Bus Voltage Fluctuation Research Articles

DC Side Bus Voltage Control of Wind Power Grid-Connected Inverter Based on Second-Order Linear Active Disturbance Rejection Control

In order to improve the dynamic response speed and the steady-state performance of the DC side bus voltage of the wind power grid-connected inverter, a mathematical model of a typical three-phase voltage type PWM (Pulse Width Modulation, PWM) grid-connected inverter was established, and its traditional voltage-current double closed loop with proportional-integral control method was analyzed. Then a second-order linear active disturbance rejection controller that does not depend on system model information was designed to replace the traditional voltage outer loop proportional-integral controller, thus a new double closed-loop control structure was formed to control it. The frequency domain theory was used to analyze the convergence of the third-order linear extended state observer and the influence of the total disturbance on the performance of the third-order linear extended state observer. The parameter tuning scheme of the designed controller was given. Finally, the 1.5 MW direct-driven permanent magnet wind power generation system was built in the Matlab/Simulink software and the control effects of the two control modes under different working conditions are compared. The simulation results show that the control scheme designed in this paper is superior to the traditional proportional-integral controller which has good anti-interference characteristics and robustness. Especially it has a good stability effect on DC side bus voltage fluctuations.

Robust Control Design for High-Power Density PV Converters in Weak Grids

This paper proposes a robust current controller (CC) and dc-link voltage controller based on $\mu $ -synthesis and $H_{\infty }$ method, respectively, for a single-phase photovoltaic (PV) converter with an LCL -filter under weak grid operation. The $\mu $ -synthesis CC guarantees system robustness against weak grid uncertainties such as grid impedance variations and voltage harmonics. Moreover, the controller compensates the system delays arising from the calculation, pulsewidth modulation, and zero order hold, as well as delay uncertainties. The CC only uses the grid current feedback which eliminates the requirement for additional loops or state measurements utilized in partial or full state-feedback systems. In addition, the $H_{\infty }$ dc-link voltage controller minimizes the bus voltage fluctuations caused by variations of PV system power generation. Also, the controller ensures system robustness against such power variations, which conventional controllers fail to perform. Moreover, power decoupling is achieved by the control system without any auxiliary circuit. In addition, the stability and robust performance of the phase-locked loop in a weak grid with major impedance changes is verified. Simulation and experimental results prove the superior performance of the proposed controllers in both cases of a nominal system and a system with deviated parameters.

Research on Virtual Inductive Control Strategy for Direct Current Microgrid with Constant Power Loads

In order to improve the stability of direct current (DC) microgrid with constant power loads, a novel virtual inductive approach is proposed in this paper. It is known that the negative impedance characteristic of constant power loads will lead to DC bus voltage fluctuation, which will be more serious when they integrate into the DC microgrid though a large transmission line inductive. For the convenience of analysis, a simplified circuit model of the system is obtained by modeling the distributed resources. Unlike the existing control strategies, the proposed control strategy constructs a negative inductance link, which helps to counteract the negative effects of the line inductive between the power source and the transmission line. Detailed performance comparison of the proposed control and virtual capacitance are implemented through MATLAB/simulink simulation. Moreover, the improved performance of the proposed control method has been further validated with several detailed studies. The results demonstrate the feasibility and superiority of the proposed strategy.

Coordinated control strategy of DC microgrid for hydrogen production load

In order to solve the problem of wind abandonment and light abandonment in DC microgrid, adding hydrogen production equipment in DC microgrid can effectively alleviate this problem. In the DC microgrid for hydrogen production load, a hierarchical control strategy of DC bus voltage is proposed based on the consideration of charging state, aiming at the fluctuation of DC bus voltage. The operation of the system is divided into seven modes, each of which adopts different control strategies. Finally, a model is built on the PSCAD/EMTDC simulation software and the experimental platform of new energy generation. The simulation and experimental results verify the effectiveness and feasibility of the proposed control strategy.

Research on Control Strategy of Grid-Connected Converter Based on Virtual Inertia

As the DC microgrid has low inertia, this paper draws on the virtual synchronous machine technology to propose and control a new virtual inertial control strategy. This control strategy enables the grid-connected converter to have both AC inertia and DC inertia. By small signal decomposition, the transfer function block diagram of the system is established. The effects of different inertial parameters on the system’s AC inertia and DC inertia are studied. The dynamic characteristic analysis and simulation results show that the proposed virtual inertial control can effectively suppress the fluctuation of DC bus voltage and AC power and increase the AC and DC inertia of the converter.

Grid connection technique based onμtheory for a two‐stage PV structure

There is greater flexibility in selecting the direct-current (DC) bus voltage in a two-stage photovoltaic (PV) grid-connected structure than in a single-stage one. The independent DC/DC and DC/alternating current (AC) control in traditional control methods changes the duty cycle of the first-stage DC/DC, which affects the control of the secondary DC/AC and even stabilises the DC bus voltage between the two stages. Therefore, two-stage converters should be considered and controlled in a unified manner. Here, the authors introduce a robust μ value control method based on the hybrid sensitivity theory to control a two-stage PV power generation system. Considering the effect of parameter perturbation on the modelling of the DC/DC average state model, the proposed method improves the DC bus voltage stability of a PV system and the power quality of the grid-connected current while achieving the maximum power point tracking performance of the PV. MATLAB simulations using three PV operating conditions demonstrate that the proposed control method can effectively reduce the fluctuation of the DC bus voltage between the two stages, the harmonic distortion rate of the grid-connected current, and the power oscillation time when the working conditions change.

A Virtual Inertia-Based Power Feedforward Control Strategy for an Energy Router in a Direct Current Microgrid Application

Due to the uncertainty of the power load and the randomness of distributed generations, low-voltage direct current (LVDC) bus voltage fluctuation will greatly affect the safety of an energy router-enabled direct current (DC) microgrid. In this paper, a power feedforward control strategy based on a dual active bridge (DAB) DC/DC converter in an energy router-based DC Microgrid is proposed. Based on this strategy, the LVDC bus voltage is controlled by virtual inertia control of the DC microgrid, instead of by the DAB converter. Thus, two benefits of the proposed strategy can be achieved: the power feedforward control can be realized, to mitigate the voltage fluctuation range of the LVDC bus; and the modulation algorithm in the DAB converter can be simplified. Experimental results verify the correctness and effectiveness of the proposed control method.

Hierarchical control of DC microgrid with dynamical load power sharing

The hierarchical control strategy is proposed for DC microgrid with distributed generators, energy storage systems and loads. In order to maintain power balance and smooth bus voltage fluctuation during system operation due to dynamic load and resource variation, the designed hierarchical control is divided into two layers. In the primary layer, the novel adaptive droop controller with voltage feedback compensation is constructed for distributed generators and energy storage systems by taking advantage of local electrical parameters, which can realize the dynamic load power sharing and reduce bus voltage deviation. The distributed generators may operate between maximum power point tracking mode and droop control mode by bus regulation units when power supply is larger than the loads demand and the charging capacity of energy storage systems. The state of charge can also be adjusted for energy storage through the designed adaptive droop scheme. On the other hand, the secondary supervisory control is designed to reduce power exchange and improve the system stability. Using the designed hierarchical control, the DC microgrid can operate in four different modes according to which source responsible for the bus voltage stabilization. Simulation and comparison results validate that the proposed strategy may smooth the bus voltage fluctuation and realize the dynamic power sharing.

Research on Theoretical Calculation Methods of Photovoltaic Power Short-Circuit Current and Influencing Factors of Its Fault Characteristics

With the substantial increase in the capacity of grid-connected photovoltaic (PV) power, the adverse effects of its complex fault characteristics on grid relay protection are increasingly highlighted. Based on the introduction of the topology and the control strategy on low-voltage ride through of PV power, a theoretical solution method for solving the fault current of PV power is proposed by taking account of the DC bus voltage fluctuation, and a theoretical calculation model of its transient and steady state is established. The correctness of the theoretical results is verified by the numerical simulation results and low-voltage ride through (LVRT) experiment results. Furthermore, to gain a better understanding of the factors influencing PV power fault characteristics, the effects of several factors including proportional integral (PI) controller parameters, fault voltage sag depth, and PV power load level on PV power fault characteristics are analyzed by using simple variable method. The obtained results can provide a theoretical reference for the control parameter design and protection research of PV power.

Double PWM coordinated control based on model predictive algorithm and power compensation

With analysis on double PWM structure though system's energy flow theory, active power and reactive power of rectifier are controlled by model predictive algorithm. System adopts the method of combine dynamic power's compensation with static power's compensation for system power in order to reduce system power error. A new power compensation algorithm is proposed to designs a new controller to replace PI controller of system's voltage loop, which can restrain fluctuation of DC bus voltage when load power suddenly varies and reduces DC-link's capacity of capacitance. As for inverter side, the system uses the method of rotor flux linkage oriented to control three phase asynchronous motor. Results of simulation can show that, system can realise the smallest tracking error of active power and reactive power by model predictive control, and restrain fluctuation of DC bus when load power suddenly changes. As the same time, the gird current's waveform is well.

Study on the Interaction Mechanism between Photovoltaic Power Generation and Power Quality

In the microgrid, the voltage of the access point fluctuates due to the PV power plant connected to the grid, and there is a risk of voltage overrun. In this article, the photovoltaic power generation system access to the power grid and the starting point of the impact of the access point voltage deviation mechanism is analysed, obtaining the two major influencing factors including light intensity and ambient humidity. It also analyzes the effect of light and humidity fluctuation on the fluctuation of bus voltage and uses PSCAD to simulate the mechanism of generating harmonics in photovoltaic power generation system.

Mitigating Distribution Power Loss of DC Microgrids With DC Electric Springs

DC microgrids fed with substantial intermittent renewable energy sources face the immediate problem of power imbalance and the subsequent dc bus voltage fluctuation problem (that can easily breach power system standards). It has recently been demonstrated that dc electric springs (DCES), when connected with series non-critical loads, are capable of stabilizing the voltage of local nodes and improving the power quality of dc microgrids without large energy storage. In this paper, two centralized model predictive control (CMPC) schemes with: 1) non-adaptive weighting factors and 2) adaptive weighting factors are proposed to extend the existing functions of the DCES in the microgrid. The control schemes coordinate the DCES to mitigate the distribution power loss in the dc microgrids, while simultaneously providing their original function of dc bus voltage regulation. Using the DCES model that was previously validated with experiments, simulations based on MATLAB/Simulink platform are conducted to validate the control schemes. The results show that with the proposed CMPC schemes, the DCES are capable of eliminating the bus voltage offsets as well as reducing the distribution power loss of the dc microgrid.

An adaptive voltage‐regulation control strategy of an electric spring based on output current feedback

With the increasing penetration of distributed generation (DG) in power systems, the voltage instability problem of power systems is receiving a great deal of attention. As new power electronics equipment, an electric spring (ES) can effectively restrain the bus voltage fluctuation caused by DG, and in addition achieve the new control paradigm of load demand following power generation. In this article, an adaptive voltage‐regulation control strategy of ES is proposed, in which the proportional integral (PI) postgain changes dynamically according to the real‐time output current of the ES. The simulation results show that, when the proposed control strategy is applied, bus voltage can still be stabilized effectively by the ES under the condition that noncritical loads change. The proposed control strategy brings the adaptive voltage‐regulation capability of the ES, and therefore, it can be in series with multiple noncritical loads reliably and safely, and accordingly the costs can be greatly reduced. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Short-Circuit Current Calculation and Harmonic Characteristic Analysis for a Doubly-Fed Induction Generator Wind Turbine under Converter Control

An accurate calculation of short-circuit current (SCC) is very important for relay protection setting and optimization design of electrical equipment. The short-circuit current for a doubly-fed induction generator wind turbine (DFIG-WT) under excitation regulation of a converter contains the stator current and grid-side converter (GSC) current. The transient characteristics of GSC current are controlled by double closed-loops of the converter and influenced by fluctuations of direct current (DC) bus voltage, which is characterized as high order, multiple variables, and strong coupling, resulting in great difficulty with analysis. Existing studies are mainly focused on the stator current, neglecting or only considering the steady-state short-circuit current of GSC, resulting in errors in the short-circuit calculation of DFIG-WT. This paper constructs a DFIG-WT total current analytical model involving GSC current. Based on Fourier decomposition of switch functions and the frequency domain analytical method, the fluctuation of DC bus voltage is considered and described in detail. With the proposed DFIG-WT short-circuit current analytical model, the generation mechanism and evolution law of harmonic components are revealed quantitatively, especially the second harmonic component, which has a great influence on transformer protection. The accuracies of the theoretical analysis and mathematical model are verified by comparing calculation results with simulation results and low-voltage ride-through (LVRT) field test data of a real DFIG.

Performance Analysis of PV Energy System for Pulse Loads

At present, there are more and more pulse loads in military weapons. The specific performance is periodic sudden increase and anticlimax in power. The capacity of photovoltaic power generation with external battery is unable to respond to frequent mutations in the load quickly, due to its limited capacity and slower chargedischarge rate of battery. This paper build the simulation model about the photovoltaic energy system for Power Supply to Weapon Equipments and study the effect law of pulse load on the photovoltaic energy system. Through the method for calculating operation parameters of microgrid,the influence of pulse load with different parameters was carried on the selective analysis. The operation of pulse load results in the fluctuation of bus voltage and the decrease of power transmission efficiency. The influence of pulse load with the different parameters on the system is not the same.

Virtual DC machine: an inertia emulation and control technique for a bidirectional DC–DC converter in a DC microgrid

In a DC microgrid (DCMG), the DC bus voltage is vulnerable to the power fluctuations caused by the variations of intermittent renewable energy sources and local loads. A concept is developed to operate a bidirectional DC-DC converter as a DC machine to mimic the inertia characteristic of a DC machine and regulate the DC bus voltage fluctuations during power fluctuations of the DCMG. The concept of operating a bidirectional DC-DC converter as a DC machine can be termed as virtual DC machine (VDCM). The proposed concept of a VDCM is basically a control technique for a bidirectional DC-DC converter, which is derived from the theory of the DC machine operation and its speed control technique. The implementation, operation, controller design and dynamics of VDCM are also discussed. A VDCM can be easily operated in both generating and motoring mode also. Being free from comparatively slower mechanical devices, a VDCM can switch over from one mode to another very quickly. Hence, it provides a useful way to control the storage devices in microgrid applications.

Balance control of grid currents for UPQC under unbalanced loads based on matching-ratio compensation algorithm

In three-phase four-wire systems, unbalanced loads can cause grid currents to be unbalanced, and this may cause the neutral point potential on the grid side to shift. The neutral point potential shift will worsen the control precision as well as the performance of the three-phase four-wire unified power quality conditioner (UPQC), and it also leads to unbalanced three-phase output voltage, even causing damage to electric equipment. To deal with unbalanced loads, this paper proposes a matching-ratio compensation algorithm (MCA) for the fundamental active component of load currents, and by employing this MCA, balanced three-phase grid currents can be realized under 100% unbalanced loads. The steady-state fluctuation and the transient drop of the DC bus voltage can also be restrained. This paper establishes the mathematical model of the UPQC, analyzes the mechanism of the DC bus voltage fluctuations, and elaborates the interaction between unbalanced grid currents and DC bus voltage fluctuations; two control strategies of UPQC under three-phase stationary coordinate based on the MCA are given, and finally, the feasibility and effectiveness of the proposed control strategy are verified by experiment results.

Network security-aware charging of electric vehicles

Large-scale integration of electric vehicles (EV) and wind power could have significantly negative impacts on power systems security. So, it is becoming an increasingly important issue to develop an effective network security-aware charging strategy of EVs. This paper proposes a multi-objective formulation for the optimal charging schedule of EVs while considering N − 1 security constraints. An EV aggregator representing a cluster of controllable EVs is modeled for determining the optimal charging schedule based on a trilevel hierarchy. On the top level, the grid control center determines the EV charging strategy from the proposed formulation, where bus voltage fluctuations, network power losses, and EV charging adjustments are considered as multi-objective functions. To reduce the computational burden, Lagrangian Relaxation (LR) is introduced to handle time coupled constraints and Benders Decomposition is introduced to handle contingencies. Case studies have been conducted on the New England 39-bus system, and the results verify the necessity of considering N − 1 security constraints and the effectiveness of the proposed formulation and solution approach.

Optimal Allocation of ESSs for Mitigating Fluctuation in Active Distribution Network

As distributed generations (DGs) are occupying an increasing proportion in active distribution network, the fluctuation of bus voltage becomes severe because of their intermittent and stochastic characteristics. Energy Storage System (ESS) can be adopted as an effective means to mitigate this fluctuation. This paper proposes a methodology for the optimal allocation of ESSs based on the novel optimal affine power flow (OAPF) approach. Affine models of wind turbine generation (WTG), photovoltaic (PV) system, and ESS are built while considering uncertainty characters of their power outputs. The objective function is set to minimize the capital investment of ESSs and the penalty costs of bus voltage fluctuations. The complex affine arithmetic based distribution power flow is used to ensure that constraints on power flow limits, voltage limits, and ESS operational limits are satisfied. The optimization problem is solved by an improved immune genetic algorithm (IIGA). The proposed approach is verified via a modified IEEE 33-bus system with a high penetration of DGs. Results show that the optimal allocation of ESSs can satisfy both the technical and economic requirements under such an uncertain environment.

Research on Short Circuit Current Characteristics of Doubly-fed Wind Power Generator Considering Converter Regulation

When fault occurs in the grid, the short circuit characteristics of doubly-fed induction generator (DFIG) are more complicated due to the coupling influence of generator electromagnetic transient and converter regulation. In view of this problem, a DFIG stator/rotor short circuit current calculation method considering the transient regulation of both rotor-side and grid-side converters is proposed in this paper. First, DFIG fault equivalent network considering the rotor-side converter control is established, and the analytical expressions of stator/rotor currents in this network are derived. On this basis, considering the DC bus voltage fluctuation, the double-loop transient control process of the gird converter in the short circuit process is described quantitatively. Thus, the mechanism of the second harmonic in the stator short circuit current is revealed, and the expression of the harmonic component is derived. Finally, the analytical expressions of stator and rotor short circuit full currents are obtained. Simulation results verify the correctness and feasibility of the proposed method.