• Energy Research

Control methods based on global positioning systems (GPS) are reported in multiple literatures recently, which achieve a fixed frequency operation of the microgrid and therefore has the tremendous benefit that any problems related to frequency instability are eliminated. However, the possible interruption of GPS timing signals may cause current circulating and finally leads to instability of the microgrid, yet it is largely neglected in literatures. This paper presents an angle synchronizing mechanism which utilizes the timing signal from GPS satellites and an auxiliary frequency droop loop to ensure synchronization during GPS offline events. Smooth transfer is guaranteed between primary and auxiliary control loops with discrete control architecture. Also, to allow microgrid using GPS-based control to work in tandem with the bulk power system or another frequency droop microgrid, an extra synchronization algorithm is proposed. The viability and performance of the proposed control structure is validated by case studies.

Global Positioning System (GPS) based control has recently been reported as a replacement of droop control to achieve fixed frequency operation in islanded microgrids. This article presents the perspective that the power sharing performance of a general microgrid synchronized by GPS is essentially determined by equivalent output impedance. A novel adaptive virtual impedance control approach, which implements different values of virtual resistance for d- and q -axis, is proposed accordingly. The virtual resistance concept is implemented comprising a basic local implementation for output impedance shaping, and a sparse resistance tuning network for the compensation of mismatched feeder impedance, which demands no knowledge of actual output impedance. The resistance tuning network utilizes the consensus protocol and only requires neighboring interactions among DG units. A complete tuning of output resistance for a given load condition results in accurate active and reactive power sharing even after communication is interrupted and will still outperform conventional droop control methods if load changes during the interruption. Small-signal analysis based on delay differential equations model of the overall microgrid is performed to investigate the adverse impact of communication delays on system stability. The efficacy of the proposed approach is validated by both simulation and experimentation.

Abstract The energy storage system is a type of equipment that is widely used to reduce peak loads, but its development is restricted by the high cost. Flexible load is a kind of load resource that can be dispatched by power grid to reduce load demand, but its development is restricted by users’ comfort degree. Therefore, a generalized energy storage system (GESS) needs to be proposed to maximize users’ comfort degree and minimize the investment cost of energy storage equipment simultaneously. GESS includes a traditional ESS, demand response and electric vehicles which can effectively reduce the load demand if uniformly dispatched. The duality theory is introduced to solve the bilevel model, which is built to uniformly optimize the GESS configuration in the upper level and scheduling in the lower level. A novel energy loss model and relaxed second order cone power flow model are embedded in the lower level to reduce the energy loss and avoid power congestion problems. The 15-bus network and modified IEEE 123-bus network are used as examples to demonstrate the advantages of the GESS and PCR transactions. The simulation results show that the investment cost of the GESS is far lower than that of the traditional ESS, and the GESS peak shaving effect is better. The application of the PCR transaction enables maximum satisfaction of the power demand of users participating in the demand response. The overall operation cost of the energy storage system is significantly reduced from the perspective of the power grid and users, which is beneficial for the further promotion of energy storage technology in the power grid.

Introducing P2P energy trading in the consumers’ local market makes the energy utilization rates of users improved significantly. However, the output uncertainty of distributed energy sources poses challenges to the normal conduct of the P2P transaction. The uncertainty sources have to pay for the reserve resources for the possible power gap. The difference between the actual output and the day ahead market planning needs to be charged to punish the uncertainty producers. Therefore, a novel pricing strategy (uncertainty marginal price, UMP) is proposed in this aper to charge the uncertainty producers and credit the reserve resources. Voltage sensitivity coefficients, power transfer distribution factors and loss sensitivity factors are introduced to linearize the power flow model. The whole problem can be formulated as a robust model and decomposed into two subproblems. CCG algorithm is applied to solve the two-stage robust model. This new pricing strategy is verified on the IEEE-33 and 69 bus systems. Numerical results indicate that the proposed price increases with the rise of uncertainty level. The reference points of the system robustness can be chosen as the inflection points of the operation cost curves under different uncertainty levels.