# Attenuation of DC-Link Pulsation of a Four-Wire Inverter during Phase Unbalanced Current Operation
## 2 Four-Wire Hybrid Converter with Multi-Resonant Regulators  
A four-wire hybrid converter is comprised of an electrochemical energy storage, a capacitor bank, two isolated DC/DC converters and a three-phase, four-wire bidirectional converter (Figure 2). Both DC/DC converters are based on two H-bridges connected through a medium frequency transformer (DAB--dual active bridge topology) [8; 9]. The converter connected to energy storage serves as a charging controller [10], while the one  
Figure 1: Energy user worst case scenario. connected to the capacitor bank is meant to take over any non-DC current, which would normally transfer to the battery. The four-wire bidirectional converter is a 3 level T-NPC topology-based converter [11; 12], with an independent control of active and reactive power in each phase. It can be viewed as three independent single-phase converters, connected to common N-wire. An additional DC/DC converter with the capacitor bank takes compensates the DC link current pulsation. Effectively, in response to this pulsation, the DC/DC converter produces a current of the same amplitude but opposite phase than the pulsation. Since capacitors do not generate losses when under AC loads, they can take over any pulsation that would negatively affect the lithium-ion energy storage. Compensating of this current could be achieved by different means; however, all those are worse compared to the dual-active bridge topology and capacitor bank. Passive compensation would require an increase by several times in capacitance to have a noticeable effect on the system, and it would draw a significant intrush current. A buck-boost converter has limited voltage gain, worse efficiency and dynamics than DAB converter. An LLC converter has worse dynamics and bidirectionality requires a specific design and is often limited. An interesting concept is used in electric vehicles, where resonant circuits are used to produce sinewaves to warm the batteries in cold environment--a similar solution could be applied in this system; however, due to low resonant frequency, large capacitors and inductors would be required. Since the dual active bridge topology is naturally bidirectional and fully symmetric, it is an optimal choice for this application. Voltage-oriented control algorithms are limited when it comes to compensating the grid asymmetry, and it is impossible to achieve different power flow direction in each of the phases using those algorithms. This forces different approach: a separate control algorithm for each of the phases of the three level-four wire inverter. With separate current references for each of the phases it is possible to achieve asymmetric operation of the inverter and balance the grid on local scale. The control of active and reactive power on the output of the four-wire converter requires correct and reliable synchronization with the grid [13]. In order to provide an accurate signal, even in presence of a significant voltage asymmetry, a DDSRF-PLL (Decoupled Double Synchronous Reference Frame--Phase Locked Loop) synchronization algorithm is used (Figure 3) [14; 15; 16]. A sawtooth signal produced by DDSRF-PLL is used to create sine and cosine wave reference signals for both active and reactive current and those values are then multiplied by amplitude references (iref_active_a, iref_reactive_a) provided by a higher-level controller, like grid operator. Those two signals are then added together and the result is a sinewave with amplitude and phase required to  
Figure 2: Three-phase, four wire converter system with energy storage and capacitor bank, where: L1, L2, R, C—passive elements of inverter output LCL sin filter, \(U_{cap}\), \(t_{cap}\)—current and voltage of bank capacitor, \(U_{listor}\), \(i_{ator}\)—current and voltage of energy storage, \(u_{g}\)—utility grid voltage, \(i_{g,\mu}\), \(u_{g,\mu}\)—phase a current and voltage, 3L T-NPC—three-level T-type converter, H- full bridge, PWMA—phase a control signal. force power flow in specific direction. This reference wave is compared against feedback.