Reducing the Energy Storage Requirements of Modular Multilevel Converters with Optimal Capacitor Voltage Trajectory Shaping

Abstract

The required module capacitance value is a driving factor of the volume and cost of modular multilevel converters (MMC). In order to minimize the required capacitance value, an optimal circulating current and common mode (CM) voltage injection strategy which keeps the conduction losses and die CM voltage as low as possible is proposed in this paper. Unlike previous methods from literature that only focus on minimizing the amplitude of energy fluctuation in the arm capacitors, the proposed optimization procedure is based on the optimal shaping of both the arm voltages and energies. As a result, the proposed optimization scheme achieves a further reduction in the required capacitance value of up to 42 % compared to existing methods. The procedure is validated with closed loop simulation results covering the full operating range of an exemplary MMC.

Pareto Frontier of the Arm Energy Ripple and the Conduction Losses of a Modular Multilevel Converter

“…in which z ∈ Z is any integer. The most restrictive requirements result from the cases k = 4 or k = 6 of the Diophantine equations (22), whose solutions are…”

“…The last approach is applied in [31,32] considering regimes with zero commonmode voltage. The solution in [22] uses the common-mode voltage to evade the impending arm voltage saturation at the upper limit allowing for a further capacitance reduction in the case of the third approach. However, this evasion reduces the arm voltage margin on the lower limit given by the half-bridge cells because it manipulates the 3rd and 9th harmonic of the common-mode voltage and moves the minimum of the required arm voltages v q1 closer to zero.…”

“…Several control schemes to reduce the capacitor voltage ripple have been proposed using only the circulating current [18,19] or using a combination of circulating current and common-mode voltage [20,21]. The use of the common-mode voltage can achieve further reduction of the voltage ripple, but it also increases the arm voltage, reducing the available modulation index required for control purposes [22].…”

Reducing Losses and Energy Storage Requirements of Modular Multilevel Converters With Optimal Harmonic Injection

“…In [17], an offline optimization procedure is proposed that identifies the optimal trade-off between the reduction in the required capacitance, the CM voltage amplitude and the mean-rectified arm current as a simplified measure for the conduction losses. The optimization procedure searches for optimal combinations of the second and fourth harmonics in the circulating current as well as the third and ninth harmonics in the CM voltage.…”

“…Therefore, in this paper, the optimization procedure from [17] is extended. Here, the optimal trade-off between the reduction in the required capacitance, the semiconductor losses, and the AC voltage or the arm current amplitude is identified.…”