Analysis of power quality problems in DC distribution network

Under the background of steady development of DC transmission and distribution technology, a series of pilot projects and demonstrations have been carried out gradually in the Engineering application of DC distribution network. In this paper, the typical mode of low voltage DC distribution network- flexible distribution system, is taken as the main research object. The interface between low voltage DC distribution and AC distribution, the distributed energy resource and electric vehicle, and the characteristics of electrical load is considered synthetically. The DC voltage level, power supply radius and capacity, the mode of grounding and connection, and topology structure are analyzed and studied. The typical DC distribution structure is analyzed by simulation. And the DC network load mutual supply experiment system suitable for different scenarios is constructed.

Wind energy is one of the fastest-growing sources of electricity in India and around the world. In spite of being cheap, clean and abundant the continuous fluctuations in the renewable energy causes significant power quality issues. This paper presents a technical review of power quality problems associated with grid connected wind power plant and describes the Dynamic Voltage Restorer that injects voltage into the system with a view to enhance the network power factor while stabilizing the voltage and reducing the network losses. On the basis of measurements and norms followed according to the guidelines specified in IEC-61400 (International Electro-technical Commission) standard, the performance of the wind turbine and thereby power quality should be determined. Wind turbines, especially of Induction type, can draw large amount of reactive power from the grid. Insufficient supply of reactive power from the grid to the wind generators may cause voltage stability problems. To minimize reactive power exchange between wind generators and grid, dynamic compensation of reactive power can be employed by using DVR. This paper also describes the analysis of Dynamic Voltage Restorer (DVR) using MATLAB/SIMULINK for the grid connected Wind Power Plant.

Electric railway power systems (ERPS) as one of the most critical and high-power end-user loads of utility grids are characterized by outlandish power quality (PQ) problems all over the world. The extension and evolution of different supply topologies for these systems has resulted in significant and various forms of distortions in network voltage and current in all ERPS, the connected power system, and adjacent consumers. During the last years, numerous studies have been offered to investigate various aspects of PQs in a specific supplying topology. Variation in the supply structure of the ERPS and different types of locomotives has propelled the observation of different PQ phenomena. This versatility and development have led to confront considerable types of two-way interactive interfaces as well as reliability and PQ problems in ERPS. In addition, the lack of standards explicitly dedicated to ERPS has added to the ambiguity and complexity of this issue. In this paper, an extensive review of PQ distortions and phenomena in different configurations of ERPS is proposed and a systematic classification is presented. More than 140 scientific papers and publications are studied and categorized which can provide a fast review and a perfect perspective on the status of PQ indexes for researchers and experts.

This paper describes the technical principles of series compensation, and introduces the series compensation device structure, and finally got through simulation effects of rapid switching Series Compensation for distribution network distance protection. Distribution network is an important network connections and power grid users, from the distribution of electric energy in the power system, the strength of its transmission capacity directly determines the power user power quality is good or bad. However, since the electricity distribution network of long-term failure to get the attention of practitioners, its construction and the relatively slow pace of reconstruction, the aging serious, increasingly unable to meet the annual growth of electricity demand.The main problem with the presence of the grid are:Distribution network construction is lagging behind, power transmission grid capacity is insufficient. Network structure is irrational, power supply radius is too long, circuitous route power line terminal voltage is too low. Part serious aging distribution equipment and lines, the line number is small, backward technology, serious power loss. Reactive power shortage, power grid is not economical and low power quality. Therefore, how to improve the status of the distribution network has become the focus of power workers

This work proposes a topology of isolated microgrids with hybrid network (part in DC and part in AC) of electric power distribution. The topology consists of using a main DC power distribution system and subsystems with AC distribution. DC/AC converters are strategically distributed along the main DC network, forming AC subsystems, in order to serve a certain number of consumers with AC loads. Although there are loads that can be fed in DC, this is not a reality for household loads in several countries, which are basically AC powered. Thus, the interest of this work was to evaluate the possible advantages of this topology as compared to the conventional AC microgrids with low voltage distribution network. In this context, the main aspect studied was to increase the extension of the distribution network without the need to use electric transformers or conductors with larger cross sections. Intrinsically, the efficiency of the energy distribution was evaluated with respect to the energy losses, and the variation of the implantation and operation costs. An isolated microgrid with a purely AC distribution network was used as reference: the microgrid with solar-wind generation of Lencois Island is located in northeastern Brazil.

During the last decade, DC power distribution represents a remarkable increase with renewable energy (RE) generation especially with photovoltaic (PV) systems. DC power distribution with PV system creates a wide prospect for using DC power for residential applications. One of the most challenges to enlarge PV generation is to reduce its cost. Inverter (DC - AC) and its annual maintenance cost represent a considerable value in a PV system, which can be eliminated in case of DC power distribution. This paper is comparing the cost of energy (KWH) in case of AC and DC power distribution for a defined PV system. The studied system can be considered as a model for PV residential application powered by a PV system in developing countries. The two AC and DC systems are simulated and costs of energy are compared to determine the generated energy cost using Homer software.

The multi-terminal flexible DC distribution network can realize the power supply of distributed power supply and the flexible operation of multiple DC loads, which is an important development direction of the future DC distribution network. The coordinated control and communication coordination of each converter station and multi-converter is the key to the safety and stability of DC distribution network. Therefore, in view of the physical information coupling characteristics of multi-terminal flexible DC systems, considering the coordinated control stability characteristics and information interaction capabilities between multi-terminal flexible converter stations, a master-slave control strategy based on multi-site cooperative communication is proposed. Then, combined with the real-time power simulation system RT-LAB and the network simulation tool OPNET, a power information hybrid real-time simulation platform for multi-terminal flexible DC distribution network is constructed to analyze the power information interaction process in different communication environments and its application to DC distribution network. Finally, with the Suzhou Four-Terminal Flexible DC Network Project as an example, the effectiveness of the proposed coordinated control strategy and power information hybrid simulation platform is verified by setting network communication congestion and delay in typical scenarios such as fault-locking and DC load disturbance of the master-slave converter station.

Due to the advantages such as low line cost, low transmission loss, and high power supply reliability, DC distribution networks have become the main development trend for future distribution networks. In this paper, a typical DC distribution network with multiple voltage levels is considered as a research object. It is proposed that the interface converters between DC buses with different voltage levels be implemented through the series-parallel combination of full-bridge LLC resonant converters. To realize the decentralized self-discipline control of DC voltage under various working conditions, different slack buses are prepared according to the voltage ranges of the DC buses, and the voltage regulation modes of the DC distribution network are divided into main voltage regulation mode, backup voltage regulation mode, and off-grid voltage droop regulation mode. By introducing a voltage coefficient related to DC voltage deviation as a basis for mode switching, the voltage fluctuations caused by slow switching between control modes in the method of traditional voltage margin control is reduced, facilitating fast and smooth switching between different voltage regulation modes. Finally, a simulation model for DC distribution networks is constructed utilizing MATLAB/Simulink. Simulation results verify the effectiveness and feasibility of the proposed voltage regulation modes and switching methods for DC distribution networks. Finally, an experimental platform is also constructed to verify the feasibility of the mode switching method proposed in this paper.

Compared to the AC distribution network, DC distribution network has the advantage on reliability, transmission capacity and power flow control. This article firstly introduces the current development of DC power distribution technology and the power distribution demonstration projects at home and abroad, and sorts out the main technical standards of DC power distribution system voltage levels. On this basis, the five typical application scenarios of DC power distribution technology are clarified and the main characteristics of various scenarios are further analyzed. Study the structural differences and configuration requirements of various typical medium-voltage DC distribution networks. By comparing and analyzing the technical characteristics of various AC/DC converters, DC transformers and DC circuit breakers, the technical requirements of various DC power distribution equipment in terms of topology, control strategy, and magnetic integration technology are put forward. Furthermore, in view of the advantages and disadvantages of different control methods, research requirements for multi-operation state conversion, circulation suppression, and energy optimization are proposed. Finally, the protection requirements of AC side faults, converter faults, DC side faults and other types of faults are analyzed, and the basic requirements for DC distribution protection configuration and technical requirements for fault identification, fault location, coordination and cooperation of multiple voltage levels are put forward.

Chapter 38 - Power quality

The increase of sustainable energy sources and integration with the power distribution grid produced power quality (PQ) problems such as voltage sag and voltage swell. These problems are produced in the power distribution systems due to multiple reasons that affect sensitive loads. Voltage sag and voltage swell mostly occurring Power quality complications in the power system. These power quality issues in the power distribution system can be mitigated by using custom power devices, Dynamic Voltage Restorer (DVR) is the newly introduced device. Different techniques and algorithms have been used to mitigate these problems. In this research work, an ultra-capacitor-based DVR and a super-twisting sliding mode controller are used for the said problem. The addition of energy storage, such as an ultra-capacitor, to a dynamic voltage restorer, creates a voltage that can alleviate voltage issues such as sag and swell. For both the voltage sags and voltage swells, a high voltage is necessary for a short spell. A dynamic voltage restorer is an apparatus that is linked between the voltage source and the load side using an injection transformer to input voltage into the power distribution side to remedy a problem that has a significant impact on the load voltage. With the inclusion of a dynamic voltage restorer and sliding mode controller, the system will be able to fix PQ issues such as voltage sag and swell on its own, rather than relying on the grid as in past years. MATLAB/Simulink R2018a has been used to conduct simulation studies. Simulation studies have demonstrated that the proposed scheme is more effective, simple, robust, and more flexible and has a better mitigation time of 2.5 milliseconds and a total harmonic distortion of less than 5% as per IEEE standards.

Voltage dips are the most common power quality disturbances. They can often cause serious damages to the end- users, especially to the industrial ones or those involved in services that today are almost entirely based on digital processes. Thus, their mitigation is assuming a key role for both customers and distributors. The first ones look for eliminating any obstacle to the correct operation of their processes, and are often willing to stipulate premium contracts that guarantee a high level of quality to their power supply. The second ones have to solve power quality problems in their networks with restrict available budget, first of all, to achieve the Regulatory targets on quality but also to satisfy the most exigent customers. In previous works, the authors proposed a distribution network planning methodology that adds to the traditional technical constraints also the target of the power quality. In terms of voltage dips, the power quality target to be achieved may be limiting the voltage dips frequency in each node of a given network within a prefixed threshold. In this paper, an improved algorithm able to find the most economical solutions to comply with this constraint has been developed. This improved algorithm considers by the same standards different corrective actions, with the aim at finding the best compromise between investment and benefits. Accordingly with this view, the goal is minimizing the cost that the distributors have to sustain for the protective measures, the cost paid by the customers for the damages caused by the not mitigated disturbances and for the premium quality contracts. The effectiveness of the proposed approach is proved by examples in a test network and in a small one derived from real cases.

Power quality of DC microgrid: Index classification, definition, correlation analysis and cases study

The power supply continuity is the key factor affecting the normal operation of the load. The complex power quality problems (such as two-frequency ripple superposition, voltage sag and ripple superposition) in the dc power supply system will directly affect the continuous power supply of the load. The internal conversion control technology of the dual power switch can guarantee the continuous power supply of the load. To solve the problem of compound power quality in dc dual power supply system, this paper proposes a dual power conversion control technique based on PSO-MP. In this technique, PSO algorithm is used to perform coarse search and MP algorithm to identify the composite power quality disturbance. The example analysis not only verifies the validity and accuracy of the method proposed in this paper, but also improves the reliability of the double power switch and quickly realizes the conversion between two dc power sources.

In the current energy landscape, power quality (PQ) emerges as a critical concern. Even when there is no fault on a line, PQ issues are common in all power networks since 90% of power systems’ loads are variable or inductive in nature. Variable loads cannot be avoided; hence, PQ concerns such as voltage swelling and sag will always arise. Voltage sag is one of the main issues within a distribution network, resulting in financial losses for the utility company and the customer. The Dynamic Voltage Restorer (DVR) effectively addresses voltage sags and minimizes total harmonic distortion (THD) in the distribution network. This paper proposed a novel control strategy to increase the PQ in a system. A Frilled Lizard Optimization-optimized fuzzy PI controller is proposed in this work to control the inverter. This proposed method improves the DVR’s ability to correct voltage sag and reduce total harmonic distortion as soon as possible. The PI control scheme is utilized initially to reduce the oscillations and remove the steady-state error. To increase the tendency rate of the error to zero, the PI method is applied to a fuzzy logic-based compensatory stage. The proposed approach is validated using pro-type models, as well as mathematical and Simulink modelling. In the Results Section, the performance of the proposed controllers with the DVR is tabulated and compared with other DVR controller schemes described in other research papers.