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.

A new control strategy forMMCunder grid voltage unbalance condition

“…First of all, the three-phase voltage and current signals are obtained from the AC side, and the voltage and current signals are separated by positive and negative sequence in the positive and negative synchronously rotating d-q frame. Secondly, according to the requirements of MMC operation under grid voltage unbalance, current reference signals are calculated based on three different control targets, and then, according to the expression (14), (20) into the controller based on Lyapunov function, and join the circulating current control. Finally, the capacitor voltage control part is added, and the carrier signal is modulated to generate a control signal, and the upper and lower arms of the MMC are controlled, respectively.…”

“…However, when the grid voltage is unbalanced, for the outside of the MMC, the current and power of the AC side will be fluctuated and the DC side voltage will also be fluctuated, which will seriously affect the stability and power quality of the system. For example, previous studies [13][14][15][16] are methods to suppress the internal circulating current of the MMC under unbalanced grid voltage. Therefore, it is very necessary to control the MMC under unbalanced grid voltage.…”

“…At present, most of the control research of the MMC under unbalanced condition are focused on the internal circulating current control. For example, previous studies [13][14][15][16] are methods to suppress the internal circulating current of the MMC under unbalanced grid voltage. These methods can well suppress the internal circulating current and effectively reduce the capacitor voltage fluctuations, in that insulated gate bipolar transistor (IGBT) will not misoperate.…”

Control design of Modular Multilevel Converters in normal and AC fault conditions for HVDC grids

“…One way to overcome this problem is to disable the AC component of the additive current upon detecting such disturbance and enabling it once the fault is cleared [35]. Even though this makes the converter unable to control the balance between upper and lower arms during the sag, it is worth noting that there are no sources of sustained drift between upper and lower arms unless faults occur within the converter itself.…”

Handling of unbalanced faults in HVDC-connected wind power plants

“…seq. is proposed and analyzed in [11,17], focusing proportional-integral (PI) control in the synchronous reference frame and proportional-resonant (PR) control in the stationary reference frame, as well as for MMCs in [23]. Ref.…”