Frequency drop due to loss of massive generation is a threat to power system frequency stability. Under-frequency load shedding (UFLS) is the principal measure to prevent successive frequency ...declination and blackouts. Based on traditional stage-by-stage UFLS scheme, a new continuous UFLS scheme is proposed in this paper to shed loads proportional to frequency deviation. The characteristic of the proposed scheme is analyzed with a closed-form solution of frequency dynamics. Frequency threshold and time delay are added to make the proposed scheme practical. A line-by-line scheme based on precise load control is introduced to implement the continuous scheme for systems without enough continuously controllable loads. The load shedding scale factor of the proposed scheme is tuned with an analytical method to achieve adaptability to different operating conditions. The adaptability of the proposed scheme is validated with 39-bus New England model and simplified Shandong Power Grid of China.
•A new strategy for maintaining frequency and voltage stability in the power system during severe disturbances.•Implementation of the Empirical Mode Decomposition based on PMU measurements to form ...coherent bus groups.•Distribution of load shedding using weight coefficients.•UFLS Strategy for N-2 Criterion.
This paper introduces an adaptive under frequency load shedding (AUFLS) scheme based on synchrophasor measurements and Empirical Mode Decomposition (EMD) to enhance power system stability during significant disturbances. The traditional UFLS methods, which rely on fixed frequency thresholds, often lead to unnecessary disconnections and a lack of flexibility. The proposed AUFLS scheme dynamically adapts to the magnitude of disturbances, frequency response, and voltage stability index, resulting in a more efficient and responsive system. Various UFLS techniques are reviewed, highlighting the advantages of adaptive strategies. The novelty of this research lies in the innovative application of the EMD algorithm within the AUFLS framework. This method identifies the center of inertia (CoI) of the rate of change of frequency (RoCoF) from frequency signals and calculates the total active power imbalance due to disturbances. Furthermore, EMD is applied to the voltage angle at each bus to identify and form coherent bus groups. These coefficients are subsequently employed to allocate the required active power shedding within each group to maintain system stability. Voltage stability indices are calculated for each group, and load shedding is distributed among the buses based on normalized voltage stability indices, thereby enhancing the precision of load shedding decisions and the overall stability of the power system. Results demonstrate that the proposed scheme provides satisfactory frequency response while requiring less load shedding compared to traditional methods, making it effective for modern power systems with high penetration of renewable energy sources (RES). The performance of the proposed scheme is assessed using the IEEE 39-bus test system, demonstrating its effectiveness in improving system resilience to frequency disturbances. The research concludes that the AUFLS scheme offers a promising approach for enhancing the resilience of power systems to frequency disturbances.
The activation of under frequency load shedding (UFLS) is the last automated action against the severe frequency drops in order to rebalance the system. In this paper, the setting parameters of a ...multistage load shedding plan are obtained and optimized using a discretized model of dynamic system frequency response. The uncertainties of system parameters including inertia time constant, load damping, and generation deficiency are taken into account. The proposed UFLS model is formulated as a mixed-integer linear programming optimization problem to minimize the expected amount of load shedding. The activation of rate-of-change-of-frequency relays as the anti-islanding protection of distributed generators is considered. The Monte Carlo simulation method is utilized for modeling the uncertainties of system parameters. The results of probabilistic UFLS are then utilized to design four different UFLS strategies. The proposed dynamic UFLS plans are simulated over the IEEE 39-bus and the large-scale practical Iranian national grid.
•The existence of the total equivalent inertia constant of microgrid is proved.•The approach to estimate the magnitude of deficiency in active power is proposed.•An equivalent inertia constant-based ...multi-stage UFLS approach is proposed.•Variation in power generation during the load shedding process is considered.•The proposed load shedding scheme is independent of MG parameters.
The imbalance between the generated power and the load demand is the major factor that is usually responsible for frequency instability in power systems, most especially islanded microgrids. To determine the size of the loads that should be shed and their appropriate locations in the power system, to maintain the system frequency within the permissible limits, this paper presents an effective adaptive control scheme. In the proposed controller, a stepwise load-shedding approach is designed in the islanded MGs to regulate the grid frequency while providing the amount of power shortage. To this achieve, it locally measures the system parameters most especially voltage and frequency signals. Thereafter, a stepwise load-shedding will take place in locations where the highest voltage drop and frequency variation are experienced. The load-shedding step changes according to certain factors such as shedding speed, location and value, and the rate of frequency change. The proposed approach eliminates the adjustable loads to return the frequency back to the desired value. Simulation results of the proposed method under different practical scenarios, when compared with the conventional PID controller, provide considerable enhancement in the power system frequency stability.
The high penetration of renewable energy (RE) significantly impacts the power system frequency stability for the following reasons: first, the replacement of conventional thermal generator by RE such ...as wind and solar degrades the frequency control capability; second, the variability of wind and solar imposes more challenges on the frequency stability. The current unit commitment (UC) program in the industrial practice cannot fully provide frequency reserve requirement to maintain the satisfactory frequency performance and the frequency reserves are not fully compensated through the existing market scheme. In this paper, a security constrained unit commitment (SCUC) model with primary and secondary frequency reserve is proposed to co-optimize the energy and frequency reserve to provide satisfactory frequency performance. Post-contingency transmission constraints are enforced to account for the deliverability of the frequency reserve. The locational based reserve pricing is proposed to differentiate the compensating prices for reserves procured at various locations. A five-bus system and a 118-bus system are used to test the proposed model. Frequency dynamic simulation is also implemented to illustrate the frequency performance improvement based on the proposed method in terms of various frequency metrics.
•Improper operation of UFLS methods due induction motor and distributed generation de-energization dynamics.•A method based on machine learning to block the undue operation of UFLS.•Improved security ...of the UFLS without the need to use time delay function.
Under Frequency Load Shedding (UFLS) is the last resource to maintain the power system frequency after contingences. Load shedding can be performed by conventional, adaptive, computational or wide area monitoring techniques. The most common is a conventional UFLS. Load shedding is accomplished when predetermined frequency thresholds are reached. Although the conventional methods are simple, easily implemented and dependable, they may operate improperly. In the Brazilian electric power system, several actions of the UFLS scheme were found to be improper owing to the dynamics of induction motors and distribution generation during the main source outage of distribution feeders. In this way, when the main source is out of service, machine inertia maintains the frequency for some electric cycles, leading the frequency relay to undue tripping. In this paper, the negative sequence voltage and the rate of change of frequency are handled by a logistic regression method to identify the events of improper action. The proposed method was evaluated through the modified IEEE 9-bus test system, and several cases have been tested. The proposed method reached a level of performance higher than 99% of correct operation, and it needs a small training set.
The increasing share of renewable distributed generations has raised a challenge about the frequency stability. Under-frequency load shedding (UFLS) can deal with this challenge, but it usually ...considers a prorated load-shedding strategy and neglects the impact of uncertainty from renewable generation. This may result in unnecessary additional economic losses from the system blackout. In this paper, a risk-decision-based optimal adaptive decentralized UFLS method is proposed to coordinate the distribution of shedding load in the smart distribution network to avoid unnecessary additional economic losses, while considering the wind power uncertainty. Firstly, the uncertainty from wind power generation is modeled by confidence intervals and evaluated by the calculation of the conditional value at risk (CVaR). Then, to economically distribute the power deficit among load agents, a decentralized consensus on the incremental cost among decentralized agents is reached, where the interruption cost of the local agent varies with its load as a nonlinear relationship. Finally, a two-layer adaptive risk-decision decentralized UFLS control framework is proposed to enable the frequency stability and economic operation of the smart distribution network. With the proposed strategy, local information sharing and important global information discovery are achieved in a distributed way. Simulations on an improved IEEE 33-bus system show that the proposed strategy can effectively avoid rigid shedding and implement the optimal allocation of the system power deficit.
•An adaptive risk-decision UFLS strategy is proposed to consider the uncertainty of wind power.•A confidence level selection method is designed to improve the accuracy of the reference of CVaR.•A CVaR-based deficiency estimation method to minimize the risk of load shedding under wind power uncertainty.•The shedding load is distributed among different load agents from the perspective of the overall social welfare.
Under-Frequency Load Shedding (UFLS) schemes are the last resort to contain a frequency drop in the grid by disconnecting part of the demand. The allocation methods for selecting feeders that would ...contribute to the UFLS scheme have traditionally relied on the fact that electric demand followed fairly regular patterns, and could be forecast with high accuracy. However, recent integration of Distributed Generation (DG) increases the uncertainty in net consumption of feeders which, in turn, requires a reformulation of UFLS-allocation methods to account for this uncertainty. In this paper, a chance-constrained methodology for selecting feeders is proposed, with mathematical guarantees for the disconnection of the required amount of load with a certain pre-defined probability. The correlation in net-load forecasts among feeders is explicitly considered, given that uncertainty in DG power output is driven by meteorological conditions with high correlation across the network. Furthermore, this method is applicable either to systems with conventional UFLS schemes (where relays measure local frequency and trip if this magnitude falls below a certain threshold), or adaptive UFLS schemes (where relays are triggered by control signals sent in the few instants following a contingency). Relevant case studies demonstrate the applicability of the proposed method, and the need for explicit consideration of uncertainty in the UFLS-allocation process.
•A chance-constrained methodology for selecting feeders contributing towards UFLS is proposed.•Uncertainty and correlation driven by Distributed Generation are explicitly considered.•A distributionally-robust option is given, requiring minimal information on uncertainty.•The method applies to several communication requirements for the operation of the power system.
•Design of a multi-stage stochastic optimal UFLS model to improve system frequency performance.•Developing a time discrete MILP model to procure near-optimal setting of UFLS relays.•Ensuring the UFLS ...scheme performance with respect to power system various uncertainties.•Efficient relay setting extraction including set point frequency, load to be shed, the time delay.
The ever-increasing penetration of Renewable Energy Sources (RESs) into the power system has faced system operators with higher risks subject to a growing level of the associated uncertainties. To preserve the system frequency security, an under-frequency load shedding (UFLS) scheme can usually be utilized as a final remedial action, which is aimed at removing the excessive load. UFLS can be managed in a multi-stage portfolio based on the priority and sensitivity of the loads under control to cope optimally with the occurring power imbalances. Design of an optimal, robust UFLS scheme is a vital procedure. To that end, the present paper proposes a new UFLS protective system which is conducted to shed the minimum optimal load after precise detection of the frequency excursions. The problem is transferred into a Mixed-Integer Linear Programming (MILP) based optimization framework, and it is to be solved in several stochastic scenarios for setting the UFLS system parameters. The analysis of the UFLS setting results, which are extracted through implementation of the model on the IEEE 39-bus test system, demonstrates the effectiveness of the proposed MILP-based methodology in dealing with the severe uncertainties resulting from RES output variations and load fluctuations.
•Proposing a UFLS plan considering a comprehensive list of credible unit schedule.•Optimizing the UFLS plan via an MIP model considering the system frequency response.•Modeling smart load in primary ...frequency control to reduce the load shedding.
Due to inertia deterioration which is caused by low inertia renewable resources, frequency-based protective schemes must be re-designed. This paper presents a multi-stage under frequency load shedding (UFLS) plan which is designed based on the yearly load duration curve (LDC) and activated according to the system frequency and the rate-of-change-of-frequency (RoCoF). In order to design a robust UFLS plan, a comprehensive list of credible operational and topological scenarios under different levels of renewables penetration are defined. For optimizing the UFLS performance over the constructed scenarios, a detailed system frequency response including the time response of governors, and natural load damping is developed. Then, the potential of frequency-dependent smart loads (SLs) as a remedy to postpone or reduce the total amount of load shedding in low inertia grids are investigated. Eventually, the proposed UFLS scheme is re-designed in the presence of SLs. The proposed method is formulated as a Mixed-Integer Linear Programming (MILP) model and solved using the CPLEX algorithm in GAMS and the effectiveness of the proposed method is investigated for the dynamic IEEE 39-bus test system.
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