Control of modular multilevel converters under singular unbalanced voltage conditions with equal positive and negative sequence components

Abstract

Abstract-This article focuses on the control of Modular Multilevel Converters (MMC) for High Voltage DC (HVDC) applications during unbalanced AC grid voltage sags where positive and negative sequence voltages are equal. The control scheme is based on six arm energy regulators, six independent current controllers and two reference calculation stages that convert the power references into grid and inner current references. Conventional inner AC currents reference calculation fails if the amplitude of the positive and the negative sequence AC grid voltages are equal, a state which is referred to in this paper as singular voltage condition. This article discusses the types of network faults that cause this condition and proposes three different solutions to operate the converter in such scenarios. The adequacy of the proposed solutions is validated through simulations considering each of the problematic fault scenarios.

Stochastic Linear Programming for Optimal Planning of Battery Storage Systems Under Unbalanced-uncertain Conditions

“…The constraints (2)-(4) confirm that the interfacing inverter can only operate on charging state or discharging state at each hour. The constraints (5) and (6) verify that the decision making binary variables can get only zero and one, and the constraints 7and (8) introduce the allowed range of the variables.…”

“…The voltage source inverters are the main technologies to deal with power quality issues. The proper control and operation of the inverters can successfully reduce the unsymmetrical loading level in power grid [6]. The active filters based on the different inverter topologies can successfully inject the desirable voltages/currents to the power grid and remove the unbalanced condition.…”

Modulation techniques applied to medium voltage modular multilevel converters for renewable energy integration: A review

“…The parallel connection between the converter legs may lead to inrush currents flowing between them, which motivate the need for balancing their energy storage (horizontal balancing). The vertical and horizontal energy distribution can be controlled by imposing the proper harmonic frequencies on the inner current flow [78,79]. The third energy balancing procedure refers to the energy distribution between the capacitors assembled on the same stack, which is achieved by a selective control of the submodules states [61,80].…”

“…In order to accomplish identical energy distribution among the six stacks, the energy deviation targets for its legs and arms, respectively modeled as E ∆ * jj and E ∆ * j , were set to zero. To address ternally flow in the converter legs are computed in stage 2 , respectively modeled as P AC * dif fj and P DC * dif fj [78,79]. In addition, in order to locally manage the pole-to-pole voltage across the dc ter- dif fj (see Figure 14) and the converter emf e * j (see Figure 15), the target values for the voltages across the arms u * jk are defined as (14).…”

Internal Balance during Low-Voltage-Ride-Through of the Modular Multilevel Converter STATCOM

“…For simplification reasons and for lower computational effort in the calculation of the references of the circulating current fundamental component, the authors assume that the positive sequence voltage is significantly larger in magnitude than the negative sequence voltage, which is the case in most AC faults, as the experiments shown in Section 4. In the rare case when the positive and the negative sequences have similar magnitudes [36], the simplification would result in an imperfect decoupling of the arm currents. However, this error would be dealt with by the action of the integral part of the arm energy balancing controller.…”