Distributed dynamic grid support using smart PV inverters during unbalanced grid faults

Abstract

A dynamic voltage support strategy using smart photovoltaic (PV) inverters during unbalanced grid faults events is proposed. It uses Karush-Kuhn-Tucker condition for finding optimal solutions to calculate the inverter's active and reactive current references. The proposed methodology also takes the X/R ratio into consideration which allows the inverter to differentiate weak or strong grid conditions and adjust its reference currents. Existing multiple-complex coefficient-filter based phase locked loop is used to extract the positive and negative sequence components. The proposed strategy deploys existing dual vector current control to ensure optimal current injection and low voltage ride through. A distributed ride-through coordination approach among multiple inverters is also proposed based on different optimisation goals-either fundamental positive or fundamental negative sequence voltage support. The strategy is simulated, and inverter's transient performance is experimentally verified on a modified IEEE-13 bus test feeder using controller hardware-in-the-loop approach. Results show substantial evidence that the proposed method can be successfully applied to support the grid during an unbalanced fault event. Table 1 Voltage ride-through requirement Voltage range, p.u. Clearing time, s

Improved LVRT Performance of PV Power Plant Using Recurrent Wavelet Fuzzy Neural Network Control for Weak Grid Conditions

“…On the basis of the grid codes, the grid-connected inverters should withstand and remain connected during certain grid faults. Many researches have been proposed in the past decade for the DGs to establish the ability to ride through the grid faults [4]- [7]. To fulfill LVRT requirements under voltage dips, three different control schemes using linear quadratic regulator and symmetrical components have been proposed in [4].…”

“…In [6], several reference current generation methods, which were developed based on the positive-negative sequence control strategy, were reported to provide the LVRT requirements for the grid-connected inverter based DGs. A Karush-Kuhn-Tucker condition for finding optimal solutions to calculate the inverter's active and reactive current references is proposed in [7]. The proposed methodology takes the X/R ratio at the point of common coupling (PCC) into consideration which allows the inverter to adjust its reference currents to ensure the LVRT [7].…”

Online Parametric Estimation of Grid Impedance Under Unbalanced Grid Conditions

“…Applications of the estimated impedance may include (1) online stability assessment based on generalised nyquist criteria to extend previous studies that considered the PRBS estimation technique only for balanced systems [7,[21][22][23] and (2) the use of the fundamental grid impedance to improve, for example, low-voltage ride-through capability of grid-connected inverters. In recent studies [11,31], it was shown that the fundamental grid impedance components (R g and L g ) are required to obtain optimal or suboptimal dq-axis current references of both the positive-and negative sequences' control loops. The work in the above studies assumed the information of the grid impedance is known and time-invariant, where this is not the case due to the dynamic properties of the power system, especially low-voltage distribution networks.…”

Voltage Support under Grid Faults with Inherent Current Limitation for Three-Phase Droop-Controlled Inverters

“…However, the tripping scenario is against the recent fault-ridethrough (FRT) requirement that demands from the DERs to support the faulty grids [5][6][7][8]. Hence, current-limiting techniques should be embedded in the control design of every inverter-interfaced DER to allow maximum power injection and avoid undesired tripping [9][10][11]. In voltage-controlled inverters, saturated integrators in the inner control loops are usually employed to accomplish the desired current limitation; however these units may suffer from integrator wind-up and eventually lead to instability [10,12].…”

“…According to the authors knowledge, this is the first time that the above properties are guaranteed in a unified control structure without switching to a different control scheme under faults. Compared to the current controllers that limit the inverter current on both sequences by limiting their reference values [11,18], or to the methods employing saturation units which can lead to instability under faults or power step changes, as showcased in [10,12,13], here a droop controller is proposed and grid current boundedness is guaranteed from the input-to-state stability (ISS) property of the closed-loop system. Furthermore, instead of using root-locus analysis to test the closed-loop system stability [5], asymptotic stability of any equilibrium point of the closed-loop system in the bounded operating range is proven without assuming knowledge of the system parameters.…”