Method for the identification of weak and/or strong branches of an electric power transmission system

• US 20030144802A1
• Filed: 12/23/2002
• Published: 07/31/2003
• Est. Priority Date: 12/24/2001
• Status: Active Grant

• Current Assignee: ABB Power Grids Switzerland AG (ABB Limited)
• Original Assignee: ABB Technology AG (ABB Limited)
• Inventors: Fulczyk, Marek, Bertsch, Joachim, Sobierajski, Marian
• Granted Patent: US 6,832,167 B2
• Time in Patent Office: Days
• US Class Current: 702/57
• CPC Class Codes: H02J 3/24 Arrangements for preventing or reducing oscillations of power in networks by control effected upon a single generator H02P9/00

Abstract

The subject of the invention is a method of identification of weak and/or strong branches of an electric power transmission system.

In the inventive method electrical parameters charactirising the nodes and branches of an electric power transmission system are subjected to computational treatment in order to obtain equations of power flow in all nodes of the system at assumed 100 percent system load value. Then an electric model of a branche is assumed and a curve P-Q is constructed which shows the functional relation between active and reactive load in the system. For the assumed branch model a branch voltage stability coefficient is determined. Then the analysed system is overloaded by increasing the total system load up to 120% base load and the branch voltage stability coefficient is determined again. The numerical values of the appropriately determined coefficients are compared with threshold values considered to be a safe margin for the maintenance of voltage stability for the given branch.

Claims

1. A method for the identification of weak and/or strong branches of an electric power transmission system which employs known analytical methods appropriate to power flow in nodes and branches of the power transmission system and which analyses functional relations between active and reactive load for this system, characterised in that the electric parameters characterising the nodes and branches of the power transmission system are subjected to computational treatment in order to obtain equations of power flow in all nodes of the system at assumed 100 percent base system load, which is used to calculate complex voltage values in those nodes, and thereafter an electrical model of a system branch situated between two receiver nodes is assumed and a limiting curve /P-Q/ is constructed, which shows the functional relation between reactive load /Q/ and active load /P/ for the assumed electrical model of the branch, and a base load point for that branch is assumed, and then a branch voltage stability coefficient for the analysed system branch is determined, after which the total system load is increased to oveload the system to 120% base load and all operations relating to the determination of the voltage stability coefficient for the analysed system branch at the predetermined system oveload are repeated, and then the numerical value of the branch voltage stability coefficient is compared with a threshold value considered to be a safe margin for maintaining voltage stability for the analysed branch, and the value of the difference between the values of the branch voltage stability coefficients determined for both types of system load is checked whether it is more than, equal to or less than zero, and on the basis of those comparisons the analysed branch is identified as weak or strong.

2. A method according to claim 1, characterised in that the branch voltage stability coefficient /c_vc/ is calculated from this relation:

c_vc−

d_vc·

(1−

p_vc),

where;

d_vc={square root}{square root over ((p_cr−

p_b)² (q_cr−

q_b)²)}—

is the distance between the base point of the branch load and the critical point on the curve P-Q,

p_cr—

is the values of the branch active load coordinates during critical operating conditions at the voltage stability limit,

q_cr—

is the values of the branch reactive load coordinates during critical operating conditions at the voltage stability limit,

p_b—

is the values of the coordinates of the base point of active load in the analysed branch,

q_b—

is the values of the coordinates of the base point of reactive load in the analysed branch,

a p_VC—

is the probability of occurrence of voltage instability in the analysed branch, according to this formula;

$p_{\mathrm{vc}}=1-\frac{{\int <br><br>}_{P_{\min }}^{\sqrt{-\frac{Q_{\min }}{1-\frac{X_b<br><br>B_b}{2}}\frac{0.25}{{\left(1-\frac{X_b<br><br>B_b}{2}\right)}^2}}}<br><br>\left(-\left(1-\frac{X_b<br><br>B_b}{2}\right)<br><br>P^2\frac{0.25}{1-\frac{X_b<br><br>B_b}{2}}-Q_{\min }\right)<br><br><br><br>P}{\left(P_{\max }-P_{\min }\right)·<br><br>\left(Q_{\max }-Q_{\min }\right)}$

where;

P_max—

is the maximum value of the coordinates of active load in the branch during critical operating conditions at the voltage stability limit,

P_min—

is the minimum value of the coordinates of active load in the branch,

Q_max—

is the maximum value of the coordinates of reactive load in the branch during critical operating conditions at the voltage stability limit,

X_b—

is the reactance of the analysed branch, and B_b is its susceptance.

3. A method according to claim 1 or 2, characterised in that the analysed branch is considered weak where the value of the branch voltage stability coefficient /c_vc/ for 100% system load is less than 0.125 while the difference between the coefficient c_vc, determined for the given node at total system load equal to 100% and the coefficient /c_vc/ determined for the given node at total system load equal to 120% is more than zero, or the analysed branch is considered strong where the value of the branch voltage stability coefficient /c_vc/ for 100% system load is less than 0.125 and the difference between the coefficient c_vc determined for the given node at total system load equal to 100% and the coefficient /c_vc/ determined for the given node at total system load equal to 120% is less than or equal to zero.