Abstract
The operation of modern distribution grids has changed due to the high integration ofDistributed Energy Sources (DER). For this reason, quantities of DERs, energy storagesystems (ESS) and loads form subsystems, called microgrids. The microgrids can operateboth grid-connected and isolated in island mode. As the majority of DERs are connectedto microgrid through DC/AC or AC/DC/AC converters, the operation of the microgrid canbe controlled by developing the proper control strategies for these converters.The operation of the microgrid can be divided in several control levels. The primarylevel aims mainly in the steady state operation. In order to avoid communicationinterconnections among the DERs or among the DERs and a central controller, the droopcontrol methodology is mainly adopted. According to this method, each DER injects activeand reactive power, forming the frequency and the voltage of each node, with respect topermissible limits. This operation emulates the parallel operation of ...
The operation of modern distribution grids has changed due to the high integration ofDistributed Energy Sources (DER). For this reason, quantities of DERs, energy storagesystems (ESS) and loads form subsystems, called microgrids. The microgrids can operateboth grid-connected and isolated in island mode. As the majority of DERs are connectedto microgrid through DC/AC or AC/DC/AC converters, the operation of the microgrid canbe controlled by developing the proper control strategies for these converters.The operation of the microgrid can be divided in several control levels. The primarylevel aims mainly in the steady state operation. In order to avoid communicationinterconnections among the DERs or among the DERs and a central controller, the droopcontrol methodology is mainly adopted. According to this method, each DER injects activeand reactive power, forming the frequency and the voltage of each node, with respect topermissible limits. This operation emulates the parallel operation of synchronousgenerators. However, the different R/X ratio of the connection lines affects the reactivepower sharing. For overcoming this problem, the virtual impedance control is added,which decouples the active and the reactive power. This dissertation proposes theadaptation of the virtual impedance, according to the power factor each DER operates.The power factor should be above 0.8. Therefore, the reactive power does not relyanymore on the impedance of the connection line.Furthermore, in order to enhance the reliable islanded operation of the microgrid, theDERs perform ancillary services, such as feeding of non-linear and asymmetrical loads.Therefore, the converters incorporate the active filter operation, in order to reduce theharmonic distortion and asymmetry of the voltage. A new control strategy is proposed,which transforms the harmonic voltage and the current in dq0 rotating frame, byimplementing the Park transformation. The advantage of the proposed control strategylies on using constant values, which are not affected by the normal frequency variations.Another issue is the cooperation of the microgrid with an Energy Storage System (ESS).The ESS is strategically placed at the Point of Common Coupling (PCC) with the main grid,controlling the connection switch and performing the following services: absorption ofany mismatches between the power production and consumption, detection of the maingrid, synchronization process of the microgrid with the main gird and loss reduction ingrid-connected mode. Contrary to the literature, the proposed strategy does not rely onany kind of physical communication, but only on local measurements. In order to performthis task, the converter of ESS adjusts the microgrid frequency and voltage at the PCC andthe synchronization takes place with seamless transient effects. Moreover, when the gridis connected, the ESS measures the reactive power exchange with the grid and injects a7th harmonic zero-sequence voltage. This voltage is identified by each DER, which in turnadjust their reactive power according to a proposed curve. Due to the load reactive powerfulfillment from the DERs, the reactive currents along the distribution lines are reduced,leading to a loss reduction, too. This dissertation deals with the protection issue of a looped islanded converterdominatedmicrogrid, which is protected by conventional overcurrent devices. In case ofa fault within the microgrid, the microgrid impedance is suddenly reduced, causing thefault detection. In order to calculate the microgrid impedance, the control angle is slightlydistorted, causing a respective distortion in the injected current. The feedback of thedistorted current can be identified in the voltage, calculating in this way the microgridimpedance at the output of each DER. Consequently, each DER injects a fault currentproportional to the measured impedance, following a proposed droop curve. The faultclearing is implemented in a selective way, as the DERs closer to the fault inject largercurrents, compared with the other DERs. Therefore, the microgrid can be protected withconventional overcurrent devices, without any further communication.Finally, a control strategy for loss reduction in island operation mode is also developed.This method utilizes the droop control with adaptive droop coefficients, according to themeasured microgrid impedance. The DERs closer to the loads inject larger currents, whilethe far ones smaller currents, leading to line loss reduction. The proposed methodologycan be implemented in every microgrid topology, irrespective of the location of the DERsand loads. Furthermore, a virtual impedance control is not necessary, as the impact of thedifferent line impedances is inherently adopted in the adaptive droop coefficients.
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