Abstract:The increasing penetration of intermittent and stochastic distributed energy resources (DERs), including renewable generations, active loads and energy storage, brings great challenges for traditional distribution networks. Active distribution systems (ADS) have coordinated controls and active managements in place and make the higher penetration and integration of distributed energy resources possible. This special issue is dedicated to reflecting the latest progress and technologies in ADS. There are 17 papers included focusing on the following 7 topics: 1) Planning; 2) Control and protection; 3) Management; 4) System analysis and market; 5) Energy storage; 6) Power quality; 7) Communication. In summary, these 17 papers are well organized to address various challenging issues in ADS. It is hoped that this issue will serve as an introduction for future research aimed at the interconnection of DERs to distribution network.

Abstract:The approach to planning, design and operation of distribution networks have significantly changed due to the proliferation of distributed energy resources (DERs) together with load growth, energy storage technology advancements and increased consumer expectations. Planning of active distribution systems (ADS) has been a very hot topic in the 21st Century. A large number of studies have been done on ADS planning. This paper reviews the state of the art of current ADS planning. Firstly, the influences of DERs on the ADS planning are addressed. Secondly, the characteristics and objectives of ADS planning are summarized. Then, up to date planning model and some related research are highlighted in different areas such as forecasting load and distributed generation, mathematical model of ADS planning and solution algorithms. Finally, the paper explores some directions of future research on ADS planning including planning collaboratively with all elements combined in ADS, taking into account of joint planning in secondary system, coordinating goals among different layers, integrating detailed operation simulations and regular performance based reviews into planning, and developing advanced planning tools.

Abstract:A substation planning method that accounts for the widespread introduction of distributed generators (DGs) in a low-carbon economy is proposed. With the proliferation of DGs, the capacity that DGs contribute to the distribution network has become increasingly important. The capacity of a DG is expressed as a capacity credit (CC) that can be evaluated according to the principle that the reliability index is unchanged before and after the introduction of the DG. A method that employs a weighted Voronoi diagram is proposed for substation planning considering CC. A low-carbon evaluation objective function is added to the substation planning model to evaluate the contribution of DGs to a low-carbon economy. A case study is analyzed to demonstrate the practicality of the proposed method.

Abstract:Distribution system planners usually provide dedicated feeders to its different class of customers, each of whom has its own characteristic load pattern which varies hourly and seasonally. A more realistic modeling should be devised by considering the daily and seasonal variations in the aggregate load patterns of different class of customers. This paper addresses a new methodology to provide integrated solution for the optimal allocation of distributed generations and network reconfiguration considering load patterns of customers. The objectives considered are to maximize annual energy loss reduction and to maintain a better node voltage profile. Bat algorithm (BA) is a new bio-inspired search algorithm which has shown an advance capability to reach into the promising region, but its exploration is inadequate. The problem is solved by proposing the improved BA (IBA). The proposed method is investigated on the benchmark IEEE 33-bus test distribution system and the results are very promising.

Abstract:An active distribution system power-supply planning model considering cooling, heating and power load balance is proposed in this paper. A regional energy service company is assumed to be in charge of the investment and operation for the system in the model. The expansion of substations, building up distributed combined cooling, heating and power (CCHP), gas heating boiler (GHB) and air conditioner (AC) are included as investment planning options. In terms of operation, the load scenarios are divided into heating, cooling and transition periods. Also, the extreme load scene is included to assure the power supply reliability of the system. Numerical results demonstrate the effectiveness of the proposed model and illustrate the economic benefits of applying distributed CCHP in regional power supply on investment and operation.

Abstract:Active distribution network (ADN) is a solution for power system with interconnection of distributed energy resources (DER), which may change the network operation and power flow of traditional power distribution network. However, in some circumstances the malfunction of protection and feeder automation in distribution network occurs due to the uncertain bidirectional power flow. Therefore, a novel method of fault location, isolation, and service restoration (FLISR) for ADN based on distributed processing is proposed in this paper. The differential-activated algorithm based on synchronous sampling for feeder fault location and isolation is studied, and a framework of fault restoration is established for ADN. Finally, the effectiveness of the proposed algorithm is verified via computer simulation of a case study for active distributed power system.

Abstract:With the growing number and capacity of photovoltaic (PV) installations connected to distribution networks, power quality issues related to voltage regulation are becoming relevant problems for power distribution companies and for PV owners. In many countries, like Italy, this has required the revision of the standards concerning the connection to the public distribution network of distributed renewable generation. The new standards require a flexible operation of generation plants that have to be capable to change the active and reactive power dynamically in function of the network parameters (i.e. frequency and network local voltage) in local control or following external commands. Therefore, this paper investigates the use of smart inverter in a critical PV installation, where relevant voltage fluctuations exist. A case study, with real network parameters monitoring data?and measurements, is discussed in the paper with the aim of showing how ‘smart’ features of new inverters can be implemented to increase PV plant integration in low voltage distribution networks.

Abstract:Smart grid constrained optimal control is a complex issue due to the constant growth of grid complexity and the large volume of data available as input to smart device control. In this context, traditional centralized control paradigms may suffer in terms of the timeliness of optimization results due to the volume of data to be processed and the delayed asynchronous nature of the data transmission. To address these limits of centralized control, this paper presents a coordinated, distributed algorithm based on distributed, local controllers and a central coordinator for exchanging summarized global state information. The proposed model for exchanging global state information is resistant to fluctuations caused by the inherent interdependence between local controllers, and is robust to delays in information exchange. In addition, the algorithm features iterative refinement of local state estimations that is able to improve local controller ability to operate within network constraints. Application of the proposed coordinated, distributed algorithm through simulation shows its effectiveness in optimizing a global goal within a complex distribution system operating under constraints, while ensuring network operation stability under varying levels of information exchange delay, and with a range of network sizes.

Abstract:An islanding detection method in ungrounded power distribution system based on single-phase operating mechanisms of the circuit breaker (CB) is proposed in this paper. When CB opens three phase circuits to form an island, one phase circuit is opened firstly and the other two phase circuits are opened secondly after certain duration. During the period when only one phase circuit is opened, negative sequence voltage with certain duration is obtained at DG side because of the asymmetric operation of the system. After all three phase circuits are opened, the voltage variation direction of the two phases that are secondly disconnected from the grid follows the voltage variation direction of the firstly disconnected phase. Based on the above voltage characteristics, an islanding detection scheme is proposed to identify genuine islanding from system disturbances. The performance of the proposed scheme is tested in PSCAD/EMTDC using a 10 kV distribution network with synchronous DG interconnection. Simulation results demonstrate the proposed method is effective and reliable to detect islanding formation.

Abstract:Active energy management is an effective way to realize the flexible utilization of distributed energy resources to suit the characteristics of active distribution system. Advanced active energy management strategies need to be designed to coordinate the optimization of ‘generation, network, load’. An active management model is built for the local distribution system integrated with the generation curtailment mechanism and the charging/discharging management of plug-in electric vehicles. Furthermore, different strategies based on the energy management model are presented. The model and strategies are tested and discussed in a modified distribution system, and the impacts with different load profiles are also analyzed.

Abstract:In this paper, a new Distribution Management System (DMS) framework based on security region is proposed. First, the concept of Distribution System Security Region (DSSR) is introduced. DSSR is capable to describe the N ? 1 security boundary of the whole distribution network, including the secure output range of DGs. This new theoretic tool provides a chance for the implementation of real-time security analysis and active controls in DMS. Second, this paper proposes and describes five security states for distribution system. Third, an upgraded DMS enhanced with DSSR is proposed, which consists of advanced security functions such as preventive and predictive control of the trajectory of operating points. Finally, a practical case is presented to simulate the proposed DSSR-enhanced DMS, in which both the security region of network and the output range of DGs are calculated. Typical security functions are also demonstrated. In conclusion, the new DMS framework aims to help operate the system closed to its security boundary in order to improve the efficiency significantly within same security standard. This work is beneficial for future low carbon distribution systems with high penetration rate of DGs.

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Abstract:Dispatch and bidding strateThe active distribution network (ADN) is able to manage distributed generators (DGs), active loads and storage facilities actively. It is also capable of purchasing electricity from main grid and providing ancillary services through a flexible dispatching mode. A competitive market environment is beneficial for the exploration of ADN’s activeness in optimizing dispatch and bidding strategy. In a bilateral electricity market, the decision variables such as bid volume and price can influence the market clearing price (MCP). The MCP can also have impacts on the dispatch strategy of ADN at the same time. This paper proposes a bi-level coordinate dispatch model with fully consideration of the information interaction between main grid and ADN. Simulation results on a typical ADN validate the feasibility of the proposed method. A balanced proportion between energy market and ancillary services market can be achieved.gy of active distribution network in energy and ancillary services market

Abstract:As the integration of distributed generations (DGs) transforms the traditional distribution network into the active distribution network, voltage stability assessments (VSA) of transmission grid and distribution grid are not suitable to be studied separately. This paper presents a distributed continuation power flow method for VSA of global transmission and distribution grid. Two different parameterization schemes are adopted to guarantee the coherence of load growth in transmission and distribution grids. In the correction step, the boundary bus voltage, load parameter and equivalent power are communicated between the transmission and distribution control centers to realize the distributed computation of load margin. The optimal multiplier technique is used to improve the convergence of the proposed method. The three-phase unbalanced characteristic of distribution networks and the reactive capability limits of DGs are considered. Simulation results on two integrated transmission and distribution test systems show that the proposed method is effective.

Abstract:An economic and environmental evaluation of active distribution networks containing lithium ion batteries (Li-ion), sodium sulfur batteries (NaS) and vanadium redox flow batteries (VRB) was carried out using the EnergyPLAN software. The prioritization schemes of the combination of energy storage systems and intermittent energy systems were studied technically and economically based on some specific situations of the grid integrated with wind power. The results suggest that the technical and economic optimal intermittent energy-storage capacity ratio was 2:1 in predetermined energy system scenarios. Li-ion batteries storage system performed the best in critical excess electricity production (CEEP) absorption, energy saving and emission reduction while NaS batteries storage system was the most competitive among the three due to its cheaper costs.

Abstract:An optimal sizing method is proposed in this paper for mobile battery energy storage system (MBESS) in the distribution system with renewables. The optimization is formulated as a bi-objective problem, considering the reliability improvement and energy transaction saving, simultaneously. To evaluate the reliability of distribution system with MBESS and intermittent generation sources, a new framework is proposed, which is based on zone partition and identification of circuit minimal tie sets. Both analytic and simulation methods for reliability assessment are presented and compared in the framework. Case studies on a modified IEEE benchmark system have verified the performance of the proposed approach.

Abstract:This paper proposes a novel approach to compensate buses voltage and current harmonics through distributed generation (DG) interfacing converter in a multi-bus microgrid. The control approach of each individual DG unit was designed to use only feedback variables of the converter itself that can be measured locally. In the proposed approach, the adjacent bus voltage is indirectly derived from the measured DG converter output voltage, DG line current and line impedance. A voltage closed-loop controller and a current closed-loop controller are designed to achieve both functions of DG real power generation and harmonics compensation. Therefore, the traditional harmonic measurement devices installed at the bus as well as the long distance communication between the bus and the DG converter are not required. The proposed approach can compensate the current harmonics, mitigate the buses voltage distortion and enable the customer devices to be operated in normal conditions within the multi-bus microgrid, and meanwhile relieve the burden of power quality regulator installed at the point of common coupling. Matlab simulations and experimental results are presented to show the operational effectiveness of the proposed approach.

Abstract:In this work, a novel performance analysis method for evaluating the robustness of emerging power distribution networks (PDNs), which involve deployable renewable energy sources, is proposed. This is realized with the aid of the outage probability (OP) criterion in the context of cooperative communications, which is widely considered in modern wireless communication systems. The main usefulness of this method is that it allows the involved components to communicate to each-other by means of a robust and flexible wireless sensor network architecture. In this context, any conventional medium voltage (MV) bus of the PDN is represented as a wireless relay node where data signals gathered from each MV bus can be forwarded reliably to a control station for the subsequent processing. The received signals at wireless nodes are decoded and then forwarded to ensure minimal errors and maximal robustness at the receiving site. The considered OP analysis denotes the probability that the power of a received information signal drops below a pre-defined threshold which satisfies the acceptable Quality of Service requirements of a reliable signal reception. To this end, simple closed-form expressions are proposed for the OP of a regenerative cooperative-based PDN in the presence of various multipath fading effects, which degrade information signals during wireless transmission. The offered results are rather simple and provide meaningful insights for the design and deployment of smart grid systems.