Steady-state Voltage Stability Research Articles

98/00383 High power factor correction circuits with space vector and hysteresis control methods

This paper presents a prototype of an expert system for monitoring and improving steady state voltage stability in power systems. The monitoring methodology is based on the L-index of load buses (Kessel and Glavitsch, IEEE Transactions, 1986, PWRD-1(3), 346–354). This index uses information on a normal load flow and is in the range from 0 (no load on the system) to 1 (voltage collapse). The control against voltage collapse is based on reducing the magnitude of the L-indices of the critical nodes for a given system operating condition based on heuristic criteria. The control variables considered are switchable VAR compensators, OLTC transformers and generators excitation. The proposed expert system technique has been tested under simulated conditions on a few practical Indian power systems. The performance of the expert system is compared with conventional optimization technique and concurrent results have been obtained. The proposed expert system technique is found to be suitable for on-line application in an Energy Control Center for voltage stability improvement as the solution is obtained fast, with a minimum of numerical computations. Results obtained for a 24 node EHV Indian power network and a modified IEEE 30 bus system are included for illustration purposes.

Voltage stability improvement: case studies of Indian power networks

It is becoming increasingly important for power system planning and operating engineers to be capable of performing comprehensive voltage stability analyses of the systems. This need is largely due to the recent trends towards operating systems under stressed conditions—as a result of increasing system loads without sufficient transmission and: or generation enhancements. There have been many failures, due to voltage instability in power systems around the world. In recent years many researchers have suggested techniques for voltage stability analysis considering both static and dynamic aspects. This paper is mainly concerned with analysis of steady state voltage stability. The contributions are: A method to compute voltage collapse proximity based on new operational load flow (OLF) and sensitivity of reactive control variables; An algorithm for reactive power optimization using LP technique to improve voltage stability. Simulated case studies conducted on two Indian power networks based on the proposed techniques.

98/00386 Load representation for steady-state voltage stability studies

98/00386 Load representation for steady-state voltage stability studies

98/00380 Enhancement of steady-state voltage stability by static VAR compensators

98/00380 Enhancement of steady-state voltage stability by static VAR compensators

Enhancement of steady-state voltage stability by static VAR compensators

Steady-state voltage instability can certainly be enhanced by static VAR compensators which can hold certain node voltages constant and create infinite buses within the system nodes. Static VAR compensator parameters needed for this purpose are found. Controller gains, droop slopes, reference voltages and compensator ratings are determined for maintaining the load node voltages constant irrespective of system loadabilities to values which lead to voltage instabilities. Influence of system equivalent impedances on these parameters is finally discussed.

Load representation for steady-state voltage stability studies

This paper presents new models for representation of different load types in steady-state voltage stability studies. The models are derived from the available measured characteristics of the power system loads. Special attention is paid to the induction motor load, with different mechanical loading as well as for special large loads such as aluminum smelter plants and are furnaces loads. Comparisons between results from the known models and the developed models are made and the necessity for using these developed models are put forward. It is also proved that erroneous results will be obtained using known available models of these loads.

An expert system for power system voltage stability improvement

This paper presents a prototype of an expert system for monitoring and improving steady state voltage stability in power systems. The monitoring methodology is based on the L-index of load buses ( Kessel and Glavitsch, IEEE Transactions, 1986, PWRD-1(3), 346–354). This index uses information on a normal load flow and is in the range from 0 (no load on the system) to 1 (voltage collapse). The control against voltage collapse is based on reducing the magnitude of the L-indices of the critical nodes for a given system operating condition based on heuristic criteria. The control variables considered are switchable VAR compensators, OLTC transformers and generators excitation. The proposed expert system technique has been tested under simulated conditions on a few practical Indian power systems. The performance of the expert system is compared with conventional optimization technique and concurrent results have been obtained. The proposed expert system technique is found to be suitable for on-line application in an Energy Control Center for voltage stability improvement as the solution is obtained fast, with a minimum of numerical computations. Results obtained for a 24 node EHV Indian power network and a modified IEEE 30 bus system are included for illustration purposes.

Voltage stability-limited interchange flow

The concept of power flow interchange limited by load voltage constraints is described via the continuation power flow (CPF). Critical flows on tie-lines are determined based on maintaining steady-state voltage stability. The most critical tie-lines are also identified via sensitivity analysis. An application to a test power system is presented.

EFFECT OF LOAD MODELING ON STEADY STATE VOLTAGE STABILITY

ABSTRACT This paper discusses the importance of load modeling in power systems for voltage stability studies. The steady-state voltage stability analysis is carried out using the available continuation power flow. Different types of load models - constant power; constant current; constant impedance; and composite load, which is a combination of the first three models, are considered. The composite load model is obtained using the LOADSYN software, where the load dependency on voltage is considered. The concept of load connectivity is defined and the confusion about the critical point with respect to PV curves is clarified. It is shown that for nonlinear loads, the critical point is not at the tip of the PV curve.

An optimal reactive power planning strategy against voltage collapse

This paper introduces a method of determining the minimum amount of shunt reactive power (VAr) support which indirectly maximizes the real power transfer before voltage collapse is encountered. Using a relaxation strategy that operates with a predictor-corrector/optimization scheme, a voltage stability index that serves as an indirect measure to the closeness of reaching the steady state voltage stability limits is obtained. Sensitivity information that identifies weak buses is also available for locating effective VAr injection sites.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The continuation power flow: a tool for steady state voltage stability analysis

The authors present a method of finding a continuum of power flow solutions starting at some base load and leading to the steady-state voltage stability limit (critical point) of the system. A salient feature of the so-called continuation power flow is that it remains well-conditioned at and around the critical point. As a consequence, divergence due to ill-conditioning is not encountered at the critical point, even when single-precision computation is used. Intermediate results of the process are used to develop a voltage stability index and identify areas of the system most prone to voltage collapse. Examples are given where the voltage stability of a system is analyzed using several different scenarios of load increase.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fast calculation of a voltage stability index

The minimum singular value of the power flow Jacobian matrix has been used as a static voltage stability index, indicating the distance between the studied operating point and the steady-state voltage stability limit. A fast method to calculate the minimum singular value and the corresponding (left and right) singular vectors is presented. The main advantages of the algorithm are the small amount of computation time needed, and that it only requires information available from an ordinary program for power flow calculations. The proposed method fully utilizes the sparsity of the power flow Jacobian matrix and the memory requirements for the computation are low. These advantages are preserved when applied to various submatrices of the Jacobian matrix. The algorithm was applied to small test systems and to a large system with over 1000 nodes, with satisfactory results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Steady state voltage instability operations perspective

This work reviews the basic theory of steady-state voltage instability; provides a physical interpretation of steady-state voltage instability; identifies some of the problems operators are presently experiencing; and gives simple procedures which can be used to determine system margin on a real-time basis. It is shown that steady-state voltage stability sensitivity investigations, using real-time data, can provide operators and system dispatchers with a viable procedure to estimate available system reserve margin to maintain adequate voltage security for a wide range of system parameters and operating conditions. The work also describes implementation of computer-based hardware for collecting real-time data which can be used to alert dispatchers of impending voltage problems and/or for conducting offline validation studies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Steady-state voltage stability regions of power systems

This paper deals with the steady-state (small-disturbance) voltage stability of power systems. A simple example is proposed to interpret the real-world phenomena of voltage instability. For general interconnected power systems, the voltage stability is studied via linearized dynamical equations of on-load tap changers and steady-state decoupled reactive power flow equations. A set of conditions is derived for a hyperbox of tap settings and load bus voltages to satisfy the steady-state stability requirements. The results can be used to construct steady-state voltage stability regions for the security monitoring of power systems.