Adaptive Load Shedding Scheme Research Articles

Innovative Load Shedding Scheme to Restore Synchronous Generator-Based Microgrids During FIDVR

Fault induced voltage recovery (FIDVR) phenomenon may occur in microgrids ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Gs) and distribution systems due to the induction motors (IMs) behavior after system voltage sags especially short circuit faults. For the case that motors reaccelerate, system voltage may recover to its nominal value after few seconds. However, it may remain low during motor stalling condition, which result in unstable (non-recoverable) FIDVR. In this paper, an adaptive load shedding scheme is proposed to mitigate unstable FIDVR in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Gs containing synchronous dispersed generators (SDGs) to enhance system dynamic behavior. Derivative of the imaginary part of equivalent admittance seen from the distribution feeders with respect to time is utilized to discriminate stable FIDVR from unstable ones. In addition, a new criterion is proposed to determine SDG stability margin during the FIDVR phenomenon. Then, a new load shedding scheme is presented based on the proposed SDG stability margin and IM reacceleration index. Moreover, the load shedding time delay is adaptively calculated based on motor reacceleration time. Performance of the proposed method is evaluated by comprehensive simulation studies on two realistic networks. These investigations fairly reveal security and reliability of the proposed method in various conditions.

Adaptive Voltage-Based Load Shedding Scheme for the DC Microgrid

The direct current (DC) microgrid requires a fast load shedding scheme that prevents instability and voltage collapse when the distributed energy resources are unable to meet the power demand. The load shedding scheme is also expected to prevent unnecessary service interruptions caused by over-shedding of loads. This paper proposes an adaptive voltage-based load shedding scheme utilizing voltage thresholds that are automatically adjusted depending on the rate of change of locally measured bus voltages. The performance of the proposed load shedding scheme is investigated and compared with that of the conventional voltage-based load shedding scheme, using a realistic and detailed study system, under various disturbances. The comprehensive simulation studies that are conducted using the PSCAD software indicate that the adaptive load shedding scheme (i) effectively maintains the DC microgrid power balance through fast and coordinated load shedding, (ii) prevents the DC bus voltages from falling below acceptable levels, (iii) ensures that the critical loads do not experience excessive steady-state voltage deviations, (iv) minimizes the durations and magnitudes of the temporary voltage drop caused by sudden disturbances, and (v) improves power supply reliability.

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.

An improved adaptive wide-area load shedding scheme for voltage and frequency stability of power systems

Conventional under-frequency and under-voltage load shedding schemes are known as effective approaches for load shedding purposes. Generally, these two schemes are deployed independently within which the combinatorial disturbances are overlooked. Contributing to this context, the ongoing study raises a high concern on developing a generalized wide-area adaptive load shedding scheme. In order to preserve the voltage and frequency stabilities, the established approach deploys the variations of voltage, frequency, active, and reactive power in all buses, simultaneously. More specifically, the reactive power variation on voltage stability is incorporated in the proposed load shedding index. This idea improves the performance of the proposed scheme, significantly. Infield phasor measurement units are establishing a wide-area basis, enabling the proposed approach in modern control centers. Extensive numerical studies are devised to assess the performance of the proposed approach encountering various disturbances. The obtained results are discussed in depth.

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.

Improved UFLS with consideration of power deficit during shedding process and flexible load selection

This study presents an improved under-frequency load shedding (UFLS) scheme that can detect power deficit during the shedding process and accordingly adjust the amount of load shedding. This is achieved by continuous monitoring of the overshooting signal of the second frequency derivative of the centre of inertia. Once detected, an equivalent system inertia constant is estimated in order to quantify the new power deficit. The scheme is also equipped with an optimisation algorithm to determine the best combination of loads that is close to the amount of power deficit, which minimises frequency overshoot/undershoot. The optimisation technique selected for this work is based on particle swarm optimisation. The performance of the proposed UFLS scheme was validated using a modified IEEE 33 bus with two mini-hydro generators and one full converter wind turbine. The system and the proposed UFLS was modelled and simulated in PSCAD/EMTDC software. The results confirmed that the proposed scheme is capable of shedding loads with minimum undershoot/overshoot, and detect and estimate a new power deficit during load shedding. The results reported by the proposed scheme proved to be significantly better than those reported by conventional and adaptive load shedding schemes.

A Plug-and-Play Selective Load Shedding Scheme for Power Systems

In contemporary power systems, conventional load shedding schemes are typically used. These load shedding schemes employ a preset step-level structure that identifies the amount of load to be shed. There are predefined candidate circuit breakers to be opened, based on the importance of loads, historical data for the load demand at each substation, and the operator's experience. Clearly, real conditions at the time of fault differ, resulting in the shedding of a larger or smaller amount of load and to a nonoptimal operation of the load shedding scheme. A second point to note is that adaptive load shedding schemes are usually only theoretically functional due to the large amount of real-time monitoring data required to perform satisfactorily. These data cannot be obtained on a large scale and in a cost effective manner. Last but not least, adaptive load shedding schemes do not always respond adequately against newly emerging or combinational events. In this paper, an adaptive load shedding scheme is proposed, based on a single measurement of the electric frequency of the power system. It approximates online the certain structure of the nonlinearities of the swing equation of the power system and adaptively bounds the load disturbances and the functional approximation errors of the nonlinearities. This way, a control law is extracted that “cancels” the nonlinearities and responds adequately to the load disturbances, achieving the frequency stability of the system effectively.

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.

An Analytical Adaptive Load Shedding Scheme Against Severe Combinational Disturbances

To diminish damaging consequences of power system major disturbances, under frequency load shedding (UFLS) plans are devised so as to prevent power networks from collapse. Since load shedding scenarios in conventional UFLS plans are specified regardless to voltage stability criteria, they cannot bring desired outcomes at the event of severe and combinational contingencies. In these circumstances, the location of load drops, in addition to their amounts, should be determined as well. This paper presents a two-unit wide-area adaptive load shedding scheme based on synchrophasors. In the first unit, an adaptive precise system frequency response (SFR) model is developed, by the help of which, the appropriate amount of load rejections is determined such that the dynamic and steady state frequency limitations are fulfilled. Incorporation of both static and dynamic loads into the adaptive SFR model results in a negligible error in calculations. Specifying the best location of load drops based on suitable voltage stability criteria is the main task of the second unit. These two units are executed in a parallel way; the computational burden of the proposed approach is thus tractable for real world applications. The new method is successfully testified in the IEEE 39-bus system and Khorasan network.

Adaptive load shedding scheme for frequency stability enhancement in microgrids

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.

Distributed Adaptive Load Shedding Scheme to Maintain Transient Stability and Prevent Overload

Abstract The paper presents the design and operation principles of the distributed adaptive load shedding scheme. In addition, consideration is given to the algorithms for the adjustment of adaptive control actions performed by this scheme to ensure transient stability of an electric power system and eliminate overcurrent in the network components. Based on the complexity of the defended system structure as well as the need to enhance the adaptability of the algorithms, the research suggests refusing to use the conventional “behavior confirmation” principle in favor of the improvement in an intelligence level of the automation.

Adaptive load shedding scheme to preserve the power system stability following large disturbances

Following large disturbances, the conventional under frequency load shedding (UFLS) relays may not operate properly, as they are designed to merely preserve the frequency stability of the system, independent of its voltage stability. In this study, an adaptive load shedding scheme is proposed to maintain the power system stability when the conventional scheme fails. In doing so, the frequency and its falling rate along with a properly defined voltage falling rate are exploited to determine closeness to the disturbance area and its severity. Accordingly, a proper amount of load is shed from the locations with the highest influence on the system stability. The proposed scheme exclusively utilises locally measured quantities and does not require a communication link. The associated logic is practical and can be easily appended to the existing UFLS relays to enhance their performance. To demonstrate the efficiency of the proposed UFLS scheme, it is applied to two large-scale power systems where the exact load model is taken into account. The obtained results verify that a large number of blackouts because of inappropriate operation of conventional UFLS relays can be prevented using the proposed scheme.

Wide-Area Adaptive Load Shedding Control to Counteract Voltage Instability

A two-level adaptive load shedding scheme against voltage instability is proposed, adjusting its actions to the severity of the situation. The lower level includes a set of distributed controllers which curtail loads once the voltages at monitored transmission buses fall and stay for some time below threshold values. The adaptive nature of the proposed scheme comes from the upper level adjusting those thresholds in real-time. At the time instant the upper level detects that the system enters an emergency condition, it sends a signal to the lower-level controllers requesting them to take their currently measured voltages as threshold values. The upper-level emergency detection can be based on various criteria, the key-point being that it takes advantage of a wide-area monitoring of the system. In particular, the paper illustrates the use of a voltage instability detection scheme based on sensitivity computation. In case the upper level fails sending its signal, the lower-level controllers act in purely distributed mode, each using its pre-set threshold voltage. The capabilities of the proposed scheme are illustrated on a small but realistic test system.

A Synchrophasor Assisted Frequency and Voltage Stability Based Load Shedding Scheme for Self-Healing of Power System

This paper presents a new scheme for load curtailment in the power system required for its self-healing under critical contingencies, which may pose a threat to frequency as well as the voltage stability of the system. The load shedding requirement in the system has been calculated based on the computed disturbance power as well as the voltage stability condition of the system, with the aid of real-time data, assumed to be available from the synchrophasor based wide area monitoring and control system (WAMCS). This scheme assesses voltage stability based on a dynamic voltage stability criterion, formulated using a Voltage Stability Risk Index (VSRI). Suitable locations for the load curtailments have been chosen according to the VSRI, calculated at each load bus. Performance of the proposed scheme has been tested on New England 39-bus system and a practical 246-bus Indian system. The results are compared with a conventional under frequency load shedding scheme and an adaptive load shedding scheme.

Monitoring the First Frequency Derivative to Improve Adaptive Underfrequency Load-Shedding Schemes

The actual activation of underfrequency load shedding is rarely needed in a power system. However, this does not diminish its significance, as it is the last barrier before a system blackout if underfrequency conditions occur. Like in all other areas of application, a tendency also exists to maximize the efficiency of the underfrequency load-shedding scheme and to bring its operation to the limits. By default, the scheme should be able to stop a frequency fall under any circumstances. However, trying to lower the quantity of the disconnected load in the same process is the purpose of this paper. After the theoretical background is summarized, in terms of the reaction of individual generators in the islanded system to a certain power imbalance, a procedure is described showing how it is possible to adjust any of the adaptive load-shedding schemes to utilize as much primary frequency control as possible. As a reference, a theoretically optimal underfrequency load-shedding scheme is presented.

Adaptive Scheme for Minimal Load Shedding Utilizing Synchrophasor Measurements to Ensure Frequency and Voltage Stability

This article presents a new scheme for load curtailment in the power system under critical contingencies that may pose a threat to the frequency and voltage stability of the system. The proposed scheme is based on a two-stage load-shedding strategy. In the first stage, disturbances have been classified based on the computed value of the disturbance power, and the load-shedding requirement in the system has been assessed based on the frequency stability, as well as dynamic voltage stability, conditions. The scheme assumes the availability of real-time data from a synchrophasor-based wide area monitoring and control system. A dynamic voltage stability criterion has been developed in this stage using a voltage stability risk index to assess the voltage stability in real time. Suitable locations for the load curtailments have been chosen according to the voltage stability risk index, which is calculated at each load bus. After the system attains transient stability, the second stage of the proposed scheme determines the optimal redispatching of generators for recovering the loads as much as possible, which had been curtailed in the first stage following the contingency. Performance of the proposed scheme has been tested on a 10-machine, New England 39-bus system, and the results are compared with a conventional under-frequency load-shedding scheme and an adaptive load-shedding scheme.

Adaptive load shedding and regional protection

The requirement for improved efficiency whilst maintaining system security necessitates the development of improved system analysis approaches and the development of advanced emergency control technologies. Load shedding is a type of emergency control that is designed to ensure system stability by curtailing system load to match generation supply. This paper presents a new adaptive load shedding scheme that provides emergency protection against excess frequency decline, whilst minimizing the risk of line overloading. The proposed load shedding scheme uses the local frequency rate information to adapt the load shedding behaviour to suit the size and location of the experienced disturbance. The proposed scheme is tested in simulation on a 3-region, 10-generator sample system and shows good performance.

Self-Healing in Power Systems: An Approach Using Islanding and Rate of Frequency Decline Based Load Shedding

This paper provides a self-healing strategy to deal with catastrophic events when power system vulnerability analysis indicates that the system is approaching an extreme emergency state. The system is adaptively divided into smaller islands with consideration of quick restoration. Then an adaptive load shedding scheme based on the rate of frequency decline is applied. The proposed scheme is tested on a 179-bus, 20-generator sample system and shows very good performance.

Adaptive load shedding for isolated power systems

Emergency control of frequency in an isolated power system is considered. In many such systems, limited load shedding is permitted under certain generation loss conditions to reduce the expense of spinning reserve provision. It is important in these circumstances to minimise consumer disruption through proper-design of the load shedding arrangements. An adaptive load shedding scheme is proposed, based on intelligent relays supplied with updated values of key system parameters in addition to the usual measurement of frequency. Results are presented to demonstrate the ability of the proposed scheme to reduce potential load shedding to a value close to the theoretical minimum.

New technique for adaptive-frequency load shedding suitable for industry with private generation

A new principle for adaptive load shedding schemes is investigated. The method requires the instant collection of data that can be utilised in analytical formulae for the frequency and its rate of change. The correctness of the method requires some sensitivity analyses towards the effectiveness of the parameters of every element of the industrial power system. Dynamic loads are represented with a new combined D-factor to account for both frequency and voltage changes. The turbine speed regulator and the generator automatic voltage regulator with a full-phase coordinate model is analysed and compared with the performance of the transient energy function and its analytical formulae. The new adaptive technique, which utilises the principle of adaptive timing setting triggered by a ROCOF relaying and, with an estimate of the lowest frequency at df/dt = 0, is compared with a modern scheme utilising ROCOF and two linear-type relays. The proposed adaptive method proved to reduce the amount of load shed, and it is inherently accurate.