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Provided is a method and device for assessing and monitoring voltage security in a power system. More specifically, a method and device for assessing and monitoring the value of reactive power load when changes in reactive power outputs of at least some of the generators in the electrical power syst

Provided is a method and device for assessing and monitoring voltage security in a power system. More specifically, a method and device for assessing and monitoring the value of reactive power load when changes in reactive power outputs of at least some of the generators in the electrical power system cause all of the generators in the system to reach the combined operating limit of their reactive power output. In response thereto, the method and device are further adapted to initiate suitable control measures such as switching of transformer banks and/or capacitor/reactor banks, as well as shedding loads whenever necessary to mitigate an impending voltage stability problem.

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1. A device for assessing and monitoring voltage stability in an electrical power system including a plurality of generators and phasor measurement units, said phasor measurement units adapted to acquire and communicate phasor measurements from various locations on the power system, the device compr

1. A device for assessing and monitoring voltage stability in an electrical power system including a plurality of generators and phasor measurement units, said phasor measurement units adapted to acquire and communicate phasor measurements from various locations on the power system, the device comprising: a communications port adapted to receive the phasor measurements from the phasor measurement units,a predictive reactive power load limit calculator to calculate a reactive power loading margin where the plurality of generators will reach their respective and combined reactive power limits using a small number of power-flow runs, the calculator in communication with the communications port and receiving the phasor measurements, and the calculator adapted to: calculate a predictive generator reactive power sensitivity indicating an anticipated change in generation reactive power output resulting from a predictive change in reactive power load at a specified reactive power load using the phasor measurements, wherein the magnitude of the sensitivity is related to the proximity to a static limit;determine, based on the sensitivity, a load increment used for calculating the specified reactive power load for the next power-flow run, and,when the sensitivity is not less than a threshold, calculate the reactive power loading margin as a difference between the specified reactive power load and a current reactive power load. 2. The device of claim 1 further comprising an apparatus for reducing the reactive power load at one or more of the generators. 3. The device of claim 2 wherein the apparatus is a load shedding apparatus. 4. The device of claim 1 further comprising an apparatus for increasing or decreasing reactive power outputs of some of the generators. 5. The device of claim 1 further comprising an apparatus for accelerating the calculation processing of the predictive reactive power load limit calculator. 6. The device of claim 1 further including a generator reactive power sensitivity calculator. 7. The device of claim 6 wherein the generator reactive power sensitivity calculator is based at least in part on power-flow Jacobian. 8. The device of claim 1 further comprising an alarm coupled to the predictive reactive power load limit calculator adapted to indicate when a plurality of generators in the electrical power system have reached their combined maximum operating limit of providing reactive power. 9. The device of claim 1 further comprising a signaling circuit coupled to the predictive reactive power load limit calculator adapted to provide a signal representative of a control or monitoring action when a plurality of generators in the electrical power system have reached their combined maximum operating limit of providing reactive power. 10. The device of claim 1 wherein the communications port is adapted to receive Supervisory Control and Data Acquisition (SCADA) measurements, and the predictive reactive power load limit calculator is adapted to calculate a reactive power loading margin based in part on the SCADA measurements. 11. A method for assessing and monitoring voltage stability in an electrical power system including a plurality of generators, including the steps of: a voltage security processor acquiring phasor or Supervisory Control and Data Acquisition (SCADA) measurements from various locations on the power system,calculating, using the voltage security processor, a reactive power loading margin where the plurality of generators will reach their respective and combined reactive power limits using a small number power-flow runs, at least one power-flow run comprising: calculating a predictive generator reactive power sensitivity indicating an anticipated change in generation reactive power output resulting from a predictive change in reactive power load at a specified reactive power load using the measurements, wherein the magnitude of the sensitivity is related to the proximity to a static limit,determining, based on the sensitivity, a load increment used for calculating the specified reactive power load for the next power-flow run, and,when the sensitivity is not less than a threshold, calculating the reactive power loading margin as a difference between the specified reactive power load and a current reactive power load. 12. The method of claim 11 further comprising the step of reducing the reactive power load at one or more of the generators. 13. The method of claim 11 further comprising the step of increasing the reactive power outputs of some of the generators. 14. The method of claim 11 further comprising the step of decreasing the reactive power outputs of some of the generators. 15. The method of claim 11 wherein the generator reactive power sensitivity is calculated based at least in part on power-flow Jacobian. 16. The method of claim 11 further comprising the step of signaling an alarm to indicate when a plurality of generators in the electrical power system have reached their combined maximum operating limit of providing reactive power. 17. The method of claim 11, wherein the step of calculating a predictive generator reactive power sensitivity comprises evaluating a power-flow solution. 18. The device of claim 1, wherein the predictive reactive power load limit calculator is adapted to calculate a predictive generator reactive power sensitivity by evaluating a power-flow solution. 19. The device of claim 1, wherein the predictive reactive power load limit calculator is adapted to calculate a predictive generator reactive power sensitivity at a particular bus by evaluating a number of power flow solutions. 20. The method of claim 11, wherein the step of calculating a predictive generator reactive power sensitivity comprises calculating the sensitivity at a particular bus by evaluating a number of power flow solutions. 21. The device of claim 19, wherein the reactive power loading margin comprises a reactive power loading margin at the particular bus calculated as a difference between the reactive power load at the particular bus and the predictive change in reactive power load at the particular bus when the next generator will cause the plurality of generators to reach a combined reactive power operating limit. 22. The method of claim 20, wherein the reactive power loading margin comprises a reactive power loading margin at the particular bus, and the step of calculating the reactive power loading margin comprises calculating as a difference between the reactive power load at the particular bus and the predictive change in reactive power load at the particular bus when the next generator will cause the plurality of generators to reach a combined reactive power operating limit. 23. The device of claim 1, wherein the static limit comprises a nose of a QV curve. 24. The method of claim 11, wherein the static limit comprises a nose of a QV curve. 25. The device of claim 1, wherein the calculator is further adapted to calculate a plurality of power-flow solutions at a plurality of different reactive power loads, and wherein the sensitivity is calculated based on the incremental difference between a second reactive power generated at a second of the plurality of different reactive power loads and a first reactive power generated at a first of the plurality of different reactive power loads. 26. The device of claim 1, wherein the calculator is adapted to directly calculate the sensitivity when it is assumed only reactive power load is changing using a sensitivity equation: Δ⁢⁢QPV=(∂qPV∂VPV)x*⁢(∂qPQ∂VPQ)x*-1⁢Δ⁢⁢QPQ wherein ΔQPV is the anticipated change in the generation reactive power output for a plurality of reactive power source buses, ΔQPQ is the predictive change in the reactive power load for a plurality of reactive power sink buses, qPV is a plurality of generation instantaneous reactive power values, VPV is a plurality of bus terminal voltage magnitudes corresponding to the plurality of reactive power source buses, qPQ is a plurality of load instantaneous reactive power values, VPQ is a plurality of bus terminal voltage magnitudes corresponding to the plurality of reactive power sink buses, the exponent (−1) represents an inverse, ∂ indicates a partial derivative, and x* is a power flow solution, and wherein the sensitivity equation is solved using a linearized power-flow equation, the linearized power-flow equation comprising a power-flow Jacobian matrix at a power-flow solution multiplied by a change in state variables minus a change in the power flow solution. 27. The device of claim 1, wherein the calculator is adapted to directly calculate the sensitivity when real and reactive load powers are changing using a first sensitivity equation: Δ⁢⁢QPV=(∂qPV∂δPV⁢∂qPV∂δPQ⁢∂qPV∂VPQ)(x*)⁢(Δ⁢⁢δPVΔ⁢⁢δPQΔ⁢⁢VPQ) wherein ΔQPV is the anticipated change in the generation reactive power output for a plurality of reactive power source buses, qPV is a plurality of generation instantaneous reactive power values, δPV is a plurality of bus terminal voltage phase angles corresponding to the plurality of reactive power source buses, δPQ is a plurality of bus terminal voltage phase angles corresponding to a plurality of reactive power sink buses, VPQ is a plurality of bus terminal voltage magnitudes corresponding to the plurality of reactive power sink buses, Δ denotes change of a quantity, ∂ indicates a partial derivative, and x* is a power flow solution, and a second sensitivity equation: (Δ⁢⁢δPVΔ⁢⁢δPQΔ⁢⁢VPQ)=(∂pPV∂δPV∂pPV∂δPQ∂pPV∂VPQ∂pPQ∂δPV∂pPQ∂δPQ∂pPQ∂VPQ∂qPQ∂δPV∂qPQ∂δPQ∂qPQ∂VPQ)(x*)-1⁢(Δ⁢⁢PPVΔ⁢⁢PPQΔ⁢⁢QPQ) wherein pPV is a plurality of generation instantaneous real power values, pPQ is a plurality of load instantaneous real power values, qPQ is a plurality of load instantaneous reactive power values, the exponent (−1) represents an inverse, PPV is a plurality of generation active power outputs, PPQ is a plurality of active power loads, and QPQ is the reactive power load for the plurality of reactive power sink buses, and wherein the first and second sensitivity equations are solved using a linearized power-flow equation, the linearized power-flow equation comprising a power-flow Jacobian matrix at a power-flow solution multiplied by a change in state variables minus a change in the power flow solution.