Voltage Reactive Power Research Articles

Power System Voltage Correction Scheme Based on Adaptive Horizon Model Predictive Control

Model predictive control (MPC) is commonly used to compensate for modeling inaccuracies and measurement noise in voltage control problems. The length of the prediction horizon and control horizon of a MPC-based method has significant impact on the control performances. In existing relevant works, those horizon parameters are determined off-line based on experience or enumeration, and keeps constant during the entire receding-horizon optimization process. This paper presents a system voltage correction scheme based on adaptive horizon model predictive control (AH-MPC). The reactive power compensation and voltage regulation devices are coordinated to maintain the system voltages within a desired range. An evaluation index is proposed to determine the horizon parameters, which reflects the maximum voltage regulation ability with the current parameter configuration. Within each sampling interval, the horizon parameters are updated according to the evaluation index and real-time measurements periodically, which comprehensively considers the system uncertainties and voltage recovery speed, and the computational effort is remarkably reduced. The validation and effectiveness of the proposed method is verified by the simulation analysis on the test system.

Transient Analysis of Quasi Oppositional Based Lightning Search Algorithm Optimized PID Controller in Isolated Small Hydro Power Plant

In this paper, Small Hydro Plant (SHP) of 1.3 MW is simulated with two conventional PID controllers in excitation system and governor to enhance the capability to handle the transiency of the generator. Excitation voltage control and turbine speed control are the two basic control schemes, to regulate reactive power or terminal voltage and real power or frequency respectively. The selection parameters of the PID controllers are significant to enhance the performance of the system. Quasi Oppositional Base­­d Lightning Search Algorithm (QOLSA) is validated in this paper to optimize the PID controllers over LSA and PSO. Renewable energy source like SHP is environment friendly and very imperative to meet the vigorously growing load demand. The simulation of the SHP is established in MATLAB/SIMULINK environment. Finally, QOLSA optimized PID controller contributes better control in terminal voltage and power over LSA and PSO algorithms. Citation: Kaushaley, S., and Shaw, B. (2018). Transient Analysis of Quasi Oppositional Based Lightning Search Algorithm Optimized PID Controller in Isolated Small Hydro Power Plant. Trends in Renewable Energy, 4, 34-43. DOI: 10.17737/tre.2018.4.3.0048

Voltage droop Coordinating Control applied in UPFC and STATCOM system

When UPFC, unified power flow controller is applied with other FACTS into power grid, it is possible that the voltage controlled vibrates constantly to response to a sudden reactive power turbulent in grid if the parameters of these FACTS are not coordinating reasonably. Moreover, the reactive power generated by these equipment will intertwine unexpectedly. The article proposes a method named voltage-reactive power droop control to allow the reference voltage fluctuating around the rating voltage so that the vibration is reduced and the power distribution is improved. Finally, the article cite a electric-magnetic simulation by EMTDC models of east-China power grid to prove it effective when applied to improve the response characteristics to sudden turbulence in power grid.

Analysis and Research on the effect of the Operation of Small Hydropower in the Regional Power Grid

The analysis of reactive power balance and voltage of power network not only affects the system voltage quality, but also affects the economic operation of power grid. In the calculation of reactive power balance and voltage analysis in the past, the problem of low power and low system voltage has been the concern of people. When small hydropower stations in the wet period of low load, the analysis of reactive power surplus and high voltage for the system, if small hydropower unit the capability of running in phase is considered, it can effectively solve the system low operation voltage of the key point on the high side.

Network-Cognizant Voltage Droop Control for Distribution Grids

This paper examines distribution systems that have a high integration level of distributed energy resources (DERs), and addresses the design of local control methods for real-time voltage regulation. Particularly, the paper focuses on proportional control strategies wherein the active and reactive power output of DERs are adjusted in response to (and proportionally to) local changes in voltage levels. The design of the voltage-active power and voltage-reactive power characteristics leverages suitable linear approximations of the ac power flow equations and is network-cognizant; that is, the coefficients of the controllers embed information on the location of the DERs and forecasted noncontrollable loads/injections and, consequently, on the effect of DERs power adjustments on the overall voltage profile. A robust approach is pursued to cope with uncertainty in the forecasted noncontrollable loads/power injections. The stability of the proposed local controllers is analytically assessed and numerically corroborated.

Passive control design for multi-terminal VSC-HVDC systems via energy shaping

This paper investigates a passive control (PC) scheme for multi-terminal voltage source converter (VSC) based high voltage direct current (VSC-HVDC) systems, which can provide a reliable and effective integration of electrical power from renewable energy. A storage function is constructed for each terminal and reshaped into a desired output strictly passive form via feedback passivation with an extra system damping. The beneficial nonlinearities are retained which results in a better transient dynamics of the active power, reactive power, and direct current cable voltage. Then the retained internal dynamics related to the direct current (DC) cable current and common DC voltage is proved to be asymptotically stable by zero-dynamics technique of output dynamics, thus the closed-loop system can be asymptotically stabilized. Case studies are carried out on a four-terminal VSC-HVDC system, under six conditions, e.g., active power and reactive power regulation with parameter uncertainties, faults at alternating current (AC) bus and DC cable, offshore wind farm connection, weak AC grid connection, and robustness of DC cable resistance uncertainties. Simulation results verify the effectiveness of PC against that of linear proportional-integral (PI) control and nonlinear feedback linearization control (FLC) under various operation conditions.

Droop Control Methods for PV-Based Mini Grids with Different Line Resistances and Impedances

Different droop control methods for PV-based communal grid networks (minigrids and microgrids) with different line resistances (R) and impedances (X) are modelled and simulated in MATLAB to determine the most efficient control method for a given network. Results show that active power-frequency (P-f) droop control method is the most efficient for low voltage transmission networks with low X/R ratios while reactive power-voltage (Q-V) droop control method is the most efficient for systems with high X/R ratios. For systems with complex line resistances and impedances, i.e. near unity X/R ratios, P-f or Q-V droop methods cannot individually efficiently regulate line voltage and frequency. For such systems, P-Q-f droop control method, where both active and reactive power could be used to control PCC voltage via shunt-connected inverters, is determined to be the most efficient control method. Results also show that shunt-connection of inverters leads to improved power flow control of interconnected communal grids by allowing feeder voltage regulation, load reactive power support, reactive power management between feeders, and improved overall system performance against dynamic disturbances.

Power Quality Improvement and Low Voltage Ride Through Capability in Hybrid Wind-PV Farms Grid-Connected Using Dynamic Voltage Restorer

This paper proposes the application of a dynamic voltage restorer (DVR) to enhance the power quality and improve the low voltage ride through (LVRT) capability of a three-phase medium-voltage network connected to a hybrid distribution generation system. In this system, the photovoltaic (PV) plant and the wind turbine generator (WTG) are connected to the same point of common coupling (PCC) with a sensitive load. The WTG consists of a DFIG generator connected to the network via a step-up transformer. The PV system is connected to the PCC via a two-stage energy conversion (dc–dc converter and dc–ac inverter). This topology allows, first, the extraction of maximum power based on the incremental inductance technique. Second, it allows the connection of the PV system to the public grid through a step-up transformer. In addition, the DVR based on fuzzy logic controller is connected to the same PCC. Different fault condition scenarios are tested for improving the efficiency and the quality of the power supply and compliance with the requirements of the LVRT grid code. The results of the LVRT capability, voltage stability, active power, reactive power, injected current, and dc link voltage, speed of turbine, and power factor at the PCC are presented with and without the contribution of the DVR system.

Novel Linearized Power Flow and Linearized OPF Models for Active Distribution Networks With Application in Distribution LMP

The locational marginal price (LMP) methodology has been discussed for distribution networks/systems under the smart grid initiative. In this paper, a new distribution LMP (DLMP) formulation is presented which includes reactive power prices and voltage constraints. To solve DLMP, three modeling tools, namely, linearized power flow for distribution (LPF-D), loss factors for distribution (LF-D), and linear optimal power flow for distribution (LOPF-D) are proposed. LPF-D solves not only voltage angles but also magnitudes through linear expression between bus injections and bus voltages, specifically for distribution systems. LF-D is solved recursively based on the radial topology of typical distribution systems. With the integration of LPF-D and LF-D, conventional optimal power flow (OPF) can be reformulated as LOPF-D which is essentially a linear programming model. Test results on various systems show that: 1) LPF-D efficiently yields very close results if compared with AC power flow; 2) LOPF-D provides very close dispatch results in both real and reactive power if compared with ACOPF; and 3) the proposed DLMPs calculated with LF-D and LOPF-D give accurate price information if compared with the prices from ACOPF. Further, these three tools are not limited to DLMP but can be potentially applied to other distribution analyses.

Multi-Objective Optimization Approach for Placement of Multiple DGs for Voltage Sensitive Loads

This paper presents the optimal placement of multiple Dispersed Generators using multi-objective optimization. The optimization is carried out with objectives namely active power loss, reactive power loss, voltage deviation and overall economy. The multi-objective optimization and accounting conflicting objectives are realized through Particle Swarm Optimization with fuzzy decision approach to find the optimal sizes and sites of Dispersed Generators for voltage dependent residential, commercial and industrial loads. The clusters of buses are formulated from base case load flow to limit the search space for finding the placement of the Dispersed Generators. The effectiveness of the proposed approach is tested on a 69-bus radial distribution. It is found that the optimal placement of the Dispersed Generators improves the overall performance of the system and the optimal allocation is affected by the type of load.

Distributed Generation Units as Ancillary Services Providers in a Pre Smart Grid Environment

AbstractNowadays, ancillary services in electrical distribution networks (e. g. voltage support and reactive power control), usually provided by capacitor banks, start to be performed by distributed generation units (DGs). In this way, several papers have been studying the use of DGs as reactive power providers, and the power electronic/market regulation involved in this new scenario. However, the authors commonly consider a full implementation of Smart Grid philosophy, i. e., there are appropriate communications between DGs and distribution network operator (DNO)’s control centers, but it is not a close reality in many developing countries, due to high costs involved in their implementation. Therefore, this paper proposes a new method in order to use DGs as ancillary services providers in a short and medium-term (called in the literature Pre Smart Grid), in which there are not effective communications between DGs and control centers of DNOs. The proposed method uses a non-uniform DGs distribution, obtained from local atlas of wind, solar, hydraulic and biomass power. The methodology presented accurate results when compared with a PSO-based method, widely used to solve optimization problems, but needs a complete Smart Grid philosophy implementation to work.

Optimal placement of distributed generator for power loss minimization and voltage stability improvement

This paper presents a solution to a problem of optimal allocation of distributed generator (DG) for multi objective minimization of real power loss, reactive power loss, reactive power generation and voltage deviation. The four objective functions are formulated and combined to form a single objective function using weighting factors. The problem is solved by using a technique formed by a combination of Particle Swarm Optimization (PSO) and Newton Raphson Power Flow (NRPF) methods. This hybrid strategy utilizes the global searching capability of PSO and its derivative free nature and the ability of NRPF method to find optimal solution when initial points are efficiently chosen to reach global optimal solution. The method is tested on IEEE 14-bus power system. To minimize cost one DG is considered for installation. The proposed method iteratively moves DG in each load bus of the test system, find optimal transformers’ taps settings and compute objective function value. The DG is assumed to generate 2.5 MW. The bus that gives the minimum value of the objective function is considered as the best candidate for DG installation. In this work, it is bus number nine. Distributed generator used is from wind energy technology.

An Improved Droop Control Strategy for Low-Voltage Microgrids Based on Distributed Secondary Power Optimization Control

To achieve accurate reactive power sharing and voltage frequency and amplitude restoration in low-voltage microgrids, a control strategy combining an improved droop control with distributed secondary power optimization control is proposed. The active and reactive power that each distributed generator (DG) shares is calculated by extracting load information and utilizing a power sharing ratio, and is reset to be the nominal power to recalculate droop gains. The droop control curves are reconstructed according to the nominal active and reactive power and the recalculated droop gains. The reconstructed active power-frequency droop control can regulate active power adaptively and keep frequency at a nominal value. Meanwhile, the reconstructed reactive power voltage droop control can reduce voltage amplitude deviation to a certain extent. A distributed secondary power optimization control is added to the reconstructed reactive power voltage droop control by using average system voltage. The average system voltage is obtained by using a consensus algorithm in a distributed, sparse communication network which is constituted by all controllers of DGs. As a result, accurate reactive power sharing is realized, average system voltage is kept at a nominal value, and all voltage amplitude deviations are further reduced. Due to the absence of a microgrid central controller, the reliability of the strategy is enhanced. Finally, the simulation results validate the proposed method.

A State-Independent Linear Power Flow Model With Accurate Estimation of Voltage Magnitude

Linearized power flow models are of great interest in power system studies such as contingency analyses and reliability assessments, especially for large-scale systems. One of the most popular models—the classical DC power flow model—is widely used and praised for its state independence, robustness, and computational efficiency. Despite its advantages, however, the DC power flow model fails to consider reactive power or bus voltage magnitude. This paper closes this gap by proposing a decoupled linearized power flow (DLPF) model with respect to voltage magnitude and phase angle. The model is state independent but is distinguished by its high accuracy in voltage magnitude. Moreover, this paper presents an in-depth analysis of the DLPF model with the purpose of accelerating its computation speed, leading to the fast DLPF (FDLPF) model. The approximation that is applied to obtain the FDLPF model from the DLPF model is justified by a theoretical derivation and numerical tests. The proposed methods are provably accurate and robust for several cases, including radial distribution systems, meshed large-scale transmission systems and ill-conditioned systems. Finally, expressions for sensitivity with regard to MW flow and bus voltage are provided as a potential application.

Analysis into the Impacts of Short Circuit Capacity and X/R Ratio on Voltage Stability in Wind Power Plants

The voltage stability of a distribution network connected Wind Power Plant (WPP) is dictated by the Short Circuit Capacity (SCC), Short Circuit Ratio (SCR), and short circuit impedance angle ratio at a given Point of Common Coupling (X/RPCC). This paper presents a comprehensive analytical study into the interconnection of WPPs to distribution networks through detailed power-voltage (PV), reactive power-voltage (QV), and SCR-voltage analysis studies. Analysis studies have been carried out using Induction Generator (IG) and Doubly-Fed Induction Generator (DFIG) WPP models connected to IEEE standard distribution systems. The study has shown that for small X/RPCC ratios, DFIG-based WPPs suffer from Power Quality (PQ) concerns. In the case of IG-based WPPs, PQ requirements can hardly be satisfied when wind power penetration is high for both large and small X/RPCC values. Analytical models, developed using QV analysis, have shown that voltage variations in response changes in wind reactive power at large X/RPCC buses is approximately two times greater than those with a small X/RPCC ratio. This research will guide decisions on the selection of feasible networks and WPP interconnection buses where steady-state voltage stability requirements can be best satisfied.

An Approach to Improve the Penetration of Sustainable Energy Using Optimal Transformer Tap Control

A method to secure the generator reactive power reserve by adjusting the transformer tap to improve the power system penetration of renewable energy has been proposed in this study. The tap is adjusted based on the voltage and reactive power sensitivity in the power system network. That is, the transformer tap sensitivity is calculated and analyzed to adjust the tap variation to gain sufficient or the least necessary amount of reactive power reserve. This method can be effective for generators without any margins in the reserves. The optimization problem based on the calculated sensitivities and effectiveness are presented. The optimum solution derived from such a problem provides the minimum control amount necessary to maintain the system voltage and dynamic reactive power reserve at their pre-specified levels to improve the power system acceptability of renewable energy. To demonstrate the effectiveness of the method proposed, a simulation has been performed for an IEEE-25 bus system. The results from simulations prove that the voltage has been well maintained while securing a dynamic reactive power reserve through optimal control based on the sensitivity analysis.

Passivity-based control of cascaded multilevel converter based D-STATCOM integrated with distribution transformer

This paper proposes a novel adaptive passivity-based control (PBC) of cascaded multilevel converter based D-STATCOM integrated with distribution transformer for medium voltage reactive power compensation. A cascaded multilevel converter-based D-STATCOM is connected to a group of taps on the primary windings to form an integrated structure. The power transformer is used as coupling transformer at the same time. The structure is helpful to achieve a flexible connection point voltage and improve the capacity utilization of the transformer. The winding currents distribution and compensation capacity constraint are analyzed in detail by steady-state phasor diagrams. A nonlinear passivity-based control is designed to achieve reference currents tracking. Furthermore, an adaptive control is also presented to attenuate the effects of parameters perturbation on the performance of the D-STATCOM. Finally, down-scaled prototype experimental results are provided to verify the steady and dynamic performances of the integrated D-STATCOM with PBC for reactive power compensation.

Analysis of droop control method in an autonomous microgrid

Abstract In this paper an analytical approach is conducted to evaluate the droop control method in an islanding microgrid. Droop control is the key solution for sharing the demand power between generators in autonomous microgrids where there is no support from the electricity distribution grid. In the paper, three important load types are investigated to verify the droop control performance. First, coupling of active power and reactive power is considered in the microgrid and a new method is proposed to facilitate separate control of powers. In the proposed method the effects of droop gains on decoupling of active power and reactive power control, voltage regulation, power oscillation and system stability are studied. In the second load type study, by applying the different types of faults, induction motor characteristics are observed. By simulation results it is shown that the fault intensity and duration will determine how the microgrid attains to fast frequency convergence and how fast protection system operation can improve system stability. In the third case, imbalanced nonlinear load is studied in the microgrid and the influences of embedded controllers on harmonic distortion, system balance and voltage regulation are observed.

A Large Scale Grid Data Analysis Platform for DSOs

The number of fluctuating distributed energy resources (DER) in electricity grids is continuously rising. Due to the lack of operational information on low-voltage (LV) networks, conservative assumptions are necessary to assess the connection of generators to the grid. This paper introduces the hosting capability (HC) as a measure to assess the amount of DER that can be integrated in LV-feeders. The HC of a feeder is the minimum amount of DER that can be hosted in a feeder without reinforcement needs for a given DER-scenario and for a given admissible voltage rise. The hosting capability assessment was performed on the entire LV-grid data of two Austrian Distribution System Operators (DSOs) with more than 36,000 LV-feeders. In total, 40 HC-scenarios were calculated with varying admissible voltage rise levels, DER-scenarios and reactive power control strategies. It turned out that only few feeder parameters such as the resistance at the end node and the lowest ampacity value of feeders show a high correlation with the calculated HC. Further, the impact of the DER-scenario on the share of voltage and loading constrained feeders is rather limited. The gathered results are suitable to validate equivalent LV-feeders models to perform integrated power flow studies on the transmission and distribution grids. Besides the results obtained for the network data of the two DSOs, a performant, modular and parallelizable tool has been developed to automatically analyze large LV network sets.

Robust Direct Power Control Schemes of DFIGs Driven by Variable-speed Wind Turbines

Despite its several advantages, a classic direct power control (DPC) technique of doubly fed induction generators (DFIGs) driven by variable-speed wind turbines has some drawbacks such as high power ripples and variable switching frequency. In this paper, two robust controllers are designed to improve the classical DPC performance without complicating the overall scheme. First, an integral sliding mode controller (ISMC) is designed to regulate the stator active and reactive powers. Two integral switching functions are selected for controlling stator active and reactive powers. The idea of total sliding mode controller is selected to avoid reaching phase stability problem. Second, a diagonal recurrent neural network (DRNN) controller is designed and trained based on DPC. The DRNN has several advantages compared to the classical static neural networks such as recurrence and simple construction. Simple off-line back-propagation algorithm is proposed to train the proposed DRNN. The stability of the proposed ISMC and DRNN controller is proved using the Lyapunov stability theorem. The grid side converter is controlled based on the DPC principle to ensure both constant DC-link voltage and grid side reactive power. The feasibility of the proposed DPC schemes is validated by simulation studies on a 1.5-MW wind power generation system. The performance of the proposed schemes is compared with a conventional DPC scheme under different operating conditions.