# Sensorless Current Pulsation Compensation in a Hybrid Energy Storage
## 1 Introduction  
The current trends in renewable energy sources have created a significant change in the operation of the utility grid. Increasing numbers of households with photovoltaic inverters installed are viewed as local energy sources, operating in grid-feed mode. Unfortunately, it is common practice to install a three-phase inverter in the household, and the loads (TVs, household appliances, and air conditioning) are rarely connected in an optimal way--shared evenly across all three phases (Figure 1). This creates a situation where a household with a symmetrical renewable source and an asymmetrical load is seen by the distribution grid as having an asymmetrical load or asymmetrical source [1]. This situation will be much more prominent in rural areas, where the distance to a power source is significant and the household is connected to the so-called, weak grid [2]. In turn, the voltage in the area will become asymmetrical and fluctuate during the day, with negative consequences for both the household and the grid infrastructure. This situation has a negative influence on both the household devices and the grid infrastructure--for example, inverter synchronization issues or reactive power flow. In order to reduce asymmetry, a three-phase, four-wire converter (3p-4w converter) is required, along with energy storage [3, 4]. Such a converter, utilizing independent power control in each of the phases, can transfer the excess energy from the underloaded phase into the overloaded one, thus achieving a reduction in the voltage asymmetry [5]. It is also possible to achieve this effect without energy storage, but such a solution would have its compensation ability limited by the DC link capacity. Asymmetric operation of an inverter causes a current flow in the neutral wire and creates current pulsation in the DC link [6]. This pulsation is added to the DC componentof the DC link current and has a frequency two times higher than the frequency of the grid (Figure 2). This creates several issues, including increased THDi on the output of the inverter or the generation of losses in the energy storage. Losses in energy storage are turned into heat, and this effect is commonly used, in a controlled manner, to preheat the batteries before rapid charging of an electric vehicle in cold environments [7]. In a grid-tied inverter application, this effect is undesirable: since the pulsation is tied to asymmetrical output currents, it is not directly controlled and can lead to overheating of the storage. Since the energy storage usually limits its current outputs as a function of temperature, the range of operation of the storage is limited as well. Another problem is that this pulsation is often not visible to the energy storage due to the current measurement sampling time and can disrupt the state of charge (SOC) estimation, creating a faulty state in the energy storage. Operation of the inverter during an asymmetric grid state is usually considered a problem of maintaining symmetric operation during an asymmetric grid state [8]. Another trend is the asymmetric operation of the inverter and the reduction of the voltage asymmetry in the grid. The first approach typically utilizes an inverter driven by Dual-Vector-Current-Control algorithms (DVCC), which are based on symmetrical decomposition and synchronization to the positive sequence to deliver symmetric power to the grid [9]. The approach focused on asymmetrical operation uses independent control loops for each of the phases of the inverter to inject asymmetrical currents into the grid to reduce voltage asymmetry, but it also uses symmetrical decomposition to synchronize with the grid. Commonly, additional switches are introduced to the topology of the inverter to reduce the voltage pulsation in the DC link [10]. In this paper, the approach to compensating the DC link current pulsation during asymmetrical operation of the 3f4p inverter is presented. The presented system is based on a dual-active bridge operating as a pulsation compensator.