     When, for example, the communication traffic increases, the determination result is not necessarily required to be transmitted in real time in order to prevent reduce the communication traffic and prevent an overload on the communication network  300 . 
     In Step S 4 , when the generator energy does not exceed a threshold until the monitoring cancellation time in the threshold determination, the power system stabilization device  10  finishes the calculation and returns to Step S 1 . 
     The control (correction control) after the threshold determination described above is conventional control for correcting the first-stage control. 
     Next, calculation processing contents of the central stabilizer  210  are described. 
       FIGS. 16 to 18  are diagrams for describing a configuration example of the central stabilizer  210 .  FIG. 19  is a flowchart for describing an overall process of the central stabilizer  210 . The overall process is briefly described. First, system data D 9 , assumed failure data D 6 ′, search range data D 10 , and determination result data D 7  that are manually input or automatically received are used to perform state estimation and power flow calculation, thereby calculating and storing an appropriate system state. Examples of the system data D 9  include system topology, active power, reactive power, voltage, impedance, earth capacitance, and a transformer tapping ratio for a substation. The assumed failure data D 6 ′ is a list of failures to be controlled among possible failures. The search range data D 10  is the range of a power flow that can flow through a control subject location in a substation, and the range of the amplitude of a load value for stability limit search is determined by the search range data D 10 . Subsequently, stability calculation is performed on the assumed failure data D 6 ′ to determine a first-stage control detail for each failure that can occur in the power system indicated by the assumed failure data D 6 ′. After that, the stability limit is searched for, and generator energy at the stability limit is calculated. The calculated generator energy is used as a threshold, and a correction processing content corresponding to the threshold is calculated. The obtained results are transmitted to the power system stabilization device  10  as the first-stage control data D 11  and the threshold and control data D 3 . The control effect and the determination control result data D 7 , which are obtained when a failure has actually occurred and the first-stage control is executed and the threshold determination is performed, are displayed on a screen. The flow of the above-mentioned processing is described in detail below. Descriptions of contents overlapping with those of the power system stabilization device  10  described above with reference to  FIG. 1  to  FIG. 15  are omitted. 
       FIG. 16  is an example of an overall configuration diagram of the central stabilizer  210  according to this embodiment. The central stabilizer  210  includes a control detail determining unit  34  and various kinds of databases. The control detail determining unit  34  includes a state estimation/power flow calculation unit  35 , a stability calculation unit  36 , a first-stage control detail calculation unit  37 , a stability limit search unit  38 , a generator phase difference calculation unit  30   b , a generator energy calculation unit  31   b , and a threshold and correction control detail calculation unit  39 . The databases included in the central stabilizer  210  are a system database  29  storing the system data D 9 , an assumed failure database  26   b  storing the assumed failure data D 6 ′, a search range database  40  storing the search range data D 10 , a determination result database  27   b  storing the determination result data D 7 , a first-stage control database  41  storing the first-stage control data D 11 , a stability limit database  42  storing stability limit data D 12 , and a threshold and control database  23   b  storing the threshold and control data D 3 . 
     Data treated by the central stabilizer  210  are the system data D 9 , the assumed failure data D 6 ′, the search range data D 10 , the determination result data D 7 , the first-stage control data D 11 , the stability limit data D 12 , the threshold and control data D 3 , and the determination control result data D 7 . 
     The state estimation/power flow calculation unit  35  in the control detail determining unit  34  calculates and stores an appropriate system state by using the system data D 9 . The appropriate system state can be obtained, for example, by determining an assumed predetermined function coefficient from measured data by the method of least squares. The first-stage control detail calculation unit  37  in the control detail determining unit  34  determines a control detail of the first-stage control by using the system data D 9 , the state estimation result, the assumed failure data D 6 ′, and the stability calculation unit  36 . The stability limit search unit  38  in the control detail determining unit  34  searches for a stability limit by using the system data D 9 , the state estimation result as the appropriate system state, the search range data D 10 , the assumed failure data D 6 ′, and the stability calculation unit  36 . The generator phase difference calculation unit  30   b  in the control detail determining unit  34  calculates a generator phase difference on the basis of the stability calculation result. The generator energy calculation unit  31   b  in the control detail determining unit  34  calculates generator energy on the basis of the generator phase difference and the stability calculation result. The threshold and correction control detail calculation unit  39  in the control detail determining unit  34  calculates the generator energy as a threshold for each period, and determines a correction processing content by using the stability limit search result, the system data D 9 , the assumed failure data D 6 ′, and the stability calculation unit  36 . The control detail determining unit  34  transmits the first-stage control data D 11  and the threshold and control data D 3  to the power system stabilization device  10 , and receives the determination result data D 7  from the power system stabilization device  10 . The power system stabilization device  10  that has received the first-stage control data D 11  and the threshold and control data D 3  executes threshold determination. 
       FIG. 17  is a block diagram illustrating an example of a hardware configuration of the central stabilizer  210  and an overall configuration of the power system. In  FIG. 17 , the central power stabilizer  210 , the power system stabilization device  10 , the power system  100 , the partial power system  101  included in the power system  100 , and a generator  110   b  are coupled to the communication network  300 . 
     The power system  100  is coupled to the generators  110   a  and  110   b , transformers  130   a  and  130   b , and measurement apparatuses  44   a  and  44   b  via the branches  140   a  and  140   b , nodes  120   a  and  120   b , and nodes  121   a  and  121   b , respectively. Although not illustrated in  FIG. 17 , anyone or more of the failure detection apparatus  150 , a load, and other measurement apparatus and control apparatus are present. The generators  110   a  and  110   b  may each be, in addition to the generator as in this example, an electric power plant including a plurality of generators or a power generating facility of a power generation operator having a plurality of electric power plants. 
     The central stabilizer  210  has a hardware configuration in which a display unit  11   b , an input unit  12   b , a communication unit  13   b , a CPU  14   b , and a memory  15   b  that are similar to those in the power system stabilization device  10  and various kinds of databases (system database  29 , assumed failure database  26   b , search range database  40 , determination result database  27   b , first-stage control database  41 , stability limit database  42 , threshold and control database  23   b , and program database  28   b ) that are different from those in the power system stabilization device  10  are coupled to a bus line  43   b.    
     The configurations of the generator  110   b , the display unit  11   b , the input unit  12   b , the communication unit  13   b , the CPU  14   b , and the memory  15   b  are similar to those of the generator  110   a , the display unit  11   a , the input unit  12   a , the communication unit  13   a , the CPU  14   a , the memory  15   a , and the like respectively. 
     Referring to  FIG. 18 , stored contents in the program database  28   b  are described.  FIG. 18  is a diagram illustrating a configuration example of program data in the power system stabilization device  210 . In the program database  28   b , for example, a state estimation/power flow calculation program P 50 , a stability calculation program P 60 , a first-stage control detail calculation program P 70 , a stability limit search program P 80 , a generator phase difference calculation program P 10   b , a generator energy calculation program P 20   b , and a threshold and correction processing content calculation program P 90  are stored. The first-stage control detail calculation program P 70  and the threshold and correction processing content calculation program P 90  have the functions of transmitting controls contents and thresholds to the power system stabilization device  10 , respectively. The program group illustrated in  FIG. 18  is an example of a program group constituting a configuration example that is not minimum but basic. In another example, a program for adjusting the threshold and/or control detail on the basis of the determination result may be further provided. 
     Returning to  FIG. 17 , the CPU  14   b  executes calculation programs (state estimation/power flow calculation program P 50 , stability calculation program P 60 , first-stage control detail calculation program P 70 , stability limit search program P 80 , generator phase difference calculation program P 10   b , generator energy calculation program P 20   b , and threshold and correction processing content calculation program P 90 ) read from the program database  28   b  into the memory  15   b , thereby executing each processing of, for example, calculating a state estimation/power flow, calculating stability, calculating a first-stage control detail, searching for a stability limit, calculating a generator phase difference, calculating generator energy, calculating a threshold and a correction processing content, instructing image data to be displayed, and searching for data in various kinds of databases. The memory  15   b  is a memory configured to temporarily store calculation temporary data and calculation result data, such as display image data, control data, and control result data. The image data generated by the CPU  14   b  are displayed on the display unit  11   b  (for example, display screen). 
     Roughly divided eight databases are stored in the central stabilizer  210 . The system database  29 , the assumed failure database  26   b , the search range database  40 , the first-stage control database  41 , and the stability limit database  42  other than the program database  28   b , the determination result database  27   b , and the threshold and control database  23   b  are described below. 
     The system data D 9  in the system database  29  includes system configuration, line impedance, system measurement data (P, Q, V, I, Φ, time stamp-added data, and PMU data), data necessary for system configuration and state estimation (such as threshold for bad data), generator data, and other data necessary for power flow calculation and state estimation/stability calculation. For example, the generator data includes the concept of time, and generator outputs and generator phases may be accumulated in time series. The measurement value may be acquired from a central load dispatching center or an EMS (Energy Management System: power system supply and demand management server), or may be directly acquired from a measurement apparatus disposed at each location in the entire system. When data is manually input, data is manually input from the input unit  12   b  and stored. For manual input, predetermined image data may be generated by the CPU  14   b  and displayed on the display unit  11   b . For manual input, a complementary function for assisting an operation by an operator may be used such that a large volume of data can be easily set by semi-automatic input. 
     The assumed failure data D 6 ′ in the assumed failure database  26   b  includes, as illustrated in  FIG. 30 , a list of failure locations, failure conditions, and failure clearance timings as assumed failure cases in the power system. For example, the assumed failures in the assumed failure data D 6 ′ may be arranged in the order of severity. Depending on the system operation, only severe failure cases may be included in the assumed failure data D 6 ′. For example, screening based on severity may be performed to classify the assumed failure data D 6 ′ into a plurality of lists. 
     The search range data D 10  in the search range database  40  includes, as illustrated in  FIG. 29 , upper and lower limit values of a power flow fluctuation range for each area for the system data illustrated in  FIG. 22 . For example, the power flow fluctuation range illustrated in  FIG. 29  may be set on the basis of the result of measuring a power flow fluctuation of the power supply and/or load that changes in a predetermined cycle. 
     The first-stage control data D 11  in the first-stage control database  41  includes data that does not include a threshold in the threshold and control data D 3  illustrated in  FIG. 6 . 
     The stability limit data D 12  in the stability limit database  42  includes the relation between a power flow fluctuation amount and a generator internal phase difference angle first wave peak value in every area in the search process, and stability limit positions. 
     As described above, in this embodiment, the first-stage control by pre-calculation and the corrective stabilization control by post-calculation based on acceleration of generators are combined for a power flow fluctuation that is not assumed by pre-calculation. Consequently, the stability of the power system can be automatically maintained with less labor. 
     In this embodiment, a time deviation of the voltage phase at the bus of a generator or an electric power plant is used, and hence accurate stabilization control of the generator or the electric power plant can be executed. 
     As described in the modified example of this embodiment, when a difference between the voltage phase of the bus at a generator or an electric power plant and the voltage phase at a bus at a predetermined distance therefrom is used as the voltage phase angle time deviation, information corresponding to the time deviation can be obtained from the difference in voltage phase at these buses by relatively simple calculation. 
     In this embodiment, the deviation between average values of phase angles of outputs of generators in a predetermined period of time, which are calculated for each predetermined period of time in the time period excluding immediately before and after the occurrence of a failure, is used as generator phase difference data. Consequently, a fluctuation in measured values due to measurement errors or the like can be removed by averaging to improve the accuracy of stabilization control. 
     In this embodiment, the energy value calculated by using the generator output and the generator phase difference data is used as an index representing acceleration of the generator, and hence the acceleration of the generator can be accurately grasped from the acceleration energy and deceleration energy. An accurate acceleration index can be obtained by integral calculation based on the generator output and the generator phase difference data. 
     Next, calculation processing contents in the central stabilizer  210  are described with reference to  FIG. 19 .  FIG. 19  is a flowchart illustrating an example of the whole processing in the central stabilizer  210 . The flow of the processing is described below. 
     First, in Step S 31 , the central stabilizer  210  receives system data D 9  from the measurement apparatuses  44   a  and  44   b . The central stabilizer  210  further receives system data D 9  that is set by manual input using the input unit  12   b . For example, the central stabilizer  210  receives the system data D 9  periodically at a predetermined cycle. 
     In Step S 32 , the central stabilizer  210  uses the system data D 9  to create a system model from system connection information, power flow information, and the like. In Step S 33 , the central stabilizer  210  executes state estimation by the state estimation/power flow calculation unit  35 , and calculates and stores an appropriate system state. In Step S 34 , the central stabilizer  210  selects an assumed failure from the assumed failure data D 6 ′. In this case, in order to reduce the calculation volume, the central stabilizer  210  may execute screening for narrowing down assumed accidents to be controlled in accordance with predetermined conditions, rather than sequentially selecting all assumed accidents. In Step S 35 , the central stabilizer  210  uses the system data D 9 , the state estimation result, and the stability calculation unit  36  to calculate a control detail of first-stage control by the first-stage control detail calculation unit  37 . The content of the first-stage control involves, for example, repeating processing of calculating transient stability for an assumed failure, and when step-out has occurred, selecting a generator to be shed that has reached a threshold most early, and calculating transient stability in the power-controlled state, until a desired system state is achieved, for example, until the power system is stabilized without any step-out. In this case, the generator phase may be calculated by either of the measurement apparatus  44   a  or the power system stabilization device  10 . Examples of desired system states include a system state in which system voltage reactive power is stable, a system state in which consignable power is maximum, and a system state in which distribution loss is minimum. Theses system states can be calculated on the basis of system constraints. 
     Next, in Step S 36 , the central stabilizer  210  searches for a stability limit in the search range data D 10  in a power flow section of the state estimation result by using the assumed failure data D 6 ′ and the system data D 9  and using the stability limit search unit  38  and the stability calculation unit  36 . 
     Reference is now made to  FIG. 20 .  FIG. 20  is a flowchart illustrating an example of the stability limit search processing. 
     In Step S 41 , the central stabilizer  210  sets data on a power flow obtained by power flow calculation using the state estimation result as an initial power flow section. The initial power flow section is a power flow section at an operating point before a failure. 
     In Step S 42 , the central stabilizer  210  searches for a transient stability deteriorating direction. 
     Reference is now made to  FIG. 21 .  FIG. 21  is a flowchart illustrating an example of the processing for searching for the transient stability deteriorating direction.  FIG. 21  illustrates an example of processing from Step S 51  to Step S 64  respectively corresponding to areas A to C obtained by dividing the power system  100  as illustrated in  FIG. 22 . The number of the divided areas is determined in advance. The stability limit is searched for by varying the load amounts of loads  170   c  to  170   e  in the respective areas. 
     First, in Step S 51 , the central stabilizer  210  changes the directions of power flows in all the areas to increasing directions. The change width in this case and the change width use thereafter are predetermined increment widths set in advance. In Step S 52 , the central stabilizer  210  calculates transient stability. Only short analysis is necessary for the stability calculation because only a generator internal phase difference angle first wave peak value needs to be grasped. 
     In Step S 53 , the central stabilizer  210  changes the direction of the power flow in the area A to a decreasing direction, and in Step S 54 , the central stabilizer  210  calculates transient stability again. Next, in Step S 55 , the central stabilizer  210  compares the stabilities obtained by the transient stability calculation before and after the change of the direction of the power flow, thereby confirming whether the stability has deteriorated. 
     When the stability has deteriorated, in next Step S 56 , the central stabilizer  210  corrects the direction of the power flow in the area A to an increasing direction. When the stability has improved, on the other hand, the central stabilizer  210  maintains the direction of the power flow in the area A to the decreasing direction. 
     Next, in Step S 57 , the central stabilizer  210  changes the direction of the power flow in the area B to a decreasing direction, and in Step S 58 , the central stabilizer  210  calculates transient stability again. Next, in Step S 59 , the central stabilizer  210  compares the stabilities before and after the change of the direction of the power flow, thereby confirming whether the stability has deteriorated. 
     When the stability has deteriorated, in next Step S 60 , the central stabilizer  210  corrects the direction of the power flow in the area B to an increasing direction. When the stability has improved, on the other hand, the central stabilizer  210  maintains the direction of the power flow in the area B to the decreasing direction. 
     Next, in Step S 61 , the central stabilizer  210  changes the direction of the power flow in the area C to a decreasing direction, and in Step S 62 , the central stabilizer  210  calculates transient stability again. Next, in Step S 63 , the central stabilizer  210  compares the stabilities before and after the change of the direction of the power flow, thereby confirming whether the stability has deteriorated. 
     When the stability has deteriorated, in Step S 64 , the central stabilizer  210  corrects the direction of the power flow in the area C to an increasing direction. When the stability has improved, on the other hand the central stabilizer  210  maintains the direction of the power flow in the area C to the decreasing direction. 
     As described above, through the processing for searching for the direction in which the transient stability deteriorates, the change direction of the power flow in each area can be automatically set to the direction in which the stability deteriorates, thereby reducing the subsequent adjustment labor.  FIG. 23  is an example illustrating a power flow fluctuation in each area in the processing of searching for the direction in which the transient stability deteriorates. The directions of the power flows in all areas are changed to increasing directions (upper right in  FIG. 23 ), and after that, the power flow in each area is reduced to confirm the direction in which the stability deteriorates (transient stability deteriorating direction). 
     Returning to  FIG. 20 , in Step S 43 , the central stabilizer  210  sets a power flow fluctuation width (value initially used for stability limit search) for the transient stability deteriorating direction determined in Step S 42 . For setting the power flow fluctuation width, a power flow fluctuation value in a selected area that reaches a step-out determination threshold is calculated and set on the basis of a relation expression among an initial power flow section (operating point before accident) determined for the search of the stability deteriorating direction, a power flow fluctuation in the selected area at a section where the stability is deteriorated, and a generator internal phase difference angle peak value (first wave). An approximation formula is used as the relational expression. The approximation may be linear approximation or quadratic approximation. 
     In Step S 44 , the central stabilizer  210  creates and saves a power flow section for the case where the power flow fluctuation set in Step S 43  occurs. 
     In Step S 45 , the central stabilizer  210  executes transient stability calculation at the power flow section created in Step S 44 . Then, the central stabilizer  210  compares the previous and current transient stabilities. When the transient stability has changed from a transient unstable state to a transient stable state or changed from the transient stable state to the transient unstable state, the central stabilizer  210  inverts the search direction. 
     Next, in Step S 46 , the central stabilizer  210  determines whether a transient unstable power flow section has appeared in the past processing process. When there is a transient unstable power flow section, the central stabilizer  210  resets the power flow fluctuation width set in Step S 44  to be halved, and proceeds to the next step. When there is no transient unstable power flow section, on the other hand, the central stabilizer  210  resets the power flow fluctuation width set in Step S 44  to be doubled, and proceeds to the next step. 
     In Step S 48 , the central stabilizer  210  compares the power flow fluctuation width set in this case with a threshold. When the power flow fluctuation width is equal to or more than the threshold, the central stabilizer  210  returns to Step S 44 . When the power flow fluctuation width is equal to or less than the threshold, in Step S 49 , the central stabilizer  210  saves the power flow section calculated last as a stability limit. At the time of saving the power flow section as a stability limit, an unstable power flow section under the last or second last search conditions is also saved. 
     The stability limit is searched for as described above. The stability limit is searched in accordance with the above-mentioned flow under constraints of the search range data D 10 . 
     In this example, a stability limit is searched for while the power flow fluctuation width is set by binary search. In another example, the power flow fluctuation width may be set by random numbers in a maximum fluctuation range, and a stability limit may be searched for by the Monte Carlo method. The search for a stability limit may employ, for example, a search method using a PSO (Practice Swarm Optimization) and optimum power flow calculation in combination. A stability limit may be searched for by another search method. 
     Returning to  FIG. 19 , in Step S 37 , the central stabilizer  210  determines a threshold and a correction processing content. 
     Reference is now made to  FIG. 24 .  FIG. 24  is a flowchart showing an example of the flow of processing from the determination of a shedding generator (control subject generator) to the calculation of generator energy and the calculation of a threshold for each period. 
     In Step S 71 , the central stabilizer  210  reads the calculation result of transient stability in an unstable power flow section closest to the stability limit calculated during the stability limit search, which is saved in Step S 49 , into the memory  15   b . The unstable power flow section is a power flow section that is saved as an unstable power flow section in the last or second last search conditions at the time of saving the power flow section as a stability limit. 
     In Step S 72 , the central stabilizer  210  performs stability analysis using the unstable power flow section read in Step S 71 , and determines generator to be controlled on the basis of the result of the stability analysis. In this case, similarly to the method of selecting a generator subjected to first-stage control, a generator that has reached a step-out determination threshold most early in the unstable power flow section is selected as a generator to be controlled. For example, the number of generators to be shed is increased, or a shedding generator with a larger capacity is selected again. 
     In another example, in Step S 71 , the power flow section at the stability limit may be used as an unstable power flow section. In this case, in Step S 72 , a generator having the largest generator internal phase difference angle first wave peak value is selected. 
     In Step S 73 , the central stabilizer  210  calculates transient stability by the stability calculation unit  36  using the system data D 9  and the assumed failure data D 6 ′ in the same unstable power flow section, and in Step S 74 , the central stabilizer  210  determines whether the transient stability as the calculation result is stable. When the transient stability is unstable, the central stabilizer  210  increases the number of generators to be shed until the transient stability is stable. In this case, an upper limit of the number of generators to be shed is set in advance. When the transient stability is stable, on the other hand, the central stabilizer  210  finally determines the selected control subject generator, and in Step S 75 , saves data on the control subject generator. 
     Reference is now made to  FIG. 25 .  FIG. 25  is a diagram for describing processing in the stability limit search unit  38 .  FIG. 25  illustrates an example of the manner of stability limit search in a relational diagram of the power flow fluctuation amount in each area and the generator internal phase difference angle first wave peak value, and an image of an unstable power flow section used for the search and the selection of stability limit and control subject generator, and an image of a method of determining a control subject generator and the determination of correction processing contents. 
     How the stability limit search is executed by binary search is indicated by the broken-line arrow. As an image of a power flow section in a region where the generator internal phase difference angle first wave peak value is equal to or more than a step-out determination threshold, a transient state in which the generator internal phase difference angle exceeds the step-out determination threshold is illustrated. As an image of a power flow section in a region where the generator internal phase difference angle first wave peak value is equal to or less than the step-out determination threshold, a transient state in which the generator internal phase difference angle does not exceed the step-out determination threshold but converges is illustrated. 
     Next, in Step S 76 , the central stabilizer  210  reads the power flow section at the stability limit and the calculation result of the transient stability into the memory  15   b . In Step S 77 , the central stabilizer  210  calculates a generator phase difference of the control subject generator saved in Step S 75  by the generator phase difference calculation unit  30   b  in the calculation result of the transient stability of the power flow section at the stability limit. The calculation of the generator phase difference and the calculation of the generator energy in this case are performed by processing similar to that in the power system stabilization device  10 . The generator phase differences are calculated until the first wave peak value, and are integrated for each period to calculate generator energy and determine a threshold. In Step S 78 , the central stabilizer  210  calculates generator energy by the generator energy calculation unit  31   b.    
     Reference is now made to  FIG. 26 .  FIG. 26  is a diagram illustrating a waveform of generator output Pg-voltage phase angle time deviation Δδv, which is illustrated by two time-series waveforms of generator output Pg-time t and voltage phase angle time deviation Δδv-time t. 
     In  FIG. 26 , a hatched region where the generator output Pg is smaller than the initial generator output Pg 0  represents acceleration energy, and another hatched region where the generator output Pg is larger than the initial generator output Pg 0  represents deceleration energy. Locations denoted by the same numerals [1] to [6] in  FIG. 26  indicate corresponding locations in the respective graphs. As illustrated at the upper right in  FIG. 26 , generator energy can be calculated by integrating the generator output Pg with the voltage phase angle time deviation Δδv. 
     The acceleration energy and the deceleration energy based on the generator output Pg and the voltage phase angle time deviation Δδv can be determined by Expression (8) and Expression (9), respectively. The generator output Pg may be the sum of outputs of a plurality of generators included in an electric power plant. The voltage phase angle time deviation may be an electric power plant bus voltage phase angle time deviation. 
     
       
         
           
             
               
                 
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     where EA represents the acceleration energy and ED represents the deceleration energy. 
     Returning to  FIG. 24 , in Step S 79 , the central stabilizer  210  calculates generator energy for each period by the threshold and correction control detail calculation unit  39 , and sets the calculated generator energy as a threshold. In this case, the threshold is determined by the sum of acceleration energy and deceleration energy, and can be determined by Expression (10). 
       [Math. 10] 
         E   A   +E   D   =E   limit   (10)
 
     where Elimit represents the threshold. 
     Reference is now made to  FIG. 27 .  FIG. 27  is a diagram for describing the processing in the threshold and correction control detail calculation unit  39 .  FIG. 27  illustrates time divided images of generator energy for calculating thresholds for the periods (1) to (3).  FIG. 27  illustrates how to calculate the thresholds for the periods (1) to (3) in ascending order. The threshold for each period is determined on the basis of generator energy that is determined by integral calculation from failure clearance to each period. Calculating generator energy in a time division manner can provide thresholds for the respective periods. This configuration enables a threshold to be set for a severe failure. Depending on a failure, there is not so much temporal margin from first-stage control to correction control, and hence it is necessary to the period to be short. 
     Returning to  FIG. 24 , in Step  79 , the central stabilizer  210  calculates and saves the generator energy as a threshold for each period in the manner described above. 
     Returning to  FIG. 19 , in Step S 38 , the central stabilizer  210  determines whether Steps S 33  to S 37  have been finished and the first-stage control data D 11  and the threshold and control data D 3  have been determined for all assumed failures. When the processing has not been finished for all assumed failures, the central stabilizer  210  returns to Step S 34 . When the processing has been finished and the first-stage control data D 11  and the threshold and control data D 3  have been finished for all assumed failures, the central stabilizer  210  proceeds to next Step S 39 . 
     In Step S 39 , the central stabilizer  210  transmits the determined first-stage control data D 11  and threshold and control data D 3  to the power system stabilization device  10 . This transmission cycle is, for example, a constant cycle determined in advance. 
     The central stabilizer  210  may display operating states, such as the state in which the power system is under monitoring, the threshold has been exceeded, and the control is being executed, on the screen. This configuration enables an operator to easily grasp the operation states of the power system stabilization device  10 . In this case, until the control is executed, the states from the reception of various kinds of data to the transmission of the control command and determination result may be repeatedly displayed on the screen. Further, the displaying of generator output, generator energy, and threshold determination result enables an operator to examine later whether the control determination was correct. 
     Reference is now made to  FIG. 28 .  FIG. 28  is an example of a time chart illustrating timings of failure occurrence and each control of the power system stabilization device  10 .  FIG. 28  is an example where correction control based on determination of threshold excess determination timing 2 is executed in addition to the first-stage control. The time chart as in  FIG. 28  may be displayed on the screen of the central stabilizer  210 . This configuration is advantageous in that an operator can easily grasp the control timings and operations therefor. 
     Reference is now made to  FIG. 31 .  FIG. 31  is a graph illustrating a temporal change of the generator voltage phase angle δv. The graph as in  FIG. 31  may be displayed on the screen of the central stabilizer  210 . An operator can grasp control effects of the power system stabilization device  10  at a glance. The operator can save and display stabilization measures for past assumed accidents, which are used as a reference for creating a system plan. 
     As described above, in this embodiment, a stability limit at which the power system becomes unstable if the power flow of the power system is further changed when the power flow of the power system is changed from a stable state such that stability is deteriorated for each failure that possibly occurs in the power system is determined, and a value of the acceleration index at the stability limit is determined as the threshold. Consequently, a threshold appropriate for each failure can be determined. 
     In this embodiment, a plurality of thresholds may be determined for an elapsed time from the occurrence of a failure. With this configuration, it can be determined a plurality of times with the lapse of time whether the acceleration index exceeds a threshold, and appropriate determination results with the lapse of time can be obtained. 
     The above-mentioned embodiments of this invention are illustrative for describing this invention and are not intended to limit the scope of this invention to the embodiments. A person skilled in the art can carryout this invention in various other forms without departing from the gist of this invention. 
     REFERENCE SIGNS LIST 
     
         
           10  Power system stabilization device 
           11   a ,  11   b  Display unit 
           12   a ,  12   b  Input unit 
           13   a ,  13   b  Communication unit 
           14   a ,  14   b  CPU 
           15   a ,  15   b  Memory 
           20  Generator phase data 
           21  Generator output database 
           22  Generator phase difference database 
           23   a ,  23   b  Threshold and control database 
           24   a ,  24   b  Program database 
           25  Control command database 
           26   a  Failure database 
           26   b  Assumed failure database 
           27   a ,  27   b  Determination result database 
           28   a ,  28   b  Program database 
           29  System database 
           30   a ,  30   b  Generator phase difference calculation unit 
           31   a ,  31   b  Generator energy calculation unit 
           32  Threshold value determining unit 
           33  Control command unit 
           34  Control detail determining unit 
           35  state estimation/power flow calculation unit 
           36  Transient stability calculation unit 
           37  First-stage control detail calculation unit 
           38  Stability limit search unit 
           39  Threshold and correction control detail calculation unit 
           40  Search range database 
           41  First-stage control database 
           42  Stability limit database 
           43   a ,  43   b  Bus line 
           44   a ,  44   b  Measurement apparatus 
           51  Generator output data 
           52  Generator phase data 
           53  Threshold and control data 
           56  Failure data 
           57  Determination control result data 
           58  Control command data 
           59  First-stage control data 
           100  Power system 
           105  Bulk power system 
           101 ,  111 - 113  Partial power system 
           110   a - 110   c  Generator 
           120   a - 120   e ,  121   a - 121   e  Node (bus) 
           130   a - 130   e  Transformer 
           140   a - 140   f ,  141   a - 141   e  Branch (line) 
           150  Failure detection apparatus 
           160  Generator control apparatus 
           170   c - 170   e  Load 
           210  Central stabilizer 
           300  Communication network