Voltage Instability Problems Research Articles

Research on Voltage Coordinated and Stability Control of Condensers Based on Reactive Power Replacement

Abstract In this research, we investigated condenser voltage coordination and stability control systems by analyzing voltage coordinated control mode among excitation, voltage coordination and stability as well as automatic-voltage reactive power control systems. By using dynamic excitation regulation and steady-state regional voltage source control replacement, dynamic reactive power quickly output under fault condition as well as optimized regional reactive power under steady state were realized. The correctness and practicability of the proposed strategy was verified through engineering application. Adopting the reactive power displacement of voltage coordination control system to solve local voltage instability problem after UHV DC blocking. The proposed method realized multi-time scale emergency, recovery and steady-state control of the system under steady and dynamic states.

SE-CNN based emergency control coordination strategy against voltage instability in multi-infeed hybrid AC/DC systems

For receiving-end power systems with multi-infeed HVDCs and large-scale renewable energy generation, the challenges of weak reactive power support and limited anti-interference capacity pose a threat to the power system voltage stability. To enhance the voltage stability under large disturbances, this paper proposes a data-driven method for coordinated emergency control strategy against voltage instability in multi-infeed hybrid AC/DC systems based on the squeeze and excitation (SE) module and convolutional neural network (CNN). Firstly, the SE module and CNN are integrated to intuitively capture correlations among various feature channels and extract key features related to voltage stability levels. Secondly, a voltage stability evaluation method is proposed based on the SE-CNN model, utilizing a dual channel structure to unveil the quantitative relationship between key response characteristics and voltage stability margin. Finally, regarding the control measures containing load shedding and DC modulation, the emergency control sensitivity index is proposed to assess the stability margin improvement effect of various control objects, and the optimal emergency control strategy is formulated to coordinate multiple voltage stability emergency control measures. Case studies were conducted on a typical China local region receiving-end power grid test system with voltage instability problems, and the simulation results verified the effectiveness of the proposed method. © 2017 Elsevier Inc. All rights reserved.

Optimal power quality improvement in distribution system with UPQC using an improved strategy

AbstractA hybrid technique is proposed to enhance the power quality (PQ) on the distribution sides of the utility grid (UG) by controlling a unified power quality conditioner (UPQC) connected to hybrid sources (photovoltaic [PV] and wind turbine [WT]). The proposed hybrid method integrates the implementation of the pelican optimization algorithm and the Aquila optimizer; hence, it is called the improved aquila optimizer (IAO) technique. The objective of the proposed method is to lessen the total harmonic distortion (THD), voltage instability, and PQ issues during load fluctuation situations. The improved Aquila Optimizer technique optimizes the control parameters of the UPQC to achieve optimal PQ development. The series controller is attached to the grid‐side to enhance grid PQ, while the shunt hybrid active power filter (SAPF) and shunt active power filter (SHAPF) generate the best control pulses based on load and source conditions. The proposed solution addresses power loss, THD, and voltage instability problems during load fluctuation conditions. The series controller reduces voltage sag by 14% and voltage swell by 15%. The THD for the proposed technique is 0.8%. The PQ of the proposed technique is improved, and various characteristics are reduced. The efficiency of the proposed technique is examined by using MATLAB and is compared to existing approaches. The PQ characteristics are significantly improved, and the proposed technique is better than the existing techniques.

Research on Black Start Control technology of Energy Storage Power Station Based on VSG All Vanadium Flow Battery

To reduce the losses caused by large-scale power outages in the power system, a stable control technology for the black start process of a 100 megawatt all vanadium flow battery energy storage power station is proposed. Firstly, a model is constructed for the liquid flow battery energy storage power station, and in order to improve the system capacity, four unit level power stations are processed in parallel. Secondly, based on the energy storage of all vanadium flow batteries, the traditional voltage and frequency stability control technology has been improved to address the characteristics of low inertia and damping in power electronic interfaces. The improved VSG strategy has been applied to the overall control of a hundred megawatt level flow battery energy storage power station, solving the voltage and frequency instability problems caused by load shocks and fluctuations. Finally, stable control of multiple parallel machines during the self starting process of the liquid flow battery energy storage power station was achieved, avoiding the occurrence of multiple machine circulation and uneven power distribution problems during the self starting process.

CNN‐LSTM based power grid voltage stability emergency control coordination strategy

AbstractThe occurrence of cascading failure and small probability contingency in hybrid AC/DC power grid aggravates the mismatch risk of traditional emergency control with offline‐predetermination–online‐practice (ODOP) mode. To ensure the voltage stability of power grid under large disturbances, this paper proposes a voltage stability emergency control coordination strategy based on convolutional neural network (CNN) and long short‐term memory (LSTM) network. First, the mismatch mechanism of traditional emergency control is revealed under the variation of operating conditions. Second, the start criterion of complementary emergency control with ODOP mode is proposed, and the CNN‐LSTM network is established to quantitatively evaluate the voltage stability margin. Finally, the emergency control sensitivity index is proposed to predict the stability margin enhancement under alternative emergency control measures, and the optimal ODOP‐based emergency control strategy is determined to coordinate multiple voltage stability emergency control measures. Case studies are performed in the actual Northwest China local region hybrid AC/DC power grid with voltage instability problems, and the simulation results verify the effectiveness of the proposed method.

An individual sequencing control strategy for three-phase four-leg inverter under unbalanced loads

Under the influence of the unbalanced load, the negative and zero sequence components in the output voltage of the three-phase four-leg voltage source inverter(TPFL-VSI) will cause asymmetry voltage. This paper designs an individual sequencing control for the three-phase four-leg inverter to address this problem. The second-order generalized integrator is adopted to extract the current’s positive and negative sequence components. The virtual oscillator control(VOC) controls the positive sequence component, and the negative sequence component is suppressed by designing the virtual impedance and the multiple proportional resonance controller. The proportional resonance control is used to control the zero-sequence voltage of the fourth bridge arm. Considering that voltage deviation is caused by the access of negative sequence virtual impedance and load step, the secondary voltage compensation strategy is further added in the individual sequencing control to improve the droop relationship in the VOC and effectively solve the problem of system voltage instability. Ultimately, a TPFL-VSI model is built in Matlab/Simulink, and simulation results verify the feasibility and effectiveness of the proposed control strategy under different conditions.

Study on the characteristics of an electromagnetic damping generator

This paper proposes an electromagnetic damping generator (EDG), which is a novel downhole generator for the rotary steerable system (RSS). The EDG is designed to solve the problem of unstable voltage output by the RSS generator. The generator is designed by integrating a traditional generator and an eddy current retarder (ECR). The system converts the excess kinetic energy of the EDG rotor into heating energy by the principle of eddy-current braking and resistance heating. The electromagnetic finite element models of the generator part and ECR part are established and solved by computer. The voltage and torque generated by the generator part and the braking torque of the ECR part are predicted. In the bench test, When the input power is 2537 W < Pinput < 3304 W, the EDG is able to output 300.8 V. When the input power is 3304 W, the output voltage of the ordinary generator is 346 V. The EDG can be adjusted by a decrement of up to 45.2 V to output the rated voltage. The problem of generator output voltage instability in the RSS is solved when mud flow is unstable.

Optimal allocation of distributed generators in radial distribution networks based on ant-lion optimizer

This paper presents the use of a recently developed algorithm inspired by the hunting mechanism of ant-lions in nature, called ant lion optimizer (ALO). The ALO algorithm is used for solving the voltage instability problem in radial distribution networks. The proposed method ensures system operation within voltage standard limits and with minimum power losses. The proposed method implies the following main tasks; load flow analysis with forwarding/backward sweep method, voltage stability index calculation and optimizing the distributed generators (size, location, and power factor) by applying the ALO. The brilliant performance of hybridizing these tasks will be able to give an overall view of voltage stability for radial distribution system; calculate the voltage stability index; detect the most sensitive node to collapse and presents a solution (to mitigate voltage instability problem and reduce system power losses). The performance and feasibility of the proposed method are demonstrated through simulation case on IEEE 69-bus. The obtained results provide the optimal solution.

Study on coordinated multi-objective reactive control system of AC/DC power grid based on new-generation synchronous condenser

With rapid development of HVDC transmission system, the coupling relationship between the AC and DC power grid has become increasingly stronger. Especially, the problem of voltage instability is prominent in HVDC receiving-end system, the dynamic reactive power performance of the receiving-end power grid is inadequate. Therefore, the new-generation synchronous condenser (NGSC) as ‘a strong reactive power support device” is widely applied to high-capacity DC transmission system. However, there is a lack of coordination control strategy between the NGSCs and AC/DC power grid, so it is difficult to give full play to the reactive power supporting capacity of the NGSCs. In this paper, a coordinated multi-objective reactive control strategy between the NGSCs and the AC/DC power grid is proposed . In steady state, the initial reactive power optimization method of a NGSC is proposed. In transient state, a commutation failure suppression scheme of DC system based on the NGSCs and the DC control system is proposed. In transition process, the coordinated control strategy of the NGSCs with capacitance and reactance is studied. The research results provide theoretical basis for subsequent study on the safety and stability of AC/DC power grid.

A novel AC-DC hybrid microgrid architecture and control strategy for electric vehicle swap station

The characteristics of small volume, modularity, and easy expansion in solid-state transformers (SST), which makes SST widely applied in electric vehicle swapping stations. However, due to various loads and pulsed loads, voltage instability and fluctuation problems inevitably occur in electric vehicle swapping stations. Aiming at these problems, a topology structure based on a Modular Multilevel Solid State Transformer (MMC-SST) is proposed. It is the first choice for realizing the flexible power allocation of AC/DC hybrid system and the interconnection of medium and low voltage. The performance of the DC bus voltage of the charging station has been improved. Based on this topology, an energy management strategy with virtual synchronous generator (VSG) control is proposed to smoothly transition the on-grid and off-grid processes of the distributed power source. In addition, due to the frequency regulation is applied in the control system, the energy storage system has damping and inertia when the output of the new energy power generation equipment changes or load fluctuation, reducing the impact on the load energy storage equipment and improving the stability and safety of the electric vehicle charging station system. Finally, the effectiveness of the proposed topology and its control strategy in different working scenarios are verified by simulation. Simulation analysis shows that the proposed topology has good advantages in DC bus voltage stability, optimization of electric vehicle charging characteristics and load management.

Voltage S tability Assessment of Sudan ese National Network within Re Evaluation of the SVC Projects

In the last ten years Sudan National Grid suffering from a steady increase in loads, with a shortage of generation and congestion in transmission substations and lines. This caused many cases of blackouts. As a solution to this situation, the Static Var Compensators (SVC) project was recommended to improve the voltage in the National Grid (NG) by providing adequate reactive power support at the specified nine locations to solve voltage instability problems. This study re-evaluates the project, especially after the delay in implementation and the major change that occurred in the NG the evaluation will cover all nine proposed substations (Port-Sudan (POR), Hag-Abdallah (HAG), Farooq (FAR), Mahadiya (MHD), Kuku (KUK), Local market (LOM), New Hasahisa (NHAS), Bagair (BAG), and Gamueia (GAM).) and show its effect on the overall National Grid voltage.

Real-Time Adaptive UVLS by Optimized Fuzzy Controllers for Short-Term Voltage Stability Control

This paper proposes a novel real-time adaptive under voltage load shedding (UVLS) method by using optimized fuzzy logic controllers to address fault-induced delayed voltage recovery (FIDVR) and short-term voltage instability (STVIS) problems. An intelligent multi-input multi-output fuzzy logic load shedding controller is developed based on a short-term voltage recovery criterion (STVRC). For the controller, the magnitude deviation and the recovery time deviation of voltage trajectory are extracted as inputs, while the load shedding amount at the current load shedding stage and the time delay of the next stage are taken as outputs. Particle swarm optimization (PSO) technique is further employed for the tuning of the controller parameters to realize the voltage recovery requirement at the lowest cost of load shedding. The proposed UVLS method is formed by a group of the fuzzy logic controllers deployed at the load buses, each monitoring its own bus voltage and shedding load in closed-loop. Responding to the severity of voltage variations, the proposed method is able to adaptively determine the location, time and amount of load shedding in real-time to make the voltage satisfy the STVRC. The effectiveness of the proposed method compared to other methods is testified by simulations on the Nordic test system.

Improving performance of transmission networks using facts through continuation power flow method

Over the past 50 years, modern electrical systems have become more complex, as they overrun the geographical boundaries of neighboring countries. The problem is that the power system faces many challenges, because it is exposed to difficult operating conditions. The phenomenon of voltage instability is the most frequent phenomenon, and this can lead to the collapse of the power system. To avoid power outages in the system (especially in blackout situations), the power system must be analyzed in order to maintain voltage stability in the expected difficult operating conditions. The main objective is to determine the maximum load capacity of the system and the causes of voltage instability. The voltage instability problem is related to the nature of nonlinear loads, so different load characteristics must be taken into consideration when analyzing voltage stability. This study aims to discover the maximum load capacity required by using the continuous power flow method (CPF) in the studied network. Then, the performance of this network using a Flexible Alternating Current Transmission System (FACTS) will be utilized. FACTS systems present a promising solution in improving the voltage stability by improving the power transmission capacity and controllability of the parameters of the existing power networks. This study will be conducted on a reference network platform under normal working conditions, then installation of one of the FACTS systems will show its effect on improving voltage stability. The continuous power flow method will be used to find PV curves, which in turn will help to determine the conditions of maximum loading while maintaining stability, and identify the bus bar with the smallest voltage, on which the flexible AC systems will be installed. The software environment MATLAB/PSAT will be used for modeling and simulation.

Reactive Power Control using STATCOM for Power System Voltage Improvement

Stability of power system is the ability of the system, with a certain initial operation conditions, to restore the operating balance conditions after exposure to disturbance such as faults or sudden load changes. More attention is required to address voltage instability problems to keep voltage profile under control during abnormal conditions. This paper proposes a Static Synchronous Compensator (STATCOM) using (MATLAB / Simulink Program) The STATCOM includes proportional-integral (PI) control model is used to control the voltage during abnormal conditions by absorbing or injecting reactive power into the power system. Simple and reliable PI controller has been used and designed to be stable under various operating conditions. Three phase Two level PWM strategy technic used in STATCOM controller to decrease the harmonics injected after adding STATCOM to the power system (THD = 2.21%). The simulation study has been done by using 9 buses IEEE system after making a disturbance such as sudden load change. Then the voltage profile during this interval is being studied with and without using STATCOM. The simulation results show that adding STATCOM to this system led to improve the voltage profile during disturbance interval and made the system more stable and reliable by preventing the disturbance effects from reaching the generation side (Vbus 5 = 0.8980 pu, during disturbance before adding STATCOM, Vbus 5 = 1.0003 pu, during disturbance after adding STATCOM). The fast response of the STATCOM controller gives the ability of injecting or absorbing reactive power during disturbances and keeping the system voltages within the IEEE standards limits (0.95 pu > V> 1.05 pu).

Optimal Placement of Capacitor Bank on Ondo 132/33KV Transmission Substation

Introduction: Relevancies of electrical power system to consumers cannot be overemphasized. This requires healthy operation of such system. 132/33KV network is an important network within the transmission system and its importance calls for effective and efficient operation. However, due to the long distance at which the transmission line span, the nature of the lay (either overhead or underground) and type of cable used, there are series of voltage instability and power loss along the transmission network which are threat both economically and technically to both electricity providers and consumers because of the cost implication of the losses and effect on the stability of the network.
 Aims: This research aims is to seek optimal placement of a capacitor bank to proffer solution to both voltage instability and power loss problem by simulating Ondo 132/33KV transmission network using NEPLAN software.
 Methodology: The network model was developed using NEPLAN software. The voltage profile and power loss with and without capacitor bank were determined from power load flow solutions using Newton-Raphson method.
 Results: Effects of optimal placements of capacitors along the studied transmission line is established.
 Conclusion: Proper installation of capacitor bank is also found to enhance performance with an accompanied improvement in voltage profile along the buses.

Research on fuzzy PID droop Control method for DC microgrid in island mode

In order to solve the problem of difficult power supply in remote mountainous areas and islands, based on the traditional drooping control of DC microgrid, this paper introduces fuzzy PID to realize the self-adjustment of drooping coefficient, which solves the problem of voltage instability of DC bus caused by external environmental conditions and load mutation. MATLAB/SIMULINK simulation results show that the fuzzy PID droop control can effectively reduce the voltage drop, maintain the bus voltage stability, and realize the current sharing of all distributed power sources in the DC microgrid under the load switching condition.

Research on improving transient voltage stability of wind power grid connected system by SFCL and STATCOM

Aiming at the problem of transient voltage instability in the grid-connected wind power system, a combined method of superconducting fault current limiter (SFCL) and static synchronous compensator (STATCOM) was proposed to suppress voltage saps. In this method, a superconducting fault current limiter (SFCL) is connected in series on the transmission line, and a static synchronous compensator (STATCOM) is connected in parallel on the bus of the wind farm. SFCL shows zero impedance when the system is in normal operation, and quickly shows high impedance when the fault occurs, so as to effectively suppress the short-circuit current and provide the bus voltage support. At the same time, STATCOM provides reactive power during the fault period, so that the system voltage can quickly recover. By establishing the corresponding simulation model, SFCL and STATCOM are used to jointly improve the voltage stability of the wind power grid-connected.

Searching for the shortest path to voltage instability boundary: From Euclidean space to algebraic manifold

Voltage instability is one of the main causes of power system blackouts. This paper focuses on finding the shortest path to the boundary of the singularity-induced voltage instability problem. Instead of using the Euclidean distance, we propose to use the arc length of the path on the network constraint manifold. This formulation is further converted into an optimal control framework to solve for the shortest path on the manifold. We rigorously show that the global solution of the proposed problem formulation always ends on the correct singular boundary. However, the traditional Euclidean distance formulation does not achieve this crucial topological property, thus, can lead to a wrong voltage collapse direction and/or a very conservative estimation of the stability margin. Numerical simulations are firstly performed on a low-dimensional example to fully visualize the algebraic manifold, the singular submanifold, and the optima for both problem formulations. Then, a larger 39-bus example is investigated in three different cases for both formulations. The results validate our theoretical statements that our proposed formulation always identifies the shortest path towards the correct voltage instability boundary. A broad range of potential applications using the proposed method are further discussed.

Automatic Voltage Regulator as a Voltage Control in 1 Phase Axial Generator System

A constant output voltage on a generator is very important to produce the expected power supply. Changes in the output voltage of a generator are influenced by various disturbing factors, one of which is load and Rotations Per Minute (RPM) which are not always constant. Therefore we need a special regulator equipment to keep the generator output voltage constant. The problem of voltage instability during load changes must be overcome to keep the voltage constant, so that equipment is needed that can control the voltage stability. This tool is the Automatic Voltage Regulator (AVR). The purpose of this study is to design a voltage control device in a single phase axial generator system. The research method used consists of 3 stages, namely: 1). Design and design of tools, 2). Stages of making tools, 3). Stages of testing the tool. The results of the research were obtained when an axial generator with an inner air gap of 0.4 cm, outer 0.5 cm and rpm 2589. The output voltage of the generator began to decrease significantly until it reached -70 volts and rpm also decreased to -200. In contrast, when the generator is operated using the AVR, the reduction only reaches -30 volts. However, when the generator is operated with AVR, the decrease in rpm is greater until it reaches -220.

A Comprehensive Review of Microgrid Control Mechanism and Impact Assessment for Hybrid Renewable Energy Integration

This paper describes a comprehensive review of microgrid control mechanism and impact assessment for hybrid grid. Building the model of sustained energy growth is one of the actions to achieve the Sustainable Development Objective (SDO) and change the global fossil fuel system. For co-operation in the development of one independent supplier of renewable energy, the microgrid is essential. Hybrid solar energy microgrid is also a solution for reducing fossil fuel consumption and providing an environmentally sustainable solution to rising rural electricity demand. The most common renewable energy options in the microgrid are solar photovoltaics, wind turbines and biomass. The environmentally sustainable and technologically innovative installation is convenient everywhere. Hybrid microgrid-based renewable energy, however, is confronted, given its intermittent and variable source efficiency, by challenges such as voltage instability, frequency instability, charge malfunction and power quality problems. The paper thus offers a critical overview of the micro grid growth, economic analysis and control strategy.