Improved Droop Control Method Research Articles

An Improved Power-Sharing Method for a Multi-Terminal HVDC Transmission System Based on Adaptive Voltage Droop Control

The prerequisite for the normal operation of a flexible high-voltage direct current (HVDC) transmission system is the maintenance of the stability of the direct current (DC)-side voltage, and droop control has a good dynamic regulation capability. In this paper, we first study the operating characteristics of droop control and derive its equivalent circuit, as well as the power distribution equation for droop control with a four-terminal system as an example. Then, based on this, an improved droop control method is proposed so that the droop factor can be adaptively adjusted according to the power change and provide corresponding characteristics under different operating conditions to enhance the power regulation capability of the controller under high power fluctuations. Finally, a power systems computer-aided design (PSCAD) electromagnetic transient model of the four-terminal, flexible, high-voltage DC transmission system was established and verified by simulation results.

Power Control Strategy for a Ferry’s DC Power System Using Supercapacitors

Integrated power systems are gaining popularity in the field of power systems and DC integrated power systems are considered promising for electric propulsion ships due to their simple grid topology, low fuel consumption, and easy access to new energy sources. However, the dynamic response characteristics of the power plant can be compromised when a variable speed generator is used in a DC power system, despite achieving energy savings. In this research, we investigate the power control strategy of a specific type of a ferry’s DC power plant. We establish a mathematical model and a Matlab/Simulink-based simulation model to analyze the performance of the proposed strategy. The research utilizes the fast charging and discharging advantages of supercapacitor storage devices to compensate for the dynamic impact delay of the power output when using the variable speed generator set. Additionally, an improved DC bus voltage droop control method that incorporates voltage compensation is proposed to mitigate problems related to large bus voltage fluctuations under sudden load change conditions, enabling better load distribution between different power sources. The simulation results confirm the effectiveness of the proposed strategy in optimizing the speed-seeking method of the variable speed diesel engine sets matching with the supercapacitor, and its positive impact on the dynamic performance of the propulsion system is demonstrated under variable load conditions resulting from ferry operations.

Research on power balance control method of microgrid energy storage unit

In the isolated island operation of microgrids, affected by the different equivalent circuit impedance between distributed generators, the traditional droop control cannot evenly divide the reactive power borne by distributed generators according to the different line impedance. Therefore, firstly, the reasons affecting power sharing are analyzed, and then an improved droop control method based on traditional droop control is proposed; that is, the time change rate of voltage is used as the droop variable, and a voltage recovery mechanism is added to the reactive droop controller, in order to adjust the reciprocal of voltage to zero in the steady state, so as to stabilize the voltage. Aiming at the problem that the droop controller cannot always realize the equal distribution of reactive power, a voltage compensation link suitable for improved droop control is designed and added to eliminate the line impedance difference between distributed generators and improve the accuracy of reactive power distribution. Finally, the effectiveness and feasibility of the proposed control strategy are verified by building a “wind light storage load” microgrid simulation model on the MATLAB / Simulink platform.

Microgrid Droop Control Based on Simulated Annealing Adaptive Particle Swarm Optimization

Droop control has been widely used and researched as a characteristic method to achieve current distribution. The traditional droop control has inherent limitations. With the development of various control algorithms, the sec-ondary control of the DC microgrid droop control is no longer limited to the use of classical deviation adjustment or the general compensation method of the associated parameters for compensation. An improved droop control method based on the optimal compensation method. Compared with the traditional particle swarm optimization, the adaptive particle swarm optimization (ASAPSO) algorithm based on simulated annealing is adopted, which enhances the ability of the particle swarm to improve the convergence speed and convergence accuracy. By improving the algorithm, the optimization results are fed back to the droop parameters in real time, so as to improve the optimization effect. The effectiveness and viability of the proposed control method are verified by simulation experiments with MATLAB/Simulink.

An improved droop control strategy for accurate current sharing and DC-BUS voltage compensation in DC microgrid

This paper addresses load current sharing among parallel-connected DC-DC converters and bus voltage drop issues caused by traditional droop method in low-voltage DC microgrid. Conventionally, I-V droop control is an effective way to parallel converter modules due to its high reliability, extendibility and economy. However, when the effect of mismatching line resistances is considered, the traditional droop control has limitations on the voltage regulation while the load current sharing accuracy is maintained. In this paper, an improved nonlinear droop control method is adopted to realize both accurate load current sharing and local output voltage improvement, which is achieved by adaptively adjusting nonlinear I-V droop curve with the information of DC bus voltage. Meanwhile, a secondary controller of DC bus voltage restoration is introduced to realize voltage drop compensation through the secondary regulation of I-V curve. The simulation model with two source converters and a single Constant Power Load (CPL) was built in Matlab-Simulink, which can be extended to multiple sources and multiple loads connected to the DC bus. Compared with the simulation results of the traditional droop control method and another improved droop control method, the better performance of the adopted control strategy is validated.

Improved Droop Control Strategy of Energy Storage in Islanded Microgrid

In islanded AC microgrid, droop control strategy is usually adopted for independent distributed generators (DG) which can realize coordinated operation of each unit under the conditions of less data communication. For different capacity of energy storages (ES) system, due to the difference of SOC value, capacity and other state parameters, some units may reach the threshold quickly, thus affecting the system stability. Therefore, on the basis of traditional droop control, combined with the SOC value of energy storage unit, the droop curve which is automatically adjusted with the change of SOC value is constructed. At the same time, the bus voltage is corrected to run at a predetermined point through the voltage feedback link to eliminate the voltage fluctuation. Finally, a microgrid model with dual energy storage system is built inMatlab/Simulink simulation environment. Theresults show that the improved droop control method can make the SOC values of the two energy storages consistent.

Power Sharing and Synchronization Strategies for Multiple PCC Islanded Microgrids

Most of researchers have already studied and discussed the power sharing and synchronization of several generation systems connected to a unique point of common coupling (PCC) to which the loads are also connected. A high penetration rate of distributed generation systems (DGs) based on renewable energies has for logic consequence the development and setting up of networked multi-PCC microgrids. In this paper an improved droop control method for synchronization as well as active and reactive power sharing of different DGs in multiple PCC islanded microgrids is proposed while the real characteristics of the line feeders are taken into account. The simulation results confirm the feasibility and effectiveness of the proposed strategies for synchronization and interconnection of different microgrid DGs, while insuring accurate sharing of the DGs active and reactive powers. 

An Improved Droop Control Method for Energy Storage Module of DC Microgrid

This paper proposes an improved droop control method with state of charge (SOC) balance function for energy storage module of DC microgrid. The average SOC of all energy storage modules is obtained by low bandwidth network. The SOC balance controller calculates the difference between each module’s SOC and average SOC and then adjust the droop coefficients according to the difference. As the coefficient changes, the output power of module is allocated according to its SOC and eventually achieve balance. Besides, secondary voltage controller is employed to compensate DC bus voltage drop. Also, the stability of the proposed method is studied by small-signal analysis. The effectiveness of the proposed method is finally verified by MATLAB/Simulink simulation.

An Improved Droop Control Method for Voltage-Source Inverter Parallel Systems Considering Line Impedance Differences

In this paper, the effect of the line impedance difference between various inverters on power sharing with the traditional droop control method is fully analyzed. It reveals that the line impedance difference causes a significant reactive power error. An improved droop control method to eliminate the reactive power errors caused by the line impedance errors is proposed. In the proposed method, a voltage compensation determined by the actual reactive power error between the local inverter and the average one is added into the local voltage reference based on the CAN communication. Even when the communication is interrupted, the controller will operate with the last value of the average power, which still outperforms the traditional method. The effectiveness of the proposed control method is verified by simulation and experimental results, which show the proposed method possesses the better power sharing performance and dynamic response.

A Compensation Control Scheme of Voltage Unbalance Using a Combined Three-Phase Inverter in an Islanded Microgrid

A large number of single-phase loads in an islanded microgrid have a bad influence on the alternating current (AC) bus voltage symmetry, which will further impact the power supply for the other loads. In this paper, the combined three-phase inverter is adopted as the distributed generation (DG) interface circuit for its independent control of each bridge. However, the combined three-phase inverter will generate an asymmetrical voltage with the traditional droop control. Moreover, the system impedance also effects the voltage symmetry. Therefore, the improved droop control method based on the self-adjusting P-f and Q-U droop curves and the system impedance voltage drop compensation are proposed. The system control scheme is also designed in detail. A simulation and an experiment under the conditions of the balanced, unbalanced loads are carried out, and the results verify the feasibility and effectiveness of the control strategy.

An Improved Droop Control Strategy for VSC-Based MVDC Traction Power Supply System

Due to the development of advanced power electronic techniques, voltage-source-converter-based medium voltage dc (MVDC) traction power supply system (VSC-based MVDC TPSS) could be an alternative supply solution for high-speed railways (HSRs) in the future. The distributed configuration of the MVDC system, along with dynamic load and catenary resistance, makes the control of such system a real challenge. In this paper, an improved droop control method is proposed to obtain a better performance of system operation for the specific supply environment without any communication links. The proposed control scheme can eliminate the voltage deviation due to droop action and improve catenary network voltage control dynamics by using voltage-shifting and load current feedforward control approaches simultaneously. Additionally, the detailed model of the proposed control scheme is derived and the stability in the case of moving load is critically tested. Finally, a simulation model based on MATLAB/Simulink and an experimental prototype have been established to validate the effectiveness of the proposed control method.

An Accurate Power-Sharing Control Method Based on Circulating-Current Power Phasor Model in Voltage-Source Inverter Parallel-Operation System

This paper proposes an optimized mathematical model of the voltage-source inverter parallel-operation system (VSIPS) and proposes an improved droop control method based on this model, which can realize accurate power sharing in VSIPS. First, this paper analyzes VSIPS as a multi-input and multioutput system and proposes a precise definition of circulating current. The circulating-current model, the steady-state model, and the small-signal model are proposed subsequently based on the optimum design of wire impedance, which constitute the optimized mathematical model of VSIPS in s -domain. The circulating-current phasor model and the circulating-current power phasor model (CCPPM) are also proposed. Second, the mathematical model of traditional droop control is built and analyzed based on CCPPM, which elaborates the tradeoff of droop control between the steady-state voltage bias and the load-sharing accuracy, and proves that the $V- Q$ droop control cannot realize accurate reactive-power sharing. Third, an improved droop control method ( $\omega - P_{{\text{cir}}}$ and $V- Q_{{\text{cir}}}$ control) is proposed, which can realize the accurate active- and reactive-power sharing and eliminate the steady-state voltage bias simultaneously. Finally, simulation and experimental results are presented, which validate the proposed mathematical model of VSIPS, the analysis of droop control, and the performance of the proposed method.

Modelling and analysis of small signal for multi-machine parallel system based on droop control

Microgrid system based on multi-machine parallel connection has complex structure and various connection modes. In the process of grid-connected splitting, because of the frequency and voltage deviation between sub-microgrid, it will produce large electric power at the PCC point, which will impact the multi-machine microgrid system and affect the dynamic stability of the system. In order to solve the problem of control accuracy and dynamic stability in this such kind of process, considering the voltage drop and voltage phase angle offset of long-distance lines, an improved droop control method is proposed based on the parallel structure of multiple microgrid, the droop control accuracy is optimised, at the same time the corresponding small signal model is established for parameter optimisation, and the simulation model of improved droop control is established on MATLAB/Simulink. The simulation results verify the rationality and effectiveness of the improved control strategy.

Improved droop control method with precise current sharing and voltage regulation

The droop control method is one of the simplest methods for load sharing control of multiple converters. Although the current sharing precision can be adjusted to be well through the droop parameter, the voltage regulation precision is hard to be maintained well due to the voltage droop. In addition, the stability and dynamic response is usually degraded since the small-signal model of multiple converter system is usually changed significantly. To cope with these two limitations, this study presents an improved droop control method with automatic master to correct the voltage regulation. A robust control scheme is also provided to maintain a good stability and dynamic response as the design of the normal condition. The proposed method is confirmed with some simulation and experimental results.

An Improved Droop Control Strategy for Reactive Power Sharing in Islanded Microgrid

For microgrid in islanded operation, due to the effects of mismatched line impedance, the reactive power could not be shared accurately with the conventional droop method. To improve the reactive power sharing accuracy, this paper proposes an improved droop control method. The proposed method mainly includes two important operations: error reduction operation and voltage recovery operation. The sharing accuracy is improved by the sharing error reduction operation, which is activated by the low-bandwidth synchronization signals. However, the error reduction operation will result in a decrease in output voltage amplitude. Therefore, the voltage recovery operation is proposed to compensate the decrease. The needed communication in this method is very simple, and the plug-and-play is reserved. Simulations and experimental results show that the improved droop controller can share load active and reactive power, enhance the power quality of the microgrid, and also have good dynamic performance.

A Control Strategy with Series-Parallel Inverter for Micro-Grid Operation

For the different operation mode of micro-grid, an improved droop control method and a parallel inverter was proposed depending on the study of micro-grid inverter. Grid-connected interface contains a series inverter and a parallel inverter, and can be switched to select different work mode. The parallel inverter can eliminate harmonic, compensate three-phase imbalanced current to improve the quality of the power delivered to the utility grid. In islanding operation mode, the improved droop control strategy was applied, where an integral controller was introduced. So it can reduce the inverter output voltage amplitude. Thereby it can restrain circulation and realize the power of self-distribution. The effectiveness and feasibility are verified by the simulation result.

An Improved Droop Control Method for DC Microgrids Based on Low Bandwidth Communication With DC Bus Voltage Restoration and Enhanced Current Sharing Accuracy

Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.