Undervoltage Load Shedding Scheme Research Articles

Inclusion of Pre-Existing Undervoltage Load Shedding Schemes in AC-QSS Cascading Failure Models

A challenging problem facing AC-Quasi-Steady-State (AC-QSS) cascading failure models of power system is the divergence issue primarily stemming from voltage collapse phenomena. In reality, there are undervoltage load shedding (UVLS) relays, which aim to prevent such a collapse by shedding a pre-specified fraction of load at buses where the corresponding voltages fall below a threshold. However, capturing the UVLS action in QSS models is very difficult, because most of the time the model cannot generate an equilibrium below the voltage threshold due to divergence. To address this problem, current models have applied different variants of uniform load shedding (ULS) till convergence is achieved, which differ from the ground truth. In order to solve this, we propose a methodology that leverages the post-ULS load flow as a starting point when divergence occurs. In this condition, a sensitivity index coupled with the voltage magnitudes of buses is used to recognize the buses that are most prone to voltage collapse. The UVLS scheme is then applied to these buses. To verify the accuracy of the results, we also present a suitable dynamic cascade model with appropriate limits and protection details that can selectively capture UVLS action, thereby revealing the proximate ground truth. Predictions of the proposed model are validated against those of the dynamic model for representative cases in IEEE 118-bus system. In addition, results of the proposed model are contrasted with two ULS schemes on the 2383-bus Polish system.

A new optimal under-voltage load shedding scheme for voltage collapse prevention in a multi-microgrid system

In this paper, a new optimal under-voltage load shedding (UVLS) method is presented to maintain voltage stability of a multi-microgrid (MMG) system. Great benefits are gained by exploiting MMGs, indicating the integration of microgrids (MGs) and distributed energy sources (DERs), at both medium voltage (MV) and low voltage (LV) levels. Advanced strategies should be employed in order to guarantee stability of a MMG. This paper, for the first time, presents an optimal UVLS scheme for the MMGs which considers voltage stability margin, minimum amount of load shedding and minimum operation cost of the system in the objective function, simultaneously. In addition, application of demand response programs (DRPs) and MGs/loads priorities have been addressed in the proposed method. The optimal load shedding problem is solved using hybrid discrete continuous exchange market algorithm (HDCEMA) with regard to system stability, voltage magnitudes and other related constraints. The proposed method is tested on a 55-bus MMG system consisting of 5 MGs with distributed and renewable energy sources. In order to compare results with previous methods, two UVLS schemes and two optimization algorithms in the literature are implemented on the simulated MMG system. Simulation results confirm the effective performance of the proposed method.

Dynamic under-voltage load shedding scheme considering composite load modeling

This manuscript recommends an under-voltage load shedding scheme based on the sensitivity of dynamic voltage curves to mitigate voltage collapse in highly stressed power systems. The developed method can be used to determine the minimum amount and proper locations of load curtailment by considering power system dynamics including dynamic load modeling. The selected Indonesian regional system has a large number of dynamic loads in form of air conditioning and water pump loads whose dynamics contribute considerably to the complexity of the power system stability. Loss of the transmission line in the system caused by the impact of the composite load after major disturbance is studied. The proposed scheme involves an iterative procedure based on the sensitivity analysis of the dynamic voltage curve to solve the problem of load shedding. Furthermore, this work developed a new index named dynamic voltage-active power sensitivity (DVPS). The DVPS is calculated in all load buses to give information about the bus that has a predominant influence on improving system voltage stability through load shedding. Dynamic simulations are carried out with an Indonesian power system, the South Sulawesi interconnection power system with the proposed load shedding scheme. The results of this work show a noteworthy improvement in the magnitude of the voltage level as well as the voltage stability margin.

A practical under-voltage load shedding strategy for regional power grid considering multiple operating modes

Nowadays, under-voltage load shedding (UVLS) schemes in China’s regional power grids are often drafted according to one certain operating mode, and then been chosen by experienced technicians based on comparison between simulation results on various operating modes. This process seriously depends on the skill and experience of the technician, and cannot guarantee the safety under some circumstances. Considering the above issues, this paper proposes a configuration method of UVLS applying to distributed control systems, and an automatic scheme formulating process. Specifically, aiming at developing a strategy suitable for multiple operating modes, Analytic Hierarchy Process (AHP) is introduced to obtain the fuzzy comprehensive sensitivities of the buses. The transient voltage stability of a regional power grid is investigated and the UVLS strategy is studied, which indicates the validity and practicality of the proposed configuration method.

Deep Feedback Learning Based Predictive Control for Power System Undervoltage Load Shedding

In practical power systems, it is still very challenging to figure out a cost-effective undervoltage load shedding (UVLS) scheme that can reliably and adaptively react to the short-term voltage stability (SVS) problem. Faced with this challenge, this paper develops an intelligent data-driven predictive UVLS scheme for online SVS enhancement. Inspired by valuable ideas in model predictive control and supplementary excitation control in power systems, a novel deep feedback learning machine (DFLM) is designed to precisely predict future voltage violations after UVLS execution. With the help of the DFLM, the UVLS scheme is aware of potential effects of various candidate UVLS actions. Owing to this desirable nature, it can adaptively respond to diverse SVS conditions and optimize UVLS decisions in a non-iterative way. Further, two well-designed strategies, i.e., stepwise constraint relaxation and incremental DFLM adaptation, are introduced to enhance the scheme's reliability and adaptability during online application. Numerical test results on the Nordic test system and the realistic North GZ Power Grid in China showcase the excellent performances of the proposed scheme.

Development and Real-Time Implementation of an Under Voltage Load Shedding Scheme Using a Real-Time Digital Simulator

Generation, transmission, and distribution of electric power systems are extremely important to our modern society. It is critical to keep the power grid stable and reliable at all times. Individual power system reliability needs to be enforced to avoid a voltage drop due to contingencies such as a loss of generation or a sudden increase in a load demand. Voltage changes for a given increase in the load demand can lead to voltage instability conditions. The main factor causing instability is the inability of the power systems to meet the demand of the reactive power. This subject has been a concern over the last two decades and a lot of research work has been done in the area of voltage stability and control. Under-voltage load shedding schemes are commonly implemented as an emergency measure in cases where the voltage conditions are drastically dropping due to a sudden increase of the load demand in the system. This paper addresses the problem of designing and developing a robust and efficient Under-Voltage Load Shedding (UVLS) scheme for a real-time implementation in the power system networks. A new multi-stage adaptive UVLS scheme is developed in order to improve the operation and increase reliability and flexibility of the existing UVLS schemes. This scheme is implemented in real-time using RSCAD software environment of the Real-Time Digital Simulator (RTDS).

Probabilistic Study of Undervoltage Load Shedding Scheme to Mitigate the Impact of Protection System Hidden Failures

The investigations on recent blackouts have disclosed that one of the main reasons for cascaded tripping of protection systems is hidden failures (HFs). This involves an in-depth study of the different modes of HF in protection systems to assess their impact on cascading events in a power grid. This paper portrays a detailed study on the modeling of different HF modes and proposes a supervised undervoltage load-shedding scheme to further reduce the likelihood of cascading events caused by HFs. The proposed methodology uses two different probabilistic models of HFs to assess HFs’ impact on cascading events. The proposed methodology is validated through numerous case studies on the IEEE 57-bus network. It is also observed that inclusion of self-monitoring and control features in digital relays significantly reduces the risk of wide-area blackouts. The methodology can be effectively adopted for planning, inspection, and repair of the protection system and the maintenance of it where investment guarantees its reliability.

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.

Mixed integer linear formulation for undervoltage load shedding to provide voltage stability

Undervoltage load shedding (UVLS) is the last resort against voltage instability in critical situations. Here, the UVLS scheme is formulated as a mixed integer programming (MIP) model. The aim of the proposed UVLS model is to provide a predetermined value of loading margin with minimum amount of load shed. The full non-linear AC power flow equations are linearised using a piecewise linear technique. The proposed linear AC power flow is then integrated into the UVLS problem. A ZIP load model is utilised to demonstrate the fulfilment of the MIP-based UVLS model under non-linear static loads. The efficacy of the developed linear AC power flow is verified under different operational conditions and contingencies. The results of the proposed MIP-based UVLS model are compared with the original non-linear programming UVLS formulation. To verify the performance of the developed load shedding strategy, the proposed MIP-based UVLS model is implemented in IEEE 14-bus and IEEE 118-bus test systems.

A Novel Load Shedding Scheme for Voltage Stability

Abstract Undervoltage load shedding serves to maintain voltage stability when a majority of loads are fast acting. An undervoltage load shedding scheme should address two tasks: the detection of voltage instability following a large disturbance and the determination of the amount of load to be shed. Additionally, in case of short-term voltage instability, the scheme should be fast. This paper proposes a method to predict voltage instability arising due to a large disturbance. The amount of load to be shed to maintain voltage stability is then determined from the Thevenin equivalent of the network as seen from the local bus. The proposed method uses local measurements of bus voltage and power, and does not require knowledge of the network. The method is validated by simulation of three test systems subjected to a large disturbance. The proposed scheme is fairly accurate in estimating the minimum amount of load to be shed to maintain stability. The method is also successful in maintaining stability in cases where voltage collapse is detected at multiple buses.

A robust undervoltage load shedding scheme against voltage instability

Undervoltage load shedding (UVLS) is an effective measure for restoring voltage stability as a last resort in emergency conditions. In this paper, a UVLS scheme is proposed considering the uncertainties of load and line parameters. The loading margin is considered as the voltage stability index. The proposed scheme determines the optimal load shedding pattern subject to technical constraints. The info-gap decision theory uncertainty modeling technique is utilized to find a robust and cost-effective pattern of load shedding. The proposed scheme is formulated as a nonlinear programming problem, which is solved using the sequential quadratic programming technique. The performance of the proposed scheme is verified for the IEEE-14 bus and IEEE-118 bus test systems.

전력계통 신뢰도 기준 분석을 통한 765kV 선로사고에 대한 부하차단 적정량 산정에 관한 연구

Load shedding is a last measure to avoid nationwide cascading collapses of power system by removing the pre-determined amount of loads from the main grid. In Korea, SPS(Special Protection System) is prepared to keep the power system stability from the extreme contingency of the critical transmission line losses. Among them, we need to pay attention to 765kV T/L’s because they have great influence on the total system stability. According to the present SPS operating guide, the total loads of 1,500MW should be removed through 2 step under-voltage load shedding(UVLS) scheme in case of 765kV T/L’s contingencies. However, it is not clear to defined how to determine the typical load reduction amounts for each case. This paper proposes a method to estimate appropriate amounts of load shed for 765kV T/L’s contingencies by analyzing the relevant national and international standards.

Implementation of Under Voltage Load Shedding for Fault Induced Delayed Voltage Recovery Phenomenon Alleviation

Significant penetration of induction motor loads into residential neighborhood and commercial regions of local transmission systems at least partially determine a vulnerability to a fault induced delayed voltage recovery (FIDVR) event. Highly concentrated induction motor loads with constant torque could stall in response to low voltages associated with system faults. FIDVR is caused by wide spread stalling of small HVAC units (residential air conditioner) during transmission level faults. An under voltage load shedding scheme (UVLS) can be an effective component in a strategy to manage FIDVR risk and limit the any potential disturbance. Under Voltage Load Shedding take advantage of the plan to recovery the voltage of the system by shedding the load ways to alleviation FIDVR.

Adaptive under-voltage load shedding scheme using model predictive control

Recent system wide disturbances have demonstrated the need for automatic system protection schemes to prevent such cascading disturbances. In this paper, an adaptive under-voltage load shedding scheme using model predictive control is proposed to protect power system against voltage instability. The proposed scheme calculates the criticality of the system based on the measurements of voltage magnitudes and reactive power generations and then in case of voltage instability a model predictive based step-sized load shedding scheme will be triggered. The speed and amount of load shedding steps are adapted to the severity of contingency in the affected region. By devising the grid into the separate regions the proposed scheme acts as a distributed controller without having a central dispatching center. The performance of the proposed model predictive based scheme is verified over a 10-bus dynamic test case. The proposed model predictive control will be solved using sequential quadratic programming technique.

A Novel Online Load Shedding Strategy for Mitigating Fault-Induced Delayed Voltage Recovery

This paper develops a novel online fast load shedding strategy aimed at shedding the most effective load to mitigate fault-induced delayed voltage recovery (FIDVR). Induction motor kinetic energy deviation has been shown to be identical to the integral of power imbalance. Instead of using voltage as an indicator, the proposed strategy applies a centralized scheme making use of equivalent motor kinetic energy to identify the most effective loads to shed. In order to derive kinetic energy, the equivalent inertia and the speed of the induction motor are needed. A methodology is proposed to obtain these variables through online measurements. Simulations on a sample power system illustrate the novel load shedding strategy and its effectiveness. The proposed strategy is compared with conventional four-stage under-voltage load shedding (UVLS) scheme. The results have validated that the proposed strategy can effectively mitigate fault-induced delayed voltage recovery and require much less load to be shed.

Application of Multi-step Undervoltage Load Shedding Schemes to the KEPCO System

This paper deals with improvements to the special protection schemes (SPS) which have been applied to the low probability and high impact contingencies in the Korea Electric Power Corporation (KEPCO) system since 2004. Among them, the SPS for voltage instability in the Seoul metropolitan area is considered in this paper, and is a form of event-based undervoltage load shedding with a single-step scheme. Simulation results based upon a recent event that occurred on 765kV lines show that the current setting values of the SPS have to be revised and enhanced. In addition, by applying response-based multi-step undervoltage load shedding (UVLS) schemes to severe contingencies in the system, more effective results than those of the existing single-step SPS can be obtained. Centralized and distributed UVLS schemes are considered in the simulation. ULTC-based load recovery models and over excitation limiters (OXL) for the KEPCO system are also included in the long-term voltage instability studies.

Electric Power System Under-Voltage Load Shedding Protection Can Become a Trap

Problem statement: Under-Voltage Load Shedding (UVLS) protection of Electric Power Systems (EPS) is frequently used against Voltage Collapse (VC), however when there is automatic bus voltage regulation with excessive capacitive compensation, the UVLS scheme may not trip. In this case, load shedding must be based in a Voltage Collapse Proximity Indicator (VCPI). Many UVLS procedures may not be appropriate today. Approach: In order to elucidate the problem stated, several studies were carried out using MatLab/SimPowerSystems. In the first case, it was simulated a reduced electric system consisting of an infinite-bus feeding a load through a large impedance line. Two other cases were simulated now including a fixed capacitive impedance (representing a saturated SVC or similar) with 25 and 60 MVAr, both with a generator regulating the load bus voltage. Graphic curves representing the load bus voltage versus time were obtained with the application of a ramp power load. Results: In all cases the curves showed if there was sufficient time to command the UVLS scheme. The usual UVLS criteria failed for the third case. As the capacitive reactive power of the saturated compensation devices was increased, their equivalent capacitance, corresponding to the sum of maximum MVAr capacities, grows. The load demand increase, after MVAr saturation, can cause a voltage decrement which is too fast for UVLS adequate operation. Conclusion/Recommendations: Based in past experiences, any operator could be confident on existing UVLS protection of some area, but a VC can occur with the current situation without UVLS trip, as stated. It was suggested to check the current UVLS operation conditions, especially in areas where there was a growth of both load demand and reactive power resources. When UVLS method is found ineffective, then a suggestion is to replace it by a technique based upon some VCPI.

A global Particle Swarm-Based-Simulated Annealing Optimization technique for under-voltage load shedding problem

In this paper, a new approach based on hybrid Particle Swarm-Based-Simulated Annealing Optimization technique (PSO-B-SA) is proposed for solving under-voltage load shedding (UVLS) problem. Under-voltage load shedding (UVLS) is one of the most important tools for avoiding voltage instability. In this paper, the UVLS problem is formulated based on the concept of the static voltage stability margin and its sensitivity at the maximum loading point or the collapse point. The voltage stability criterion is modeled directly into the load-shedding scheme. In any UVLS scheme finding the global point is very important for having cost effective economy. The proposed PSO-B-SA methodology is implemented in the under-voltage load shedding scheme for IEEE 14 and 118 bus test systems. In addition to having better solution, the global property of the proposed approach plays an important role in on-line applications. Simulation results show the efficacy and advantage of the proposed scheme.

An enhanced under-voltage load-shedding scheme to provide voltage stability

Under-voltage load shedding is one of the most important tools for avoiding voltage instability. In this paper, an optimal load-shedding algorithm is developed. This approach is based on the concept of the static voltage stability margin and its sensitivity at the maximum loading point or the collapse point. The traditional load-shedding objective is extended to incorporate both technical and economic effects of load shedding. The voltage stability criterion is modeled directly into the load-shedding scheme. The proposed methodology is implemented over the IEEE 14 and 118 bus test systems and solved using a mathematical (GAMS/CONOPT) and two heuristic (PSO and GA) methods. The heuristic techniques are employed only to validate the results.

Summary of "System protection and voltage stability"

This paper is a summary of "System Protection and Voltage Stability", a special publication prepared by Protection Aids to Voltage Stability Working Group of the Substation Protection Subcommittee of the IEEE Power System Relaying Committee. This summary describes the risk and mechanism of voltage collapse and suggests some operating, system upgrades, and protection solutions. Various undervoltage load shedding schemes are discussed, including actual or planned installations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>