On-Line Transient Stability Assessment and Enhancement by Pattern Recognition Techniques

The k-Nearest Neighbour (k-NN) rule is one of the most popular techniques in Pattern Recognition. This technique requires good prototypes in order to achieve good results with a reasonable computational cost. When objects are represented by strings, the Median String of a set of strings could be the best prototype for representing the whole set (i.e., the class of the objects). However, obtaining the Median String is an NP-Hard problem, and only approximations to the median string can be computed with a reasonable computational cost. Although proposed algorithms to obtain approximations to Median String are polynomial, their computational cost is quite high (cubic order), and obtaining the prototypes is very costly. In this work, we propose several techniques in order to reduce this computational cost without degrading the classification performance by the Nearest Neighbour rule.

Online transient stability assessment and preventive control are important measures to prevent power system blackouts. Based on the deep cross network (DCN), an online assessment and preventive control method for power system transient stability is proposed. The deep cross network is developed to estimate the critical clearing time (CCT) of the credible fault contingencies. Based on the deep cross network model, the sensitivity of the active power of the generator set to the fault critical clearing time is calculated and then the DCN model-embedded transient stability constrained optimal power flow (TSCOPF) model is proposed to compute the preventive generation re-dispatch strategy. This TSCOPF model is computed by successive linear programming assisted by the DCN. The simulation results of the IEEE 39-bus test system show that this method can give early warning to the operation mode with instability risk and generate preventive control strategies, which can effectively improve the safety level of power grid operation.

In a high percentage of new energy-islanded microgrids, the overall inertia of the system gradually decreases, and the transient stability requirements of the microgrid frequency and voltage become more and more demanding under low-inertia conditions. To improve the transient stability of low-inertia islanded microgrid frequencies and voltages, this paper proposes a transient stability enhancement strategy for islanded microgrids based on energy storage system (ESS)–virtual synchronous generator (VSG) control. Model predictive control (MPC) is added within the active control loop of the VSG to achieve dynamic correction of the active power reference value of the VSG; PI control link is added within the reactive control loop to achieve a fast dynamic response of the reactive power command value. The ESS achieves fast and accurate regulation of frequency and voltage according to the power reference value of the VSG active control loop and the power command value of the reactive control loop simultaneously. Considering the need to ensure the ability of VSG to operate stably during transients, a comprehensive current-limiting technique combining virtual impedance and phase limiting is used to limit the fault current of VSG and maintain its synchronization and stability. Finally, the simulation results verify the strategy’s effectiveness and the superiority of the transient stability enhancement effect.

This paper focuses on the transient stability analysis and enhancement evaluation of an actual cogeneration plant. The cogeneration system is constructed in the ElectroMagnetic Transient Program (EMTP) environment for analyses. Simulation results show that the needed CCT curves for successful grid-disconnection operation are much lower than calculated CCT curves without executing grid-disconnection operation. Therefore, three schemes including installations of power factor correction capacitor, static var compensator (SVC) and static synchronous compensator (STATCOM) for enhancement of grid-disconnection operation are studied, respectively. Simulation results are given and discussed.

Superconducting magnetic energy storage (SMES) and superconducting fault current limiters (SFCL) can be used to stabilize the wind generator system. While both SMES and SFCL use superconducting coils for their operations, this paper makes a comparative study of these two devices in case of their applications in the wind generator system. The comparison is done on the basis of transient stability enhancement, controller complexity, and cost. In order to analyze the effect of the SMES and SFCL on wind generator stabilization in more detail, several performance indices are considered. Simulation results show that both the SMES and SFCL can stabilize the wind generator system during a severe fault. However, the performance of the SMES is better than that of the SFCL. Also, from the controller viewpoint, the SMES system is more complex than the SFCL system. Moreover, from the perspective of cost, the SMES might be costlier than the SFCL. As a whole, this study would help the readers understand the relative effectiveness of the SMES and SFCL methods, and then select a suitable method for wind generator stabilization.

This paper proposes an adaptive sparse polynomial chaos expansion(PCE)-based method to quantify the impacts of uncertainties on critical clearing time (CCT) that is an important index in transient stability analysis. The proposed method can not only give fast and accurate estimations for the probabilistic characteristics (e.g., mean, variance, probability density function) of the probabilistic CCT (PCCT), but also provides crucial information about the sensitivity of random inputs with respect to the variance of PCCT. Utilizing the sensitivity information, mitigation measures can be developed for transient stability enhancement. Numerical studies on the WSCC 9-bus system demonstrate the high accuracy and efficiency of the proposed method compared to the Monte Carlo simulation method. The provided sensitivity information and the effectiveness of mitigation measures in transient stability enhancement are also verified.

On-line steady-state security assessment of power systems

This paper describes the on-line voltage security assessment environment developed within the framework of the EU-sponsored OMASES project, as well as its application to the Greek Interconnected Power System. The heart of all computations is a fast time-domain method. Security is analyzed with respect to power transfers in critical corridors or power consumption in load areas. Results take on the form of either pre-contingency secure operation limits, or post-contingency load-ability limits, together with various diagnosis tips. Typical results obtained during the test phase of the project are reported.

With the integration of large scale wind power, the characteristics of power system operation has been changing, and, some problems come forth. On-line security assessment and control decision support system provides the stability and security state message for dispatcher in time, and, some suggestions to improve the level of power system stability and security. To suit the requirements of large amount wind power integrated power system, the technology of on-line security assessment and control decision support must be modified. This paper focuses on the technology of on-line security assessment and control decision for operation of power system with large scale wind power and designing the support system. In the paper, the study and application of on-line security assessment and control decision support technology in China are introduced. Considering the wind power in China having some distinct characteristics such as large scale, centralized development, long-distance and high voltage transportation, the new requirements to on-line security assessment and control decision support are analyzed. They includes need for new information, on-line modelling with large scale wind farm, transfer limitation calculation under the constraint condition of wind power integration requirement, auxiliary preventive control decision based on the forecasting and adjustment properties of wind power output, coordinated emergency control wind power with hydraulic and thermal power. On the basis of technical rule for connecting wind farm into power grid and the interaction between wind power and power grid, the advices on engineering application of on-line security assessment and control decision support technology for large amount wind power integrated power system, are proposed, for example, with respect to the standard of security assessment and selection of pre-contingency sets. (7 pages)

The transient angle instability and overcurrent of the virtual synchronous generator (VSG) under severe fault are risks for the stable and safe operation. Existing works have proposed some efficient methods for the transient stability enhancement or current limitation of VSG. Yet, the consideration for both of them is rarely studied. Moreover, the reasonable parameter design method for the control is usually lacked. In this paper, with the large signal model of VSG, a dual-loop PQ control scheme is introduced. With the proposed method, both the transient angle stability and overcurrent can be controlled simultaneously. In addition, how to design the reasonable coefficients of the proposed control scheme with additional requirement of output active power is also presented. The effect of the proposed method can be achieved with the coefficients in the allowable range. Finally, this method is validated by simulation results.

The problem of transient stability and voltage regulation for a single machine infinite bus (SMIB) system is addressed in this paper. An improved Backstepping design method for transient stability enhancement and voltage regulation of power systems is discussed beginning with the classical Backstepping to designing the nonlinear excitation control of synchronous generator. Then a more refined version of this technique will be suggested incorporating the sliding mode control to enhance voltage regulation and transient stability. The proposed method is based on a standard third-order model of a synchronous generator connected to the grid (SMIB system). It is basically implemented on the excitation side of the synchronous generator and compared to the classical Backstepping controller as well as the conventional controllers which are the automatic voltage regulator and the power system stabiliser. Simulation results prove the effectiveness of the proposed method which ameliorates to a great extent the transient stability compared to the other methods.

The Unified Power Flow Controller (UPFC) can inject voltage with controllable magnitude and phase angle in series with a transmission line. It can also generate or absorb controllable reactive power. UPFC is expected to be able to damp power system oscillations more effectively than power electronics devices such as SVG and TCSC. In this paper, a control system design of a UPFC for power system damping enhancement based on the eigenvalue control method is proposed. It is made clear that the best design method for the power system damping enhancement is to determine steady-state values of the UPFC control variables and the control parameters of the UPFC such as gains and time constants simultaneously, because the controllability of UPFC depends on the steady-state values of UPFC and the power flow condition. The effectiveness of the proposed control system taking into account UPFC inverter ratings is verified by digital time simulation. Furthermore the effects of the input signals to the UPFC controller on small-signal stability and transient stability enhancement are studied, and it is made clear that UPFC controllers using global information are more effective for power system damping enhancement than those using local information because global information has stronger observability for power system oscillations than local information. © 2000 Scripta Technica, Electr Eng Jpn, 133(3): 35–47, 2000

Transient Stability Enhancement in Power System Using DSSC and DSSC with FLC

In recent years, power demand has increased day by day while the expansion of power generation and transmission has been limited due to limited resources and environmental restrictions. Flexible Alternating Current Transmission Systems (FACTS) devices play an important role in improving the power system performance. Flexible AC transmission systems devices have been mainly used for solving various steady states, dynamic and transient control problems of power system. TCSC is an important member of FACTS family. In a power system, TCSC device has high potential in applications because it may improve power system performance including power flow control, transfer capability enhancement of the transmission system, transient stability enhancement and sub-synchronous resonance (SSR) mitigation etc. As other FACTS devices, TCSC is also a costly device. Therefore this is utmost essential to find its optimal size and location in a power system to achieve maximum benefit of this device. This complicated problem of finding optimal location and sizing of TCSC has been solved by researchers in different ways by considering different objectives. This paper presents a brief review of the work carried out by researchers in this field.

Flexible AC Transmission System (FACTS) controllers are advanced power electronic devices employed in electrical power systems to enhance system stability, reliability, and power transfer capabilities. These controllers enable dynamic and flexible control of various transmission parameters such as voltage, current, impedance, and phase angle, thereby improving the overall power system performance. FACTS devices are broadly classified into series, shunt, series- series, and series-shunt configurations, each designed to manage reactive power and regulate voltage either by injecting voltage in series or current in shunt with the transmission line. Well-known FACTS devices include Static Var Compensators (SVC), Static Synchronous Compensators (STATCOM), Thyristor Controlled Series Capacitors (TCSC), Static Synchronous Series Compensators (SSSC), and Unified Power Flow Controllers (UPFC), each serving specific roles in power flow control, voltage regulation, and transient stability enhancement. The integration of these controllers provides significant benefits such as increased transmission line capacity, improved voltage stability, minimized transmission losses, and damping of power system oscillations. They are capable of rapid response during transient disturbances, effectively mitigating voltage collapses and enhancing system resilience against faults and contingencies. FACTS controllers not only improve steady-state operational parameters but also address dynamic stability and transient conditions, enabling power systems to operate closer to their stability limits while maintaining security. Their deployment helps avoid costly infrastructural expansions by optimizing existing transmission assets, which is critical given environmental and economic constraints. Recent advances in semiconductor technologies and control algorithms have made FACTS devices more reliable, efficient, and adaptable to modern grids, including renewable integration and deregulated power markets. In summary, FACTS controllers play a vital role in modern power systems by providing flexible, efficient, and reliable solutions for power flow management, voltage control, and system stability enhancement, thereby supporting the evolving demands of today's complex electrical networks.