| With the increasingly serious issues on environment and energy, national governmentshave attached great importance to the electric vehicles (EV) and renewable energygeneration (REG). The EVs have the advantages on energy saving and environmentalprotection, while the REG can achieve the sustainable development of the energy strategy,and both have promoted the construction of electric vehicles charging stations and energystorage power stations, as well as the development of their industry chains. Combining theadvantages of electric vehicles charging station and energy storage power station, thecharge-discharge-storge power station (CDSPS), with various functions and low runningcosts, is able to realize bidirectional scheduling of battery energy. This paper presents thestudy on power conditioning system (PCS) in the CDSPS, where the system consists ofmultiple paralleled PCS sub-modules and a hierarchical control structure is introduced.Consequently, according to the management and scheduling of the upper computer, PCScan work in a variety of operating modes. In this paper, issues on several pivotal operatingconditions are discussed and some concrete and effective solutions are provided.Aiming at the grid-connected operation, a mathematical model of PCS is establishedand a multi-mode control strategy for charging and discharging is designed. The systemworks in constant current mode or constant power mode when the battery voltage is lowerthan the rated voltage, while it changes to the constant voltage mode after the batteryvoltage reached to rated voltage. The operating characteristic of positive and negativesequence phase-locked loop (PLL) of PCS is analyzed under the weak grid conditionwhere the voltages at the point of common coupling (PCC) are distorted and unbalanced.A compound synchronous reference frame PLL (CSRF-PLL) is proposed, whichcomposed of a T/4delay positive sequence (PS) PLL and a T/3&T/6delay negativesequence (NS) PLL. The CSRF-PLL can adapt to the unbalanced voltages and eliminatethe impacts of some specific harmonics. Characteristics of the DC port are analyzed viathe instantaneous power theory when the voltages are unbalanced, and then an unbalancedcontrol strategy based on the CSRF-PLL is designed by injecting a NS current into the PSreference current via a negative-sequence-to-positive-sequence transformation (NS-PST).It can well suppress the battery current ripples under weak grids.For the island operation mode, a mathematical model of the PCS is established first.To eliminate the resonance peak of the output filter, a detailed analysis of the virtualimpedance method (VIM) is given out in this paper, and it is concluded that the active damping method using resistor in series with inductor is best for the three-phase PCS. Thecontrol strategies are discussed under the nonlinear load conditions, and then a controlstrategy combined with the VIM and load current full feedforward is given out. Thediscrete time mathematical model of PCS is established. The relationship between thedegree-of-freedom (DOF) of controller and the DOF of system is discussed when one-step-delay is considered. It demonstrated that the DOF of system is always one more thanthe DOF of controller in the discrered inverter system. Therefore, a control strategy basedon the augmented state variable (ASV) is proposed. By introducing the output ofcontroller as an ASV, the desired control characteristic is obtained by means of poleassignment without adding any sensors.Contraposing the parallel-island operation mode, construction form of the parallelsystem and interaction mode of the data information are discussed. Study shows that whenthe independent dc-bus parallel scheme is adopted, the traditional load sharing strategybecomes invalid since the state of charge (SOC) of each battery is different. Therefore, apower allocation strategy (PAS) based on SOC and module capacity is designed, whichcan make full use of the total capacity of the parallel system. Then, the productionmechanism of the high-frequency harmonics circulating current (HFHCC) is analyzedwhen the SOC of each module is different, the mathematical statement and the relevantsuppression method is put up also. Finally, on the basis of the traditional centralizedcontrol scheme, an indirect control scheme with power errors based on the dq coordinatesdecoupling is proposed, and the impact of various parameters on the system stability andPDS is analyzed in detail.The basic principle of the active phase shift (APS) islanding detection method (IDM)is analyzed and a mathematical model of the three-phase PCS under islanding condition isestablished. Analysis shows that the islanding detection algorithm by using the grid-connected reactive current to control the frequency is more reliable, as it can decouple theeffects betweent the amplitude and frequency of voltage at PCC. An IDM based on activephase shift with positive feedback (APSPF) is proposed, which is simple to implement andcan be well adapted to different grid-connected control strategies for both single-stage anddouble-stage three-phase PCS. The none-detection zone (NDZ) of APSPF is described andits impact on reactive power control accuracy is discussed as well. Finally, we extendedthe APSPF IDM to the multiple PCS parallel system, and discussed the performances ofthe APSPF IDM in several different cases, study shows that the APSPF IDM is welladapted to the parallel system with good robustness. |
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