Positive and Negative Sequence Control Strategies to Maximize the Voltage Support in Resistive–Inductive Grids During Grid Faults

Abstract

Grid faults are one of the most severe perturbations in power systems. During these extreme disturbances, the reliability of the grid is compromised and the risk of a power outage is increased. To prevent this issue, distributed generation inverters can help the grid by supporting the grid voltages. Voltage support mainly depends on two constraints: the amount of injected current and the grid impedance. This paper proposes a voltage support control scheme that joins these two features. Hence, the control strategy injects the maximum rated current of the inverter. Thus, the inverter takes advantage of the distributed capacities and operates safely during voltage sags. Also, the controller selects the appropriate power references depending on the resistive-inductive grid impedance. Therefore the grid can be better supported since the voltage at the point of common coupling is improved. Several voltage objectives, which cannot be achieved together, are developed and discussed in detail. These objectives are threefold: a) to maximize the positive sequence voltage, b) to minimize the negative sequence voltage, and c) to maximize the difference between positive and negative sequence voltages. A mathematical optimal solution is obtained for each objective function. This solution is characterized by a safe peak current injection, and by the optimization of the voltage profile in any type of grid connection. Therefore, the proposed control scheme includes advanced features for voltage support during voltage sags, that are applicable to different power facilities in different types of networks. Due to system limitations, a suboptimal solution is also considered, analyzed and discussed for each of the optimization problems. Experimental results are presented to validate the theoretical solutions.

Performance Comparison of Grid-Faulty Control Schemes for Inverter-Based Industrial Microgrids

“…Usually, the reference currents are designed by considering a current injection angle of 90 degrees (i.e., by assuming that the impedance is dominantly inductive). In this study, the injection angle is calculated for each injection point using the impedance seen from the output side of the generators [25]. The control parameters for this impedance are and for its real and imaginary parts, as shown in Figure 3b.…”

“…To this end, the scheme is based on injecting the maximum current during the voltage sag via negative sequence only. The optimal injection angle and the reference currents are defined in [25] as…”

“…In fact, if the angle between the sequence voltages is out of the range 5π/3 ≤ ϕ ≤ π/3, Equations (14) and (15) produce correct results only if the following correction is applied [25] Energies 2017, 10, 2096…”

“…In GCCS3 control scheme, the reference currents for injecting maximum current via positive and negative sequences can be written as [25] …”

“…For angles inside the range 5π/3 π/3, the value obtained from Equation (18) must be directly used in Equations (14) and (15). In GCCS3 control scheme, the reference currents for injecting maximum current via positive and negative sequences can be written as [25 …”