Low-voltage AC-DC hybrid distribution network for large-scale photovoltaic consumption Space-time Coordinated Optimization Method

Local control method and simulation analysis of hybrid AC/DC low-voltage distribution networks with high-proportion photovoltaics

This paper addresses common-mode (CM) and radio frequency (RF) electromagnetic interferences (EMI) in a converter-fed low-voltage direct current (LVDC) distribution research network. The LVDC distribution is a novel approach to public electricity distribution, and a research site is set up to enable comprehensive studies concerning the LVDC distribution. Because the customers in the LVDC system are supplied with customer-end inverters (CEI), CM and RF EMI generated by the CEIs have to be analyzed. To this end, common-mode currents in the DC network and the customer-end networks are studied by on-site measurements. The objective is to determine whether there are disturbances that exceed the requirements of the standards or whether the CM current could cause safety issues in the converter-fed user-end network. In the DC network, the effect of the CM current on the feasibility of power line communication (PLC) in a DC network is analyzed. Finally, RF disturbances are measured and discussed.

A dynamic reference voltage adjustment strategy for Open-UPQC to increase hosting capacity of electrical distribution networks

The low-voltage distribution networks have some structural features, such as short supply line, more lines branch and influence of transition resistance of the short-circuit current. These characteristics seriously affect the development of fault location technology. The paper, based on the basis of the structural features about the low-voltage distribution networks, create the distribution networks of description matrix by knowledge of mathematical topology and use the distribution network fault location algorithm based on web-based structure matrix. By the algorithm, the area of fault judgment and the actual distribution network fault location are fully consistent. The new theory provides a new way of thinking for fault location in low-voltage distribution networks.

The three-phase imbalance in low-voltage distribution networks (LVDNs) seriously threatens the security and stability of the power system. At present, a standard solution is automatic phase commutation, but this method has limitations because it does not address the branch imbalance and premature convergence or instability of the commutation algorithm. Therefore, this paper proposes a novel refined regulation commutation system, combined with a modified optimized commutation algorithm, and designs a model and simulation for feasibility verification. The refined regulatory model incorporates branch control units into the traditional commutation system. This effectively disperses the main controller’s functions to each branch and collaborates with intelligent fusion terminals for precise adjustment. The commutation algorithm designed in this paper, combined with the above model, adopts strategies such as symbol encoding, cubic chaotic mapping, and adaptive adjustment based on traditional genetic algorithms. In addition, in order to verify the effectiveness of the proposed method, this paper establishes a mathematical model with the minimum three-phase imbalance and commutation frequency as objectives and establishes a simulation model. The results of the simulation demonstrate that this method can successfully lower the three-phase imbalance of the low-voltage distribution network. It leads to a decrease of the main circuit’s three-phase load imbalance rate from 27% to 6% and reduces each branch line’s three-phase imbalance ratio to below 10%. After applying the method proposed in this paper, the main and branches circuit three-phase imbalance are both lower than the limit ratio of the LVDNs, which can improve the quality and safety of electricity consumption. Additionally, the results also prove that the commutation algorithm under this method has faster convergence speed, better application effect, and better stability, which has promotion and application value.

The high proportion of distributed photovoltaics (PVs) connected to distribution substation areas causes complex operation of the medium- and low-voltage distribution network. New power electronic devices represented by soft open point (SOP) can achieve a flexible interconnection between distribution substation areas. In this paper, a cooperative operation optimization method for medium- and low-voltage distribution networks considering flexible interconnected distribution substation areas is proposed. Firstly, interval affine is used to model the resource aggregation and obtain the flexibility region constraints of distribution substation areas. Then, a multi-scale cooperative operation framework for medium- and low-voltage distribution networks is constructed. The medium-voltage distribution network adopts the centralized method to solve and issue operation strategy instructions. The distribution substation area generates a voltage-reactive power adaptive regulation curve and performs intra-day rolling control. Finally, the case studies show that the proposed method has efficient operation strategy of medium- and low-voltage distribution networks and can alleviate the voltage fluctuation caused by high integration of PVs.

There is a problem of three-phase unbalanced power quality for the power electronic interface of the low-voltage distribution network. If reasonable and effective means are not used to control the three-phase imbalance in the distribution network, it may affect the safe and stable operation of the entire power system. Therefore, the problem of three-phase unbalance in the low-voltage distribution network has become a hot topic of research. Generally, for the treatment of three-phase load imbalance in the low-voltage network segment, a reactive power compensation device is commonly used. In this paper, the vector analysis compensation strategy is used to detect the active and reactive components of the fundamental current related to the compensation capacitance value. The current detection method of space vector transformation is designed. The feasibility of the scheme is verified by simulation analysis.

In this study, a mechanism is developed to support the distribution system operators (DSOs) with a better grid management scheme regarding congestion management by incorporating a data-driven congestion scheme and thus reducing the congestion. The objective of the work is to avoid/reduce the thermal overloading of the transformer with an affordable approach. A data-driven model is developed, which can help DSOs to solve congestion problems in the low-voltage (LV) distribution network. Demand response is applied for the procurement of flexibility, using available flexibility to mitigate the congestion in the distribution network. Once the congestion occurs, the DSO can refer to the data-driven classification model and mitigate the congestion with the minimised cost. A typical Dutch LV distribution network is considered for analysis. It is observed that the DSO has significant savings and reduction of congestion occurrence in the LV distribution network. The performance and accuracy of the developed data-driven model are also found to be well within satisfactory limits.

The rapid increase of distributed energy resources (DERs) installation at residential and commercial levels can pose significant technical issues on the voltage levels and capacity of the network assets in distribution networks. Most of these issues occur in low-voltage distribution networks (LVDNs) or near customer premises. A lack of understanding of the networks and advanced planning approaches by distribution network providers (DNSPs) has led to rough estimations for maximum DERs penetration levels that LVDNs can accommodate. These issues might under- or over-estimate the actual hosting capacity of the LVDNs. Limited available data on LVDNs' capacity to host DERs makes planning, installing, and connecting new DERs problematic and complex. In addition, the lack of transparency in LVDN data and information leads to model simplifications, such as ignoring the phase imbalance. This can lead to grossly inaccurate results. The main aim of this paper is to enable the understanding of the true extent of local voltage excursions to allow more targeted investment, improve the network's reliability, enhance solar performance distribution, and increase photovoltaic (PV) penetration levels in LVDNs. Therefore, this paper reviews the state-of-the-art best practices in modeling unbalanced LVDNs as accurately as possible to avoid under- or over-estimation of the network's hosting capacity. In addition, several PV system modeling variations are reviewed, showing their limitations and merits as a trade-off between accuracy, computational burden, and data availability. Moreover, the unbalanced power flow representations, solving algorithms, and available tools are explained extensively by providing a comparative study between these tools and the ones most commonly used in Australia. This paper also presents an overview of unbalanced optimal power flow representations with their related objectives, solving algorithms, and tools.

The possibility of high Photovoltaic (PV) and Electric Vehicle (EV) penetration in Low-Voltage (LV) Distribution Networks (DN) is examined in this research utilising a typical DN found in Malta as a case study. The European Union (EU) has established goals and directives that make it quite probable that DNs will include a lot of PVs, causing overvoltages, and a lot of EVs, charging erratically, causing undervoltages. A DN may encounter both unfavourable scenarios on the same day since PV generation and EV charging often occurs simultaneously, particularly in residential case studies. Utilising dispersed or centralised storage effectively is one method to address these problems, however, doing so may postpone the need for costly DN expenditures. The paper demonstrates the level of automatic mitigation achieved in Malta residential areas. The information provided is mostly by actual measurements taken at the start of the LV feeder.

The development of a low-voltage dc distribution network has advanced the research on dc circuit breakers. Compared with the Z-source circuit breakers, T-source circuit breakers offer more advantages and greater research value. However, the existing bidirectional Z-source and T-source circuit breakers still suffer from limitations, such as high conduction loss, complex topology, and no physical isolation. Therefore, to solve these limitations, in this article, a novel low-loss bidirectional T-source circuit breaker with physical isolation is proposed. The proposed circuit breaker employs mechanical switches to reduce conduction loss, realize physical isolation, and change the power-flow direction. Since mechanical switches do not need to operate in the fault-clearing stage, the novel circuit breaker does not generate a dc arc and retains the fast self-shut <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-off</small> characteristic of T-source circuit breakers. Moreover, soft-starting circuit and manual-tripping circuit are integrated into the proposed circuit breaker. The detailed analysis and design equations of the circuit breaker are provided. Finally, the capabilities of the novel circuit breaker are verified by simulation and experiment.

Electric vehicles (EV) have gained global attention due to increasing oil prices and rising concerns about transportation-related urban air pollution and climate change. While mass adoption of EVs has several economic and environmental benefits, large-scale deployment of EVs on the low-voltage (LV) urban distribution networks will also result in technical challenges. This paper proposes a simple and easy to implement single-phase EV charging coordination strategy with three-phase network supply, in which chargers connect EVs to the less loaded phase of their feeder at the beginning of the charging process. Hence, network unbalance is mitigated and, as a result, EV hosting capacity is increased. A new concept, called Maximum EV Hosting Capacity (HC max ) of low voltage distribution networks, is introduced to objectively assess and quantify the enhancement that the proposed phase-shifting strategy could bring to distribution networks. The resulting performance improvement has been demonstrated over three real UK residential networks through a comprehensive Monte Carlo simulation study using Matlab and OpenDSS tools. With the same EV penetration level, the under-voltage probability was reduced in the first network from 100% to 54% and in the second network from 100% to 48%. Furthermore, percentage voltage unbalance factors in the networks were successfully restored to their original values before any EV connection.

The paper investigates the power losses for real distribution networks in harmonic polluted state. Actually, the low voltage (LV) distribution networks supply a significant number of nonlinear consumers which causes a distorted and unbalanced state. A negative effect of these states is represented by the increase of power losses in distribution network elements. Even if the symmetrical sinusoidal state is initially considered for the power losses, in reality this often leads to power lines overloading, especially for the neutral. The method for power losses estimation in LV distribution network in harmonic polluted environment is very difficult to use. The aim of this paper is to present a reasonably accurate approach to evaluate the effects of harmonic currents on power losses for LV distribution networks using fuzzy techniques. Moreover, to highlight the proposed methodology, a study case for a real low voltage distribution network in order to compute the additional power losses due to the distorted state is provided.

With mature of the direct current (dc) distribution grid technology, the distributed energy is embraced as much as possible, and reducing network loss becomes an effective way to increase energy efficiency and system stability. The conventional reducing loss approaches include complex algorithms, loss modeling in power devices and adding hardware, which lead to costs and system response time increase and system stability reduce. Considering the Intermittent characteristics of distributed energy, for reducing network loss, the effective option is to control error and response speed directly through the upper control, when distributed energy accesses on dc distribution network, which is no need to change grid topologies, run cumbersome power flow algorithms, nor add additional equipment. A reducing network loss method is proposed based on this idea. The details are as follows: Firstly, the network loss formula is derived based on power flow calculation, and the network loss change rules are analyzed. Secondly, one optimal power flow mathematical model of dc distribution network is established, which takes the minimum network loss as the objective functions and conforms to constraints of system security and components operating limit. The tide optimization is solved by using the artificial bees (ABC) algorithm. Thirdly, the network loss reduction method is proposed in the dc distribution network by using master–slave control through real-time control instruction optimization. The node voltage, branch current and main voltage source converter power are precisely regulated to control the power flow; thus, the network loss of multiple distributed energy access dc distribution network can be reduced. Fourthly, when wind and solar energy is connected to the dc distribution network, a typical IEEE16 node case is demonstrated to verify the feasibility of the proposed method using software MATLAB/SIMULINK. Fifthly, evaluation and prospect are made for the research.

Distributed photovoltaic (PV) in the distribution network accounted for an increasing proportion of the distribution network, and the power quality of the distribution network of the power quality problem is more and more significant. In this paper, the voltage regulation methods for low-voltage distribution networks containing high-penetration PV are investigated. First, the working principles of the four voltage control methods are introduced: energy storage system configuration, regulating the reactive power output of PV inverters, restricting the active power output of PV, adjusting the switching positions of on-load regulator trap changer and distribution network reconfiguration, and then, in combination with the recent related research, the optimization of each method is compared horizontally with its respective concerns and characteristics. The optimization of each method is compared horizontally with the recent studies to find out the focus and characteristics of each method, and the shortcomings of each method are explored. Coordinated voltage control through multiple flexibility resources has become the mainstream voltage regulation scheme, and distribution network voltage regulation is considered from the perspective of flexibility resources. The three types of flexibility resources, namely, source, network, and storage, have been widely used in distribution network voltage regulation. Although load-side resources have become one of the main regulation resources of the new type of power system, the current study introduces less about the participation of load-side flexibility resources in voltage regulation of LV distribution networks and advancing the application of load-side resources in voltage regulation of LV distribution networks is the focus of future research. Then, the important role of load-side flexibility resources in voltage regulation is described in three parts, namely, the important role of load-side resources, the development trend, and the suggestions for promoting the coordination of source-network-load-storage flexibility resources, aiming to promote the application of load-side resources in voltage regulation in LV distribution networks, and the suggestions and programs are proposed for the technological challenges faced by voltage regulation. In the context of today’s new power system emphasizing the interaction of source, network, load, and storage, new technologies and methods for solving voltage problems in LV distribution networks are prospected, with a view to providing certain reference value for the actual operation and optimization of distribution network systems.