This paper analyses the requirements for load balancing of a new transformer-less grid interface topology for large-scale electric vehicle (EV) charging infrastructures. The proposed configuration utilizes a Modular Multilevel Converter (MMC) to supply wireless EV chargers from each module. The inherent galvanic isolation provided by wireless inductive power transfer and the scalability of the MMC topology enable transformer-less connection to medium voltage (MV) distribution grids. This can reduce the footprint and copper volume of the internal power distribution for the parking infrastructure. The load distribution within the MMC topology depends on the location and power requirements of each EV to be charged. Requirements for load balancing by controlling the internal circulating currents of the proposed topology when supplying unevenly distributed loads are derived. It is also demonstrated how a second harmonic component of the circulating currents can be utilized to ensure balancing capability within each MMC arm, and how its required amplitude depends on the load distribution. The theoretical analysis and the performance of a corresponding control strategy are first verified by time-domain simulations of a large-scale infrastructure. Experimental results from a small-scale prototype based on an MMC where each arm has 12 modules with individual controllable loads are presented.

Thermal design is particularly important for high-power and compact inductive power transfer (IPT) systems having a limited surface area for heat dissipation. This article presents the thermal design and optimization of a 30-kW IPT system for electric vehicles. An improved analytical thermal model with high accuracy for liquid-cooled magnetic couplers was proposed by using the thermal network method (TNM). It considers heating components and thermal interface materials. Then, the multiobjective thermal optimization procedure of the liquid-cooled magnetic coupler was conducted with the presented model. Tradeoffs among temperature rise, weight, and cost were discussed, and an optimized solution was selected. The thermal FE models were established and compared with the thermal networks. Subsequently, the thermal performance of the system at different power levels and misaligned conditions was analyzed. The experimental setup based on the fiber Bragg grating sensors was built, and the prototypes were tested with an output power of around 28 kW. The error of stable temperature between the experiment and the prediction was less than 10% at the measurement points, verifying the thermal models. The proposed thermal models and optimization procedure accelerate the thermal design of IPT systems, toward higher power density.

This article identifies how constant voltage load characteristics cause inductive power transfer systems to exhibit a poorly damped oscillation mode. When operated with pulse skipping strategies such as pulse density modulation (PDM), the skipped voltage pulses can excite this mode and cause severe oscillations that do not appear in systems with a constant resistance load. The critical mode is identified from a linearized state-space model of the system and two control approaches are proposed for attenuating the oscillations in current amplitude and power flow. First, the influence of the operating frequency on the critical eigenvalue is analyzed and it is shown how slightly off-resonant operation can increase the damping of the oscillation mode. Second, an active damping method based on sending current feedback control is studied. The active damping is based on phase shift modulation with limited phase shift angles applied to the PDM signal when oscillations are detected. The effectiveness and feasibility of the proposed methods are validated by simulations and experimental results from a small-scale laboratory prototype.

This letter proposes a conditional pulse density modulation (PDM) method for inductive power transfer (IPT) systems, which can automatically adapt the generated output patterns to effectively suppress current/power ripples. The proposed conditional PDM introduces a sending current limitation as an additional condition to skip pulses, which modifies the output patterns generated by conventional delta-sigma-based PDM. Thus, the PDM pulse pattern is continuously adapted to attenuate any excited current/power oscillations. The presented modulator is simple to implement and well-suited for IPT systems with poorly damped constant voltage load characteristics. Simulations and experiments demonstrate the effectiveness of the proposed modulation method.

This paper presents a sending-side gain-scheduled controller for improving the dynamic response of Inductive Power Transfer (IPT) systems with sub-resonant frequency control. The presented method is intended for inductive battery charging in high-power transport applications which are required to operate under highly variable conditions, including a wide range of coupling and output power. For such applications, the use of fixed controller gains selected to ensure the system stability in the full range of operation causes a slow response under low coupling or light load conditions. For implementing the gain-scheduling, the output power and coupling coefficient of IPT systems with the sub-resonant frequency control are estimated in real-time by using only sending-side information. The parameters of the gain-scheduled controller are then determined by the output power value and the estimated coupling, to ensure the rapid response and stability of the system. Moreover, the bandwidth limitations and risks of instability caused by dual-side communication in closed-loop power control is avoided. Effectiveness and feasibility of the proposed method are validated by time-domain simulations and experimental results from a small-scale laboratory prototype.

The Cascaded H-Bridge (CHB) multilevel converter is an attractive solution for integrating Photovoltaic (PV) generators with the ac grid. However, the power generated in the PV panels may be unequal due to partial shading, temperature change, and dust layer accumulation leading to unequal and unbalanced power between the phases and possible violation of power quality constraints. Fundamental Frequency Zero-Sequence Voltage (FFZSV) injection can be applied but this method can produce overmodulation in conditions with high PV power mismatch. A new FFZSV is proposed in this paper assuming a CHB with a Battery Energy Support (BES) to compensate for the effects of unbalanced PV power generation. A barycentric coordinate representation helps to determine the capacity of the BES required. A coordinated control between the PV system and BES is also proposed in the paper. Additionally, a novel approach to deliberate FFZSV injection is discussed in this paper by which the maximum charge and discharge rate for the BES can be obtained along with balanced current injection into the grid. The effectiveness of the proposed technique is verified in numerical simulations and with experimental results.