2020, 44(7):1-14. DOI: 10.7500/AEPS20190507007
Abstract:Intelligent distribution transformer is a key device to construct the new generation of intelligent distribution network with the rapid development of smart grid. This paper analyzes the main functions of the intelligent distribution transformer combining the development trend of the distributed generation of renewable power and AC/DC hybrid distribution power system, and states that the key of realizing the intelligent distribution transformer is the high controllability, which can be achieved by utilizing the power electronic devices. Based on this, the analysis shows that the power electronic transformer (PET) and hybrid distribution transformer (HDT) are two alternative circuit schemes for the intelligent distribution transformer. The basic working principle of PET and HDT are introduced, and the several typical topologies of the intelligent distribution transformer based on PET, HDT and the extended HDT are presented and their characteristics are analyzed. Based on a specific application scenario in the distribution network, the working principle, reliability, economy, controllability of the intelligent distribution transformer based on PET and HDT are compared, and the variation trend of each performance is presented when the ratio of the conversion power increases. Finally, the future development trend of the intelligent distribution transformer are analyzed.
2020, 44(7):15-22. DOI: 10.7500/AEPS20190308006
Abstract:Microgrid is effective to realize the complementary advantages, efficient accommodation, and grid-connected long-distance transmission of renewable energies. Based on the distributed topological structure of blockchain similar to that of microgrid market, the overall framework of microgrid market is designed. Taking a full consideration of economy and energy utilization of microgrid market, this paper introduces the conceptions of game theory and potential game to convert constraints to state space, and build a microgrid operation optimization model based on the ordinal potential game of state variables with two-way time-varying information interaction. The state balance of this model is proved. In different information interaction conditions of microgrid market, the strategic learning algorithm of games is used to solve this potential game model, and the operation of microgrid market is optimized. Simulation results show that the application of this potential game model with rational microgrid participants enables all devices of the microgrid to operate at the optimal level, increases the revenue of the microgrid, and improves the utilization rate of renewable energy.
2020, 44(7):23-30. DOI: 10.7500/AEPS20190501002
Abstract:Aiming at the topology architecture of a hybrid combined multi-microgrid and distribution network, the virtual coordinator and the interconnection line equivalence are introduced and make the topology become three-layer system, which consists of component, subsystem and main system. Therefore, analytical target cascading (ATC) algorithm can be used to distributedly solve the corresponding economic dispatching problem. Meanwhile, robust optimization can deal with the uncertainty caused by renewable energies and loads. Specifically, the built robust economic dispatch model can be distributedly solved by combined column-and-constraint generation and ATC algorithm. To get the optimal value, proposed distributed method only needs small amount of data exchange of every layer among distribution network operator, virtual-zone coordinator and multi-microgrid operator, which effectively guarantees the privacy of different entities. Finally, a case study is carried out on an improved IEEE 33 system with six microgrids. The results verify the effectiveness of the proposed distributed algorithm.
2020, 44(7):31-37. DOI: 10.7500/AEPS20190326010
Abstract:Aiming at the problems that the energy supply of multi-energy microgrid involves in coupling of multiple kinds of energy sources and the energy supply capability of each energy load is hard to be quantificationally evaluated. The evaluation method of feasible and reliable energy supply intervals for multi-energy microgrid are proposed. Firstly, taking the multi-energy microgrid as the research object, a typical model of microgrid system is constructed based on the energy hub model, and the concept of energy supply capability of multi-energy microgrid is proposed. Secondly, the model of feasible energy supply interval of multi-energy microgrid is constructed, the constraints of energy supply reliability is further considered, and the model of reliable energy supply interval for multi-energy microgrid is built. Thirdly, based on NSGA-2 multi-objective optimization algorithm, the solution method and process of the model of reliable energy supply interval is proposed. Finally, the effectiveness and practicability of proposed models and methods are verified by a case.
2020, 44(7):38-45. DOI: 10.7500/AEPS20190108003
Abstract:To realize the stable and optimal operation, a strategy of multi-level power management is proposed for the islanded AC/DC hybrid microgrid cluster. The proposed strategy consists of two parts: distribution control strategy and interaction management strategy of multi-level power. Through the three levels of power autonomy, power interaction and power balance, decentralized management of system power is realized by the distribution control strategy of multi-level power. Considering the two operation cases of normal communication and communication fault, an interaction management strategy of multi-level power is proposed for AC/DC hybrid microgrid cluster, which can realize reasonable switching management between power distribution levels. Meanwhile, the proposed strategy can ensure the power quality of the islanded system, and reduce interaction loss of system power. To lower the system requirements for the communication network, the event-triggered controller is designed to realize data exchange only at the moment of the event triggering. Finally, the effectiveness of the proposed control strategy is verified by MATLAB/Simulink simulation platform.
2020, 44(7):46-52. DOI: 10.7500/AEPS20190708002
Abstract:Firstly, the thermodynamic model of typical temperature-controlled load is described. Based on this, the variation of load temperature distribution and the change of load switch states with the control strategy of common temperature setting are discussed. Then, based on the temperature queue, a cluster control strategy of density clustering based on load temperature (DCBLT) is proposed. This strategy considers the distribution of the loads in the temperature interval, clustering analysis of load cluster is made by DCBLT method and the dense region and non-dense region are determined. Temperature nodes and compensation nodes are calculated, and device response group and temperature setting value are determined. Finally, the load group is controlled by taking the set temperature as a response signal. The electric heat pump of common temperature-controlled load is used as the control object to make the simulation analysis. Compared with the switch changing times of the load group with the common temperature setting control, the results show that the changing times of switching states is obviously reduced during the control and the number of loads in which the switch state changes is significantly reduced.
2020, 44(7):53-60. DOI: 10.7500/AEPS20190603004
Abstract:In order to improve the performance of frequency regulation and power optimization for microgrid, a two-layer control structure is proposed, in which the primary control converts the traditional power-frequency droop into the power-increment factor droop to accommodate this structure. The secondary control uses a consensus protocol to achieve the goals of frequency synchronization, frequency no-difference and power optimization in a distributed manner. In addition, it is found that if only the saturator is used for power constraint, it will lead to conflicts between frequency regulation and power optimization. To solve this conflict, a corresponding power constraint control method is proposed. The stability of the proposed control strategy in the system is demonstrated by the Lyapunov direct method. Finally, the effectiveness of the strategy is verified by a simulation example of 5-node island microgrid.
2020, 44(7):61-69. DOI: 10.7500/AEPS20190515005
Abstract:By studying on the distributed energy storage system in DC microgrid, the asymmetrical characteristics in performance, parameters and information between the energy storage units are analyzed. Consequently, the maximum output power and the equivalent state of charge (SOC) evaluation methods for the hybrid energy storage units considering the multiple operation modes are proposed. On this basis, a hierarchical control strategy for load power distribution of distributed energy storage system is proposed based on the discrete consensus algorithm, which includes two layers. In the lower layer, based on multi-mode parameter evaluation of energy storage unit, the dynamic droop control is used to allocate the output power of each energy storage unit. In the upper layer, with the purpose of reducing the difference of equivalent SOC of distributed energy storage system and using the weak communication between adjacent units, the current correction signals are generated by discrete consensus algorithm to directly adjust the droop control current reference value and dynamically adjust the output power of each hybrid energy storage unit in an asymmetric working state. Finally, a simulation model is built in MATLAB/Simulink to verify the proposed control strategy.
2020, 44(7):70-78. DOI: 10.7500/AEPS20190519002
Abstract:Firstly, this paper proposes a dispatching architecture for load aggregator, and the step function characteristic of the average air-conditioning load and the set temperature difference between beginning moment and end moment in an hour is obtained by optimizing the fixed-frequency air-conditioning load minutely. Then a temperature difference and power model for air-conditioning, which is suitable for hourly scheduling, is established by linearizing the feature of the step function above. Secondly, according to the feature of temperature difference and power model for the air-conditioning, the applicability grouping method for air-conditioning is proposed. Then, this paper proposes a non-cooperative game model of air-conditioning load aggregator for the multiple load aggregators in the distribution area, and introduces a continuous intermediate variable for the problem of control variable discretization in the game profit function. A hierarchical optimization model of the game is set up, and the proof and solution method for the Nash equilibrium solution of unique pure strategy for the hierarchical optimization of the game are given. Finally, the feasibility of the proposed method is verified by a case simulation.
2020, 44(7):79-86. DOI: 10.7500/AEPS20190314001
Abstract:Power-to-gas (P2G) technology provides a new way for the renewable energy accommodation. However, the application and development of P2G technology are limited by the problems of high investment cost and poor economic characteristics. In order to realize the accommodation of curtailed wind power and improve the economy of P2G, the paper firstly proposes to use curtailed wind power to produce natural gas through P2G technology. Natural gas is stored in gas storage tank, and participates in the operation scene of frequency regulation through the nature gas generation unit (NGGU).The coordinated operation strategy of each equipment is proposed. Secondly, according to the above operation strategies, the cost-benefit model of P2G, gas storage tank and NGGU participating in the frequency regulation process is constructed with the goal of maximizing the income, in order to achieve the optimal capacity configuration of each equipment. Finally, the actual curtailed wind power of a certain wind farm and the actual dispatching commands of an automatic generation control (AGC) unit are used as a reference, and the particle swarm optimization algorithm is used for planning calculation. The calculation results analyze the economy and feasibility by using curtailed wind power and P2G technology to participate in frequency regulation auxiliary service of the electricity market with the optimal configuration of P2G, gas storage tank and NGGU capacity.
2020, 44(7):87-92. DOI: 10.7500/AEPS20190714001
Abstract:Recently, the oscillations with the interconnection of the large-scale renewable energy have received extensive attention. At present, the small signal model near the stable operation point, which ignores the nonlinear characteristics such as limits of converter, is very popular. In fact, the influence of the limits of the converter on the oscillation characteristics cannot be ignored under certain conditions. This paper reveals that one of the mathematical mechanisms of the subsynchronous band oscillation(SSBO) of the power system interconnected with direct-driven wind turbine is the non-smooth bifurcation related to oscillation limits with the phase portrait analysis. Firstly, the nonlinear non-smooth bifurcation theory related to the limits is introduced. Secondly, the limit links of the power system interconnected with direct-driven wind turbine and the subsynchronous frequency band oscillation phenomenon related to the limits is presented. And then, the influence of different limits on the characteristics of the SSBO is analyzed, and the phenomenon of saturation of the stator DC voltage limit is the main cause of the SSBO is revealed. Furthermore, the characteristics of SSBO with different short circuit ratios (SCRs) are analyzed and the non-smooth bifurcation caused by changing parameters is revealed. Finally, the effect of AC fault duration and grounding resistance on SSBO is analyzed, and the non-smooth bifurcation caused by caused by initial value is revealed. The analysis shows that the SSBO in the power system interconnected with direct-driven wind turbine after the fault cleared is induced by multiple limits with a sufficiently large disturbance, which causes large deviation from the stable equilibrium point. Mathematically, the occurrence of the SSBO is corresponding to the non-smooth bifurcation in the dynamic system.
2020, 44(7):93-99. DOI: 10.7500/AEPS20190803005
Abstract:Ultra-low frequency oscillation is a small disturbance stability problem in the active frequency control process. Power system stabilizer (PSS), which is traditionally used to suppress low frequency oscillations, can also be used to suppress frequency oscillations due to load voltage regulation effects that cause the coupling of reactive voltage control and active frequency control. A method for selecting the PSS which can suppress frequency oscillations in a multi-machine system is proposed. This method combines the effects of PSS on low-frequency oscillation and frequency oscillation. A PSS parameter optimization model considering frequency oscillation suppression is constructed. The model still uses the damping ratio of the low-frequency oscillation mode as the optimization target, but adds the phase requirement of the generator excitation system in the corresponding band of the frequency oscillation as a constraint to ensure that the unit excitation system provides sufficient positive damping for the frequency oscillation. The particle swarm optimization algorithm is used to solve the model to obtain PSS optimization parameters. The effectiveness of the proposed method is verified by the simulation results.
2020, 44(7):100-106. DOI: 10.7500/AEPS20190301001
Abstract:In order to calculate the static voltage stability boundary rapidly and precisely, an algorithm of fitting static voltage stability boundary based on asymptotic numerical method is proposed. By analyzing the equations formulated by the voltage collapse point, the general formula of the high-order partial derivatives of the static voltage stability boundary is obtained. Based on the asymptotic numerical method, the relationship between the fitting error and the boundary area is analyzed. The method avoids the multiple power flow calculations in traditional method, and has higher accuracy and lower time cost. On the other hand, the factor table of the existing coefficient matrices can be reused when calculating the high-order partial derivatives, which decreases the calculation burden. Finally, the related applications based on the method are presented. The cases based on IEEE 118 node system are established to verify the effectiveness of the proposed method.
2020, 44(7):107-114. DOI: 10.7500/AEPS20190409001
Abstract:The controllable resources in both transmission grid and distribution grid should be considered coordinately in the transmission congestion management in the spot power market. In view of the ability of a virtual power plant (VPP) to effectively integrate distributed generators (DGs) to participate in power market, a bi-level optimization model based on transmission and distribution coordination for transmission congestion management in the spot power market is established, in which a distribution grid is equivalent to a VPP. The transmission congestion caused by voltage over-limit and branch/interface power over-limit are considered at the transmission grid level, in order to realize the coordination optimization of VPPs and thermal units. The VPP level considers the participation of controllable DGs and interruptible load, and satisfies the security constraints of distribution grid. Through the interaction between the two levels, economic utilization of global resources of transmission and distribution grids is realized. Meanwhile, a more reasonable price signal is given. Simulation results on an integrated test system of IEEE 30-node transmission grid and 3-feeder 16-node distribution grid show that the proposed model and method are effective.
2020, 44(7):115-122. DOI: 10.7500/AEPS20190118007
Abstract:In the environment of transmission and distribution electricity price reform, the profit mode of power grid enterprises has been changed. It is of great significance for the future development of power grid enterprises to improve the precision investment ability. Firstly, based on the Logistic model, this paper analyzes the development stage of city region according to the electricity consumption per capita, and introduces the adaptability of city regional economy and power grid development to revise the results. Secondly, the dynamic adjustment mechanism of transmission and distribution price is analyzed, and the evaluation model and process of power grid investment planning strategy are constructed. The differential investment strategy of power grid is designed in three typical modes, i.e., all new construction, all expansion and parallel construction. Finally, four typical cities are selected for the calculation and analysis. The results show that on the one hand, the city regions in the post-rapid development stage should adopt all new investment strategies, on the other hand, the city regions in the saturated growth stage should adopt all expansion models, and the optimal investment strategies of the city regions in the rapid growth stage should be analyzed concretely. Based on the traditional investment decision-making, the strategy of grid differentiated investment considering transmission and distribution price verification and city regional development stage can provide reference for the resource optimization of provincial power grid among cities.
2020, 44(7):123-129. DOI: 10.7500/AEPS20190612007
Abstract:In order to ensure the safety and power quality, the distributed generation system based on grid-connected inverters needs islanding detection function. The conventional islanding detection methods are not suitable for the grid-connected inverter with droop control, and the existing islanding detection methods designed for grid-connected inverters with droop control have to change the droop characteristics and exist non-detection zone. To solve the problems above, this paper proposes an islanding detection method for the grid-connected inverter with droop control based on impedance identification. The method injects voltage disturbance at the point of common coupling, identifies the impedance by voltage and current disturbance signals, and then decides whether the islanding effect occurs according to the impedance variation. In order to improve the detection sensitivity, the harmonic disturbance injection is adopted to amplify the differences of characteristic parameters before and after islanding. In addition, the disturbance signals are extracted by recursive discrete Fourier transform, which improves the accuracy and timeliness of impedance identification. The experimental results verify the effectiveness of the proposed method.
2020, 44(7):130-138. DOI: 10.7500/AEPS20190531003
Abstract:Due to the simple control and wide soft switching range of dual active bridge series resonant converter (DABSRC), it can be used in the isolation DC-DC stage of the cascaded H-bridge (CHB) converter based power electronic transformer (PET) in power transformer link. Considering there exists instaneous power fluctuation of twice power grid frequency at each phase of PET, it causes the high-frequency current fluctuation in the DABSRC with the same frequency at isolation stage. In addition, it would increase the conduction loss and current stress of power semiconductor devices in the isolation stage DABSRC. In order to solve this issue, aiming at optimizing the valid values of high-frequency current, a zero sequence voltage injection (ZSVI) control strategy is proposed. By injecting zero sequence voltage into AC side of the CHB converter, the current stress of switching device for isolation stage DABSRC is reduced. Finally, simulation and experimental results on a demonstration prototype of 10 kV AC/750 V DC 3 MVA PET in Tongli, Suzhou city of China verify the feasibility of the control method.
2020, 44(7):139-144. DOI: 10.7500/AEPS20190613006
Abstract:Flicker envelope tracking is the key to analyze voltage fluctuation and flicker. The square detection method recommended by IEC has a large error in the low-frequency range of voltage flicker. It is not suitable for the envelope tracking of voltage flicker caused by wind power integration. Aiming at the problem of voltage flicker caused by wind power integration, this paper proposes an envelope tracking method of voltage flicker based on the sparse recovery algorithm. The extreme points of the voltage flicker signal are taken as the observation sequence, and a discrete cosine transform base with varied band width is constructed according to the smoother variation characteristics of the flicker envelope at the low-frequency band. Then the orthogonal matching pursuit algorithm is used to recover the optimal flicker envelope. By analyzing several types of voltage flicker signal with single-frequency flicker, multi-frequency flicker, fundamental frequency variation, harmonics and noise, the accuracy of the proposed algorithm to track the flicker envelope is demonstrated. Finally, the measured voltage fluctuation signals of the wind farm in Zhangbei flexible DC grid of China are analyzed, which verifies the feasibility and effectiveness of the proposed algorithm for tracking the voltage flicker envelope caused by wind power integration.
2020, 44(7):145-152. DOI: 10.7500/AEPS20190619009
Abstract:In view of the fact that the conventional semi-bridge modular multilevel converter (MMC) cannot block the fault current, and the existing MMC with the fault clearing capability takes too long to clear the fault, an active-grounded MMC (AG-MMC) topology with DC fault clearing capability is proposed. On the basis of conventional semi-bridge MMC, a current transfer branch is added outside the inductors on the upper and lower bridge arms of each phase of the converter, and a cut-off branch and an energy absorption branch are added at the DC outlet of the converter. When a bipolar short circuit occurs on the DC side of the MMC, the cut-off branch can effectively isolate the circuit breaker and the DC line. The current transfer branch can consume AC current and inductor current, and the energy absorption branch can quickly clear the fault current. The topology of AG-MMC and the process of fault isolation and clearing are analyzed in detail, and the design and calculation method of key parameters are given. Finally, the two-terminal MMC and four-terminal MMC simulation systems are built by using RT-LAB OP5607 software. Comparative analysis shows that the proposed AG-MMC can clear faults within ten milliseconds, which has great advantages in terms of economy and practicality.
2020, 44(7):153-160. DOI: 10.7500/AEPS20190511001
Abstract:Two key problems faced by meshed DC power grids are insufficient freedom of DC power flow control and DC line faults. In view of the above two problems, this paper proposes an integrated high voltage direct current (HVDC) circuit breaker with current flow control capability for DC power grid. Firstly, the basic structure and the working principle of the modular multi-level power flow controller (MMPFC) and hybrid HVDC circuit breaker are introduced. Then, the integrated HVDC circuit breaker is proposed, and the basic structure, the configuration principle, control mode and operation principle are introduced, respectively. Based on the analysis model of DC fault for single inverter, the capability to isolate the DC fault by integrated HVDC circuit breaker is analyzed. Finally, a four-terminal flexible HVDC grid is built on the PSCAD/EMTDC simulation platform to verify the current regulation capability and DC fault handling capability of integrated HVDC circuit breaker. The simulation results show that the integrated HVDC circuit breaker can control the DC power flow effectively and has the ability to deal with DC line faults.
2020, 44(7):161-168. DOI: 10.7500/AEPS20190320013
Abstract:High impedance grounding faults occur frequently in distribution network with arc suppression coil grounding. There are some problems in existing fault detection and location methods, such as low accuracy and difficulty in implementation. Parallel resonance process is analyzed when high impedance grounding fault occurs in distribution network with arc suppression coil grounding， and the distribution characteristics and the phase differences of the declining periodic component of zero sequence current at the upstream and downstream of the fault points are also investigated. A fault section location method for a high impedance grounding fault based on the declining periodic component of zero sequence current is proposed in this paper. The main station in distribution automation collects the phases of declining periodic component of zero sequence current calculated by the fault location units of each fault section in the feeder, then the phases and estimated transition resistance are used to select the fault section. The proposed fault location method is verified by RTDS simulation, and the results show that the proposed method is accurate and feasible.
2020, 44(7):169-177. DOI: 10.7500/AEPS20190123010
Abstract:In order to eliminate the influence of neutral grounding modes, transition resistance and the blind spot of linear correlation method on the location accuracy,when the single-phase grounding fault occurs in the system, a fault location method based on amplitude feature and Hausdorff distance algorithm is proposed. Firstly, the fault feature of zero-sequence current transient components are analyzed, and the relationship between the amplitude characteristics of zero-sequence current transient components at both ends of the section is summarized. Secondly,the amplitude feature matching degree of the zero-sequence current transient components at both ends of the feeder section is measured by introducing the Hausdorff distance algorithm, and the fault section can be effectively distinguished by comparing with the set threshold. Finally, the simulation rusults verify that the proposed method has no location blind zone, and the fault zone can be accurately located in different system grounding modes and different grounding transition resistances.
2020, 44(7):178-186. DOI: 10.7500/AEPS20190824004
Abstract:All the existing quantification methods of harmonic responsibility of distributed multiple harmonic sources use the external port current (hereinafter referred to as the harmonic source port current) of the harmonic source equivalent circuit (Norton equivalent model in common use) to calculate the harmonic contribution of the harmonic source to the concerned bus. Since the harmonic source port current is constrained by the Kirchhoff’s law, as long as the harmonic source port current changes in the power grid, the harmonic contributions of all harmonic sources to the concerned bus will change accordingly. However, the harmonic source port current usually is the result of the coupling of harmonic source and background harmonic, so the harmonic contributions calculated by this current cannot truly reflect the harmonic emission level of the harmonic source. Therefore, this paper quantifies the harmonic responsibility of each harmonic source to the concerned bus by harmonic source current itself rather than harmonic source port current. The proposed method directly calculates the complex harmonic voltage contribution of each harmonic source to the concerned bus, and it does not need to solve the harmonic transfer impedance of the harmonic source to the concerned bus, which reduces the effect of cumulative errors and can more fairly and reasonably reflect the harmonic responsibility of each harmonic source. The effectiveness and accuracy of the proposed method are verified by simulation and experimental cases.
2020, 44(7):187-199. DOI: 10.7500/AEPS20190602001
Abstract:With the continuous expansion of renewable energy market in China, the pressure of government’s subsidy for renewable energy is gradually increasing due to the existing feed-in tariff (FIT). Currently, renewable portfolio standard (RPS) has become the targeted renewable energy incentive mechanism in China. Firstly, the implementation of two main types of renewable energy incentive policies are discussed, including government-subsidized type and market-oriented type, and the FIT and RPS which are involved in the transformation are compared. Secondly, the overall framework, core design elements and supporting measures of RPS are introduced， respectively. Then, according to the implementation and research analysis of RPS in various countries, the influences of RPS mechanism on the behavior of market participants and the overall performances of power market and green certificate market are summarized. Finally, the analysis shows that China should promote RPS mechanism stage by stage. The FIT-RPS double-track mechanism should be considered, with options on either FIT or RPS implemented for different types of power generation enterprises and various kinds of renewable energy sources. Meanwhile, China should take supporting measures such as penalty and carbon emission trading.