Steady-state Stability Margin Research Articles

Enhancement of power system stability by real-time prediction of instability and early activation of steam turbine fast valving

Turbine FV is a stability enhancing method used to protect power systems against the loss of synchronism after extreme contingency events. To avoid unnecessary FV action, smart control schemes can be used based on real-time instability prediction. In the last decade, several WAMS-based instability prediction methods have been developed. However, relatively long cumulative latencies in WAMS structures significantly reduce the efficiency of preventive control and can lead to system instability. To speed up FV action initiated by smart control scheme, this paper proposes a fast stability prediction method which is completely different from other methods described in literature. It uses only local measurements and requires performing simple mathematical operations. The basis of this method is the prediction of the magnitude of the power-angle characteristic. Just after the fault clearance, the method allows to predict transient instability and initiate MFV action. Based on the magnitude of the power-angle characteristic, the steady-state stability margin can be also computed. When this margin is too small, SFV action can be initiated as well. The validity of the proposed method has been verified by simulations performed for a large-scale real power system and detailed models of power system elements.

Исследование позиционной модели энергетической системы

The position model specific features are investigated; the eigenvalues and eigenvectors of the small perturbation equation matrix are analyzed, and a criterion of limiting operation modes in terms of aperiodic steady-state stability is obtained, which generalizes the classic Wagner - Evans criterion for complex multimachine systems. It has been found that the position model, i.e., a model in which the powers and torques of generators depend only on the relative angles of machine rotors, implicitly uses the distributed slack bus philosophy, and its limiting modes in terms of aperiodic steady-state stability are exactly identical with the limiting modes of a load flow model with a distributed slack bus in which the coefficients characterizing the participation of nodes in active power balancing are assigned to be directly proportional of the synchronous machines’ inertia constants. It has been found from a comparative analysis of the limiting mode parameters in the position model and the model with an infinite bus that the position model yields a conservative (underestimated) assessment of the electric power system steady-state stability margin. It is shown that the steady-state stability margins are estimated in the most adequate manner by means of the load flow model with an infinite bus, in the limiting modes of which the relative power loss increments do not exceed unity for all nodes.

Application of Unified Power Flow Controller to Improve Steady State Voltage Limit

<span>This paper utilizes the voltage source model of Unified Power Flow Controller (UPFC) and examines its abilities in mitigating the steady state stability margins of electric power system. It analyzes its behavior for different controls strategies and proposes the most efficient mode of controlling the controller for voltage stability enhancement. A systematic analytical methodology based on the concept of modal analysis of the modified load flow equations is employed to identify the area in a power system which is most prone voltage instability. Also to identify the most effective point of placement for the UPFC, a computer program has been developed using MATLAB. The results of analysis on 14 bus system is presented here as a case study.</span>

Псевдомодальное оценивание запасов статической устойчивости электроэнергетических систем

Псевдомодальное оценивание запасов статической устойчивости электроэнергетических систем

An approach to optimal allocation of SVC in power systems connected to DFIG wind farms based on maximization of voltage stability and system loadability

This paper proposed an approach to determine optimal location of SVC (Static Var Compensator) to improve the voltage profile and maximize system loadability in power networks connected to a doubly-fed induction generator (DFIG)-based wind farm. A variable reactance model for SVC is presented in steady state studies and implemented in the load flow program with embedded Flexible AC Transmission Systems (FACTS) devices. The continuation power flow method is used to determine optimal location of SVC and steady state stability margin, based on closing to point of voltage collapse. As an important result in this paper, we obtained an optimal location of SVC devices on wind farm by calculating the new indices such as maximum loading point (MLP) and mega watt margin (MWM). A case study and simulation are done on modified IEEE14 bus test system.

Study on the Steady-State Stability Margin of a Weak Section by Using the Online Data of WAMS

To realize the on-line quick analysis of the steady-state stability margin for the weak section of a power grid, a method of dividing the important generator groups according to the impact of the system equivalent impedance on the static stability limits of the section is presented. By using the dynamic information provided by the Wide Area Measurement System (WAMS), the center of inertia (COI) of the section of the two sides system is determined. Then the steady-state stability limits of the transmission section and the static stability margin can be calculated. The algorithm is verified by taking the out-play cross-section of West Inner Mongolia as an example, the results show that the static stability power limit and the steady-state stability margin of the section can be obtained accurately according to the presented algorithm. This algorithm can provide an effective analytical method for the online static stability security analysis of a power system.

A Novel Fuzzy-based Power System Stabilizer for Damping Power System Enhancement

A Power System Stabilizer (PSS) is the absolute most cost effective approach of increase the system positive damping; enhance the steady-state stability margin. The stability of power electrical networks is really a key factor for the delivery of top quality energy. This study compares the power system stabilizer based on fuzzy logic control with the conventional power system stabilizer for Single Machine Infinite Bus System (SMIB). The objective of designing the fuzzy stabilizer could be the increased stability and improving the dynamic response of the single machine system linked to the infinite bus. A Fuzzy Power System Stabilizer (FPSS) design using discrete inputs is presented. To offer satisfactory behavior in the whole region of operation of the generator, additional signals namely, generator's active and reactive power are utilized as inputs to FPSS. The results of simulations performed on the model of nonlinear system shows good performance of the Fuzzy controller. SMIB system was selected due to the simple structure, that will very useful in understanding the effects and implications of the PSS.

Stability Margin of DFIG Wind Turbines: Effects and Solutions

Doubly Fed Induction Generator (DFIG) based wind farms are one of the major contributors to wind power generation. Even though they possess advantages such as the capability of generating active power over an extended range of speed and the reduction of mechanical stresses in the turbine and the coupling, achieving maximum power extraction at high wind speeds while satisfying the present grid code requirements is a major issue. This is because the stability of the DFIG imposes a limitation on extracting maximum power at super synchronous speeds. An operating chart of the DFIG has been derived incorporating a steady state stability margin for safe operation. The PI limits of the active (P) and reactive (Q) power controllers have been derived by considering stator current, the rotor side converter power ratings and the stability margin of the generator. Studies also have been carried out to investigate what factors could be considered to improve the P and Q capability of the DFIG wind turbine.

An Improved Simple Adaptive Control Applied to Power System Stabilizer

A conventional power system stabilizer (PSS) can improve the steady-state stability margin and increase the system positive damping, but it has some drawbacks, such as the time-consuming tuning and nonoptimal damping in the entire operating process. A proportional--integral (PI) adaptive control law in simple adaptive control (SAC) can overcome these drawbacks. However, it can only be applied to the plants with almost strict positive realness (ASPR), and the computation of adaptive control law is really complex. A new improved SAC (ISAC) based on quadratic performance index was proposed. A new PSS can be designed using the ISAC. The simulation and experimental results show that this stabilizer possesses good dynamic characteristics and regulation precision. They also show that the new PSS can improve the dynamic stability of the power system and suppress the low-frequency oscillation effectively.

A model experiment of current limiting characteristics of superconducting fault current limiter in the demand side with distributed generator

Superconducting Fault Current Limiters (SCFCLs) are expected to improve both reliability and stability of power systems. Specifications of SCFCLs, such as trigger current level, recovery characteristic and impedance must be considered precisely in introducing SCFCLs to a power system. In this paper, current limiting characteristics of SCFCL in the demand side, (that is, a customer who has distributed generators and loads), is studied and demonstrated experimentally by use of a small 18kVA synchronous test generator with a test three-phase SCFCL unit of a transformer type and an artificial transmission line. In general, a current limiting reactor of normal conductor is installed in order to reduce the large fault current from the generator, however, it also decreases the maximum output power of the generator to the grid. The current limiting tests were carried out by replacing the reactor with the SCFCL. It was experimentally confirmed that SCFCLs can suppress the fault current considerably and can improve the transient stability at a certain fault without reducing the steady state stability margin.

MEASURES OF VOLTAGE INSTABILITY AND VOLTAGE COLLAPSE, PART I: THEORETICAL DEVELOPMENT

The purpose of this paper is to present a theoretical foundation for modeling, analysis, and testing of systems experiencing voltage collapse. The boundary theorem of the load flow feasibility region (FR), the steady-state stability margin and the sensitivity of the stability margin to bus voltages and bus injections are defined. The theoretical concept of feasibility region and practical multiple load flow solutions are unified and presented in this paper. The voltage-weak points and key contributing factors affecting the system voltage instability can be identified according to the values of the sensitivity of the stability margin to bus voltages and bus injections.

Predicting steady-state instability

The ability to know how much load can be met and how much power can be sold while maintaining a safe distance from the point of voltage collapse and steady-state instability is of great concern both in the real-time dispatch and during the operations planning and scheduling activities of power systems. This is particularly true in an increasingly deregulated electric utility industry where power systems are, and will be, dispatched within smaller voltage and steady-state stability margins than in the past. The authors describe a new application of REI-Dimo techniques which targets power system operation close to the maximum power transfer capability while maintaining a safe distance from critical states.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monitoring and Control of Power Station Stability Margin

Many power stations need control systems to improve the steady state and transient stability. The paper presents a simple device which measures the level of a steady state stability margin. It is designed as a two input comparator with input quantities being the bus voltage and tne sum of currents of all outgoing feeders. The proper selection of the comparator characteristic makes it only slightly sensitive to the variation of primary system parameters. Another identical comparator with higher setting may be used to monitor “insecure states” which may lead to instability in the case of a certain disturbance.

Steady State Stability Margin Index of Power Systems

Steady State Stability Margin Index of Power Systems

Steady State Stability Margin Index Based on Liapunov Function

Steady State Stability Margin Index Based on Liapunov Function