Grid Voltage Drop Research Articles

A Stabilization Control Strategy for Wind Energy Storage Microgrid Based on Improved Virtual Synchronous Generator

In high-penetration renewable-energy grid systems, conventional virtual synchronous generator (VSG) control faces a number of challenges, especially the difficulty of maintaining synchronization during grid voltage drops. This difficulty may lead to current overloads and equipment disconnections, and it has an impact on the security and reliability of the system, as well as limiting the dynamic reactive power support capability of the system. To solve this problem, in this study, a wind–solar hybrid power generation system is designed with a battery energy storage device connected on the DC side, and proposes a low voltage ride-through (LVRT) control strategy for the grid-connected inverter based on an improved VSG. The control strategy employs an integrated current limiting technique combining virtual impedance and vector current limiting to ensure that the VSG exhibits good dynamic power support characteristics during symmetrical faults by adjusting the setpoint value of reactive power. At the same time, it maintains the synchronization and power angle stability of the VSG itself to achieve the goal of LVRT. Simulation results show that the proposed control strategy can effectively suppress the renewable power fluctuations (about 30% reduction in fluctuations compared to the conventional strategy) and ensure the safe and reliable operation of the renewable energy sources and VSGs during grid-side faults. In addition, it provides a given reactive power support and stable grid voltage control (voltage dips reduced by about 20%), which significantly enhances the LVRT capability of the hybrid wind–solar-storage generation system.

Low voltage ride through enhancement of a permanent magnet synchronous generator based wind energy conversion system in an islanded microgrid: A dynamic matrix controlled virtual DC machine

In an isolated microgrid, the wind energy conversion system based on direct-drive permanent magnet synchronous generator may experience fluctuations in the DC bus voltage due to wind variations and grid voltage drop, thereby compromising system reliability. To address this issue, the scheme of the supercapacitor energy storage system is proposed. The existing double closed-loop PID control, which cannot provide inertia support, has shortcomings such as difficult parameter setting and slow dynamic response. To solve this problem, the dynamic-matrix-control based predictive control for a virtual DC machine method is proposed. Firstly, the concept of virtual DC machine control is firstly introduced in supercapacitor energy storage system. Secondly, according to the unit step response of virtual DC machine, the prediction model of virtual DC machine is established. Finally, the desired torque increment reference of virtual DC machine is calculated by the dynamic-matrix-control controller to change the input torque reference. Hardware-in-the-loop experiment verifies the effectiveness of the proposed method: Compared with traditional double closed-loop PID control, the maximum voltage fluctuation is reduced by 67.9 % under random wind fluctuation and the inhibition effect on DC voltage surge and plunge when fault occurs in the power grid is increased by 61.4 % and 57.9 %.© 2017 Elsevier Inc. All rights reserved.

Passivity-based Rieman Liouville fractional order sliding mode control of three phase inverter in a grid-connected photovoltaic system.

Photovoltaic (PV) system parameters are always non-linear due to variable environmental conditions. The Maximum power point tracking (MPPT) is difficult under multiple uncertainties, disruptions and the occurrence of time-varying stochastic conditions. Therefore, Passivity based Fractional order Sliding-Mode controller (PBSMC) is proposed to examine and develop a storage function in error tracking for PV power and direct voltage in this research work. A unique sliding surface for Fractional Order Sliding Mode Control (FOSMC) framework is proposed and its stability and finite time convergence is proved by implementing Lyapunov stability method. An additional input of sliding mode control (SMC) is also added to a passive system to boost the controller performance by removing the rapid uncertainties and disturbances. Therefore, PBSMC, along with globally consistent control efficiency under varying operating conditions is implemented with enhanced system damping and substantial robustness. The novelty of the proposed technique lies in a unique sliding surface for FOSMC framework based on Riemann Liouville (R-L) fractional calculus. Results have shown that the proposed control technique reduces the tracking error in PV output power, under variable irradiance conditions, by 81%, compared to fractional order proportional integral derivative (FOPID) controller. It is reduced by 39%, when compared to passivity based control (PBC) and 28%, when compared to passivity based FOPID (EPBFOPID). The proposed technique led to the least total harmonic distortion in the grid side voltage and current. The tracking time of PV output power is 0.025 seconds in PBSMC under varying solar irradiance, however FOPID, PBC, EPBFOPID, have failed to converge fully. Similarly the dc link voltage has tracked the reference voltage in 0.05 seconds however the rest of the methods either could not converge, or converged after significant amount of time. During solar irradiance and temperature change, the photovoltaic output power has converged in 0.018 seconds using PBSMC, however remaining methods failed to converge or track fully and the dc link voltage has minimum tracking error due to PBSMC as compared to the other methods. Furthermore, the photovoltaic output power converges to the reference power in 0.1 seconds in power grid voltage drop, whereas other methods failed to converge fully. In addition power is also injected from the PV inverter into the grid at unity power factor.

Low-voltage ride-through capability improvement of Type-3 wind turbine through active disturbance rejection feedback control-based dynamic voltage restorer

Abstract Disconnections due to voltage drops in the grid cannot be permitted if wind turbines (WTs) contribute significantly to electricity production, as this increases the risk of production loss and destabilizes the grid. To mitigate the negative effects of these occurrences, WTs must be able to ride through the low-voltage conditions and inject reactive current to provide dynamic voltage support. This paper investigates the low-voltage ride-through (LVRT) capability enhancement of a Type-3 WT utilizing a dynamic voltage restorer (DVR). During the grid voltage drop, the DVR quickly injects a compensating voltage to keep the stator voltage constant. This paper proposes an active disturbance rejection control (ADRC) scheme to control the rotor-side, grid-side and DVR-side converters in a wind–DVR integrated network. The performance of the Type-3 WT with DVR topology is evaluated under various test conditions using MATLAB®/Simulink®. These simulation results are also compared with the experimental results for the LVRT capability performed on a WT emulator equipped with a crowbar and direct current (DC) chopper. The simulation results demonstrate a favourable transient and steady-state response of the Type-3 wind turbine quantities defined by the LVRT codes, as well as improved reactive power support under balanced fault conditions. Under the most severe voltage drop of 95%, the stator currents, rotor currents and DC bus voltage are 1.25 pu, 1.40 pu and 1.09 UDC, respectively, conforming to the values of the LVRT codes. DVR controlled by the ADRC technique significantly increases the LVRT capabilities of a Type-3 doubly-fed induction generator-based WT under symmetrical voltage dip events. Although setting up ADRC controllers might be challenging, the proposed method has been shown to be extremely effective in reducing all kinds of internal and external disturbances.

Rotor Voltage Compensation Control Strategy of Double-fed Wind Turbine Considering Stator Flux Transient Characteristics

The stability of the grid voltage is directly related to the stability of the DFIG operation, which is related to the topology of the DFIGURE After the grid failure, the rotor current will be too large if the stability threshold is exceeded continuously. The current low voltage control uses additional hardware measures to make the wind turbines survive under the impact of grid voltage drop. Because additional hardware measures will increase the investment of the system, this paper proposes an additional control of the rotor-side converter of the double-fed wind turbine considering the stator flux linkage. The output voltage of the converter is increased through the compensation of additional control to suppress the excessive rotor current. Firstly, the dynamic process of the double-fed wind turbine can be obtained from the relationship between stator flux linkage and rotor voltage of the circuit before and after the fault, and the rotor equivalent circuit is obtained. Combined with the actual control of the rotor converter, the rotor current composition in different coordinate systems is obtained. Compared with the two current components, the equivalent relationship can be obtained to get the transient voltage to be compensated. The rotor converter increases the corresponding output voltage to prevent excessive rotor currents from affecting wind power systems. The simulation proves that the proposed additional control can limit the transient current of the fan rotor in a safe range when the grid voltage drops, which enhances the anti-interference ability of the double-fed wind turbine.

Current limiting control method with adaptive virtual impedance for grid-forming STATCOM

Current limiting is an important issue in the static synchronous compensator (STATCOM) with grid-forming (GFM) control. However, the conventional methods are difficult to deal with the application scenarios of different grid faults. In this paper, an improved current limiting method with the adaptive virtual impedance is proposed for the GFM STATCOM. The specific implementation strategy of the GFM control is introduced firstly. The generation method of the adaptive virtual impedance and the realization of current limiting strategy are also presented in detail. The proposed adaptive virtual impedance can be adjusted automatically by following voltage sag, which realizes the adaption of different grid faults situations. The proposed method is realized by the GFM STATCOM simulation platform with PSCAD/EMTDC, it is confirmed that the proposed method has a faster current limiting response speed when the voltage sag is larger, which can improve the supporting effect of GFM STATCOM for the grid voltage drop.

Coordinated Rotor-Side Control Strategy for Doubly-Fed Wind Turbine under Symmetrical and Asymmetrical Grid Faults

In order to solve the problems of rotor overvoltage, overcurrent and DC side voltage rise caused by grid voltage drops, a coordinated control strategy based on symmetrical and asymmetrical low voltage ride through of rotor side converter of the doubly-fed generator is proposed. When the power grid voltage drops symmetrically, the generator approximate equation under steady-state conditions is no longer applicable. Considering the dynamic process of stator current excitation, according to the change of stator flux and the depth of voltage drop, the system can dynamically provide reactive power support for parallel nodes and suppress the rise of DC side voltage and rotor over-current. When the grid voltage drops asymmetrically, the positive and negative sequence components are separated in the rotating coordinate system. The doubly fed generator model is established to suppress the rotor positive sequence current and negative sequence current respectively. At the same time, the output voltage limit of the converter is discussed, and the reference value is adjusted within the allowable output voltage range. In order to adapt to the occurrence of different types of power grid faults and complex operating conditions, a fast switching module of fault type detection and rotor control mode is designed to detect the type of power grid faults and voltage drop depth in real time and switch the rotor side control mode dynamically. Finally, the simulation model of the doubly fed wind turbine is constructed in Matlab/Simulink. The simulation results verify that the proposed control strategy can improve the low-voltage ride through performance of the system when dealing with the symmetrical and asymmetric voltage drop of the power grid and identify the power grid fault type and provide the correct control strategy.

Dynamic Pricing Strategy of Charging Station Based on Traffic Assignment Simulation

The number of electric vehicles is increasing rapidly worldwide, leading to increasing demand for charging. This will negatively impact the grid. Therefore, it is essential to relieve the power grid operation pressure by changing the charging behaviour of users. In this paper, the charging behaviour of electric vehicles was guided by price instruments to maintain grid balance This paper uses travel simulation to establish the relationship between travel demand and electricity prices. The results were evaluated through the amount of grid voltage drop and network loss. Furthermore, we used the differential evolutionary algorithm to calculate the optimal operation status of the grid, which contains the minimal network loss and maximal voltage drop at different charging stations and the charging price. Finally, the effectiveness of the mechanism proposed in this paper was compared with other simulation examples. The results showed that the pricing strategy could guide users’ charging choices and maintain the grid load balancing. The simulation results show that the average bus voltage increases by 1.26% and 6.59%, respectively, under different requirements.

Optimum Control of Grid-Connected Solar Power System Under Asymmetrical Voltage Drop

Solar power systems are now gradually dominating in providing clean, environmentally friendly energy and human health. In areas with a large share of solar power, grid connection control plays a key role in ensuring operational quality and stability, especially in the event of a grid failure. In case of asymmetrical voltage drop, the control system needs to maintain operation and create a function to assist in restoring the power grid. This study proposes a method to control the solar power system in the condition of asymmetric grid voltage drop based on the method of controlling symmetrical components. Controllers for each of the forward and inverse components are built to limit the effects of failures. The optimal control parameter calculation method is also proposed to improve the overall quality and minimize the undesired variation of the electromagnetic quantities. The simulation and experimental results are verified to evaluate the effectiveness of the grid-connected control method in converting DC power to three-phase power.

Feedforward effect on the transient response during low voltage ride through

Suitable control methods and strategies applied in power electronic converters are vital to interconnecting renewable energy sources to the utility grid. Furthermore, grid codes and international standards are introducing new specifications according to the rapid changes the electric power systems are experiencing in the last decades. Therefore, developing, testing, and upgrading the control methods to fulfil those new standards is essential. This paper focuses on the low voltage ride through capability applied in an industrial inverter connected to the grid through an LCL filter with a maximum admissible inverter-side current of 1.23 pu. The main challenge is that the protective circuit shuts the inverter down once the maximum current is exceeded as a result of a sudden drop in grid voltage and consequently disconnects it from the grid, making it not compliant with the standards. This paper investigates the effect of the feedforward voltage loop on the inverter side peak current and suggests a solution to limit the current based on modifying the feedforward structure and tuning the controller parameters. Simulation results show a reduction in the inverter side current from 1.89 pu to 1.095 pu.

Improvement of voltage adaptability for decentralised doubly‐fed induction generator wind power system based on nine‐switch converter

This paper proposes a method to improve the voltage adaptability of doubly fed induction generator (DFIG) wind turbines in a decentralised scenario by incorporating a nine-switch converter (NSC). With the proposed scheme, the DFIG can realise low voltage ride-through (LVRT) and continuous operation under the grid voltage harmonic, drop, and rise. The topology of the NSC-DFIG system is introduced and is used to solve the circuit model. Control strategies for voltage compensation and current compensation are based on the circuit model. The corresponding voltage compensation scheme and the NSC DC voltage control scheme were designed according to the specific capacity configuration of the NSC in this study. To reduce the DC-side voltage of the NSC, a third harmonic-injection modulation method and a dynamic modulation ratio allocation strategy were incorporated. Simulations under four power grid voltage conditions were used to validate the scheme. This method can improve the voltage adaptability for decentralised DFIG wind power systems, which can enable their successful interactions with the power grid.

Impact of on-grid solar energy generation system on low voltage ride through capability

This paper represented a control strategy for photovoltaic (PV) system, this control strategy was referred to as a low voltage ride through (LVRT), it had been achieved by using three phase (3-PH) (PV) grid-connected system, where this paper discussed the way to achieve maximum output active power from the solar system, while the solar system remained connected to the grid with voltage decreasing controller techniques and this was valid until certain amount of voltage decreasing which was clarified in the results. The main goal of low voltage ride through depended on injecting reactive power to the grid, amount of the injected reactive power depended on regulations of the grid code using the control of the inverter and the strategy depended on the grid voltage drop amount. MATLAB simulation had been used to achieve what was mentioned above, which led to present various cases of achieving maximum output active power with grid voltage drops by using conventional proportional integral (PI) control of the inverter. Finally, another control method, which was proportional integral genetic algorithm (PI-GA), had been used to improve value of the generated output active power.

Research on Low Voltage Ride through Control of a Marine Photovoltaic Grid-Connected System Based on a Super Capacitor

With the increase of photovoltaic penetration rate, the fluctuation of photovoltaic power generation affects the reliability of ship power grids. Marine PV grid-connected systems with high penetration rates should generally have a low voltage ride-through capability. In the present paper, a strategy in which super capacitors are applied for energy storage in a marine photovoltaic grid-connected system is proposed, and an inverter adopts independent decoupling control of active and reactive currents to improve the LVRT capability of photovoltaic grid-connected systems. In addition, a comprehensive control strategy is also designed to control the supercapacitor, to regulate the active power through the control method of the voltage outer loop and the current inner loop, in order to maintain the DC bus voltage stability. At the same time, the inverter can increase the reactive power output to support the grid voltage. The advantage of this system is in smoothing the power imbalance in a short time, enhancing the low voltage ride-through capability of the photovoltaic grid-connected system, improving the power quality, and ensuring the safety and stability of the ship’s power grid. MATLAB/Simulink were employed to establish a ro-ro ship super capacitor–marine photovoltaic grid-connected power system model and to carry out simulation experiments by setting the grid voltage drop. The results show that when the grid voltage drops, the inverter adjusts the distribution of active and reactive power. The power factor drops from 1 to 0.77, and the effective value of the voltage drop increases from 150 V to 156 V, which proves that this strategy effectively reduces the depth of the grid voltage drop and improves the low voltage ride-through capability of the photovoltaic grid-connected system.

Enhancement of a Voltage Drop in a Low-Voltage Grid by the Contribution of a Hybrid PV-Wind Turbine Generator

Enhancement of a Voltage Drop in a Low-Voltage Grid by the Contribution of a Hybrid PV-Wind Turbine Generator

Super-twisting SMC for MPPT and grid-connected WECS based on SCIG

This paper proposes a high-order sliding mode control (HO-SMC) with the super-twisting (ST) algorithm for maximum power point tracking (MPPT) and grid-connected wind energy conversion system (WECS), based on squirrel-cage induction generator (SCIG). The main features of this control strategy are attenuation of the chattering phenomenon inherent in first-order sliding mode control and its robustness against external and internal disturbances encountered by the wind power system. The simulation is carried out under SimPowerSystems of MATLAB/Simulink to evaluate the performance and effectiveness of the proposed control compared with conventional and fuzzy logic proportional-integral (PI) controllers for three different scenarios of disturbances, a fluctuating wind speed, a grid voltage drop, and parametric variations.

Mechanism of second harmonic generation of photovoltaic grid-connected system faults and its impact on transformer protection

The short-circuit transient characteristics of photovoltaic(PV) power system are different from traditional power sources. When the grid voltage drops, the output current of PV will contain a high proportion of second harmonics which cannot be ignored. This paper takes a single-phase turn-to-turn short-circuit fault inside the PV grid-connected transformer as an example. First, it analyzes the mechanism of single-frequency input and double-frequency output of a three-phase grid-connected inverter based on a phase-locked loop (PLL), and secondly, considering the comprehensive grid-connected control strategy, the principle of current second harmonic in the minor faults is analyzed and the analytical expression of second harmonics is derived. It is pointed out that the transient dc component will produce second harmonic component in the grid-connected current through control system of the inverter. This current induces a second harmonic voltage in the impedance, and as a disturbance term it generates a new harmonic current through the control link. Superimposed second harmonic currents in two scenarios may threaten the reliability of transformer differential protection. Finally, the correctness of the theoretical analysis is verified by simulation based on MATLAB/Simulink.

Control strategy of doubly-fed induction wind turbine

Based on the principles of DFIG, this paper establishes a DFIG model in a synchronously rotating dq coordinate system. For the grid-side control system, the vector control technology facing the grid voltage is used to realize the decoupling of DFIG active power and reactive power. The methods of feedforward compensation and PI control are used to control the rotor side of the DFIG. When the voltage drops above the low voltage ride-through operating standard curve, DFIG should not leave the grid. When the grid voltage drops below 0.9 times the standard unit value, DFIG should also provide reactive power to the grid to support the grid voltage. Since the reactive power that can be provided by the stator side is much greater than the reactive power on the rotor side, when the grid voltage drop is small, the stator side first provides reactive power to the grid. When the grid voltage drops more, the stator side and the rotor side share power to the grid to provide reactive power. Finally, using MATLAB/Simulink software to simulate DFIG, verify the effectiveness of the control strategy proposed in this paper, and lay a foundation for fault characteristic analysis.

Asynchronous Grid Connection of a Cage Induction Generator Excited by a Power Electronic Converter

This article presents an analysis of grid connection of an excited induction generator, where an inrush current is significantly reduced and voltage drop in the local grid is eliminated. In the proposed method, generator connection to the grid is done at the moment when the generator voltage vector equals the grid voltage vector, but the generator frequency differs from the grid frequency. Excitation of the induction generator is provided by a power electronic converter, which is rated only on the rated reactive power of the generator. Moreover, when the generator is connected to the grid, the converter satisfies reactive power demand of an induction generator, according to the requirements from the distribution system operator. The simulation and laboratory tests were conducted with a 7.5 kW induction generator.

Transient Stability Enhancement Control Strategy Based on Improved PLL for Grid Connected VSC during Severe Grid Fault

In this article, the transient synchronization process of the grid-connected voltage source converters (VSC) is studied detailly. Firstly, the phase-locked loop (PLL)-synchronized VSC is modeled according to the rotor motion equation of synchronous generator (SG). Furthermore, the VSC's damping ratio is derived, and the effects of the VSC's control parameters and the operation status on the equivalent power angle (EPA) is investigated. Moreover, the influence of the PLL's parameters and the voltage dips degree on the VSC's transient frequency behavior are analyzed. Analysis result reveals that the VSC's damping ratio decreases once the grid voltage drops, so that the EPA and frequency of the VSC may appear large overshoot and oscillation, which may trigger the frequency protection and deteriorate the VSC's transient stability. In order to ensure that the VSC can smoothly operate to new equilibrium points, an improved PLL is proposed, which can adaptively adjust the VSC's damping ratio for different voltage sags. The proposed method can not only reduce the overshoot of the EPA but also restrain the VSC's frequency dips degree. Finally, the simulations and experimental tests validate the effectiveness of the theoretical analysis.

Impact of Energy Storage Devices on the Distribution System Stability with Distributed Generation. (Dept. E.)

Renewable energy sources have environmental and economic advantages, but it causes many fluctuations and severe problems on the power system. One solution to decrease these problems are designing renewable sources as distributed generations (DGs) on the power distribution system. But it is necessary to add energy storage systems (ESS) to the power system to improve the impact of DGs on the power system stability. In this paper, the photovoltaic system (PV) with energy storage devices such as battery or superconducting magnetic energy storage systems (SMES) is added to the power system and analyzed the stability of the system by using the software program, “MATLAB/Simulink®”. The transient stability of the power system is studied based on many factors as maximum rotor speed deviation, the drop-in grid voltage, and the drop in DC voltage of the PV. A different abnormal system states are simulated to show the effect of system configuration on its stability. For validation, a comparative study is implemented. From the results, it is found that the stability of the power system is improved through the adding of the PV system and increased by adding energy storage systems.