This article proposes a new architecture for the hybrid distribution transformer (HDT) with a partially rated power converter connected in series with the existing distribution transformer. The proposed single-phase system is constructed by adding a three-legged power converter in series to the existing transformer's primary side. Here, the way the converter is connected to the transformer's two taps provides the following advantages: 1) It is possible to reduce the power converter's power rating, and 2) An additional current loop, which gives a degree of freedom, is added. For better power quality, the proposed system synthesizes a voltage vector to compensate for the voltage sags/swells and the grid voltage harmonics. Furthermore, it improves the grid's power factor to increase the power transmission efficiency. The proposed system meets the needs of smart grids in terms of resilience. The configuration, control strategies, analysis, and activation/deactivation sequence of the proposed system are discussed in this article. Real-scale simulations and lab-scale experiments were performed to validate the proposed system, and the system's effectiveness was verified through the obtained results.

This article proposes an offline compensation method for current scaling gains in the AC motor drive systems with three-phase current sensors. Many articles have been presented about the current scaling gain compensations of inverters. Most of them can be adopted during rotations. Those were studied for two-phase current sensing. Applications, such as elevators and hoist cranes, generally measure three-phase currents directly using three current sensors in preparation for a fault in any one of them. In this article, we present a compensation method for a three-phase current sensing system. This article analyzes the effect of current scaling gain errors on the neutral current that is calculated with measured stator currents. Based on the analysis, a compensation strategy for the current scaling gains is presented. The proposed method is applicable only at a standstill. It can be used for initial commissioning since it works at a standstill regardless of whether the rotor is locked or not. The experiments that demonstrate the validity of the proposed method were carried out in an elevator system as well as a laboratory. The proposed method is simple to be implemented and it requires no additional hardware.

This article proposes an improved neutral-point voltage balancing control (NPVBC) with time delay compensation and an antiwindup loop based on a dynamic change limit for a three-level neutral-point clamped (NPC) voltage-source inverter. NPVBC is essential for a three-level NPC inverter since it has a separate dc link. In an NPVBC based on the carrier-based pulsewidth modulation (CBPWM), the neutral-point (NP) voltage is regulated by synthesizing the NP current. The conventional NP voltage balancing controller is designed as a proportional-integral controller; however, time delays due to the digital control and pulsewidth modulation, and antiwindup loop were not considered despite the fact that digital control delay disturbs the accurate synthesis of NP current and the windup phenomenon may appear on the regulator since synthesizable NP current range can be limited by operating condition. To solve these problems, this article proposes improvement methods for NPVBC with digital delay compensation and an antiwindup loop based on CBPWM. Analysis on the effect of digital delay is described, and the time delays compensation method is proposed. Furthermore, an NP voltage balancing controller with an antiwindup loop is proposed along with an analysis of the synthesizable NP current range. With the proposed method, NP voltage ripples can be reduced, and control stability of the NPVBC can be greatly improved by preventing the windup phenomenon that may occur when the inverter output is insufficient. To verify the proposed methods, simulations and experiments, including elevator tower test, were conducted, and the results verify the effectiveness of the proposed methods.

This paper proposes a new unified power flow controller (UPFC) topology. A single phase of them system with the proposed topology consists of an N:2 transformer with a center tap at the low-voltage side and a power converter module comprising full- and half-bridge converters. A three-phase system can be implemented with three devices. While the conventional UPFC topology uses two three-phase transformers, which are called series and parallel transformers, the proposed topology utilizes three single-phase transformers to implement a three-phase UPFC system. By using an autotransformer structure, the power rating of the transformers and the voltage rating of switches in the power converter module can be significantly decreased. As a result, it is possible to reduce the installation spaces and costs compared with the conventional UPFC topology. In addition, by adopting a full- and half-bridge converter structure, the proposed topology can be easily implemented with conventional power devices and control techniques. The techniques used to control the proposed topology are described in this paper. The results obtained from simulations and experiments verify the effectiveness of the proposed UPFC topology.

This article proposes a standstill sensorless self-commissioning strategy for synchronous machines, including the initial positioning and torque canceling techniques. The proposed identification consists of the self-axis test to identify self-saturation and the cross-saturation test to identify cross-saturation. An inverter feeds the machine during the process. The induced current and the flux data can be gathered from multi-points within the current constraints to identify the flux saturation characteristics. The induced current generates the torque, including the magnetic and reluctance torque. The generated torque moves the rotor, and the data would be distorted since the data gathering assumes the rotor stands still. However, the rotor position can be maintained if each torque is canceled. The proposed method considers not only the magnetic torque but also the reluctance torque, which is not considered in the conventional method. Initial positioning with zero speed is achieved with the proposed techniques. So, more accurate data can be obtained, and the self-commissioning time is significantly reduced since the additional alignment process can be removed. Experiments were performed on a 1.5 kW SynRM and an 11 kW IPMSM, and the results proved the effectiveness of the proposed method.

This article proposes a parameter estimation technique for a self-saturation model of synchronous motors. The standstill self-identification method requires an equation-based flux saturation model for flux saturation modeling. The selected self-saturation model has the form of a polynomial representing a current with respect to a magnetic flux. This flux saturation model contains unknown parameters, including exponents, coefficients, and a current source for a permanent magnet. The exponent is a crucial parameter representing the nonlinear current and magnetic flux relationship. Therefore, the estimated flux saturation model only represents the flux saturation well if an appropriate exponent is selected. The proposed method can estimate the exponent of the self-saturation model for synchronous reluctance motors (SynRMs) using a linear least squares method (LSM). In addition, parameters of the self-saturation model for permanent magnet synchronous motors (PMSMs), including the exponents and a constant current source, can be estimated simultaneously. The rest of the coefficients of the flux saturation model are estimated using a linear LSM based on the pre-estimated exponents and a constant current source. The proposed parameter estimation method can be implemented in embedded systems. The performance of the proposed parameter estimation method was verified in 1.5-kW SynRM and 11-kW interior PMSM.