Abstract:
A system for real-time monitoring of synchrophasors from an electrical power system serving multiple geographic areas can control system instability, without preset parameters or thresholds, detects power flow between the generation sources. If a change in power flow of at least 5% over at time period of 1 second or less occurs and the response to the change in power flow is greater than the change in power flow a signal is generated for corrective action.

Description:
BACKGROUND 
       [0001]    This invention pertains to the field of electrical power systems, and more specifically to a system for remedying electrical power system instability. 
         [0002]    Traditionally, electric utilities utilize complex system models of electrical power systems to examine thousands of simulated events that have the potential to cause reliability problems in delivering electricity to customers. One event that causes problems for systems having connected generation in different geographic areas is transient instability. For example, power swings caused by faults on transmission lines can result in synchronism loss (out-of-step) between electrical power systems. A swing relay is typically installed to detect such power swings. A modeling system instructs the swing relay to take appropriate action when synchronism loss is detected. The swing relay can attempt to remedy the power swings by running back generators or tripping generators offline to restore the electrical power systems to a stable state. 
         [0003]    Referring to  FIG. 1 , there is shown a flowchart of a prior art monitoring system  100  for monitoring electrical power system stability. System status data  102  is compared with a simulated model  104  of instability and a result is produced. The result of the comparison is evaluated  106  to determine if a parameter or a criterion of the model has been violated. If a parameter or criterion has been violated  106 , then a corrective action instruction  108  is transmitted to maintain system stability. The corrective action instruction  108  can be, for example, a generator rollback or a generator trip based upon the simulated model. 
         [0004]    There are deficiencies in the prior art systems. For example, there is a delay inherent in the system because the simulated models require time consuming calculations to instruct the swing relay. This delay can cause a system crash with attendant blackouts. Also, the modeled events cannot cover all possible real-world events and thus can leave the electrical system vulnerable to non-predicted impact events that can cause power outages, blackouts or brownouts. 
         [0005]    Accordingly, there is a need for a system for stabilizing electrical power systems that identifies impact events and quickly responds to the impact event in real-time. 
       SUMMARY 
       [0006]    A method of determining instability in an electrical power system serving at least first and second geographic areas, each geographic area comprising at least one generation source. The method comprising the steps of a) receiving first synchrophasor measurements from a first generation source in the first geographic area; b) receiving second synchrophasor measurements from a second generation source in the second geographic area, the second generation source being geographically separated from the first generation source; c) comparing the first synchrophasor measurements to the second synchrophasor measurements in a first time period to yield a first comparison measurement and then again in a second time period to yield a second comparison measurement; and d) transmitting a remedy control command to the first generation source, the second generation source or both the first generation source and the second generation source when a difference between the first and the second synchrophasor comparison indicates an instability. 
         [0007]    A method of determining instability in an electrical power system serving at least first and second geographic areas, each geographic area comprising a generation source. The method comprising the steps of: a) receiving first synchrophasor measurements from a first generation source in the first geographic area; b) receiving second synchrophasor measurements from a second generation source in the second geographic area, the second generation source being geographically separated from the first generation source; c) comparing the first synchrophasor measurements to the second synchrophasor measurements in a first time period to yield a first comparison measurement and then again in a second time period to yield a second comparison measurement, the first and second comparison measurements differing by a sufficient amount to indicate an instability in the system; and d) transmitting a remedy control command to remedy the instability. 
         [0008]    Transmitting comprises transmitting the remedy control command to the first generation source, the second generation source or both the first generation source and the second generation source. The remedy control command is transmitted when the first and second comparison measurements differ by at least 5%. The remedy control command transmitted is a trip generator command to take the first generation source, the second generation source or both the first generation source and the second generation source offline, a trip load command to take the load in the first generation source, the load in the second generation source or both the load in the first generation source and the second generation source offline or a run-back generator command to reduce energy flow from the first generation source, the second generation source or both the first generation source and the second generation source. The remedy control command transmitted is to generate an alarm. 
         [0009]    A system for controlling an electrical power system serving at least first and second geographic areas, each geographic area comprising a generation source, the system comprising: a) one or more transceivers for receiving first synchrophasor measurements from a first generation source in the first geographic area and second synchrophasor measurements from a second generation source in the second geographic area, the second generation source being geographically separated from the first generation source; b) a monitor communicatively coupled to the transceivers and configured for comparing the an absolute value of the first synchrophasor measurements to the absolute value of the second synchrophasor measurements in a first time period to yield a first comparison measurement and then again in a second time period to yield a second comparison measurement, the first and second time periods differing by at least 1/30 of a second; and c) an alarm communicatively coupled to the monitor for generating a remedy control command when the first and second comparison measurements differ by a sufficient amount to indicate an instability in the system. The system monitor is configured to indicate an instability when the second comparison measurement is at least 5% greater than the first comparison measurement. The system alarm is configured to transmit a remedy control command when an instability is indicated. The remedy control command comprises a rollback command or a disconnect command to one or more of the generation sources, or a disconnect command to one or more of loads. 
         [0010]    A method of controlling an electrical power system serving two geographic areas, each geographic area having a generation source, the method comprising the steps of: a) monitoring power flow between the geographic areas using synchrophasors at a location between the generation sources; b) detecting a change in the monitored power flow; c) detecting a system response to the change in power flow using synchrophasors; d) comparing the change in power flow and the system response; and e) generating a corrective signal if the system response is greater than the change in the monitored power flow. The step of comparing comprises determining that the system response is greater than the change in power flow, and wherein a corrective signal is generated. 
         [0011]    A system for controlling an electrical power system serving two geographic areas, each area having a generation source, comprising: a) a first generation source; b) a second generation source electrically connected to the first generation source; c) a power flow detector electrically connected to both the first generation source and the second generation source, the power flow detector being between the generation sources, where the power flow detector is capable of detecting a change in power flow using synchrophasors; and d) a transmitter communicatively coupled to the first generation source and the second generation source capable of generating a remedy signal for taking corrective action. 
         [0012]    A method of controlling an electrical power system serving two geographic areas, each geographic area having a generation source, the method comprising the steps of: a) monitoring power flow between the geographic areas using synchrophasors at a location between the generation sources; b) detecting a change in the monitored power flow using synchrophasors; c) calculating an impact energy and a reverse energy; d) comparing the reverse energy and the impact energy; and e) generating a corrective signal if the reverse energy is greater than impact energy. The step of comparing comprises determining that the reverse energy is greater than the impact energy, and wherein a corrective signal is generated. 
     
    
     
       DRAWINGS 
         [0013]    These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0014]      FIG. 1  is a flowchart of a prior art monitoring system for controlling electrical power system stability; 
           [0015]      FIG. 2  is a block diagram of an electrical power system and control station for monitoring electrical power system stability, the system having features of the present invention; 
           [0016]      FIG. 3  is a Venn diagram of an electrical power system for which the system of  FIG. 2  is useful; 
           [0017]      FIG. 4  is a block diagram of a generator system of the electrical power system of  FIG. 3 ; 
           [0018]      FIG. 5  is graph of power versus time showing impact energy and recovery energy for the electrical power system of  FIG. 2  in condition related to instability; and 
           [0019]      FIG. 6  is a flowchart of a system for monitoring and controlling electrical power system stability according to one embodiment of the present invention. 
       
    
    
     DESCRIPTION 
       [0020]    The present invention overcomes limitations of the prior art by providing a real time system to detect and remedy electrical power system transient instability, preferably without the use of simulated equivalent models requiring preset parameters or criteria. The present invention effectively remedies more electrical power system instabilities, in real-time, than can be predicted using simulated events alone. 
         [0021]    As used in this disclosure, except where the context requires otherwise, the term “comprise” and variations of the term, such as “comprising”, “comprises” and “comprised” are not intended to exclude other components or steps. 
         [0022]    The term “syncrophasor” means the phasor portion of an alternating current (AC) power signal representing both the magnitude and phase angle as referenced to an absolute point in time. Techniques for measuring sychrophasors are described in IEEE Std C37.118™-2005 and IEEE Std 1344-1995. 
         [0023]    The term “power flow” refers to the rate of flow of electrical energy transmitted from a first location to a second location utilizing the electrical grid. 
         [0024]    The term “trip generator command” refers to a command to disconnect a generation source from an electrical load connected to the generation source. 
         [0025]    The term “roll back generator command” refers to a command to a generation source to reduce the amount of power flow such as, for example, slowing down an electrical turbine to reduce the amount of electrical energy produced, shunting a portion of the electricity to ground, and the like. 
         [0026]    The term “absolute value” refers to a numerical value without regard to its sign, such as, for example, 1 is the absolute value of both 1 and -1. 
         [0027]    The term “impact event” refers to a positive or negative electrical phase swing between two or more electrical power generation sources that can be measured, where the power flow from a first generation source to a second generation source is denoted as a function of time: P(t). 
         [0028]    The term “impact energy” (IE) means the measurable area of a change in power (ΔP) between two or more generation sources in the first half cycle of an impact event for a given time period (t 0  to t 1 ) as denoted in the equation: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0029]    The term “reverse energy” (RE) means the measurable area of a change in power (ΔP) between two or more generation sources in the second half cycle of an impact event for a given time period (t 1  to t 2 ) denoted in the equation: 
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         [0030]    The term “alarm” refers to an audio, visual and/or tactile device that serves to call attention, to warn, or provide information of instability in an electric power system. 
         [0031]    The term “geographically separate” refers to a child region that is physically separated from, yet not autonomous of its parent region. 
         [0032]    Referring to  FIG. 2 , a system  200  for measuring synchrophasors to detect instability in an electrical power system and signaling that instability is presented. A first electrical load  202  is electrically connected to a first generation source  204 , a second electrical load  206  is electrically connected to a second generation source  208 , continuing through to an nth electrical load  210  that is electrically connected to an nth generation source  212 . The generation sources  204 ,  208  and  212  can be any source of electricity that provides electricity to an electrical power distribution system such as, for example, a cogeneration plant, a fossil fuel generation plant, a geothermal generation plant, a solar generation station, a solar turbine station, wind turbines, and combinations thereof. The generation sources  204 ,  208  and  212  can be located in separate geographical locations or within the same geographical location such as, for example, a micro-cogeneration facility in a commercial building, or a residence with solar panels that each provides electricity to the same transmission lines that a utility uses to transmit electricity. 
         [0033]    The generation sources  204 ,  208  and  212  further comprise generator transceivers  214 ,  216  and  218 , respectively, that are communicatively coupled to a control station  220 . The control station  220  comprises a control station transceiver  222 , a monitoring device  224 , and an alarm  226 . The control station transceiver  222  is communicatively coupled to the generator transceivers  214 ,  216  and  218 . The generator transceivers  214 ,  216 ,  218  and  222  are configured to transmit synchrophasor measurements and receive remedy control commands such as, for example, roll back generator, or turn off generator commands, etc. The monitoring device  224  is configured to perform synchrophasor calculations as described herein. The monitoring device  224  comprises hardware, software, or both hardware and software for performing calculations for comparing the synchrophasor measurements such as, for example, a computer with software, a microprocessor or a hardwired analog computer. 
         [0034]    The remedy control command can be a roll-back command, for slowing down the generation sources  204 ,  208  and  212 , a trip generator command to turn off at least one of the generation sources  204 ,  208  and  212  or load trip to disconnect end users from the electrical power grid  204 ,  208  and  212  for remedying an electrical power instability in at least one of the generation sources  204 ,  208  and  212  and the electrical loads  202 ,  206  and  210 . Optionally, the control station  220  can activate an alarm  226  to alert a user that there is a an impact event that is being corrected or that the user is to perform manual intervention, such as, for example, trip at least one of the generation sources  204 ,  208  and  212 , to remedy the electrical power instability. 
         [0035]    With reference to  FIG. 3  an electrical power system can be considered as two portions, a first area  302  and a second area  304 . The first area  302  can comprise multiple generation sources where an electrical problem is located. The second area  304  can also comprise multiple generation sources that would be affected if the electrical problem in the first area  302  is not remedied. The areas  302  and  304  can be geographically separated from each other or one of the areas  302  and  304  can be located in the same geographical area as the other area (i.e. one area is nested inside the other area). 
         [0036]    Each area  302  and  304  can be represented as an equivalent generator system  400 , as shown in  FIG. 4 , the first area  302  is represented by a first generation source  402  and the second area  304  is represented by a second generation source  404 . Power flow (P) transmitted from the first generation source  402  to the second generation source  404  is a function of time, denoted as P(t), and can be measured using synchrophasors. 
         [0037]    A technique, according to one embodiment, that identifies impact events and quickly responds to remedy the impact event in real-time, comprises the steps of calculating the change in energy (ΔE) between two or more generation sources by subtracting the absolute value of the impact energy (IE) found in equation (1) from the absolute value of the resultant energy (RE) found in equation (2) as shown in the following equation: 
         [0000]      Δ E=|IE|−|RE|   equation (3) 
         [0038]    One method of calculating IE and RE is to calculate an area under each curve, measured in real-time by synchrophasors, that is produced by an impact event as shown in  FIG. 5 . The chart  500  is a plot of the areas comprising the impact energy  502  (IE), determined by receiving first synchrophasor measurements from a first generation source  402  in the first geographic area and second synchrophasor measurements from a second generation source  404  in the second geographic area in a first time period, and the reverse energy  504  (RE), determined by receiving first synchrophasor measurements from a first generation source  402  in the first geographic area and second synchrophasor measurements from a second generation source  404  in the second geographic area in a second time period, versus time. The impact energy  502  is the energy received in a first-half-swing of an electrical power system disturbance, measured between the first area  402  and the second area  404 . The reverse energy  504 , is the energy the electrical power system releases, measured between the first area  402  and the second area  404 , in the second-half-swing of an electrical power system disturbance measured between the first area  402  and the second area  404 . 
         [0039]    The area under the plot of the impact energy  502  is calculated by: 
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         [0000]    where ΔP is the change in power measured by a synchrophasor, t is the time in seconds, t 0  is the time when the impact starts, and t 1  is the time when the impact energy  502  crosses the power flow P(t 0 ) measured at the beginning of an impact event. The area under the plot of the reverse energy  504  is calculated by: 
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         [0000]    where ΔP is the change in power measured by a synchrophasor, t is the time in seconds, t 2  is the time when the change in power re-crosses the power flow P(t 0 ) measured at the beginning of the impact event. 
         [0040]    Referring now to  FIG. 6  there is shown a flowchart  600  of a system for monitoring electrical power system stability according to one embodiment of the present invention. First, synchrophasor data  602  is measured. Then, the impact energy of the first swing is calculated  604  by the following: 
         [0000]      Δ E=|IE|−|RE|   equation (3) 
         [0041]    Then it is determined  606  if the change in energy is greater than zero, ΔE&gt;0. If ΔE is greater than 0, sequential power swings can drive the power flow to divergence and system instability. Thus, if the result of the calculation, ΔE, is greater than zero, then a remedy command  608  is transmitted. Otherwise, when the energy flow difference is less than zero, ΔE&lt;0, sequential power swings are suppressed to convergence and system stability, and thus no remedy is required. The calculation of the energy flow difference, in this embodiment, can be completed quickly and efficiently in real-time with dedicated hardware, such as, for example, an analog style computer, a digital microprocessor, or a computer with software programmed to perform the calculations. Additionally, there is no need for simulated models or parameters because the calculations are effective for power instability occurrences. However, simulated models can be used as a backup or as a first line of defense. Optionally, a threshold can be set to trigger an alarm for user intervention or notification of an impact event. 
         [0042]    Although the present invention has been discussed in considerable detail with reference to certain preferred embodiments, other embodiments are possible. Therefore, the scope of the appended claims should not be limited to the description of preferred embodiments contained in this disclosure. All references cited herein are incorporated by reference in their entirety.