The increasing penetration of renewable energy sources (RES) poses a major challenge to the operation of the electricity grid owing to the intermittent nature of their power output. The ability of utility-scale battery energy storage systems (BESS) to provide grid support and smooth the output of RES in combination with their decrease in cost has fueled research interest in this technology over the last couple of years. Power electronics (PE) is the key enabling technology for connecting utility-scale BESS to the medium-voltage grid. PE ensure energy is delivered while complying with grid codes and dispatch orders. Simultaneously, the PE must regulate the operating point of the batteries, thus for instance preventing overcharge of batteries. This paper presents a comprehensive review of PE topologies for utility BESS that have been proposed either within industry or the academic literature. Moreover, a comparison of the presently most commercially viable topologies is conducted in terms of estimated power conversion efficiency and relative cost.

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A Review of Power Electronics for Grid Connection of Utility-Scale Battery Energy Storage Systems

Lifecycle cost assessment and carbon dioxide emissions of diesel, natural gas, hybrid electric, fuel cell hybrid and electric transit buses

Multilevel cascaded H-bridge converters are promising candidates for large-scale photovoltaic power plants. They allow direct connection to medium-voltage distribution networks without the presence of bulky line frequency power transformers. Owing to the stochastically variable nature of irradiance level, ambient temperature, and other factors, power levels in the three phases are expected to be unequal. The power imbalance condition creates unexpected problems with this topology, which was initially designed to operate under balanced power conditions. To deal with this issue, the paper proposes three novel zero-sequence injection methods as an expansion to the conventional zero-sequence injection method. Results obtained from simulations and a 430-V 8-kW three-phase seven-level cascaded H-bridge prototype are presented to verify the effectiveness and feasibility of the proposed methods.

Power Balance of Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Integration

Model predictive control (MPC) methods for applications in power converters have received considerable attention in recent years. The idea behind MPC is to use a mathematical model of the system to predict its future behavior within a predefined time. An optimization problem that includes the control objectives, the predicted variables, and possible constraints of the system is solved, yielding the control actions to be applied.

Assessing Finite-Control-Set Model Predictive Control: A Comparison with a Linear Current Controller in Two-Level Voltage Source Inverters

Implementation of proportional-integral controllers in synchronous reference frame is a well-established current control solution for electric machines. Nevertheless, their gain selection is still regarded to be poorly reported, particularly in relation to the influence of the computation and modulation delay. To fill this gap, a design procedure to set the maximum gains for an acceptable damped response, with the delay being considered, has been recently proposed. In contrast, this paper presents a simple rule of thumb to achieve nearly the minimum settling time in combination with negligible overshoot for reference changes. This conclusion is theoretically demonstrated by the analysis of root locus diagrams and of overshoot versus settling time trajectories for sweeps of gain values. The design approaches aimed at gain maximization and the one developed here are compared, revealing that the latter provides shorter settling time and much lower overshoot in the command tracking response, while allowing greater stability margins. On the other hand, the proposed tuning method leads to a worse disturbance rejection, but by including an active resistance with enhanced pole/zero cancellation as a second degree of freedom, both design procedures attain comparable and optimized attenuation of disturbances. Matching simulation and experimental results validate the theoretical study.

Tuning Method Aimed at Optimized Settling Time and Overshoot for Synchronous Proportional-Integral Current Control in Electric Machines

This paper analyzes the behavior of discrete-time current regulators for ac machines operating at high ratios of fundamental-to-sampling frequencies, a situation common for high-speed automotive drives and large-traction drives. At high ratios of fundamental-to-sampling frequencies, highly oscillatory, or unstable, response can occur if the current regulator design does not properly incorporate the effects of the discrete nature of the controller, including delays between the sampling of signals and the application of the voltage commands through pulse-width modulation (PWM). This paper investigates these issues for different design methods and current regulator topologies. As part of this investigation, a simple discrete-time domain ac machine model is developed that includes the delays associated with PWM. This model is then used to design a discrete-time domain version of the complex vector PI current regulator that demonstrates improved response compared with the other regulators studied. Simulation and experimental results are provided to compare the performance, stability, and robustness of the current regulators analyzed.

Discrete-Time Current Regulator Design for AC Machine Drives

This paper analyzes the behavior of discrete-time current regulators for AC machines operating at high ratios of fundamental-to-sampling frequencies, a situation common for high speed automotive drives and large traction drives. At high ratios of fundamental-to-sampling frequencies, highly oscillatory, or unstable, response can occur if the current regulator design does not properly incorporate the effects of the discrete nature of the controller, including delays between the sampling of signals and the application of the voltage commands through pulse-width modulation (PWM). This paper investigates these issues for different design methods and current regulator topologies. As part of this investigation a simple discrete-time domain AC machine model is developed that includes the delays associated with PWM. This model is then used to design a discrete-time domain version of the complex vector PI current regulator that demonstrates improved response compared with the other regulators studied. Simulation and experimental results are provided to compare the performance, stability, and robustness of the current regulators analyzed.

Discrete-time current regulator design for AC machine drives

Evaluation of cascaded H-bridge inverter for utility-scale photovoltaic systems

A system and method of controlling a multi-phase DC-DC converter having a first phase-leg and a second phase-leg using a single current sensor and a controller is provided. The single current sensor senses an amount of current flowing through the direct current link. Based on the amount of current sensed, the controller controls current flowing through the first phase-leg as well as current flowing through the second phase-leg.

Multi-phase dc-dc converter control

A power generation system includes at least one generator having at least two sets of stator windings, an active rectifier comprising power cell based modular converters associated with each set of generator windings. Each set of windings is connected to an AC voltage side of the associated active rectifier, with each active rectifier having a positive DC voltage output and a negative DC voltage output. The DC voltage outputs of active rectifiers are connected to each other in series. A medium voltage DC (MVDC) collection network comprises positive pole cables and negative pole cables, wherein each positive pole cable is connected to the positive DC voltage output of a first active rectifier and each negative pole cable is connected to the negative DC voltage output of a last active rectifier. A substation receives the negative and positive pole cables of the MVDC collection network for further transformation and transmission.

Hongrae Kim

Papers

Discrete-Time Current Regulator Design for AC Machine Drives

Discrete-time current regulator design for AC machine drives

Evaluation of cascaded H-bridge inverter for utility-scale photovoltaic systems

Multi-phase dc-dc converter control

Medium voltage dc collection system