Applications and methods for voltage instability predictor (VIP)
First Claim
1. A method for protecting an electrical power system, comprising the acts of:
- (a) measuring current and voltage phasors at a point on the system;
(b) based on the current and voltage phasors, determining an apparent impedance ({overscore (Z)}app) associated with a load region and a Thé
venin impedance ({overscore (Z)}Thev) associated with a source region, wherein {overscore (Z)}Thev is tracked using a ratio of a rolling sum of voltage (Trek(V)) to a rolling sum of current (Trek(I));
(c) comparing the Thé
venin impedance and apparent impedances; and
(d) deciding whether to initiate a prescribed action based on the relationship of the apparent impedance to the Thé
venin impedance.
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Accused Products
Abstract
A Voltage Instability Predictor (VIP) estimates the proximity of a power system to voltage collapse in real time. The VIP can be implemented in a microprocessor-based relay whose settings are changed adaptively to reflect system changes. Only local measurements (voltage and current) at the bus terminal are required. The VIP detects the proximity to collapse by monitoring the relationship between the apparent impedance {overscore (Z)}app and the Thévenin-impedance. In addition, we disclose: (1) that the VIP may be used in connection with non-radial topologies; (2) a new, more robust method to track voltage collapse in terms of impedance using rolling sums; and (3) a new method for representing distance to voltage collapse in terms of power margins.
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Citations
18 Claims
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1. A method for protecting an electrical power system, comprising the acts of:
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(a) measuring current and voltage phasors at a point on the system;
(b) based on the current and voltage phasors, determining an apparent impedance ({overscore (Z)}app) associated with a load region and a Thé
venin impedance ({overscore (Z)}Thev) associated with a source region, wherein {overscore (Z)}Thev is tracked using a ratio of a rolling sum of voltage (Trek(V)) to a rolling sum of current (Trek(I));
(c) comparing the Thé
venin impedance and apparent impedances; and
(d) deciding whether to initiate a prescribed action based on the relationship of the apparent impedance to the Thé
venin impedance.- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14)
Step 0;
Let y_ and x_ be the most recent values stored in the circular arrays, for voltage and current respectively;
Step 1;
Let y=present value of Trek(V);
x=present value of Trek(I);
Step 2;
If (y−
y_) is greater than a prescribed threshold dv or if (x−
x_) is greater than a prescribed threshold dI then (a) store both y and x in respective circular arrays, and (b) continue with Step 3;
otherwise, go to Step 1;
Step 3;
Calculate ZThev;
Step 4;
Replace y_ and x_ with the most current values of y and x, respectively;
Step 5;
Go to Step 1.
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13. A method as recited in claim 1, further comprising a digital filtering step.
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14. A method as recited in claim 13, wherein the digital filtering step comprises the following computation:
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15. A method for protecting an electrical power system, comprising the acts of:
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(a) measuring current and voltage at a point on the system;
(b) based on the current and voltage measurements, determining a Thé
venin impedance associated with a source region, and determining a power margin in accordance with a prescribed formula, wherein {overscore (Z)}Thev is tracked using a ratio of a rolling sum of voltage (Trek(V)) to a rolling sum of current (Trek(I)); and
(c) deciding whether to initiate a prescribed action based on the power margin. - View Dependent Claims (16, 17, 18)
obtaining data representing voltage and current at a plurality of points in time;
determining the power observed at the present time;
forecasting a maximum available power at a future time, based on the plurality of data points;
computing a difference between the forecasted maximum available power and the observed current power; and
defining the power margin based on the computed difference.
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17. A method as recited in claim 15, wherein said prescribed action is load shedding.
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18. A method as recited in claim 15, wherein said prescribed action is controlling on-load tap-changing (OLTC) transformers.
Specification