Showing papers on "Power-system protection published in 2011"
01 Jan 2011
TL;DR: It is expected that WAMPAC systems will in the future reduce the number of catastrophic blackouts and generally improve the reliability and security of energy production, transmission, and distribution, particularly in power networks with a high level of operational uncertainties.
Abstract: Wide-area monitoring, protection, and control (WAMPAC) involves the use of system-wide information and the communication of selected local information to a remote location to counteract the propagation of large disturbances. Synchronized measurement technology (SMT) is an important element and enabler of WAMPAC. It is expected that WAMPAC systems will in the future reduce the number of catastrophic blackouts and generally improve the reliability and security of energy production, transmission, and distribution, particularly in power networks with a high level of operational uncertainties. In this paper, the technological and application issues are addressed. Several key monitoring, protection, and control applications are described and discussed. A strategy for developing a WAMPAC system in the United Kingdom is given as well.
682 citations
TL;DR: In this article, a new protection algorithm for DC line faults in multi-terminal high voltage DC (MTDC) systems is proposed, which uses wavelet analysis to detect the fault location based on local measurements.
Abstract: A new protection algorithm for DC line faults in multi-terminal high voltage DC (MTDC) systems is proposed in this study A four-terminal MTDC model is used to investigate fault behaviour and detection using the simulation program PSCAD/EMTDC The simulation results are post-processed using Matlab The fault clearing must be done very rapidly, to limit the effect of the fault on neighbouring DC lines because of the rapid increase in DC current However, before clearing the line, the fault location must be detected as soon as possible A rapid fault location detection algorithm is therefore needed, preferably without communication The protection algorithm proposed in this study uses wavelet analysis to detect the fault location based on local measurements The protection algorithm consists of three independent fault criteria, of which two use wavelet analysis The third criterion is based on a detection method in the time domain The latter is an additional detection method independent of wavelet analysis Using a two out of three selection criteria results in an increased reliability of the whole protection algorithm The final objective is to implement a protection algorithm which allows to detect a DC fault within 1 ms without using communication between the participating converter stations
292 citations
TL;DR: In this article, the authors proposed a protection strategy based on microprocessor-based relays for lowvoltage microgrids, which is to a large extent independent of the fault current magnitude and the mode of operation.
Abstract: One of the major challenges associated with microgrid protection is to devise an appropriate protection strategy that is effective in the grid-connected as well as islanded mode of operation. This paper proposes a protection strategy based on microprocessor-based relays for low-voltage microgrids. Further, the structure of a new relay enabling the proposed protection strategy is presented. One of the salient feature of the developed protection scheme is that it does not require communications or adaptive protective devices. Moreover, it is to a large extent independent of the fault current magnitude and the mode of operation. Transient time-domain simulation studies are conducted to demonstrate the effectiveness of the proposed protection strategy and its enabling relay, using the PSCAD/EMTDC software package.
280 citations
TL;DR: A new smart grid infrastructure for active distribution systems that will allow continuous and accurate monitoring of distribution system operations and customer utilization of electric power is presented.
Abstract: This paper presents a new smart grid infrastructure for active distribution systems that will allow continuous and accurate monitoring of distribution system operations and customer utilization of electric power. The infrastructure allows a complete array of applications. The paper discusses four specific applications: a) protection against downed conductors; b) load levelization; c) loss minimization; and d) reliability enhancement.
224 citations
20 Mar 2011
TL;DR: Different options and ideas for VSC system protection are explored, which have become an attractive option for renewable energy applications or for distribution in large cities.
Abstract: Currently classical thyristor-based high voltage direct current (HVDC) systems hold the market in bulk power transmission. However, recent advances in semiconductor technology have led to voltage source converter based HVDC (VSC-HVDC) systems becoming a viable competitor. Not only is VSC-HVDC a competitor for transmission but it can also be used in multi-terminal systems, which have become an attractive option for renewable energy applications or for distribution in large cities. As more and more VSC-HVDC systems are installed, the protection of these systems must be taken into account. This paper explores different options and ideas for VSC system protection.
210 citations
01 Jan 2011
TL;DR: In this article, the authors identify factors that contribute to lightning-caused faults on the line insulation of overhead distribution lines and suggested improvements to existing and new constructions, as well as suggested improvements for new and existing constructions.
Abstract: Factors that contribute to lightning-caused faults on the line insulation of overhead distribution lines and suggested improvements to existing and new constructions are identified in this guide.
193 citations
TL;DR: In this article, the authors describe a general method to locate faults in a distributed generation (DG) system using synchronized voltage and current measurements at the interconnection of DG units and is able to adapt to changes in the topology of the system.
Abstract: It has been shown that coordination between protective devices in distribution systems in the presence of significant distributed generation (DG) will be disrupted. With the recent trend of adopting and integrating renewable resources and microgrids with distribution systems, it is probable that distribution systems will have significant and arbitrary penetration of DG in the near future. This will change the distribution systems to multisource unbalanced systems where protective devices may not coordinate. The fault location in this type of system will be a challenge. This paper describes a general method to locate faults in this type of system. The method uses synchronized voltage and current measurements at the interconnection of DG units and is able to adapt to changes in the topology of the system. The method has been extensively tested on a 60-bus distribution system for all types of faults with various fault resistances on all sections of the system, with very encouraging results.
189 citations
TL;DR: A conceptual layered framework for protecting power grid automation systems against cyberattacks without compromising timely availability of control and signal data is proposed and the proposed “bump-in-the-wire” approach provides security protection for legacy systems which do not have enough computational power or memory space to perform security functionalities.
Abstract: The smart grid moves new power grid automation systems from being proprietary and closed to the current state of information technology (IT) which is highly interconnected and open. But open and interconnected automation platforms bring about major security challenges. The power grid automation network has inherent security risks due to the fact that the systems and applications for the power grid were originally designed without much consideration of cybersecurity. This paper first introduces scope and functionalities of power grid, its automation and control system, and communications. Potential cyberattacks and their adverse impacts on power grid operation are discussed, a general SCADA cyberattack process is presented. This paper discusses the major challenges and strategies to protect smart grid against cyberattacks and finally proposes a conceptual layered framework for protecting power grid automation systems against cyberattacks without compromising timely availability of control and signal data. The proposed “bump-in-the-wire” approach also provides security protection for legacy systems which do not have enough computational power or memory space to perform security functionalities. The on-site system test of the developed prototype security system is briefly presented as well.
162 citations
TL;DR: In this paper, a novel anti-islanding method (AIM) is proposed, which injects the output current with a little harmonic current into the grid and monitors the harmonic components of the voltage at the point of common coupling using the Goertzel algorithm.
Abstract: Islanding refers to a condition of a distributed generator (DG) in that it continues to power a location even though power from the grid is no longer present. This condition can be dangerous to grid workers who may not realize that the load is still powered even though there is no power from the grid. Adverse effects of islanding are low power quality, grid-protection interference, equipment damage, and personnel safety hazards. For these reasons, DG systems must detect an islanding condition and immediately stop producing power; this is referred to as anti-islanding. Islanding detection methods can be categorized into two major approaches: the passive and active methods. The passive methods are based on measurement of the natural effects of islanding. The active methods use intentional transients or harmonic effects. When the power generated by the DG matches the load power consumption, passive methods fail due to the small natural effects of islanding. Therefore, the passive methods have a nondetection zone (NDZ). The active methods can reduce the NDZ size. However, these methods reduce the grid power quality. In this paper, a novel anti-islanding method (AIM) is proposed. A single-phase DG using the proposed AIM injects the output current with a little harmonic current into the grid and monitors the harmonic components of the voltage at the point of common coupling using the Goertzel algorithm. The Goertzel algorithm is a kind of discrete Fourier transform. It extracts the magnitude and phase of the desired frequency from the input signal, with a minimum computation. The proposed islanding detection algorithm resolves the NDZ but also the bad effects on the grid power quality due to injecting harmonic components qualified by the interconnection standard. The proposed islanding detection method was verified using PSIM (see www.powersimtech.com) simulations and experimental results.
146 citations
TL;DR: In this paper, a comprehensive scheme to study transmission vulnerability based on the fault chain theory of security science is proposed, where the cascading failure process and its generic features are described according to a fault chain and a new vulnerability assessment index is presented to identify critical lines and sections contributing to failure extension and system instability.
Abstract: Vulnerable transmission lines or sections have great impacts on large-scale blackouts and cascading failures in power grids. A comprehensive scheme to study transmission vulnerability based on the fault chain theory of security science is proposed in this paper. In this scheme, the cascading failure process and its generic features are described according to a fault chain. Power flow transfer, sequent overload of lines, and damage of transmission sections are modeled and normalized to predict all the fault chains. On the basis of the fault chains, a new vulnerability assessment index is presented to identify critical lines and sections contributing to failure extension and system instability. The effectiveness of the proposed assessment scheme is demonstrated via numerical examples on three power systems. Considering operation properties and transient impact, the proposed scheme can be executed in an efficient way.
TL;DR: In this article, the authors developed new models and concepts for incorporating the effect of protection system failures into power system reliability evaluation, and proposed a Markov model of power systems with protection failures.
Abstract: This paper develops new models and concepts for incorporating the effect of protection system failures into power system reliability evaluation. The two types of protection failures, i.e., undesired-tripping mode and fail-to-operate mode, and their impact on reliability modeling are discussed. A complete Markov model for current-carrying components is established and its simplified form appropriately describes the overall reliability situation of individual components. Concepts of self-down state and induced-down state are introduced and used to build the composite unit model for quantitatively assessing the influence of protection failures on modeling system states. Finally, a Markov model of power systems with protection failures is proposed for system reliability evaluation. The proposed methodology is then illustrated in detail.
19 Jun 2011
TL;DR: The Distributed Energy Technologies Laboratory at Sandia National Laboratories (SNL) has used multiple reconfigurable arrays with a variety of module technologies, inverters, and balance of system (BOS) components to characterize new Photovoltaic DC AFCIs and Arc Fault Detectors (AFDs).
Abstract: The 2011 National Electrical Code® (NEC®) added Article 690.11 that requires photovoltaic (PV) systems on or penetrating a building to include a listed DC arc fault protection device. To fill this new market, manufacturers are developing new Arc Fault Circuit Interrupters (AFCIs). Comprehensive and challenging testing has been conducted using a wide range of PV technologies, system topologies, loads and noise sources. The Distributed Energy Technologies Laboratory (DETL) at Sandia National Laboratories (SNL) has used multiple reconfigurable arrays with a variety of module technologies, inverters, and balance of system (BOS) components to characterize new Photovoltaic (PV) DC AFCIs and Arc Fault Detectors (AFDs). The device's detection capabilities, characteristics and nuisance tripping avoidance were the primary purpose of the testing. SNL and Eaton Corporation collaborated to test an Eaton AFD prototype and quantify arc noise for a wide range of PV array configurations and the system responses. The tests were conducted by generating controlled, series PV arc faults between PV modules. Arc fault detection studies were performed on systems using aged modules, positive- and negative-grounded arrays, DC/DC converters, 3-phase inverters, and on strings with branch connectors. The tests were conducted to determine if nuisance trips would occur in systems using electrically noisy inverters, with series arc faults on parallel strings, and in systems with inverters performing anti-islanding and maximum power point tracking (MPPT) algorithms. The tests reported herein used the arc fault detection device to indicate when the trip signal was sent to the circuit interrupter. Results show significant noise is injected into the array from the inverter but AFCI functionality of the device was generally stable. The relative locations of the arc fault and detector had little influence on arc fault detection. Lastly, detection of certain frequency bands successfully differentiated normal operational noise from an arc fault signal.
TL;DR: In this paper, a ZigBee monitoring and protection system for building electrical safety is proposed, which can dynamically set the overload limit of outlets and avoid the effects on other equipments in the same branch circuit when the outlet disconnects the power.
TL;DR: A hybrid protection scheme for bipolar HVDC line has been implemented with field-programmable gate array (FPGA) and evaluation of its performance in real time is described in this paper.
Abstract: A hybrid protection scheme for bipolar HVDC line has been implemented with field-programmable gate array (FPGA) and evaluation of its performance in real time is described in this paper. The protection scheme amalgamates a traveling wave protection unit, a boundary protection unit, and a lightning unit. A stationary wavelet transform (SWT) and its wavelet modulus maxima are used to remove noise and represent a useful traveling wave signal. Boundary protection based on SWT is used jointly with traveling wave protection to distinguish internal faults from external faults. The lightning unit is used to distinguish non-fault transients and to identify the faulty pole. Xilinx XtremeDSP Development Kit-4 is employed for the hardware implementation, with coding done using the Xilinx System Generator and AccelDSP. The cost-efficient hardware co-simulation approach is adopted here to evaluate the real-time performance of the protection scheme.
08 Jun 2011
TL;DR: In this article, the authors discuss the current developing stage of dc distributed power system by the following contents: the basic structure and characteristics; the benefits of the dc distribution system; the development of current techniques adopted in DPS and its analysis of protection and system interaction.
Abstract: The history of the centralized and distributed power system is introduced far away from the power system being first built by Edison, which started the first confrontation between ac and dc power system. Although ac power system has dominated for a long century because of easy transmission and some other benefits, dc power system has still gave some good performances and attracted more attention during the past three decades. This paper will discuss the current developing stage of dc distributed power system by the following contents: the basic structure and characteristics; the benefits of the dc distribution system; the development of current techniques adopted in DPS and its analysis of protection and system interaction. In additional, the comparisons between dc and ac distributed power system are presented in detail, and finally, the challenge in future is also summarized.
TL;DR: In this article, the authors developed a new algorithm to improve the differential protection performance by using fuzzy logic and Clarke's transform, which was used to model an electrical power system using Alternative Transients Program (ATP) software.
Abstract: The power transformer is a piece of electrical equipment that needs continuous monitoring and fast protection since it is very expensive and an essential element for a power system to perform effectively. The most common protection technique used is the percentage differential logic, which provides discrimination between an internal fault and different operating conditions. Unfortunately, there are some operating conditions of power transformers that can affect the protection behavior and the power system stability. This paper proposes the development of a new algorithm to improve the differential protection performance by using fuzzy logic and Clarke's transform. An electrical power system was modeled using Alternative Transients Program (ATP) software to obtain the operational conditions and fault situations needed to test the algorithm developed. The results were compared to a commercial relay for validation, showing the advantages of the new method.
TL;DR: In this article, a hybrid multiagent framework with a Q-learning algorithm is presented to support rapid restoration of power grid systems following catastrophic disturbances involving loss of generators, which integrates the advantages of both centralized and decentralized architectures to achieve accurate decision making and quick responses when potential cascading failures are detected in power systems.
Abstract: This paper presents a hybrid multiagent framework with a Q-learning algorithm to support rapid restoration of power grid systems following catastrophic disturbances involving loss of generators. This framework integrates the advantages of both centralized and decentralized architectures to achieve accurate decision making and quick responses when potential cascading failures are detected in power systems. By using this hybrid framework, which does not rely on a centralized controller, the single point of failure in power grid systems can be avoided. Further, the use of the Q-learning algorithm developed in conjunction with the restorative framework can help the agents to make accurate decisions to protect against cascading failures in a timely manner without requiring a global reward signal. Simulation results demonstrate the effectiveness of the proposed approach in comparison with the typical centralized and decentralized approaches based on several evaluation attributes.
TL;DR: A novel wide area backup protection algorithm to identify fault branch based on the fault steady state component is proposed and the simulation results for the 10-generator 39-bus system verify that this method is able to easily identify fault Branch with limited measurement points.
Abstract: A novel wide area backup protection algorithm to identify fault branch based on the fault steady state component is proposed. Under normal conditions of the power system, subsets of buses called protection correlation regions (PCRs) are formed on the basis of the network topology and phasor measurement unit (PMU) placement. After the fault occurs, by analyzing the fault steady state component of differential current in each PCR, the fault correlation region is confirmed and then a fault correlation factor (FCF), is calculated in real time to locate the fault branch. The simulation results for the 10-generator 39-bus system verify that this method is able to easily identify fault branch with limited measurement points.
TL;DR: In this paper, the problem of detecting a fault on a transmission line from the residual vector of a synchrophasor state estimator (SynSE) was considered and a remote backup protection scheme for supervised zone-3 operation of distance relays was proposed.
Abstract: We consider the problem of detecting a fault on a transmission line from the residual vector of a synchrophasor state estimator (SynSE). For a transmission line under fault, we identify six operating modes depending upon location of the phasor measurement units (PMUs). We show that in five operating modes, the fault is seen in the residual vector while in one mode, the fault will not reflect in the residual vector. The analysis leads us to propose a remote backup protection scheme for supervised zone-3 operation of distance relays. In case of inadequate PMU penetration in the system, the implementation of SynSE can even be restricted to a small subsystem of critical lines.
07 Nov 2011
TL;DR: In this paper, a microprocessor-based sub-harmonic protection relay is proposed to detect subharmonics in the range of 5 -25 Hz using current and voltage level detectors, subharmonic calculations of individual subharmons, and total sub harmonic distortion factor (TSHD) to generate an alarm and/or trip.
Abstract: A routine switching event in the Xcel Energy transmission system resulted in sustained oscillations between wind generators and the system connected through a series compensated line. This event led to the development of a new microprocessor based sub-harmonic protection relay that could detect such conditions and take preventative measures. This paper discusses a new technique to detect sub-harmonics in the range of 5 – 25 Hz using current and voltage level detectors, sub-harmonic calculations of individual sub-harmonics and total sub-harmonic distortion factor (TSHD) to generate an alarm and/or trip to isolate the faulty system. Various test cases are presented to illustrate the effectiveness of the new technique.
TL;DR: The authors propose the integration of the self-healing capability against faults of the measurement system in power system fault detection and protection systems and the proposed architecture is based on exchange of information between energy conversion and measurement devices.
Abstract: Improved reliability and safety of the medium-voltage dc power distribution systems on board of all electric ships are the objectives of this paper. The authors propose the integration of the self-healing capability against faults of the measurement system in power system fault detection and protection systems. While most of previous work in the literature focuses on either one aspect independently, here, the two are integrated. On one hand, our approach addresses also the case of concurrent power system fault and measurement system fault. On the other hand, the proposed approach must be capable of distinguishing between the two types of failure. The proposed architecture is based on exchange of information between energy conversion and measurement devices. This makes the impact of communication delays critical, so its analysis is provided for the proposed case study. The impact on the performance of the measurement validation and protection systems is derived and can provide hints on the design. The protection method used as case study consists in controlling power converters to ride through the power system fault while maintaining power supply to the vital loads. To overcome failures of the measurement system, invalid data were detected and reconstructed through their expected value.
Book•
05 Aug 2011
TL;DR: The information provided in Digital Signal Processing in Power System Protection and Control can be useful for protection engineers working in utilities at various levels of the electricity network, as well as for students of electrical engineering, especially electrical power engineering.
Abstract: Digital Signal Processing in Power System Protection and Control bridges the gap between the theory of protection and control and the practical applications of protection equipment. Understanding how protection functions is crucial not only for equipment developers and manufacturers, but also for their users who need to install, set and operate the protection devices in an appropriate manner.After introductory chapters related to protection technology and functions, Digital Signal Processing in Power System Protection and Control presents the digital algorithms for signal filtering, followed by measurement algorithms of the most commonly-used protection criteria values and decision-making methods in protective relays. A large part of the book is devoted to the basic theory and applications of artificial intelligence techniques for protection and control. Fuzzy logic based schemes, artificial neural networks, expert systems and genetic algorithms with their advantages and drawbacks are discussed. AI techniques are compared and it is also shown how they can be combined to eliminate the disadvantages and magnify the useful features of particular techniques.The information provided in Digital Signal Processing in Power System Protection and Control can be useful for protection engineers working in utilities at various levels of the electricity network, as well as for students of electrical engineering, especially electrical power engineering. It may also be helpful for other readers who want to get acquainted with and to apply the filtering, measuring and decision-making algorithms for purposes other than protection and control, everywhere fast and on-line signal analysis is needed for proper functioning of the apparatus.
TL;DR: In this article, a method to resolve fault localization problems in power system was developed based on real-time measurement of phasor measurement units, which used mainly pattern classification technology and linear discrimination principle of pattern recognition theory to search for laws of electrical quantity marked changes.
TL;DR: In this article, the applicability of decision tree, hidden Markov model, and probabilistic neural-network (PNN) classification techniques to distinguish the transients originating from the faults from those originating from normal switching events was investigated.
Abstract: This paper investigates the applicability of decision tree, hidden Markov model, and probabilistic neural-network (PNN) classification techniques to distinguish the transients originating from the faults from those originating from normal switching events. Current waveforms due to different types of events, such as faults, load switching, and capacitor bank switching were generated using a high-voltage transmission system simulated in PSCAD/EMTDC simulation software. Simulated transients were used to train and test the classifiers offline. The wavelet energies calculated using three-phase currents were used as input features for the classifiers. The results of the study showed the potential for developing a highly reliable transient classification system using the PNN technique. An online classification model for PNN was fully implemented in PSCAD/EMTDC. This model was extensively tested under different scenarios. The effects of the fault impedance, signal noise, current-transformer saturation, and arcing faults were investigated. Finally, the operation of the classifier was verified using actual recorded waveforms obtained from a high-voltage transmission system.
TL;DR: This work presents the development and implementation of an artificial neural network based algorithm for transmission lines distance protection that does not need any topology adaptation or ANN parameters adjustment when applied to different electrical systems.
TL;DR: In this paper, a novel current-only directional detection possibility is highlighted, where the fault direction may be forward (between relay and grid), or reverse (between rel and source), the normal power flow being from source to the grid.
Abstract: Overcurrent relays are widely used for protection of power systems, directional ones for transmission side, and nondirectional ones for distribution side. The fault direction may be forward (between relay and grid), or reverse (between relay and source), the normal power flow being from source to the grid. Known directional overcurrent relays rely on a reference voltage phasor for estimating the direction of the fault, requiring both current and voltage sensors. This increases the cost of the relays, prohibiting the utilization of such relays in the distribution side protection and automation, which is going to be a key part in the smart grid initiative. In this paper, a novel current-only directional detection possibility is highlighted.
01 Sep 2011
TL;DR: The use of back-propagation (BP) neural network architecture is presented as an alternative method for fault detection, classification and isolation in a transmission line system and provides a reliable and an attractive alternative approach for the development of a protection relaying system for the power transmission systems.
Abstract: Transmission lines, among the other electrical power system components, suffer from unexpected failures due to various random causes. These failures interrupt the reliability of the operation of the power system. When unpredicted faults occur protective systems are required to prevent the propagation of these faults and safeguard the system against the abnormal operation resulting from them. The functions of these protective systems are to detect and classify faults as well as to determine the location of the faulty line as in the voltage and/or current line magnitudes. Then after the protective relay sends a trip signal to a circuit breaker(s) in order to disconnect (isolate) the faulty line.The features of neural networks, such as their ability to learn, generalize and parallel processing, among others, have made their applications for many systems ideal. The use of neural networks as pattern classifiers is among their most common and powerful applications. This paper presents the use of back-propagation (BP) neural network architecture as an alternative method for fault detection, classification and isolation in a transmission line system. The main goal is the implementation of complete scheme for distance protection of a transmission line system. In order to perform this, the distance protection task is subdivided into different neural networks for fault detection, fault identification (classification) as well as fault location in different zones. Three common faults were discussed; single phase to ground faults, double phase faults and double phase to ground faults. The result provides a reliable and an attractive alternative approach for the development of a protection relaying system for the power transmission systems.
15 Dec 2011
TL;DR: This paper defines security metrics that quantify the importance of individual substations and the cost of attacking individual measurements, and uses the metrics to show how various network layer and application layer mitigation strategies can be used to decrease the vulnerability of the state estimator.
Abstract: The power system state estimator is an important application used to calculate optimal power flows, to maintain the system in a secure state, and to detect faulty equipment. Its importance in the operation of the smart grid is expected to increase, and therefore its security is an important concern. Based on a realistic model of the communication infrastructure used to deliver measurement data from the substations to the state estimator, in this paper we investigate the vulnerability of the power system state estimator to attacks performed against the communication infrastructure. We define security metrics that quantify the importance of individual substations and the cost of attacking individual measurements. We provide efficient algorithms to calculate these metrics, and use the metrics to show how various network layer and application layer mitigation strategies can be used to decrease the vulnerability of the state estimator. We illustrate the efficiency of the algorithms on the IEEE 118 and 300 bus benchmark power systems.
TL;DR: In this paper, a new combinational load shedding method is proposed to increase the security of power system during large disturbances, which uses locally measured frequency and voltage signals and does not need any communication link.
Abstract: In the recent years, operation of power systems at lower stability margins has increased the importance of system protection methods that protect the system stability against various disturbances. Among these methods, the load shedding serves as an effective and last-resort tool to prevent system frequency/voltage instability. For major combinational disturbances, the active power deficit is usually accompanied by reactive power deficit. Therefore frequency stability and voltage stability of the system are jeopardised simultaneously. For such cases, the conventional non-adaptive underfrequency load-shedding scheme cannot offer adequate protection to prevent system collapse. In this study a new combinational load shedding method is proposed to increase the security of power system during large disturbances. The proposed method uses locally measured frequency and voltage signals and does not need any communication link. Performance of the conventional scheme and the proposed load-shedding method using different load shedding criterions have been simulated in two test systems: an actual large network and a standard test system. Obtained simulation results confirm that the proposed schemes provide considerable enhancement in the power system voltage stability margin and by using the proposed method various power system blackouts could be prevented.