A GPS-Based Control Framework for Accurate Current Sharing and Power Quality Improvement in Microgrids
Abstract:
This paper proposes a novel hierarchical control strategy for improvement of load sharing and power quality in ac microgrids. This control framework is composed of a droop based controller at the primary level, and a combination of distributed power sharing and voltage conditioning schemes at the secondary level. The controllers in the primary level use GPS timing technology to synchronize the local reference angles. The voltage reference of each Distributed Generation (DG) is adjusted according to a voltage-current (V-I) droop characteristic to enable proper current and power sharing with a fast dynamic response. The droop coefficient, which acts as a virtual resistance is adaptively changed as a function of the peak current. This strategy not only simplifies the control design but also improves the current sharing accuracy at high loading conditions. The distributed power sharing scheme uses consensus protocol to ensure proportional sharing of average power. The voltage conditioning scheme produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.
EXISTING SYSTEM:
The virtual impedance schemes achieve a fast dynamic response by modifying the DG voltage according to the DG output current. Furthermore, proper sharing of negative sequence and harmonic currents is achieved by selecting the virtual impedance of each unit inversely proportional with its power rating. However, in weak islanded MG, where the line impedance is considerable, accurate load current sharing requires large virtual impedances which may produce a large voltage distortion [7]. Therefore, there is a trade-off between current sharing accuracy and power quality. To compensate for the voltage drop on the lines, a virtual capacitance [8] or an adaptive negative virtual resistance [9] can be employed. However, those schemes require the knowledge of line impedances and network topology. An alternative approach is using a hierarchical control structure, composed of primary and secondary levels [10]. The primary controller comprises local DG controllers, which use a combination of droop control method and virtual impedance to coordinate the power generation of DGs and share the harmonic loads between them. The secondary controller produces compensating signals so as to improve the voltage quality in a so-called Sensitive Load Bus (SLB). The compensation signals are broadcasted to the local controllers to adjust the DG reference voltage accordingly. The hierarchical control scheme has been further elaborated in [11] to enhance the frequency regulation .
PROPOSED SYSTEM:
In order to improve the current sharing accuracy in islanded MGs while ensuring high power quality, a novel control strategy is proposed in this paper. The proposed control method for a general MG consisting of N voltage-controlled DGs and several loads, which can be balanced, unbalanced, linear or nonlinear is depicted in Fig. 3. The control framework is comprised of primary and secondary control levels. At the primary level, a new droop controller is proposed to enable sharing of load current a the DG unit with a fast dynamic response. The secondary control level includes a centralized voltage conditioning module and distributed power sharing control agents. The individual control agents and the voltage conditioning module are interconnected through a low bandwidth communication (LBC) network. Additionally, the DG units are synchronized by GPS timing technology. The inverter reference voltage is obtained as the summation of the droop and secondary control signals. A cascaded control scheme comprising Proportional-Resonant (PR) voltage and current controllers is used in the inner control loop to track the reference voltage.
CONCLUSIONS:
The islanded MGs solely rely on the local DG units for voltage support and load/generation balance. On the other hand, the individual power electronic interfaced DG units have a relatively small capacity and are susceptible to overcurrent stresses. Therefore, an accurate load sharing strategy is crucial to prevent activating the overcurrent protection systems and possible damages. In this paper, a new hierarchical control structure is proposed for improving power quality and current sharing accuracy of MGs. The control framework is comprised of primary level, which is responsible for fast and accurate sharing of instantaneous load current a the DG units and the secondary control, which facilitates accurate sharing of active power as well as compensating voltage distortions caused by nonlinear and unbalanced load currents. The proposed control framework takes advantage of GPS timing technology as a means for achieving fixed frequency operation and eliminating the transformation errors resulting from Park / inverse Park Transformations. The proposed control architecture is independent of the system topology and does not require knowledge of line impedances. Since the current sharing is managed by the primary control level, which has a fast dynamic response, transient currents are also properly shared a the units. On the other hand, the large time constant of the secondary level enables implementation of the method with a low bandwidth communication network. Experimental results demonstrate the efficacy of the presented approach in terms of current sharing accuracy and power quality The proposed method is a forward step towards the integration of GPS technology with the state of the art control strategies in smart MGs. A future step is the incorporating of the GPS timing into grid connected control applications.
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