Repetitive control scheme for circulating current suppression in modular multilevel converters

Abstract

Modular multilevel converters (MMC) are intensively researched and developed in high power, high/medium voltage applications. However, the interaction between switching action and fundamental-frequency pulsation in capacitor voltage of submodules will induce 2 ndorder and other even-harmonic harmonics in circulating current. These harmonic currents will introduce extra power loss, increase current stress of power devices and even jeopardize safe operation of MMC. Traditional methods are usually designed under single harmonic frequency, resulting in increased complexity for an integrated solution. This paper presents a repetitive control scheme based on unified model of circulating currents to solve the problem. It is simultaneously effective for multi-harmonic suppression, easy to implement and can be widely applied to single-phase or three-phase MMC. Simulation and experimental results of a MMC operating at inverter mode prove the feasibility of proposed methodology.

Analysis and Control of Modular Multilevel Converters under Asymmetric Arm Impedance Conditions

“…Due to the MMC's unique configuration, there are complex interactions involving different currents and voltages in the MMC, and extensive research has been conducted on the modeling and control strategy of the MMC [11][12][13][14][15][16][17][18][19][20][21][22]. The relationship between the arm current and capacitor voltage was analyzed in [11] and [12].…”

“…Many control strategies have been developed such as circulating current control [15][16][17][18] and energy-based modeling control [19][20][21][22]. The circulating current control method suppresses the even-order common-mode harmonic current by adding extra controllers, such as a proportional-integral (PI) controller in a double-frequency rotational frame [15], a series of resonant controllers tuned at even-order harmonic frequencies [16], or a repetitive controller [17][18]. The energy-based modeling control is based on the schemes of regulating the total energy and energy balancing [19][20][21][22][23].…”

“…Yao is with China Electric Power Research Institute, Xiaoying Road, Beijing, 100192, China (email: yaoliangzhong@epri.sgcc.com.cn) the MMC, affecting the capacitor voltage in the sub-modules (SM) and the energy variation in each arm, but it does not affect the ac side current (differential-mode current). Many control strategies have been developed such as circulating current control [15][16][17][18] and energy-based modeling control [19][20][21][22]. The circulating current control method suppresses the even-order common-mode harmonic current by adding extra controllers, such as a proportional-integral (PI) controller in a double-frequency rotational frame [15], a series of resonant controllers tuned at even-order harmonic frequencies [16], or a repetitive controller [17][18].…”

Eliminating the influence of capacitor voltage ripple on current control for grid-connected modular multilevel converter

“…Although the capacitor voltage ripple as well as arm current harmonics can be suppressed by installing larger capacitance in the submodules, the hardware requirement and cost can be reduced by adopting harmonic suppression controllers in the control system. [9] [10] The interaction between the capacitor voltage and the arm current can be decoupled by incorporating real-time capacitor voltage information for calculating the modulating signals, either by estimation algorithm or measurement. [5] [11] However, direct modulation is still advantageous because it is straight-forward, easy to implement, and no estimation or additional measurement is needed.…”

VHDL Implementation of Capacitor Voltage Balancing Control with Level-Shifted PWM for Modular Multilevel Converter

“…Therefore, an FPGA has been coupled with DSP boards, mainly to distribute these control signals [16]. In [17] - [19], an FPGA is also used to shorten the processing time by executing simple logic functions such as performing carrier-based PWM.…”