A Generalized Model For Unified Ac-Dc Load Flow Analysis

Conventionally, the power systems are operating separately as AC or DC grids. Both systems have their advantages and disadvantages; however, if the AC loads need to be connected with the DC grid or DC loads with the AC grid, then DC-AC and AC-DC converters are required that results in conversion losses and an increase in the overall cost. Therefore, a comprehensive solution is the hybrid AC-DC grid in which DC loads relate to the DC grid and AC loads relate to the AC grid and one bi-directional converter joins both AC and DC grids and provides need-based power sharing between them. In future, it will be beneficial to operate big organizations, such as data centers, industry, and telecom exchanges under hybrid AC-DC grids because of the ever-increasing DC load demand and to avoid multiple AC-DC-AC conversion losses. Moreover, in recent times the use of Electric Vehicles (EVs) is increasing rapidly and now workplaces are required to have their own EV fast charging docking stations that adds an extra DC load demand for the organizations. Therefore, by keeping the facts in view, we proposed a hybrid AC-DC nano-grid based distributed power generation and power consumption is proposed as a suitable solution to reduce multiple conversion losses within the organizations. The AC and DC load of the Lahore's central telecom exchange is modeled for the simulations. For hybrid AC-DC nano-grid, a droop controller based bi-directional converter is designed for need-based power sharing between AC and DC nano-grids. The proposed hybrid AC-DC nano grid eliminates the excessive voltage conversion problem of telecom exchanges; provide fuel cell and battery bank-based emergency backup solution along with DC fast charging system for the EVs.

In recent years, modern distribution networks are rapidly evolving toward the new concept of Smart Grid (SG) due to the increasing level of penetration of distributed generation (DG) units, storage systems and information and communication technologies. In the framework of low voltage distribution systems, SGs based on either AC or DC grids have been proposed and compared in the technical literature, evidencing their advantages and drawbacks. Only recently, hybrid SGs that include both DC and AC grids have been proposed with the aim of optimizing the whole system behavior. In this paper, an innovative hybrid low voltage SG that includes properly controlled AC and DC grids is analyzed. The DC grids - including storage systems and interconnected to the AC grid through AC/DC PWM-controlled static converters - highly contribute to improve both quality and continuity of supply to AC and DC sensitive loads. The AC grid is optimally controlled to perform several system services (i.e., minimize power losses and optimize voltage profiles at fundamental frequency and at harmonic frequencies). Time domain simulations on a 30-node low voltage hybrid SG are presented and discussed to evidence the effectiveness of the considered solution.

Line-commutated converter based high voltage direct current (LCC-HVDC) transmissions are widely applied in delivering large-capacity electricity in China. Multiple large-capacity LCC-HVDC transmissions strengthen dynamic coupling between AC and DC networks. Existing literature has reported some stability problems caused by dynamic interactions between AC and DC power grids, however, the mechanism behind these problems has not been totally understood and revealed. To analyze dynamic interactions between AC and DC power grids, this article presents a small-signal model to understand the external characteristics of LCC-HVDC stations based on the motion equation concept. Comparison of time-domain simulation response with the detailed switching model shows that the proposed model can hold main behaviors of concern. Dynamic interactions between AC and DC power grids are studied based on the proposed model in multi-infeed LCC-HVDC systems. Analytical results demonstrate that there exists power dynamics in receiving AC grid when sending AC grid subjects to a disturbance, and effects of converter stations’ controller parameters on power dynamics are explained by the physical relationship among state variables of the proposed model.

This paper proposes a hybrid AC/DC micro-grid which consists of an AC grid and a DC grid and operates in both grid-tied and autonomous mode. Wind turbine generators (WTGs), diesel generators (DGs) and conventional AC loads are connected to the AC grid whereas photovoltaic arrays with boost converter (PVBC), fuel cell generators and DC loads are tied to the DC grid. AC bus and DC bus are coupled through a four-quadrant operating three phase converter which can act as an inverter or rectifier. Battery is connected to the DC bus through a charging/discharging converter to maintain energy requirement when the system operates in the isolated operation mode. A prototype of the hybrid system has been established and simulated using Matlab Simulink. The coordination control algorithms are proposed to balance power flow between AC and DC grids and to maintain both DC and AC voltages. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient temperature and load capacities are also considered in system control and operation. The simulation results show that system is stable under various load and supply conditions. Moreover, the hybrid grid has higher efficiency than individual DC or AC micro grid due to avoidance of multiple DC-AC-DC conversions.

Introduction: In a hybrid AC/DC receiving-end grid, the proportion of external power injection continues to increase. The receiving-end AC grid may develop new weak nodes in grid operation. To investigate the voltage violation problem caused by the high penetration of external power injection in multi-infeed hybrid AC/DC receiving-end grids, this study proposes an adaptive spectral clustering approach for dynamic partitioning and weak node identification in hybrid AC/DC grids. Methods: Considering the degree of coupling between the AC grid and DC grid, the indices of short-circuit ratio (SCR) and multi-infeed short-circuit ratio (MISCR) are used to describe the electrical distance. To reduce the voltage control complexity, the zonal voltage regulation capability is integrated into the improved similarity matrix. Based on the spectral clustering algorithm, an improved similarity matrix is proposed, incorporating the similarity of operational state, electrical distance, and zonal voltage regulation capability, to classify the grid into several partitions. The optimal number of partitions can be determined automatically by identifying the maximum eigengap. Finally, the node with the largest singular value entropy in each partition is identified as the weak node, allowing for the quantification of its impact on system stability under disturbances. Results: The modified IEEE 118 bus system yields results that validate the feasibility of the proposed method. Discussion: This work aims to automatically determine the partitions of the hybrid AC/DC grid and identify the weak node within each partition, considering the mutual influence of the AC and DC grids and the zonal voltage regulation capability, in order to facilitate further operation and control. Conclusion: The proposed method incorporates the SCR, MISCR, and zonal reactive power compensation capability. The SCR and MISCR are used to describe the correlation mechanism between AC and DC grids, while the zonal reactive power compensation capability is utilized to enhance the accuracy of partitioning for hybrid AC/DC grids. By incorporating the eigengap in the spectral clustering method, the weak nodes in each partition are identified for further operation and control of the hybrid grid.

This article presents a novel concept of a universal solar converter suitable for application in both in the dc or single-phase ac grids using the same terminals. The idea lies in the utilization of the same semiconductors in the dc–dc and in the dc–ac configuration, resulting in minimal redundancy. Possible semiconductor stages are considered. The particular attention is focused on the output filter design along with proper protection circuit selection for dc and ac grids. The design example and comparative analysis between dc–dc, dc–ac, and universal solutions are given. The experimental prototype of the universal solar converter that is rated for 3.6 kVA power in the ac mode and 5 kW in the dc mode is presented. The experimental results demonstrate the ability of operation in ac or dc grids with main correspondent modes. Possible fields of application along with main benefits are addressed in conclusions.

This paper presents small signal stability analysis of a hybrid microgrid from three different aspects. First, a generalized droop control of interlinking converter (ILC) is proposed which defines voltage and frequency droops at its terminals to achieve a bidirectional, autonomous power flow between grids. The impact of the droop gains on the stability is discussed and an operational range is defined for a stable system operation. Second, contrary to previous literature which have modeled the ac and dc grids by ideal sources and converters, the ac grid in this scheme is represented as a second-order synchronous generator and the dc grid is modeled as a PV system so as to analyze the dc grid dynamics in detail. It is observed that the dc link capacitor and inductor filter can lead to large system oscillations which can compromise stability. Third, the stability of the hybrid grid is analyzed from the aspect of short circuit ratio (SCR). Both rectifier and inverter mode of operation are used to understand how stability is affected with strength of ac grid. The time-domain simulations in PSCAD are used to confirm the validity of results obtained in frequency domain.

This paper proposes a hybrid ac/dc micro grid to reduce the processes of multiple dc-ac-dc or ac-dc-ac conversions in an individual ac or dc grid. The hybrid grid consists of both ac and dc networks connected together by multi-bidirectional converters. AC sources and loads are connected to the ac network whereas dc sources and loads are tied to the dc network. Energy storage systems can be connected to dc or ac links. The proposed hybrid grid can operate in a grid-tied or autonomous mode. The coordination control algorithms are proposed for smooth power transfer between ac and dc links and for stable system operation under various generation and load conditions. Uncertainty and intermittent characteristics of wind speed, solar irradiation level, ambient temperature, and load are also considered in system control and operation. A small hybrid grid has been modeled and simulated using the Simulink in the MATLAB. The simulation results show that the system can maintain stable operation under the proposed coordination control schemes when the grid is switched from one operating condition to another.

Numerous promising features of Medium Voltage DC Distribution System (MVDC-DS) make it highly probable that it would be extensively integrated with the future AC power grid to enhance power availability. However, issues may arise during interconnection such as voltage incompatibility between the two grids i.e. MVDC-DS and AC grid. Keeping this in mind, in this paper an Improved Modular Multilevel Converter topology (I-MMC) is considered and a novel voltage balancing algorithm (VBA) is proposed. I-MMC offers several unique features like wide range of voltage boosting capability, increased number of levels in the output voltage, low switch count and enhanced THD compared to the conventional MMC. Thus, the I-MMC ensures the interconnection of the AC and DC grids and also ensures low foot print size, high power density and low infrastructure cost. I-MMC is equally effective for applications like electrical machine drive systems, High Voltage DC (HVDC) systems, and other high voltage, high power applications. I-MMC and VBA are simulated in MATLAB/Simulink environment. Representative waveforms for a wide range of loading conditions are included to validate the theoretical claims.

High Voltage Direct Current (HVDC) grid concept has recently been introduced as the next step to the point-to-point HVDC links to maximise the benefits of more power flow share, and reduce the impact of renewables intermittency. However, the integration of DC grid within existing AC systems is complex and very challenging. The compatibility between the two systems under different operating conditions needs to be fully understood. This will require sophisticated models and analysis of their behaviour using powerful tools. This paper presents a representative dynamic model of a hybrid AC-DC gird, and investigates the transient and dynamic interaction between the AC and DC grids under different fault conditions. The model represents a reduced AC UK power system interfaced to a detailed four-terminal DC grid. The developed mode is built in Real Time Digital Simulation (RTDS) using dual time steps simulation techniques.

The structure of a DC grid enables control and protection in a different way, as usually employed in AC grids. Power electronic converters combine the control and protection. The structure of the DC grid is also determined by the type of power electronic converters being centralized or decentralized. Power congestion management greatly improves the DC grid. Although power electronics means extra losses per converter, so a lower efficiency, the DC grid can be used more efficient than the AC grid.

This paper presents an integrated traction machine and converter topology that has bidirectional power flow capability between an electric vehicle and the DC or AC supply or grid. The inductances of the traction motor windings are used for bi-directional converter operation to transfer power eliminating the need for extra inductors for the charger and vehicle-to-grid (V2G) converter operation. The electric powertrain system size and weight can be minimized with this approach. The concept has been analyzed with finite element coupled simulation with dynamic analysis software. Experimental results are also provided with an electric machine. The interleaving technique has been used with the inductors to share the current and reduce the converter switching stresses. The proposed integrated converter can operate in the traction mode and in the bidirectional power flow mode when energy can be transferred between the vehicle and the DC or single phase AC supply.

This study proposes a novel control scheme for frequency support in multi-terminal dc grid-based hybrid ac/dc networks. This control is based on the multi-agent system paradigm with an adaptive frequency droop control and is distributed in all grid-tied converters. The objective of this control is to enable the ac grids to intelligently decide on their participation on frequency support, considering their power reserves, demand, and relevant technical constraints and requirements. The main achievement of the proposed control is to minimise the rate of change of frequency and frequency undershoot (f nadir ), and also providing a systematic enhancement in frequency stability, particularly in the disturbed and weak ac grids. A stability analysis is performed for the proposed control, using vector Lyapunov method for interconnected systems, deriving the conditions for stable operation. Test scenarios are conducted on a modified seven-terminal CIGRE grid, connecting five ac grids. Results under different scenarios show the performance and robustness of the proposed control in providing significant enhancement in the frequency regulation for the connected ac grids.

In recent years, most of the loads and distributed generations are connected to the AC grid through the power electronic converters. Using the DC grid beside the AC grid can reduce the conversion stages and power losses. Protection of the DC grids is a challenging issue because of the new structures of DC grids and fast transients of the DC faults. This paper studies the protection of the low voltage DC (LVDC) system in the presence of the photovoltaic (PV) and energy storage systems (ESS). An LVDC system consisting of a DC microgrid is considered and Different operating modes are analyzed. DC faults behavior and protection challenges are discussed for each mode through simulations employing MATLAB software. Finally, some methods are presented to solve the protection challenges. The results show that changing the protection arrangement of the system and choosing suitable control logics for the ESS and the PV prevent the unwanted outage of the loads and provide the possibility of the microgrid operation in islanded mode.

The main aim of this paper is to improve dynamic performance of a multi‐terminal DC (MTDC) grid in a wind‐integrated AC power system through DC voltage control. The proposed approach is based on design of robust H∞ controller and loop shaping along with mutual quality decomposition method (MQDM). For extracting transfer function of the plant, model of Master‐Slave control system is converted to standard feedback control system. Then, based on the obtained difference between singular‐value curves of the main open‐loop system and the proposed method, the weighting functions of robust H∞ controller are selected. In the next step, by using the MQDM technique, the transfer function ofthe system in state‐space form is evaluated to extract gain of the robust controller. For practicality of the conducted approach, singular‐value order reduction Hankel algorithm along with frequency response analysis have been used for designing the gain of the robust controller. The developed method has been implemented on the MTDC converter in the wind farm side to control the voltage of DC grid. The simulations are accomplished in MATLAB, where by considering different uncertainties in the DC grid, wind unit, and the AC grid, robustness of the proposed approach has been clearly demonstrated.