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This paper presents a new capacitively-aided zero voltage switching (ZVS) technique for isolated bus converters. The proposed technique helps to achieve ZVS conditions for both inverter and rectifier switches with the aid of magnetizing current of the transformer and capacitors interconnecting primary and secondary side switching nodes. With the proposed topology, the ZVS conditions for inverter and...
DC system is promising in data centers and future homes. It can be interconnected with ac system through power converters. One typical case is connecting the 380 V dc grid to the single-phase ac utility through a transformerless two-stage ac/dc converter. In such configuration, the ac and dc system common mode (CM) quantities are coupled through the common ground. While the high frequency noise is...
In this paper, an adaptive synchronous rectifier (SR) driving scheme for the LLC resonant converters using the ripple counter concept is proposed, along with two methods of implementation. With the proposed scheme, the SR drain to source voltage is sensed to detect the body diode conduction, based on which the SR on-time can be well tuned to eliminate the body diode conduction. One proposed implementation...
AC power electronics system are prone to instability due to the interaction between the power converters, and terminal characteristics of individual power converter based criteria are very attractive for analyzing stability of whole system. However, more and more AC power electronics systems emerge with the feature of variable fundamental frequency, while existing approaches for stability analysis...
A controller is proposed for a medium voltage medium frequency (MVMF) AC collection grid for utility scale Photovoltaic (PV) farms. A new MVMF AC collection grid was presented in previous work. It enhanced the system modularity, increased the system efficiency and reduced the passive component sizes. The MVMF AC collection grid comprises modules of inverter and medium frequency transformer that are...
One important type of power conditioning systems in photovoltaic (PV) application is the string inverter which requires small input voltage and current ripple. In addition, high-efficiency and high-power density are also the critical requirements for string inverter system. So the input voltage and current ripple cannot be easily rejected by paralleling large conventional electrolytic capacitors in...
For constant voltage constant frequency pulse-width modulation (PWM) inverter system, repetitive control (RC) can achieve zero steady-state tracking error for any periodic signal. Multi-rate repetitive control (MRC), which is featured by a fast system sampling rate and a reduced RC rate, is able to lower CPU computation load while achieving low tracking error and fast convergence speed. To accurately...
A series distributed generation (DG) units based islanding microgrid configuration and the corresponding power sharing method are proposed in this paper. Unlike a conventional islanding microgrid where parallel DG units are connected to the point of common coupling (PCC) to supply electricity to loads in a cooperative manner, this paper discusses the possibility of using series connected low voltage...
Traditional droop-controlled systems assume that the generators are able to provide sufficient power as required. This is however not always true, especially in renewable systems, where the energy sources (e.g., photovoltaic source) may not be able to provide enough power (or even loss of power generation) due to the intermittency. In that case, unbalance in active power generation may occur among...
Microgrids with increased penetration of renewables have significant frequency excursions to power generation changes. A straight forward approach to solve this problem is by enhancing the inertia of the system using energy storage (ES). They provide Dynamic Frequency Control (DFC) support to the interconnected Distributed Energy Resources (DERs) in the microgrid. However, the amount of inertial support...
Wireless power transfer is a promising alternative option for electric vehicle charging due to its non-contact operation. However, the magnetic coupling variation caused by misaligned coils limits its practical application. The output voltage can significantly drop due to the coupling change, lowering the power transfer capability. A uniform-gain frequency tracking control is proposed to keep the...
Increasing the penetration of distributed renewable sources, including photovoltaic (PV) sources, poses technical challenges for grid management. The grid has been optimized over decades to rely upon large centralized power plants with well-established feedback controls, but now non-dispatchable, renewable sources are displacing these controllable generators. By programming autonomous functionality...
This paper presents a high power density 2 kW single-phase inverter, with greater than 50 W/in3 power density and 90% line-cycle average efficiency. This performance is achieved through innovations in twice-line-frequency (120 Hz) energy buffering and high frequency dc-ac power conversion. The energy buffering function is performed using an advanced implementation of the recently proposed stacked...
This paper analyzes first the stability of single-loop digital voltage control scheme for the LC-filtered voltage source inverters. It turns out that the phase lag, caused by the time delay of digital control system and by the use of integral controller, can stabilize the voltage loop without damping of LC-filter resonance. The stability regions are then identified with alternative voltage controller...
In this paper, the stability of solid state transformer (SST)-paralleled inverters system is investigated by using the unified impedance-based stability criterion (UIBSC). Theoretical analysis reveals that the output impedance characteristics of SST are frequency-dependent and multiple resonances can happen in the SST-paralleled inverters system. A lead-lag controller and a negative impedance feedback...
In this paper, control of a grid-tied converter as a voltage controlled converter is investigated for distribution level grids. Small signal model of the converter is presented and a control structure is designed to deliver desired active and reactive power. This controller is using power flow equations to control the converter as a synchronous machine and without the need for a dedicated synchronization...
Integrated starter/generator based on the wound-rotor synchronous machine is becoming increasingly popular in modern aircraft due to the advantages of high safety and low cost in maintenance. In this paper, three detailed excitation control methods for the two-phase brushless exciter of a wound-rotor synchronous starter/generator in the starting mode were proposed and compared. In order to get constant...
Open-end winding topology with a common DC source has path for zero-sequence current. Zero-sequence current needs to be suppressed because it does not contribute to the drive torque but has harmful effects such as loss and torque ripple. Both inverter and motor have sources of the zero-sequence component. The zero-sequence source on the inverter side is the voltage error due to dead time in switching...
The PI-based current control scheme in dq-frame and PR-based one in αβ-frame are the two most popular current control schemes. It is commonly known that there exists axes cross-coupling in dq-frame, while in αβ-frame it doesn't. And decoupling terms are usually performed to decouple dq-axes cross-coupling. However, PR control scheme presents different performance, when compared with dq-frame PI control...
High switching frequency is one of the effective methods to improve power density for a LLC resonant converter. However, conventional controllers such as a digital signal processor (DSP) and an analog controller have a performance of the limited frequency resolution which introduces high primary and secondary side current variations and poor output voltage regulation at high switching frequency. In...
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