Power Flow Control Strategy Research Articles

Special Issue “Plug-In Hybrid Electric Vehicle (PHEV)”

Climate change, urban air quality, and dependency on crude oil are important societal challenges. In the transportation sector especially, clean and energy-efficient technologies must be developed. Electric vehicles (EVs) and plug-in hybrid electric vehicles (PHEVs) have gained a growing interest in the vehicle industry. Nowadays, the commercialization of EVs and PHEVs has been possible in different applications (i.e., light duty, medium duty, and heavy duty vehicles) thanks to the advances in energy-storage systems, power electronics converters (including DC/DC converters, DC/AC inverters, and battery charging systems), electric machines, and energy efficient power flow control strategies. This Special Issue is focused on the recent advances in electric vehicles and (plug-in) hybrid vehicles that address the new powertrain developments and go beyond the state-of-the-art (SOTA).

Power Flow Control Strategy and Reliable DC-Link Voltage Restoration for DC Microgrid under Grid Fault Conditions

In this paper, an effective power flow control strategy (PFCS) based on the centralized control method and a reliable DC-link voltage (DCV) restoration algorithm for a DC microgrid (DCMG) under grid fault conditions are proposed. Considering the relationship of supply-demand power and the statuses of system units, thirteen operating modes are presented to ensure the power balance in DCMG under various conditions. In the PFCS, the battery charging/discharging procedure is implemented considering the battery power limit to avoid overheating and damage. Moreover, load shedding and load reconnection algorithms are presented to maintain the system power balance, even in critical cases. To prevent the system power imbalance in DCMG caused by the delay of grid fault detection, a reliable DCV restoration algorithm is also proposed in this paper. In the proposed scheme, as soon as abnormal behavior of the DCV is detected, the battery or wind power generation system instantly enters a local emergency control mode to restore the DCV rapidly to the nominal value, regardless of the control mode assigned from the central controller. Comprehensive simulations and experiments based on the DCMG testbed are carried out to prove the effectiveness of the PFCS and the proposed DCV restoration algorithm.

MODELING OF DVSI WITH HYBRID ENERGY SYSTEM AND VOLTAGE OPTIMIZER ACTIVE POWER CONTROLLER

Technological progress and environmental concerns drive the power system to a paradigm shift with more renewable energy sources integrated to the network by means of distributed generation (DG). These DG units with coordinated control of local generation and storage facilities form a micro grid. A voltage regulation and power flow control scheme for a wind energy system (WES) WITH solar system is proposed. The simulation result shows that the system with the proposed voltage optimizer active power control strategy proposed is better as compared to the basic phase locked loop control. Also the system was made efficient by integrating it with the wind system and making it a hybrid power source. On comparing the active power outputs from the system with phase locked loop control with proposed voltage optimizer active power control, it was found to be that the proposed system gives 4 MW output which is considerably more than the 0.5 MW output of the system with basic PLL control. Thus it can be drawn from this work that while designing an inverter control strategy the proposed voltage optimizer active power controller can serve the purpose with better results

Distributed Optimal Tie-Line Power Flow Control for Multiple Interconnected AC Microgrids

In a multi-microgrid system (MMG), the microgrids (MGs) are normally managed by independent operators. Distributed energy trading/scheduling schemes via interactions of these MG operators have been extensively investigated. How to coordinate these MGs to implement the acquired optimal schedule in the real time under constant load fluctuation while guaranteeing operational stability is seldom reported. Due to the intrinsic advantages of scalability, robustness, and fast response in comparison to the centralized scheme, a multi-agent based distributed optimal tie-line power flow control strategy is proposed to achieve this objective, which is facilitated by a regional communication network overlapping each MG and distributed sensors monitoring the tie-line power flows. When the MMG is operated in the grid-connected mode, the proposed scheme can maintain the scheduled tie-line power flows among the MGs in the presence of any disturbance by adjusting the real-time power outputs of the distributed energy resources proportionally. When the MMG is islanded, frequency recovery can be further achieved via a local frequency feedback mechanism. Convergence of the proposed approach is analytically proved. Simulation results in a four-MG system modified from the IEEE 34-bus test feeder system validate the effectiveness and efficiency of the proposed approach in both grid-connected and islanded modes.

Comprehensive Power Flow Analyses and Novel Feedforward Coordination Control Strategy for MMC-Based UPFC

This paper aims to discover the general steady-state operation characteristics, as well as improving the dynamic performance, of the modular multilevel converter (MMC)-based unified power flow controller (UPFC). To achieve this, first, we established a detailed power flow model for MMC-based UPFC containing each critical part and made qualitative and graphical analyses combining 2-dimensional operation planes and 3-dimensional spatial curve surfaces comprehensively to derive general power flow principles and offer necessary references for regulating UPFC. Furthermore, to achieve better performance, we designed a feedforward control strategy for the shunt and series converters of UPFC, both comprising two feedforward control blocks with the introduction of necessary compensating branches, and analyzed the performance in complex and time domain, respectively. The proposed power flow principles and control strategies were validated by a (power systems computer aided design) PSCAD model of 220 kV double-end system; the results reveal the MMC-based UPFC can realize the power flow principles and improve the control speed, stability, and precision of the power flow regulations under various conditions.

A Post-Contingency Power Flow Control Strategy for AC/DC Hybrid Power Grid Considering the Dynamic Electrothermal Effects of Transmission Lines

The blocking of large-capacity HVDC or the tripping of AC transmission lines by accident will cause power flow transfer sharply and increase the risk of cascading faults in AC/DC hybrid power grid. Therefore, it is needed to take reasonable safety control measures in time to interdict the spreading of the accident. In this paper, a post-contingency power flow control (PPFC) strategy for AC/DC hybrid power grid was proposed. This strategy brings the dynamic electrothermal effects of the conductor into the safety constraints to fully excavate the short-time overload endurance capability of transmission lines, and coordinates the active power adjustment of generators, power transmission adjustment of HVDC, and load shedding measures, to realize the dynamic optimal power flow control in multiple time sections. The proposed optimization control model takes minimum total cost as the objective function, and accounts for the influence of different weather conditions along the transmission lines and load fluctuations after the accident on the electrothermal safety of transmission lines, which can quickly provide the economical and reasonable power flow control scheme, to prevent the AC/DC hybrid power grid from unexpected cascading tripping effectively. Finally, the effectiveness and superiority of the proposed strategy were verified through case studies on the modified IEEE 10-machine 39-node power systems.

Optimal Power Flow and Unified Control Strategy for Multi-Terminal HVDC Systems

This paper presents an automation strategy for multi-terminal HVDC (MT-HVDC) systems combining a dc optimal power flow (dc OPF) routine and a unified reference controller (URC). In the presented automatic framework, the dc OPF algorithm is implemented at the power dispatch center (PDC) of the MT-HVDC system to find optimal reference operation points of the power converters to minimize the losses during the operation of the MT-HVDC grid and solves the contradiction between minimizing losses and preventing commutation failure. At the local control systems, the operating points of the voltage-source converter (VSC) stations are tuned based on the calculations executed in the PDC, which enables fast response to power fluctuation and ensures a stable dc voltage. However, if the communication between the two control layers is lost, the MT-HVDC grid remains stable based on the pre-defined V-P droop characteristics for the power converter stations till the connection establishes again, and a set of new operating points is generated and sent. The static and dynamic simulations conducted on the CIGRE B4 HVDC test grid establish the efficient and effective grid control performance with the proposed automation strategy. The analysis shows that the proposed control scheme achieves the desired minimum losses while, at the same time, satisfying the system constraints.

Improved power flow control strategy of the hybrid AC/DC microgrid based on VSM

The hybrid AC/DC microgrid with different types of distributed generations (DGs) and load demands is considered to be the preferred microgrid mode in the future. For hybrid AC/DC microgrid, autonomous and bidirectional power flow between AC and DC subgrids with superior dynamic performance is the fundamental objective to realise proportional power sharing and robust operation. In this study, an improved active power control strategy of the bidirectional AC/DC main converter (BMC) based on virtual synchronisation machine (VSM) for inertia improvement of the AC bus frequency and DC bus voltage of the hybrid microgrid is proposed. The VSM control is derived through the virtual inertia equation and the virtual capacitance equation, which can manage the AC frequency and DC voltage simultaneously. Then, the small signal and large signal characteristics analysis of the BMC are conducted to analyse the VSM control feature with different control parameters. On the basis of the small signal analysis, a proportional power sharing principle to select the control parameters is given to make DGs share the power commensurate with their power ratings. Finally, simulation cases are carried out to show the validity and efficiency of the proposed control strategy.

Power flow control scheme for multiport power electronics transformers

Development of power electronics transformers (PETs) has made dc distribution and hybrid ac/dc systems a competitive solution for future network expansion. This study focuses on the functions of a multiport PET interfacing medium-voltage and low-voltage networks. A comprehensive power flow control scheme based on a globalised multiport transmission model of the PET and droop control is derived. This allows for a coordinated energy exchange between the ports of the PET, which enables autonomous operation. Simulations of the proposed approach in a hybrid ac/dc distribution network with different levels of distributed generation and loads verify the effectiveness of the method.

Novel Economic Analysis to Design the Energy Storage Control System of a Remote Islanded Microgrid

This paper presents a novel power flow control strategy, combined with an economic analysis, for an energy management system (EMS) involving a hybrid energy storage. The EMS operates a remote microgrid and directs the power flow to either batteries or supercapacitors to increase the life of the batteries. This paper demonstrates how the use of supercapacitors increases the lifetime of the batteries and ultimately how it affects the economics of the system. The proposed EMS controller also compensates for the 120-Hz ripple on the dc bus. Modeling, simulations, and experimental verification are presented together with the procedure to perform the assessment of the battery lifetime, according to the tuning parameters of the controller.

Peak Energy Management Using Renewable Integrated DC Microgrid

Thirst for energy is increasing significantly with increase in economy around the world which burdens conventional grid (CG) due to peak demand, energy deficit, and carbon emission, but major part of continents has plentiful wind and solar energy which can be harnessed locally to minimize it. Major objective of this research is to reduce the peak power deficit present in CG system and to provide a reliable power supply even in case of grid failure or during blackout. In this paper, development and evaluation of a small scale grid interactive dc microgrid for residential houses, telecommunication systems, and data center has been proposed. The objective is being achieved by designing a comprehensive power flow control strategy and explored for different practical scenarios through real time simulation in RSCAD/RTDS platform. A prototype is developed to validate the simulation results presented during grid connected and isolated mode.

Power Flow Control Scheme for Hybrid Single-Phase Energy System Using Droop Control: a Comprehensive Survey

This paper performs a comprehensive survey on power flow control scheme and is applied to a single-phase hybrid energy system. The survey is made based on the performance of the system during operation in islanding and grid-tied modes. It covers the coordination of control strategies and the stability and the efficiency of the controller techniques among the hybrid systems. At the same time, the system must be able to stabilize the voltage level as well as to control the flexibility of power flows. Therefore, this review is based on droop control concept and has been coordinated with adaptive and robust control mechanisms. The importance of these controllers is to see the adaptability to various conditions practically, such as disconnection of transition of those modes whenever there are dynamic changes to the hybrid system. For this reason, the droop control scheme must able to fulfil the performance requirements to achieve faster and smoother transition associated with islanded and grid configurations. Moreover, it also should provide an effective system performance in terms of setting up the power control strategy to manage the power flows among the inverters connected to local loads. As a conclusion, this paper has tabulated several important tables on the variety of droop control techniques that are suitable for different case systems for the hybrid-islanded-grid connection. As an ultimate goal, it is also to provide information covered from the system configuration to choose the right controller that can balance the power in the system and generate a continuous load voltage waveform during transient.

Hierarchical Power Flow Control of a Grid-tied Photovoltaic Plant Using a Battery-Supercapacitor Energy Storage System

A two-layer power flow control for a grid-tied photovoltaic plant equipped with battery-supercapacitor energy storage system is presented in this paper. The proposed strategy aims at simultaneously handling the intermittent nature of the solar radiation received by the photovoltaic collectors and the need to consistently supply the specified power taking advantage of the complementary characteristics of the energy storage technologies. It relieves the batteries from stress through the use of the photovoltaic and the supercapacitor to supply the high frequency power demands by means of hierarchical control. Extensive simulation results confirm the effectiveness of the proposed power flow control strategy.

Model Predictive Control for Distributed Microgrid Battery Energy Storage Systems

This paper proposes a new convex model predictive control strategy for dynamic optimal power flow between battery energy storage systems distributed in an AC microgrid. The proposed control strategy uses a new problem formulation, based on a linear d-q reference frame voltage-current model and linearised power flow approximations. This allows the optimal power flows to be solved as a convex optimisation problem, for which fast and robust solvers exist. The proposed method does not assume real and reactive power flows are decoupled, allowing line losses, voltage constraints and converter current constraints to be addressed. In addition, non-linear variations in the charge and discharge efficiencies of lithium ion batteries are analysed and included in the control strategy. Real-time digital simulations were carried out for an islanded microgrid based on the IEEE 13 bus prototypical feeder, with distributed battery energy storage systems and intermittent photovoltaic generation. It is shown that the proposed control strategy approaches the performance of a strategy based on non-convex optimisation, while reducing the required computation time by a factor of 1000, making it suitable for a real-time model predictive control implementation.

Fault Ride-Through Enhancement for Grid-Tied PV Systems With Robust Control

In this paper, a new control structure is proposed for grid-tied photovoltaic (PV) systems where the dc bus voltage is regulated by the dc/dc converter controller, whereas the maximum power point tracking (MPPT) function and the power flow control are embedded into the dc/ac converter controller. A PV voltage-regulation is designed to build the linkage between MPPT function and power flow control. In this way, the dc/dc converter controller and the dc/ac converter controller are decoupled, which naturally provides the dc bus voltage protection. In particular, an uncertainty and disturbance estimator (UDE)-based current-mode controller (CMC) is proposed for accurate voltage regulation of the dc/dc converter. And a bounded-voltage power flow control strategy is proposed for the dc/ac converter to improve the existing UDE-based robust power flow control for ac voltage protection. The effectiveness of the proposed method is experimentally validated in a lab-environment grid-tied PV system platform with the fault ride-through capabilities. In addition, simulation studies are also provided to demonstrate the need of the PV voltage-regulation between MPPT and power flow control, and the advantages of the bounded-voltage design in the power flow control.

Performance Analysis of Grid connected PV/Wind Hybrid Power System during Variations of Environmental Conditions and Load

This survey investigates a dynamic modeling, simulation and control of Photovoltaic (PV)-wind hybrid system connected to electrical grid, considering changes of environmental conditions. In addition, the daily variations of critical load power are considered. The studied hybrid system consists of two Photovoltaic (PV) stations placed at different locations and one wind farm are integrated into main AC bus to enhance the system effectiveness. The PV/wind hybrid system feeds large plant with critical variable loads and electrical utility grid. The technique of extracting maximum power point is applied for both photovoltaic stations and wind farm to capture maximum power under varying climatic conditions. Modeling and simulation of the studied hybrid system is performed using matlab-Simulink software. The reliability of the studied hybrid system is analyzed under various operating conditions such as changes of solar irradiation and wind speed. Control strategy for power flow is proposed to supply critical load demand of plant. The simulation results show that when the injected power from hybrid system is larger than critical load power, the excess power will be injected to electrical grid. Otherwise, when injected power is lower than critical power demand, electrical utility grid in cooperated with hybrid power system will supply the critical load power. In addition, when the injected power from hybrid system is unavailable, load demand is entirely fed by electrical utility.

Decoupled TAB converter with energy storage system for HVDC power system of more electric aircraft

It is an important trend to develop the more electric aircraft (MEA) ±270 V high-voltage direct current (HVDC) power system because of its better reliability, power quality and power density. However, there also exists the low-voltage 28 V DC system in HVDC power system, for the aviation battery and some equipment still rely on it. In traditional MEA power systems, battery is only used as a backup power supply, hardly participating in the adjustment of the power system. On the other hand, the function of turbine engine is unidirectional. Hence, it's necessary to add an energy storage port. In this research, the authors put forward the problems of power coupling and strong non-linearity in traditional three-port triple active bridge (TAB) converters, and propose a decoupled TAB topology, which has the feature of decoupled power flow. The topology is compatible with ±270 V DC, 28 V DC and energy storage system with high-power density and efficiency. In addition, the authors unify the efficiency optimisation problem of dual active bridge topology mathematically and provide an optimal parameter design approach for inductance and turns ratio. Simulation results show the power flow control strategy is appropriate for the topology.

Distributed Real-Time Optimal Power Flow Control in Smart Grid

Conventionally, power system has a hierarchical control structure including primary, secondary, and tertiary controls. The drawbacks of this hierarchical scheme are manifest: 1) it lacks flexibility and scalability, which is against the trend toward an open-access power system; 2) load forecast as the basis of tertiary control could be inaccurate and infeasible, especially in microgrid for example; 3) as the penetration of renewable energy increases, the relatively long time-scales of secondary and tertiary controls cannot accommodate to more severe power fluctuation within the system. To avoid these drawbacks, a distributed real-time optimal power flow control strategy is introduced in this paper. With the aid of up-to-date smart grid technologies such as two-way communication and distributed sensor, the proposed approach can avoid the need of load forecast and achieve the same objective as hierarchical control with a feedback mechanism in real time, that is to recover the nominal system frequency and maintain the active power of the generators close to the optimal operational condition in the presence of any disturbance. Convergence of the proposed approach is analytically proved. Simulation results in a 34-bus islanded microgrid and the IEEE 118-bus bulk power grid validate the effectiveness and efficiency of the proposed approach.

Optimal Power Control Strategy for Wind Farm with Energy Storage System

The use of energy storage systems (ESSs) has become a feasible solution to solve the wind power intermittency issue. However, the use of ESSs increases the system cost significantly. In this paper, an optimal power flow control scheme to minimize the ESS capacity is proposed by using the zero-phase delay low-pass filter which can eliminate the phase delay between the dispatch power and the wind power. In addition, the filter time constant is optimized at the beginning of each dispatching interval to ensure the fluctuation mitigation requirement imposed by the grid code with a minimal ESS capacity. And also, a short-term power dispatch control algorithm is developed suitable for the proposed power dispatch based on the zero-phase delay low-pass filter with the predetermined ESS capacity. In order to verify the effectiveness of the proposed power management approach, case studies are carried out by using a 3-MW wind turbine with real wind speed data measured on Jeju Island.

Varying phase angle control in isolated bidirectional DC–DC converter for integrating battery storage and solar PV system in standalone mode

This study proposes a varying phase angle control (VPAC) in isolated bidirectional dc-dc converter (IBDC) for integrating battery storage unit to a DC link in a standalone solar photovoltaic (PV) system. The IBDC is capable of power transfer using high step up/down ratio between DC link and battery. The VPAC control proposed in this study effectively manage the power flow control between the battery storage unit and the solar PV fed DC link by continuously varying the phase angle between high voltage and low voltage (LV) bridge voltage of the IBDC. The solar PV system is incorporated with the maximum power point tracking using DC-DC converter. In order to control the voltage across the AC load a voltage source inverter is used. The study also presents the design aspects of the IBDC converter for the application considered. The performance of the proposed power flow control strategy has been studied through PSCAD/EMTDC simulation and validated using LPC 2148 ARM processor.