Branislav Hredzak

A Model Predictive Control System for a Hybrid Battery-Ultracapacitor Power Source

A model predictive control system for a hybrid battery-ultracapacitor power source is proposed and experimentally verified in this paper. The main advantage of the proposed system is that the battery current, the battery state of charge, and the ultracapacitor current and voltage are maintained within predefined limits during the operation. In addition, the controller allocates fast current changes to the ultracapacitor while the battery responds mainly to slow current changes which helps to increase the battery lifetime. The presented experimental results verify operation of the proposed system.

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

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.

A proximate-time-optimal-control design and its application to a hard disk drive dual-stage actuator system

We propose a compensator-based strategy for design of a track-seeking and track-following control system for a dual-stage servo actuator in hard disk drives. A well-known decoupling structure is employed to disconnect the control of the primary voice coil motor (VCM) actuator from the loop for a secondary high-bandwidth actuator. The compensator is placed in the secondary loop and suitably combined with a saturation nonlinearity in order to obtain actuator signal boundedness. The design procedure consists of four steps: 1) design of an established nonlinear seek-settle-track following controller for the VCM; 2) design of a linear track following controller for the secondary actuator; 3) observer design; and 4) design of a compensator to retain global stability and to improve performance. The proposed control system improves performance of both long-span seeking (proximate-time-optimal controller) and short-span seeking. In addition, it achieves high-bandwidth track following performance. The experimental results show good track-following performance, and short-span/long-span-seeking performance with fast settling time. The overshoot during track seeking can be made negligible for a suitably tuned VCM-actuator control loop.

A Low Complexity Control System for a Hybrid DC Power Source Based on Ultracapacitor–Lead–Acid Battery Configuration

A dc hybrid power source based on the combination of ultracapacitor and lead-acid battery is considered in this paper. The various control systems for such hybrid power source reported in the technical literature thus far are rather complex. A low complexity control system for such hybrid power source is proposed in this paper. The key feature of the proposed control system is its capability to maintain operation of the hybrid power source within all important operational limits. The proposed control system allows one to allocate the high-frequency current demands to the ultracapacitor and specify the current limits for both the battery and the ultracapacitor. It also maintains operation of the battery within its state of charge limits and the ultracapacitor voltage at a predefined value while charging the ultracapacitor from the battery rather than from the common dc bus. Presented experimental results verify the satisfactory operation of the power source utilizing the proposed control system.

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

Multilevel-cascaded H-bridge converters are promising candidates for next generation photovoltaic power converters. They feature reduced switching losses and higher conversion efficiency with modular structure; characteristics vital for large-scale photovoltaic power plants. However, the stochastically-variable nature of irradiance levels and ambient temperatures affects the normal operation of this topology, because power levels in the three phases can be unequal. The existing zero sequence injection method can deal with the power imbalance problem, but it is limited in its application. The paper proposes a zero sequence injection method to optimize the converter power balance, extending the converter operation with severe power imbalance. Based on the proposed optimal method, a simplified optimal zero sequence injection method requiring less calculation effort is derived and compared with the optimal method. Simulation and experimental results validate the effectiveness and feasibility of the proposed methods.

Single-Phase Grid-Connected LiFePO

AC line integrated energy storage systems are attractive as they increase the system efficiency by reducing the number of required power processing stages. In this paper, operation of a recently proposed battery-supercapacitor hybrid energy storage system (HESS) comprising two DC/AC boost converters, battery, supercapacitors, grid connection, state of charge (SOC) estimation, and associated control systems is experimentally verified and further improved. The improvement is achieved by a phase-shifted interleaved operation of the boost converters. The proposed phase-shifted interleaved operation reduces the switching frequency current ripple component in both the battery and supercapacitor currents. Experimental results show that during the interleaved operation, the HESS operates as expected and allocates all fast current variations to the supercapacitor, while the battery responds to slow varying current demands. At the same time, the control system maintains the supercapacitor voltage at around a predefined value and the batterys SOC, estimated using an extended Kalman filter, is maintained within the specified SOC limits.

_{\bf 4}

This paper proposes dynamic energy level balancing between distributed storage devices as a strategy to improve frequency regulation and reliability in droop controlled microgrids. This has been achieved with a distributed multi-agent cooperative control system which modifies the output power of droop controlled storage devices so that they reach a balanced energy state. As the storage devices approach a common energy level they are able to contribute their full power capacity to deal with generation and demand fluctuations in the microgrid. The cooperative control system also provides secondary frequency control, restoring the microgrid to the reference frequency. Simulations have been completed showing that the cooperative control system improves frequency regulation compared to traditional droop control strategies when the storage devices begin at different energy levels and the microgrid experiences generation or demand variability. A control input saturation constraint has been developed which ensures that the cooperative control system will not overload the storage devices.

Battery–Supercapacitor Hybrid Energy Storage System With Interleaved Boost Inverter

A cascaded H-bridge (CHB) converter is one of the viable options for next-generation large-scale photovoltaic (PV) power conversion. Owing to the increased number of components involved, converter reliability and fault-tolerant control are important issues. A CHB converter must also manage unequal power generation among bridges, which is inherent in PV applications because of unequal solar irradiance and/or module temperatures. This paper proposes a fault-tolerant strategy, which maintains balanced three-phase grid currents during faults with unequal power generation in healthy H-bridges. The effectiveness of the presented fault-tolerant control is verified using the results obtained from a 430-V 10-kW laboratory prototype.

Distributed Cooperative Control of Microgrid Storage

This paper proposes a unified distributed control strategy for DC microgrid operating modes, without bus voltage signaling or mode detection mechanisms that are normally required for decentralized control strategies. The proposed control strategy is based on the novel integration of distributed controllers for energy balancing between DC microgrid energy storage systems with distributed controllers used to regulate the average DC microgrid bus voltage, and a new method for controlling the grid connected rectifier that maintains the distributed control structure. Under the proposed control strategy, seamless mode transitions are achieved between qualitatively different operating modes, namely, 1) grid connected operation with the rectifier providing load balancing, 2) grid connected operation with the rectifier charging the energy storage systems, and 3) islanded operation. In all operating modes, the average DC microgrid bus voltage is regulated to the microgrid voltage reference, and the energy storage systems are controlled independently of the operating mode to achieve and maintain a balanced energy level. Simulations are presented demonstrating the performance of the proposed control strategy for a 380 VDC datacenter with intermittent photovoltaic generation and communication delays expected from a WiFi control network implementation.

Operation of Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Power Plants Under Bridge Failures

This paper proposes a multi-agent control strategy to coordinate power sharing between heterogeneous energy storage devices distributed throughout a DC microgrid. Without requiring a central controller, the proposed control strategy extends the benefits offered by hybrid energy storage systems to DC microgrids with batteries and ultracapacitors spatially distributed at different levels of the power distribution hierarchy. The proposed control strategy has the following advantages. 1) The high frequency microgrid load is provided by the ultracapacitors. 2) The low frequency load is provided by batteries used for bulk energy storage during islanded mode, and the main grid during grid connected operation. 3) The ultracapacitor voltages are regulated at a desired reference. 4) State of charge balancing is provided between the batteries. 5) The energy storage systems cooperate based on neighbor-to-neighbor output feedback over a sparse communication network. The only communication requirement is a spanning tree from the ultracapacitor leaders and battery leaders to their respective followers. Simulations are presented demonstrating the performance of the proposed control strategy for a 380 VDC datacenter during grid connected and islanded operation.