Applications and methods for voltage instability predictor (VIP)

• US 6,249,719 B1
• Filed: 06/14/1999
• Issued: 06/19/2001
• Est. Priority Date: 05/15/1998
• Status: Expired due to Term

• Current Assignee: ABB Incorporated (ABB Limited)
• Original Assignee: ABB Power T&D Co Inc (ABB Limited)
• Inventors: Julian, Danny E., Saha, Murari M., Gjerde, Jan Ove, Vu, Khoi Tien
• Primary Examiner(s): Shalwala, Bipin
• Assistant Examiner(s): Patel, Nitin
• Application Number:
  US09/333,185
• Time in Patent Office: 736 Days
• Field of Search: 700/294, 700/292, 700/295, 700/286, 700/293
• US Class Current: 700/292
• CPC Class Codes: H02J 3/00 Circuit arrangements for ac mains or ac distribution networks
• CPC Class Codes: H02J 3/0012 Contingency detection
• CPC Class Codes: H02J 3/12 for adjusting voltage in ac networks by changing a characteristic of the network load
• CPC Class Codes: H02J 3/242 using phasor measuring units [PMU]
• CPC Class Codes: Y02E 40/70 Smart grids as climate change mitigation technology in the energy generation sector
• CPC Class Codes: Y02E 60/00 Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
• CPC Class Codes: Y04S 10/22 Flexible AC transmission systems [FACTS] or power factor or reactive power compensating or correcting units

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.

Claims

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.

2. A method as recited in claim 1, wherein said prescribed action is load shedding.

3. A method as recited in claim 1, wherein said prescribed action is controlling on-load tap-changing (OLTC) transformers.

4. A method as recited in claim 1, wherein said prescribed action is initiated if a difference between the magnitudes of the Thé

venin impedance and apparent impedance is less than a predetermined amount.

5. A method as recited in claim 1, wherein the apparent impedance represents a ratio between the voltage and current phasors.

6. A method as recited in claim 1, wherein the method is applied to a non-radial power system topology.

7. A method as recited in claim 6, wherein the non-radial topology includes a tie line coupling the source region to the load region, and the method is applied by detecting voltage instability at a point on the tie line.

8. A method as recited in claim 1, wherein Trek(V) is computed as Trek(V)=Σ

a_kV_k and Trek(I) is computed as Trek(I)=Σ

a_kI_k, wherein k is an index related to measurement times, a_k are coefficients, and V_k are values of voltage measured at the point on the system.

9. A method as recited in claim 8, wherein the coefficients (a_k) are selected in accordance with an n-size FIR filter, where n is an integer.

10. A method as recited in claim 8, wherein the coefficients (a_k) are selected by taking n equidistant points on a unit circle, where n is an integer.

11. A method as recited in claim 8, further comprising a preliminary smoothing step.

12. A method as recited in claim 11, wherein the preliminary smoothing step comprises the following steps:

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 d_v or if (x−

x_) is greater than a prescribed threshold d_I 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 Z_Thev;

$Z_{\mathrm{Thev}}=\frac{\left(y-y_{\mathrm{—<br><br>}}\right)}{\left(x-x_{\mathrm{—<br><br>}}\right)}$

Step 4;

Replace y_ and x_ with the most current values of y and x, respectively;

Step 5;

Go to Step 1.

13. A method as recited in claim 1, further comprising a digital filtering step.

14. A method as recited in claim 13, wherein the digital filtering step comprises the following computation:

15. A method for protecting an electrical power system, comprising the acts of:

(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.

16. A method as recited in claim 15, wherein the power margin is determined in accordance with the following process:

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.

17. A method as recited in claim 15, wherein said prescribed action is load shedding.

18. A method as recited in claim 15, wherein said prescribed action is controlling on-load tap-changing (OLTC) transformers.