Voltage Instability Detection Research Articles

Detecting Power System Voltage Stability in Large Test System

The unpredictable changes in electricity demand, along with potential risks, require power system operators to act quickly to prevent voltage instability. To do this, real-time monitoring is crucial, especially in larger systems, but it is a challenging task for power system engineers. The existing methods for detecting voltage instability need to be tested and validated for complex systems. This study aims to validate a voltage instability detection index called SQLVIDI in a larger test system with 118 buses (IEEE 118 bus test system). The simulation results manifest that this index has the capability to detect potential voltage instability in the power system with larger number of buses.

Overview, comparison, and extension of emergency controls against voltage instability using Inverter-Based Generators

This paper provides a general overview of some previously reported schemes for the on-line detection and corrective emergency control of long-term voltage instability in a power system with a significant share of Inverter-Based Generators (IBGs) connected at the distribution level. Next, it proceeds to introduce new wide-area emergency controls to avoid an imminent voltage instability. Reactive support from IBGs is utilized as part of voltage stability emergency control, while in normal operation IBGs are operating at unity power factor. Results are shown for the IEEE Nordic Test System with increased IBG penetration, through feeders adapted from Hellenic Interconnected System wind farms. The newly proposed wide-area controls are compared with existing local emergency control schemes and are shown to have superior performance.

Real Time Voltage Instability Detection in DFIG Based Wind Integrated Grid with Dynamic Components

Real Time Voltage Instability Detection in DFIG Based Wind Integrated Grid with Dynamic Components

Validation of Voltage Instability Detection and Control Using a Real Power System Incident

This paper applies a voltage instability monitoring method based on voltage and current measurements from a transmission bus PMU on the Hellenic Interconnected System using both unstable and marginally stable scenarios, derived from the historical 12 July 2004 blackout of the Athens area. The effectiveness, selectivity and reliability of the proposed monitoring method is clearly demonstrated, allowing its integration into a System Protection Scheme with direct load shedding. It is shown that the proposed instability detection and control scheme could have prevented the voltage collapse if applied at the time of the event.

Voltage stability assessment accounting for current-limited converters

This paper investigates how current-limited converters influence system stability. An improved approach for assessing voltage stability is introduced, which determines the maximum deliverable power to a load by accounting for changes in the Thévenin voltages. The new approach is able to detect voltage instability well before the traditional approach based on Thévenin impedance matching. A new stability boundary is discovered that describes the aperiodic small signal stability boundary for current-limited converters. The improved approach was modified to account for current-limited converters. The detection of voltage and rotor angle instability during current limitation were demonstrated on a seven bus system, where test cases were performed. The test cases showed that voltage instability can lead to aperiodic small signal instability, when the converter has reached its current limit and loss of synchronism in the unstable region during current-limitation causes the system voltages to collapse.

Voltage instability detection using synchrophasor measurements: A review

Voltage instability detection using synchrophasor measurements: A review

Combined analysis of distribution‐level PMU data with transmission‐level PMU for early detection of long‐term voltage instability

In the present study, a new method is proposed for early voltage instability detection based on the statistical analysis of the data obtained from PMUs and micro-PMUs. Although, the use of high sampling rate measurements helps operators to assess the dynamic behavior of the systems, but it may lead to processing a large volume of data, which is a main challenge in this regard. Here, K-Medoid partitioning method is used for clustering and reducing the computational burden. Clustering is done based on the analysis of the voltage magnitude variance in unstable scenarios. Based on the critical slowing down phenomenon, the voltage magnitude variance in the critical transmission-level busbars and in the power plant busbars are used as instability detection indices. The data measured by PMUs give information about severity of the events, and micro-PMUs data provide information on operating status of the over-excitation limiters as well as the resiliency of network to keep voltage. In different conditions of the Nordic test system, all contingencies are considered for data training. Efficiency of the proposed method for early detection of instability in online operation is evaluated by AdaBoost algorithm, and the obtained results are compared by those of other classifiers.

A Fast Online Voltage Instability Detection in Power Transmission System Using Wide-Area Measurements

Monitoring voltage stability in real time is a pressing issue for large complex power systems. In this work, reactive power losses calculated from synchrophasor measurements are used to develop a novel voltage instability detection index named SQLVIDI. The proposed index is fast enough to be used for real-time monitoring of voltage stability and for dispatching mitigating control actions in time. The prowess of the proposed index has been tested rigorously on New England 39 bus test system and IEEE 118 bus test system by running time-based simulations. The index successfully detects the impending long-term voltage instability in a timely manner. This timely detection of voltage instability ensure system operator to take preventive control actions to minimize the operational risk. Simulation results show’s that proposed index is simple, reliable, computationally inexpensive and therefore suitable for online real-time detection applications in power systems.

Agent‐based identification and control of voltage emergency situations

Changing dynamics of power systems caused by the migration from conventional to distributed energy sources increase the risk of blackouts due to voltage instability, especially in case of unforeseen network conditions (e.g. (N-k)-cases). To enable both secure and efficient power supply, novel monitoring and emergency control systems for the identification of voltage emergency situations as well as the execution of control actions are required that react reliably in due time and adaptively in the case of changing network situations. This study presents a distributed agent-based approach to counteract voltage instability that is based solely on local measurements and limited inter-agent communication. Distributed agents located at substations in the (sub-)transmission network monitor distribution and transmission voltages, as well as, load tap changer positions and are able to autonomously curtail load in case system stability is endangered. The applicability of the approach is demonstrated in a co-simulation environment interfacing the multi-agent system with a dynamic power system simulation. The presented approach allows for an early detection of voltage instability as well as a coordinated execution of available control actions.

Voltage Stability Monitoring From a Transmission Bus PMU

This paper examines the problem of monitoring long-term voltage instability from a phasor measurement units installed at any transmission bus. It is shown that the well-known impedance matching condition on a radially connected load bus, as well as the local identification of voltage emergency situations (LIVES) method based on load tap changing transformers can be generalized into a New LIVES Index suitable for any transmission bus feeding a weak area of a system. Results on an IEEE voltage stability test system validate the dependability and security of the new index for the early detection of voltage instability. Moving average, as well as a new filtering method based on short-time Fourier transform are employed to reduce the effect of noise and short-term transients.

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.

Voltage instability detection based on the concept of short circuit capacity

Summary This paper proposes an online detection method for power system voltage instability, which considers the internal impedances of generators and the voltage-dependent and recovery characteristics of the electric loads. First, a novel voltage stability analysis method based on the concept of short circuit capacity (SCC) is established. Based on it, a SCC-based voltage stability index (VSI) is defined as the ratio of the SCC supplied by the combined generation-transmission system and the SCC required by load bus to maintain voltage stability. Then, for the calculation of SCC-based VSI, a two-step equivalent method of power system is presented to obtain the SCC supplied by the system side, in which the generator is modeled as a time-varying internal voltage behind constant internal impedance. Meanwhile, a modification method of load power is proposed to deal with the voltage-dependent characteristics and the restoration dynamics of system loads. As a result, the SCC required by loads can be precisely calculated. At last, the 5-bus system and the New England 39-bus system are used to validate the effectiveness of the proposed voltage instability detection method. Copyright © 2015 John Wiley & Sons, Ltd.

Tracking the maximum power transfer and loadability limit from sensitivities-based impedance matching

This paper is concerned with voltage instability detection in power systems. The Impedance Matching (IM) equations are formulated as sensitivities-based equations and practical issues about impedance matching are addressed. This research presents a direct relationship between IM and power injection sensitivities, and proposes the use of a Sensitivity Impedance Matching–(SIM) for detection of instability conditions. SIM may detect the Maximum Transfer Point (MTP) and the Saddle-Node Bifurcation (SNB – the maximum loadability point). Prediction capabilities of SIM based on anticipation parameters and reactive generation limitation in advance are presented. Approximated active, reactive, and also apparent margin indices are proposed. The paper introduces novel theoretical results for designing reliable Voltage Instability Detection (VID) schemes.

Parameter Identification With PMUs for Instability Detection in Power Systems With HVDC Integrated Offshore Wind Energy

Large-scale offshore wind farms can be integrated with onshore ac grids by the voltage source converter based high voltage direct current (VSC-HVDC) technology. The resulting impact on the security of ac grids can be significant. Therefore, it is important to develop an HVDC model for online stability monitoring of the integrated ac/dc systems. This paper proposes a new HVDC model based on a circuit-theoretic foundation. With the available phasor measurement units (PMUs) at VSC stations, the parameters of the HVDC equivalent model can be identified in real-time by synchronized voltage and current phasor measurements at VSC ac terminals. The proposed HVDC model is applied to online voltage instability detection for integrated ac/dc systems by the Thevenin impedance matching. The HVDC equivalent circuit enables the Thevenin equivalent impedance of the HVDC-connected offshore wind farm to be determined directly from the PMU measurements. Numerical simulations are performed on the IEEE 39-bus system with an HVDC-connected offshore wind farm to validate the effectiveness of the proposed HVDC model.

Online Assessment of Voltage Stability in Power Systems with PMUs

Abstract: For the assessment of voltage instability which comprises the detection of voltage instability and identification of critical buses, two indices namely, system wide Qtax, and bus-specific qtax, are proposed. The Qtax, based on the sensitivity of the reactive power injections to the loading in the system provides early detection of impending voltage instability. The computed qtax indices identify the critcal buses among the load buses in the system. The identified critical buses provided optimal lacations for the corrective control actions for averting voltage instability. Additionally, for voltage stability monitoring, determining the poing of exhaustion of the reactive reserves in system is also crucial. This is addressed by proposed Q-Monitoring Index (QMI), which is the ratio of the reactive component of the source current to the sink current that flows through the adjacent transmission line. These proposed indices together can provide early indication to impending voltage instability. This has been illustrated on IEEE-39 bus system. The reactive support on identified critical buses results in maximum increase in the loadability of the system.

A Simple Scheme for Wide Area Detection of Impending Voltage Instability

This paper proposes a synchrophasor-assisted simple scheme suitable for on-line anticipation of the impending long-term voltage instability in the power system. The proposed scheme utilizes the bus voltage magnitude and the consecutive bus voltage deviations in the system. These two criteria are combined by means of their weighted sum, to define a Synchrophasor Based Voltage Instability Monitoring Index (SVIMI). A new algorithm is also developed for updation of the criteria weights. The performance of the proposed scheme is demonstrated on New England (NE) 39-bus system and Northern Regional Power Grid (NRPG) 246-bus Indian system. The test results are also compared with two existing voltage stability monitoring schemes to demonstrate the effectiveness of the proposed method.

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.

Early Detection of Voltage Instability in Distribution System utilizing Phasor Measurement Units

Power system automation leads to a great jump in detecting and solving the system problems. Due to the serious problems of system voltage collapse, an early prediction online voltage instability detector is discussed in this paper. The voltage instability detector utilizes the voltage and current phasor readings of connected phasor measurement units to the distribution terminals. The detector works according to two parallel algorithms. The first algorithm is a comparison for terminal current and voltage phasors with an offline calculated Look-up table, in which particle swarm optimization technique is used. The Look-up table represents the permissible maximum current and minimum voltage phasors for each terminal. The second algorithm is based on the online computation of the ratio of the Thevenin equivalent system impedance to the terminal load impedance. The simulated voltage detector is applied to two different unified power systems. It gives good and reasonable results.

Wide-Area Detection of Voltage Instability From Synchronized Phasor Measurements. Part I: Principle

This two-part paper deals with the early detection of an impending voltage instability from the system states provided by synchronized phasor measurements. Recognizing that voltage instability detection requires assessing a multidimensional system, the method fits a set of algebraic equations to the sampled states, and performs an efficient sensitivity computation in order to identify when a combination of load powers has passed through a maximum. The important effects of overexcitation limiters are accounted for. The approach does not require any load model. This first part of the paper is devoted to theoretical foundations of sensitivity calculation along the system trajectory, derivation of the algebraic model, and illustration on a simple five-bus system involving the long-term dynamics of a load tap changer and a field current limiter.

Wide-Area Detection of Voltage Instability From Synchronized Phasor Measurements. Part II: Simulation Results

This two-part paper deals with the early detection of an impending voltage instability from the system states provided by synchronized phasor measurements. Recognizing that voltage instability detection requires assessing a multidimensional system, the method fits a set of algebraic equations to the sampled states, and performs an efficient sensitivity in order to identify when a combination of load powers has passed through a maximum. This second part of the paper presents simulation results obtained from detailed time-domain simulation of the Nordic32 test system, without and with measurement noise, respectively. Several practical improvements are described such as anticipation of overexcitation limiter activation, and use of a moving average filter. Robustness to load behavior, non-updated topology and unobservability is also shown. Finally a comparison with Thevenin impedance matching criterion is provided.