Frequency Load Shedding Scheme Research Articles

Enhancing power system stability with adaptive under frequency load shedding using synchrophasor measurements and empirical mode decomposition

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.

A multi‐area design of under frequency load shedding schemes considering energy storage system

AbstractThis paper presents a comprehensive under frequency load shedding (UFLS) model that can be implemented in a multi‐area power system with real network characteristics. Conventional single‐area UFLS models operate on the basis of an equivalent center‐of‐inertia (COI) model, which ignores the local dynamics of system frequency response (SFR) and the impacts of load shedding location. Unlike the single‐SFR model, that is commonly utilized in previous works, the suggested multi‐area or multi‐SFR UFLS plan of this research has the distinct benefit of taking into account the dynamics of power transfer across different electric areas. The proposed multi‐area UFLS design incorporates a flywheel energy storage system (FESS) to support the inertial system frequency response and alleviate more than 30% load shedding while improving the frequency nadir by 25%. In order to investigate the performance of the proposed method under high penetrations of inertia‐free renewables, the inertia of the power network is reduced by around 30%; therefore the proposed UFLS scheme is assessed under a low inertia scenario. The proposed multi‐stage UFLS scheme is formulated as a mixed‐integer linear programming problem, and the optimal settings, including frequency set‐points and load shedding, are then optimized. The efficiency of the proposed model is verified using the IEEE‐118 Bus dynamic test system.

Review of Low Inertia in Power Systems Caused by High Proportion of Renewable Energy Grid Integration

With the power industry moving toward a green and low-carbon direction, renewable energy is occupying an increasingly larger share in the power system. However, compared with traditional thermal power generation, the instability of new energy generation is very prominent, which also leads to a decrease in the inertia of the power system after the grid integration of a high proportion of renewable energy. If no measures are taken, this may lead to frequency collapse accidents. Therefore, this paper first introduces two international major power outage accidents that have occurred in recent years, analyzing the causes, and then summarizes the inspiration obtained from the accidents. Subsequently, some research results on low inertia-related issues in the power system caused by the high proportion of new energy grid integration in recent years were summarized and analyzed from three aspects: inertia evaluation methods, optimal operation measures for the power system, and under frequency load-shedding (the abbreviation “ULFS” in the following text stands for it) schemes. Finally, suggestions were made for future research directions.

Hierarchical under frequency load shedding scheme for inter-connected power systems

Severe disturbances in a power network can cause the system frequency to exceed the safe operating range. As the last defensive line for system emergency control, under frequency load shedding (UFLS) is an important method for preventing a wide range of frequency excursions. This paper proposes a hierarchical UFLS scheme of “centralized real-time decision-making and decentralized real-time control” for inter-connected systems. The centralized decision-layer of the scheme takes into account the importance of the load based on the equivalent transformation of kinetic energy (KE) and potential energy (PE) in the transient energy function (TEF), while the load PE is used to determine the load shedding amount (LSA) allocation in different loads after faults in real-time. At the same time, the influence of inertia loss is considered in the calculation of unbalanced power, and the decentralized control center is used to implement the one-stage UFLS process to compensate for the unbalanced power. Simulations are carried out on the modified New England 10-generator 39-bus system and 197-bus system in China to verify the performance of the proposed scheme. The results show that, compared with other LSA allocation indicators, the proposed allocation indicators can achieve better fnadir and td. At the same time, compared with other multi-stage UFLS schemes, the proposed scheme can obtain the maximum fnadir with a smaller LSA in scenarios with high renewable energy sources (RES) penetration.

The growth of distributed generation and associated challenges: A Great Britain case study

Great Britain has reached high penetrations of distributed generation (DG). Historically, there has been a lack of technical requirements for DG to provide system support and for network operators to monitor, control and gather detailed information on DG installations. As a result, much of it is unobservable and uncontrollable. This work has analysed the available data sources for the amount, size and type of DG installations in GB. It is found that the lack of transparency and consistency of data are likely to act as an obstacle to the development of a more active distribution network. The impact that high DG has on system operation and system stability is discussed, including the potential for DG to offer flexibility services. A system disturbance in GB on August 9th 2019 brought a number of issues into focus: Slow action to change inappropriate settings of DG loss-of-mains protection and the uncertainty that DG adds to the effectiveness of under frequency load shedding schemes. Finally, it is argued that coordination between transmission and distribution networks is central to addressing many of the challenges and is a key enabler to utilising the flexibility available from DERs and releasing capacity for more DERs to be connected.

A New Adaptive Underfrequency Load Shedding Scheme to Improve Frequency Stability in Electric Power System

This paper proposes a new adaptive underfrequency load shedding scheme (UFLS) to avoid frequency instability in electrical power system during abnormal wide area disturbances. The developed scheme is based on online monitoring of the distance relay zone 3 decisions of some tie lines using (WAMS) and SCADA/EMS systems, to check rapidly and reliably the uncontrolled islanding conditions and, permit an automatic load shedding action to maintain frequency stability of power system. Simulation results on 400 kV Turkish transmission systems demonstrated the effectiveness of the proposed scheme compared to current frequency load shedding schemes which they cannot consider all possible circumstances because of their limited access to the power network data. Simulation results clearly indicate that large disturbances in power systems can be avoided and their propagation can also be stopped using the proposed scheme.

Underfrequency Load Shedding Using Locally Estimated RoCoF of the Center of Inertia

The conventional Under Frequency Load Shedding (UFLS) scheme could result in unacceptably low frequency nadirs or overshedding in power systems with volatile inertia. This paper proposes a novel UFLS scheme for modern power systems whose inertia may vary in a wide range due to high penetration of renewable energy sources (RESs). The proposed scheme estimates the rate of change of frequency (RoCoF) of the center of inertia (CoI), and consequently, the loss of generation (LoG) size, using local frequency measurements only. An innovative inflection point detector technique is presented to remove the effect of local frequency oscillations. This enables fast and accurate LoG size calculation, thereby more effective load shedding. The proposed UFLS scheme also accounts for the effect of the inertia change resulting from LoG events. The performance of the proposed scheme is validated by conducting extensive dynamic simulations on the IEEE 39-bus test system using Real Time Digital Simulator (RTDS). Simulation results confirm that the proposed UFLS scheme outperforms the conventional UFLS scheme in terms of both arresting frequency deviations and the amount of load shed.

A new real-time multi-agent system for under frequency load shedding in a smart grid context

Automatic under frequency load shedding schemes need to be carefully designed in order to reduce the risk of widespread system collapse. This paper proposes a centralized hierarchical multi-agent system that coordinates various stages of monitoring and decision making processes. The main contribution is to improve traditional contingency response algorithms such as load shedding schemes, taking advantage of the future smart grid infrastructure. The multi-agent system seeks for a minimum amount of load disconnection in a short period of time, causing the least possible disturbance in the system frequency. A hardware-in-the-loop simulation of a full electric power system using a real time digital simulator was utilized. The solution was embedded in a real time system, consisting of hardware and software, to test and validate the proposed methodology. In addition, the studied methodology was compared with two other load shedding philosophies through a load shedding metric score. Shedding was carried out in a single step and the amount of disconnected load was close to the dynamic power unbalance. The results show that it is possible to improve the traditional load shedding philosophy schemes and use advanced communication infrastructure, monitoring and embedded processing capabilities to provide better stability and reduce unnecessary load disconnections from the system.

Long-Term Impact Evaluation of Advanced Under Frequency Load Shedding Schemes on Distribution Systems With DG Islanded Operation

This paper presents a long-term impact evaluation algorithm to assess advanced under frequency load shedding (UFLS) schemes on distribution systems with intentional islanding of distributed generation (DG). The algorithm is based on a combined discrete-continuous simulation model which is utilized to verify the effect of the schemes on reliability indices such as the system average interruption frequency index, system average interruption duration index, and energy not supplied. Moreover, a polynomial neural network-based approach to advanced load shedding is implemented to support DG islanding in order to illustrate the applicability of the evaluation. Simulation results highlight the long-term effect of employing UFLS to support intentional islanding of DG using an actual network from the South of Brazil.

Decentralized Adaptive Under Frequency Load Shedding Scheme Based on Load Information

The inertia of the power system changes when the generators in the system are disturbed or tripped. However, in some systems, there is no control center or communication device falling behind. It makes real-time inertia difficult to obtain and affects the adaptive under frequency load shedding (UFLS) scheme. This paper proposes an adaptive decentralized UFLS scheme based on load information to overcome this problem. First, a calculation method is derived according to the load information after the disturbance to identify the real-time of load equivalent virtual inertia. Then, the power deficit of load is estimated and the optimal adaptive load shedding scheme is obtained. Finally, compared with conventional UFLS schemes, the proposed scheme can be performed merely by load information. Hence, the requirement of communication conditions and the dependence on the control center are reduced. The simulation is carried out on the island utilizing the IEEE39-bus system. Furthermore, the accuracy of real-time identification of load equivalent virtual inertia and the validity of the load shedding scheme are verified.

Application of Hybrid Meta-Heuristic Techniques for Optimal Load Shedding Planning and Operation in an Islanded Distribution Network Integrated with Distributed Generation

In a radial distribution network integrated with distributed generation (DG), frequency and voltage instability could occur due to grid disconnection, which would result in an islanded network. This paper proposes an optimal load shedding scheme to balance the electricity demand and the generated power of DGs. The integration of the Firefly Algorithm and Particle Swarm Optimization (FAPSO) is proposed for the application of the planned load shedding and under frequency load shedding (UFLS) scheme. In planning mode, the hybrid optimization maximizes the amount of load remaining and improves the voltage profile of load buses within allowable limits. Moreover, the hybrid optimization can be used in UFLS scheme to identify the optimal combination of loads that need to be shed from a network in operation mode. In order to assess the capabilities of the hybrid optimization, the IEEE 33-bus radial distribution system and part of the Malaysian distribution network with different types of DGs were used. The response of the proposed optimization method in planning and operation were compared with other optimization techniques. The simulation results confirmed the effectiveness of the proposed hybrid optimization in planning mode and demonstrated that the proposed UFLS scheme is quick enough to restore the system frequency without overshooting in less execution time.

Integration of Distributed PV in Existing and Future UFLS Schemes

In recent years, the amount of photovoltaic panels in residential areas has steadily increased. As these areas are usually the first to be disconnected by under frequency load shedding schemes, the adequacy of the scheme comes under pressure. Already now, during some hours of the day, several feeders have more local generation than load. Disconnecting those feeders during an under frequency event, would aggravate the situation. Furthermore the impact of disconnecting feeders with significant load and generation, is much higher than is expected based on the netted load. Therefore, this paper proposes several new feeder ranking methodologies to take the distributed generation into account in the under frequency load shedding scheme. For each of the proposed methodologies, the advantages, disadvantage and hardware requirements are listed. Finally, these methodologies have been simulated on data provided by the French DSO.

An Adaptive Wide-Area Load Shedding Scheme Incorporating Power System Real-Time Limitations

Under frequency load shedding (ULFS) schemes are devised to help power system maintain the load-generation balance by shedding adequate loads to match the generation. In this paper, a new centralized adaptive load shedding scheme is proposed in three stages. The first stage determines the requirements needed for implementation of the proposed scheme. The optimal amount and location of load drops and post load shedding strategies are specified in the second stage. In addition to consideration of operational limitations and voltage stability criteria, this stage takes the interruption cost of dropped loads into account. In the third stage, the event type is determined in real time and the pre-specified optimal load shedding scheme and post load shedding strategies associated with the event are implemented. The proposed methodology is successfully testified in the IEEE 39-bus system through several case studies.

A Smart Under-Frequency Load Shedding Scheme based on Takagi-Sugeno Fuzzy Inference System and Flexible Load Priority

This paper proposes a new smart under frequency load shedding (UFLS) scheme, based on Takagi-Sugeno (TS) fuzzy inference system and flexible load priority. The proposed scheme consists of two parts. First part consists of fuzzy load shed amount estimation module (FLSAEM) which uses TS-fuzzy to estimate the amount of load shed and sends its value to accurate load shedding module (ALSM) to perform accurate load shedding using flexible load priority. The performance of the proposed scheme is tested for intentional islanding case and increment of sudden load in the system. Moreover, the response of the proposed scheme is compared with adaptive UFLS scheme to highlight its advantages. The simulation results show that the proposed UFLS scheme provides the accurate load shedding due to advantage of flexible priority whereas adaptive UFLS scheme due to fixed load priority does not succeed to achieve accurate load shedding.

Adaptive Under-Frequency Load Shedding Scheme in System Integrated with High Wind Power Penetration: Impacts and Improvements

As the requirements of economical operation and reliability on power grid are enhanced gradually nowadays, the existing under frequency load shedding (UFLS) scheme is not quite fit for the modern power system that integrates high wind power. In this paper, the impacts of high wind power penetration on the UFLS are discussed thoroughly. A novel adaptive load shedding (LS) scheme is presented taking the high wind power penetration into account. In the proposed scheme, the equivalent inertia constant (EIC) is calculated accurately to improve the power deficit accuracy so as to reduce the error of LS. The dynamic correction of power deficit is able to solve the negative effects of the wind power output random reduction/the wind generator tripping. Besides, the locking criterion is capable of avoiding the influences of the wind power output random increase on the LS, thus cutting down the LS costs and even preventing the frequency overshoot. Moreover, in terms of the LS parameters setting, the coordination of the low frequency protection of the wind generator and the frequency threshold is addressed. The location and capacity model of LS, which is based on the load characteristics, can ameliorate the frequency recovery process. Finally, the validity and robustness of the proposed scheme are verified in the simulations on the IEEE-39 bus system with high wind power penetration.

Multi-layer fuzzy-based under-frequency load shedding in back-pressure smart industrial microgrids

In industrial microgrids (MGs), various operating conditions may lead to severe frequency fluctuations. If no any particular remedy is thought, these fluctuations may jeopardize the system stability against which load shedding is recognized as an effective countermeasure. This issue has not been properly addressed in back-pressure MGs where cogeneration of steam and electricity is required. If the permissible operating range is violated, under frequency load shedding (UFLS) scheme propagates suitable commands to disconnect a set of loads and hence assuring the frequency stability. This manuscript proposes an efficient multi-layer fuzzy-based load shedding (MLFLS) approach to enhance the MG overall performance. In comparison with conventional UFLS scheme, the amount of shed load is determined through a multi-layer mechanism. Frequency deviation and steam pressure are selected as two control inputs and the output signal is a control command to keep a specific load in connected or disconnected mode. To this end, suitable rule basis is embedded in MLFLS inference system to determine the amount of required load to be shed. The proposed approach is tested on a simulation model of a real-world autonomous MG and a suitable comparison is performed against the conventional UFLS scheme. The obtained results are discussed in depth.

Application of Meta-Heuristic Techniques for Optimal Load Shedding in Islanded Distribution Network with High Penetration of Solar PV Generation

Recently, several environmental problems are beginning to affect all aspects of life. For this reason, many governments and international agencies have expressed great interest in using more renewable energy sources (RESs). However, integrating more RESs with distribution networks resulted in several critical problems vis-à-vis the frequency stability, which might lead to a complete blackout if not properly treated. Therefore, this paper proposed a new Under Frequency Load Shedding (UFLS) scheme for islanding distribution network. This scheme uses three meta-heuristics techniques, binary evolutionary programming (BEP), Binary genetic algorithm (BGA), and Binary particle swarm optimization (BPSO), to determine the optimal combination of loads that needs to be shed from the islanded distribution network. Compared with existing UFLS schemes using fixed priority loads, the proposed scheme has the ability to restore the network frequency without any overshooting. Furthermore, in terms of execution time, the simulation results show that the BEP technique is fast enough to shed the optimal combination of loads compared with BGA and BPSO techniques.

Impact of Large-scale Wind Power Integration on the UFLS Scheme

This paper analyzes some major factors for the impact of wind power integration on power system dynamic frequency characteristics in theory. Combining with an actual grid, test scenarios with various scales of doubly fed induction generator (DFIG)-based wind power integration are formulated. Through simulation in the scenarios with FASTEST software, the impacts on the action of under frequency load shedding (UFLS) scheme are displayed in two aspects, the rate of change of frequency (ROCOF) and wind turbine generator own protection. Simulation result shows that the ROCOF will not affect the lock action of UFLS devices in the test system of this paper. Wind turbine generator protection may worsen frequency responses, so it is proposed to be in harmony with the design of UFLS scheme.

Coordinated under frequency load and capacitor shedding for bulk power systems

Under frequency load shedding (UFLS) scheme has been widely implemented as a safety net to prevent system collapse following major disturbances which results in large mismatch between load and generation. However, voltage related issues post activation of UFLS stages are also vital concerns in preserving system stability and should be properly analysed. Fixed and switched shunt capacitors that are in service during normal operation for maintaining system voltage and dynamic MVAR reserve can generate surplus reactive power post operation of UFLS relays which may result in over voltage issues, generator under excitation and some undesirable conditions such as transformer saturation, ferro-resonance etc. This study addresses the need for coordinated under frequency load and capacitor shedding and its implementation approach to effectively preserve system stability following small and large disturbances. To confirm the feasibility of the approach, the proposed method has been used to design coordinated UFLS and under frequency capacitor shedding (UFCS) schemes for a real and actual power network. In addition, the proposed coordinated UFLS and UFCS scheme has been combined with automatic switching of shunt reactors to optimise the performance of the scheme.

Centralized Adaptive Under Frequency Load Shedding Schemes for Smart Grid Using Synchronous Phase Measurement Unit

Under frequency load shedding (UFLS) is an effective way to prevent system blackout after a serious disturbance occurs in a power system. A novel centralized adaptive under frequency load shedding (AUFLS) scheme using the synchronous phase measurement unit (PMU) is proposed in this paper. Two main stages are consisted of in the developed technique. In the first stage, the active power deficit is estimated by using the simplest expression of the generator swing equation and static load model since the frequency, voltages and their rate of change can be obtained by means of measurements in real-time from various devices such as phase measurement units. In the second stage, the UFLS schemes are adapted to the estimated magnitude based on the presented model. The effectiveness of the proposed AUFLS scheme is investigated simulating different disturbance in IEEE 10-generator 39-bus New England test system.