Power System Operation and Control

A Presentation on the Research Paper:PREVENTIVE CONTROL APPROACH FOR VOLTAGE STABILITYIMPROVEMENT USING VOLTAGE STABILITY CONSTRAINEDOPTIMAL POWER FLOW BASED ON STATIC LINE VOLTAGESTABILITY INDICES

Presented by:Debasish Choudhury

Abstract:

This paper presents a novel voltage stability constrained optimal power flow (VSC-OPF) approach based on static line voltage stability indices to simultaneously improve voltage stability and minimise power system losses under stressed and contingency conditions.

The proposed methodology uses a voltage collapse proximity indicator (VCPI) to provide important information about the proximity of the system to voltage instability. The VCPI index is then incorporated into the optimal power flow (OPF) formulation.

Introduction:

As several blackouts around the world have been related to voltage instability phenomena much interest has been devoted by planning engineers to the voltage stability constrained OPF (VSC-OPF) problem.

The present paper deals with including the voltage stability issue in the conventional OPF to effectively improve system voltage stability as well as to reduce power losses when subject to unexpected contingencies such as generation outages, tripping of a transmission line in a heavily loaded system or an unpredictable increase in load demand.

Contd…

In the literature, many methods and techniques have been developed for voltage stability analysis and voltage collapse prediction.

Moreover a number of static voltage stability indices have been widely used for evaluating and predicting the proximity of the system to voltage instability. These indices are of two types: Bus Voltage Stability Indices and Line Indices.

Contd…

Much work have been done in order to investigate the most efficient and accurate method to incorporate these voltage stability criteria into the OPF formulation. Many researchers have even introduced bus voltage indices into the OPF problem.

Reasons for using VCPI(Voltage Collapse Proximity Indicator) The VCPI has the best accuracy and

robustness in predicting voltage collapse. Also, the VCPI index has some strong

points such as simplicity, fast numerical calculation, flexibility for simulating any type of

topological and load modifications in the network and

its application for real time simulation

Single Transmission line model:

Line VSI: VCPI The VCPI index is based on the concept of

maximum power transferred through the lines of the network shown in the previous slide and is defined by:

where the numerators are the real and reactive powers transmitted to the receiving end and the denominators are the maximum amount of real or reactive power that can be transmitted though the line.

The maximum active or reactive powers may be given by:

where: Vs=Sending end voltage

Zs= Line Impedence

Ф= load impedance angleѲ= line impedance angle

The VCPI increases with increasing power flow, varies from 0 to 1.

The critical line will be the line with the highest VCPI value and the load bus connected to the line will be the vulnerable bus in the system.

From the above equations, it is clear that the rise in voltage increases the values of Pr (max) and Qr (max) and as such decrease VCPI improving the voltage stability of the system.

OPF formulation

The OPF problem is generally formulated as:

minf (x,u) ---------- (4)subject to

g(x,u)=0 ---------- (5 )and h(x,u)>=0 --------- (6)

where (4) is the objective function to be minimised

(5) represents the equality constraints and(6) represents the inequality constraints.

Objective functions: Fuel Cost:

Where:FC= total fuel costNg= Total No. of generators

ai, bi, ci = fuel cost coefficients of the ith generator.

PGi = Real power output of the ith generator.

Objective function (proposed) Voltage Stability Index:

where: VCPIT = sum of the voltage stability indices

for all the lines of the system VCPIi = VCPI for line I

Ni = No. of transmission lines in the system

Conventional Constraints: Equality constraints: represent the

non-linear power flow equations:

where symbols carry their usual meanings.

Conventional Constraints(contd…)

Inequality constraints: include the system operating and the security limits: PGi,min <= PGi <= PGi,max ---------------- 11

QGi,min <= QGi <= QGi,max ---------------- 12

Vi, min <= Vi <= Vi,max ---------------- 13

where symbols carry their usual meanings.

Constraint (proposed) Voltage stability constraint: Voltage limits

alone are not sufficient to guarantee an acceptable voltage stability level of the system under different operating conditions. In this paper, a voltage stability constraint based on the VCPI is added to the classical OPF.

VCPImax <= VCPIlimit

where VCPIlimit is a desired threshold value to ensure a certain system security level and VCPImax is the maximum value of the VCPI index

VSC-OPF approach for preventive control

The purpose of the VSC-OPF based on the VCPI index is mainly to move the power system operation state far away from the voltage collapse by increasing the system stability margin. The proposed algorithm is incorporated into an automatic security monitoring and control system (ASMCS) as shown in the next slide:

Implementation of the VSC-OPF algorithm

The computational algorithm is shown in the next slide:

Results and Discussions

The main purpose of this paper is the performance evaluation of the proposed VSC-OPF approach based on the line VSI (VCPI), the OPF problem has been solved for different cases described as follows:

1) Cost function 2) Cost function with voltage stability

constraint 3) Total sum of VSI

IEEE 30 bus system

System performance under stressed condition:

Conclusion:

A novel VSC-OPF approach for voltage stability preventive control has been proposed.

The effectiveness and robustness of the proposed OPF based on VCPI index are tested and demonstrated.

The proposed technique is based on a simple concept and can be practically applicable for the online voltage security assessment.

THANK YOU