Generator Outages Research Articles

Enhancing power grid resilience to winter storms via generator winterization with equity considerations

We develop two-stage stochastic programming models for generator winterization that enhance power grid resilience while incorporating social equity. The first stage in our models captures the investment decisions for generator winterization, and the second stage captures the operation of a degraded power grid, with the objective of minimizing load shed and social inequity. To incorporate equity into our models, we propose a concept called adverse effect probability that captures the disproportionate effects of power outages on communities with varying vulnerability levels. Grid operations are modeled using DC power flow, and equity is captured through mean or maximum adverse effects experienced by communities. We apply our models to a synthetic Texas power grid, using winter storm scenarios created from the generator outage data from the 2021 Texas winter storm. Our extensive numerical experiments show that more equitable outcomes, in the sense of reducing adverse effects experienced by vulnerable communities during power outages, are achievable with no impact on total load shed through investing in winterization of generators in different locations and capacities.

A hierarchical optimization technique for placement of battery energy storage system to improve grid transient stability

AbstractA battery energy storage system (BESS), due to its very fast dynamic response, plays an essential role in improving the transient frequency stability of a grid. The performance of the BESS varies with the system's installation site. Hence, the optimal location of the BESS is of utmost importance for improving transient frequency stability. Therefore, this paper presents a hierarchical approach for optimizing the BESS placement to improve a grid's transient frequency stability. In most research, frequency nadir and rate of change of frequency (ROCOF) have been considered for studying frequency stability. This paper considers two more parameters, along with frequency nadir and ROCOF, to study the transient frequency stability, settling time, and decay ratio. A novel frequency stability index (FSI) using the four transient frequency parameters has been developed. After a significant disturbance in a benchmarked test system, the FSI was used to identify the optimal location of the BESS for stabilizing the frequency. It has been observed that, after a sudden generator outage, the ROCOF and the frequency nadir improve the best when the BESS is located at the bus closest to the generator experiencing the outage. However, considering the other two parameters as well, the value of the FSI is the minimum; that is, the optimum solution is when the BESS is located at the bus that is the second closest to the generator experiencing the outage. Results of similar studies validate the proposed FSI in indicating the optimal location of the BESS in improving the transient frequency behavior of the system.

Enhancement of ATC with FACTS Devices in Deregulated Power System Considering Various Contingency and Benefit Margins

The global electric power systems are under stressed condition because of increased per capita power consumption and ever-growing load demand. It is difficult to modify existing infrastructure of transmission systems to meet increasing load demand. The volume of electric power transmitted depends on the real and reactive power supply and the availability of margin in transmission system. Available Transfer Capability is an estimation of additional power transfer capability of the existing transmission system for a further market activity over and above the already committed power transactions. Factors such as weather, line and generator outages must be taken into account when computing the rigorous value of ATC. This paper focuses on enhancing the ATC limit of a system by employing STATCOM and Whale Optimization Algorithm for tuning system variables and to determine optimal size and location. An investigation has been carried out to understand the impacts of ATC by implementing line and generator outages in the test system. Additionally, an investigation has been done to realise the various benefit margins like ETC, TRM and CBM. The competence of the proposed methodology is validated by conducting different case studies on IEEE-30 bus test system using MATLAB for various bilateral and multilateral transactions.

Effect of PV Plant on Frequency Stability in IEEE12 Bus System for Different Penetration Levels and Depth of Frequency Support

The utilization of renewable energy sources, particularly large-scale photovoltaic plants (PVPs), has been increasing in power systems. However, the high penetration of PVPs in power systems has implications for the system's moment of inertia and frequency response. Therefore, it is crucial to thoroughly investigate the impact of PVPs on system frequency. This study focuses on investigating the effects of a large-scale PVP at various penetration levels on frequency and the depth of frequency support. The study proposes a strategy aimed at maximizing power generation from PVPs while also providing support to system frequency. To assess the effects of PVPs, the study examines events such as load outages, generator outages, load changes, and three-phase short circuits. Additionally, the relationship between voltage and frequency during these events is considered, with the load represented by an impedance model. By studying the depth of frequency support alongside different penetration levels, the study provides a detailed understanding of the impact of PVPs on frequency. The study aims to determine the threshold at which the penetration level of PVPs becomes significant in terms of frequency support. This analysis sheds light on the point at which the presence of PVPs begins to exert a notable influence on frequency stability and support within the power system. Overall, the study contributes to the integration of PVPs in power systems by providing insights into their impact on frequency dynamics and proposing strategies to enhance power generation and frequency support simultaneously.

Nataf-KernelDensity-Spline-based point estimate method for handling wind power correlation in probabilistic load flow

Modern power systems integrated with renewable energies (REs) contain many uncertainties. The proposed method introduces a novel approach to address the challenges associated with wind power generation uncertainty in probabilistic load flow (PLF) studies. Unlike conventional methods that use wind speed as an input, the paper advocates for utilizing wind generator output power (WGOP) as an input to the point estimate method (PEM) in solving PLF. The uniqueness lies in recognizing the distinct behavior of wind power uncertainty, where not all random samples of wind speed contribute to actual wind power production. The paper suggests a Nataf-KernelDensity-Spline-based PEM, combining the Nataf transformation, Kernel density estimation (KDE), and cubic spline interpolation. This innovative integration effectively manages wind power correlation within the analytical framework. By incorporating spline interpolation and kernel density estimation into the traditional PEM, the proposed method significantly enhances accuracy. To validate the effectiveness of the proposed approach, the method is applied to IEEE-9 and IEEE-57 bus test systems, considering uncertainties related to load, wind power generation (WPG), solar power generation (SPG), and conventional generator (CoG) outages. Comparative analysis with Monte Carlo simulation (MCS) results demonstrates that the proposed method outperforms the conventional PEM in terms of accuracy. Overall, the paper contributes a pioneering solution that not only highlights the importance of using WGOP as an input in PLF but also introduces a sophisticated method that surpasses traditional approaches, improving accuracy in power system studies involving renewable energy integration. The accuracy of the proposed method is validated by comparing its results with those obtained through Monte Carlo simulation (MCS), where the proposed method yields more accurate results than the conventional PEM.

Optimisation of topology, rating, control and insulation coordination of a large series–parallel DC windfarm

AbstractThis study presents the design of a large DC series‐parallel windfarm that considers the key operational challenges in determining the optimal generator equipment rating and DC transmission voltage control. With this topology, offshore platform is not needed, but wind speed dispersion over windfarms and generator outage possibilities demand generator DC output voltages beyond their ratings. An optimization algorithm is developed along with a complete cost model, which includes operational and capital costs with all topology options, in order to inform design decisions. The results are evaluated on a 100‐generator, 25 × 4, 1 GW, 525 kV test wind farm and performance is verified using detailed PSCAD/EMTDC time‐domain simulation. The key finding is that a positive net present value (NPV) is achieved, avoiding power curtailment even under 3% generator outage, when an overvoltage coefficient of 1.145 pu is utilised on all wind generators, assuming use of a suitable DC transmission voltage control. Further, a wide‐ranging parameter sensitivity analysis is performed, and tower arrangement for insulation requirements is proposed.

Koopman model predictive control based load modulation for primary frequency regulation

AbstractConventional power systems function under the assumption that loads are uncontrollable, and that the generation control is the primary means of preserving system voltage, frequency, and stability. Thanks to recent advancements of power electronics technology and communication schemes, demand‐side resources are now capable of providing fast frequency regulation. Controllable loads can provide upward/downward reserve during frequency excursions. In this paper, the authors consider collective contribution of large clusters of controllable loads which modulate their aggregate demand power to regulate the primary frequency. Koopman model predictive control is designed to handle local frequency variations caused by various disturbances at each load bus, considering uncertain load models. The efficacy of the proposed method has been validated using the New‐England power system considering two scenarios, namely, load variation, and generation outage.

Localisation of facts compensator using reptile search algorithm for enhancing power system security under multi‐contingency conditions

AbstractDue to the improvement in power demand usage, the system causes more stress. Suitable placement of FACTS is introduced to improve power flows, stability, and power system security. An advanced optimal method has been introduced to resolve optimal power flow under various conditions. Five varied FACTS devices were positioned inside the power system to improve system safety at the least cost. Selecting the suitable location and size of FACTS device enabling high security and less cost was proposed using RSA. The objective functions were mitigated based on the RSA's inspiration to improve system security. The placement of the FACTS compensator was based on objective functions like LOSI, voltage deviation, real power loss, investment cost, sensitivity index, fuel costs, and constraints. The proposed model was validated under three conditions, namely generator outage, line outage, and both outage in IEEE 118 and IEEE 30 bus‐systems. In the IEEE 30 bus system, TCSC provides better security of 1.4 severity at normal conditions and 1.3 severity in contingency conditions. In the IEEE 118 bus system, UPFC has less severity of 2.4 at normal conditions, and STATCOM has the least severity of 3 at contingency conditions. The proposed model provides enhanced security in all circumstances and reduces overall costs.

Optimal location of multiple FACTS devices in N-1 contingency conditions using traditional genetic algorithm

<span lang="EN-US">Transmission systems are prone to contingency conditions that would either occur using a generator outage or line outage. Placing and sizing the flexible ac transmission systems (FACTS) devices appropriately can reduce the effects of the contingency condition. This paper optimally locates FACTS devices in a transmission system under the N-1 contingency condition. The genetic algorithm (GA) technique is used to locate different, multiple FACTS devices (thyristor-controlled series capacitor and static VAR compensator) optimally in a power system. This optimization technique is used to locate FACTS devices on the IEEE 9 bus system. MATLAB simulation is developed and checked for both single and multiple FACTS placements. Simulation results obtained for generator outage and line outage are tabulated with the type of FACTS device/rating, location, and generation cost with line loss reduction. The optimized results observed for the cost-optimized FACTS placement problem are found to be satisfactory. The results obtained in the IEEE 9 bus system have shown improvement in a decrease of generation cost and system loss component while placement and sizing of both the FACTS devices.</span>

A robustness-enhanced frequency regulation scheme for power system against multiple cyber and physical emergency events

Emergency events, e.g., accidental generator outages and abrupt steeps in electric load in physical layer, and unexpected communication time delays in cyber layer, are raising increasing concerns in modern power systems, and can cause system frequency deterioration, thus threatening power energy security. However, currently there is few discussions on dealing with multiple emergency events in power system comprehensively and simultaneously. In order to counter the adverse effects caused by these various emergency events and maintain the system frequency, a robustness-enhanced frequency regulation scheme is proposed in this article based on the coordinate transformation technique and Lyapunov theorem. First, to analyze the dynamic process of the power system in contingency conditions, the model of the power system frequency regulation is reconstructed considering multiple emergency events. Furthermore, to guarantee the power system frequency stability, a virtual auxiliary surface is constructed based on the coordinate transformation technique, which can provide the control reference for frequency regulation. On this basis, a robust controller is developed to drive the system’s trajectory to the proposed virtual auxiliary surface, thus guaranteeing the robustness of power system frequency, e.g., minor frequency deviation and faster recovery speed, even with emergency events. In addition, the stability of the system with the proposed controller is rigorously proved by Lyapunov theorem. Finally, the effectiveness of the proposed scheme is verified by case studies. The results show that by using the proposed scheme, the maximum frequency deviation and recovery time can be improved to about 61.11% and 46.40% of the original value under multiple emergency events, respectively. Therefore, the proposed scheme can counter well against emergency events and contribute to the power system’s frequency regulation and energy security.

Evaluation of wind-solar hybrid power generation system based on Monte Carlo method

<span lang="EN-US">The application of wind-photovoltaic complementary power generation systems is becoming more and more widespread, but its intermittent and fluctuating characteristics may have a certain impact on the system's reliability. To better evaluate the reliability of stand-alone power generation systems with wind and photovoltaic generators, a reliability assessment model for stand-alone power generation systems with wind and photovoltaic generators was developed based on the analysis of the impact of wind and photovoltaic generator outages and derating on reliability. A sequential Monte Carlo method was used to evaluate the impact of the wind turbine, photovoltaic (PV) turbine, wind/photovoltaic complementary system, the randomness of wind turbine/photovoltaic outage status and penetration rate on the reliability of Independent photovoltaic power generation system (IPPS) under the reliability test system (RBTS). The results show that this reliability assessment method can provide some reference for planning the actual IPP system with wind and complementary solar systems.</span>

Optimal deloading of PV power plants for frequency control: A techno-economic assessment

Due to the lack of inertial support from photovoltaic (PV) systems, large-scale PV integration into the existing grid poses a major threat to frequency stability. To address this, numerous techniques for incorporating energy storage systems have been proposed in the existing literature, but they incur significant operational costs. PV systems with deloading mechanisms have been researched as an alternative efficient strategy in many studies as a key frequency response support after a disturbance. However, deloaded PV systems incur a cost burden due to idle reserve capacity during regular operation. Consequently, an optimum deloading of PV systems is required to enhance the grid's frequency stability while minimizing its total operating cost. To that aim, this paper proposes a gradient descent-based optimization method to determine the optimum deloading of PV systems. The optimization considers both frequency response constraints (frequency nadir and rate of change of frequency – ROCOF) and operating cost constraints (generation, up-regulation, and value of lost load cost). The proposed technique is employed in a modified IEEE 39 bus New England System for varying levels of PV integration under two distinct cases. The optimization and relevant simulations are performed in Python and DIgSILENT PowerFactory platforms. In the case of a major generator outage with PV penetrations of 30%, 40%, and 50%, the optimal deloading percentages are 6.96%, 5.931%, and 4.968%, respectively, with no load shedding. Moreover, in the interconnection outage case, the optimal deloading percentages increase to 14.92%, 13.3%, and 12.42%, respectively, with load shedding. Nevertheless, in each case, deloaded PV systems consistently preserve frequency stability in the grid while incurring minimized operating costs. Furthermore, the performance and cost of the proposed solution are compared to two other primary frequency response support systems, viz. Battery Energy Storage System (BESS) and Synchronous Condenser (SC). The optimally deloaded PV system outperforms both these devices considering both technical and economic aspects.

Reliability and economics evaluation for generation expansion planning incorporating variability in wind energy sources

Generation expansion planning is a strenuous process encompassing technical, economic, geographical, and climatic factors. Due to the fast growth of grid-integrated renewable energy sources, planning for operational measures and generation techniques needs to be more flexible. Unlike conventional energy sources, renewable energy sources are fluctuating in nature, and a suitable methodology has to be adopted for efficient generation expansion planning considering reliability and economic analysis. The multistate generation models for representing wind energy sources make the reliability evaluation more challenging. The present work incorporates the impact of wind power generation variability and wind turbine generator outages on system reliability, an essential aspect of generation planning. In this paper, the techno-economic feasibility assessment model has been incorporated to ensure a reliable power supply, strengthening the grid's economic feasibility and lowering the energy generation and interruption costs and annualized system costs. The proposed model for reliability evaluation involves Fourier domain analysis. The model has been validated to study the wind energy sources dynamics and economic behavior, i.e., annualized operational maintenance cost and annualized fuel cost on the revised IEEE RTS with wind power, for the sensitive assessments for generation expansion planning.

Enhanced probabilistic contingency analysis considering fast ramping events of renewable generation

One of the major challenges in transmission planning is accounting for the impacts of renewable generation. Contingency Analysis (CA) is a technique used by utilities to meet transmission planning criteria mandated by the North American Electric Reliability Corporation (NERC). In existing CA practices, renewable generation is treated like conventional generation and does not consider the intermittency and fluctuations inherent to renewables. As renewable generation continues to grow, considering the impacts of renewable intermittency is becoming increasingly important. In this paper, an innovative approach is proposed to model the renewable intermittency as generator outages by, for the first time, introducing the concept of intermittency-induced outages (IIOs) for a single renewable plant and common mode outages (CMOs) for multiple renewable plants. A probabilistic outage model features different outage modes and the associated parameters that can be derived based on historical renewable generation data. The IIOs and CMOs can be seamlessly integrated into the existing probabilistic contingency analysis (PCA) framework. This paper presents the details for modeling and parameterization of the renewable outage modes. Such probabilistic outage models have been implemented using a Python-based enhanced PCA (ePCA) tool that drives and enhances the capability of the built-in PCA module of the PSS/E software platform, a widely used tool by the industry. Using the Western Electricity Coordinating Council (WECC) system, a proof-of-concept case study is presented and discussed to illustrate the unique impacts of increased penetration levels of renewables on transmission planning studies.

A streamlined and enhanced iterative method for analysing power system available transfer capability and security

Available Transfer Capability (ATC) is a crucial indicator for ensuring the reliable and efficient electric grid as it helps to manage congestion and ensure that the power flows are kept within safe operating limits. This article presents a novel approach named as modified repeated alternating current power flow (MRACPF) with step-size control mechanism and compares it with the existing techniques like direct current power transfer distribution factors (DCPTDF) and repeated alternating current power flow (RACPF) for analyzing the ATC of the power system during bilateral wheeling transactions in MATLAB environment. The study further assesses the ATC in different multilateral wheeling transactions of two standard test systems (IEEE 30-bus and IEEE 118-bus) and evaluates inter-area ATCs for the IEEE 118-bus system under various operational scenarios. The Composite Security Index (CSI) which is based on both line flow limit and bus voltage limit violations is used to identify and rank the most severe (k-1) line contingencies, and is able to differentiate between secure and insecure state of the power system and furthermore, MRACPF with step-size control mechanism is used to assess ATC during the severe (k-1) line outage and generator outage scenarios for various power transactions. The MRACPF method produces comparable results to the RACPF method for obtaining the Available Transfer Capability (ATC), but it takes less time. This time savings can be significant, especially for larger power systems, where the execution time can be reduced by more than 50%.

TCSC Application for Transient Stabilization of Wind Turbine with Fixed Speed Induction Generator

Wind turbines (WTs) are a desirable alternative to traditional nonrenewable power resources as a result of recent environmental concerns. Some of them are fixed speed wind generator (FSWG) and have been integrated to power system by squirrel cage induction generator (SCIG). Induction machine absorbs reactive power during all operating conditions, especially at fault condition may result in severe voltage drop which can lead to generator outage. This outage disconnects a significant amount of active power and consequently leads to frequency instability. In order to prevent induction generator (IG) outages in short circuit failures, this paper investigates thyristor-controlled series capacitor (TCSC) device as a candidate solution. TCSC compensates the IG terminal voltage drop by adjusting transmission line impedance at fault condition. In the proposed method, a metaheuristic technique means that shuffled frog leaping algorithm (SFLA) has been utilized to optimize the TCSC controller gains. The proposed scheme can be applied for both SCIGs and wound rotor induction generators (WRIGs) which is another advantage of this method. Single-machine infinite bus system is considered as case study, and various operating conditions and disturbances have been considered to verify the effectiveness of the proposed method.

Two‐stage risk‐constrained stochastic optimal bidding strategy of virtual power plant considering distributed generation outage

AbstractThis paper presents an optimal bidding strategy for a technical and commercial virtual power plant (VPP) in medium‐term time horizon. A VPP includes various distributed energy resources (DERs) that can participate in the Pool and Futures markets. Although medium/long‐term scheduling provides the opportunity to participate in the futures market, it also raises the possibility of unit failure. In this regard, the impact of distributed generation (DG) units’ failure, as an important challenge in VPP, is incorporated in the proposed model. The model is formulated as a risk‐constrained two‐stage stochastic problem. The VPP signs futures market contracts in the first stage, and in the second stage, it participates in the day‐ahead (DA) market and manages its DERs. Long short‐term memory neural network and scenario generation and reduction methods are used to capture the uncertainty parameters of electrical load, DA market prices, wind speed, and solar radiation in the proposed model. The performance of proposed model is investigated in different cases. The obtained results show that the VPP can compensate the losses caused by the DG units’ failure through taking advantage of the arbitrage opportunity.

Generic security-constrained inertia emulation scheme for VSI-based DC system using supercapacitor

The large-scale grid integration of renewable energy source via a voltage source inverter (VSI)-based DC system results in a continuous reduction in system kinetic inertia, deteriorating grid frequency stability. To mitigate this problem, this paper proposes a generic security-constrained inertia emulation (SIE) scheme, which integrates an appropriately-sized supercapacitor (SC) bank with a VSI-based DC system via a bidirectional DC/DC converter. Inertial response provision is enabled by an SIE control loop developed for the DC/DC converter controller to adjust the SC voltage along with the grid frequency. It is further underpinned by adding an inertia adjustment algorithm to cope with potential security risks of VSI overcurrent and DC system overvoltage. The SIE scheme features a plug-and-play advantage without modifying the original system. The SIE controller parameter optimization is also carried out through the small-signal stability analysis. The effectiveness of the SIE scheme is evaluated using a modified IEEE 39-bus test system in the presence of generator outage and grid fault.

Simulating the GB power system frequency during underfrequency events 2018–19

AbstractLightning hit a transmission power line outside London, England on 9 August 2019. There followed a loss of power from a cascade of generator outages that exceeded contingency reserves, leading to an exceptional fall in grid frequency causing widespread transport disruptions and the disconnection of over 1 m households. Simulating such events typically involves a system of differential equations representing the overall generation and load present at the time. A standard model based on the swing equation assumes unlimited capacity in aggregated resources, and the availability of these services throughout the duration of the frequency deviation. In simulating the effect of outages on the GB Grid frequency on 2019/8/9, the effect of limiting these services to the capacity of resources engaged during the event is examined. It is shown that by taking these refinements into account the timing and extent of the frequency nadir can be successfully estimated. Insight is gained into the responses of various grid characteristics and how they interact with unplanned generation imbalances. Using this adapted model, further events on the GB grid are examined to validate the influence of these features. With the model’s effectiveness validated, novel mitigating measures to preserve the stability of a low-inertia grid can be evaluated.

A novel collapse prediction index for voltage stability analysis and contingency ranking in power systems

Voltage instability is a serious phenomenon that can occur in a power system because of critical or stressed conditions. To prevent voltage collapse caused by such instability, accurate voltage collapse prediction is necessary for power system planning and operation. This paper proposes a novel collapse prediction index (NCPI) to assess the voltage stability conditions of the power system and the critical conditions of lines. The effectiveness and applicability of the proposed index are investigated on the IEEE 30-bus and IEEE 118-bus systems and compared with the well-known existing indices (Lmn, FVSI, LQP, NLSI, and VSLI) under several power system operations to validate its practicability and versatility. The study also presents the sensitivity assumptions of existing indices and analyzes their impact on voltage collapse prediction. The application results under intensive case studies prove that the proposed index NCPI adapts to several operating power conditions. The results show the superiority of the proposed index in accurately estimating the maximum load-ability and predicting the critical lines, weak buses, and weak areas in medium and large networks during various power load operations and contingencies. A line interruption or generation unit outage in a power system can also lead to voltage collapse, and this is a contingency in the power system. Line and generation unit outage contingencies are examined to identify the lines and generators that significantly impact system stability in the event of an outage. The contingencies are also ranked to identify the most severe outages that significantly cause voltage collapse because of the outage of line or generator.