Generalized Switching Method for DC-link Capacitor RMS Current Reduction in Battery Energy Storage System

Abstract

The indirect DC-DC-AC power conversion structure, consisting of a bi-directional boost converter and three-phase inverter, is commonly used structure because it satisfies the step-up or step-down requirement between input and output and is easy to use. This structure is widely employed in photovoltaic, battery energy storage system, hybrid electric vehicle, and fuel cell applications. The DC-link capacitors are used between the converters for efficient power conversion of the power converters by minimizing the influence of the instantaneous power imbalance between the converters due to the structural characteristics of the multi-stage indirect power conversion scheme. The reliability of such a typical multi-stage converter system is influenced by various components such as switching elements, inductors, capacitors, connectors, resistors, and gate drivers. Among these devices, the capacitor is the main factor that reduces the overall reliability of the system. One of the main factors determining the lifetime of a capacitor is its temperature, which is affected by the temperature around the capacitor, the resistance inside the capacitor, and the RMS value of the current flowing into the capacitor. The studies for increasing the lifetime of capacitors are classified into the passive method, active method, control method, and switching method. The passive method is a method in which a heat dissipation device or a capacitor is added to reduce the temperature of the capacitor. The active method uses a passive device including switches to reduce the current flowing into the capacitor, thereby reducing the ohmic loss of the capacitor. In addition, the control method minimizes the average power difference across the capacitors using the control in the form of feedforward and feedback. The switching method reduces the instantaneous current difference of the switching harmonic components caused by the switching of the converter. In particular, the switching method has excellent performance in reducing the instantaneous current flowing into the capacitor without additional hardware. In the boost converter and the three-phase inverter structure used in this paper, the conventional switching method has a characteristic that the inverter has a suitable performance only when it is a unit power factor and SPWM. Therefore, in the situation where the inverter supplies the reactive power for supporting the grid voltage or controls the motor, RMS current reduction performance of the capacitor is reduced. In addition, it has a disadvantage in that the performance is reduced even when a PWM method other than SPWM is used. In this paper, a novel switching method is proposed to improve the lifetime of the DC-link capacitors of the boost converter and three-phase inverter structure in various power factors. The proposed method analyzes the nonlinear characteristics of switching by analysis of the instantaneous current across the capacitor. In addition, to ensure the ease of digital implementation, the Fourier Series approximation is used to find the travel time of the PWM carrier of the boost converter to synchronize the converter and inverter currents. To verify the effectiveness of the proposed method, 1 kVA class simulation and prototype experiments are conducted and the performance is shown. The proposed switching method has the maximum reduction performance of about 45% for SPWM and about 37% for SVPWM.