Control Strategy for Distribution Generation Inverters to Maximize the Voltage Support in the Lowest Phase During Voltage Sags

Abstract

Voltage sags are considered one of the worst perturbations in power systems. Distributed generation power facilities are allowed to disconnect from the grid during grid faults whenever the voltage is below a certain threshold. During these severe contingencies, a cascade disconnection could start, yielding to a blackout. To minimize the risk of a power outage, inverter-based distributedgeneration systems can help to support the grid by appropriately selecting the control objective. Which control strategy performs better when supporting the grid voltage is a complex decision that depends on many variables. This paper presents a control scheme that implements a smart and simple strategy to support the fault: the maximum voltage support for the lowest phase voltage. Therefore, the faulted phase that is more affected by the sag can be better supported since this phase voltage increases as much as possible, reducing the risk of under-voltage disconnection. The proposed controller has the following features: a) maximizes the voltage in the lowest phase, b) injects the maximum rated current of the inverter, and c) balances the active and reactive power references to deal with resistive and inductive grids. The control proposal is validated by means of experimental results in a laboratory prototype.

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

“…When current allocation has taken place, the value S + max can be easily selected as 3V + I max+ grms and the positive sequence reference powers can be calculated according to the FRT guidelines in Equation (28), while when I max+ grms = I max grms from Equation (32) or due to the saturator, then S + max = S max and I max− grms = 0. Since the maximum current for each sequence is now defined according to the voltage drop, the proposed controller dynamics in (16), (17), (23) and (24) can be adapted online through the expressions…”

“…After applying the proposed controller into the three-phase inverter system, the closed-loop dynamics are given by (16), (17), (23)- (26). The state vector of the closed-loop system, for both sequences, takes the form…”

“…Under balanced fault conditions, the limitation of the total current and the maximum power injection for supporting the grid are the two main tasks of the inverter. However, when unbalanced faults appear at the grid, the selection of the appropriate strategy to optimally provide grid support is a complicated problem [17,18]. Significant amount of research has addressed the inverter response through current controllers that inject both positive and negative sequence currents, in order to provide voltage support in terms of positive sequence voltage support and negative sequence voltage elimination [19,20].…”

Fault Analysis Method Considering Dual-Sequence Current Control of VSCs under Unbalanced Faults

“…In recent years, the potential of VSCs to provide positive-and negative-sequence short circuit currents simultaneously has received increasing attention [9][10][11][12][13][14][15]. In [9,10], a negative-sequence current was injected to help reduce the active power or reactive power oscillations.…”

“…The focus was on the VSC itself and its impact on the grid was not investigated. The work in [11,12] aimed to regulate the grid phase-voltage under unbalanced fault conditions to comply with pre-defined voltage limits. Multi-objective control strategies were proposed in [13][14][15] considering the characteristics of VSC output powers and their capability to regulate grid voltage at the same time.…”