Forward Power Flow Research Articles

A novel multi-port high-gain bidirectional DC–DC converter for energy storage system integration with DC microgrids

Bidirectional converters have often been used in numerous applications like DC microgrids, renewable energy, hybrid energy storage systems, electric vehicles, etc. The paper proposes a novel multi-port high-gain (NMPHG) bidirectional DC–DC converter that supports DC microgrid (DC-MG) applications. The main contributions of the proposed converter are high step-up/step-down conversion gain, multiple input ports, lower switch voltage stress, and lower component count owing to the single converter with multiple input ports for DC microgrid applications. The detailed operational principle, analysis, and design considerations of proposed NMPHG bidirectional DC–DC converters are discussed. Furthermore, the loss analysis, detailed comparison with similar works, and efficiency analysis with non-modalities during forward power flow (LV to HV) and reverse power flow (HV to LV) modes are presented. The efficiency of the proposed converter is found to be 93.8% in forward power flow and 92.9% in reverse power flow modes at rated power. Finally, a hardware prototype of the proposed NMPHG bidirectional DC–DC converters is implemented with 100 W in FPF mode and 200 W in RPF mode with a TMS320F28335 processor and validated with theoretical counterparts.

Multi-Objective Unified Optimal Control Strategy for DAB Converters with Triple-Phase-Shift Control

To solve the problems of large current stress, difficult soft-switching of all switches, and slow dynamic response of dual active bridge converters, a multi-objective unified optimal control strategy based on triple-phase-shift control was proposed. The forward power flow global modes of triple-phase-shift control were analyzed, and three high-efficiency modes were selected to establish the analytical models of current stress and soft-switching. Combined with these models, the optimal solutions in different modes were derived by using the cost function-optimization equation to overcome the limitation of the Lagrange multiplier method, such that the DAB converter achieved the minimum current stress, and all switches operated in the soft-switching state over the entire power range. At the same time, the virtual power component was introduced in the phase-shift ratio combination, which improved the dynamic response of output voltage under the input voltage or load steps changed by power control. The theoretical analysis and experimental results show that the proposed control strategy can optimize the performance of the DAB converter from three aspects, such as current stress, soft-switching, and dynamic response, which achieves multi-objective optimization of the steady-state and dynamic performance of DAB converters.

Analysis of a Three-Level Bidirectional ZVS Resonant Converter

A bidirectional three-level soft switching circuit topology is proposed and implemented for medium voltage applications such as 750 V dc light rail transit, high power converters, or dc microgrid systems. The studied converter is constructed with a three-level diode-clamp circuit topology with the advantage of low voltage rating on the high-voltage side and a full-bridge circuit topology with the advantage of a low current rating on the low-voltage side. Under the forward power flow operation, the three-level converter is operated to regulate load voltage. Under the reverse power flow operation, the full-bridge circuit is operated to control high-side voltage. The proposed LLC resonant circuit is adopted to achieve bidirectional power operation and zero-voltage switching (ZVS). The achievability of the studied bidirectional ZVS converter is established from the experiments.

Asymmetrical Bidirectional DC–DC Converter With Limited Reverse Power Rating in Smart Transformer

The increasing penetration of distributed generators in low-voltage distribution grids may lead to distribution transformers, also a smart transformer, experiencing the reverse power flow, which is less as compared to the forward power flow in the current electric grid. This article proposes an asymmetrical bidirectional dc-dc (AB-DC/DC) converter, which has two partial-scale rectifiers of an active bridge and a diode bridge connected in parallel at the secondary side, with less cost of power semiconductors and the capability of bidirectional operation to satisfy the realistic condition. The proposed AB-DC/DC converter works as a single active bridge under forward power flow condition, while it works as a dual active bridge under reverse power flow condition. The solutions to share the active power under forward operation and suppress the circulating power under reverse operation are proposed, respectively. The simulation and experimental results both clearly validate the effectiveness and feasibility of the proposed AB-DC/DC converter.

Research on Distributed Source-Load Interaction Strategy Considering Energy Router-Based Active Distribution Network

With increasing penetration of distributed generation, surplus energy at some critical buses in distribution network will extremely challenge the stable operation of conventional power system. Energy routers (ERs) can be placed at the critical buses so as to achieve flexible management of power flow in active distribution network. It will offer more flexibility to realize the interaction between source and load. In this paper, energy routers are mainly catalogued as source-type and load-type, and then a novel distributed source-load interaction strategy for energy router based active distribution network is proposed, which only requires information exchange between neighbor routers. The interaction strategy includes three parts: the strategy for source-type ERs, the strategy for load-type ERs and the cooperation strategy for multiple source-type and load-type ERs. Moreover, a novel calculating method of power loss is proposed to make strategies applicable considering reverse and forward power flow. Finally, the correctness and effectiveness of the proposed strategies are verified by comparison with optimal routing paths derived by the typical centralized Dijkstra algorithm in the case study.

Optimal placement and sizing of PV systems and electric parking lots considering reactive power capability and load variation

ABSTRACTPower systems should operate in reliable, stable, and efficient conditions. Addition of new generation units or loads to the power systems may change their performance. Therefore, appropriate decisions should be made to manage these elements to improve the power system performance. In this study, optimal placement and sizing of photovoltaic systems and electric parking lots (EPLs) are studied considering the reactive power capability of the inverters and load variation in a 24-h period. For the EPL, a proper charge/discharge scheme (CDS) is initially proposed to flatten the daily load profile; then the EPL with the associated CDS is considered to find its optimal location. Voltage profile, energy losses, bus, and line voltage stability are considered as the objectives of the problem. Genetic algorithm and backward–forward power flow method are utilised to solve the problem considering the IEEE 33-bus system. The results show that all objectives are improved utilising the proposed method.

Detection of Fault Direction and Location in Compensated System using Sequence Component

Objectives: The proposed scheme presents a fault direction estimation as well as fault location determination technique for short transmission line using positive sequence component. An objective of the suggested work is to recognize direction of fault and locate it in short transmission line using positive sequence component. Methods/Analysis: Solution is given based on difference between positive sequence post-fault and steady state current phasor and difference of magnitude among post-fault and steady state positive sequence voltage phasor for fault direction estimation. Fault location is estimated by two-terminal method. Fast Fourier Transform is considered to detect the direction of fault and Discrete Fourier Transform is used to find the location of fault. The proposed work is estimated by simulated data in PSCAD and MATLAB/SIMULINK for a compensated line. Findings: Fault detection as well as fault location estimation in both sections of compensated line considering transmission line issues is identified properly. Many techniques are available for fault direction estimation and fault location determination. Sometimes available methods do not provide proper result of different issues which are present in compensated system. The proposed method is tested for forward power flow, reverse power flow, voltage inversion and current inversion cases. Simulation results show that proposed technique properly detects the direction of fault and locating the exact location of fault. Novelty/ Improvement: This approach deals with detection as well as location identification of fault in compensated system, so it is computationally efficient.

Fundamental Study of Voltage Stability in a Power System with Large Penetration of Photovoltaic Generation Considering Induction Motor Dynamics

SUMMARYBecause of global warming, attention to photovoltaic (PV) generation has increased, and considerable PV generation capacity will be installed in the power system. In Japan, most PV generation will be connected to the distribution system, which will cause changes in power flow in the power system. Forward power flow (flow from the higher voltage system to the lower voltage system) will be reduced and reverse power flow may occur. Because of the change in power flow, the voltage characteristics will also be changed. Consequently, it is expected that the penetration of PV generation will have some impact on voltage stability. In previous studies, the voltage characteristics in a simple power system model consisting of one infinite bus, one load, and one PV power source, have been investigated. It was found that there is not only a forward power flow limit but also a reverse power flow limit (the left nose in the PV curve), and that when the output power from PV generation is very large, the left nose sometimes has some impact on voltage stability. In this paper, the voltage stability in a simple power system model considering the dynamics of an induction motor is discussed.

Application of DSTATCOM for surplus power circulation in MV and LV distribution networks with single-phase distributed energy resources

Single-phase distributed energy resources (DERs), such as rooftop photovoltaic arrays, are usually installed based on the need and affordability of clients without any regard to the power demand of the connected phase of a three-phase system. It might so happen that the power generation in a particular phase is more than its load demand. This may cause a reverse power flow in a particular phase, especially in a three-phase, four-wire distribution system. If now the load demand in the other two phases is more than their respective generations, then these two phases will see a forward power flow, while there will be a reverse power flow in the third phase. This will create severe unbalance in the upstream network. In this paper, a distribution static compensator (DSTATCOM) is used to circulate the excess generation from one phase to the others such that a set of balanced currents flow from or into the upstream network. Two different topologies of DSTATCOM are proposed in this paper for the low and medium voltage feeders. Two different power circulation strategies are developed for this purpose. Furthermore, a suitable feedback scheme is developed for each topology for power converter control. The performance of the proposed topologies and the control schemes for the DSTATCOM is evaluated through computer simulation studies using PSCAD/EMTDC.

Fundamental Voltage Characteristics in Power System with a Large Penetration of Photovoltaic Generation

Because of global warming, the attention to the photovoltaic generation has increased, and a lot of photovoltaic generations will be installed to the power system in future. In Japan, most photovoltaic generations will be connected to the distribution system. This will make the power flow in the power system changed. That is to say that the forward power flow (flow from higher voltage system to lower voltage system) reduces, or, moreover, that the reverse power flow occurs. Because of the change in power flow, the voltage characteristics will be also changed. Consequently, it is expected that the penetration of photovoltaic generation has some impacts on voltage stability. In this paper, simple power system model which consists of one infinite bus, one load, and one photovoltaic generation is considered, and the fundamental voltage characteristics are discussed by PV curves and phasor diagram. The voltage characteristics considering the current limit by PCS (Power Conditioning System) in the photovoltaic generation are also discussed.

Power Flow Calculation in Distribution System Considering Different Load Model

In traditional power flow calculation used constant power load model, it was completely unreasonable to assume that all load nodes in power system can be classified as PQ nodes.In that the result of constant power load model does not accurately reflect the distribution characteristics of network power flow. To put forward power flow calculation considering load models ,such as constant power load ,constant current load and constant resistance load, derives error equations of the node power, calculates element of Jacobian matrix. So, by using MATLAB simulation software ,the program for power flow are presented, which based on the method of different load models, taking 21-node system for example and makes comparative calculations. The research shows that power flow calculation considering different load models can get more reasonable solution.

Surface modes along two parallel circular ferrite rods magnetized longitudinally

A method is presented for solving the problem of propagation along many parallel ferrite rods magnetized axially. The characteristic determinant is derived from the reciprocity theorem where auxiliary electric and magnetic current sources are suitably selected. Using this method, the dispersion relations and the mode configurations of surface modes along two rods are shown and discussed in detail for both the cases of parallel and antiparallel magnetizations. For each case there generally exists a pair of modes with the same mode number which are backward waves. For the parallel magnetization, the field patterns in the transverse plane are symmetric and antisymmetric, respectively, about the point bisecting the straight-line segment that connects the centers of two rods. For the antiparallel case, the respective modes have the symmetrical and antisymmetrical electric field patterns about the vertical bisector of the same segment; and the results are reversed for the magnetic field patterns. Therefore, the field patterns of the latter first (or second) mode in the half-space exactly coincide with those of the corresponding mode along a single rod over a plane magnetic (or electric) conductor. The latter first mode disappears as the spacing between two rods becomes narrower than a limit. The distributions of the time-averaged power density for all the above modes are found to be symmetric about the above-mentioned vertical bisector. For some modes, there exists a slight local forward power flow in rods, accompanied with a much larger backward flow in the other region. This region with forward flow is found to appear when the strong coupling between two rods occurs. The coupling effects between two rods are more remarkable for the antiparallel case than for the parallel case, and they tend to become weaker for higher modes and for the larger spacing.