Traditional 10kV distribution network runs in the open-loop mode and the contact switch between regions is in cold standby state. If a transformer fault occurs, before the contact switch closes, the load is out of power. Meanwhile, the main transformer in substation is designed with large capacity, and the average load rate of the main transformer is low. Its a good solution to establish AC/DC hybrid distribution network by adding the multi-terminal flexible DC interconnection into the traditional 10kV AC distribution network. According to the characteristics of AC/DC hybrid distribution network, this paper proposes a new reliability evaluation method for AC/DC hybrid distribution network, in order to explore whether the transformer capacity redundancy can be reduced with keeping the reliability of the system. We construct the reliability model of multi-terminal flexible DC interconnection with considering the spare components, and adopt network reconfiguration to restore power to load with using branch exchange method during transformer fault. According to the reliability evaluation of a 10kV distribution network with multi-terminal flexible DC interconnection, the results prove that the reliability of distribution network increases after adding the multi-terminal flexible DC interconnection. Meanwhile, the results of the network reconfiguration prove that the designed capacity of the main transformer can decrease.
The partition operation problem of existing AC urban power grid can be probably solved by applying flexible DC electronic devices for flexible interconnection. This paper introduces the siting of the first flexible interconnection demonstration application of urban power grid partition at home and abroad and proposes a practical siting method. Firstly, we construct a siting model of flexible interconnection device in partition with considering 7 parameters such as static security, power capability, short-circuit current, power stability, etc. Since index calculation such as the power stability requires a series of case fault simulation, the amount of calculation will be too huge for flexible interconnection siting in big cities, and the model is difficult to solve directly. Therefore we present a practical siting method. Firstly, we give the active and reactive power demand indexes of partition for the preliminary screening of schemes, and further obtain seven screening indies through detailed simulation analysis. Then we adopt analytic hierarchy process to determine the optimal scheme. Finally, we introduce the siting discussion of demonstration application. The results show that the siting scheme determined by the proposed method has good interconnection effect. And the method finds more good sites in addition to the demonstration project location, which can provide more than one choice for following interconnection projects. Meanwhile, the study shows that not all the flexible interconnections can bring obvious effects. Therefore the distribution features suitable for flexible interconnections are concluded. The proposed method and practice in this paper are of great significance to the development of partition flexible interconnection for urban electric power.
Most of exiting economic evaluation method for power system is based on the deterministic method, whose calculation results can not provide sufficient decision information as the technical economy variables are indeterminable in economic evaluation. According to AC/DC hybrid distribution network, this paper proposes a probabilistic life cycle cost method. Firstly, we establish a probabilistic life cycle cost model. Then, we evaluate economic benefits based on probabilistic method；and present a probabilistic analysis method of economic evaluation indexes. Finally, the feasibility and validity of the proposed evaluation method are verified by simulation results of AC/DC hybrid distribution network.
To solve the capacity optimal configuration problem of hybrid energy storage system for smoothing wind power fluctuating in the DC power distribution network, this paper proposes the multi-objective capacity configuration model with taking hybrid energy storage system which composed by Vanadium redox flow battery and ultra capacitor as research objects. Firstly, we take the real output of wind power and the charge state of energy storage as indicators to establish the desired output curve of wind power. Then, we construct the multi-objective capacity optimal configuration model which takes the minimum annual cost considering DC distribution benefit and the maximum percent of pass of wind power output power as objective functions, the charge state and power output as constraints. Finally, we adopt adaptive genetic algorithm to solve the proposed model. The results show that DC distribution benefit is mainly reflected in the income of reducing transmission loss, and has a limited impact on saving converter cost and increasing DC converter station cost.
To solve the voltage deviation caused by the fluctuation of generation and variety of load in AC/DC active distribution network (ADN) with distributed generation (DG) of high-permeability, this paper proposes an ADN voltage control strategy composed of multiple reactive power source such as flexible DC device, DGs and discrete reactive power compensation equipment, etc. We divide the voltage running state of distribution network into three states. (1) In normal state, global optimization is conducted by putting into the discrete reactive power source and using its static reactive power to replace the dynamic reactive power of flexible DC device to improve the dynamic reactive power reserve capacity of distribution network. (2) In early warning state, the coordinated strategy of centralized control and local control is implemented through multiple reactive power source to adjust the node voltage which is slightly deviated. (3) In emergency state, the flexible DC device is used to rapidly support the deviating node and transform the state of distribution network into early warning state or normal state. The simulation and experimental validation based on IEEE 33-bus network are performed and the result shows that the proposed strategy can improve the reserve margin of the dynamic reactive power and realize the effective control of the distribution network voltage.
With the connection of new elements such as distributed generation, energy storage and flexible load and so on, the active distribution network (ADN) with flexible DC devices has become the future development direction of distribution network, which can adjust the power flow flexibly, realize the mutual assistance of power flow between areas and optimize the controllable resources in a wider scope. Considering local assumption of distributed generation, this paper establishes a "regional autonomy - global coordinating optimization" hierarchical scheduling architecture for the ADN with flexible DC devices. Regional autonomy takes the minimum scheduling cost as the goal to optimize the controllable distributed generation, energy storage and flexible load by the means of distributed model predictive control (DMPC). Based on regional autonomy, global coordinating optimization establishes a global coordinating optimization objective function in order to realize minimum cost, and utilizes flexible DC devices to realize the coordinative optimization of inter-area power and the greatest consumption of the distributed power. Finally, the efficiency of DMPC, which control the output of regional resources active power, is verified by the simulation；at the same time, the multi-scenario simulation results show that the global coordinating optimization can realize the mutual support between regional power in a greater scope by using flexible DC devices, promote the distributed powers consumption in the maximum degree, and improve the economy and security in the operation of active distribution network.
This paper proposes coordinated control strategy for energy storage DC/DC converter and interface converter in AC/DC hybrid microgrid. In every state of the system, energy storage DC/DC converter always performs voltage control to realize zero deviation at DC side.Interface converter can regulate their work modes by detecting the state of AC/DC hybrid microgrid to realize the stable operation of microgrid system under grid-connected mode and isolated mode, and the stable and fast switching between these two operation modes. Through the verification of computer software simulation and physics experiment, it can be proved that the proposed control strategy can realize the zero deviation of DC-side voltage of AC/DC hybrid microgrid under the isolated mode,as well as the good stability in operation and mode switching.
The operation control strategy of AC/DC active power distribution network of distributed generation with high permeability is the key to realize fully distributed generation supply and ensure the safe and reliable operation of power grid. This paper divides the running states of AC/DC active power distribution network into norm state；early warning state and emergency state. Cycle global optimization is implemented in the norm state, and the optimization model considers distributed generation consumption, load balancing and the minimum loss of active network. The early warning states including real-time early warning and situational awareness early warning use coordinate correction control strategy to improve grid safety margin. The emergency state uses flexible DC device for emergency power support, which can make the power grid to restore to the normal state or turn to the early warning state. Finally, the correctness and effectiveness of the proposed operation control strategy are validated through examples.
The change of user load characteristic and the development of power electronic technology make DC distribution network to get attention. This paper constructs a new type of AC/DC hybrid distribution network which contains distributed wind power, analyzes the cause of the bus voltage fluctuation and proposes the control strategy suitable for the features of AC/DC hybrid distribution network, which can stabilize the bus voltage of wind farm. We establish the simulation model of AC/DC hybrid distribution network with wind farm in PSCAD/EMTDC, and verify the proposed control method. The results show that, compared with the traditional distribution network, the new AC/DC hybrid distribution network can better absorb wind wave power and the bus voltage stability of wind farm is better.
The interconnection of medium-voltage DC (MVDC) distribution network with DC load, DC micro grid is the key to realize DC distribution network. Therefore, it is an urgent demand for a DC transformer which is adaptable to MVDC and LVDC (low-voltage DC) with a large voltage change (DC/DC converter). The three-phase dual-active bridge (DAB) converter has advantages of galvanic isolation, bidirectional power flow and small filter size, etc. Based on the three-phase DAB converter topology, considering the high voltage stress of high voltage side and the high current stress of low voltage side in DC/DC converter of MVDC distribution network, this paper designs a three-level DC/DC converter module. And then this paper uses a modular input-series-output-parallel (ISOP) connection to design DC/DC converter of MVDC distribution network. Finally, this paper presents a coordinate control method of high voltage side and low voltage side of converter module and a control method for the equipartition of input voltage and output current, whose feasibility is verified in MATLAB/Simulink.
According to the instability problem of DC microgrid system when Boost converter carries constant power load (CPL), this paper proposes a control strategy of converter based on sliding-mode control. Firstly, according to system transfer function, the results of mechanism analysis point that the negative resistance characteristics of CPL in converter will cause the instability of the system. Therefore, we use the sliding-mode control to construct the switching function that is the difference between the reference value and actual value of converter output power, and calculate the control law. Then, we choose the exponential approach law to improve the convergence rate, and adopt the PWM control to suppress buffeting caused by the stochastic of switching frequency. Finally, the simulation experiment proves that the proposed strategy has great response speed, and can effectively ensure the stability of the system, which can provide theoretical guidance for engineering practice.
The traditional AC/DC power flow calculation method is mainly used for transmission network, but the AC /DC distribution network needs to consider many problems, such as three-phase unbalance on the AC side, low voltage multi-terminal DC (MTDC) access in to distributed generation on the DC side, and so on. To solve those problems, this paper proposes the MTDC power flow calculation sequential method for AC/DC hybriddistribution networkwith considering three-phase unbalance.Firstly, we construct the unbalanced three-phase model for AC system and converter station. Then,we derive DC power flow equation with considering different control modes of converter station. On this basis, we derive the modified equation for DC power flow calculation with DC/DC converter. According to different control modes for DC/DC converter and converter station, we propose the different equivalent solution ways for AC/DC power flow calculation. At last, an improved IEEE 34 node example is presented to verify the proposed algorithm. The simulation results show that the proposed power flowcalculation method of AC/DC distribution network for processing three-phase unbalance, distributed generation for DC and other problems has better convergence performance, under different control modes of MTDC.
The AC-DC hybrid microgrid is the good solution for distributed generation (DG) paralleling in grid. Therefore, it is of great value to study the state estimation of AC-DC hybrid microgrid with DG. According to the control characteristic diversification of inverter and the integration of DC grid, this paper proposes a novel state estimation algorithm for the AC-DC hybrid microgrid considering operating status. Firstly, we establish the Droop-type node model of DG based on the islanded operating characteristics of microgrid. Furthermore, we formulate the appropriate pseudo-measurements to improve the system observability, according to the state estimation features of the AC-DC hybrid microgrid. Meanwhile, we use the branch current amplitude measurement conversion to reduce the difficulty of solving Jacobian matrix. Finally, we adopt the least square estimation to solve the proposed state estimation model of the AC-DC hybrid microgrid. The proposed models on grid-connected and islanded operation modes are tested on the modified IEEE-13 bus distribution system, whose results demonstrate the effectiveness of the proposed model in engineering applications.
To explore the energy savingpotential of AC/DC hybriddistribution network andmeet usersrequirements of power supply reliability and voltage quality, this paper proposes a comprehensive valuation method for the energy efficiency of AC/DC hybrid distribution network. Firstly, we analyze the traditional and new factors affecting the energy efficiency of AC/DC hybrid distribution network, and deduce the quantitative models for each impact factor. Secondly, we construct the evaluation index system for the energy efficiency of AC/DC hybrid distribution network from four aspects of medium voltage distribution network,distribution transformers,low-voltage DC distribution network and low-voltage AC distribution network. Finally, taking a distribution network as example, we estimate the energy efficiency of AC hybrid distribution networkbeforetransformationand that of AC/DC hybrid distribution networkafter the transformation. The results show that the energy efficiency of AC/DC hybrid distribution networkis better than that of traditional AC distribution network, and the performance indicators have obvious advantages.
DC distribution network has drawn widespread attention globally with its advantages such as larger transmission capacity and suitability for the access of distributed generation (DG). The present study on the DC distribution network is mainly in topology, energy efficiency, control and protection. According to the problem of power flow calculation in DC distribution network, this paper establishes the steady-state model of non-ideal converter with considering the switching loss and the parasitic parameters of inductance and capacitance, and gives the steady-state model parameters of buck, boost and other 6 kinds of converters under continuous current mode (CCM). According to the characteristics of DC grid, which means that in DC distribution network only active power and voltage need to be considered, and the converters have different control strategies, we propose a new power flow calculation model with Newton-Raphson Jacobian matrix including DC distribution network. We adopt the proposed model and method in the power flow calculation of typical 14-bus DC distribution network, and compare the results with that of PSCAD simulation, which verify the correctness of the proposed model and algorithm.
The introduction of numerous distributed resources (DR) with small capacity into distribution network will significantly change the distribution of short circuit current. Traditional short circuit current calculation requires the nodal admittance Y matrix of systems passive network. However, numerous DR with small capacity will significantly increase the dimension of the nodal admittance Y matrix, which will cause the increase of the cost of storage and inverse computing of matrix so as to reduce the efficiency. Aiming at the problem, based on the second-order difference equation with variable coefficients, this paper proposes a calculation method for the short circuit current of distribution network integrated with numerous DR with small capacity. The advantage of the proposed method is that the dimension of nodal admittance Y matrix doesnt increase correspondingly with the amount increase of DR, which can reduce the storage space and computation time significantly. The simulation example in IEEE test system verifies the correctness and efficiency of the proposed method.
With the development of low-voltage DC (LVDC) power distribution system and more and more strict requirements on power quality of consumers, it has been more and more important to give a power quality comprehensive evaluation method for LVDC power distribution system. However, the existing comprehensive method of power quality, which focuses on AC power system, is not applicable to DC power distribution system well. In order to study DC power distribution system better and improve the power quality problems effectively, a new power quality comprehensive evaluation method for LVDC power distribution system should be put forward. This paper establishes the comprehensive evaluation index system for LVDC power distribution system, adopts the comprehensive weighting method combined with analytic hierarchy process (AHP) and entropy weight coefficient method, and applies grey relational analysis (GRA) in the comprehensive evaluation of power quality. Finally, a case study shows that the proposed comprehensive evaluation method can effectively evaluate the power quality of the object.
With the continuously increasing power demand and expanding scale of distribution network in big cities, the high voltage distribution network is partitioned and the medium voltage network is operated in open-loop due to the constraint of short circuit capacity and electromagnetic looped network, which results in the decreases of system equipment utilization and reliability. If the partitioned high and medium voltage distribution networks are interconnected by VSC-HVDC, the system reliability and equipment utilization will be greatly improved, which can relieve the contradiction between the limited corridor of urban power network and the high load density, and provide dynamic reactive power support in the load center at the same time. This paper investigates the application of VSC-HVDC back-to-back device in high and medium voltage distribution network in big cities and the system design scheme. In addition, the Beijing high voltage distribution network is taken as an example to demonstrate the advantages of VSC-HVDC back-to-back device in interconnecting divided sub-networks, from the aspects of reducing short circuit current and providing dynamic reactive power support.
The voltage selection is the basement of many research contents in DC distribution network which concerns the structure and layout of grid, the design and manufacture of electrical equipment, and the operation and management of power system. Taking the voltage selection method of VSC-DC distribution network as purpose, this paper studies the internal and external factors which can impact the voltage selection, analyzes the constraint conditions including the transmission capacity and distance of equipment and DC distribution system, and confirms the DC voltage selection principle and the process of middle-low voltage DC voltage selection. According to the research results, we calculate and determine that the middle voltage in engineering project is ±10kV and the low voltage is 750V and 240V, based on the load distance, the load capacity, the load voltage, the access requirements of distributed generation and other factors in data center. Combined with the reliability level requirements of power supply and distribution in data center, we propose the topological structure of AC/DC distribution network data center. The research results can provide theoretical reference for the voltage selection and planning of DC distribution power system in the future.
Flexible loop network control device without transformer shares the same ground branch with the AC system, whose neutral point is grounded by arc suppression coil. The grounding fault characteristic of the device is different from that of the conventional device with transformer. It is necessary to analyze the characteristics of the grounding fault, which provides the basis for fault diagnosis and protection design. This paper analyzes the fault mechanism of the single phase grounding fault in the AC side of the flexible loop network control device based on the modular multilevel converter, and describes the electrical characteristics of the fault side and the non-fault side of the converter. Then, this paper analyzes the characteristics of DC monopolar grounding fault in the flexible loop network control device, derives the analytical equations of fault current in different operating modes of the converter, and points out that the increase of damping resistance of arc suppression coil grounding branch can speed up the fault current attenuation. Finally, this paper constructs the PSCAD/EMTDC simulation model of the flexible loop network control device based on the modular multilevel inverter, and verifies the correctness and validity of the fault characteristic analysis.
